tag:blogger.com,1999:blog-53133964953352195682024-03-06T01:21:31.369-07:00Scientific ExplorerFor those who are curious about the nature of matter and energy in our universe.Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.comBlogger164125tag:blogger.com,1999:blog-5313396495335219568.post-75852596517938289002021-03-06T13:35:00.000-07:002021-03-06T13:38:47.194-07:00 Electrical Current<p><span face="Calibri, sans-serif">This is a question on many student’s minds: what are the electrons actually doing inside a current-carrying wire? </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span lang="EN-US">The answer might seem straightforward at first glance. An <a href="https://en.wikipedia.org/wiki/Electric_current">electrical current</a> is a flow of charge moving through an electrical conductor or through space. From <a href="http://electronics-notes.com">electronics-notes.com</a>, we have a practical definition of electrical current: it is “the rate of change of flow past a given point in an electric circuit.” From <a href="http://physicsclassroom.com">physicsclassroom.com</a> (an excellent teaching site), the definition alludes to a bit of scientific history: An electric current is “by convention the direction in which a positive charge would move.” Electrons “move through the wires in the opposite direction.” Confusing for many students, <i>conventional</i> current in a wire moves from the positive terminal of a battery to the negative terminal. Electrons, being negative charge carriers, actually move in the opposite direction. This might be where many of us stop in our quest to understand current, which is unfortunate because this is a fascinating exploration.<o:p></o:p></span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span lang="EN-US"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span lang="EN-US">The silver lining of this conventional terminology is that it nudges us toward the history, which shows an incredible advancement in solid-state physics. How was electrical current <a href="https://pwg.gsfc.nasa.gov/Education/woppos.html">discovered</a></span><span lang="EN-US">? We probably all know about Benjamin Franklin’s <a href="https://www.fi.edu/benjamin-franklin/kite-key-experiment">famous kite experiment</a> in 1752. We might not know that he didn’t actually discover electricity with this experiment, nor was he the first to discover that lightning was actually a type of electricity. Electrical forces had been known for hundreds of years by his time. I think, however, we can fairly credit him for contributing to our confusion about current. He studied <a href="https://en.wikipedia.org/wiki/Static_electricity">static electricity</a> by <a href="https://www.livescience.com/51656-static-electricity.html">producing a static charge</a> on the surface of glass, amber and other materials by rubbing them with fur or a dry cloth. This resulted in an exchange of electrons from one material to another. At that time, electrical current was called “electrical fluid,” and as such, Franklin guessed that some materials (such as glass that’s rubbed) contained more of this fluid than others. To his thinking, these charged objects contained excess, or positive, electricity, while others contained a deficiency of the fluid, or negative electricity. Electric batteries were developed soon afterward and it seemed natural to assign the direction of electrical flow from positive to negative (excess to deficient). It was only when electrons, the <a href="https://en.wikipedia.org/wiki/Subatomic_particle">subatomic particles</a> responsible for static charge, were <a href="https://en.wikipedia.org/wiki/J._J._Thomson#Discovery_of_the_electron">discovered</a> about one hundred years later, that scientists realized these particles move in the opposite direction. It is an excess of electrons that produces a negative charge, so the flow from excess to deficient must actually be from a negative terminal to a positive terminal. <o:p></o:p></span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span lang="EN-US"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span lang="EN-US">This “<a href="http://web.engr.oregonstate.edu/~traylor/ece112/beamer_lectures/elect_flow_vs_conv_I.pdf">conventional current</a>” (positive to negative) had staying power. The conventional terminal designation is still used worldwide. It’s not a problem to work with as long as it is consistent, but it can present a problem when we try to understand what is actually happening inside the <a href="https://en.wikipedia.org/wiki/Electrical_conductor">conductor</a>.<o:p></o:p></span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span lang="EN-US"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">What then is going on inside an electrical current-carrying wire, for example? Consider an everyday power cord on a vacuum cleaner. If we could zoom into a cross-section of that cord, we would see wires made of copper through which electrons travel easily and with little resistance, surrounded by material that resists current flow and provides good electrical insulation. We can imagine electrons flowing from the electrical outlet in the wall, through the cord, and into the appliance. But where do these flowing electrons end up? Do they get used up in the process of doing work somehow? When the power is shut off, are we left with an alarming reservoir of electrons somewhere inside the vacuum motor? Where do the electrons in the wall socket originate from? These great questions start us on our journey. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">When we think of electrical current as a physical flow of negatively charged particles through a material or through space, we might think of something analogous to water molecules flowing in a stream, and when we do, a number of pressing questions come to mind. Like the out-dated convention of positive to negative terminal current flow, the terms “current” and “flow” themselves lead us away from clear modern evidence that electrical current is a not a physical flow of particles at all. Many physics classrooms begin their discussion of electrical current with a <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html">water analogy</a> and it is a good place to go to get a feel for how simple <a href="https://en.wikipedia.org/wiki/Electronic_circuit">electrical circuits</a> work. But this analogy, while a good start, proves misleading as we deepen our understanding. And there is much more to this fascinating story than this. To learn it we must upgrade our understanding of what electrons are doing <i>at the subatomic level </i>inside a conductor.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">What Is an Electron?<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">All materials are made of <a href="https://en.wikipedia.org/wiki/Atom">atoms</a>. All atoms consist of a <a href="https://en.wikipedia.org/wiki/Atomic_nucleus">nucleus</a> surrounded by electrons. The nucleus is composed of neutrally charged particles straightforwardly called <a href="https://en.wikipedia.org/wiki/Neutron">neutrons</a> and positively charged <a href="https://en.wikipedia.org/wiki/Proton">protons</a>. It therefore has a positive charge. These particles are bound tightly together by a fundamental force called the strong force (strong force). This force is indeed strong. It easily overcomes the repulsive forces between the positively charged protons, but it only acts over an extremely short distance, at the scale of the nucleus itself, and from there, its influence drops off dramatically to zero. Negatively charged particles called <a href="https://en.wikipedia.org/wiki/Electron">electrons</a> surround the nucleus. They are attracted to the nucleus through the attraction of opposite electrical charges. An electrically neutral atom contains equal numbers of electrons as protons. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">We might imagine electrons moving around the nucleus in planet-like circular orbits, except that here, the atomic force is <a href="https://en.wikipedia.org/wiki/Electrostatics">electrostatic</a> rather than gravitational. This is the familiar <a href="https://en.wikipedia.org/wiki/Bohr_model">Bohr model</a> (below) introduced by Niels Bohr and Ernest Rutherford in 1913. This model of the hydrogen atom shows three possible energy shells for the electron. At n=1, the electron is at its lowest energy (ground) state. If the atom is in an excited state, the electron will be in a higher energy shell. It will emit a photon of light as it returns to a lower energy state.</p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><br /></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgfhveTESJvGQSEcrBQLHzdg3DRRtj5hjhxxO6yz8UUkpcNH9T19GbDE3bzpXJKzJvPqFbo7oeza56gkccZBOndLXt37zj6axQJ8anB-PNk3Ouj7xd1DUOsMqyFrw_lMlwzRwjeS40hiY-w/" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img alt="" data-original-height="270" data-original-width="310" height="240" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgfhveTESJvGQSEcrBQLHzdg3DRRtj5hjhxxO6yz8UUkpcNH9T19GbDE3bzpXJKzJvPqFbo7oeza56gkccZBOndLXt37zj6axQJ8anB-PNk3Ouj7xd1DUOsMqyFrw_lMlwzRwjeS40hiY-w/" width="276" /></a></div><div>The idea that electrons orbit the nucleus in specific stable orbits came from necessity. The researchers knew that atomic electrons can release energy in the form of <a href="https://en.wikipedia.org/wiki/Electromagnetic_radiation">electromagnetic radiation</a> (light) but they also knew that if an electron loses energy, it should quickly spiral into the nucleus, and in the process it would emit increasingly high frequency radiation (called the <a href="https://en.wikipedia.org/wiki/Ultraviolet_catastrophe">ultraviolet catastrophe</a>) No atom would be stable for more than a few trillionths of a second. They also knew, from experiments a few decades prior, that atoms emit light only at specific frequencies. An <a href="https://en.wikipedia.org/wiki/Excited_state">excited</a> (high energy) hydrogen atom will emit only purple, blue, aqua or red (the <a href="https://en.wikipedia.org/wiki/Balmer_series">Balmer series emission spectrum</a>, shown below), depending on the energy level of its excited-state electron, as it returns to its rest state. </div><div><br /></div><div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhPpwa4jh5ibedvMO50Xs0I5EtPnvErSlqUhB4MldcyexvJyPqh-n6Yui-Yuf4UjsF6wFlfJuX358WB0uf1vrTr0jKuHDDc2-wXVy83jM18F6vdbuqDm7meSzbVgWMPuY_bS3kbcIFPoeQv/" style="margin-left: 1em; margin-right: 1em;"><img alt="" data-original-height="123" data-original-width="1200" height="66" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhPpwa4jh5ibedvMO50Xs0I5EtPnvErSlqUhB4MldcyexvJyPqh-n6Yui-Yuf4UjsF6wFlfJuX358WB0uf1vrTr0jKuHDDc2-wXVy83jM18F6vdbuqDm7meSzbVgWMPuY_bS3kbcIFPoeQv/w640-h66/1200px-Visible_spectrum_of_hydrogen.jpg" width="640" /></a></div><br />In an excited pure hydrogen gas, you will see the whole spectrum but hydrogen will never emit green or yellow, for example. These scientists figured out that electrons in atoms must occupy discrete <a href="https://en.wikipedia.org/wiki/Bohr_model#Electron_energy_levels">energy levels</a>. They orbit at specific stable distances from the nucleus. The farther an electron is form the nucleus, the higher its energy level. An electron can move up or down only by jumping to/from specific energy levels. Energies are therefore <i>quantized</i>. They come in quanta or packets. This model is <a href="https://courses.lumenlearning.com/astronomy/chapter/spectroscopy-in-astronomy/">useful for predicting the spectral phenomena</a> of simple atoms with few electrons, like hydrogen, but it cannot explain the spectra of large complex atoms. Nor can it explain the different intensities of spectral lines for any given atom. It is also still used as a simplified <a href="https://en.wikipedia.org/wiki/Bohr_model_of_the_chemical_bond">model for chemical bonding</a> between atoms, in which atoms share one or more electrons located at their outermost energy levels.</div><p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">The modern model of the atom, the <a href="https://www.khanacademy.org/science/physics/quantum-physics/quantum-numbers-and-orbitals/a/the-quantum-mechanical-model-of-the-atom">quantum physical model</a>, describes the positions of atomic electrons not as being precisely located within energy shells but as <i><a href="https://en.wikipedia.org/wiki/Atomic_orbital">clouds of probability</a>.</i> The hydrogen atom is shown below right. The shapes of the electron orbitals are shown in yellow and blue. Denser regions of colour indicate a higher probability of the electron's location. The energy shells (1, 2 and 3) are arranged in increasing energy from top to bottom. The orbital shapes are s, p and d, shown from left to right.</p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><br /></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjq9Ep2HahvcbLMsnn1HTIRY7dLvYAPLxcjBynEkeK9a5bM1wI2feLx7e8oDGu-YODkAamPCfTNiOWEH4dB7hSOpbQCP1cWr146JQF7at981ZAmM2E5bReTsbVzsLGPyyk9IHelDFDEDX7X/" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img alt="" data-original-height="1800" data-original-width="1800" height="240" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjq9Ep2HahvcbLMsnn1HTIRY7dLvYAPLxcjBynEkeK9a5bM1wI2feLx7e8oDGu-YODkAamPCfTNiOWEH4dB7hSOpbQCP1cWr146JQF7at981ZAmM2E5bReTsbVzsLGPyyk9IHelDFDEDX7X/" width="240" /></a></div><p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">This model incorporates the fact that we no longer understand electrons to be just point charges, or tiny charge-carrying billiard balls. Under certain circumstances they do act as such. For example, an electron has a measurable <a href="https://en.wikipedia.org/wiki/Momentum">momentum</a> and it can take part in <a href="https://en.wikipedia.org/wiki/Elastic_collision">elastic collisions</a>. But electrons also act as waves and that wave nature also shows up under certain circumstances, such the creation of wavelike <a href="https://en.wikipedia.org/wiki/Electron_diffraction">interference patterns</a>. We now think of the electron as both <a href="https://www.space.com/wave-or-particle-ask-a-spaceman.html">particle <i>and</i> wave</a>, which is not easy to grasp. These qualities seem to be mutually exclusive. Electrons in conductors, however, do display both wave and particle behaviours. To predict such behaviours, we must go quantum and understand electrons as <i><a href="https://en.wikipedia.org/wiki/Wave_function">wave functions</a></i> that exhibit both particle AND wave behaviour. An electron’s position and momentum (or velocity) are now <a href="https://www.preposterousuniverse.com/blog/2014/07/24/why-probability-in-quantum-mechanics-is-given-by-the-wave-function-squared/">defined as probabilities</a>. Where and how fast an electron is going are assigned probability amplitudes, rather than specific values. Furthermore, thanks to Werner Heisenberg’s <a href="https://en.wikipedia.org/wiki/Uncertainty_principle">uncertainty principle</a> of quantum mechanics, these are <a href="https://en.wikipedia.org/wiki/Complementarity_(physics)">complementary variables</a>, which means we can only know one value at the expense of another (this part was actually formulated by Bohr). If we want to know precisely where an electron is, we cannot know its momentum at that same moment, and vice versa. Likewise, the electron’s wave and particle properties are also complementary. A single electron cannot simultaneously exhibit both its full wave-like and particle-like nature, with the exception of the famously fascinating <a href="https://en.wikipedia.org/wiki/Double-slit_experiment">double-slit experiment</a>, in which electrons show some of both behaviours at the same, which I encourage you to look up.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">This is a difficult conceptual step to make but we must because the easy-to-visualize Bohr model, as useful as it is, leaves something missing. By using <a href="https://en.wikipedia.org/wiki/Schrödinger_equation">Schrodinger’s equation</a> to mathematically describe atomic electron behaviour as a wave function, physicists can predict many of the spectral phenomena that the Bohr model cannot. It’s not easy to do in practise either; the calculations are very complex.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: red;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">What is an Electrical Conductor?<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">The atoms that make up a good <a href="https://en.wikipedia.org/wiki/Electrical_conductor">electrical conductor</a> have an atomic structure that allows the outermost electrons, called <a href="https://en.wikipedia.org/wiki/Valence_electron">valence electrons</a>, to be loosely bound to the nucleus. Valence electrons are outermost energy shell electrons. These are the electrons that take part in chemical bonding. Now we can describe chemical bonding more precisely, in quantum mechanical terms, as two or more <a href="https://en.wikipedia.org/wiki/Atomic_orbital">atomic orbitals</a> combining to form a <a href="https://en.wikipedia.org/wiki/Molecular_orbital">molecular orbital</a>. An atomic orbital is a region of space around the nucleus where an electron is most likely to be. It can be a simple spherical shape or a more complex shape depending on its energy level. The orbital shapes come from solving the Schrodinger equation for electrons bound to their atom through the electric field created by the nucleus. The orbital is part of the electron wavefunction that describes the electron’s location boundary and its wavelike behaviour.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: red;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Molecular orbitals, on the other hand, come in three different kinds: 1) <a href="https://en.wikipedia.org/wiki/Bonding_molecular_orbital">bonding orbital</a>, which has a lower potential energy than the atomic orbitals it is formed from, 2) <a href="https://en.wikipedia.org/wiki/Antibonding_molecular_orbital">antibonding orbital</a>, which has a higher energy than the atomic orbitals, so it opposes chemical bonding and 3) <a href="https://en.wikipedia.org/wiki/Non-bonding_orbital">nonbonding orbital</a>, which has the same energy so it has no effect on chemical bonding either way. A chemical bond is a constructive, in-phase, interaction between two valence electrons of two atoms. An antibonding orbital is an interaction between atomic orbitals that is destructive and out of phase. The wavefunction of an antibonding orbital is zero between the two atoms. This means there are no solutions to the Schrodinger equation for this orbital and therefore there is no probability of an electron being available to bond. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Metals tend to be good conductors because their valence electrons are loosely bound to their nuclei. They are <a href="https://en.wikipedia.org/wiki/Delocalized_electron">delocalized electrons</a>, which means that they are not associated with a particular atom or chemical bond. These molecular orbital electrons extend outward over many atoms. Although the electrons are delocalized, the metal atoms themselves are bound tightly together through <a href="https://en.wikipedia.org/wiki/Metallic_bonding">metallic bonding</a>, by electrostatic attractions between the positive nuclei and the “sea” of electrons in which they are embedded. The atoms are held tightly together, which means metals tend to have high melting and boiling points. The nuclei in metals act like positive ions in this arrangement, which means that metallic bonding is similar in this sense to <a href="https://en.wikipedia.org/wiki/Ionic_bonding">ionic bonding</a>. The resulting metal structure is a tight three-dimensional lattice arrangement, similar to the atomic structure of <a href="https://en.wikipedia.org/wiki/Ionic_crystal">ionic crystals</a> such as sodium chloride (table salt). In contrast to ionic compounds, valence electrons in the metallic lattice form molecular orbitals that extend across the entire metal. The bonding electrons themselves do not orbit the entire metal but <i>their influence</i> extends across the metal. Valence electrons in metals act more like a collective than they do in conventional chemical bonds. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">All of the valence electrons in the metal participate in molecular bonding. As vast as this collection of delocalized electrons is, the number of possible delocalized electron <i>energy states</i> is far greater. All metal atoms contain <a href="https://en.wikipedia.org/wiki/Electron_deficiency">few electrons in their valence energy orbitals</a>. <a href="https://en.wikipedia.org/wiki/Transition_metal">Transition metal</a> atoms, for example, can hold up 18 electrons in the outermost energy shell (which consists of 5 d orbitals, one s orbital and three p orbitals), but they are barely filled. If this is confusing, we will be exploring this in more detail later. The point here is there are many more possible energy states than there are electrons available to fill them. These empty available states, which are all similar in terms of energy, will become important when we look at band theory later on in this article.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">What Happens When an Electric Potential is Applied?<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">When a metal with high conductivity is placed in an electric field, all the valence electrons tend to move against the direction of the field. An <a href="https://en.wikipedia.org/wiki/Electric_field">electric field</a> is a vector force field that surrounds an electrical charge and exerts a force on other charges nearby. The electrons themselves each generate an electric field, as do the positively charge nuclei in the metal. When electrons within the metal move, they also generate a <a href="https://en.wikipedia.org/wiki/Magnetic_field">magnetic field</a>. We can begin to see electric current as a complex interplay between multiple local electric and magnetic fields.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">An analogy of a waterfall is often used to describe the movement of charged particles, such as electrons, as moving along <i>or down</i> an <a href="https://en.wikipedia.org/wiki/Electric_potential">electric potential</a>. An electric potential might at first sound the same as an electric field but the electric potential expresses the <i>effect</i> of an electric field at a particular location in the field. We could place a test charge within an electric field and measure its potential energy as a result of being in that field. (A positive test charge is usually used and this is why electrons move <i>against</i> the direction of an electric field.) The potential energy of the electric field will differ, depending on the location within that field. A negative test charge, for example, will have high potential energy close to a negative source charge and lower energy further away from it. In other words, the electron tends to move from where it would have high potential energy toward lower potential energy. Inside a copper wire attached to a battery, for example, the electron is pushed away from the high-energy negative terminal (labelled positive!) and toward the region of lowest electric potential energy, the positive terminal (labeled negative!). This difference in electric potential energy is called <a href="https://en.wikipedia.org/wiki/Voltage">voltage</a>. Voltage is defined as the amount of work required per unit of charge to move a test charge between two points. 1 volt = 1 joule of work done per 1 coulomb of charge. For example, if a 12-volt battery is used in a wire circuit to power a light bulb, every coulomb of charge in the circuit gains 12 joules of potential energy it moves through the battery. Every coulomb in turn loses 12 joules of energy into the environment as light (and some heat) as it powers the light bulb.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">The waterfall analogy, as mentioned earlier, can be misleading. Within the wire, the electrical current does not depend on electrons wriggling free from copper atoms and flowing down the wire, like water molecules would flow down a waterfall. The description of a “delocalized sea of electrons” can naturally lead to such an assumption about current. Instead, each valence electron is held loosely enough to its nucleus to “nudge” a neighbouring electron in a neighbouring atom and so on. A better analogy for electrical current might be that of fans in a football stadium row standing up one after another to do the wave. The people stay in place but the wave moves down the row. If we want to stick with water, we could say that the current is analagous to the wave travelling across a sea. Electric current can be defined as the <i>rate</i> at which <i>charge</i> (not electrons) flows past a point in a circuit. 1 ampere of current = 1 coulomb of charge/second.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">It’s easy to imagine a flow of electrons under pressure (an analogy for voltage) gliding along inside a wire like water flowing through a pipe, but current is not the physical flow of electrons, but rather the flow of the energy of their movement. We can put this idea more scientifically as a <a href="https://en.wikipedia.org/wiki/Momentum_transfer">momentum transfer</a>. Put yet another way, it is a transfer of <a href="https://en.wikipedia.org/wiki/Kinetic_energy">kinetic energy</a> from one electron to another and another and so on. We can think of nudged electrons transferring energy from one to the next like a billiard ball hitting an adjacent billiard ball along a line of billiard balls. This analogy alludes to the particle nature of the electrons.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: red;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Electrical Conduction: The Drude Model<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">The “billiard ball” description of electrical conduction is called the <a href="https://en.wikipedia.org/wiki/Drude_model">Drude mode</a>l, proposed in 1900. This model was a bit before it’s time because it predated even the <a href="https://en.wikipedia.org/wiki/Rutherford_model">(Rutherford) atomic model</a> by a few years. The “sea of electrons” embedded within a positive matrix that Rutherford envisioned (also called the raisin bun or plum pudding model) just happens to align pretty well with the Bohr valence electron model for metals, where a sea of valence electrons surrounds each atom. The Drude model is essentially a description of how kinetic energy is transferred within a conductor by treating electrons like tiny billiard balls. For those science history buffs like me, we can follow how the Drude model evolved into our modern model. We can read into it how our understanding of electron behaviour evolved. The Drude model was not a modern quantum mechanical model of what is happening, but it could predict some electron conduction behaviour by understanding conduction as a momentum transfer between electrons exhibiting particle-like behaviour. In fact, Paul Drude used <a href="https://en.wikipedia.org/wiki/Maxwell–Boltzmann_statistics">Maxwell-Boltzmann statistics</a> to derive his model. These statistics describe an average distribution of non-interacting particles at <a href="https://en.wikipedia.org/wiki/Thermal_equilibrium">thermal equilibrium</a>. This is a classical description of how an <a href="https://en.wikipedia.org/wiki/Ideal_gas">ideal gas</a> behaves, and it was the model available to him at the time. In this view, each atom in a conducting metal such as copper contributes valence electrons to a sea of non-localized, non-interacting electrons. It turns out he was accidentally right. In most cases, at least in the case of copper, we can effectively neglect the interactive forces between the electrons because they are shielded from those forces. All the relatively stationary (much more massive and more highly charged) atomic nuclei in the copper present an overwhelming influence on them. It is this <a href="https://en.wikipedia.org/wiki/Shielding_effect">shielding effect</a>) that allows us to model metal electrons fairly accurately as an ideal gas, a cloud of particles that do not interact with each other. However, the particles, in this case electrons, are far more concentrated together than the atoms in any gas would be. This means that this model doesn’t predict <i>all</i> conduction behaviour. Electrical conduction exhibits behaviours that are too complex to be described through classical ideal gas theory. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: red;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Maxwell-Boltzmann statistics were replaced around 1926 by <a href="https://en.wikipedia.org/wiki/Fermi–Dirac_statistics">Fermi-Dirac statistics</a>. Rather than focusing on the non-interaction between particles, these statistics describe a distribution of particles over a range of energy states in a group of identical particles that all obey the <a href="https://en.wikipedia.org/wiki/Pauli_exclusion_principle">Pauli exclusion principle</a>. This effectively upgraded the rule for interaction. Rather than saying the electrons don’t interact, we now say they cannot occupy the same <a href="https://en.wikipedia.org/wiki/Quantum_state">quantum state</a>. We now have to treat electrons as quantum particles. Everything about a quantum particle is described by four <a href="https://en.wikipedia.org/wiki/Quantum_number">quantum numbers</a> (spin, magnetic, azimuthal and principle). Electrons belong to a special division of quantum particles called <a href="https://en.wikipedia.org/wiki/Fermion">fermions</a>. Fermions can share any three quantum numbers but they can’t overlap in the same location at the same time. A <a href="https://en.wikipedia.org/wiki/Boson">boson</a> such as a photon can, and it is governed by a different set of statistics. We are introducing the Pauli exclusion principle to the gas-like behaviour of valence electrons inside a metal. The Pauli exclusion principle is a critically important rule that electrons in atoms (and all fermions) must obey. It is a <i>quantum mechanical</i> principle. The implications are quite profound. It means that the <a href="https://www.physicsclassroom.com/class/energy/Lesson-1/Potential-Energy">potential energy</a> of the gas-like electrons must now be taken into account, and by doing so, it limits the numbers of electrons that occupy each orbital in an atom. This rule forbids valence electrons from moving down to occupy already filled lower energy states in the atom. This means there is a lower limit on the potential energy of the atom’s (lowest energy) <a href="https://en.wikipedia.org/wiki/Ground_state">ground state</a>. By incorporating quantum mechanical effects, this model significantly improved the predictions we can make about conducting electron behaviour. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Band Theory of Metals<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">The Drude model, as good as it is, is still missing a component we need to take into account with conduction in metals. We need to consider the effects of how molecular bonding relates to the conductivity of the metal. We’ve been modelling the conduction electrons as particles that have limited occupied energy states but otherwise act like strangers to each other. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Molecular orbitals in metals are much larger than atomic orbitals, which are confined within the atom. In fact, molecular orbitals extend and overlap each other throughout the whole material. Bearing this in mind, the word “orbital” here can seem misleading. There is no orbiting implied here. Better said, the set of molecular orbitals that are created in the metal are <i>a set of interactions</i> that are generated by the valence orbitals of the interacting atoms. This molecular i<i>nteraction</i>, rather than the electrons themselves, extends throughout the material. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Metallic bonding means that a very high number of atomic valence orbitals interact <i>simultaneously</i> within a metal. This is a critical key to explaining why metals can be such good electrical conductors. Within a free atom, an atom that is not chemically bonded to any other atom, the energy that an electron can possess must fall into one of several possible <i>discrete</i> energy levels. Within a conductive metal, on the other hand, due to the overlapping of a huge number of molecular orbitals, an enormous number of new possible energy states opens up. The available energies of the valence electrons are no longer confined to discrete energies but instead now to a <a href="https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/11%3A_Chemical_Bonding_II%3A_Additional_Aspects/11.7%3A_Bonding_in_Metals"><i>band</i> of available energies</a> that can vary in width. This molecular orbital approach is called <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/band.html">band theory</a>. It’s a very useful way to visualize how conduction works, and to understand why some materials are better conductors than others. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Why are the valence energies spread out into a band? First, we must distinguish between an orbital and a shell. Inside any atom, valence electrons are confined to the outermost (highest energy or bonding) valence <a href="https://en.wikipedia.org/wiki/Electron_shell">energy shell</a>. An electron shell is all about the energy. An <a href="https://en.wikipedia.org/wiki/Atomic_orbital">atomic orbital</a> is about where an electron is <a href="https://en.wikipedia.org/wiki/Atomic_orbital#/media/File:Atomic-orbital-clouds_spdf_m0.png">most likely to be found</a>. To distinguish these two concepts, we can imagine that we are adding electrons to an atom. First we fill up a sphere-shaped 1s-orbital, which can hold two (opposite-spin) electrons. Then we fill up the next highest energy 2s spherical orbital with two electrons Then we start to fill three 2p orbitals (three dumbbells shapes in three-dimensions) with 2 electrons each, six electrons total, and so on. Every atom of every element possesses the same <i>number</i> of potential orbitals and shells, but they differ in the number of electrons inhabiting them and the orbitals themselves can differ in size. The 1s electrons belong to the lowest energy K shell. The 2s and 2p electrons belong to the next higher energy M shell. The M energy shell, which is just a circle in a Bohr diagram, can now described in three-dimensional detail, as a double-lobbed <i>and </i>spherical orbital. The three 2p orbitals (which extend along the x, y and z axis in three-dimensional space) and 2s orbital hold a total of eight electrons in the M energy shell. A simplified diagram is shown below left.</p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><br /></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEikxtI82_3buSP1FgzVhIWdk5EmNHPZ7H84O7RuYGhSPb803WJGEMncWAwDKBU3WigL9cvQnolvHjaDDe0XQ0AvG4Gq8VVw1lqDab4LJ8VIWsgrsZVtnp3WKWa_JNP7JENsrwY6ZFH9WGH1/" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img alt="" data-original-height="1554" data-original-width="2048" height="240" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEikxtI82_3buSP1FgzVhIWdk5EmNHPZ7H84O7RuYGhSPb803WJGEMncWAwDKBU3WigL9cvQnolvHjaDDe0XQ0AvG4Gq8VVw1lqDab4LJ8VIWsgrsZVtnp3WKWa_JNP7JENsrwY6ZFH9WGH1/" width="316" /></a></div>The same energy level can contain multiple orbitals. This is <a href="https://en.wikipedia.org/wiki/Hund%27s_rule_of_maximum_multiplicity">Hund’s rule</a>. <p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Let’s look at the <a href="https://en.wikipedia.org/wiki/Electron_configuration">electron configuration</a> of copper as an example. A copper atom, with 29 electrons in total, has an unexpected electron configuration that hints at why it is so conductive. If we simply fill up orbitals in order (1s, 2s, 2p, 3s, 3p, 4s, 3d; a 4-lobed d orbital can contain up to 10 electrons) we will have 1s<sup>2</sup>2s<sup>2</sup>2p<sup>6</sup>3s<sup>2</sup>3p<sup>6</sup>4s<sup>2</sup>3d<sup>9</sup>. This is actually wrong, based on experimental evidence. For copper and other transition metals, we must write the 3d orbital before the 4s orbital even though 3d is considered to be higher energy than 4s. This is an oddity of the transition elements. In these elements, the 4s orbital behaves like an outermost highest energy orbital. This short 3-minute youtube video explains why:</p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><br /></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><iframe allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/5hzv5KQ4LoM" width="560"></iframe></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><br /></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Following the transition element rule, we should have 1s<sup>2</sup>2s<sup>2</sup>2p<sup>6</sup>3s<sup>2</sup>3p<sup>6</sup>3d<sup>9</sup>4s<sup>2</sup>, thinking (correctly) that 4s should still fill up before 3d starts to fill. This is still wrong. According to experimental evidence the correct configuration is 1s<sup>2</sup>2s<sup>2</sup>2p<sup>6</sup>3s<sup>2</sup>3p<sup>6</sup>3d<sup>10</sup>4s<sup>1</sup>. In these metals, the 3d orbital is just slightly larger than the 4s orbital, but it is still higher energy than 4s the same as it is in other atoms. However, it needs only one more electron to be filled (with 10 electrons total). This filled d orbital state is a more stable lower energy configuration, so it pulls up an electron from the <i>slightly</i> <i>lower</i> energy 4s orbital to achieve it. That electron, moving up from 4s to 3d, increases in energy but the increase is very small. That small cost pays off by significantly lowering the atom’s total potential energy. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">At first glance you might conclude that copper has one valence electron, just one electron available to delocalize and form extensive molecular orbitals that take part in electrical conduction. You cannot read it from the configuration, but the 3d and 4s orbitals are <i>very</i> similar in energy. That’s why the electron swap is possible. This means that in practice, copper has eleven electrons available to contribute to the valence energy shell, and therefore to molecular orbitals and to electrical conduction.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">If we go back to our copper wire example, these valence electrons will fall into a range of many possible energy states due to the overlapping of a huge number of molecular orbitals. This range is called the <a href="https://energyeducation.ca/encyclopedia/Valence_band">valence band</a>. In conductive metals, there are many empty valence vacancies with energies <i>just above </i>the filled orbitals that electrons, can move into and out. This range of energies just above the valence band is called the <a href="https://energyeducation.ca/encyclopedia/Conduction_band">conduction band</a>. In fact, in metals, these two bands of energies overlap, so that some electrons are always present in the conduction band.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Another energy level that is very useful to understand is the <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/Fermi.html">Fermi level</a>. This can be defined as the maximum available energy level for an electron in a material that is at <a href="https://en.wikipedia.org/wiki/Absolute_zero">absolute zero</a>. Absolute zero is the coldest possible temperature of a material. It is the lowest possible potential energy state. In a conductor at absolute zero, the valence electrons are all packed into lowest available valence orbitals. Thanks to the Pauli exclusion principle, the electrons must retain a range of base level energies. We can call this range a “Fermi sea” of electron energy states. The Fermi level is like the surface of this sea, where no electron has enough energy to rise above the surface, an analogy I lifted from the Hyperphysics link above. I think this analogy can lead to some confusion. We might assume that the Fermi level is simply the top of the valence band, which is incorrect. From Wikipedia, we can understand the Fermi level more deeply as “a hypothetical energy level of an electron, such that at thermodynamic equilibrium this energy level would have a 50% probability of being occupied at any given time.” It’s critical to note that the Fermi level does not correspond to any <i>actual </i>electron energy level. Instead it is an energy state that lies between the valence and conduction band energies, or where they overlap in the case of metals. In metals, there are an equal number of occupied and unoccupied energy states at the Fermi level. There are many electrons and there are many free states, and therefore, conductivity is maximized. In contrast, in a material where all the energy states are occupied at the Fermi level, there is nowhere for electrons to move to, so it cannot conduct electricity. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">At room temperature some copper electrons are already in the conduction band. We might guess that the conductivity of a copper wire would simply continue to increase as the temperature increases above room temperature, but the relationship is more complex. As the temperature goes up, as electrons in the copper atoms gain <a href="https://en.wikipedia.org/wiki/Thermal_energy">thermal energy</a>, they also gain non-directional kinetic energy and therefore lattice vibrations in the copper grow. This kind of electron motion is random and “jittery.” Electrical conduction depends on the motion of electrons in one direction. As the temperature increases, electrical conduction becomes increasingly hindered by internal collisions between delocalized electrons, and conductivity therefore decreases. If we cool copper down toward absolute zero, we would expect fewer and fewer electrons with enough energy to conduct electricity as they fall below the Fermi level. At the same time, thermal vibrations within the metal lattice calm down so conducting electrons are less likely to be scattering by other electrons. If it were possible to reach absolute zero, we would expect no electrons at all above the Fermi level and there would be no conduction possible. It seems to me we could never apply an electric potential to test for current without adding the energy of an electric field that would excite some electrons into conduction. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: red;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Electric Insulators<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Every element and material has a valence band and a conduction band. In terms of electrical conductivity, materials fall into one of three categories: <a href="https://en.wikipedia.org/wiki/Electrical_conductor">conductor</a>, <a href="https://en.wikipedia.org/wiki/Insulator_(electricity)">insulator</a> and <a href="https://en.wikipedia.org/wiki/Semiconductor">semiconductor</a>. Even the best insulator is not a perfect insulator. It too has a conduction band that, under the right conditions, can be populated by a few mobile electrons. There is a significant difference between insulators, semiconductors and conductors in terms of how far apart the energies of the valence and conduction bands are. In the diagram below the energy is increasing from bottom to top. </p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><br /></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhfODjFDs1RePntJeezqo0oXgRZrl7qPL0TrZRDRQnH2pCk_bo9nAinKMcZtLnlSIg1LCabiRgWMjhIo8zDmlqAjKiQ6wnyNBNA5eads6LoU_5_Ej9IOmm_kqGZrbtjScyYwoQ2prm3WeEl/" style="margin-left: 1em; margin-right: 1em;"><img alt="" data-original-height="208" data-original-width="601" height="139" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhfODjFDs1RePntJeezqo0oXgRZrl7qPL0TrZRDRQnH2pCk_bo9nAinKMcZtLnlSIg1LCabiRgWMjhIo8zDmlqAjKiQ6wnyNBNA5eads6LoU_5_Ej9IOmm_kqGZrbtjScyYwoQ2prm3WeEl/w400-h139/band.png" width="400" /></a></div><br />We can describe the valence electrons in every material as having two possible ranges of energy levels, in other words, two energy bands: a valence band and a conduction band. Between these bands <a href="https://en.wikipedia.org/wiki/Band_gap">is a gap</a>, a range of energies that no valence electrons can occupy. Its width varies greatly between materials. Each element or material therefore has a unique band structure. In all insulators, there is a large range of energy above the valence band where no electron energy orbitals are available. They have large band gaps. In an insulator, the organization of the atoms does not allow for free electrons. All of the electrons are tightly bound to their nuclei (there is no shielding effect) and they form tight localized bonds between atoms. A great deal of energy must applied to an insulator before its valence electrons have enough energy to jump the band gap into the conduction band. Once enough energy is applied, however, even an insulator will start to conduct electricity. At ground state, valence electrons are tightly bound to the atoms in the insulator material. In a sufficiently high energy environment, they become <a href="https://en.wikipedia.org/wiki/Excited_state">excited</a> and delocalized into conduction electrons. <p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">I am being careful to write “energy” rather than “temperature” here because a strong electric field is much better than heat for tearing valence electrons away from atoms and turning them into conduction electrons. Heat is a randomly directed influence that generates random jittery electron movements in the material. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">To see what happens when an insulator becomes a conductor, consider a resistor hooked up to a circuit. Let’s apply a voltage much higher than it is rated for. The electric field created by the applied voltage will eventually overcome the material’s resistance. We call this resisting property its <a href="https://en.wikipedia.org/wiki/Dielectric_strength">dielectric strength</a>. When its dielectric strength is overcome, the material breaks down into a conductor. In a solid, the breakdown is physical and permanent. Our little resistor is burned and ruined. We can also observe this breakdown as an <a href="https://en.wikipedia.org/wiki/Electrostatic_discharge">electrostatic discharge</a>, a familiar example being lightning. A sudden giant spark is emitted as air, normally a strong insulator, breaks down through ionization. Electrons stripped from the gas atoms become conduction electrons when a sufficient electric potential difference between one area and another builds up during a storm. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Semiconductors<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Semiconductors are widely used in <a href="https://en.wikipedia.org/wiki/Electronic_circuit">electronic circuits</a>. A semiconductor is any material that conducts current but only partially. Its conductivity falls between an insulator and a conductor. Most are made of crystals, so they have lattice structures similar to metals. In semiconductors, the band gap is smaller than in insulators, so a smaller amount of energy (a small amount of heat in many cases) is required to bridge the band gap into the conduction band. Some semiconductors can be chemically altered to further enhance their conductivity, through a process called <a href="https://en.wikipedia.org/wiki/Doping_(semiconductor)">doping</a>. In all materials, the Fermi energy level is somewhere in the middle of the band gap and if there is no band gap, as in metals, it is near the top of the valence band.</p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><br /></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjkZeD2Zw6yWxbnaVIumCEAmufdlQ8tb4lgMbC2Z1j2nAl3yV5RYnN2Feh8mIkgGoG3UTbO4KLFuA9aO6P65uHYsC7z5lY54-DU_HgpA6oz_rpoSwI9ysJW8SRzOT0X98BU_mzhLUW9JIYP/" style="margin-left: 1em; margin-right: 1em;"><img alt="" data-original-height="328" data-original-width="600" height="350" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjkZeD2Zw6yWxbnaVIumCEAmufdlQ8tb4lgMbC2Z1j2nAl3yV5RYnN2Feh8mIkgGoG3UTbO4KLFuA9aO6P65uHYsC7z5lY54-DU_HgpA6oz_rpoSwI9ysJW8SRzOT0X98BU_mzhLUW9JIYP/w640-h350/band3.png" width="640" /></a></div><br />In semiconductors, doping can shift the Fermi level either much closer to the conduction band (N type) or much closer to the valence band (P-type). Doping is done by adding a few foreign atoms into the lattice structure of the material. These impurities add extra available energy levels. In <a href="https://simple.wikipedia.org/wiki/N-type_semiconductor">N-type semiconductors</a> (shown below left), energy levels are added near the top of the band gap (along with additional free valence electrons that contribute to conduction). <p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><br /></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjsVZ8neMDVt96z5kzqDYElOj88_ko6EVKPuc9_xpVodzkfUYUAoXEK1wuk01Wqj-fngHlt07tdpivFn3f4rGUQj1zeZk4EPf4CXWcJ2-VDUIGdJpzmTnrgVLVuebXgtCeV3Ngd0Ruj1wmB/" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img alt="" data-original-height="142" data-original-width="207" height="221" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjsVZ8neMDVt96z5kzqDYElOj88_ko6EVKPuc9_xpVodzkfUYUAoXEK1wuk01Wqj-fngHlt07tdpivFn3f4rGUQj1zeZk4EPf4CXWcJ2-VDUIGdJpzmTnrgVLVuebXgtCeV3Ngd0Ruj1wmB/w320-h221/nsem2.gif" width="320" /></a></div>These additional energy levels just below the conduction band, mean that the electrons occupying them can be easily excited into the conduction band. In <a href="https://simple.wikipedia.org/wiki/P-type_semiconductor">P-type conductors</a> (shown below right), extra empty energy levels are created as mobile energy “holes.” <p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><br /></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjmqZY5MpasKAa5GUo2vyLr-5i7yJ_wgNpMUf0FpfrxnsNowjSttP6UHOetZ0MPbwDDhzBmpsq5XhPqypJ90pXCGxDcRPM8NEqz8o2oqIiUFjzZpTs5jhW179kT1smGe8PoVHuOAbNyBrpF/" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img alt="" data-original-height="147" data-original-width="214" height="220" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjmqZY5MpasKAa5GUo2vyLr-5i7yJ_wgNpMUf0FpfrxnsNowjSttP6UHOetZ0MPbwDDhzBmpsq5XhPqypJ90pXCGxDcRPM8NEqz8o2oqIiUFjzZpTs5jhW179kT1smGe8PoVHuOAbNyBrpF/" width="320" /></a></div><p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">These are holes that electrons would normally occupy in the top of the valence band of the material. Electrons can jump in and out of them. N-semiconductors are much more conductive than P-type semiconductors because there are many more electrons available in the conduction band in the N-type than there are holes in the valence band in the P-type. </p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: red;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Conductors<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">In conductors, there is no energy gap at all. The valence band overlaps the conduction band. At room temperature, some electrons are energetic enough to be conduction electrons. When an electric potential is applied, current is generated in the material. When we want to know how good an electrical conductor is, we can ask how many electrons are available in the conduction band. Copper, as we discovered, has eleven electrons per atom that populate the valence band. These electrons are delocalized in the lattice as molecular orbitals. That’s a large number of electrons available to generate a current when an electric potential is applied. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: red;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">If all metals have overlapping valence and conduction bands, why are some better conductors than others? The answer can <a href="https://chemistry.stackexchange.com/questions/58847/why-is-copper-a-better-conductor-than-iron">be quite complicated</a> but we can get some idea by comparing copper with iron, which is much less conductive. Copper has a conductivity of<a href="https://www.thoughtco.com/table-of-electrical-resistivity-conductivity-608499"> 5.96 x 10<sup>7</sup> S/m</a> (Siemens per metre) at 20°C. Iron’s is 1.00 x 10<sup>7</sup> S/m at 20°C, about six times lower. Copper displays an almost perfect <a href="https://en.wikipedia.org/wiki/Fermi_surface">Fermi surface</a> which means it acts like a hypothetical free electron sphere would act. Its valence electrons are all lined up close to the Fermi level, the line between occupied and unoccupied energy shells. They move easily in any direction. Iron has two main differences. Iron atoms have a strong magnetic moment; they act like tiny magnets. This splits the band structure into two parts, based on the two magnetic spin states. This splitting separates the electrons, reducing the ways they can move. The second difference is the Fermi surface isn’t smooth for iron. A Fermi surface is an abstract geometric representation of all occupied versus unoccupied electron energy states in a metal at absolute zero. The shape is derived from the symmetry and periodicity of the metal lattice. In iron, it is broken up into many disconnected pockets, rather than a smooth electron sphere. Electrons have to jump from pocket to pocket in order to move. Even though iron has free (delocalized) electrons in molecular orbitals just as copper does, these two factors make it much harder for the electrons to move and generate current.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: red;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">To sum up, when wondering about the conductivity of a material, a crucial question to answer is how close the conduction band is to the valence band. In a conductive material the two bands are very close and in metals they overlap. In a semiconductor at room temperature, there is a small gap in energy between the valence band and the conduction band with the Fermi level within that gap. Doping adjusts the Fermi level and may provide additional conducting electrons. In an insulator at room temperature, the energy difference between valence band and conduction band is so large that at room temperature no electrons in the valence band can absorb enough energy to populate the conduction band. If the energy of an insulator is increased (usually be applying very high voltage), some of the conductor’s valence electrons may absorb enough energy to cross the Fermi level and jump the gap into the conduction energy band. The dielectric strength of the material is overcome and the material breaks down.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">There is a finer difference between conductors and semiconductors, in terms of the density of energy states crossing the band energy gap. In a semiconductor, the conduction band is above the Fermi level, so as its energy goes up, the conduction band begins to get populated by electrons as they cross the gap, starting from zero to one electron to two and so on. Conductivity gradually increases starting from zero. In a good conductor, the Fermi level is in the conduction band because it overlaps with the valence band at room temperature. As the energy rises, the number of conduction electrons starts with an already populated conduction band and increases from there. This is additional reason why metal conductors conduct current so readily.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">In addition to their conductivity, band theory also explains many physical properties of metals. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Metals conduct heat better than other materials because their delocalized electrons can easily transfer <a href="https://en.wikipedia.org/wiki/Thermal_energy">thermal energy</a> from one region to another. Generally, the better a metal is at conducting electricity, the better it is at conducting heat. Metals at room temperature feel cool to the touch because the electrons in the metal readily absorb the thermal energy in your warmer fingertips. This energy absorption represents a very small jump in energy to slightly higher available energy levels. For the same reason, metals under a hot sun tend to feel hotter than other objects nearby. The electrons easily transfer thermal energy to relatively cooler objects such as your fingertips. Metals tend to have a small <a href="https://en.wikipedia.org/wiki/Specific_heat_capacity">specific heat capacity</a>, which is a measure of how much heat must be added to a material to raise its temperature. In a similar way, metals are lustrous because numerous similar energy levels available to the valence electrons means they can absorb the energy of various wavelengths of visible light (they absorb various colours simultaneously). When electrons inevitably decay back to lower rest state energy levels, they emit light across the same wide range of wavelengths. As light is continuously absorbed and re-emitted from the surface of the metal, we see it as lustrous and shiny.<span style="color: #c00000;"></span><o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Three Different Velocities <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: red;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Now that we know how electrical conduction starts, we can focus on what’s happening when the current flows. Here we can further our quantum mechanical understanding of how the electrons in a conductor behave. A metal, as we know, is organized into a three-dimensional lattice of atoms. Each atom has an outer energy shell of delocalized valence electrons. These electrons are somewhat free of the attractive influence of their nuclei and they interact with each other, forming molecular bonds between atoms within the lattice. This “sea of electrons” is free to move and conduct an electrical current through the metal. For example, when a voltage is applied to a circuit, electrons in a copper wire drift from the negatively charged terminal toward the positive charge. The electrons <i>themselves</i> <a href="https://en.wikipedia.org/wiki/Drift_velocity">drift very slowly</a> through the metal, on the order of a just few metres per hour. If we looked inside the copper wire with the circuit switch turned off, we would see electrons continuously making microscopically short trips in random directions, changing direction as they strike other electrons and bounce off them. The net velocity of the these electrons is zero when no voltage is applied. When a voltage is applied, there is a net flow, a drift, of electrons in the opposite direction (negative to positive) to the electrical field (positive to negative) that is superimposed over the random drift motion. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Wikipedia supplies an interesting mathematical <a href="https://en.wikipedia.org/wiki/Drift_velocity#Numerical_example">example</a> of electron drift velocity, worked out for a 2-mm diameter copper wire. The drift velocity (which is proportional to the current; here 1 ampere) works out to be 23 um/s. That’s <i>micrometres</i>. For a typical household 60 Hz alternating current, the electrons in the wire drift less than 0.2 um per half cycle. This means that the electrons flowing across the contact point in a switch, for example, never even leave the switch. This doesn’t mean that the electrons are individually lumbering along within the wire. Individual electrons in any material at room temperature have a tremendous amount of kinetic energy, called <a href="https://en.wikipedia.org/wiki/Fermi_energy">Fermi energy</a>. The Fermi velocity of electrons is about 1570 km/s! The drift, the net movement of electrons in one direction under an electric potential, however, is extremely slow. The <a href="https://en.wikipedia.org/wiki/Speed_of_electricity">speed of electricity</a> (the speed at which an electrical <i>signal</i> or energy travels) through a copper wire is yet again different. The energy that runs the motor in a vacuum travels as an <a href="https://en.wikipedia.org/wiki/Electromagnetic_radiation">electromagnetic <i>wave</i></a> from the socket, through the cord, and to the motor. The wave’s propagation speed is close to but not quite the speed of light. This is why the vacuum starts up immediately once you flip the switch. And this is where our simpler billiard ball explanation of current propagation falls down. If electrons were like tiny balls striking one another down a line, like football fans doing the wave in a stand, delays caused by the inertia of each electron as it is put into directed motion would build up quickly and significantly and slow the current to a stop. The electric signal instead travels as propagating synchronized oscillations of electric and magnetic fields generated by the oscillations of electrons in conducting atoms.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">The wire <i>guides</i>, rather than physically carries the wave of electromagnetic energy. This wave of energy, in turn, generates an electromagnetic <i>field </i>that propagates through space, responding to the (just preceding) energy flow. The time required for the field to propagate along the metal means that the electric field can lag slightly behind the electromagnetic wave, an effect that grows with the length of the wire, but the lag is immeasurably small at the scale of a vacuum cleaner cord. The vacuum is switched on and the wave of energy, which we can think of as an electromagnetic field signal, travels at near light speed down the cord. All the mobile electrons in the copper wires immediately get the signal to start oscillating along the electrical circuit. The electric signal, as an electromagnetic (EM) wave, is composed of oscillating electric and magnetic fields. The electromagnetic energy flows as oscillating electric and magnetic fields that are generating just ahead of it. The electrons, acting as moving charge carriers and magnetic diploes, generate the oscillations of the (EM) signal. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">The EM wave loses energy along the cord, as energy is transferred into work done by the charge carriers in the wire. Energy is always lost during the transmission of electricity. Using <a href="https://en.wikipedia.org/wiki/Ohm%27s_law">Ohm’s law</a>, we know that voltage, current and resistance <a href="https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance">are related to each other</a>. Losses square with the current, which means that a small jump in current through a wire leads to a big jump in loss (through increased electrical resistance). This is why, for example, long-distance transmission lines are so high voltage, to minimize current loss due to resistance. A lot of energy is lost as microscopic friction (electrons bumping into electrons) inside the wire, and it is released as heat. A current-carrying copper wire warms up. Because the atoms are bound tightly together and there are many electrons in close proximity to one another, electrons will experience some resistance even in a good conductor such as copper.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Bloch Theorem<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">We haven’t explored yet how the three-dimensional lattice arrangement of atoms in a metal impacts its conductivity. In 1928, Felix Bloch formulated his <a href="https://en.wikipedia.org/wiki/Bloch%27s_theorem">Bloch theorem</a> to deal with these effects. <o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">To use this tool to understand how current propagates within a metal lattice, we need to deepen our understanding of <a href="https://en.wikipedia.org/wiki/Quantum_mechanics">quantum mechanics</a> once again. Electrons, like any matter particles, have a <a href="https://en.wikipedia.org/wiki/Wave–particle_duality">dual wave and particle nature</a>. Resistivity can be explained by electron-electron collisions, a particle-like phenomenon. Conductivity, however, is better understood as the transmission of energy waves between electrons. Bloch theorem incorporates both natures simultaneously by treating electrons mathematically as <a href="https://en.wikipedia.org/wiki/Wave_function">wavefunctions</a>. Put more precisely, this theorem is set up in quantum mechanics as special limitations put on <a href="https://en.wikipedia.org/wiki/Schrödinger_equation">Schrodinger’s famous equation</a>. This equation describes the wave function of a quantum mechanical system. A wave function, in turn, is a mathematical description of an <i>isolated</i> quantum state. The wavefunction is useful because it gives us measurable information about that quantum state. I think it’s fair to say that the wavefunction, a complex function of space and time, pins down the un-pin-able physical properties of the electron. These physical properties are position, momentum, energy and angular momentum, the four quantum numbers mentioned previously. The “pin” is mathematical. If you know these four numbers, you know everything there is to know about any particular individual electron. In a quantum mechanical system the set of possible values for these physical properties cannot be specific values as they would be in a classical system. Here, they are expressed instead as eigenvalues, or as ranges of possible values.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">You will notice I said “isolated state” when we really want to describe what is happening in a complex system containing many electrons. None of these electrons are acting as perfectly isolated particles, but again we start by simplifying matters. We make the assumption that the electrons are acting like free particles and we ignore their interactions with each other. Copper, as mentioned earlier, comes pretty close to this hypothetical ideal state. An electron acting like a free electron in a metal such as copper can be treated mathematically as a wavefunction. In this case, we put limits on the wavefunction, so that our solutions to Schrodinger’s wave equation take into account the periodic nature of a three-dimensional lattice. Put more precisely, these solutions form a <a href="https://en.wikipedia.org/wiki/Plane_wave">plane wave</a> that is modulated by a <a href="https://en.wikipedia.org/wiki/Periodic_function">periodic function</a>. A plane wave is set up as a set of two-dimensional wavefronts traveling in forward through three-dimensional space in a perpendicular direction. A periodic function is a mathematical way to describe a system that repeats its values at regular intervals, like the regular intervals of a crystalline arrangement in a metal. These functions are called Bloch functions and we can use them to describe the special wavefunctions, the special quantum states, of electrons in a metallic crystalline solid. The Bloch function itself is not periodic but its <i>probability</i> wavefunction includes the periodicity of the lattice, which tells us that the probability of finding an electron is the same at any <i>equivalent</i> position in the lattice.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">This mathematical description of electrons underlies the band theory of electron conduction that we discussed. Band gaps, energy states where electrons are forbidden, can now be described mathematically as values for energy, E, where there are no <a href="https://en.wikipedia.org/wiki/Eigenfunction">eigenfunction</a><a href="https://en.wikipedia.org/wiki/Eigenfunction">s</a> in the Schrodinger equation. An eigenfunction, also known as a Bloch function here, is the mathematical description of any particular electron, as a wavefunction within a crystalline (metal) solid. We can predict the energy range where electrons are forbidden (the band gap) by working out where there are no eigenfunctions (no wavefunction solutions). The actual calculations are exceedingly complex and I don’t pretend to understand them. I do think, though, that it’s quite astounding that we have a way to precisely predict and describe a very complex behaviour that is critical to understanding how electric conduction works.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><span style="color: #c00000;"> </span></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Emergent Behaviours and Properties<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">It might strike us as surprising that the Bloch theorem actually works so well, considering that we are ignoring many complications that could arise from electron-electron interactions. Complex interactions between subatomic particles in a metal can lead to the <a href="https://en.wikipedia.org/wiki/Emergence">emergence</a> of unexpected properties at the larger macro- or everyday scale.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">The electron, as a elementary particle, has a charge and a mass. But, because it is a <i>quantum</i> particle, its charge and mass are qualities that can act in independent and unexpected ways. This is where our notion of the electron as a tiny physical ball of charge is really challenged. The Bloch theorem works because the electron’s <i>charge</i> moves within a periodic electrical potential as if it were a free electron in a vacuum. It’s mass, in contrast, becomes an <i><a href="https://en.wikipedia.org/wiki/Effective_mass_(solid-state_physics)">effective mass</a></i>. The mass of an electron at rest is always about 9 x 10<sup>-31</sup> kg. Inside a metal, however, an electron can <i>seem</i> to have a different mass based on how it reacts to various forces acted upon it. Effective mass can range considerably, from zero to around 1000 times the electron rest mass, and this can have a big influence on how the metal behaves. For example, “<a href="https://en.wikipedia.org/wiki/Heavy_fermion_material">heavy fermion“ metals</a>, those with an effective electron mass in the 1000 range, can exhibit <a href="https://en.wikipedia.org/wiki/Superconductivity">superconducting</a> properties, such as zero electrical resistance below a low critical temperature. Effective mass can even be negative as in the case of P-semiconductor <a href="https://en.wikipedia.org/wiki/Electron_hole">electron holes</a> mentioned earlier. An electron hole is a lack of an electron mass where one should exist in the lattice, and leaving a local net positive charge. Each hole acts like a particle and is referred to a <a href="https://en.wikipedia.org/wiki/Quasiparticle">quasiparticle</a>, in this case a positively charged one. It is phenomenon that arises from a complex system. This behaviour plays an important role in current conduction through semiconductors. Excited electrons leave behind holes in their old ground state energy level, and they can move just like electrons do, resulting in an electric current moving in the opposite direction.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Conclusion<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;">Electric current is perhaps the most basic concept at the heart of the science of electricity. It’s a rare day that goes by when we don’t make use of at least one light bulb or our mobile phone. It’s so familiar to us that we might not give it much thought. Yet, electrical current is mysterious. It cannot be directly seen, heard or felt. It’s not easy to gain an idea of what it actually is. In order to do this we had to dive deeper and deeper into theory, from classical to quantum mechanical, while we updated our mental snapshot of the electron along the way, from the Rutherford haze to the tiny charged billiard ball to a modern quantum cloud of mathematical probabilities.<o:p></o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p><p class="MsoNormal" style="font-family: Calibri, sans-serif; margin: 0cm;"><o:p> </o:p></p>Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-86104871747998213692020-02-01T15:14:00.000-07:002020-02-02T10:54:36.611-07:00Time as a DimensionTo explore this, we are really exploring the model that incorporates <a href="https://en.wikipedia.org/wiki/Time_in_physics">time</a> as a dimension. We have no doubt heard of <a href="https://en.wikipedia.org/wiki/Space-time">space-time</a>, which we might imagine as some kind of four-dimensional fabric that permeates the universe, or as something that sets the stage upon which the universe exists. We might think of <a href="https://www.space.com/15524-albert-einstein.html">Albert Einstein</a> as the father of space-time, and though several physicists took starring roles in developing this theory, Einstein no doubt brought the theory together.<br />
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All events in the universe take place in space-time. Space-time is actually a <a href="https://en.wikipedia.org/wiki/Mathematical_model">mathematical model</a>. It fuses three spatial dimensions with one time dimension into a geometric whole. How can we conceptualize this? There is a common trap in physics, which is to confuse the theory or mathematical model with the actual thing, something physical that can be observed and measured. A wealth of experimental evidence, listed in the 4-minute video below, supports the <a href="https://en.wikipedia.org/wiki/Special_relativity">theory of special relativity</a>, which describes the space-time model:<br />
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<iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/lPagWmsrC4I" width="560"></iframe>
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This is good evidence that the space-time model used in special relativity accurately accounts for real observable and testable phenomena.<br />
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What is a Dimension?<br />
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Mathematically speaking, a <a href="https://en.wikipedia.org/wiki/Dimension">dimension</a> is the smallest number of coordinates you need to specify the location of a point. A line, for example, has a dimension of 1 because you only need one coordinate to specify a point on it. A surface or plane has a dimension of two and the inside of a sphere has three dimensions. To describe the location of a point inside a sphere, for example, you need three coordinates - we usually call them x, y and z coordinates. Building on this we can say that a point within a sphere (or any three-dimensional space) can potentially move in any combination of three possible directions - in the x, y and/or z direction. Borrowing from statistics, we can say that point possesses three <a href="https://en.wikipedia.org/wiki/Degrees_of_freedom_(statistics)">degrees of freedom</a>.<br />
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It's easy to visualize the three dimensions of space we live in. We've got up/down, latitude and longitude. An everyday example of a location in three-dimensional space might be "on the third floor in the northwest corner of the Black Building." We can pinpoint exactly where to go if we are given these directions. In our everyday world, time, however, feels different. Locations in space can be fixed but we experience time as fluid. It flows as events constantly recede into our past and reach into our future. Now when we revisit our directions, we notice that we need a "when." What if these are instructions for a dentist appointment, for example? The dentist wants me at a specific location at a specific time, such as in his chair at 2 pm on Wednesday. I've got all four coordinates you need. At 1 pm or 3 pm, someone else will be occupying that chair, those identical spatial coordinates. But only I will be in that chair at this specific time coordinate of 2 pm Wednesday. Therefore, we can see that to describe any <a href="https://en.wikipedia.org/wiki/Event_(relativity)">event</a> we'll need both spatial coordinates and a time coordinate.<br />
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But how is time related to space? I can explore this by describing my walking path to the dentist's chair. I will need to describe both space and time. I enter the east door of Black Building at 1:45, approach the elevators at 1:50, wait there for one for a minute for the door to open and then travel up to the third floor during the next minute before I walk down a corridor to the dentist office at 1:55. I plunk myself down in the dentist chair at 2 pm and remain there for 50 minutes. I am describing a series of events that flow through space AND time.<br />
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During these events spanning between 1:45 and 2:50 pm, I am moving through space and time, in other words, I am moving through four changing coordinates, or dimensions. However, I notice that I may be moving through one or two spatial dimensions simultaneously but in every case as I move through space I must also move through time. Time seems to have some rules that space doesn't. I can never "stop" in a time dimension. I can never move into a negative time dimension. Time, then, appears to have fewer degrees of freedom than any spatial coordinate. I, or anyone, can only move in one direction along this timeline and at a rate I can't control. Is time a dimension? If it is, it does not appear to be a fourth dimension in the same way as the other three spatial dimensions.<br />
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We can map out my progress between 1:45 and 2:50 pm on Wednesday using a framework that describes changes in all four coordinates of time (1) and space (3). Yet, as we just noticed we encounter some problems when we think about time as a fourth dimension equivalent to a spatial dimension. We can describe this impasse mathematically. As we will get into later, it was <a href="https://www.newscientist.com/article/mg13718543-900-pay-attention-albert-einstein/">Hermann Minkowski</a>, not Einstein, who formulated the dimensions of space-time, and these four dimensions are NOT equivalent to <a href="https://en.wikipedia.org/wiki/Four-dimensional_space">four-dimensional (Euclidean) space</a>. Space-time is not, for example, this albeit mesmerizing four-dimensional rotating cube:<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">JasonHise;Wikipedia</span></span></td></tr>
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This cube above is described by <a href="https://en.wikipedia.org/wiki/Euclidean_geometry">Euclidean</a> space. There is no time dimension to it. Space-time does have geometry but it is different in important ways. The non-Euclidean mathematics used to describe space-time describes how time works with space. To make this connection conceptually, we can trace how the concept of space-time came about. <br />
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Time Was Once an External Stage<br />
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Isaac Newton: We probably all started our exploration of physics with <a href="https://en.wikipedia.org/wiki/Isaac_Newton">this great man</a>, a key figure in the scientific revolution in mid-seventeenth century Europe. In his time, a four-dimensional framework for any physical event would seem unnecessary and probably ridiculous. To describe his <a href="https://www.grc.nasa.gov/www/k-12/airplane/newton.html">laws of motion</a>, he needed only three spatial dimensions, the ones we experience every day, and he assumed it all happened while an absolute time progressed at a specific rate independent of everything else going on. Even physical space was likewise treated as outside all events. Every object either had an absolute state of motion or an absolute state of rest relative to the absolute space it found itself in. Time and space were treated <a href="https://en.wikipedia.org/wiki/Absolute_space_and_time">much like an external stage</a> on which all phenomena in the universe take place. The assumption made sense and it worked for a long time. It is how we experience space and time every day.<br />
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The idea was basically unquestioned until the mid-nineteenth century, shortly after <a href="https://en.wikipedia.org/wiki/James_Clerk_Maxwell">James Clerk Maxwell</a> and others, started to tinker with electricity, magnetism and light. Maxwell discovered that all three were a) related to each other and b) <a href="https://en.wikipedia.org/wiki/History_of_electromagnetic_theory">traveled as disturbances</a> through (Newtonian three-dimensional) space.<br />
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Maxwell worked out that "light and magnetism are <a href="https://en.wikipedia.org/wiki/A_Dynamical_Theory_of_the_Electromagnetic_Field">affectations of the same substance</a> and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws." The "field" in this statement is the epicentre of what would become one of the most hotly contested debates in the history of science. What exactly was this field? His work would ultimately lead to the connection between space and time. To start with, he and others knew only two well-established facts: 1) light seemed to have a constant velocity through air and 2) light slowed down when it traveled through other transparent media such as water.<br />
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Aether Was the Medium of Space<br />
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Scientists at this time knew that electromagnetic disturbances act like waves so they figured they must travel through some kind of medium, which they called <a href="https://en.wikipedia.org/wiki/Luminiferous_aether">aether</a>. Experimental results on aether, however, were confounding. For example, the speed of light through air was constant in any direction. Wind or air density had no effect. Even more confusing, the speed of an electromagnetic disturbance seemed to be independent from speed of the source of that disturbance. In other words, light seemed to disregard Newtonian physics. How? What about this aether substance could accommodate such findings? It proved frustratingly impossible to define electromagnetic waves mechanically. They did not act like other <a href="https://en.wikipedia.org/wiki/Mechanical_wave">mechanical waves</a> such as sound waves or water waves.<br />
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Like many other scientists at the time, Einstein, pondered how aether worked. Like physicists <a href="https://en.wikipedia.org/wiki/Paul_Dirac">Paul Dirac</a>, <a href="https://en.wikipedia.org/wiki/Louis_de_Broglie">Louis de Broglie</a>, Maxwell and others, he thought that aether was some kind of medium with physical properties filling empty space. There must be something there that carries the electromagnetic disturbance, and perhaps the results could be explained by some kind of elastic force through which the waves are propelled, analogous to water waves or sound waves. Even in 1920, <i>after</i> he developed special relativity, he stated that there must be something that allows for the "existence of standards of space and time (measuring-rods and clocks)" to allow for space-time intervals in the physical sense.<br />
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Aether Theories Run Into Trouble<br />
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Various aether experiments designed to find out how it worked mechanically, and there were many, yielded either contradictory or nonsensical results. An interesting and well-known example of this problem was the <a href="https://en.wikipedia.org/wiki/Fizeau_experiment">Fizeau experiment</a>, conducted in 1851. At that time, a number of scientists were comparing the speed of light (an electromagnetic disturbance) in air versus water. They could observe that a beam of light slowed down in water. They wondered what process slowed the light-bearing aether down in the denser medium.<br />
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<span style="font-size: x-small;">If you are wondering how light slows down when it has one invariant velocity, you ask a fantastic, often overlooked, question. A good concise answer can found <a href="https://en.wikipedia.org/wiki/Refraction#General_explanation">here</a>.
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This phenomenon, called <a href="https://en.wikipedia.org/wiki/Refraction">refraction</a>, has been observed since ancient times and was described mathematically in the 1600's by <a href="https://en.wikipedia.org/wiki/Snell%27s_law">Snell's Law</a>. A number of researchers revisited refraction as possible evidence that <a href="https://en.wikipedia.org/wiki/Aether_drag_hypothesis">aether could be partially dragged</a> by moving matter such as water. The idea being tested was that aether moving against the direction of water flow might be slowed down. Vice versa, aether dragged along with the water flow might boost the flow rate of the aether and therefore the speed of light. Fizeau designed an experiment that compared the speed of light through water moving in the same direction as the light beam with the light's speed moving against the direction of the water flow. If their theory was correct, light would move faster along the same direction of water flow and slower when it's against the flow. They didn't know how or if aether interacted with matter but this experiment was designed as a first step to the answers. They made the assumption that because light could penetrate all transparent media, such as water, those media were permeable to the aether. If the medium is moving, does it carry the aether along with it? Is the aether partially dragged or is not affected at all?<br />
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<a href="https://en.wikipedia.org/wiki/Hippolyte_Fizeau">Hippolyte Fizaeu</a> got puzzling, but not entirely unexpected, results from his experiment. He showed that the speed of light in same-flow water was boosted but it was less than the sum of the speed of light in air plus the speed of the water flow, as Newton's laws would have predicted. The aether appeared to be dragged along, but only partially. It turned out that <a href="https://en.wikipedia.org/wiki/Augustin-Jean_Fresnel">Augustin-Jean Fresnel</a> had already established a dragging coefficient in the late 1700's, based on several earlier aether experiments that appeared to support the idea of partial aether dragging. All of these experimental results seemed to suggest that the aether might be denser inside mediums such as water than it was in air or in a vacuum and that light traveled more slowly through denser media. Fresnel's dragging coefficient was proportional to the refractive index of the medium.<br />
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Scientists at the time knew that the reduction in the speed of a beam of light depends on the index of refraction, which in turn depends on the light's wavelength. The refractive index decreases with increasing wavelength, so, for example, blue light bends more than red light when a light beam passes from air into water. This is why white light <a href="https://en.wikipedia.org/wiki/Dispersion_(optics)">disperses</a> into a rainbow when it passes through a prism. This presented a snag. The experimental data pointed to a seemingly complex scenario where aether is partially dragged by matter and the aether must flow (simultaneously!) at different rates for different colours of light, as a white light beam, containing all the colours, was used in their experiments. Were there a seemingly infinite number of aethers, one for each wavelength of light? The use of <a href="https://www.physicsclassroom.com/class/light/Lesson-1/Polarization">polarized light</a> in this experiment presented the same problem. Light polarized in opposite directions both exhibited the same partial aether drag, suggesting that the aether was carrying two opposite directions of motion at the same time. These partial-dragging results were confirmed by numerous other experiments as well. Aether theory was offering complication rather than simplification.<br />
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Electromagnetism Re-imagined<br />
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It wasn't until around 1892 that <a href="https://en.wikipedia.org/wiki/Hendrik_Lorentz">Hendrik Lorentz</a> approached these baffling experimental results from a new angle and a solution began to take shape. He assumed, first of all, that the aether was completely stationary. He was then able to derive Fresnel's coefficient by using Maxwell's equations and an undragged aether. He looked at the problem as one of light speed transitioning between two reference frames, one where the system is at rest in the aether and the other where the system is in motion in the aether. By rest, he meant absolute rest in the absolute space of Newton. By doing this he introduced a clear distinction between matter (this time in the form of electrons) and aether. This meant a departure from any mechanical theory of aether.<br />
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Referring the work of Maxwell and others, he described the aether as "states" in an electromagnetic field. By doing so, he introduced an abstract aether replacing the previous and problematic mechanistic model. He also planted the seed for special relativity: A moving observer with respect to the aether will observe the same electromagnetic phenomena as an observer at rest in the stationary aether.<br />
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Lorentz's <a href="https://en.wikipedia.org/wiki/Lorentz_ether_theory">interpretation</a> was that partial dragging was something that happened to the electromagnetic wave <i>itself</i> and not something that happened to the aether, which was stationary. This proved to be a critical first step in the evolution toward our modern theory of space-time. It was, however, a first step. It transformed mechanical aether into an abstract electromagnetic aether, but it still held onto the presence of some kind of aether and it held onto Newton's concept of absolute space.<br />
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The idea that space must contain something to support the propagation of electromagnetism (and gravity as well) was extremely difficult to let go. By 1901, as Lorentz was developing the theory underlying his famous <a href="https://en.wikipedia.org/wiki/Lorentz_transformation">Lorentz transformation</a>, which would provide a bedrock for Einstein's theory of special relativity, <a href="https://en.wikipedia.org/wiki/Henri_Poincar%C3%A9">Henri Poincaré</a> wrote (in a state of philosophical angst?) that there must be no absolute space nor absolute time. Even so, Poincaré would not let go of aether: "If light takes several years to reach us from a a distant star, it is no longer on the star, nor is it on the Earth. It must be somewhere, and supported, so to speak, by some material agency."<br />
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Aether Evolves into Space-time and Fields<br />
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Quoting from Einstein once again in 1920, after he published his theory of relativity, "To deny the aether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view." Again, one can detect the angst.<br />
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Today, we can argue that Poincaré and Einstein were correct about space, but only in a sense. We can argue that the <a href="https://en.wikipedia.org/wiki/Field_(physics)">field</a> itself has replaced the aether. Electromagnetic radiation, such as visible light, which was the focus of Fizeau's and other aether experiments, is carried as waves through an electromagnetic field. These waves <a href="https://en.wikipedia.org/wiki/Wave_function">operate by quantum rules</a> rather than mechanical rules. We now have a concept of a physical universe that is described by the mathematics of space-time geometry, and within that geometry we describe various fields carried by force-carrying particles called <a href="https://en.wikipedia.org/wiki/Boson">bosons</a>. Photons are the force-carrying particles of the electromagnetic field. Starting with the development of the <a href="https://en.wikipedia.org/wiki/Electromagnetism">theory of electromagnetism</a> in the late 1800's and continuing through the development of <a href="https://en.wikipedia.org/wiki/Quantum_mechanics">quantum mechanics</a> later in the early 1900's, physicists gradually <a href="https://en.wikipedia.org/wiki/Luminiferous_aether#End_of_aether">abandoned the idea</a> of a background medium altogether. Aether isn't required to explain how special relativity works.<br />
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How "real" is a field? Is it physical or strictly a mathematical construct? The <a href="https://en.wikipedia.org/wiki/Field_(physics)#Field_theory">concept of the field</a> arose as a fundamental physical quantity that independently exists. For example, physicists envisioned the <a href="https://en.wikipedia.org/wiki/Electromagnetic_field">electromagnetic field</a> extending indefinitely throughout space in all directions as a physical field interacting with matter. Maxwell's electromagnetic field equations were developed using <a href="https://en.wikipedia.org/wiki/Classical_field_theory">classical field theory</a> which obeyed Newton's laws. Later on they were refined further by incorporating special relativity and quantum mechanics. We now know that an electromagnetic field is carried by force-carrying photons. Subatomic particles, such as photons, are treated as excited states in the quantum field that obey the laws of quantum mechanics.<br />
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As treated in quantum field theory, a field is strictly mathematical and doesn't physically exist. The field still extends all over space and we can make an observation of the field by taking a measurement of it at a particular moment and location. In this sense, the field interacts with matter and we can argue that the field is physically real. We experience evidence of it all the time, such as when we see a burst of visible light photons when we turn on a light. We feel a static charge or watch iron filings arrange themselves according to the lines of force exerted by a magnet. In the case of magnetism, for example, we indirectly detect the <a href="https://en.wikipedia.org/wiki/Virtual_particle">virtual</a> photons that carry the magnetic force. We understand these photons as mathematical <a href="https://en.wikipedia.org/wiki/Wave_function">wave functions</a>.<br />
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The Speed Of Light Forces a Shift In Thinking<br />
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Einstein, wondering about space and time and perplexed by the nature of aether, turned his focus to the experimental findings. He knew that Lorentz was beginning to approach the aether problem in terms of changing frames of reference. He focused on one blaring observation. The speed of light appeared is a universally unchanging value. In itself this was a truly mind-blowing observation in a then largely Newtonian universe.<br />
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Imagine the headlight of a train approaching at the speed of light. The photons in that light beam would also be travelling at the speed of light, and never exceeding it. Why don't these two velocities add together, as they would if a man threw a ball forward from a train travelling forward at everyday speed? If photons followed the same rules of Newtonian dynamics as balls do, the two speeds would add up. What is it that slows the light down and keeps it in check? What is that process if it is not the work of some kind of elastic aether? Einstein knew that something in the description of this thought-experiment must give. One thing he could conclude with some certainty was that he did not yet have the entire picture of space as the medium through which light travels. By following a tactic similar to Lorentz by allowing the question of medium to take a back seat, he could reframe the problem. If the speed of light never changes, then time or space, or both, must. Put mathematically, space and/or time must <a href="https://en.wikipedia.org/wiki/Transformation_(function)">transform</a>.<br />
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Time Can Vary<br />
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The concept of transformation itself isn't new in physics. <a href="https://en.wikipedia.org/wiki/Galilean_transformation">Galilean transformations</a> operate in Newtonian physics. They tell us that any event that takes place in one <a href="https://en.wikipedia.org/wiki/Frame_of_reference">frame of reference</a> will operate under the same physical laws if it takes place in a different frame of reference. For example, barring all other sight cues, a car traveling at 50 km/h passing a car traveling at 30 km/h in the same direction will appear to the passengers of the 30 km/h car to be traveling at 20 km/h. It's the basic addition/subtraction of velocity vectors, operating under the same rules as the ball being thrown from a train example above. These kinds of transformations presume that the passage of time is the same for observers in different frames of reference. They presume that time is absolute in other words. These Newtonian rules are still useful and that's why we learn them. They work perfectly until we are dealing with velocities approaching light speed (or near gravitational fields). Lorentz and others tried to understand how the speed of light breaks these well-established common-sense rules. In any reference frame the speed of light is always the same. It does not obey the Newtonian laws that underlie a Galilean transformation.<br />
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A Galilean transformation holds up for events that happen at everyday velocities, but as an object approaches the speed of light in one reference frame compared to a stationary reference frame (we can call this frame a stationary observer), both space and time, for that observer, transform. Space and time depend on the reference frame. Any object approaching the speed of light experiences <a href="https://en.wikipedia.org/wiki/Time_dilation">time dilation</a> (time stretching or slowing down) and <a href="https://en.wikipedia.org/wiki/Length_contraction">length contraction</a> as observed relative to a stationary observer. To that observer, the object contracts in the direction it is traveling* and a clock attached to that object slows down.<br />
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*<span style="font-size: x-small;">An object travelling near light speed will actually appear rotated even though its measured length will be contracted. The object is moving so fast that light from the along the object reaches the observer at slightly different times. A receding object will appear contracted and an approaching object will appear elongated, while a passing object will appear skewed or twisted. This optical effect is called <a href="https://en.wikipedia.org/wiki/Terrell_rotation">Terrell rotation</a>).</span><br />
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This means that observers moving at different speeds relative to one another can observe different distances, different elapsed times and even, as a result of these transformations, experience different orderings of events. These transformations are not illusions. At the expense of getting ahead of myself, consider an example of a proton (a particle of matter) in the Large hadron Collider. It is accelerated to almost light speed and as it does so it experiences a Lorentz factor of about 10,000. The <a href="https://en.wikipedia.org/wiki/Lorentz_factor">Lorentz factor</a> is the factor by which time and length change for an object that is moving. To put this in perspective, if you could shrink down and ride on top of this proton from Earth to Alpha Centauri, your trip would you take only a couple of days. Alpha Centauri is four light-years away, which means it takes (traveling at light speed!) four years for its photons to make that same length of trip. An observer on Earth would record that your trip to Alpha Centauri took a little over four years.<br />
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Putting <a href="https://en.wikipedia.org/wiki/Einstein%27s_thought_experiments#Special_relativity">his thought-experiment observations</a> into a formal framework, Albert Einstein published his game-changing theory of <a href="https://en.wikipedia.org/wiki/Annus_Mirabilis_papers#Special_relativity">special relativity in 1905</a>. It incorporated Lorentz transformations in space and time. A few years later, <a href="https://en.wikipedia.org/wiki/Hermann_Minkowski">Hermann Minkowski</a> formulated a geometric interpretation of the Lorentz transformations, and this is now the mathematical structure, called Minkowski space-time, on which the theory of special relativity rests.<br />
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<a href="https://en.wikipedia.org/wiki/Minkowski_space">Minkowski space-time</a> mathematically combines three-dimensional Euclidean space with time to create a four-dimensional structure called a <a href="https://en.wikipedia.org/wiki/Manifold">manifold</a>. A manifold is a strictly mathematical concept that is nicely explained <a href="https://physics.stackexchange.com/questions/298022/what-is-a-manifold">here</a>.<br />
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The Space-time Interval<br />
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If we take the simple concept of distance, we can get a feel for how Minkoswki space-time works. In Newtonian physics, the distance between two points is invariant. It will be the same regardless of reference frame. In special relativity, however, that distance will depend on whether the observer is moving or not (length contraction). In four-dimensional space-time, a new invariant "yardstick" called the <a href="https://en.wikipedia.org/wiki/Spacetime#Spacetime_interval">space-time interval</a> replaces distance. Whereas in Newtonian physics, time and space (distance) are invariant, in Minkowski space-time, time dilates as distance contracts. These measurements are dependent on the frame of reference. The space-time interval of an event, which combines space and time, is the same in any frame of reference. A space-time interval extends from one place and time to another place and time. We can even build space-time by taking successive snapshots of space over time and adding them all together. Measurements of space and time can vary between observers but the space-time interval, obtained by measuring the distance and time between two events, will always be the same in every frame of reference. It doesn't matter how fast or in what direction an object is traveling with respect to the observer. The space-time interval displays Lorentz invariance.<br />
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To visualize how time and distance relate to one another geometrically in a space-time interval, as well as how the speed of light is constant in every frame of reference, try this 7-minute video:<br />
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<iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/zScn3tV9YPU" width="560"></iframe>
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How To Get From Length to a Space-time Interval<br />
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How did we get to this new invariant idea of "length" in space-time? When Minkowski developed the space-time manifold, he imagined that the space-time interval could be related to <a href="https://en.wikipedia.org/wiki/Pythagorean_theorem">Pythagoras' theorem</a>, but in four dimensions rather than the two we are all probably familiar with when we draw a right triangle on a sheet of paper, shown below right.<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJp2gEOHiDgFiRl812ZyOox7j9gJL1JFHtAVtNeU5YzmqAA3rMvr27MmAcWPde-TiaAGEq5cD_Ze2DQB3K6efVbdZDeEcweqpFRKgDAzEhZDLkBsSklBXVhtKoqWfZaSrs32csTCqaHLfg/s1600/righttriangle.png" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" data-original-height="199" data-original-width="369" height="172" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJp2gEOHiDgFiRl812ZyOox7j9gJL1JFHtAVtNeU5YzmqAA3rMvr27MmAcWPde-TiaAGEq5cD_Ze2DQB3K6efVbdZDeEcweqpFRKgDAzEhZDLkBsSklBXVhtKoqWfZaSrs32csTCqaHLfg/s320/righttriangle.png" width="320" /></a></div>
The Pythagorean theorem states that the length of the hypotenuse, z, is given by the square root of x<sup>2</sup> + y<sup>2</sup> where x is the horizontal measurement and y is the vertical measurement in a two-dimensional coordinate system such as a sheet of graph paper. If we want to describe z's length in three dimensions, we just add a measurement along an additional horizontal axis, w, which we can imagine as a line coming out of the page. Then we get z<sup>2</sup> = w<sup>2</sup> + x<sup>2</sup> + y<sup>2</sup>. We can now describe line z's length and position in 3-dimensional space. To measure z's coordinates in time as well as in space, Minkowski introduced a time dimension, (ct) to the equation. Here, c is a conversion constant, which is the speed of light in a vacuum (metres per second), and t is the time interval (seconds) spanned by the space-time interval. We can think of it as the distance light travels in t seconds. This is a way to incorporate a new "length" along a new axis, and as we do this we are switching to a four-dimensional coordinate system. By doing so we are bringing time, as a unique dimension, into our geometry. The speed of light conversion constant makes this dimension uniquely different from the other three spatial dimensions. It also tells us that the speed of light can be used as an invariant measurement of time called <a href="https://en.wikipedia.org/wiki/Proper_time">proper time</a>.<br />
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With a little mathematical finesse, we end up with three dimensions of space and one dimension of time: s<sup>2</sup> = w<sup>2</sup> + x<sup>2</sup> + y<sup>2</sup> + (ict)<sup>2</sup>. We've changed our variable z to s, to show that we are now measuring distance as a space-time interval. We've also added a new variable, i, to our time axis. The term i is an imaginary unit, also known as √ (-1). The old-fashioned picturesque descriptor "imaginary" doesn't mean an imaginary number is made up. It just helps us find solutions to mathematical problems. In our case, <a href="https://en.wikipedia.org/wiki/Imaginary_time">imaginary time</a> is real time that undergoes a mathematical transformation called a <a href="https://en.wikipedia.org/wiki/Wick_rotation">Wick rotation</a>. A Wick rotation is a way to convert a problem in Euclidean four-dimensional space into a problem in Minkowskian four-dimensional space-time. It trades one spatial dimension for a time dimension and allows the dimension to undergo a Lorentz transformation, which mathematically is a rotation of coordinates.<br />
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Since the ict term is squared we can multiply (ct) by -1. We end up with s<sup>2</sup> = w<sup>2</sup> + x<sup>2</sup> + y<sup>2</sup> - (ct)<sup>2</sup>. Again, we can think of this (ct)<sup>2</sup> variable as "distance" along the time axis.<br />
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The introduction of an imaginary unit hints to us that even though we've put our equation into the form of a Pythagorean equation, the time dimension in it doesn't "act" like the other spatial dimensions. It does not have a simple Euclidean geometrical relationship with space.<br />
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Physicists now describe space-time in terms of a newer mathematical construct called a <a href="https://en.wikipedia.org/wiki/Metric_tensor">metric tensor</a>. <a href="https://en.wikipedia.org/wiki/Introduction_to_general_relativity">General relativity</a> also describes space-time, but in that case, the space-time needs to curve under gravity. We can think of the metric tensor is a device that makes corrections to Pythagoras' theorem to enable the right triangle we used as our starting point to map onto curved space-time. It also does away with the imaginary unit (i) we discussed earlier by describing events in real time instead. The negative sign, however, is preserved in the metric tensor but it now describes how distance changes with time when space-time curves. The original Minkowski equation describes an incorrect but simplified flat space-time.<br />
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By creating a space-time interval, we can understand both the invariance of the speed of light as well as time dilation and length contraction.<br />
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Speed of Light Invariance<br />
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Imagine a photon traveling at the speed of light, c. All observers will observe that same velocity no matter what their velocity might be relative to it. The distance traveled by the photon (let's say it's traveling in the x direction so we'll call it distance x) for t seconds can be written as:<br />
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x = vt where x is the distance traveled, v is velocity and t is time<br />
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Let's begin to transform this simple equation for distance into one for a space-time interval. First we'll incorporate it into the Pythagorean theorem in three dimensions, like we did earlier. I'll start using s for the distance even though I'm not quite correct yet because we haven't incorporated time.<br />
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s<sup>2</sup> = x<sup>2</sup> + y<sup>2</sup> + w<sup>2</sup><br />
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There is no motion in any direction except the x direction so the w and y axes are zero. We need to describe this relationship in terms of space-time so we add the time dimension [-(ct)<sup>2</sup>]. We can now properly describe the distance (x) in terms of a space-time interval (s):<br />
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s<sup>2</sup> = x<sup>2</sup> + 0<sup>2</sup> + 0<sup>2</sup> - (ct)<sup>2</sup><br />
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We can swap out x by incorporating our earlier equation x = vt.<br />
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s<sup>2</sup> = (vt)<sup>2</sup> - (ct)<sup>2</sup>. Our object is traveling at the speed of light so we know v = c.<br />
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s<sup>2</sup> = (ct)<sup>2</sup> - (ct)<sup>2</sup>. We get s<sup>2</sup> = 0 so s = 0.<br />
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This means that the space-time interval for any object traveling at light speed is zero. It is invariant. It doesn't matter what reference point you measure the object from. You could be accelerating in the w or y direction as you measure its velocity. It will always be light speed and its space-time interval will always be zero. Put another way, only an object traveling at light speed will have a zero space-time interval. All observers will observe the same (zero) space-time interval for that object, which means they all observe it to have a velocity of c.<br />
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How does a photon experience the universe? We can get a feel for this surprisingly complex situation by comparing the <a href="https://en.wikipedia.org/wiki/World_line">world lines</a> of three objects, all traveling at different constant velocities in the same direction, shown below in a simple space-time graph.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj2vmIS6TW7pQHFE7FhQc7RLupxHrsp3M37Ncq02HwTCjq8xWgi79kdzu5ckag9mk6R3LkSqqEKAeNbgFNKfmIIHZKU666qMAN1qvON4dHHw6ggVRJKxl27rzr-3IXj9qd4nxqMYiUuOnYR/s1600/Worldlines1.jpg" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="257" data-original-width="400" height="255" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj2vmIS6TW7pQHFE7FhQc7RLupxHrsp3M37Ncq02HwTCjq8xWgi79kdzu5ckag9mk6R3LkSqqEKAeNbgFNKfmIIHZKU666qMAN1qvON4dHHw6ggVRJKxl27rzr-3IXj9qd4nxqMYiUuOnYR/s400/Worldlines1.jpg" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Jheise;Wikipedia</span></span></td></tr>
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We have to be careful because we are not representing velocity or position versus time. Each line, called a world line, is built from a sequence of space-time events for each object. Each point on each line is a four-dimensional space-time event. An event in space-time is a specific location in three-dimensional space at a specific time. The t in the graph depicts proper time.<br />
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In this graph, t is time and x is distance along one space coordinate. We could draw a more complex space-time graph by incorporating all three space coordinates with one time coordinate, representing Minkowski space-time.<br />
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An object at rest would be a vertical line originating at the same origin point as the coloured lines and where x = 0. Its world line is space-like. A space-like world line could likewise describe the length of a physical object such as a ruler, as the distance between two space-like events. Each of the three coloured lines represents the world line of an object traveling at a specific constant velocity (hence all the straight lines). Their world lines, and the world lines of any objects traveling less than the speed of light, are time-like curves in space-time. Even though only straight lines are drawn here, any world line is considered to be a special type of curve in space-time.<br />
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This graph represents all times (future and past) and all possible distances along x in space-time. It is a simple representation because all three objects are traveling along in the same direction, along the x-axis. They all originate at the origin of time and distance on the graph. At that origin point, they share the same space-time interval. A physical example might be a single particle decaying into three particles, each having a different velocity.<br />
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A stationary object moves in time but not in distance. A slow object moves further in time than it does in distance. A faster object moves further in distance than it does in time. A very fast object moves in distance but very little in time. A photon has the fastest possible velocity. It moves in distance but not in time. It follows a light-like curve, which would be represented as a horizontal line moving along the x-coordinate, where t = 0. A light-like curve is a straight line in this simple graph where two spatial dimensions are not shown. Often the convention is to draw this horizontal line at a fixed 45-degree angle. By doing this we can draw the light-like curve in three spatial dimensions as an easier-to-visualize three-dimensional cone, directed upward into the future and downward into the past, shown below.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiQMIfeEUchKqfuSlsbVfulgwcwoiyITfbBbOgwArX7ErOqAkJwMa0KgpjTBBvzL2-ZTDx3uzSENhnrgiRekz4goh-mohyphenhyphen96kTWhwrkjgDOpooBZ2nBqhHMjBDOwCRFUv6kNYlmQxH0YbY6/s1600/2560px-World_line2.svg.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1423" data-original-width="1600" height="568" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiQMIfeEUchKqfuSlsbVfulgwcwoiyITfbBbOgwArX7ErOqAkJwMa0KgpjTBBvzL2-ZTDx3uzSENhnrgiRekz4goh-mohyphenhyphen96kTWhwrkjgDOpooBZ2nBqhHMjBDOwCRFUv6kNYlmQxH0YbY6/s640/2560px-World_line2.svg.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">MissMJ;Wikipedia</span></span></td></tr>
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Put more sophisticatedly, we can say the photon approaches the limit of proper time. A photon might be emitted from a distant star and travel through space for 4 billion years before it is absorbed. We can measure that journey as taking 4 billion years (a great "distance" in time), but for the photon there is no distance along the time axis. It is emitted and absorbed instantaneously. Remember Henri Poincaré's troubling question? He asked where the light beam is in the space between stars. He wondered what medium was carrying it. Can we say that the photon even has a journey? We observe photons of starlight traveling across great distances over light-years of time. But in the photon's frame of reference the universe does not seem to consist of the space-time we experience.<br />
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Time Dilation<br />
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Imagine setting up an array of synchronized clocks over a very large table in space, a table on the scale of thousands of kilometres across with no gravitational field are nearby. From one edge of the vast table you take a photo of all the clocks. You find that the clock closest to you is running a little faster than those furthest away. After a little thinking, you realize you need to take into account the transition time for the light from each clock to reach you. The speed of light is constant so this is a fairly straightforward synchronization calculation. You go back and adjust all of your clocks. Now they all read the same time when you take your photo of them. A friend flies past your clock arrangement at 0.95% light speed and takes his own photo of the array at exactly the same time you take your next photo. Comparing photos, you notice that his clocks are all a bit behind yours. Your frame of reference is at rest compared to your clock table so you don't experience time dilation. However, you and your clock table are in motion compared to his frame of reference. He records time dilation. His present moment was not the same as your present moment. The two events you experienced were desynchronized.<br />
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If an additional initially synchronized clock were glued to the outside hull of your friend's ship beforehand and you repeated the experiment, you might guess that you will see it as running faster than your clocks. Instead, you read it as running slower as he flies past you. For you, he is the moving frame of reference. For him, once again you are the moving frame of reference. You once again see in your photos that his clocks are slower. And yet, for him, your clocks were slower. This counter-intuitive effect is known as the <a href="https://en.wikipedia.org/wiki/Twin_paradox">twin paradox</a>.<br />
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What is different in the two frames of reference has nothing to do with the mechanisms of the clocks. A moving mechanism doesn't get heavier or something such as that. The key difference is that the moving clock is traversing a longer distance between events (ticks). The events do not have to be hands moving on a clock face. The clocks could be mechanical, quartz digital, atomic or even hourglasses.<br />
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To get a feel for this it might be easier to imagine that our clocks are made of pulses of light bouncing between two mirrors. One trip from mirror to mirror is equivalent to one tick of our earlier clock. The speed of light is invariant, so the ticking mechanism of this clock will be perfectly constant. The clock moving with respect to the stationary clock will tick slower. The moving clock will, in its frame of reference, experience the stationary clock as the one moving and it will tick slower than the former one. This brings home the fact embedded in special relativity that there is no absolute motion and there is no absolute rest. The only absolute is the speed of light. We can visualize time dilation (and the twin paradox) in the set-up in the gif below.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhG9PmGc6cqi_faT0t_6RVQGSTPFqKuewfb0vhFkvPiXipBWP_uZUrQd3FYzIteoKqgxfqFTzcxEy3d63077qzYCak5TQpke_0FkTA25wBW5f3naHrPMei8qIUsfLfWHNU6FRsr8ObZIO7X/s1600/Time_dilation02.gif" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="352" data-original-width="279" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhG9PmGc6cqi_faT0t_6RVQGSTPFqKuewfb0vhFkvPiXipBWP_uZUrQd3FYzIteoKqgxfqFTzcxEy3d63077qzYCak5TQpke_0FkTA25wBW5f3naHrPMei8qIUsfLfWHNU6FRsr8ObZIO7X/s400/Time_dilation02.gif" width="315" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Cleoris;Wikipedia</span></span></td></tr>
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Each blue dot represents a pulse of light. Each pair of dots (red pairs and green pairs) are mirrors bouncing light pulses back and forth. Each pair is a clock. If we measure the time it takes for one light pulse to reach from one mirror to the other mirror we will always get the same result as long as we are in the same frame of reference. This is called proper time. It is the fastest possible time because it is the shortest distance traveled by the light pulses. For each group of clocks the other group ticks more slowly because the light pulse has a longer distance to travel when it is moving. Time dilation works not just for light. It works for any series of events. The physics of any event or process is constrained by relativity. In other words, special relativity forces all other laws in physics to obey. For example, a man in motion with respect to a stationary man ages more slowly.<br />
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Length Contraction<br />
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We can do another thought experiment to explore how length contraction occurs. Imagine two light clocks like the ones described above in which light bounces between two mirrors. We can put them close together or far apart and we can orient them in any way we want with respect to each other. If they are both at rest with respect to us, as observers, they will run at the same rate. Any direction or location in space (barring any and all gravitational influences) is physically the same and physical laws work the same anywhere in the universe. If we set them perpendicular to one another and then set them both in motion at 99 % light speed in the direction of one of the clocks, we get some interesting results.<br />
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We, as observers, remain at rest with respect to the clocks. This means that as the clocks fly by us, the light is bouncing parallel to the motion in one clock and the light is bouncing perpendicular to the motion in the other clock. We will find that they will both run slower, as we expect, but we also find that they are still both running at the same rate. This observation is not what we expected.<br />
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The clock that is perpendicular to the motion should slow down, as we figured out above. We can imagine that those light pulses must travel a longer distance because they are making a stretched out zigzag path of motion between the mirrors. The stretching is where the extra distance comes from. But what happens to the clock that is parallel to the motion? Aren't those light pulses going to take a much longer time to reach the front-facing mirror that's going just 1 % slower than them?<br />
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We can put together a more concrete example to show what's going on here. Let's say the mirrors in the clocks are 300,000 km apart, the distance light travels in one second. At rest with respect to us, the light will take one second to travel from one mirror to the other. Now we set them in motion like we did above, with one set of mirrors oriented perpendicular to the motion and one parallel to it. If the clocks are now moving at 99 % light speed and the light is moving perpendicular to the direction of motion in one of them, we can calculate that the light will now take about 10 times longer, or 10 s, to make one trip between mirrors. The clock is now going 10 times slower relative to us.<br />
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What about the parallel clock? At rest, the light takes one second to go from one mirror to the other. If the light pulses in the clock are moving in the same direction, at 99 % light speed, the light has to chase a rapidly receding mirror in one direction. We can figure out that it will take about 100 s to reach the front mirror when it's moving away at 99 % light speed. It will take just a tiny fraction of a second to make its return trip to the other mirror because the back mirror is approaching at almost light speed. We discover that the light takes ten times longer to bounce mirror to mirror in the parallel clock (100s rather than 10 s). Yet we measured them and they are both running at the same rate - 10 times slower than they did at rest with us. How is the parallel clock still keeping the same time as the perpendicular one? The only way it can is to physically shrink in the direction of the motion, shortening the bounce distance. In fact, it will shrink to 1/10th of its rest length at 99 % light speed. Length contraction and time dilation have a perfectly inverse relationship. Length and time compensate one another to preserve the invariance of the space-time interval we explored earlier.<br />
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If one of the clocks could travel at light speed (and it cannot because it has mass) the light pulses in it would not move at all. There would be no ticking forward in time. If we could somehow ride along with a photon of light, we would discover that it does not experience time. If we consider the effect of length contraction as well, we come to a startling conclusion. There is no distance at all between the two mirrors in the <i>parallel</i> clock. In that clock the mirrors themselves would have no depth. Time slows down and length contracts for objects travelling very fast. A photon's path of travel is shortened to zero. Proper distance, like proper time, does not exist for a photon. Realizing this gives new weight to the concept that light has a space-time interval of zero.<br />
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Some Parting Thoughts<br />
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To accept the well-established fact that we live inside time as part of four-dimensional space-time is a bit like accepting that we experience only a tiny sliver of visible colour within a far vaster array of electromagnetic radiation. We know that it exists but we don't directly experience it in our everyday lives. We intuitively understand the universe in three-dimensional space but it is almost impossible to conceptualize four-dimensional space-time.<br />
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The movie Interstellar plays with the fact that time is a dimension. In that movie, a future "us" has figured out how to manipulate time to make it act like a tangible physical dimension. We currently can't do that and I don't know if that ever could be possible, but the mathematical formulation of space-time, in particular the Wick and Lorentz rotations, appear to treat time and space as two facets of the same thing. We got to our understanding of time as a dimension of space-time by way of the speed of light. At the speed of light, both time and space reach their limits. Does space-time exist at the speed of light? Do space and time fully unfold to our perception only when we experience the universe at rest?<br />
<br />Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-57984281410282112332019-09-29T14:42:00.001-06:002019-09-29T14:42:56.644-06:00Rocket ScienceInterstellar Rocket Propulsion: An Exploration of the Options and Challenges<br />
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There is considerable scientific interest in designing an interstellar spacecraft, and the technologies required to build one are theoretically feasible. If we ever venture <a href="https://en.wikipedia.org/wiki/Interstellar_travel">to explore a distant stellar system</a>, one of these technologies might get us there.<br />
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Proxima Centauri b: A Destination Case Study<br />
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It might go without saying, that we want to look for life elsewhere in the universe. We all look to the heavens and wonder: are we alone? To answer this question, we are going to need to develop new technologies. We now know that the majority of stars in our universe have a complement of orbiting planets. There are several known possibly Earth-like planets we could explore. The problem is that they are so far away. Our nearest star, <a href="https://en.wikipedia.org/wiki/Proxima_Centauri">Proxima Centauri</a>, is 4.25 light years away, shown as a yellow dot below. This might be the first exoplanet system we actually get to.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgK4h6MUrnWk52Uf7m5xHjxAcine4ZTjV5Qeh8jc7dBuH-gnmhjyV2BYHrmVCX6g1sFY6aTnxYuaGS4ZIOpfnexfWTMgnK5ZxZAXphi9mloroU1qgnOk9pxpQ9Dw5Xg2lsLeJuLnLUFnnf1/s1600/PIA18003-NASA-WISE-StarsNearSun-20140425-2.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="900" data-original-width="1200" height="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgK4h6MUrnWk52Uf7m5xHjxAcine4ZTjV5Qeh8jc7dBuH-gnmhjyV2BYHrmVCX6g1sFY6aTnxYuaGS4ZIOpfnexfWTMgnK5ZxZAXphi9mloroU1qgnOk9pxpQ9Dw5Xg2lsLeJuLnLUFnnf1/s640/PIA18003-NASA-WISE-StarsNearSun-20140425-2.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">NASA/Penn State University; Wikipedia</span></span></td></tr>
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The year when the distance to each star or system was calculated is listed after each name. Proxima Centauri, as seen through the eye of Hubble Telescope, shines bright white, see below left. Although it is our nearest stellar neighbor, it is invisible to the naked eye. Its luminosity is very low because it is a small star, just 1/8 the mass of our Sun.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh-BvfTE5-4jOhBIgMBHkvUK1VoQ6ihuFqOsofZXQwtP0sOHDJ53ugi9oBiPRpL-MzOzaV3qZ1cEfh03ARuRJRt8qskEPukOj9z3HDPWRvXwrbsl-xNEo2ukWSm_H2AH5l094v2P4l286dc/s1600/New_shot_of_Proxima_Centauri%252C_our_nearest_neighbour.jpg" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="592" data-original-width="604" height="313" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh-BvfTE5-4jOhBIgMBHkvUK1VoQ6ihuFqOsofZXQwtP0sOHDJ53ugi9oBiPRpL-MzOzaV3qZ1cEfh03ARuRJRt8qskEPukOj9z3HDPWRvXwrbsl-xNEo2ukWSm_H2AH5l094v2P4l286dc/s320/New_shot_of_Proxima_Centauri%252C_our_nearest_neighbour.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">ESA/Hubble;Wikipedia</span></span></td></tr>
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Proxima Centauri is slightly closer to us <a href="https://en.wikipedia.org/wiki/Alpha_Centauri">than its two partner stars</a>, a binary pair called Alpha Centauri AB, or simply Alpha Centauri as shown in the diagram above.<br />
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Proxima Centauri is a small low-mass <a href="https://en.wikipedia.org/wiki/Main_sequence">main sequence</a> <a href="https://en.wikipedia.org/wiki/Red_dwarf">red dwarf</a> star. It is seen as fairly white through the Hubble lens because even though it is technically a “red” dwarf, its surface temperature is 3040 K (as hot as a light bulb burning warm white).<br />
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Proxima Centauri has at least one confirmed planet orbiting it, called <a href="https://en.wikipedia.org/wiki/Proxima_Centauri_b">Proxima Centauri b</a> (or Proxima b for short). Below is an artist’s impression of the planet orbiting its star. Based on one of several possible models of formation, it could be an arid, but not water-free, super-Earth (1.3 times Earth’s mass).<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjLJcrwKPVG7eD2g8jDzb_cYEVLrIoxt94SITGbLqKdWp_INCrQzDB-O9BR-rjopYhnl9XAnPE00MZmTWtJrB_dVyCF4iiJi7AgwTJ38W3frTN9B1Bdkq6Jzx_2sOPOyp7NBypi5fYcB_1f/s1600/Artist%25E2%2580%2599s_impression_of_Proxima_Centauri_b_shown_hypothetically_as_an_arid_rocky_super-earth.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="900" data-original-width="1600" height="360" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjLJcrwKPVG7eD2g8jDzb_cYEVLrIoxt94SITGbLqKdWp_INCrQzDB-O9BR-rjopYhnl9XAnPE00MZmTWtJrB_dVyCF4iiJi7AgwTJ38W3frTN9B1Bdkq6Jzx_2sOPOyp7NBypi5fYcB_1f/s640/Artist%25E2%2580%2599s_impression_of_Proxima_Centauri_b_shown_hypothetically_as_an_arid_rocky_super-earth.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Artist’s impression; ESO (European Southern Observatory)/M. Kommesser; Wikipedia</span></span></td></tr>
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Whether this planet is habitable or not is <a href="https://phys.org/news/2018-09-closest-planet-solar-habitable-dayside.html">an open question</a>. Even if it is devoid of life, it is an intriguing planet. Discovered in 2016 by <a href="https://www.eso.org/public/teles-instr/lasilla/36/harps/">HARPS Spectrometer</a>, <a href="https://scitechdaily.com/earth-sized-planet-proxima-b-might-be-habitable/">this planet orbits</a> in what is called the <a href="https://en.wikipedia.org/wiki/Circumstellar_habitable_zone">habitable zone</a>. The habitable zone is the distance from a star where a planet’s surface temperature could support liquid water. The radiant heat from the star is just right, not too cold (a frozen world) and not too hot (a world where any surface water would turn to steam and leave the atmosphere). Such a world could support the kind of water-based biochemistry used by all living creatures on Earth. There could be life based on non-water chemistry, using a solvent <a href="https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry">other than water</a>, but this is a marker scientists tend to agree on because we have proof that water-based biochemistry works. Even though it is within the habitable zone, this planet is so close to its dim star, Proxima Centauri, that it is likely to be tidally locked, meaning that the same face of the planet always faces its star. It doesn’t rotate, so there is no night and no day. The star-facing side would likely be too hot to support life as we know it and the night side would be too cold. The strip of area in between, which could be warm enough for liquid surface water, might support life.<br />
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A significant challenge faced by any life on Proxima Centauri b is the radiation <a href="https://en.wikipedia.org/wiki/Habitability_of_red_dwarf_systems">it would endure from its star</a>. Red dwarfs are more violent and less stable than larger stars such as ours. They are covered in starspots that can dim their output light up to 40% for months on end. At other times the planet’s atmosphere, assuming it has one, would face erosion from frequent and powerful radiation flares. Does it have a magnetosphere that would protect an atmosphere?<br />
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Shielded from the direct impact of its star’s unpredictable radiation, the “in between” zone might stand a chance to support life. It’s a bit of a slim shot at alien life, but fortunately it isn’t our only shot. There are numerous other exoplanets, which might be more suitable for life to evolve, and they might all be within our eventual physical reach. The first question, however, is how do we find them?<br />
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The Search For Exoplanet Candidates<br />
<br />
It is startlingly amazing what we can glean through indirect observation. Proxima Centauri b, our case study planet, was discovered by ESO’s HARPS Spectrometer, located at La Silla observatory in the Chilean desert. Like other planets, it is illuminated by reflected light only and it is far too faint to be been by the naked eye. High-precision HARPS detects minute Doppler shifts in the radiation from its parent star, Proxima Centauri. These <a href="https://en.wikipedia.org/wiki/Doppler_spectroscopy">tiny shifts in the star’s radial velocity</a> are in reality the tiny regular wobbles it makes as its planet orbits around it. Each time it orbits, the planet shifts the center of gravity of this simple two-body system back and forth. Even though this detection method detects only a fraction of possible planets - those planets whose orbital plane lines up with Earth - in just two decades, <a href="https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html">more than 4000 exoplanets have been confirmed</a>. We are in the golden age of exoplanet discovery.<br />
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HARPS isn’t alone. Sophisticated ground-based telescopes of various kinds (listed <a href="https://en.wikipedia.org/wiki/List_of_exoplanet_search_projects">here</a> in Wikipedia), have detected exoplanets. New ones are detected as our ability to look improves. HARPS, so far, has discovered over 130 exoplanets. Many additional observatories are located in orbit around Earth, such as the <a href="https://en.wikipedia.org/wiki/Kepler_space_telescope">Kepler Space Telescope</a>, for example, with 2421 exoplanets detected and counting. And now <a href="https://en.wikipedia.org/wiki/Transiting_Exoplanet_Survey_Satellite">TESS</a> has detected 9 exoplanets during its first operational year. The tantalizing question is what kinds of alien life could some of these exoplanets harbour?<br />
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These sophisticated observations can answer questions about planetary mass, orbit, the atmosphere, and even possible surface composition regarding planets that are far too distant, too small and too dim for us to observe directly in the night sky. This information is tantalizing. Yet there is no substitute for being there. It is a human need to see with ones own eyes, feel with ones own fingers. Perhaps someday a manned interstellar mission to a planet harbouring alien life will happen. It’s perhaps more likely that we might never surmount the daunting technical obstacles faced with such a long manned spaceflight, one that out of necessity would take many years, perhaps even longer than a single human lifetime to accomplish. Wikipedia lists our <a href="https://en.wikipedia.org/wiki/Interstellar_travel#Proposed_methods">manned travel options</a>, and obstacles, in this regard.<br />
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There is hope. A future robotic mission equipped with suites of sensory instruments that could see, hear, physically feel, chemically taste and smell for us might be feasible in the near future. Besides crafting a vehicle that can house these instruments AND withstand years of interstellar travel, the principle challenge facing us is the time it will take to get to any distant solar system. Even traveling at near light speed to our closest exoplanet system, Proxima Centauri, would take at least four years. Assuming we have the propulsion technology to make this happen, imagine how difficult it would be to get funding for a certainly expensive exploratory mission that would take over eight years to receive any data back IF nothing goes wrong.<br />
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Spacecraft Propulsion: A Very Brief History<br />
<br />
Perhaps the most daunting question is how to propel the spacecraft, and that is really what I’d like to explore in this article. To put this problem in perspective, consider the chemical propulsion technology we currently use. In 2013, <a href="https://en.wikipedia.org/wiki/Voyager_1">Voyager 1</a> (shown below) entered interstellar space at a velocity of 62,000 km/h (about 17 km/s).<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjbCseRER1clUFJwe7CtIg_slHzF7_YaQQnkBMXKUjRbzfwiyBH5j_ltw8uocgdjQMBPzg2OD4Kbs7odEXcit9-gI_AbWnbMEb_TxMvuQf4dkKmI8w1lFLbq2_J9A6cMholGUskeGIoZGdG/s1600/PIA17049_hires.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="901" data-original-width="1600" height="360" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjbCseRER1clUFJwe7CtIg_slHzF7_YaQQnkBMXKUjRbzfwiyBH5j_ltw8uocgdjQMBPzg2OD4Kbs7odEXcit9-gI_AbWnbMEb_TxMvuQf4dkKmI8w1lFLbq2_J9A6cMholGUskeGIoZGdG/s640/PIA17049_hires.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Artist’s rendition; NASA/JPL- Caltech</span></span></td></tr>
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That’s fast, about 52 times the speed of sound. Yet, at this rate it would take about 70,000 years to reach Alpha Centauri from Earth, just not feasible. As you can imagine, chemical propulsion technology has improved since 1977, when Voyager was launched. Accelerated by Jupiter’s gravity, the <a href="https://en.wikipedia.org/wiki/Juno_(spacecraft)">Juno probe</a>, launched in 2011, (see image below) briefly clocked in at 266,000 km/h as it locked into orbit around Jupiter. A 35-minute insertion burn decelerated it to a manageable velocity of about 2000 km/h.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg6RSo4cnLXMzcFRFSLVwFlaFteXFFW37u2vjPlGcOWfozX1rOTZvdiNysa_pVCqWtc3vgyIMOREy2a4KLLacJf6DEh7dpfflpdIlftOhcGoi7ZBkRUeV1wD1xCQ0A4lrfBGpLgxcYUDBmt/s1600/dims.jpeg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="984" data-original-width="1600" height="392" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg6RSo4cnLXMzcFRFSLVwFlaFteXFFW37u2vjPlGcOWfozX1rOTZvdiNysa_pVCqWtc3vgyIMOREy2a4KLLacJf6DEh7dpfflpdIlftOhcGoi7ZBkRUeV1wD1xCQ0A4lrfBGpLgxcYUDBmt/s640/dims.jpeg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Artist’s impression; NASA</span></span></td></tr>
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The <a href="https://www.nasa.gov/content/goddard/parker-solar-probe">Parker Solar Probe</a>, launched last summer (2018), (shown below) should reach a velocity about three times higher, 692,000 km/h, as it slingshots around the Sun in 2025, setting a new velocity record that is almost ten times the velocity of Voyager 1.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiLrrZm7bkqzwtQsc-EBiY0Q-nYGoamHr50rEQJ09pD8zDtJM4U9bHR-Yh-r8eegL3VpOd1rEifnjRw1K1hanWBMDpYKSVDcUdu2DDw8LpNFsqLwqLjt4Chn5_jDPcWrZ8bSKQ8Q4fsKXfs/s1600/2560px-Parker_Solar_Probe.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1108" data-original-width="1600" height="442" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiLrrZm7bkqzwtQsc-EBiY0Q-nYGoamHr50rEQJ09pD8zDtJM4U9bHR-Yh-r8eegL3VpOd1rEifnjRw1K1hanWBMDpYKSVDcUdu2DDw8LpNFsqLwqLjt4Chn5_jDPcWrZ8bSKQ8Q4fsKXfs/s640/2560px-Parker_Solar_Probe.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Artist’s rendition; NASA/John Hopkins APL/Steve Gribben</span></span></td></tr>
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The fundamental physics behind these speed records is the same for all these craft. They make use of a phenomenon called <a href="https://en.wikipedia.org/wiki/Gravity_assist">gravity assist</a>, which can really got things moving.<br />
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Current Technique: Gravity Assist<br />
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Let’s start at the beginning. In order to successfully launch, a rocket must overcome Earth’s <a href="https://en.wikipedia.org/wiki/Gravity_well">gravitational well</a>. A gravity well? We can think of a mass “denting” the space around it, as depicted below right, with a mass such as a planet, at the bottom of the well. The gravity well is a conceptual model arising from Einstein’s <a href="https://en.wikipedia.org/wiki/Introduction_to_general_relativity">theory of general relativity</a>, which predicts that gravity arises from the curvature of space-time. Technically it is four-dimensional space-time, not three-dimensional physical space, that curves .<br />
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhOgOvLqsWV0kTzohG7BjzijbM2qMg0fe5oiKpIQpVZbxg5NoNko-3E_t6Lxb3Y5QDCw1yEsOPNiCRgBjJamNwRc1z4dvJG73Qt75p8fNW05Cf0t-sqMjmtpP9apQ2b3aOzV7QSnTGyJCKh/s1600/GravityPotential.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="462" data-original-width="796" height="185" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhOgOvLqsWV0kTzohG7BjzijbM2qMg0fe5oiKpIQpVZbxg5NoNko-3E_t6Lxb3Y5QDCw1yEsOPNiCRgBjJamNwRc1z4dvJG73Qt75p8fNW05Cf0t-sqMjmtpP9apQ2b3aOzV7QSnTGyJCKh/s320/GravityPotential.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">AllenMcC;Wikpedia</span></span></td></tr>
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In rocket science, it helps to conceptualize gravity as a geometry (Einstein) rather than <a href="https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation">an attractive force</a> (Newton). Both Voyager 1 onboard a <a href="https://en.wikipedia.org/wiki/Titan_IIIE">Titan 3E rocket</a> and the Parker Probe onboard the current <a href="https://en.wikipedia.org/wiki/Delta_IV_Heavy">Delta IV Heavy rocket</a> accelerated to velocities of approximately 130,000 km/h (36 km/s) as they left Earth’s atmosphere. Consuming chemical rocket fuel, the rockets achieved enough velocity to climb out of Earth’s gravity well.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhoCj2wbJ1qwVtNCF5XoLP2Q3GGkaPmb6mOBelA4M29wazGeXq1vfQhPj_QrUC7qe4JahL8r_mAE6ebc3LUI6loNiPE9w5vyupBWztTxTZhK-AVwSeROm0x0aM4Iv5PDMlS1qbWxB4VgRx4/s1600/1920px-Titan_3E_Centaur_launches_Voyager_2.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1600" data-original-width="1206" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhoCj2wbJ1qwVtNCF5XoLP2Q3GGkaPmb6mOBelA4M29wazGeXq1vfQhPj_QrUC7qe4JahL8r_mAE6ebc3LUI6loNiPE9w5vyupBWztTxTZhK-AVwSeROm0x0aM4Iv5PDMlS1qbWxB4VgRx4/s640/1920px-Titan_3E_Centaur_launches_Voyager_2.jpg" width="482" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">An image of Voyager 2 aboard Titan III-Centaur. Both Voyager probes were launched by Titan III rockets, NASA/MSFC</span></span></td></tr>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhTdyRIdeATPNN_mzf85yKK1PdwVU-7JmVzuCjwRTCQxrOUztU8ReJuqk-DuSa1masqwcLZKkS97Z7_i-IvY6_noXwAYKXrPm88XZRMg9KnWeEZlUWcVha9sPywhBI5xuvKJVp3qwoPRtHh/s1600/1920px-Delta_IV_launch_2013-08-28.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1600" data-original-width="1280" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhTdyRIdeATPNN_mzf85yKK1PdwVU-7JmVzuCjwRTCQxrOUztU8ReJuqk-DuSa1masqwcLZKkS97Z7_i-IvY6_noXwAYKXrPm88XZRMg9KnWeEZlUWcVha9sPywhBI5xuvKJVp3qwoPRtHh/s640/1920px-Delta_IV_launch_2013-08-28.jpg" width="512" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Delta IV Heavy Rocket launch in 2013; U.S. Air Force/Joe Davila. This rocket is currently in use with several launches expected in 2020-2023.</span></span></td></tr>
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A future interstellar rocket will have to climb out of Earth’s gravity well and then climb out of the massive Sun’s gravity well. Such climbs are steep; chemical fuel is rapidly used up. That will be a challenge for a future interstellar probe, as it was for Voyager 1, the only craft so far to climb out of our solar system gravity well. Earth makes an indent in space that a rocket must climb out of. The Sun makes a far deeper indent, which any spacecraft must overcome in order to leave our solar system. Velocity is key. Consider a black hole, by comparison. Its mass is so great and concentrated in such small volume that its gravitational well is inescapable, even to light. Photons of light, traveling at 300,000 km PER SECOND, are too slow to escape it.<br />
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One key problem with chemical rocket propulsion is fuel weight. Voyager 1 was launched on the expendable 4-stage rocket, <a href="https://en.wikipedia.org/wiki/Titan_IIIE">Titan 3E</a>, which used solid and liquid chemical fuels in its engines, providing enough thrust to lift the fairly heavy (823 kg) Voyager probe. At lift-off, the fully fueled rocket weighed about 633,000 kg, almost all of which was fuel. It needed most of it just to escape Earth’s gravity, with its fuel weight working against it. Even starting out with a respectable launch velocity of 130,000 km/h, Voyager 1 would have quickly petered out to become a dead weight in space were it not for the fortuitous and anticipated line-up of massive planets at that time.<br />
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Gravity Assist<br />
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Once boosted to a certain velocity, any body will maintain that velocity in a vacuum, and space is an almost perfect vacuum. Nonetheless, gravity acts on it. In Voyager 1’s case, gravity was working against it and for it. It had to climb up the Sun’s gravity well but it had several gravity assists to help it along. It reached its current velocity of 62,000 km/h, by making use of how the planets lined up in 1977. It passed very near Jupiter and Saturn, and then Uranus and Neptune, using each of their gravity wells to slingshot around them. It went with the planet’s spin to gain momentum at the expense of the planet’s momentum in order to boost its velocity. The decrease in spin of a planet would be immeasurably miniscule because its momentum is so much greater than a relatively infinitesimal probe. Voyager 1 was able to enter interstellar space by escaping the Sun’s gravitational well, which we can think of as that same inverted cone with a gradually widening, flattening base. An object traveling near the edge of that cone base, far from the Sun, experiences a much weaker gravitational pull. The solar gravity well isn’t so steep near the edge. Very close to the Sun, where the well is steepest, an object needs to travel at 525 km/s, or almost 2 million km/h, (away from it) to escape its gravity. Escape velocity then drops exponentially with distance. To help visualize how this works, see the graph below. Using data extrapolated from Voyager 2’s similar statistics, Voyager 1’s launch speed was about 36 km/s (about 130,000 km/h) (red line), very close to solar system escape velocity (blue line) at Earth’s orbital distance from the Sun. Its initial thrust had to work against the Earth’s gravity and then against the Sun’s enormous gravity, rapidly decelerating it to about 10 km/s by the time it approached Jupiter. If it kept experiencing that rate of deceleration it would never escape the Sun’s pull. Jupiter’s gravity boosted it back up to about 28km/s, well above solar escape velocity at that distant orbit. Shortly afterward, and with the help of subsequent planetary boosts, Voyager 1 kept getting boosted to keep it well above solar escape velocity. And that’s how Voyager was able to leave our solar system behind for the adventure of interstellar space beyond.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjEeFxEuNZDANlWhqNUIRxEQiYhWE6W3v36vwuMhTYO6QwSI3xM4NCxDBwKQXP2XKmX-oP45eEfNvaPMxxu0XwozhEdUGFVOM1gJNW8j6MPj66EgOuNshc69uWza7eLvcqxK67EppJCquRB/s1600/2560px-Voyager_2_velocity_vs_distance_from_sun.svg.png" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1600" data-original-width="1600" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjEeFxEuNZDANlWhqNUIRxEQiYhWE6W3v36vwuMhTYO6QwSI3xM4NCxDBwKQXP2XKmX-oP45eEfNvaPMxxu0XwozhEdUGFVOM1gJNW8j6MPj66EgOuNshc69uWza7eLvcqxK67EppJCquRB/s400/2560px-Voyager_2_velocity_vs_distance_from_sun.svg.png" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Cmglee;Wikipedia</span></span></td></tr>
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Velocity and Travel Time<br />
<br />
We have solved the issue of leaving our solar system but the issue of achieving a reasonable journey time to an extremely distant solar system remains. There is a limit to what kind of velocity a craft can achieve using chemical fuel for an initial thrust followed by gravity assists. Even if a spacecraft uses the Sun as its gravity assist, like the Parker probe will, reaching a velocity of 692,000 km/h, it would still take about 7000 years to reach Proxima Centauri, 4.25 light years away. The only way to shorten that travel time is to go faster, much faster.<br />
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Thrust/Weight Ratio<br />
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There is a built-in limit to the usefulness of any chemical rocket fuel, and that is its <a href="https://en.wikipedia.org/wiki/Thrust-to-weight_ratio">thrust-to-weight (T/W) ratio</a>. This is a very useful dimensionless ratio calculated by dividing engine thrust (in units of force called <a href="https://en.wikipedia.org/wiki/Newton_(unit)">newtons</a> (N) by the weight of the fully fueled rocket at sea level (again in newtons), This is an example of weight treated as a force. It is generated by Earth’s gravitational field. Thrust is directly proportional to the acceleration of the rocket (force = mass x acceleration). <br />
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A high T/W ratio means high acceleration. Three force vectors work on a rocket as it lifts off: thrust (upward), weight (downward) and drag (downward). <a href="https://www.grc.nasa.gov/WWW/K-12/airplane/drag1.html">Drag</a> is a mechanical force. It is the friction created as a solid object (the rocket) moves through a fluid (air in this case). Drag diminishes as the rocket exits Earth’s atmosphere. Thrust is the force generated by the rocket’s propulsion system.<br />
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If the ratio is greater than 1 and the drag is minimal, the rocket can lift off and accelerate upward. The T/W ratio is used as a static measure for a rocket at sea level. In operation, this ratio changes constantly as the rocket uses up fuel, reducing weight, and as drag decreases up through the atmosphere. It also changes as engine efficiency changes. To get an idea of the values involved, we can compare an Airbus A380 fully fueled and loaded at takeoff with a T/W ration of 0.227. It can take off and get airborne but with a T/W less than 1 but it can’t accelerate straight upward. To get a little more nuanced, its weight overcomes its thrust so airspeed always decays during a climb, whether it’s vertical or not. Fighter jets, in contrast usually achieve a T/W of close to 1, which affords them much greater maneuverability and speed. You might find it interesting to compare the W/T ratios of various aircraft <a href="https://en.wikipedia.org/wiki/Thrust-to-weight_ratio#Aircraft_2">here</a>. The Space Shuttle fully fueled at takeoff had a T/W of 1.5 (using three main engines and two solid rocket boosters), plenty of vertical thrust to escape Earth’s gravity and reach <a href="https://en.wikipedia.org/wiki/Orbital_speed">orbital speed</a>. To achieve this, the shuttle would launch vertically for a few kilometres while performing a gravity turn. That means the shuttle allows gravity to gradually bend its trajectory from straight up into horizontal to the ground as it accelerates up to the speed and altitude where it can maintain its orbit around Earth once its rockets shut off.<br />
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You won’t find a simple T/W ratio for modern rockets at takeoff because this ratio isn’t all that useful. They are all over 1. More important are the individual thrusts of the engines and the overall weight of the fuel, which is constantly changing, and the payload weight. You might guess that a higher T/W ratio is always better for a rocket launch but it isn’t. Instead there is an optimal acceleration rate. A higher T/W ratio means you build up great velocity in the lower atmosphere where the air is thickest. This means the rocket is subject to a lot of drag and therefore heating. These two factors increase wear and tear and engine inefficiency. A lower T/W ratio, on the other hand, means it will take longer to reach orbital speed so more fuel is used up fighting against gravitational forces. There is a sweet spot. Most rockets are designed for a T/W ratio of slightly higher than 1 with a maximum acceleration of about 4 <a href="https://en.wikipedia.org/wiki/G-force">g</a> for a few seconds at the end of the first stage rocket burn, when most of the rocket fuel is burned and the weight is lowest compared to the upward thrust. 4 g means that our bodies feel 4 times heavier than they normally do (at 1 g). That is about the maximum acceleration that is <a href="https://www.pbs.org/wgbh/nova/article/gravity-forces/">“comfortable” for astronauts</a>. To achieve the very high velocity required for a feasible interstellar mission, such a rocket will need to continue to accelerate after it has achieved orbital speed, and it will also need more boost than a gravity assist from our Sun. Considering that the nearest system is over 4 light years away, it will need to be boosted to a significant fraction of the speed of light. It will also need enough reserve thrust to maneuver into orbit around its target planet, but I am getting ahead of myself.<br />
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Rocket Equation<br />
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Ultimately no chemical rocket fuel can provide the kind of long-term thrust required to reach a significant fraction of the speed of light, often loosely called <a href="https://www.quora.com/What-is-relativistic-velocity">relativistic velocity</a>. Why? To answer this question we can start by exploring the <a href="https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation">ideal rocket equation</a>. This equation describes how a rocket accelerates itself using thrust created by shooting out part of its mass at high velocity in the opposite direction. That’s the Sun-bright blast from the rocket’s engines as it lifts off its pad. The rocket moves according to the common-sense principles of <a href="https://www.grc.nasa.gov/www/k-12/airplane/conmo.html">the conservation of momentum</a>, Newton’s second law. But there is a bit more to it than what at first seems quite simple.<br />
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The rocket equation steps in where Newton’s second law cannot. Put formally, <a href="https://www.physicsclassroom.com/class/newtlaws/Lesson-3/Newton-s-Second-Law">Newton’s second law</a> states that the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. Or, simply put, it is force = mass x acceleration. In a rocket system, mass is always changing, so Russian scientist Konstantin Tsiolkovsky, in 1903, derived a formula using straightforward calculus, that describes its unique motion. It’s a lovely example of why calculus is good, and it’s a good equation <a href="https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation#Derivation">on which to practice your calculus skills</a>. Below is a screenshot of the equation <a href="https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation">taken from the Wikipedia page</a> to show you what it looks like:<br />
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In order to describe the motion of our relativistic interstellar rocket system, we need not only the rocket equation but also to juice it up further to describe a system that is acquiring relativistic mass as it approaches light speed. This kind of transformation is described by Einstein’s <a href="https://en.wikipedia.org/wiki/Special_relativity">special relativity theory</a>. This more complex rocket equation is derived for a rocket accelerating close to light speed. The mass, still changing, is also acquiring significant momentum as it approaches the speed of light, <a href="https://en.wikipedia.org/wiki/Mass_in_special_relativity">becoming relativistic mass</a> according to the law of special relativity. For those interested in the mathematics of this derivation, <a href="http://www.relativitycalculator.com/images/rocket_equations/AIAA.pdf">this paper</a> offers a side-by-side comparison between the classic rocket equation and its relativistic counterpart.<br />
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The rocket equation describes the motion of a rocket system. If you’d like to see how it is derived (in its non-relativistic form), this <a href="https://www.grc.nasa.gov/WWW/K-12/rocket/rktpow.html">NASA site</a> goes through it well. For our needs, it’s good enough to simply know the rocket equation relates a vehicle’s maximum possible change in velocity (<a href="https://en.wikipedia.org/wiki/Delta-v">delta-v</a>) to the engine exhaust velocity, as the vehicle’s mass changes while fuel is consumed. It is from this equation that we get such terms as delta-v and impulse, which we will get into. Using this equation we can explore how the momentum of a rocket system changes as its fuel is consumed. We can compare the efficiencies of various rocket fuels in a rocket system. We can calculate how much fuel is required to change the velocity of a system over time, and what the maximum change in velocity is. Ultimately this equation can tell us which kinds of fuel will get us the acceleration and maximum velocity we need to achieve interstellar travel.<br />
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Impulse<br />
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If you are sci-fi fan like me, you’ve probably heard or read, “Go to full impulse!” In physics, <a href="https://en.wikipedia.org/wiki/Impulse_(physics)">impulse</a> is another outgrowth of Newton’s second law. It is the integral of a force over a time interval. In other words, we use calculus to describe a force changing over time. As a result, impulse creates a change in momentum of a system. To see how these two concepts are related to each other, check out this <a href="http://physicsclassroom.com/">physicsclassroom.com</a> <a href="https://www.physicsclassroom.com/class/momentum/Lesson-1/Momentum-and-Impulse-Connection">link</a>.<br />
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Impulse can also be used to figure out the efficiency of the fuel used in the rocket system if the mass of the fuel (also called <a href="https://en.wikipedia.org/wiki/Rocket_propellant">propellant</a>) is taken into account. It is total impulse per unit of fuel used up. More specifically, we can measure how many seconds a fuel can accelerate its own initial mass at 1 g. The resulting value, in seconds, is called the <a href="https://en.wikipedia.org/wiki/Specific_impulse">specific impulse</a>. If we turn this relationship around slightly, we also determine how much of a particular fuel is needed for a given delta-v. There are several different ways to calculate specific impulse. Another method brings us back to the rocket equation. It can be calculated as the generated thrust of a system per unit mass fuel flow rate. This gives us the effective fuel exhaust velocity relative to the rocket. Effective exhaust velocity is proportional to the specific impulse, and both can be used to measure the efficiency of a rocket fuel. To get a feel for how these two values relate to each other, consider a fighter jet’s rocket-like engine running within Earth’s atmosphere. We can calculate the jet’s effective exhaust velocity from the rocket equation. Its actual exhaust velocity, however, will be reduced by atmospheric pressure. This pressure works against the exhaust, and therefore the momentum, and the delta-v. In return, the specific impulse of the engine’s fuel, its efficiency in other words, is also reduced under these real conditions. In jet engines, there is a big difference between effective and actual exhaust velocity. In rockets operating in the vacuum of space, there is none. If you go back to the <a href="https://en.wikipedia.org/wiki/Delta_IV_Heavy">Delta IV Heavy rocket </a>Wikipedia page, on the right you can compare the specific impulses of its engine stages at sea level (in air) and in vacuum.<br />
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Specific impulse can be derived from yet another rocket science term called <a href="https://en.wikipedia.org/wiki/Thrust">thrust</a>. Thrust is a mechanical force. It is equal and opposite to the force generated by the mass of gas accelerated and shot out as the rocket’s fuel exhaust. This force, an example of <a href="https://www.physicsclassroom.com/class/newtlaws/Lesson-4/Newton-s-Third-Law">Newton’s third law of motion</a>, is in the forward direction of the rocket. Newton’s third law states that for every action (or force), there is an equal and opposite reaction (force). Specific impulse can be calculated by dividing the thrust by the rate at which the fuel is used up.<br />
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We can compare the efficiencies of various rocket fuels by comparing their specific impulse values. Most rocket fuels <a href="https://history.nasa.gov/conghand/propelnt.htm">range from about 175 s to 300 s</a>. “Seconds” might seem like a weird unit to measure how good a fuel is, but it is quite practical, and it makes sense now that you know how it is derived. In practice, thrust and efficiency (which usually increase with the price) tend to trade off one another. Solid rocket fuels tend to have high thrust and relatively low cost. They are usually used in the first stage engines on modern rockets because you need a lot of force to achieve lift off and its okay, even advantageous, to burn off a lot of heavy fuel right away. The more expensive higher impulse fuels (many of these being liquid hydrogen-based) are saved for higher stages. These fuels have a higher specific impulse (more seconds) than solid rocket fuels. If you are curious to compare efficiencies and other features among an extensive list of rocket fuels, see NASA’s list <a href="https://history.nasa.gov/conghand/propelnt.htm">here</a>,<br />
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No chemical rocket fuel is efficient enough to accelerate to a relativistic velocity. It is tempting to think that, because there is no air resistance in space and, away from stars and planets, there is no significant gravity to work against, it would be easy to eventually accelerate a spacecraft to near light speed. The problem is that in space, there is also nothing to push against to make the craft go faster except the fuel it brings along with it. The mass of the fuel resists being accelerated. It has its own <a href="https://www.physicsclassroom.com/class/newtlaws/Lesson-1/Inertia-and-Mass">inertia</a> to overcome. If the fuel is too inefficient, you can’t pack in enough fuel to accelerate the spacecraft long enough to reach relativistic speed and overcome the inertia of the fuel mass itself. (We’ve now covered all three of Newton’s laws.)<br />
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In order to achieve a reasonable interstellar velocity, we need a fuel with a much higher efficiency, or specific impulse, than any chemical combustion reaction can provide. I’d like to caution us here because I mentioned exhaust velocity earlier. It would be easy to assume that the final velocity of a rocket is limited by its exhaust velocity. This calls for a close examination of Newton’s third law. The forces must balance out, but not the velocities. The fuel in the engine accelerates from rest to whatever the engine exhaust velocity happens to be. This accelerating mass of fuel creates the equal and opposite force of thrust. The force of thrust will continue to accelerate the rocket as long as the engine keeps working – in a vacuum. A jet flying through the atmosphere, on the other hand, will continue to accelerate until air resistance balances out the thrust of the aircraft. As a little test of this, consider a jet with enough thrust to go supersonic. This doesn’t mean its engine exhaust (which unlike a rocket is almost entirely accelerated air) is supersonic.<br />
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Fuel Options for Interstellar Flight<br />
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Most of these options are based on the likelihood that our first interstellar exploratory mission to another planetary system will be <a href="https://en.wikipedia.org/wiki/Interstellar_probe">an unmanned probe</a>. In theory, interstellar (unmanned) exploration is now possible, but much needs to be worked technically out before we can send off our first interstellar probe. We need a promising target planet, one that we will be reasonably certain has liquid water and an atmosphere containing oxygen, the bare essentials for life as we know it. New exoplanets are discovered almost every day, and new details about them are being discovered. We don’t know enough about any one of them to know if it could support life. To get there, a daunting number of technical problems need to be solved, such as propulsion and communications, but also protection from the brutal micro-impacts and radiation of interstellar space. Today’s new projects are at the brain-storming-for-ideas stage. Breakthroughs scientists are making right now are built on from the successes of previous projects, and it’s a very exciting time to follow this progress.<br />
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How It All Started: Orion<br />
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A silver lining emerged from the aftermath of WWII as some scientists began to wonder if they could harness the awesome power of a nuclear explosion to explore the depths of space. Since then, a number of additional novel propulsion concepts for such as mission have been proposed. To get an idea of them all, see <a href="https://en.wikipedia.org/wiki/Interstellar_probe#Interstellar_concepts">this list</a> in the Wikipedia article linked in the paragraph above. One of the most intriguing possibilities uses either a fission or fusion chain reaction. This concept is a powerful one. It offers a perfect scenario by combining two essential qualities into one: high thrust AND high specific impulse. It can be very efficient and it can also perform the high delta-v maneuvers required to insert itself into orbit or to land onto an exoplanet. <br />
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This dream took root in the late 1950’s, at the peak of the <a href="https://en.wikipedia.org/wiki/Atomic_Age">atomic age</a> epitomized by Disney’s (original) Tomorrowland and later on by The Jetsons, well before news of the Three Mile Island accident, the Chernobyl meltdown and the Fukushima disaster soured public opinion in the decades to come. The dream was <a href="https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)">Project Orion</a>, developed by theoretical physicist, <a href="https://en.wikipedia.org/wiki/Freeman_Dyson">Freeman Dyson</a>.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">An artist’s conception of an interplanetary Project Orion spacecraft; NASA</span></span></td></tr>
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The fascinating story of Project Orion is fleshed out in the BBC4 2002 1-hour documentary, “The Secret History of Project Orion: To Mars by A-Bomb.” So well told and painting such a vivid picture of that time, this video is one of my favourites:<br />
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In 2002, George Dyson gave an excellent 8-minute TED talk about his late father’s work. With dry wit, he displayed an original chart of wildly oscillating g-forces the passengers would have experienced, exposures of 700 rads/explosion at the crew station, and permanent eye burn for the spectators.<br />
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Originally cloaked in government top-secrecy, the Orion concept nonetheless eventually emerged to capture the imagination of the world. Stanley Kubrick’s 1968 movie, “2001: A Space Odyssey,” utilized a fictitious nuclear-powered high-performance <a href="https://www.space.com/32258-orion-space-plane-2001-space-odyssey-photo-essay.html">Orion III</a> to shuttle passengers from Earth to (also fictitious) Space Station V.<br />
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The actual Orion spacecraft would have been propelled by a series of nuclear fission explosions (nuclear pulse propulsion). Of course, one chief challenge was how not to blow the whole ship apart (how to harness and dampen almost instantaneous surges in momentum). This challenge would be revisited and refined decades later. The Partial Test Ban Treaty of 1963 as well as concerns about the radioactive fallout from its propulsion system, really a series of full-scale nuclear bombs, led to its fall from favour in the 1970’s.<br />
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But the idea didn’t fall away completely. This concept has been refined over the decades since into smaller better-controlled fission propulsion systems that use tiny pellets of fuel to generate chains of micro-explosions. Advances in techniques to confine and direct the explosions would further increase the efficiency of the system. The size of the original Orion interplanetary prototype vehicle was a huge full-scale manned 90 metre high 4000 tonne rocket. Requiring 800 0.14-kilotonne nuclear bombs detonating in rapid succession just to lift off, the original idea was to use this behemoth to get to Mars. While Orion scientists were focused on Mars, a final nail in the Orion coffin came from NASA’s decision to focus instead on (chemical propulsion) missions to the Moon.<br />
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From the grand-plan-anything-goes mindset of the 50’s, a once crazy-sounding idea is gathering new public interest. Building from the Orion days, a possible future micro-fission, or even micro-fusion, version of this system will be far smaller, lighter, and less dangerous, thanks to advances in technology. A nuclear pulse engine could consist of a series of anti-hydrogen pellets as small as 0.1 mm in diameter suspended within 100-gram hollow shells of nuclear fuel. A series of <a href="https://en.wikipedia.org/wiki/Explosive_lens">explosive lenses</a> (shaped focusing charges) could break a <a href="https://en.wikipedia.org/wiki/Penning_trap">Penning trap</a> used to suspend <a href="https://en.wikipedia.org/wiki/Antimatter">antimatter</a> in a vacuum, triggering an <a href="https://en.wikipedia.org/wiki/Annihilation">annihilation explosion</a> with enough energy to trigger nuclear fission or even fusion, which in the latter case would create a shower of fast neutrons and very little radioactive fallout. <a href="https://en.wikipedia.org/wiki/AIMStar">Project AIMStar</a> and <a href="https://en.wikipedia.org/wiki/ICAN-II">Project ICAN</a>, both proposed in the 1990’s, depend on versions of this much more compact <a href="https://en.wikipedia.org/wiki/Antimatter-catalyzed_nuclear_pulse_propulsion">antimatter-triggered nuclear pulse system</a>. Meanwhile, <a href="https://en.wikipedia.org/wiki/Project_Daedalus">Project Daedalus</a> and <a href="https://en.wikipedia.org/wiki/Project_Longshot">Project Longshot</a>, projects in the1970’s and 1980’s respectively, refined the concept of a new type of fusion energy called <a href="https://en.wikipedia.org/wiki/Inertial_confinement_fusion">inertial confinement fusion</a> for use in a nuclear pulse propulsion system.<br />
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1) Nuclear Photonic Propulsion<br />
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This theoretical system introduces an interesting twist to nuclear propulsion. Like other nuclear systems, it offers very high efficiency. The maximum specific impulse of any chemical propellant is only about 400 seconds. A variety of theoretical propulsion systems can achieve specific impulses magnitudes higher, with some also potentially achieving relativistic velocities. To compare, an <a href="https://en.wikipedia.org/wiki/Ion_thruster">ion propulsion</a> system, which I will describe, can theoretically achieve a specific impulse of about 10,000 s. With this efficiency such a craft could achieve much faster speeds, but probably not relativistic, velocity. A <a href="https://en.wikipedia.org/wiki/Photon_rocket">photon rocket</a>, however, could top this, potentially achieving an impulse of 30,000,000 s. This type of propulsion could propel a spacecraft up to 1/10 the speed of light. <br />
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A robotic probe trip to Alpha Centauri, for example, could become feasible, at a little over 40 years one-way with an additional 40 years to begin receiving information back on Earth. A <a href="https://en.wikipedia.org/wiki/Nuclear_photonic_rocket">nuclear photonic rocket</a> would have an onboard nuclear (fission) reactor, which would create enough heat to emit a broad band of very intense <a href="https://en.wikipedia.org/wiki/Black-body_radiation">blackbody radiation</a>. Analogous to the radiant heat you feel near red-hot glowing embers, the thermal radiation from nuclear fission works exactly the same but consists of highly energetic gamma and X-ray photons rather than infrared photons. If all the nuclear fuel on board a spacecraft could be converted into photon energy and it was all directed in a perfect beam out the back of the craft, it could provide an enormous impulse. However, because thrust is derived from massless photons, their overall momentum is limited. Therefore, this seemingly ideal scenario, although it is nuclear, has the problem of low thrust.<br />
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Low thrust is a problem built into photonic systems. This is actually a classic Newton’s Third Law problem. Chemically fueled rockets shoot out literally tons of mass to achieve high thrust (high acceleration), even though the spent fuel is exhausted at a comparatively low – chemical explosion-level – velocity. A rocket driven by a flow of very low-mass ions or, going further, massless photons, has an extremely low propellant mass flow (low acceleration) but very high exhaust velocity. That means that these systems can eventually achieve very high rocket velocity, but it would take a long time to reach it. A question inherent with such low-thrust systems is how to change velocity quickly enough to fall into orbit around a planet and also possibly land on a planet surface, all things that a probe mission to an exoplanet would likely have to do. It would seem a massive waste to send a craft all that way only to do a <a href="https://en.wikipedia.org/wiki/Flyby_(spaceflight)">fly-by</a>. These space flight <a href="https://en.wikipedia.org/wiki/Orbital_maneuver">orbital maneuvers</a> require steep changes in velocity, which in turn require massive thrust.<br />
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On looking up “<a href="https://en.wikipedia.org/wiki/Nuclear_photonic_rocket#Energy_requirements_and_comparisons">Energy requirements and comparisons</a>” on the Nuclear Photonic Rocket Wikipedia page linked above, it paints a rather bleak picture for our hopes of a photon propulsion system for interstellar space flight, at least for a manned flight. An example 300,000 kg spacecraft set up with reasonable perimeters, will indeed reach 1/10th speed of light. That’s feasible. However, nuclear energy is not nearly as efficient as we might assume it is. Using even the highest grade of nuclear fuel possible, the fission process itself never converts all of its fuel mass into photons. It converts only about 0.10 % of it. Reaching final velocity would require a conversion of 240 grams of mass into photon energy, which would require an enormous 240,000 kg of nuclear fuel, almost all of which would have to be carried the whole journey. We can assume the full-size reactor itself would contribute significant additional mass. Even with such a high specific impulse, and here the term is estimated because fuel is not consumed in the same way as it is in a chemical reaction, it would take a year to escape Earth’s gravity from a starting position in low orbit and it would then take 80 years to reach its final speed, accelerating at a very anemic rate of 10<sup>-5</sup> g.<br />
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2) Micro-Probe With a Laser-Driven Sail<br />
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We could totally switch up the game by going small and moving away from nuclear energy <a href="https://en.wikipedia.org/wiki/Laser_propulsion">to a highly focused laser</a> for the photon drive. It would be much easier to focus the photon beam, perhaps into a sail to push the spacecraft and it would obviously do away with the need for a nuclear reactor and all that fissionable material. However, a limitation of lasers is that they are much less efficient at converting energy into light than blackbody radiation emitters. The photon energy will also be many scales smaller than any nuclear system, even with the most powerful laser that is currently technically possible.<br />
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Still, it is a <a href="https://cosmosmagazine.com/space/laser-powered-travel-to-nearby-stars-a-step-closer">tantalizing possibility</a>. There is no need to carry any fuel, so the idea here is to go as small as possible. The 2016 <a href="https://en.wikipedia.org/wiki/Breakthrough_Starshot">Breakthrough Starshot</a> privately funded initiative describes a series of ultra-light probes (on the scale of a gram) accelerated toward Alpha Centauri by beaming laser photons into their relatively large (a few meters wide) sails from Earth. The mission plans to send back flyby photographs of Proxima Centauri b.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">An artists conception of the Starshot solar sail deploying near Earth; Kevin Gill; Wikipedia</span></span></td></tr>
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To quickly accelerate a sail deployed in orbit around Earth, a massive laser array would be required and it would have to be perfectly focused in order to strike the small sail, a technology not yet realized. Theoretically, these probes could accelerate very quickly because they are so low in mass. Pushed by a laser pulse lasting just a few minutes, they could top out at around 20% light speed, bringing transit time down to about 20 years. Researchers are hopeful they can overcome the obstacles associated with this plan. The probe would have to be robust to handle such rapid acceleration. The sail material would have to be perfectly reflective to the laser beam. If it absorbed even a tiny fraction of such intense photon energy, it would vaporize. Another problem is getting all the analytic tools required to explore an exoplanet down to nano-technology level. Finally there is a problem inherent with any spaceflight that is fast enough to reach another star system in a reasonable time frame. Interstellar space is full of dust (small molecules and ions) and radiation (gamma rays, etc, from various cosmic sources). These particles, though generally extremely sparse, would generate significant drag on a spacecraft travelling close to light speed, slowing it down. As well, even relatively stationary microscopic particles in space would act like high-energy bombs striking a craft going so fast, and any burst of cosmic radiation could destroy the sail. A 2-minute silent animation created by its founder company, <a href="https://breakthroughinitiatives.org/">Breakthrough Initiatives</a>, shows how the StarShot mission to Alpha Centauri might work.<br />
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3) Ion Propulsion<br />
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An <a href="https://en.wikipedia.org/wiki/Ion_thruster">ion thruster</a> ionizes propellant to produce a beam of ions. The thrust from an ion propulsion engine is almost imperceptible. It is equivalent to the weight of a piece <a href="https://en.wikipedia.org/wiki/Dawn_(spacecraft)#Propulsion_system">of copy machine paper on your hand</a> (about 90 mN), and this means that even a small light spacecraft would take a very, very long time to accelerate to near light speed. On the upside it is extremely efficient, achieving a specific impulse of about 10,000 seconds.<br />
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3a) Electrostatic Ion Thruster<br />
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One way to do this is to use a <a href="https://en.wikipedia.org/wiki/Gridded_ion_thruster">gridded ion thruster</a> design, schematically shown below.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEidaQ89FyWbrrZaVHW1sK9oA7buArqEl77SL9-Ko01PVzjXaVtJhhXSnJNZ7yrwp2P1clv4YsJjpzXa951p_v6xvwVuD2zq8kTlWwJuZp0NBke80IFRdW8k6SASNtiqyi8fMf1mKUaK6Bp-/s1600/2560px-Electrostatic_ion_thruster-en.svg.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1106" data-original-width="1600" height="442" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEidaQ89FyWbrrZaVHW1sK9oA7buArqEl77SL9-Ko01PVzjXaVtJhhXSnJNZ7yrwp2P1clv4YsJjpzXa951p_v6xvwVuD2zq8kTlWwJuZp0NBke80IFRdW8k6SASNtiqyi8fMf1mKUaK6Bp-/s640/2560px-Electrostatic_ion_thruster-en.svg.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Oona Raisanen;Wikipedia</span></span></td></tr>
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Basically, this engine uses an electron gun to direct an electron beam at the propellant material. Xenon is a good candidate for the fuel because it ionizes readily and it has the bonus of having a high atomic mass, 131.293 u (remember Newton’s third law) to maximize (albeit still very low) thrust. It’s an inert gas that is easy to store as well. Electrons bombard neutral xenon atoms in a chamber to produce positive ions as well as more electrons (to produce a <a href="https://www.livescience.com/54652-plasma.html">plasma state</a>). Utilizing the <a href="https://en.wikipedia.org/wiki/Coulomb%27s_law">Coulomb</a> force, a series of highly charged electrical grids direct and accelerate the plasma to a very high-velocity ion beam that propels the craft.<br />
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh50aVS1slnwzLD9h2VU2V3o8z5ittAJov5KT1LgDnTEmer495HO9Z0VTJ0d8dPdbzXnYxBoIUHSs9X0rfPPJslT1lyPjPe03FBPQFlTKgAXaBx5X9QsXrZVdiT4hyphenhyphen1VWfA35eWSLl1Azdt/s1600/2560px-CoulombsLaw.svg.png" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1280" data-original-width="1600" height="256" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh50aVS1slnwzLD9h2VU2V3o8z5ittAJov5KT1LgDnTEmer495HO9Z0VTJ0d8dPdbzXnYxBoIUHSs9X0rfPPJslT1lyPjPe03FBPQFlTKgAXaBx5X9QsXrZVdiT4hyphenhyphen1VWfA35eWSLl1Azdt/s320/2560px-CoulombsLaw.svg.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Dna-Dennis;Wikipedia</span></span></td></tr>
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An <a href="https://en.wikipedia.org/wiki/Electric_field">electric field</a> applies an <a href="https://en.wikipedia.org/wiki/Electrostatics">electrostatic</a> force. The electric fields around like charges repel each other (the two blue positive charges, right) while the fields around two opposite charges (the blue and red charges) supplies an attractive force. This force (denoted F) can be used to accelerate and direct charged ions.<br />
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The exhaust velocity of such an engine system can approach about 160,000 km/h. Still, the thrust of such a system would be small. Even using heavy xenon ions, the ion beam would not be able to overcome ordinary air resistance. Such a craft would have to be launched into orbit, and even once in the near vacuum of interstellar space, acceleration would be vey low. On the upside, this technology is very doable, <a href="https://en.wikipedia.org/wiki/Ion_thruster#Comparisons">based on test data from a variety of ion thruster engine designs</a>.<br />
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In fact, xenon ion thrusters, as part of an <a href="https://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness">NSTAR engine</a> system, were already used very successfully <a href="https://en.wikipedia.org/wiki/Dawn_(spacecraft)#Propulsion_system">to propel the recent </a><a href="https://en.wikipedia.org/wiki/Dawn_(spacecraft)#Propulsion_system">Dawn probe</a> to Vesta and Ceres in the asteroid belt. <a href="https://en.wikipedia.org/wiki/Dawn_(spacecraft)">Arriving at Ceres in 2015</a>, it is currently in an uncontrolled orbit around Ceres, having exhausted all of its fuel as of late 2018.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjV15VOqaTe4q4LrkWPlw9PBv_crKEtH9GZxHF0ntSy42RZea7y-SJ6d9uGRCJ1AeGIUlPonxGaoLB5ehMPCNc0AsQzlzN1NpL3GKbVlyoMZr2EjqIEd6FkjMwhv5gMZhe2H3gU7kPrO6xQ/s1600/pia19598-16.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="576" data-original-width="1024" height="360" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjV15VOqaTe4q4LrkWPlw9PBv_crKEtH9GZxHF0ntSy42RZea7y-SJ6d9uGRCJ1AeGIUlPonxGaoLB5ehMPCNc0AsQzlzN1NpL3GKbVlyoMZr2EjqIEd6FkjMwhv5gMZhe2H3gU7kPrO6xQ/s640/pia19598-16.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Artist’s concept of NASA’s Dawn spacecraft above Ceres; NASA/JPL-Caltech. The blue glow comes from excited ions in the engine outflow.</span></span></td></tr>
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Powered by a 10kW photovoltaic solar array (the two long panel arms, above, powering the thrusters and all the other instruments onboard), and carrying 425 kg of xenon, it was launched in 2007 and its mission was extended until it ran out of fuel. Due to its low thrust, it took four days at full throttle after separation from its launch rocket just to accelerate from zero to 100 km/h.<br />
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3b) Electromagnetic Ion Thruster<br />
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While an electrostatic thruster uses the Coulomb force from an electrical field to accelerate ions, an electromagnetic thruster uses the <a href="https://en.wikipedia.org/wiki/Lorentz_force">Lorentz force</a> in which both electric and magnetic fields exert forces on the ions. An electric field can be produced by a stationary charge, whereas a magnetic field is produced by a moving charge. The electric and magnetic fields, in either of these technologies, are generated using a power source, which can be electric solar panels if used near the Sun. Far from the Sun, another source such as nuclear power could be used instead.<br />
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In an electromagnetic thruster, an ion (+q, shown below left) in motion (vector v) is acted upon by two force fields. The magnetic field q(v x B) exerts a perpendicular force (vector B) while an electric field (q E) exerts a force in the same direction as the field (vector E). The Lorentz force is the force responsible for the circular trace patterns you see after particles are bombarded in accelerators.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiY0N-6_IgKlJXpXScP8spn23J-stOHFrOwHH2KZSphQq1m_Dhe93WnAje7VA-6wOXyPCUfZAdJ8ZaSYmknMtxkZdf4OWFpxQicsDQ3JP7_I4x43MjnBJcRq2awYqyzNBvIdg9gB0SVWS-E/s1600/1920px-Lorentz_force_particle.svg.png" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1600" data-original-width="1508" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiY0N-6_IgKlJXpXScP8spn23J-stOHFrOwHH2KZSphQq1m_Dhe93WnAje7VA-6wOXyPCUfZAdJ8ZaSYmknMtxkZdf4OWFpxQicsDQ3JP7_I4x43MjnBJcRq2awYqyzNBvIdg9gB0SVWS-E/s320/1920px-Lorentz_force_particle.svg.png" width="301" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Maschen;Wikipedia</span></span></td></tr>
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A <a href="https://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster">magnetoplasmadynamic (MPD) thruster</a> is one type of electromagnetic thruster, the other being the <a href="https://en.wikipedia.org/wiki/Pulsed_inductive_thruster">pulsed inductive thruster</a>. I’ll focus on the MPD thruster here.<br />
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Like the electrostatic ion thruster, a gas is ionized. It could be xenon once again but lithium so far has been a better performer in tests on this technology. Ionized gas particles accelerated by an <a href="https://en.wikipedia.org/wiki/Electromagnetic_field">electromagnetic field</a> could in theory generate an extremely high specific impulse, achieve an exhaust velocity about three times higher a xenon ion thruster and produce a respectable thrust of 20 N and possibly even up to 200 N, which is far higher than electrostatic ion propulsion. Like an electrostatic ion thruster, an electric field is generated by a power source. The magnetic field can be externally applied to the particles through magnetic rings around the exhaust chamber. Or, the field can be induced by the ion’s electrical current while the ions are accelerated. In this case, a cathode extends down through the middle of the ion chamber. At lower power levels, the magnetic field must be externally applied because the self-induced field is too weak. A CGI rendering of Princeton University’s lithium self-field MPD thruster is shown below.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgVN1VII7uFWiC7p6NLXjAPx8XhRgwxjSmMDn-DStfR-ogZ61B1nnnioDFQwYjiH0qZnwt3bRSOl6E1e6mNyeDfqG7clcQa-KA238g3WhDIMAroNjHVgxd8DCak9tOmOcuNJvuAN6O12A8I/s1600/mpdthruster-1.jpeg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="243" data-original-width="324" height="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgVN1VII7uFWiC7p6NLXjAPx8XhRgwxjSmMDn-DStfR-ogZ61B1nnnioDFQwYjiH0qZnwt3bRSOl6E1e6mNyeDfqG7clcQa-KA238g3WhDIMAroNjHVgxd8DCak9tOmOcuNJvuAN6O12A8I/s640/mpdthruster-1.jpeg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">NASA;Wikpedia</span></span></td></tr>
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By achieving this thrust in practice, this system, unlike a small light Dawn-type spacecraft, could potentially power a future heavy cargo, or even manned, flight to Mars (or beyond) because it would have enough thrust capacity to perform the kind of quick delta-v maneuver required to lock into orbit around a distant planet and land. Such a propulsion system, if used to travel to Mars, for example, would also have much higher fuel efficiency than any chemical fuel.<br />
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A problem with this propulsion system is that it would require far more power to operate than electrostatic ion propulsion, on the order of hundreds of kilowatts (kW) compared to 1-7 kW. As with all ion thrusters, thrust increases with power input. There are no current spacecraft power systems that can provide hundreds of kilowatts of power. For a manned trip to Mars, a Earth-based laser or even a very large solar panel array might do the job. For a much longer interstellar flight, however, nuclear power would be a reasonable choice. NASA’s <a href="https://en.wikipedia.org/wiki/Project_Prometheus">Project Prometheus</a>’ reactor, dropped in 2005, would have been a small nuclear reactor that could generate electrical energy in this power range to run such an ion engine. (These nuclear power generators should not to be confused with nuclear thermal propulsion, described earlier). This technology would also have been useful for deep space probes in our system that are too far from the Sun to use photovoltaic panels.<br />
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Several countries around the world have experimented with MPD thruster technologies. Russia, then the Soviet Union, flew some experimental prototypes on their spacecraft and more recently Japan successfully <a href="https://en.wikipedia.org/wiki/Space_Flyer_Unit">operated an MPD thruster in space</a> as part of an experiment. Research on this kind of ion thruster is still being carried out today at the <a href="https://alfven.princeton.edu/research/ep">Electric Propulsion and Plasma Dynamics Lab</a> at Princeton University as well as at NASA’s <a href="https://sec353ext.jpl.nasa.gov/ep/">Jet Propulsion Laboratory</a> and <a href="https://www.nasa.gov/centers/glenn/about/fs21grc.html">Glenn Research Center</a>.<br />
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5) Antimatter Propulsion<br />
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An antimatter rocket might sound a bit like science fiction but this technology could be one of the most promising for interstellar travel, provided a few practical matters are sorted out. Antimatter has the highest energy density of any proposed rocket fuel. In fact, <a href="https://www.nasa.gov/sites/default/files/atoms/files/2015_nasa_technology_roadmaps_ta_2_in-space_propulsion_final.pdf">up to 75%</a> of the mass of the matter/antimatter fuel mixture is directly converted into energy. This is an incredibly high figure but it is not 100% as one might at first guess. Only the annihilation of electrons with positrons yields 100% energy. In practice, other particles and their antimatter twins annihilate each other as well, yielding both energy and a variety of yet more particles, representing mass that is not converted to energy. Condensed antihydrogen fuel might someday be feasible. However, at least with current technology it would be <a href="https://www.sciencefocus.com/science/how-can-we-make-antimatter/">extremely difficult to create any sizeable quantity</a> of antihydrogen. First, antiprotons and positrons (anti-electrons) need to be created. Positrons are relatively very easy to make. Physicists at Lawrence Livermore National Laboratory in California use a special laser to irradiate a plate of gold t<a href="http://www.nbcnews.com/id/27998860/ns/technology_and_science-science/t/laser-technique-produces-bevy-antimatter/#.XSjvfC3MyQ4">o create billions of positrons per pulse</a>. <br />
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Antiprotons are not easy to make. A few antiprotons are created for each one million particle collisions in a particle collider. These rare particles are separated out from other particle products using a magnetic field. Then, these very fast antiprotons need to be slowed down using electric and magnetic fields so that they can capture positrons in order to make antihydrogen atoms. Then the atoms need to be trapped in an ultracold magnetic “bottle.” So far, a bottle of a few (less than 100) antihydrogen atoms have been stored for almost 17 minutes before they annhiliated. If CERN used its colliders only to make antimatter, it could <a href="https://angelsanddemons.web.cern.ch/antimatter/making-antimatter">only make 1 billionth of a gram per year</a>! It takes about a billion times more energy to make antimatter than what is stored in its mass. One (likely very generous) estimate puts the price of 1 gram <a href="https://www.ijser.org/researchpaper/Antimatter-Rockets.pdf">at about a trillion dollars US</a>. NASA estimates that a trip to Mars <a href="https://www.nasa.gov/exploration/home/antimatter_spaceship.html">would take around 20 milligrams of antimatter</a>. The xenon or lithium required for ion thruster engines, described above, is quite expensive too but nowhere near this scale. This being said, the tiny amounts of antihydrogen mankind might someday feasibly manufacture would provide one hell of a wallop, on the scale of several billion times more than the most efficient chemical rocket fuel.<br />
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An antimatter rocket could use the products of annihilation for direct propulsion or it could be used indirectly to power a different drive, for example, to heat a fluid such as liquid hydrogen, which would be expelled, and which would potentially supply more thrust.<br />
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The theoretical antimatter <a href="https://en.wikipedia.org/wiki/AIMStar">AIMStar</a> (Antimatter-Initiated Microfusion) rocket, developed in the 1990’s, would use gamma rays created from matter/antimatter annihilation to trigger a fission nuclear reaction in a deuterium/tritium mixture that would in turn <a href="http://large.stanford.edu/courses/2011/ph241/palke1/">start a fusion burn within that mixture</a>. That superheated plasma would be ejected to propel the rocket using a series of fusion pulses. It would have a specific impulse of about 61,000 s and achieve an exhaust velocity of about 1/3 the speed of light. AIMStar propulsion technology might be feasible in a few decades. <a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.577.1826&rep=rep1&type=pdf">Developed by Penn State University</a>, it could be used to visit the distant Oort Cloud, 10,000 A(astronomical units) away. <a href="http://ffden-2.phys.uaf.edu/213.web.stuff/scott%20kircher/microfusion.html">The mission itself </a>would take several decades, accelerating constantly for 22 years to achieve 3/1000th light speed, and require 28.5 micrograms of antimatter, more than we can create with current technology.<br />
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Fusion Propulsion<br />
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I briefly mentioned fusion as part of a theoretical antimatter engine earlier, but I think the fascinating possibility of <a href="https://en.wikipedia.org/wiki/Nuclear_fusion">nuclear fusion</a> as a practical energy source for propulsion deserves a closer look. One reason that <a href="https://en.wikipedia.org/wiki/Fusion_rocket">fusion propulsion</a> theory is attractive is it is fuel-efficient. A typical chemical rocket engine has a specific impulse of about 450 s. A fission-based design, such as <a href="https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)">Orion</a>, would have <a href="http://www2.ee.ic.ac.uk/derek.low08/yr2proj/nuclearpulse.htm">maximum specific impulse of about 10,000 s</a>. By contrast, a fusion rocket would have a specific impulse over ten times higher, <a href="https://science.howstuffworks.com/fusion-propulsion2.htm">at about 130,000 s</a>.<br />
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This option comes with serious challenges, however. One of them is low thrust. The first fission drive concept, Project Orion, demonstrated the potential power of nuclear energy for space travel. It promised high specific impulse AND thrust. Expelled from a series of fission detonations, debris particles, radiation and heavy radioactive nuclei could theoretically achieve an engine output average velocity <a href="https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Theoretical_applications">of around 100,000 km/h</a> and achieve an eventual maximum velocity in interstellar space of an impressive 36 million km/h, just over 3% light speed.. Because the debris would contain lots of large atomic nuclei (lots of mass), it would have high thrust as well. It is one of the few theoretical propulsion systems that could achieve significant thrust and impulse at the same time, a chief advantage of a fission system.<br />
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The bulk of the blast from a fusion reaction, in contrast, is made up of massless gamma rays and (low mass) protons. It’s very low in mass but it’s going very fast (on average close to slight speed), delivering an enormous specific impulse but low thrust. There are two general options for nuclear fusion propulsion. Direct propulsion means that the plasma itself is directed out the back of the craft: low thrust. Indirect propulsion means that fusion energy is used to generate electricity, perhaps to power an MPD ion drive. This is a propulsion system that is very efficient but requires at least a few hundred input watts of electricity. Fusion power could supply that, and the efficient MPD drive could achieve enough thrust to push a heavy cargo rocket or even a manned spaceflight over very long distances at significant velocity.<br />
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Besides low thrust, the biggest challenge of fusion is achieving <a href="https://en.wikipedia.org/wiki/Fusion_ignition">ignition temperature</a>. A fusion rocket will, out of necessity, use <a href="https://www.iter.org/sci/MakingitWork">fusion plasma</a> because that is where a fusion reaction takes place, in extremely hot pressurized plasma (millions of degrees C). In fact, in nature one must look into the interior of a star to find matter in this extreme state. In order to get plasma energetic enough to initiate a fusion reaction, it must be confined under extreme pressure – pressure equivalent to the interior of a star as massive as our Sun – and kept stable. The problems with fusion, in a nutshell, are how to heat gas into a fusion state, how to confine it and how to keep it stable. You must keep nuclei energetic and in close enough proximity to fuse together in an ongoing stable reaction.<br />
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Nuclear plasma cannot be confined physically. It is far too hot and under too much pressure for any material in contact to withstand, even momentarily. However, there are other ingenious ways to tame it, and these methods come with the added advantage that they can be very lightweight and compact. Weight is an overarching concern with long-term spaceflight. Everything is designed to minimize the mass that must be hauled across space, so that energy expenditure can be minimized. Fuels can be heavy, adding their own load, and their supply is limited. The main advantage of a fusion propulsion system is that it can be very efficient, requiring a smaller and much lighter fuel supply than a fission system. The fuel would consist of hydrogen isotopes such as deuterium or tritium rather than a heavy fissile isotope such as uranium-235 or plutonium-239. In addition to this advantage, more fuel mass would be turned into energy in a fusion drive than in a fission drive. In a fusion drive, 676 units of energy would be converted from 1 kg fusion fuel. A fission reaction <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin.html#c3">would yield 176 units of energy/kilogram of fuel</a>. See also that hyperphysics website link to see how these values are calculated. Furthermore, because fusion produces less radiation than fission does, less, necessarily heavy, shielding would be required to protect sensitive hardware. A fusion drive will still require a reactor, but it could be built much lighter and smaller than a fission reactor if technologies such as <a href="https://en.wikipedia.org/wiki/Magnetic_confinement_fusion">magnetic</a> or <a href="https://en.wikipedia.org/wiki/Inertial_confinement_fusion">inertial</a> plasma confinement are used. Not only can these methods heat the plasma but they also keep it under pressure.<br />
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Magnetic confinement works by inducing a circular electrical current in plasma. The current creates a magnetic field that squeezes the plasma into a thin ring. This results into two kinds of heating: Joule heating and adiabatic heating. <a href="https://en.wikipedia.org/wiki/Joule_heating">Joule heating</a> is microscopic friction caused when a current passes through a conductor. During <a href="https://en.wikipedia.org/wiki/Adiabatic_process#Adiabatic_heating_and_cooling">adiabatic heating</a>, the thermal energy (temperature) of a gas or plasma increases as it is compressed, according to the <a href="https://en.wikipedia.org/wiki/First_law_of_thermodynamics">first law of thermodynamics</a>. A technology that currently uses magnetic fusion confinement is the <a href="https://en.wikipedia.org/wiki/Tokamak">tokamak</a>, a Russian-designed device that confines and stabilizes plasma by winding magnetic field lines in a helix around a toroid (doughnut) shape. Several tokamaks are now working around the globe. To make such a device feasible as a generator, it must produce more energy than is required to maintain the fuel in a fusion state, a tall order. The first fusion tokamak-type full-size reactor, called <a href="https://www.iter.org/">ITER</a>, just achieves this and is currently under construction in France. The problem with current tokamak technology is that the toroidal reactor is very heavy, severely reducing a potential craft’s weight/thrust ratio.<br />
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A different and more feasible approach for a fusion drive would be to go small and introduce a different confinement method. One could focus intense energy onto a very small target, using a powerful laser to heat and compress a tiny fuel pellet of the hydrogen isotopes tritium and deuterium until the pellet ignites a fusion reaction. The fuel is relatively cheap and plentiful, not just on Earth but elsewhere in the universe as well, and the system could be highly efficient, perhaps used for a power source for a secondary thrust technology. An intense focused blast of laser photons heats the outer layer of the fuel pellet exploding it outward in all directions, producing an equal reaction force inward in the form of shockwaves. The shockwaves compress and heat the interior of the pellet enough to ignite fusion. This is an example of Newton’s third law in action.<br />
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This type of fusion requires a very powerful and focused laser in the vicinity of megajoules (MJ), which is doable. The <a href="https://en.wikipedia.org/wiki/National_Ignition_Facility">National Ignition Facility</a> (NIF), which researches inertial confinement fusion, has such a laser in use. This system also requires a perfect sphere of fuel in order to create a shockwave that is properly focused inward. A way around this issue has been to surround the fuel in a tiny metal cylinder. The laser focuses on the inner surface of the metal, heating it into plasma, which then radiates X-rays. The thermal energy from the X-rays is absorbed by the fuel sphere (more efficiently than by the photons). It then perfectly implodes into fusion plasma.<br />
<br />
To directly produce an impulse from this reaction, a magnetic field could confine and direct the intense blast energy of the fusion explosion. The field could also be set up to form a pusher plate. A tremendous amount of energy, in the form of gamma rays, released from a series of fusion pulses could push against a pusher plate, in this case a magnetic field, in order to translate force into acceleration. The plate could also be designed to absorb the shock of the individual pulses to allow for smooth acceleration of the craft. The drawback of this system, as mentioned, is its very low thrust.<br />
<br />
Relativistic Travel And Einstein<br />
<br />
It would seem that once we work out extremely efficient and continuous fuel consumption, our future interstellar spacecraft, perhaps even manned, should be able to accelerate across the light years of space, perhaps at a comfortable rate of 1 g, the same as Earth’s gravity, to eventually reach light speed. After all, can’t we simply use the formula f = ma? Newton’s second law states that force equals mass times acceleration. Imagine the scenario. We can use the rocket equation to refine our calculations. The rocket equation deals with the fact that we must accelerate both ship and fuel mass and the fuel mass is decreasing as we go. Then imagine that we can reverse the engine thrust and decelerate at 1 g, over several years, to reach planet X. We can assume there is no friction in this space vacuum to work against us. We will assume there are no particles in interstellar space (which is not the case and would cause a serious problem at relativistic velocity). Here, then, is our system in effect: the propellant removes momentum in one direction so the ship can gain momentum in the other direction.<br />
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The problem is in the assumption of f = ma. This <a href="https://en.wikipedia.org/wiki/Classical_mechanics">Newtonian equation</a> assumes that the relationship between mass and acceleration always holds up. And it does, but only until we get to relativistic velocities, a scenario Newton, I suspect, never thought of. At these velocities, the relationship starts to break apart. We must translate our motion equations from <a href="https://en.wikipedia.org/wiki/Galilean_transformation">Galilean transformations</a> into <a href="https://en.wikipedia.org/wiki/Lorentz_transformation">Lorentz transformations</a>. This sounds complicated, but it means that at relativistic velocities, space-time itself starts to makes itself known within our accelerating spaceship scenario. For Newton, space and time are invariant. You can always count on them to be the same. A ruler is always one foot long and time always flows at the same rate. A second lasts a second. For Einstein, <a href="https://en.wikipedia.org/wiki/Special_relativity">space and time are no longer invariant</a> and we can no longer think of them as separate from each other either. Now they must be treated as part of a 4-dimensional single continuum, <a href="https://en.wikipedia.org/wiki/Space-time">space-time</a>. Two things to consider: space-time can stretch and warp and, second, we now must think of any situation in terms of <a href="https://en.wikipedia.org/wiki/Frame_of_reference">frame of reference</a>. The ONLY constant that doesn’t change with frame of reference is the speed of light.<br />
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As our ship revs up to, say 30% light speed, we would begin to notice changes. <a href="https://en.wikipedia.org/wiki/Time_dilation">Time dilates</a> – clocks on the ship slow down compared to those on Earth. Conversely, measured from the ship, Earth’s clocks speed up. <a href="https://en.wikipedia.org/wiki/Length_contraction">Length contracts</a> in the direction of motion. This means that even though length contracts, from Earth the ship would look fine. From a dock in which the ship whizzed past, however, you would see the ship look shorter or squished. If the ship could travel exactly at light speed, it would have no length at all in this reference point. <a href="https://en.wikipedia.org/wiki/Mass_in_special_relativity">Relative mass increases</a>. The ship, as measured from Earth, would be growing more massive.<br />
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What does all this mean for Newtonian dynamics? At lower velocities, Newtonian physics holds up. As velocity becomes relativistic, the ship’s momentum/velocity/acceleration begins to lose what looked like a linear relationship to one another. This happens, according to Einstein, because the time coordinates between the ship and the observer (such as us on Earth) are no longer the same. Time, distance, and mass are now relative, that is, their values depend on your frame of reference. Only the speed of light and the basic laws of physics remain constant from every reference point in the universe.<br />
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This isn’t intuitive. The ship’s changes we would measure from a different reference point such as Earth - its mass, its physical dimensions and its time dilation - are not optical illusions. They are real valid measurements. Yet, their values depend entirely upon where we measure them. So, if you were inside the ship, your clock would be running normally, your mass would be normal and the ship would be of the same dimensions as always. Looking out the front window, however, you would notice that your frame of reference is now drastically different from the relatively stationary space around you. The stars whizzing past the ship would begin to bunch up closer, the distances between them shrinking, while the starlight reaching the front window of the ship shifts to blue (<a href="https://en.wikipedia.org/wiki/Doppler_effect">Doppler effect</a>). In your reference, they are approaching relativistic velocity toward you.<br />
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The spaceship can never exceed light speed. Witnessed from Earth, the reason is clear. As the ship approaches light speed, more and more of the force of 1 g acceleration is converted into RELATIVISTIC mass rather than velocity, until, as the theory goes, the relativistic mass approaches infinity, an impossibility, at light speed. This doesn’t mean that, at any and all reference points, your ship is now so massive it would take infinite force to continue to accelerate it (although that’s what we would correctly observe from Earth). At the same time, all processes on the ship would appear from Earth to slow down to a stop. Frozen there, there would be no way to advance further.<br />
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Time, mass and distance in space-time change as your reference changes. Inside the ship, now approaching light speed, you would see your star field bunching up, turning blue and then falling entirely beyond your visual range into an almost perfectly black invisible surface in front of your window. Unfortunately you would soon need to install a powerful blast shield because the photons of starlight will eventually transform into X-rays and then deadly gamma rays as the Doppler shift continues to shorten the wavelength of this electromagnetic radiation. Is there a limit to the Doppler shift of a photon? The wavelength of a photon should theoretically approach zero as the ship approaches light speed but there is no theoretical description that makes sense for a shorter-than-zero photon wavelength. The photon energy would approach infinity. The speed of the photons, however, will always be the same, because light speed is invariant.<br />
<br />
Time would seem to speed up for all the processes going in space in front of your ship, say an exploding supernova somewhere. At even 90% light speed, you might still be able to perceive the explosion by instruments. Once your ship passes the supernova you might see it – a Doppler-shifted slow motion explosion – in you rearview mirror. You will have to trust Einstein to continue accelerating to a final velocity of, say 99% light speed, and then decelerate when you approach your destination. As you do so, the star field will deepen, come back into view, as the reel of time once again becomes your time. Remember, everything inside your ship is normal for the whole journey. It’s the objects and their motions outside it, moving near light speed relative to your ship that will seem weird.<br />
<br />
A tantalizing feature of near light-speed travel comes from time dilation. As time relatively slows down inside the spacecraft, including for any occupants, time relatively speeds up outside of it, including on Earth. A trip to a star system 100 light years away could be shortened into, perhaps a few years FOR the occupants. This seems to break the rules of faster-than-light travel, but in the reference frame of the ship and its occupants, space contracts and stars appear much closer to each other. What, for Earth would be at least a 100-year trip, would be experienced as much shorter by the occupants because their clocks are running very slowly compared to the universe (and normally for them). To them, the distance to the star system would be just a few light years away, perhaps less, depending on how close to light speed the ship traveled and how long it would take to accelerate and decelerate. Consider that if a ship could travel at exactly light speed (which it cannot), the universe would be completely length contracted (there would be no distance between stars or galaxies at all in the direction that it is moving) and time would not exist, as time in the universe surrounding the ship would run infinitely fast, an infinite nonsense answer to the equation that describes relativistic motion. To a photon, then, <a href="https://phys.org/news/2014-05-does-light-experience-time.html">there is no time or distance between</a> its emission and its eventual absorption. That is its universe, a mind-bending thought for the next time you are in the shower. Sadly for any ultrafast interstellar travellers, Earth will have run like a movie on extreme fast forward, a minimum of over 200 years would have passed by the time the travellers returned, a few years later.<br />
<br />
Unfortunately, this journey is unfeasible. It is impossibly perilous. If your ship strikes even a cold gas cloud in space, it will blast into smithereens. Every gas atom will be a significantly massive relativistic missile striking your ship hull. Unless the ship has a powerful magnetic field to deflect particles away perhaps . . .Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-51798478808129534162019-02-10T09:38:00.001-07:002019-02-10T09:43:33.591-07:00Relaxation Yoga and the Autonomic Nervous System: Maximizing Training SuccessOne of the best functions of yoga is its effect on our central nervous system. Learning how to relax, which is the focus of <a href="https://en.wikipedia.org/wiki/Yoga_for_therapeutic_purposes">relaxation yoga</a> in particular, helps our nervous system to return to its resting state.<br />
<br />
The <a href="https://www.medicalnewstoday.com/articles/307076.php">central nervous system</a> (CNS; which consists of the brain and spinal cord) is <a href="https://en.wikipedia.org/wiki/Autonomic_nervous_system">autonomic</a>. This means that it operates outside of our conscious control. However, our conscious input (thoughts and emotions) has a significant influence on it. And vice versa: the state of our CNS affects how we feel and how we think. This is one of the keys to how relaxation yoga works.<br />
<br />
Our CNS is <a href="https://faculty.washington.edu/chudler/auto.html">regulated by two complementary systems</a> - the parasympathetic (PNS) and the sympathetic (SNS) systems. Both systems are always active and when they are in balance they keep our body functions in a state of balance. The sympathetic system is easy to remember: "flight or fight." We evolved an emergency system to help us survive a sudden perceived threat, like a big dog growling at us.<br />
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The hippocampus in the brain rings the alarm bell, sending off a variety of nervous impulses and a cascade of <a href="https://qbi.uq.edu.au/brain/brain-physiology/what-are-neurotransmitters">neurotransmitters</a> and <a href="https://www.hormone.org/hormones-and-health/hormones/hormones-and-what-do-they-do">hormones</a>. This warning blast immediately diverts our body resources to deal with an emergency. It stimulates the <a href="http://www.yourhormones.info/glands/adrenal-glands/">adrenal glands</a> above our kidneys to release <a href="http://www.yourhormones.info/hormones/adrenaline/">adrenaline</a> and <a href="http://www.yourhormones.info/hormones/cortisol/">cortisol</a> into our blood.<br />
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Our breathing rate increases, our blood pressure goes up, the bronchioles in our lungs dilate, our liver releases glucose into our bloodstream, our muscles tense to protect themselves from potential injury, we start sweating and our pupils dilate to help us see better.<br />
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Meanwhile, peristalsis in our intestines decreases, our mouths go dry, and a wide variety of restorative processes in our body temporarily take a back seat, such as cell division, cellular waste removal and immune processes.<br />
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The parasympathetic system is our "rest and digest" or "feed and breed" system. When our body is at rest, all of our restorative functions kick into gear. Regular day-to-day cellular functions resume. We eat, sleep, recover from physical activity, we fight infections and disease, and, yes, perhaps we enjoy a little romp when the opportunity arises.<br />
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At the cellular level, our cells are busy performing their assigned metabolic jobs. They grow and divide, repair DNA, remove waste products, break down and remove dead cells, build tissues and so on. Our immune system is at work repairing injuries and patrolling our blood stream for invaders. Our neurons repair any damage and build new connections. We make hormones, digest food, and feed our cells.<br />
<br />
When the SNS and PNS are in balance, our bodies are incredible machines! When they are not, we have a problem. Most often this comes in the form of a chronically activated sympathetic nervous system (SNS). This imbalance often <a href="https://www.mayoclinic.org/healthy-lifestyle/stress-management/in-depth/stress/art-20046037">results from long-term stress</a>. Some knowledge about how our CNS works and having some relaxation skills under our belts can give us a significant edge during training.<br />
<br />
First, let's map out how our CNS ebbs and flows over a typical day, and it does so significantly. In deep sleep, around 2 to 4 am, our SNS is at its most inactive level. Our brains are busy making new connections and removing waste molecules. Our skeletal muscles are disengaged from our brains, and while we are essentially paralyzed, our muscle cells are repairing all the cellular damage and micro-tears that occurred during the previous day's workout. Excess lactic acid is being removed and glucose is being stored up. The cells are busily dividing, creating new muscle tissue. Our bones are busy making new bone cells. About an hour before we wake up, our SNS starts to rev up a little bit, increasing our breathing rate, alerting our brain to wake us up. On the way to the gym, our mind signals to our body that we are going to work out. Like <a href="https://www.simplypsychology.org/pavlov.html">Pavlov's dog</a>, our heart rate increases in anticipation. Once there, we get our sweat on. Early morning is a great time to work out because it fits in beautifully with our natural <a href="https://www.sleepfoundation.org/articles/what-circadian-rhythm">circadian rhythm</a>. Our SNS activity naturally peaks early in the morning. After workout, we have a nutritious breakfast. The process of eating as well as food in our stomach triggers our PNS. Our body sequesters those adrenaline-type hormones from our blood stream and we return to a resting state. Having seen our friends, having a laugh or two and having accomplished something are additional emotional signals to our bodies to relax and rest. By around bedtime, after a good supper, our bodies should be naturally starting to gear down into sleepiness as SNS activity further quiets down.<br />
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When it all works smoothly, this is a fantastic long-term plan that keeps our bodies (and minds) in balance, or homeostasis. It helps us build healthy tissue and fight disease. It has even been shown to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3370421/">keep our cellular telomeres longer</a>. This is technical jargon that means our individual cells act like they are younger and they can divide more times before they die. I think you can see where I'm going here. This is a recipe for optimal training.<br />
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Homeostasis gets bumped off kilter when we experience a big life stress, but there is not much we can do but allow ourselves heal. The more insidious and more dangerous situation for our health is chronic low-level stress. This is the kind of stress we may not even know we have. It can build up from our day-to-day work environment, money stress, family life, a too-long honey-do list, a too busy schedule, and even just an attitude that we must attack our entire day in full-on warrior mode. It is all too easy to forget that we need comfort and rest, as well, EVERY DAY. Modern life all by itself, I think, is enough to get us out of whack.<br />
<br />
And when we're out of whack, our adrenaline and cortisol levels don't drop down to rest level during the day after our workout. Our bodies handle short-term elevations, such as during a workout or a temporary upset, very well. We evolved for that. However, our bodies don't handle long-term elevated stress levels well at all. A chronically stimulated SNS wrecks havoc on our bodily systems. Our muscles don't have a chance to repair and rebuild. We don't digest our food well and we don't get the nutrients we need to restore our bodies. Our immune system doesn't function optimally, leaving us easier prey to <a href="https://www.acsh.org/news/2016/01/23/reduce-stress-if-you-want-to-reduce-flu-risk">flu</a> and <a href="https://globalnews.ca/news/4596541/stress-common-cold/">colds</a> and possibly even at least some kinds <a href="https://www.cancer.gov/about-cancer/coping/feelings/stress-fact-sheet">of cancer</a>, although that scientific link is weak. Our blood has too much glucose in it, which <a href="https://insulinnation.com/treatment/how-stress-hormones-raise-blood-sugar/">can lead to diabetes</a>. Our blood pressure stays high and does its <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2633295/">own damage to our arteries and heart</a>. We don't sleep well, and this means our brain doesn't have a chance to restore and repair. The resting tone in our muscles is high throughout the day, and that puts extra stress on connective tissues such as ligaments and tendons. This is a recipe for poor training performance, and ultimately it can lead to over-training and a reduction in performance. Often when we notice a lack of progress, it's natural to stress out about that and train even harder, leading to a downward spiral. Everything we work at so hard in the gym is undone by our out-of-balance CNS. When we find ourselves in this frustrating situation, it might be good practise to first take a metal check on where we are at.<br />
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It is very difficult to know when our body is under chronic stress. We don't directly "feel" an overactive SNS. What we do feel are symptoms and we can watch out for them: headaches, racing and/or circular thoughts, feelings of anxiety, restlessness, feeling overwhelmed, irritability, trouble getting to sleep, trouble staying asleep, a tight-feeling chest, feeling rigid, drinking too much after work are all signals to reduce stress. We can't will it down, in the same way that we can't will ourselves to sleep when we've got insomnia. However, ust knowing how our system works and how important rest is puts us on the right track. When you find yourself thinking you don't have time for that relaxing . . . pause for a second thought. Keep life simple as you can while you're in serious training mode, rest often and deeply, eat well.<br />
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And now back to the bullet point in my title: Relaxation yoga is one ingredient can <a href="https://www.ptonthenet.com/articles/activate-the-parasympathetic-nervous-system-to-improve-recovery-3910">rebalance our CNS</a>. In these kinds of classes we learn to use our minds and our bodies to slow our breathing rate down. Different kinds of yoga also use various poses and movements to facilitate relaxation. A slowing of breath and a deepening of breath sends an unconscious signal to our CNS to calm itself and shift toward the PNS side of the spectrum. There are lots of relaxing yoga routines available to experiment with. <a href="https://www.medicinenet.com/stress_meditation_may_reduce_stress/views.htm">Meditation</a> is another practise that allows our nervous system to find its way back into balance. These ancient practises are enjoyable to learn and they are generally safe.Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-69135407718106181922018-11-11T11:23:00.000-07:002018-11-11T11:23:07.970-07:00BioluminescenceAbout 150 million years ago, some living creatures started evolving ways to make their own light, and this is what bioluminescence is: The ability of an organism to create and emit light. Having your own "flashlight" happens to be very useful. Depending on the creature, it is used for mimicry, for camouflage, to warn away predators, for communication among one's species and to find mate.<br />
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Just as bioluminescence has a multitude of functions, it appears to have evolved in a multitude of different ways. In bioluminescent bacteria, the bioluminescent machinery appears to be borrowed from its machinery for <a href="https://en.wikipedia.org/wiki/Cellular_respiration#Aerobic_respiration">cellular respiration</a>. In marine organisms, it seems to have evolved from once-essential cellular detoxification machinery. In at least one dinoflagellate species, the photosynthesis mechanism has been fine-tuned toward light production. In the firefly, molecules that once broke down fatty acids for energy storage now emit that energy as light instead. In this article we will explore the mystery of how bioluminescence evolved and how it offers important life advantages of non-glowing relatives.<br />
<br />
What Is Bioluminescence (And What Isn't)?<br />
<br />
Although bioluminescence is quite rare if you measure it as a percentage of the total number of all of Earth's species, it is quite common among marine species and it is surprisingly diverse among different distantly related groups of organisms, including land organisms. Most bioluminescent species (about 70%) are marine and these include ocean-living bacteria, dinoflagellates (these are the tiny creatures that make disturbed ocean water sparkle with tiny green-blue lights), marine crustaceans such as some krill and shrimp, some echinoderms such as sea stars, sea lilies and sea cucumbers, as well as sea squirts, and a great many species of fish, and even a few sharks. Bioluminescence has also found its expression on land but it is not as common. Land species include some bacteria and fungi as well as some insects, annelids (worms) and arachnids (spiders, scorpions, ticks and mites etc.). Groups that do not contain any bioluminescent species are the land vertebrate classes such as amphibians, reptiles, birds and mammals to which we belong.<br />
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There are, however, several notable <a href="https://en.wikipedia.org/wiki/Fluorescence"><i>fluorescent</i></a> land animals, such as the polka dot tree frog, a reddish yellow frog that glows bright green under ultraviolet light. Bio<i>fluorescence</i> is also seen in many fish and corals, in jellyfish, butterflies, parrots, spiders and even in the flowers of the common four o'clock plant (Mirabilis jalapa). Unlike bioluminescent species, which make their own light, these organisms never glow in complete darkness. They absorb light and then emit it immediately once it is absorbed, usually at a longer (lower energy) wavelength. It is important to distinguish between a fluorescent species and a bioluminescent species. They are easily mixed up.<br />
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NOT BIOLUMINESCENT:<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEia-DKFy-A6BLHbsHT3sv2wAlMWXHR4-Ls-oink2JYbiLcEg_l0EKie3iRSiGql7O2VH9FblsdJtqwyBvDs6_XiNd-SKc3ZTj1G3IsT5B45CbFtgmf3tgKKxIZ60IXZSjKy8bTHWsy0G542/s1600/Sorpion_Under_Blacklight_edit.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="768" data-original-width="1024" height="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEia-DKFy-A6BLHbsHT3sv2wAlMWXHR4-Ls-oink2JYbiLcEg_l0EKie3iRSiGql7O2VH9FblsdJtqwyBvDs6_XiNd-SKc3ZTj1G3IsT5B45CbFtgmf3tgKKxIZ60IXZSjKy8bTHWsy0G542/s640/Sorpion_Under_Blacklight_edit.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Jonbeebe;Wikipedia</td></tr>
</tbody></table>
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All scorpions such as this (actually black) one fluoresce bright aqua blue under an ultraviolet lamp. They glow more faintly in nature, in the dim ultraviolet light reflected by the moon. Their exquisitely sensitive eyes also happen to see this particular colour best.<br />
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Biologist Edie Widder offers us a taste of bioluminescence across the marine world with some incredible footage of bioluminescent marine creatures in this 13-minute video, part of her TED talk in 2013:<br />
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<iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/lKeDBpkrDUA" width="560"></iframe>
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<br />
BIOLUMINESCENT:<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjYr0bkbM3cy-944gEua1qbxGHvNpEQ2LyItkFBbIxstOoFrY9jxDUEu2gvLdiZIEaR63q5ixEq-Rzpb2UkYKpemr6EO9cscDqmLmqGYPIWFvFeLc_WHyH2ffxpoSnULImk4IH0uKe9PAjf/s1600/Photinus_pyralis_Firefly_glowing.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="703" data-original-width="765" height="588" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjYr0bkbM3cy-944gEua1qbxGHvNpEQ2LyItkFBbIxstOoFrY9jxDUEu2gvLdiZIEaR63q5ixEq-Rzpb2UkYKpemr6EO9cscDqmLmqGYPIWFvFeLc_WHyH2ffxpoSnULImk4IH0uKe9PAjf/s640/Photinus_pyralis_Firefly_glowing.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">art farmer:Wikipedia</td></tr>
</tbody></table>
<br />
Common Eastern Firefly (Photinus pyralis)<br />
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This firefly species is common in North America. At twilight, males use flashes of greenish-yellow light to attract females, who will respond with an answering flash of their own.<br />
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BIOLUMINESCENT:<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgaG1KaixiWWeM0h4IZ73Ra75-cXnAzYvT9OsAe9NLyfI_SGclFBBSLBcCnpWCOGKz73oBRti9XSD5EFLgsQ1XG_xuxxwyIIKIqEPH0CIuuvUB7AJ6gdbuuPJWfCny_A6SRJHEjACOkqTzC/s1600/Lampyris_noctiluca.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="492" data-original-width="408" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgaG1KaixiWWeM0h4IZ73Ra75-cXnAzYvT9OsAe9NLyfI_SGclFBBSLBcCnpWCOGKz73oBRti9XSD5EFLgsQ1XG_xuxxwyIIKIqEPH0CIuuvUB7AJ6gdbuuPJWfCny_A6SRJHEjACOkqTzC/s640/Lampyris_noctiluca.jpg" width="530" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Wofl-commonswiki; Wikipedia</td></tr>
</tbody></table>
<br />
Female glowworm (Lampyris noctiluca)<br />
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This glowworm is actually a beetle because it has a hard shell or carapace. In this case it is the much larger wingless female rather than the male that uses light to attract males. The brighter her glow, the more fertile she is.<br />
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In addition to these well-known land examples, bioluminescence has found widespread use among marine species that utilize it in ocean water. They use it near the water surface on dark moonless nights or very deep in the ocean, where sunlight can't penetrate.<br />
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NOT BIOLUMINESCENT (trick example):<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhxEYZ_S5es5txPiPtKjmHsz0JsIA6bU4-UQys44-7qWvyIbA0e3f1RAnGgUZW2fofjXErhBcNa6rg311BvCtM5pJcwvlx4s5zkzOi6KywqtjCxH1RcAQWzS-r7FxVrFKY3920do9NBp_fJ/s1600/Aequorea3.jpeg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1264" data-original-width="1544" height="522" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhxEYZ_S5es5txPiPtKjmHsz0JsIA6bU4-UQys44-7qWvyIbA0e3f1RAnGgUZW2fofjXErhBcNa6rg311BvCtM5pJcwvlx4s5zkzOi6KywqtjCxH1RcAQWzS-r7FxVrFKY3920do9NBp_fJ/s640/Aequorea3.jpeg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Sierra Blakely;Wikipedia</td></tr>
</tbody></table>
<br />
Crystal Jelly (Aequorea victoria)<br />
<br />
This jellyfish is commonly found floating and swimming off the west coast of Canada and the northern United States, especially in Puget Sound. This photograph shows you what the colourless animal looks like but you are actually seeing light reflected from the camera, a common misconception in such photographs found online. <a href="https://www.conncoll.edu/ccacad/zimmer/GFP-ww/shimomura.html">Its bioluminescence</a> is only visible in its outer ring, as a faint blue-green glow (which is emitted only when it is disturbed), shown below.<br />
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEikD_P8fEkHOfCvLsjF4jBvP_8Dm0KrucaG_BiMSBG9UY9n0RcwQ4GXCjevYE9zOvEcPjK_CXObqiKF_fN-QB0Lw6VemNPXQs9sR4rwcH2ZEd1bL7lr59eruH45RQPvSovbS25HXqysjatA/s1600/ring.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="152" data-original-width="166" height="366" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEikD_P8fEkHOfCvLsjF4jBvP_8Dm0KrucaG_BiMSBG9UY9n0RcwQ4GXCjevYE9zOvEcPjK_CXObqiKF_fN-QB0Lw6VemNPXQs9sR4rwcH2ZEd1bL7lr59eruH45RQPvSovbS25HXqysjatA/s400/ring.jpg" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Photo taken by Osamu Shimomura</td></tr>
</tbody></table>
<br />
<a href="https://en.wikipedia.org/wiki/Osamu_Shimomura">Dr. Shimomura</a>, a famous organic chemist and marine biologist, isolated two luminescent proteins from this species of jellyfish. One of them is a bioluminescent protein called <a href="https://en.wikipedia.org/wiki/Aequorin">aequorin</a>. It emits blue light. He also found trace amounts of another protein, and this one is fluorescent rather than bioluminescent. It's called <a href="https://en.wikipedia.org/wiki/Green_fluorescent_protein">green fluorescent protein (GFP)</a>. This protein absorbs the blue light emitted from the aequorin complex and re-emits it as green light. The aequorin complex mentioned here is a substrate/enzyme complex - more specifically a luciferin/luciferase complex. We will get better acquainted with these two evocative-sounding words later on. GFP is now widely used in biological and medical research. It is this discovery for which Dr. Shimomura received the Nobel Prize in Chemistry in 2008.<br />
<br />
Exactly what does this jellyfish uses bioluminescence for? That <a href="https://faculty.washington.edu/cemills/Aequorea.html">remains a mystery</a>. Perhaps it's a warning to potential predators. Researchers know that individuals do not flash at each other and they do not glow continuously. They can be stimulated to glow when they are disturbed but it is rarely observed in undisturbed individuals.<br />
<br />
Marine bioluminescence is very prevalent among those species that live in very deep pitch-black ocean water were no sunlight penetrates, a kilometre or more beneath the ocean surface. One could imagine that these animals would have lost their sight over time, like blind cave-dwelling salamanders did. Many of them have, relying on pressure changes and smell instead. However, some species evolved extreme light sensitivity to the faint bioluminescent light shows they encounter. Down here, water is cold, pitch-black and under tremendous pressure. Here in this inhospitable environment, a myriad of organisms communicate and navigate through a wide variety of beautiful and ephemeral displays of coloured lights.<br />
<br />
Some interesting examples include the 246 species of <a href="https://en.wikipedia.org/wiki/Lanternfish">lanternfish</a> (members of the Myctophidae family), which live deep in the all of the world's oceans, making up an astonishing 600 million metric tonnes of biomass. This is <a href="https://worldoceanreview.com/en/wor-2/fisheries/state-of-fisheries-worldwide/">about 10 times the world's yearly catch of fish</a>! These abundant but small (most are just 6 cm long) deep-sea fish play an essential ecological role as food for larger fish. Myctophum punctatum, pictured below, lives a vertically mobile life. It rises over a kilometre to reach waters near the ocean surface every sundown, following the also vertical daily migration of zooplankton, its food source.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhOq5ETDRZ_OpGKqRozvDiLu92f8or34ehHpseY_AGkaowy4kRpcYipKJ-MoM29Rcx3DVoh5_k86NSMlySm9xRxzmzvJ8j0RZCaeve3o7C9gJkUuye0CNtoEeNc3VOhBMXsVCY3eSYiT_hE/s1600/Messina_Straits_Myctophum_punctatum.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="508" data-original-width="1026" height="316" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhOq5ETDRZ_OpGKqRozvDiLu92f8or34ehHpseY_AGkaowy4kRpcYipKJ-MoM29Rcx3DVoh5_k86NSMlySm9xRxzmzvJ8j0RZCaeve3o7C9gJkUuye0CNtoEeNc3VOhBMXsVCY3eSYiT_hE/s640/Messina_Straits_Myctophum_punctatum.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Edd48;Wikipedia</td></tr>
</tbody></table>
Lanternfish (Myctophum punctatum)<br />
<br />
Although not readily visible in the photograph above, most of these species luminesce through <a href="https://en.wikipedia.org/wiki/Photophore">photophores</a> (light-producing organs) arranged in rows along the belly (they are not the reflective blue upper dorsal spots you see above). Photos of its bioluminescence are rare but the digital model of this species shown below offers an idea of how these bioluminescent photophores are arranged.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhigdqgSP7p7kC6rVrBwqrlf7r3__qmfsqTUXMRNsRr7T93MFJFtt2J8lHyuwYXbRy7Rkqkm0KDybu4uvdBQA-I4q6fz5Uamxx3Y9lyCjTOzDeQHkwlEAHVjZSV1kJC8gn7foY3SVC_3xOV/s1600/LanternFish.jpgc381fb6d-26d6-4f70-9b69-96a8112018bbOriginal.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="800" data-original-width="1422" height="360" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhigdqgSP7p7kC6rVrBwqrlf7r3__qmfsqTUXMRNsRr7T93MFJFtt2J8lHyuwYXbRy7Rkqkm0KDybu4uvdBQA-I4q6fz5Uamxx3Y9lyCjTOzDeQHkwlEAHVjZSV1kJC8gn7foY3SVC_3xOV/s640/LanternFish.jpgc381fb6d-26d6-4f70-9b69-96a8112018bbOriginal.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">JBD5;Turbosquid</td></tr>
</tbody></table>
<br />
3D Digital Model of Myctophum punctatum<br />
<br />
Myctophum punctatum uses its bioluminescence as a special type of camouflage called <a href="https://en.wikipedia.org/wiki/Counter-illumination">counter-illumination</a>. The fish regulates the brightness of the bluish light emitted by its photophores to match the blue wavelengths of the faint sunlight light streaming from above. This masks its silhouette from predators swimming underneath it. Some species also emit green or yellow light, which might be used for communication or courtship.<br />
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Just as fascinating are the <a href="https://en.wikipedia.org/wiki/Anglerfish">anglerfish</a>, an order of fish (Lophoformis) comprised of more than 200 species. All anglerfish are carnivores that use bioluminescence as a type of fishing lure. Representatives of these macabre-looking fish are shown below.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiM4J49bpy6BBMbMdIYnyukbl50aGSjJ57xO2bXbcT-34j9EQco5bRUnN5ucYGNyO2CgH82iW0N9qGVEBHR9nVgq-7ONuoJKH0zqpajB5uU1dzZjHY3174DZhCJ6jIZTkq6DL6UJ2QyUAFC/s1600/Representatives_of_ceratioid_families.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1600" data-original-width="1036" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiM4J49bpy6BBMbMdIYnyukbl50aGSjJ57xO2bXbcT-34j9EQco5bRUnN5ucYGNyO2CgH82iW0N9qGVEBHR9nVgq-7ONuoJKH0zqpajB5uU1dzZjHY3174DZhCJ6jIZTkq6DL6UJ2QyUAFC/s640/Representatives_of_ceratioid_families.jpg" width="414" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Masaki Maya et al., Wikipedia</td></tr>
</tbody></table>
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Representatives of Anglerfish Order Lophoformis<br />
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Most, but not all, of these species live deep in the ocean, where the water is pitch black, extremely cold and under intense pressure. In these species, a piece of dorsal spine has evolved into a protruding "fishing pole" tipped with a luminous bulb. Not all individuals of each species have a fishing pole, however. Those that possess one are all females. Males are much smaller and have no need to fish for food because they are <a href="https://www.nationalgeographic.com/animals/fish/group/anglerfish/">completely parasitic upon the females</a>. (Click this link: what you first see is awesome!) Bulb-less, a male latches onto a female with its sharp teeth and eventually fuses with the female's body. It connects to her bloodstream, and eventually loses its eyes and internal organs, everything except its testes. Females are known to carry more than six males on their bodies at one time.<br />
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Some rare footage of an anglerfish mating pair reveals that these strange fish not only have bioluminescent lures but a surrounding net of fine bioluminescent filaments as well. They were filmed in 2016 at 792 metres deep off Portugal's coast.<br />
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<iframe allow="autoplay; encrypted-media" allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/anDIlMVgNwk" width="560"></iframe>
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<br />
The 3-minute video above was published by Science Magazine in 2018.<br />
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A vast number of species (in the thousands) that use bioluminescence, as well as a large number of variations in the chemical reaction that produces the light, suggest that bioluminescence evolved independently many different times throughout history. Experts estimate that <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4898709/">it evolved at least 40 different times</a> starting at least 150 million years ago, near the beginning of the <a href="https://en.wikipedia.org/wiki/Cretaceous">Cretaceous Period</a>.<br />
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In fish alone, bioluminescence evolved <a href="https://mashable.com/2016/06/08/bioluminescence-fish-evolution/#dGo59BAgskqs">at least 27 different times</a>. All this <a href="https://en.wikipedia.org/wiki/Convergent_evolution">convergent evolution</a> is a testament to its usefulness. Bioluminescence is a very useful jack-of-all-trades system, <a href="https://biolum.eemb.ucsb.edu/functions.html">having numerous functions</a> such as lures for prey, predator warning systems, mate attraction and communication. Bioluminescence is autogenic in many fish, which means that the animal itself produces the light. Examples are the lanternfish, which emit light from light-production organs called photophores. Many other marine species have instead left the job of light production to internalized luminescent bacteria. To further enhance this <a href="https://en.wikipedia.org/wiki/Symbiosis">symbiotic relationship</a>, they have <a href="https://en.wikipedia.org/wiki/Coevolution">co-evolved</a> mechanisms within their bodies to turn these bacteria off and on at their will. They evolved a handy light switch for them in other words. Anglerfish belong to this group. Their mutually beneficial relationship with their luminescent bacteria <a href="http://news.cornell.edu/stories/2018/07/genetics-shed-light-symbiosis-anglerfish-and-glowing-bacteria">is especially interesting</a>. Recent genetic studies reveal that these bacteria have lost almost half of their genome compared to their free-living close relatives, an example of <a href="https://www.nature.com/articles/s41467-018-03667-1">adaptive gene loss</a>, a use-it-or-lose-it principle in genetic evolution. The bioluminescent bacteria appear to be able to swim in and out of the bulb freely using their flagella, but they have <a href="https://en.wikipedia.org/wiki/Genome_evolution#Genome_reduction_and_gene_loss">lost all the genes </a>associated with sensing and digesting food sources. The fish, instead, supply all the cellular nutrients the bacteria requires while the bacteria provide light to help the fish lure food.<br />
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Perhaps one of the eeriest examples of bioluminescence is <a href="https://en.wikipedia.org/wiki/Foxfire">foxfire or faerie fir</a><a href="https://en.wikipedia.org/wiki/Foxfire">e</a>. Produced by some species of fungi in decaying wood, the light might be used to attract insects to spread the fungus? spores or as a warning to any foraging animals nearby. Omphalotus olearius, by day, is an ordinary looking (but poisonous!) European mushroom that looks much like a chanterelle:<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjlhvjsJQ9RUkgocZBXhnpKTEhb-2pjhJO2bYwFjbFRsDQzaTed2ebkYvZdUMXcmzw86iQRSFceFXcoTBRaoRhMoGY4mDUDoni2hnWCABzEnXcyoJZ8yVseL8-hNTVdQ0qe40mfZyngXOGS/s1600/Omphalotus_olearius.JPG" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1200" data-original-width="1600" height="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjlhvjsJQ9RUkgocZBXhnpKTEhb-2pjhJO2bYwFjbFRsDQzaTed2ebkYvZdUMXcmzw86iQRSFceFXcoTBRaoRhMoGY4mDUDoni2hnWCABzEnXcyoJZ8yVseL8-hNTVdQ0qe40mfZyngXOGS/s640/Omphalotus_olearius.JPG" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Abbatiello;Wikipedia</td></tr>
</tbody></table>
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Fungus Omphalotus olearius During the Daytime<br />
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By night, it reveals why it is also called the jack-o-lantern mushroom. Its orange gills glow an eerie green. It's easy to imagine faeries holding their mysterious night-meetings here.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhGLameQq0_d1kYlD0b7Ko-AiWRCNm5ZGtsmTUpcmdACqVqxmAKzJi_DXCPT4IL09vhnXiKbf7Ma4P0QbkqdsvpayApwBUc1pzeclW3ZIzMbfGRjW8EqP_aT98cSU4cFF48_38uFA91vMdf/s1600/Omphalotus_olearius_33857.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="504" data-original-width="768" height="420" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhGLameQq0_d1kYlD0b7Ko-AiWRCNm5ZGtsmTUpcmdACqVqxmAKzJi_DXCPT4IL09vhnXiKbf7Ma4P0QbkqdsvpayApwBUc1pzeclW3ZIzMbfGRjW8EqP_aT98cSU4cFF48_38uFA91vMdf/s640/Omphalotus_olearius_33857.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Noah Siegel;Wikipedia</td></tr>
</tbody></table>
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Omphalotus olearius at Night<br />
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How Does Bioluminescence Work? Where Did It Originally Come From?<br />
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Like a glow stick, bioluminescent animals utilize a chemical reaction that produces light. Because bioluminescence evolved independently in a wide range species across the evolutionary spectrum, different species use different chemical reactions to make light. The general mechanism, however, is the same throughout. It involves three essential ingredients: a light-emitting molecule, an enzyme, and molecular oxygen, O<sub>2</sub>. Inside the light-emitting cell, a special protein reacts with a charged oxygen ion and undergoes an <a href="https://en.wikipedia.org/wiki/Redox">oxidation reaction</a>. During the reaction, an intermediate molecule in an excited state is produced. What does this mean? A molecule enters an <a href="https://en.wikipedia.org/wiki/Excited_state">excited state</a> when one or more electrons in its atoms absorb enough energy to jump outward to a higher energy <a href="https://en.wikipedia.org/wiki/Quantum_state">quantum state</a>. Light is produced during the reaction when an excited electron in the intermediate molecule emits a photon of visible light. By dong so, it loses just the right amount of energy to drop to its ground (resting or lowest energy) state. This scenario isn't unusual in biochemistry. These chemical reactions often involve a short-lived <a href="https://en.wikipedia.org/wiki/Reaction_intermediate">intermediate molecule</a>. It is usually highly reactive and it is often in an excited state. What is unique here is that the intermediate complex releases its excitation energy is as a photon rather than sequestering it in the potential energy of one or more new chemical bonds. The special protein in this reaction is generically called a <a href="https://en.wikipedia.org/wiki/Luciferin">luciferin</a> (which means a light-emitting substance*). The light-emission reaction is catalyzed by a protein enzyme, generically called a luciferase. The trigger for the reaction can be a mechanical, neurological or chemical change.<br />
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*<span style="font-size: x-small;"><i>I was so curious I had to look up "Lucifer." How is this word also the word for devil? According to Wikipedia, <a href="https://en.wikipedia.org/wiki/Lucifer">Lucifer</a> originated as the Latin word for "bringer of light?" and as a metaphor for "morning star," which, in Christianity, also appears to have been the original name for Satan before his fall from grace. It then became a common byword for Satan or Devil.</i></span><br />
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Luciferin in Many Marine Organisms<br />
<br />
The shape and size of the luciferin molecule <a href="https://en.wikipedia.org/wiki/Luciferin#Types">varies widely</a> among the various bioluminescent phyla. However, some luciferins are more commonly found than others. For example, a luciferin called <a href="https://en.wikipedia.org/wiki/Coelenterazine">coelenterazine</a> is found in most bioluminescent marine organisms. It may have evolved into its luciferin role from an earlier role in detoxifying cells. Oxygen is essential for most life on Earth, but inside cells it is a hazardous molecule, especially when it is in the form of an even more reactive <a href="https://en.wikipedia.org/wiki/Reactive_oxygen_species">oxygen free radical</a>. Coelenterazine is a strong anti-oxidant. It reacts with free oxygen radicals and neutralizes them before they can react with and seriously damage proteins and DNA inside cells. This might have been coelenterazine's original function. When ancient ancestors of marine organisms moved down into deeper sea habitats, where it is cold and oxygen levels are low, the metabolic rate of organisms would have gone down, requiring less intercellular coelenterazine. It was a molecule, already present, <a href="https://www.researchgate.net/publication/51307188_The_origins_of_marine_bioluminescence_Turning_oxygen_defence_mechanisms_into_deep-sea_communication_tools">ready to evolve into its new luciferin function</a>.<br />
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The enzyme luciferase, which catalyzes the oxidation of the luciferin, also varies widely. Each unique luciferin/luciferase system represents a unique evolutionary origin of bioluminescence. How did these molecules arise inside living cells? As suggested above, they appear to have mutated over time from biological molecules already present in the organism. Molecules very similar to the luciferin molecules seem to have already been present in many non-luminescent living organisms, doing some other function.<br />
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Luciferin in Some <a href="https://en.wikipedia.org/wiki/Dinoflagellate">Dinoflagellates</a><br />
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Another intriguing example to illustrate this theory is a <a href="https://en.wikipedia.org/wiki/Dinoflagellate#Bioluminescence">bioluminescent marine dinoflagellate</a>. The luciferin used by most of these tiny unicellular organisms <a href="https://biolum.eemb.ucsb.edu/chem/detail2.html">closely resembles chlorophyll</a>, an even more ancient and biologically important molecule. <a href="https://en.wikipedia.org/wiki/Chlorophyll">Chlorophyll</a> is also present because many dinoflagellates are <a href="https://en.wikipedia.org/wiki/Photosynthesis">photosynthetic</a>. In some of these species, luciferin even seems to retain some of the light-absorption function of chlorophyll. Some species, such as P. lunula, bioluminesce only at night, and after sunny days they glow more brightly. The luciferin of this species <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5029497/">might be a photo-oxidized chlorophyll</a>. This chlorophyll molecule fluoresces blue when it is exposed to UV light (a part of sunlight) but the fluorescence eventually stops when all the chlorophyll molecules become degraded by photo-oxidation (oxidation in the presence of sunlight). Once it is photo-oxidized, it?s function changes and it becomes a bioluminescent luciferin, which glows blue. Even though this species offers a tantalizing clue about the origin of bioluminescence, researchers know that several other bioluminescent dinoflagellate species don't use this biochemical mechanism.<br />
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Luciferase in Fireflies<br />
<br />
Luciferase, like luciferin, seems to have evolved from molecules that were already present in the organism. Over time, the molecule's function shifted toward to light-production, offering a new selective advantage to those individuals. This process of evolution toward bioluminescence is still far from understood but some headway has been made from studying coelenterazine luciferin evolution in marine organisms and by studying the chlorophyll-like mechanism for bioluminescence in a dinoflagellate species. Luciferases in beetles such as the firefly have also been the subject of close study, and again, the bioluminescent machinery might be borrowed from the cell's general biochemistry machinery. The firefly luciferase enzyme seems to have evolved from another enzyme called <a href="https://en.wikipedia.org/wiki/Long-chain-fatty-acid%E2%80%94CoA_ligase">AMP-CoA-ligase</a>. This means that somehow this enzyme evolved from breaking down fatty acids <a href="https://www.ncbi.nlm.nih.gov/pubmed/23705763">into an oxygenase/light-production function</a>. Just to note here, the original ligase with its original function still operates in the organism. Its ligase role is still essential to its cellular biochemistry, especially in a number of regulative cellular functions. We can assume that the change in function toward producing light gave bioluminescent beetles such as fireflies a significant evolutionary advantage over their non-bioluminescent relatives, by enhancing their reproductive success. The numerous evolutionary paths of luciferin and luciferase are wonderful examples of Mother Nature re-purposing items at hand into new and amazing living tools.<br />
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Two things are fairly certain: First, bioluminescence evolved independently at least 40 separate times and second, it first evolved many millennia ago. There is evidence that <a href="https://phys.org/news/2016-06-evolutionary-variety-bioluminescent-ocean-fishes.html">it evolved in marine fish between 150 and 60 million years ago</a>. In just under half of these marine fish, bioluminescence evolved not inside of them <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0155154">but in their symbiotic bioluminescent bacteria</a>.<br />
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Evolution of Bioluminescence in Bacteria<br />
<br />
It's possible, but not proven by any means, that bioluminescence first appeared in bacteria living in the early <a href="https://en.wikipedia.org/wiki/Early_Cretaceous">Cretaceous period</a> (about 145 million years ago). <a href="https://en.wikipedia.org/wiki/Aerobic_organism">Aerobic</a> bacteria evolved once Earth accumulated enough oxygen in the atmosphere to support them. These bacteria had a metabolic advantage over their anaerobic cousins. They could now use the high-energy chemical bonds in oxygen to oxidize glucose (cellular food) during <a href="https://en.wikipedia.org/wiki/Cellular_respiration">cellular respiration</a>. Those products could then be used to make <a href="https://en.wikipedia.org/wiki/Adenosine_triphosphate">ATP (adenosine triphosphate)</a>, a very important energy molecule used by all cells.<br />
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Aerobic bacteria, and most, but not all, living organisms, also use oxygen to obtain energy from other molecules in addition to glucose, such as <a href="https://en.wikipedia.org/wiki/Fatty_acid_metabolism">from fatty acids</a>. Fatty acids, which I very briefly mentioned earlier, are all-around energy storage molecules in aerobic cells. Their energy, too, can be ultimately captured in ATP. When the aerobic pathway isn't used in the cell, anaerobic processes use fatty acids to make a variety of important molecules like phospholipids, messengers and hormones. Even the aerobic <a href="https://en.wikipedia.org/wiki/Eukaryote">eukaryotic</a> cells of multicellular animals carry out anaerobic processes.<br />
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Early aerobic bacterial species, living about a billion years ago at the end of the <a href="https://en.wikipedia.org/wiki/Great_Oxygenation_Event">Great Oxygenation Event</a>, likely lived in a changing environment where oxygen levels went up and down. This environmental stress might have been the first trigger for the evolution for bioluminescence. When oxygen levels fell, a mutation in an enzyme called riboflavin oxygenase (think of this molecule as an early luciferase) might have allowed these organisms to oxidize aldehyde molecules, which would accumulate under those conditions. By oxidizing the aldehydes, they could be turned into useful fatty acids. A supply of these molecules under low oxygen conditions would be a great evolutionary advantage. These bacteria would have that extra energy boost to keep reproducing and accumulating in numbers until oxygen became plentiful again. If some of these mutants also used another molecule, the already abundantly present reduced flavin mononucleotide (FMNH<sub>2</sub>) as a substrate or cofactor (an early luciferin in other words), then they could have been "accidentally" luminous, because this reaction creates an excited intermediate molecule that gives off light when it returns to its ground state.<br />
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If these early individuals were also light-sensitive, as many bacteria are, luminance among them might have been very useful in helping individuals recognize the presence of others nearby. It might have offered them a brand new selective advantage by helping them to disperse and/or colonize more easily. In bacteria, this behaviour, called <a href="https://en.wikipedia.org/wiki/Quorum_sensing">quorum sensing</a>, allows bacteria to detect and respond to their neighbours through <a href="https://en.wikipedia.org/wiki/Regulation_of_gene_expression">gene regulation</a>. The bacteria can switch genes on or off in order to optimize the population for changing conditions. Offspring might start producing a <a href="https://en.wikipedia.org/wiki/Biofilm">biolfilm</a>, for example, so they can stick to an optimal rock surface. Or the population might switch to <a href="https://en.wikipedia.org/wiki/Endospore">spore encapsulation</a> when conditions become harsh. You could think of this new advantage like a military troop receiving radio communication on the battlefield - very useful under quickly changing conditions. All bioluminescent bacteria have a few characteristics in common: they are rod-shaped, gram-negative, they have flagella to move around with, and most importantly to the evolution argument I just laid out, they are all <a href="https://en.wikipedia.org/wiki/Facultative_aerobic_organism">facultative anaerobes</a>, which means they can live and grow when oxygen levels are high and also when they are low or zero.<br />
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The Genetics of Bioluminescence<br />
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Even though a single general pathway toward bioluminescence might have originated in facultative anaerobic bacteria, the chemistry of bioluminescence in bacteria varies depending on the bacterial strain or species. This suggests that bacterial bioluminescence <a href="https://en.wikipedia.org/wiki/Bioluminescent_bacteria#Evolution">evolved independently numerous times</a> just as it did in other phyla. While the chemistry each time might be unique, all the luminescent bacterial species <a href="https://en.wikipedia.org/wiki/Aliivibrio_fischeri#Genetics_of_bioluminescence">share the same gene sequence called the lux gene sequence, or lux operon</a>, again offering an argument for the evolution of a single general pathway toward bioluminescence, after which the chemistry evolved independently numerous times. The lux operon might be the code left over for an ancient DNA repair system. Now this sequence <a href="http://photobiology.info/Lin.html">codes for all the proteins involved in the luminescent mechanism</a>. It is a short fragment of DNA just 9 kilobases long, and it contains just 5 genes that code for the proteins required for bioluminescence. A few additional genes regulate the operon (they turn it on or off). Several other enzymes, substrates and co-factors used in the production of light are already present in the cell. This short gene sequence can be isolated and inserted into normally non-luminescent bacterial and eukaryotic cells to make them bioluminescent. A non-luminous bacterium, such as Escherichia coli for example, can be transformed into a bioluminescent one simply by the insertion of the lux gene sequence. As you can imagine, this has a myriad of potential uses in medicine such as imaging and in research.<br />
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Bacteria tend to evolve fast because their life cycle is short and there are many of them. If a random mutation in the genetic code introduces a new advantage, survivors pass it on and it quickly increases in the population. The lux operon sequence is strongly conserved (which means it stays much the same with few surviving mutations) among bioluminescent bacteria. This suggests that it must have a significant selective advantage even though it is an expensive option for a cell to choose. There is a very high energy cost to emit light. A green light photon, for example, has <a href="https://pdb101.rcsb.org/motm/78">about the same energy as the chemical bonds of 8 ATP molecules</a>. That is a significant energy commitment for a microscopically small organism like a bacterium.<br />
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Bioluminescence Chemistry<br />
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Let's focus now on the light-producing reaction itself. The light-emitting reaction in bacteria <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC372803/">has been studied extensively</a>. Reduced riboflavin phosphate (FMNH<sub>2</sub>) and a long-chain fatty aldehyde (RCHO) are both oxidized, and oxygen diffused from the environment into the cell is the perfect oxidizer to do the job. During this oxidation reaction, blue-green light is emitted. The reaction is catalyzed by various enzymes called luciferases. The luciferase used depends on the species. FMNH<sub>2</sub> and RCHO are already present in all aerobic bacteria (and eukaryotic cells) because they are part of the <a href="https://en.wikipedia.org/wiki/Electron_transport_chain">electron transport chain</a>.<br />
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I was inspired to research bioluminescence after exploring the electron transport chain in the previous article, <a href="http://sciexplorer.blogspot.com/2018/10/ozone.html">Ozone</a>. This chain is part of the process of <a href="https://en.wikipedia.org/wiki/Cellular_respiration#Aerobic_respiration">aerobic cellular respiration</a>. "Aerobic" means requiring oxygen. Through aerobic cellular respiration, cells use food and oxygen and turn it into the energy needed to grow, multiply and move. The electron chain carries out one of the processes of aerobic respiration, called <a href="https://en.wikipedia.org/wiki/Oxidative_phosphorylation">oxidative phosphorylation</a>. Its main purpose is to make an important energy molecule called ATP.<br />
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The electron transport chain in bacteria (or in mitochondria in eukaryotic cells like ours) transfers electrons from donor molecules to acceptor molecules in a series of redox reactions. It is coupled with the movement of protons (H<sup>+</sup> ions) pumped back across the cell membrane so that the cell and its environment remain neutrally charged. The chain drives the production of energy-rich ATP and it oxidizes a variety of enzymes and other proteins along the way. The final electron acceptor is oxygen, which is the perfect molecule for this because it is a powerful oxidizer (or "electron-grabber" if you want).<br />
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In non-luminescent bacteria, FMNH<sub>2</sub> (riboflavin phosphate) simply diffuses into the cytoplasm. In bioluminescent bacteria this is where the luciferase enzyme and enzymes that catalyze the creation of the intermediate complex are also present. They channel FMNH<sub>2</sub> into forming part of a light-emitting complex. In the process, FMNH<sub>2</sub> is reduced to FMN and a long-chain fatty aldehyde (RCHO) is reduced to a carboxyl acid (RCOOH). ("R" is organic chemistry shorthand for any carbon-hydrogen group)<br />
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This is what the generic reaction looks like:<br />
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FMNH<sub>2</sub> + RCHO + O<sub>2</sub> → FMN + H<sub>2</sub>O + RCOOH + <i>hv</i> (490 nm)<br />
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The products of the reaction are water, RCOOH, a carboxyl acid, and flavin mononucleotide (FMN), which is an electron carrier in the electron transport chain of every living cell. Energy is released in the reaction in the form of a blue-green photon [<i>hv</i> (490 nm)].<br />
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During the reaction, the about-to-be-reduced FMNH<sub>2</sub> binds to the luciferase enzyme. An enzyme is a biological <a href="https://en.wikipedia.org/wiki/Catalysis">catalyst</a>. It increases the rate of the reaction but it isn't consumed during it. The luciferase catalyzes the reaction by reacting with oxygen and then interacting with the aldehyde to form a fairly stable intermediate complex in an excited state. It decays, or returns to ground state, slowly. This means that light can be emitted over a significant period of time by numerous complexes rather than just in a single brief flash (although some species do flash). The particular enzyme utilized by each specific species can have a significant effect on the decay rate (duration of light production) and the turnover rate (how soon it can glow again) of this light-emitting complex.<br />
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Although bioluminescence might have evolved during exposure to low-oxygen environments, molecular oxygen (O<sub>2</sub>) is essential for the light emission reaction pathway. Marine organisms get it from the oxygen in seawater. Land organisms get it from air. The reduction of oxygen ultimately transforms potential chemical bond energy into light energy.<br />
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Although this reaction produces blue-green light, bioluminescence can show up in a variety of colours. Blue-green or blue light is most common. In a marine environment where light levels are very low, blue light is the only visible wavelength short enough to remain streaming through the water after longer wavelengths have been scattered by water molecules. This makes light in the blue spectrum very useful for communication in dark deep marine waters.<br />
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Single mutations in luciferase can modify the chemical binding sites on the light-emitting complex just enough to distort the emission colour. Some bacteria also carry fluorescent proteins that change the emission colour, for example, to yellow. These proteins absorb blue light and re-emit it as less energetic wavelengths like yellow. A few organisms emit red. In each case, the basic chemical reaction that produces the light remains the same.<br />
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Bioluminescence Is Useful To Humans<br />
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Now that the mechanics of luminescence in various species are being worked out, an increasing number of new technologies are being developed to take advantage of it. <a href="https://en.wikipedia.org/wiki/Bioluminescence_imaging">Bioluminescent imaging</a>, for example, allows scientists to non-invasively study biological processes while they are taking place inside live subjects. One especially promising new idea is to use it to track the progress of cancer metastasis in the living body in order to understand the progress of cancer better. Bioluminescent DNA machinery, the lux operon, can be inserted into various types of non-luminous cells.<br />
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To start, the lux operon can be spliced into a virus's genome. The virus then infects a cell <a href="https://en.wikipedia.org/wiki/Recombinant_DNA">and inserts that DNA into the cellular DNA</a>. The cell can then translate and <a href="https://learn.genetics.utah.edu/content/basics/transcribe/">transcribe</a> the luciferase/luciferin proteins that emit detectable light. If, for example, the lux operon is inserted into the cancer cells of a primary tumour of a laboratory animal such as a mouse, that tumour will light up and glow. As the cancer cells spread into the blood stream and invade other organs over days and weeks, the process can be observed and studied simply by observing where the mouse glows. Of course, visible light can?t pass through an animal but researchers got around this problem by culturing bioluminescent cells and selecting for lux operon mutations that emit near-infrared light rather than blue or yellow light. Cancer cells emitting this wavelength of light can be imaged and tracked over time using an infrared camera placed outside the mouse's body. This kind of imaging can be so sensitive it can detect the glow from a single cell. It is a way to see exactly where cancers spread in the living body over time.<br />
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Bioluminescence can also be used <a href="https://www.promega.ca/resources/pubhub/enotes/bioluminescent-reporter-genes/">as a very sensitive assay</a> in genetic research. Researchers can take the lux genetic code for firefly luciferin/luciferase, for example. This genetic sequence can be isolated and cloned onto any DNA sequence of interest and then inserted into a virus, which has all the cellular machinery to make proteins from the inserted genetic code. The protein that is transcribed can be measured itself or its enzymatic activity can be measured by the intensity of its bioluminescence, in this case, a yellow glow. Each protein molecule transcribed from the DNA sequence of interest, perhaps it is code for a specific enzyme or a regulatory protein, now has a glowing tag attached to it. It is a very sensitive assay.<br />
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The idea of using tiny glowing markers to visualize specific protein molecules in a petri dish or in tissue or in a living organism is not new. I already briefly mentioned <a href="https://en.wikipedia.org/wiki/Green_fluorescent_protein">green fluorescent protein</a> (GFP), a breakthrough assay protein developed in 2008, and now widely used in medical and scientific research. The genetic code from GFP is also used as a genetic assay, but in its case the marker protein fluoresces green when exposed to blue to ultraviolet light. Both bioluminescent protein complexes and fluorescent proteins emit visible light as the result of an excited electron in an energized molecule returning to its ground state. The bioluminescent marker protein, however, luminesces in total darkness and does not need to be in a lit environment. This means there is no background to eliminate when measuring the light emitted from an assay result and this makes it up to a thousand times more sensitive than GFP assays. It means as well that extremely small changes in light emission now become measurable by using ultra-sensitive cameras to detect changes too small for our eyes to detect.<br />
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A Glimpse at Future Technologies<br />
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Bioluminescence has fascinated humans for millennia. Technologies that use bioluminescence are probably still in their infancy and the future looks bright (sorry). Think of trees <a href="https://www.fastcompany.com/40571215/imagine-a-city-lit-by-glowing-trees-instead-of-streetlights">gently lighting future streets at night</a> à la Avatar. What a way for a city to go green and save electricity costs! The myriad possibilities of bioluminescent technologies are limited only by the imagination.Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-34330285315693991492018-10-08T11:21:00.000-06:002018-10-08T11:21:21.610-06:00Ozone<a href="https://en.wikipedia.org/wiki/Ozone">Ozone</a> is a duplicitous character. We hear that it is bad for us (as a component of air pollution) and we hear it is good for us (as an atmospheric layer that protects us from harmful solar UV radiation). Which one is it? Where does it come from? How does it work? In this article I hope I can dispel some ozone mystery. The journey will take us through complex and interesting terrain: atmospheric chemistry, biochemistry and free radical chemistry.<br />
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The word ozone comes from a Greek word meaning "to smell," so named because this clear pale blue gas has a peculiar odour. You may have smelled ozone while operating a poorly working (sparky) appliance or you might have noticed it as that unique fresh scent right after a thunderstorm. I find it a bit sharp on the nose and weirdly pleasant.<br />
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Ozone is produced on Earth in three ways. First, when air is subjected to an electric spark such as lightning, ozone is created. Second, ozone is produced as a byproduct of fuel combustion - from car engines, forest fires and industry. If ozone is present in high enough concentration over time, it is harmful to animals, plants and humans. This is called ground level, or <a href="https://en.wikipedia.org/wiki/Tropospheric_ozone">tropospheric ozone</a>, after the atmospheric layer in which it exists. A third method of production is natural and it occurs at very high altitude. Ozone is created when ultraviolet (UV) light from the Sun passes through oxygen in the stratosphere. This ozone <a href="https://en.wikipedia.org/wiki/Ozone_layer">forms a protective blanket</a> that strongly absorbs UV light, a harmful form of radiation, preventing it from reaching Earth's surface. To see where the troposphere and stratosphere exist, the layers of Earth's atmosphere are shown below. The <a href="https://en.wikipedia.org/wiki/Troposphere">troposphere</a> is the bottom layer in which we live. This relatively thin layer contains about 80% of the atmosphere's mass and almost all of its water vapour. It's where all of our storms occur. The <a href="https://en.wikipedia.org/wiki/Stratosphere">stratosphere</a> is the almost cloud-free layer just above it. Air here is much thinner. Atmospheric pressure here is about 1/1000 that at sea level. Jets rarely fly above the lowest part of the stratosphere.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjvAcd2pLq4h2XxzsCzO1roSWvk22cwHzcVj_aG2s1vaEhbIb28_Jg_6OWzcXfsMiC4YAi-qkNZ3h4ZxUOLt1KJQrcThP_oqUpi0D28Qx9C0_IXCAcaI3aBFPQb3eULTTBIesdJs9rNxZWq/s1600/image_gallery.jpeg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1204" data-original-width="959" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjvAcd2pLq4h2XxzsCzO1roSWvk22cwHzcVj_aG2s1vaEhbIb28_Jg_6OWzcXfsMiC4YAi-qkNZ3h4ZxUOLt1KJQrcThP_oqUpi0D28Qx9C0_IXCAcaI3aBFPQb3eULTTBIesdJs9rNxZWq/s640/image_gallery.jpeg" width="508" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: x-small;">Kelvinsong;Wikipedia</span></td></tr>
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Ozone Is Composed of Oxygen Atoms<br />
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Ozone is an <a href="https://en.wikipedia.org/wiki/Allotropy">allotrope</a> of the element, <a href="https://en.wikipedia.org/wiki/Oxygen">oxygen</a>. <a href="https://en.wikipedia.org/wiki/Allotropes_of_oxygen">Oxygen allotropes</a> are different ways in which oxygen atoms bond together. Oxygen can exist in a <a href="https://www.nasa.gov/topics/technology/features/atomic_oxygen.html">highly reactive atomic form, O<sub>1</sub></a>, or as the familiar stable colourless O<sub>2</sub> gas we breathe in. Liquefied oxygen gas is pale blue. Oxygen can also exist as unstable and reactive ozone, O<sub>3</sub>, a pale blue gas or dark blue liquid under pressure. Oxygen can even exist as a dark red metallic solid, O<sub>8</sub>, under immense pressure.<br />
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Oxygen is a reactive element, in any allotropic form. O<sub>2</sub> binds readily with most other elements and compounds to form <a href="https://en.wikipedia.org/wiki/Oxide">oxides</a> - chemical compounds that contain oxygen atoms. About half of Earth's crust consists of oxides and about one fifth of our atmosphere consists of O<sub>2</sub>. Having so much of this reactive gas in our atmosphere indicates that our planet bears life. No abiotic (non-life) processes are known to constantly replenish oxygen that is constantly sequestered into oxides by rock and minerals. The source is green plants. Green plants release oxygen gas into the atmosphere as a waste product of <a href="https://en.wikipedia.org/wiki/Photosynthesis">photosynthesis</a>, a process that uses the Sun's energy to grow and make food. Oxygen-breathing animals like us evolved to use oxygen. We can trace our origins to ancient unicellular life forms that gained the ability to utilize this chemically reactive gas in order to power a series of reactions called <a href="https://en.wikipedia.org/wiki/Cellular_respiration">cellular respiration</a>. This process turns the food we eat and the air that we breathe into the power to move, grow and repair our bodies. Plants in turn utilize our waste gas, carbon dioxide, along with the Sun's energy, to grow and make food for us, in what is a wonderfully elegant <a href="https://en.wikipedia.org/wiki/Symbiosis">symbiotic</a> partnership.<br />
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Why Is O<sub>2</sub> So Important to Life?<br />
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All of the oxygen allotropes mentioned earlier are present on Earth naturally. Each allotrope has different physical properties and chemical reactivity due to the unique structures and strengths of the chemical bonds between the atoms. O<sub>2</sub> is the most chemically stable oxygen allotrope at atmospheric pressure and temperature. Essential for life for animals, fungi, protists and some bacteria, O<sub>2</sub> has a unique electron configuration that keeps it stable in the air but reactive enough for life to exploit.<br />
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In O<sub>2</sub>, two oxygen atoms are bound together by a <a href="https://www.chemguide.co.uk/atoms/bonding/doublebonds.html">covalent double bond</a>. This means that two pairs of electrons are shared between the two atoms. The diagram below gives you an idea of how this works. Electrons are the small green and purple circles below. The large central circles represent atomic nuclei. The oxygen atom belongs to group 6 on the periodic table, which means it has 6 outer electrons, available for bonding. These are called <a href="https://en.wikipedia.org/wiki/Valence_electron">valence electrons</a>.<br />
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Each oxygen atom has 8 electrons, 6 of which are valence electrons. The valence electrons have the same energy so they belong to the same (outermost) shell below. All atoms are most stable when the valence energy shell is full, with 8 electrons. Energy shells are shown as rings in the simple O<sub>2</sub> <a href="https://en.wikipedia.org/wiki/Electron_shell">electron shell</a> diagram below. By sharing two pairs of electrons, two oxygen atoms are stabilized in a molecular structure that fills each valence shell. By bonding, each atom can attain 8 valence electrons.<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgVnKdijvXt8adCRuoDcSlgdmuhrzF1E3m54CZV0GgUR1Mj3tjd2CVNk8zthQTwPs0l3JSuSGNpiSLH8AbTfNzCWpTtbD5JpGHlurqh99uTJazjNPj78EsOPrggEsfM2VRzHV5oBAOKiPno/s1600/main-qimg-807f056a8a4aef48e389e9093a737e77.png" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" data-original-height="285" data-original-width="500" height="182" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgVnKdijvXt8adCRuoDcSlgdmuhrzF1E3m54CZV0GgUR1Mj3tjd2CVNk8zthQTwPs0l3JSuSGNpiSLH8AbTfNzCWpTtbD5JpGHlurqh99uTJazjNPj78EsOPrggEsfM2VRzHV5oBAOKiPno/s320/main-qimg-807f056a8a4aef48e389e9093a737e77.png" width="320" /></a></div>
The diagram left is a very simple way of looking at the bond in an oxygen molecule. This diagram is helpful but it doesn't show us one important fact. It doesn't show us how the electrons pair up with each other, as electrons tend to do. The oxygen molecule is quite unusual in fact because it has two unpaired valence electrons. These unpaired electrons explain why O<sub>2</sub> is so useful to life. We can show this additional fact by drawing a modified <a href="https://en.wikipedia.org/wiki/Lewis_structure">Lewis diagram</a>:<br />
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"A" to the left represents an ordinary Lewis diagram of O<sub>2</sub>. There is a double bond between the two oxygen atoms, drawn using two parallel lines. The un-bonded electrons of each atom are shown as two electron pairs. It makes sense - we learn early on in chemistry that electrons like to pair up. But it isn't quite accurate. If we refine our viewing lens once again, this time from a Lewis structure to a more complex and accurate atomic orbital representation, we discover that the double bond is actually composed of two orbital bonds: a sigma bond and a pi bond. An <a href="https://brilliant.org/wiki/atomic-orbitals/">atomic orbital</a> is a three-dimensional shape outlining where a particular electron might be orbiting a nucleus. This updated version takes into account that electrons are actually <a href="https://en.wikipedia.org/wiki/Electron#Quantum_properties">quantum wave functions</a>. This model describes electron energy in much better detail and this helps us understand the bonding behaviours of the atom.<br />
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Every single chemical bond consists of one <a href="https://en.wikipedia.org/wiki/Sigma_bond">sigma bond</a>. It is the strongest bond and it is simply the head-on overlapping of two atomic orbitals. A double bond has an additional <a href="https://en.wikipedia.org/wiki/Pi_bond">pi bond</a>. This bond is the sideways or lateral overlapping of atomic orbitals and it makes the bond stronger. A triple bond, stronger still, consists of a sigma bond and two pi bonds. To help you visualize these orbital-overlapping bonds, check out the simple diagrams on <a href="https://brilliant.org/wiki/sigma-and-pi-bonds/">this site</a>.<br />
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O<sub>2</sub> bonding is rather unique. It is a double bond but in this case, the pi bond acts like two half-pi bonds plus two unpaired electrons. Take a look again at right hand drawing in the diagram above. The unpaired electrons are shown in red for emphasis in "B". This unusual configuration leaves two unpaired electrons with equal energy. Having two unpaired electrons allows O<sub>2</sub> to reach the lowest potential energy state possible. This is something all atoms and molecules tend to do. If some heat were applied to the O<sub>2</sub> molecule, those two electrons would pair up and the molecule would have slightly more potential energy. What we learned in school still holds up: these two unpaired electrons "want" pairing. This means that an oxygen molecule greedily accepts electrons from other atoms and molecules. It is chemically reactive in other words.<br />
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Most molecules tend to have paired electron spins. O<sub>2</sub>'s unpaired electrons don't match up well with the valence electron pairs of other molecules. They are an awkward "third wheel" in the interaction. The consequence of this is that atmospheric O<sub>2</sub> reacts slowly with most other substances, rather than rapidly. This is good because otherwise the oxygen in our atmosphere would trigger spontaneous combustion. An example in nature is the gradual process of <a href="https://en.wikipedia.org/wiki/Rust">rusting</a>, the oxidation of iron exposed to air, into ferric oxides (rust). Oxygen is an <a href="https://en.wikipedia.org/wiki/Oxidizing_agent">oxidizing agent</a>, which means it causes other substances to lose electrons. By doing so, oxygen itself gains electrons. This makes sense when you look at its two unpaired electrons. They "want" electrons so they can become pairs. The word "oxidation" was coined by Antoine Lavoisier, while observing reactions with oxygen. It is a bit of a misnomer because these types of reactions, more accurately called <a href="https://en.wikipedia.org/wiki/Redox">redox (reduction/oxidation) reactions</a>, simply involve electron transfer. They don't have to involve oxygen.<br />
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How do our bodies utilize oxygen's unusual unpaired electrons? <a href="https://en.wikipedia.org/wiki/Mitochondrion">Mitochondria</a>, the tiny "power plants" inside our cells, use oxygen as a final and powerful electron acceptor along a string of reactions called an <a href="https://en.wikipedia.org/wiki/Electron_transport_chain">electron transport chain</a>. An electron transport chain is a series of redox reactions. Electrons are transferred from one molecule to the next. Differences in <a href="https://en.wikipedia.org/wiki/Gibbs_free_energy">Gibbs free energy</a> (chemical energy available to do work) between the reactants and the products drives this process forward. The beauty of this set-up is a) it's spontaneous and b) it transfers chemical bond energy to a molecule that can store and readily release it when required. Along the way, a molecule called <a href="https://en.wikipedia.org/wiki/Adenosine_triphosphate">ATP</a> is produced. ATP (adenosine triphosphate) is the all-important energy storage molecule for all life - plant and animal. Like a tiny battery or fuel cell, ATP powers almost all cellular reactions. Driven backwards, the electron transport chain "burns up" ATP to provide energy for growth and for mechanical energy such as a sperm's flagellum or a contracting heart muscle.<br />
<br />
Trivalent Oxygen, Ozone, O<sub>3</sub><br />
<br />
Now that we understand the chemical nature of O<sub>2</sub>, how does O<sub>3</sub> compare? Ozone is a bent molecule. It has a triangular shape like water, H<sub>2</sub>O. The three oxygen atoms bond with a double and a single bond that resonates back and forth. Ozone is an example of a <a href="https://en.wikipedia.org/wiki/Resonance_(chemistry)#Resonance_hybrids">resonance hybrid</a>.<br />
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhjUTV5RcmFYYrGMBTcbtMDq-xk5zHJf3jf1mzLIIL7NxgE_IQ4Z4l36pZOhhYTjBzkpKqFIKJhW2ETu0nSh0SCGzoXvEBGGJP99hstTSKRV1wG_mh3E08izIcclGH-BRYMQF6QIBbca1eD/s1600/800px-Ozone-resonance-Lewis-2D.png" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" data-original-height="215" data-original-width="800" height="86" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhjUTV5RcmFYYrGMBTcbtMDq-xk5zHJf3jf1mzLIIL7NxgE_IQ4Z4l36pZOhhYTjBzkpKqFIKJhW2ETu0nSh0SCGzoXvEBGGJP99hstTSKRV1wG_mh3E08izIcclGH-BRYMQF6QIBbca1eD/s320/800px-Ozone-resonance-Lewis-2D.png" width="320" /></a></div>
The O-O bonds are a hybrid between a single sigma bond and a double sigma-pi bond. This means that the bond strength is in between that of a double and a single bond. Ozone's hybrid bonds are slightly weaker than O<sub>2</sub>'s double bond.<br />
<br />
Unlike its somewhat tamer cousin, ozone is a chemically unstable molecule. A resonant structure tends to stabilize a molecule but it is not enough to make ozone stable. The valence electrons in ozone are shared across three nuclei rather than two. Six valence electrons each are fighting for space while packed into a bent shape. Electrons with opposite spins like to pair up but electrons in general don't like to be too close to one another. This bent shape is the most stable lowest-energy arrangement possible but it still has high potential energy and that means it is unstable.<br />
<br />
The hybrid bond structure means that the valence electrons in ozone are <a href="https://en.wikipedia.org/wiki/Delocalized_electron">delocalized</a>. These delocalized electrons spread out to form a loose molecular orbital cloud. Their enhanced motility allows them to react more readily than the localized electrons in O<sub>2</sub> do, even though O<sub>2</sub> has reactive unpaired electrons and ozone does not.<br />
<br />
Ozone is one of the strongest oxidizers known, much stronger than O<sub>2</sub>. Because ozone is unstable, it readily decomposes into stable O<sub>2</sub> gas and extremely reactive chemically unstable lone oxygen atoms. These lone atoms are the key to why ozone is such a strong oxidizer. Although these atoms have 6 valence electrons each, they don't all pair off. Two form pairs and two exist as lone electrons. Those two lone electrons mean that these atoms "want" electrons very intensely in order to stabilize themselves. They will immediately react and borrow electrons from almost any other substance they come across. Ozone is a much stronger oxidizer than O<sub>2</sub> because O<sub>3</sub> is less chemically stable. O<sub>3</sub> offers up lone oxygen atoms that form as soon as ozone decomposes. Lone oxygen atoms are much stronger oxidizers than O<sub>2</sub> oxygen molecules, because they so unstable (and therefore reactive).<br />
<br />
O<sub>3</sub> is formed when O<sub>2</sub> reacts with highly reactive atomic oxygen, O<sub>1</sub> (or just O). Fee atomic oxygen reacts and disappears almost instantly from Earth's lower atmosphere, but in the stratosphere it is continuously replenished. Stratospheric O<sub>2</sub> is bombarded by UV (ultraviolet) radiation from the Sun, cleaving its bond into two free O<sub>1</sub> atoms. In the stratosphere, O<sub>3</sub>, O<sub>2</sub> and O<sub>1</sub> all exist and are all part of a cycle. In low-Earth orbit, far above the stratosphere, the very sparse atmosphere is almost entirely composed of atomic oxygen, O<sub>1</sub>. This diffuse but highly reactive gas corrodes all the outer materials on spacecraft that pass through low Earth orbit. It is a <a href="https://en.wikipedia.org/wiki/Corrosion_in_space">significant challenge</a> that all space agencies must take into account.<br />
<br />
In the stratosphere, the O<sub>2</sub> + O → O<sub>3</sub> synthesis reaction is triggered whenever O is available. O is created when solar UV radiation breaks apart the O<sub>2</sub> molecular bond into two free oxygen atoms. Whenever atmospheric O<sub>2</sub> comes into contact with free atomic oxygen it quickly combines into O<sub>3</sub>, ozone. Because ozone is a much more powerful oxidant (electron acceptor) than O<sub>2</sub> is, it is much too reactive to be useful in any cellular electron transport chain. In fact, its oxidizing action makes ozone pollution a serious health hazard. It can damage respiratory systems in animals and cause tissue damage in plants.<br />
<br />
Although O<sub>2</sub> is less reactive than ozone, it too is an oxidation hazard inside living cells, and this is something life has learned to live with. Various intracellular sequestering processes reduce this hazard. It is an evolutionary trade-off between cell damage and oxygen's electron-acceptor powers.<br />
<br />
Free Radicals<br />
<br />
You may have heard of how bad <a href="https://en.wikipedia.org/wiki/Radical_(chemistry)">free radicals</a> are for our health. These mysterious-sounding chemicals are simply atoms or molecules (or even ions) that have an unpaired valence electron. Radicals are an important part of biochemistry and atmospheric science. We already explored a radical when we looked at the molecular bonding of O<sub>2</sub>. The O<sub>2</sub> molecule is a di-radical. It has two unpaired valence electrons. We also came across free monatomic oxygen, which is another di-radical, and a much more powerful oxidizing agent.<br />
<br />
Like O<sub>2</sub>, all radicals are reactive, some more than others depending on their stability. Radicals are always oxidants because they accept electrons. Inside cells, free radicals can cause <a href="https://en.wikipedia.org/wiki/Oxidative_stress">oxidative stress</a>. Oxidative stress is basically a disturbance in the normal intracellular redox balance. The mitochondrial (and in plants, <a href="https://en.wikipedia.org/wiki/Chloroplast">chloroplast</a>) electron transport chain is an ingenious natural invention, but it's not perfect. A few electrons always "leak" out of the chain and react directly with O<sub>2</sub> at the end. This reaction creates negatively charged O<sub>2</sub>, which is a free radical called <a href="https://en.wikipedia.org/wiki/Superoxide">superoxide</a>. This is the Lewis diagram for it:<br />
<br />
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjO09kpto-bDocrw-l0wXeDTAcWCNALZd4evpA7lcU7V8FeDKHVUmtYbai3QPqE97ZkXaIy5jrW_epT0T8Nb5Lcstl86q-5x7S9mi2yLiuHbZ6J2HqCykpCmuw8hzMNmLF2j8Ixq0GxLxS5/s1600/260px-Superoxide.svg.png" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img alt="" border="0" data-original-height="160" data-original-width="260" height="123" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjO09kpto-bDocrw-l0wXeDTAcWCNALZd4evpA7lcU7V8FeDKHVUmtYbai3QPqE97ZkXaIy5jrW_epT0T8Nb5Lcstl86q-5x7S9mi2yLiuHbZ6J2HqCykpCmuw8hzMNmLF2j8Ixq0GxLxS5/s200/260px-Superoxide.svg.png" title="" width="200" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">DoSiDo;Wikipedia</td></tr>
</tbody></table>
This highly reactive charged molecule causes oxidative stress inside cells. It reacts with biologically important molecules such as DNA and proteins. Like a bull in a china shop, it breaks DNA strands haphazardly so they cannot replicate and transcribe accurately and it denatures proteins, so they can no longer function as enzymes, hormones, antibodies and so on. This microscopic damage gradually builds up at the cellular level and in the body as a whole, an overall effect <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3184498/">we observe as aging</a>.<br />
<br />
Stratospheric Ozone<br />
<br />
Stratospheric ozone is the "good" ozone. It makes all surface life on Earth possible. In the stratosphere, ozone forms and breaks apart continuously. When atomic oxygen (O) reacts with molecular oxygen (O<sub>2</sub>), ozone (O<sub>3</sub>) forms:<br />
<br />
O + O<sub>2</sub> + M → O<sub>3</sub>+ M<br />
<br />
A significant amount of energy is released during this reaction. It requires an additional body (M), such as a non-reacting molecule nearby that can carry that energy away. There are two reasons why energy must be released. First, the chemical bond energy of O<sub>2</sub> (498 kJ/mol) is slightly higher than that of O<sub>3</sub> (445 kJ/mol) so some energy must be released. Second, the free oxygen atom in the reaction is in an <a href="https://en.wikipedia.org/wiki/Excited_state">excited (high-energy) state</a>, and that energy must be released as well.<br />
<br />
Excitation can be explained using <a href="https://en.wikipedia.org/wiki/Electron_configuration">atomic orbital notation</a>. An orbital, once again, is a three-dimensional cloud where an electron can be found. When atoms and molecules react with one another, their outermost electrons interact to form or break chemical bonds. In its ground (lowest energy) state, the electrons in an oxygen atom occupy the three lowest energy orbitals available: 1s<sup>2</sup>2s<sup>2</sup>2p<sup>4</sup>. The lowest energy 1s orbital can hold two electrons; it's full. The next higher energy 2s orbital can hold 2 electrons; it's full too, so these two orbital clouds hold 4 electrons in total. Next, the p orbital starts to fill up. In an oxygen atom, it holds 4 of 6 possible electrons. Oxygen's outermost orbitals are those with n=2 orbital energy. These are the 2s and the 2p orbitals. These n=2 orbitals equip oxygen atom with a total of 6 electrons available to react chemically. These are the valence electrons. In theory, electrons could occupy any of a very large number of possible orbitals in any atom but in a ground state (lowest energy) atom, electrons always minimize energy by occupying the innermost orbitals possible. In an excited state, one or more electrons move outward into higher energy orbitals. An excited oxygen atom is most simply denoted as 1s<sup>2</sup>2s<sup>2</sup>2p<sup>3</sup>3s<sup>1</sup> where one outermost (valence) electron has jumped up to a higher-energy 3s (n=3 energy) orbital. This electron configuration contains one unpaired valence electron, which makes it a <a href="https://en.wikipedia.org/wiki/Radical_(chemistry)">radical</a> too. Oxygen radicals are denoted as O(<sup>1</sup>D). I won't go into the reason for the "D" here, but if you want to know, <a href="https://three.jsc.nasa.gov/articles/RadChemO2Sidebar.pdf">this NASA page</a> explains it well. Not all radicals are in an excited state. Recall that a lone ground-state oxygen atom is a radical too; in fact it's a di-radical. The whole story of oxygen radicals can get pretty confusing. The important thing to remember is that all radicals are highly reactive because the unpaired electron always "wants" to pair up with a valence electron in another atom or molecule. When it does so, the system releases potential energy and stabilizes.<br />
<br />
Atomic oxygen radicals (O(<sup>1</sup>D)) are very reactive because they are radicals and they are very energetic because they are excited. The unpaired electron in the valence shell of this atom combines rapidly with any O<sub>2</sub> molecule it slams into to form ozone. In order to exist at any concentration in the stratosphere, energetic free oxygen atoms must be continuously made. They come from the <a href="https://en.wikipedia.org/wiki/Photodissociation">photodissociation</a> of molecular oxygen, O<sub>2</sub>. Photodissociation means the splitting of molecules by electromagnetic radiation, or light ("photo"). High-energy, and therefore short wavelength, UV radiation pierces the stratosphere and cleaves O<sub>2</sub>. UV photons with wavelengths shorter than 242 nm (nanometres) have enough energy to break the bond between two oxygen atoms in an O2 molecule. This energy corresponds to 498 kJ/mol. That's the bond energy of O<sub>2</sub>. The two oxygen atoms released in the reaction absorb some UV energy, leaving them in an excited state.<br />
<br />
In the stratosphere, ozone cycles continuously, forming and decomposing O<sub>3</sub>. Both processes absorb harmful solar UV radiation, particularly of wavelengths shorter than 242 nm. This is why the ozone layer is a protective blanket against UV radiation. As we just learned, ozone absorbs short (<242 nm) wavelength UV radiation when it is produced. Ozone isn't chemically stable, so it doesn't stay around for long in the stratosphere. When it itself is bombarded with stratospheric UV radiation, it readily photodissociates back into O<sub>2</sub> and O. The molecular bond energy of ozone is 445 KJ/mol, which is less than that of O<sub>2</sub> (498 kJ/mol). This means that less energetic UV wavelengths will break ozone apart, those between 240 and 320 nm. The ozone photodissociation reaction formula looks like this:<br />
<br />
O<sub>3</sub> + UV (240nm -320 nm) → O<sub>2</sub> + O(<sup>1</sup>D)<br />
<br />
Excited free oxygen atoms from this reaction continue the cycle, creating ozone once again.<br />
<br />
Stratospheric Ozone Absorbs Deadly UV Radiation<br />
<br />
All of these reactions are fast; a whole cycle takes place in just over a minute. It is a very effective life-protecting "UV absorption machine" that converts UV radiation into <a href="https://en.wikipedia.org/wiki/Thermal_energy">thermal energy</a>. That energy is carried by the excited fast-moving free oxygen atoms. The stratospheric layer, above the thermosphere, ranges from about 20 km in altitude in the tropics to just 7 km in altitude at the poles. It is a generally stable layer of air that ranges from about -51°C at the top of the troposphere to just -3°C at the top of the stratosphere. You expect the temperature to go down as you move upward through the atmosphere, but the thermal energy created by the ozone cycle is most active at the top of the stratosphere where incoming solar UV radiation bombards oxygen.<br />
<br />
The Sun bombards Earth with all wavelengths of <a href="https://en.wikipedia.org/wiki/Ultraviolet">UV radiation</a> (and other EM radiation as well). UV radiation <a href="https://en.wikipedia.org/wiki/Ultraviolet#Subtypes">ranges from 10 nm to 400 nm</a>. Wavelengths shorter than 121 nm <a href="https://en.wikipedia.org/wiki/Ionization">ionize</a> air so strongly that they are absorbed long before they can harm life on the surface. An atom or molecule is ionized when it gains or loses electrons to form charged ions. Another short mini-lesson here: What makes an atom an ion is when the number of electrons doesn't match the number of protons, so the atom therefore has an unbalanced charge. High-energy UV photons have enough energy to cleave various atmospheric molecules apart into ions while the photons are absorbed in the process. A radical is an atom that has at least one unpaired electron. In this case the electron number may still match the proton number and in that case it isn't electrically charged, but it is very reactive. The charged superoxide radical we encountered earlier is both an ion and a radical.<br />
<br />
UV radiation between 100 and 280 nm is deadly to almost all life on Earth. This is the wavelength range, especially 230 to 270 nm, utilized in special mercury, LED and xenon germicidal lamps. It kills almost all known microorganisms. Most microorganisms have not evolved protection against concentrated mid-range UV bombardment. It is the right energy to break apart chemical bonds in DNA, proving deadly. Rare exceptions <a href="https://www.ncbi.nlm.nih.gov/pubmed/23271672">are extremophiles</a> and <a href="https://www.sciencedaily.com/releases/2015/10/151026093045.htm">ancient bacteria</a> that lived before Earth had a protective ozone blanket. These organisms oxidized iron and built protective "rust blankets" around themselves to shield them from UV radiation. There is evidence that photosynthesis evolved in these bacteria.<br />
<br />
Fortunately for us, the most DNA-damaging UV range (130 nm and 260 nm) is completely absorbed by stratospheric ozone. However, a small amount of slightly longer wavelength UV radiation, between about 260 and 300 nm, does make it to the surface. This is the UV radiation (especially between 265 and 275 nm) that causes sunburns and can lead to deadly melanoma. It also causes eye cataracts and other eye damage.<br />
<br />
As you might have noticed, it is within the range that is germicidal. How does it kill germs but not us? We and other multicellular life survive because, first of all, the natural solar bombardment of this UV radiation is far less intense than a concentrated beam from a lamp. Our cells therefore have a chance to repair the damage as it happens. Secondly, we have evolved some protection through our skin. A pigment called <a href="https://medium.com/getsundots/how-does-our-skin-protect-us-b93bdd4e00e2">melanin absorbs UV radiation</a>, directing it away from vulnerable cellular proteins and DNA. Our skin even makes use of some UV exposure (280 to 315 nm) <a href="https://en.wikipedia.org/wiki/Vitamin_D#Biosynthesis">to make Vitamin D</a>.<br />
<br />
Tropospheric Ozone<br />
<br />
Tropospheric or surface ozone is the "bad" ozone. It is a respiratory irritant and it can cause plant tissue damage as well.<br />
<br />
Ozone As Ground Level Pollution<br />
<br />
Ground level ozone is a pollutant and it is a key ingredient in <a href="https://en.wikipedia.org/wiki/Smog">smog</a>. A <a href="https://en.wikipedia.org/wiki/Pollutant">pollutant</a> is a substance that is introduced into an environment that has undesirable effects on it or on the life that depends on it. We tend to think of pollutants as man-made but not all of them are. Some are created naturally such as volcanic dust and volcanic gases. Ozone is technically called a <a href="https://energyeducation.ca/encyclopedia/Secondary_pollutant">secondary pollutant</a> because it is created in the atmosphere when - react in sunlight. These primary pollutants come from combustion engines in vehicles, from industry and from forest fires.<br />
<br />
The reactions that create ozone occur best on hot sunny summer days, when there is plenty of solar (UV) radiation. High temperatures promote ozone accumulation by increasing the rates of reactions that form ozone and by reducing the ability of plants nearby to absorb ozone out of the atmosphere. Plants absorb a variety of air pollutants including as much as <a href="https://www.york.ac.uk/news-and-events/news/2013/research/heat-ozone/">20% of atmospheric ozone production</a>. However, during heat waves, stressed plants close their stomata (https://en.wikipedia.org/wiki/Stoma) (epidermal pores) in order to conserve water and this means that they cannot absorb ozone and other pollutants.<br />
<br />
Both NO<sub>x</sub> and VOCs come from natural sources as well as man-made sources. A significant amount of VOCs is released from coniferous forests, volcanoes and wildfires. NO<sub>x</sub> compounds are released during lightning storms and wildfires. There are many man-made sources of these pollutants, ranging from motor vehicle exhaust, oil refining, paints, insecticides and industrial solvents to chemical manufacturing, but most man-made NO<sub>x</sub> and VOCs come from motor vehicle exhaust. Motor vehicles are responsible for at least half of the concentration of these pollutants, especially in large cities even though <a href="https://en.wikipedia.org/wiki/Catalytic_converter">catalytic converters</a> have been mandatory since 1975 (at least in North America).<br />
<br />
In Canada, ground level ozone advisories are issued when average levels per hour exceed 82 parts per billion. Toronto, for example, typically experiences about <a href="http://www.ec.gc.ca/media_archive/press/2001/010601_b_e.htm">10 ozone advisory days each summer</a>. To see live readings for various Ontario cities, check <a href="http://www.airqualityontario.com/history/summary.php">this government website</a>. In Edmonton, close to where I live, ozone pollution risk is usually low. Our cities are a bit smaller than Toronto but just as importantly we don't tend to experience summer days as hot as Toronto does.<br />
<br />
This past summer saw Edmonton and the surrounding area blanketed in thick haze blown in from several large forest fires to the west in British Columbia. We experienced many consecutive days in August where air quality health indexes (AQHI) sat over 10+ (very high risk). The air quality health index measures the combined health risk of all fine airborne particulate matter as well as ozone and nitrogen dioxide. If at any time fellow Albertans want more specific information than an AQHI reading, check out <a href="https://aqicn.org/city/canada/alberta/edmonton-central/">this Alberta website</a> that shows current Edmonton/central Alberta levels of ozone, NO<sub>2</sub>, fine particulate matter, sulphur dioxide and carbon monoxide. To get an idea of how major cities in Canada stack up internationally, <a href="https://www.canada.ca/en/environment-climate-change/services/environmental-indicators/international-comparison-urban-air-quality.html#r2">this Canada government website</a> compares the average annual ozone levels (in ppb) of various Canadian cities with selected international cities. Across the globe, <a href="http://elte.prompt.hu/sites/default/files/tananyagok/AtmosphericChemistry/ch08s02.html">tropospheric ozone levels range</a> between less than 10 ppb over remote tropical oceans and over 100 ppb downwind of large metropolitan cities in hot weather.<br />
<br />
Some ozone can enter the troposphere from the stratosphere through disturbances such as hurricanes that can draw some lower level stratospheric air downward. However, the vast majority of ground level ozone is created when it forms from reactions of precursor compounds such as VOC's and nitrogen oxides. Ozone is highly reactive so it leaves the troposphere quickly, but plants, animals and people downwind from large cities on hot days or downwind from large forest fires can face a significant ozone hazard.<br />
<br />
In the stratosphere, we now know that ozone forms from the photodissociation of O<sub>2</sub> into oxygen atoms, which recombine with O<sub>2</sub> to form ozone. However, this reaction doesn't happen where plants, animals and humans live because short wavelength UV light (<242 nm) doesn't penetrate down into the lower troposphere. As in the stratosphere, the production of ozone <a href="http://elte.prompt.hu/sites/default/files/tananyagok/AtmosphericChemistry/ch08s02.html">requires atomic oxygen</a>. Here, surface nitrogen dioxide (NO<sub>2</sub>) does the job of supplying it. Its photodissociation requires much less UV photon energy than molecular oxygen does - anything under about 420 nm (slightly more energetic than visible violet light) will work:<br />
<br />
<br />
NO<sub>2</sub> + UV (<420 nm) → O(<sup>1</sup>D) + NO<br />
<br />
The oxygen radical produced in this case is not in an excited state. It will react with oxygen gas to create ozone:<br />
<br />
O(<sup>1</sup>D) + O<sub>2</sub> + M → O<sub>3</sub> + M<br />
<br />
However, in unpolluted air, there is no net production of O<sub>3</sub> because O<sub>3</sub> quickly reacts with the product NO to create O<sub>2</sub> and NO<sub>2</sub> once again, in a cyclic reaction (which isn't shown).<br />
<br />
When other ozone precursors such as man-made pollutants like carbon monoxide (CO) and hydrocarbons such as methane (CH<sub>4</sub>) are also present in the air, net ozone build-up can occur. This is when ozone can spike to unhealthy levels. Some of these reaction mechanisms are extremely complex, but two fairly simple surface ozone production pathways, using carbon monoxide and methane as precursors, are fairly easy to show. Their formulae are written below.<br />
<br />
Both reactions require hydroxyl radicals (*OH).<br />
<br />
A hydroxyl radical, denoted *OH, is the electrically neutral form of the hydroxide ion (OH<sup>1</sup>). It has one unpaired electron as shown in the Lewis diagram below left.<br />
<br />
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: left; margin-right: 1em; text-align: left;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgkkDuIwtvR5o7dlROO-CJyzYv_GzKOe2x_R8lxTrcRdIb29l3aXynpa4QsiL2EHIaXxDX1gQL7eX6AXYz3oeO2ZJSEg2jpBwxskOunrTdP8lOC5BhSVj7qCUspByC0qT2oUo7fLpnkOHIG/s1600/HydroxideVsHydroxyl.png" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="236" data-original-width="404" height="116" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgkkDuIwtvR5o7dlROO-CJyzYv_GzKOe2x_R8lxTrcRdIb29l3aXynpa4QsiL2EHIaXxDX1gQL7eX6AXYz3oeO2ZJSEg2jpBwxskOunrTdP8lOC5BhSVj7qCUspByC0qT2oUo7fLpnkOHIG/s200/HydroxideVsHydroxyl.png" width="200" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Attys;Wikipedia</td></tr>
</tbody></table>
Hydroxyl radicals are created when surface ozone is exposed to the longer UV radiation that reaches Earth's surface:<br />
<br />
O<sub>3</sub> + UV (240nm -320 nm) → O<sub>2</sub> + O(<sup>1</sup>D)<br />
<br />
The free radical oxygen atom produced then reacts with water vapour to create hydroxyl radicals and oxygen gas:<br />
<br />
O(<sup>1</sup>D) + H<sub>2</sub>0 → 2*OH + O<sub>2</sub><br />
<br />
<br />
Hydroxyl radicals are highly reactive and they are an important part of atmospheric chemistry. Denoted *OH, hydroxyl is sometimes called an atmospheric detergent because it reacts with many pollutants, <a href="https://en.wikipedia.org/wiki/Chemical_decomposition">decomposing</a> them into smaller less harmful compounds. In this case, however, *OH is a step in producing a pollutant: ground level ozone. Compounds commonly present in combustion vehicle exhaust, such as nitrogen monoxide (NO), nitrogen dioxide (NO<sub>2</sub>) and hydroperoxyl radicals (HO<sub>2</sub>), serve as reactants and as <a href="https://en.wikipedia.org/wiki/Catalysis">catalysts</a>. Catalysts in this case increase the reaction rate of surface ozone formation reactions. Faster production means that ozone can build up temporarily even though it is unstable.<br />
<br />
Example 1: Carbon monoxide in an NO-rich environment:<br />
<br />
CO + *OH → H + CO<sub>2</sub><br />
H + O<sub>2</sub> → HO<sub>2</sub><br />
HO<sub>2</sub> + NO → *OH + NO<sub>2</sub><br />
NO<sub>2</sub> + UV (<420 nm) → O(<sup>1</sup>D) + NO<br />
<br />
Notice that the fourth reaction just above us is exactly the same reaction as in the natural O<sub>3</sub> production reaction (the one that cycles and doesn't build up ozone). Here, however, the reaction takes place in polluted air where various pollutants catalyze ozone production. Ozone is therefore produced faster than it can be removed (which is by reacting with NO to create oxygen and nitrogen dioxide). The reaction scheme continues as the oxygen radical reacts with oxygen gas to create ozone:<br />
<br />
O(<sup>1</sup>D) + O<sub>2</sub> → O<sub>3</sub><br />
<br />
Example 2: Methane (CH<sub>4</sub>) in an NO-rich environment:<br />
<br />
CH<sub>4</sub> + *OH → CH<sub>3</sub> + H<sub>2</sub>O<br />
CH<sub>3</sub> + O<sub>2</sub> → CH<sub>3</sub>O<sub>2</sub><br />
CH<sub>3</sub>O<sub>2</sub> + NO → CH<sub>3</sub>O + NO<sub>2</sub><br />
CH<sub>3</sub>O + O<sub>2</sub> → CH<sub>2</sub>O + HO<sub>2</sub><br />
HO<sub>2</sub> + NO → *OH + NO<sub>2</sub><br />
NO<sub>2</sub> + UV (<420 nm) → O(<sup>1</sup>D) + NO<br />
O(<sup>1</sup>D) + O<sub>2</sub> → O<sub>3</sub><br />
<br />
In this case, <a href="http://hc-sc.gc.ca/ewh-semt//air/in/poll/construction/formaldehyde-eng.php">both ozone and formaldehyde (CH<sub>2</sub>O) rapidly build up</a> in the lower atmosphere. Ozone is gradually removed as it reacts with hydroperoxyl radicals. As ozone blows into non-polluted air where NO levels are low, it will react with HO<sub>2</sub> generated in the reaction "line 4" above to create *OH radicals and oxygen gas. Further ozone depletion occurs when the *OH created then reacts with additional ozone to create new HO<sub>2</sub> and more oxygen gas. Ozone is also deposited onto surfaces, where it can react with the surface it lands on. This is how plants are damaged by ozone. Ground-level ozone <a href="https://www.ars.usda.gov/southeast-area/raleigh-nc/plant-science-research/docs/climate-changeair-quality-laboratory/ozone-effects-on-plants/">causes more plant damage than all other air pollutants combined</a>.<br />
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Ozone pollution levels peak in the late afternoon, when solar UV radiation and therefore photochemical reactions peak. Ozone is much more likely to be a hazard near large cities and factories on long sunny days in calm air, rather than during short winter days, even though general air pollution levels might be similar or even higher as during a <a href="https://en.wikipedia.org/wiki/Inversion_(meteorology)">temperature inversion</a>.<br />
<br />
Ozone Is An Oxidation Threat To Our Bodies<br />
<br />
Ozone is a toxic gas. That being said, we breathe in a tiny amount of it every day. In fresh unpolluted air at sea level, natural ozone makes up about 10-15 parts per billion (ppb) which means that every 15 billion air molecules will include an ozone molecule, on average. Our lungs breathe it in, handling it without noticeable damage. Highly polluted stagnant air, however, can contain more than 125 ppb ozone. Exposure to this much ozone over several hours or days can significantly harm humans and other animals. Long-term exposure essentially <a href="https://earthobservatory.nasa.gov/Features/OzoneWeBreathe">causes premature aging in our lungs</a>. It can inflame lung tissues, cause throat irritation and shortness of breath, increase one's susceptibility to respiratory infections and it can aggravate asthma and COPD (chronic obstructive pulmonary disease). Ozone reacts with both the epithelial cells of the respiratory tract and with the molecules in the fluid that coats the tract, creating a variety of free radicals and other oxidant molecules that damage epithelial cells by causing oxidative stress. An enzyme released from the cytoplasm (cellular fluid) leaked from damaged epithelial cells attracts inflammatory cells, leading to reddening and swelling of the respiratory tract. This can in turn <a href="https://www.epa.gov/ozone-pollution-and-your-patients-health/health-effects-ozone-general-population">lead to difficulty breathing</a>. Ozone also stimulates special nerve cells that exist in between the epithelial cells lining the respiratory tract. This stimulation causes the respiratory pathways to constrict. It also induces coughing and a reflex that reduces one's ability to inhale fully. All of these respiratory effects of ozone are reasons why it is a good idea to avoid strenuous activity outdoors during an air quality alert, even when you are healthy and especially when you already suffer from a respiratory problem. You can expect high ozone levels whenever pollution levels from combustion engines or industry are high or when you live downwind in the smoky hazy air blowing in from forest fires. Although individuals vary widely in their sensitivity to ozone, most of us recover completely from short-term exposure that lasts a few hours or less. Our respiratory tissues repair themselves quickly and they usually recover completely in about 48 hours.<br />
<br />
While environmental ozone is a potent health threat, some cells in our bodies have actually evolved ways <a href="https://www.ncbi.nlm.nih.gov/pubmed/12434011">to use it and other similar oxidizing molecules to their benefit</a>. For example, during an infection, activated white blood cells, called <a href="https://en.wikipedia.org/wiki/Neutrophil">neutrophils</a>, produce ozone and ozone-like oxidizing molecules. These potent oxidizers kill the bacteria invading our system by using a process that is sometimes called an <a href="https://en.wikipedia.org/wiki/Respiratory_burst">oxidative burst</a>. How it all works <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3109634/">is still quite mysterious</a> because, for one thing, it is difficult to examine what happens chemically during a process that occurs very rapidly inside living cells. Ozone and ozone-like radicals appear to be used in the creation of deadly nitric acid that is stored up inside tiny intercellular sacks called <a href="https://en.wikipedia.org/wiki/Phagocyte">phagocytes</a>. Phagocytes are like the trash compactor units of the cell. A phagocyte will engulf a bacterium into a nitric acid bath that denatures its DNA, killing it. It is also possible that the radicals themselves directly destroy the engulfed bacterial DNA.<br />
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Tropospheric Ozone Damages Plants<br />
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Plants species vary in their sensitivity to ozone but when ground level ozone exceeds 80 ppb for over four hours, <a href="https://www.bartlett.com/resources/Ozone-Injury-to-Landscape-Plants.pdf">plant damage can generally occur.</a> This is the same ozone concentration that prompts human health advisories in Canada, so when we are at risk so are many of our plants. You will see broadleaf damage first show up as clusters of tiny reddish or purple dots in between the veins of leaves that are most directly exposed to sunlight.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiP3KKU4ZVbOLhiP1EEVi3a0bUBQHkc64HA8TbCuEd-cPssDgUs2OZRZkbVnFksHQeysWIBNh9zcHiH1fhfRkCES1DGpJ3_NFz1zTViBgTKPJh017msiJihO4IKuEh1RUvLnGmyAKSd7r03/s1600/Alder_showing_ozone_discolouration.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="240" data-original-width="350" height="436" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiP3KKU4ZVbOLhiP1EEVi3a0bUBQHkc64HA8TbCuEd-cPssDgUs2OZRZkbVnFksHQeysWIBNh9zcHiH1fhfRkCES1DGpJ3_NFz1zTViBgTKPJh017msiJihO4IKuEh1RUvLnGmyAKSd7r03/s640/Alder_showing_ozone_discolouration.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Stippling on a red alder leaf caused by ozone pollution. Pat Temple, U.S. Forestry Service; Wikipedia</td></tr>
</tbody></table>
Often, leaves subjected to accumulating long-term exposure eventually turn autumn-like colours or brown prematurely and drop off. They basically succumb to oxidative stress. Plants look like they do at the end of their season, which makes it difficult to distinguish ozone damage that occurs in late summer. Plants already under stress, such as from drought for example, show more pronounced damage.<br />
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Here in Alberta, damage is first noticed on sensitive plants such as blackberries, ash trees and big-leaf lindens rather than more tolerant trees like spruce, pine and birch trees. Generally, the leaves on sensitive plants have more and/or larger <a href="https://simple.wikipedia.org/wiki/Stomata">stomata</a>, pores that open to allow the plant to transpire (exchange gases) so they allow more ozone in. Ozone enters leaves like other gases do, through the numerous stomata. Once inside the leaf, ozone dissolves in the water inside the plant and reacts with other chemicals. It is a powerful oxidant <a href="https://www.ncbi.nlm.nih.gov/pubmed/19778374">that damages the photosynthetic apparatus</a> inside the leaf. Once this damage happens, carbon dioxide levels begin to rise inside the leaf because it is not being consumed in photosynthesis. This stimulates the leaf to close its stomata, which further reduces photosynthesis. The plant, as a result, can longer make sugars effectively to maintain its health. There is evidence that leaves higher in antioxidants such as vitamin C <a href="https://asknature.org/strategy/apoplasts-detoxify-ozone-o3-inside-leaves/#.W6vb_C8ZOQ4">have some resistance to ozone damage</a>. By reacting with ascorbate (Vitamin C) in the watery cytoplasm inside the plant leaf cell, ozone is transformed into a variety of nontoxic products that the cell can handle.<br />
<br />
Ozone damage to our global food supply is significant. A 2011 article suggests that global yield from three ozone-sensitive crops - wheat, soybean and maize - <a href="https://www.sciencedirect.com/science/article/pii/S1352231011000070">could be reduced by between 17% and 26% by 2030</a> based on projected upper and lower estimates of carbon-based emissions by the IPCC (Intergovernmental Panel on Climate Change). An effective way to reduce ozone crop loss is to move toward non-combustion green technologies in vehicles and in industry.<br />
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Ozone Threat is Close To Home<br />
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We might think of large international cities when we think of the threat of ozone pollution but the problem of ozone (and all air pollution) sits close to home. Alberta, famous for its clear blue skies, is also known for its oil, natural gas and coal production, and for its vast agricultural lands, all of which contribute to significant air pollution and, therefore, ozone pollution. In Alberta, we can expect higher ozone pollution downwind of our two major cities, Edmonton and Calgary, based on contributions of primary pollutants from vehicle exhaust and industries within city limits. However, ozone pollution can also be expected downwind of the oil sands in northern Alberta. The oil sands pump out <a href="https://www.theglobeandmail.com/news/national/oil-sands-found-to-be-a-leading-source-of-air-pollution-in-north-america/article30151841/">between 45 and 84 tonnes of organic aerosols per day</a>, a level comparable to that produced by the entire Toronto metropolitan area (about 67 tonnes per day). <a href="https://www.epa.gov/air-research/secondary-organic-aerosol-soas-research">Organic aerosols</a> are a poorly understood highly complex series of air pollutants, many of which interact with sunlight to create additional secondary pollutants, and which make up most of the fine particulate matter in air pollution.<br />
<br />
Perhaps surprising is the fact that the smaller Alberta city of Red Deer has the worst air quality in Canada, according to <a href="https://www.cbc.ca/news/canada/edmonton/alberta-on-track-to-have-worst-air-quality-in-canada-1.3221336">numerous reports that came out in 2015</a>. Current studies are still being done to figure out what the pollution consists of and where it comes from but the results so far <a href="https://www.reddeeradvocate.com/news/initiatives-started-to-improve-air-quality-in-red-deer-area/">seem to focus on two culprits</a> - nitrogen dioxide and volatile organic compounds, compounds associated with industry and key ingredients of ozone production. Contributing to the problem is the fact that Red Deer sits in a bowl between river valleys, where air can sit and stagnate, and on hot summer days, one could expect high ozone levels as well. Finally, if British Columbia continues to suffer from devastating wild fires every summer (the last two summers were record-breaking), Alberta will be blanketed by the smoke and haze as most wind flow is from west to east here. Fires tend to coincide with hot dry sunny weather, so Alberta will also suffer from seasonal ozone pollution.<br />
<br />
Ozone is a fascinating Jekyll and Hyde type of molecule. Understanding how ozone works means understanding how chemical reactions work as well as how energy affects molecular interactions. The complex machinery inside our cells can make use of the redox chemistry that utilizes various oxygen allotropes but at the same time all living cells must protect themselves from the powerful oxidative activity of these same molecules. Ozone, originating from oxygen photosynthesized by plants, protects all life from deadly solar UV radiation. Yet, when it makes direct contact with the cells of life, it is a poison. An appreciation of the dual nature of ozone paves the way to an introduction of three challenging branches of chemistry: atmospheric and radical chemistry and biochemistry. It highlights how intimately these different branches are linked.Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-86720531904937383292018-02-17T12:23:00.000-07:002018-02-17T12:23:33.782-07:00The FluThis winter (2017/2018) has been a particularly bad flu season around the world, and as of mid-February, <a href="https://www.npr.org/sections/health-shots/2018/02/09/584537788/spot-shortages-of-antiviral-drugs-seen-as-flu-season-drags-on">it continues to get worse in the United States</a>. One in ten deaths last week in the US were caused by either the flu or from pneumonia, a complication from the flu. As of February 1st, it was widespread across Canada and the United States and there are serious widespread outbreaks in Japan, the Koreas and in Europe. More than one strain of influenza appears to be responsible for these outbreaks. Here in Canada and in the US, the main culprits appear to be a strain of H3N2 as well as type B influenza, <a href="http://www.cbc.ca/news/canada/kitchener-waterloo/flu-shot-confusion-mcmaster-influenza-infectious-disease-1.4533092">both of which are spreading at the same time</a>. The current season's flu vaccine is estimated to be about 55% effective against type B influenza but only about 15% effective against the H3N2 type A virus. <br />
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What Is the Flu (Influenza)?<br />
<br />
Both the <a href="https://en.wikipedia.org/wiki/Common_cold">common cold</a> and the <a href="https://en.wikipedia.org/wiki/Influenza">flu</a> are caused and spread by <a href="https://en.wikipedia.org/wiki/Introduction_to_viruses">viruses</a>. Sometimes it is hard to tell the difference between flu and cold symptoms, especially if the flu is mild. According to webmd.com, with either the flu or a cold, you typically feel congested. You have a sore throat and tend to sneeze. These symptoms as well as headache, coughing and chest discomfort are common to both the flu and the cold. If you also have a high fever and experience extreme fatigue and muscular weakness, you more likely have the flu. Young children might also suffer from vomiting as well, according to the <a href="https://www.cdc.gov/flu/index.htm">CDC webpage on influenza</a>. Another aspect that sets the flu apart is its symptoms tend to come on very rapidly. You feel hit by a bus. Neither virus is any fun but the flu is the one that tends to put us out of commission for 1-2 weeks, in bed, almost unable to get up. It can also be deadly, especially for those of us in high-risk groups, which I will detail. <br />
<br />
While an ordinary bout of the flu is generally just awful, the flu can also become dangerous when complications develop as a result of the original viral infection of the upper respiratory tract. Anybody can become severely ill with the flu but most often complications from the flu strike the very young, the elderly, people suffering from chronic medical conditions and pregnant women. These complications can range from sinus or ear infections to pneumonia or inflammation of the heart or brain or muscle tissues. The latter complications can be life-threatening, and they obviously require hospitalized care. Severe <a href="https://www.webmd.com/arthritis/about-inflammation#1">inflammation</a> of body tissues can be very dangerous, possibly leading to <a href="https://en.wikipedia.org/wiki/Multiple_organ_dysfunction_syndrome">multi-organ dysfunction syndrome</a>. In rare cases, the body's immune reaction to the virus rather than the virus itself can trigger an inflammatory response so severe that it leads to <a href="https://en.wikipedia.org/wiki/Sepsis">sepsis</a>, which can rapidly (within hours) lead to death. It is difficult to square the fact that influenza, an annoying illness that always seems to wreck havoc with Christmas plans, is also the same virus that killed about 50 million people in 1918, some of them in matter of a day or two after contact with the virus.<br />
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This 4-minute National Geographic video offers a rather-sobering primer on how the influenza virus attacks:<br />
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<iframe allowfullscreen="" frameborder="0" height="360" scrolling="no" src="//assets.nationalgeographic.com/modules-video/latest/assets/ngsEmbeddedVideo.html?guid=ebdb6313-3c1b-42bf-a9de-b9ca8ca88a5d" width="640"></iframe>
<br />
<br />
I hope that this article will offer you the power of knowledge against this common and nasty virus. I found this to be true for myself, after doing the background research. There is hope that influenza will be eradicated once and for all, like small pox was and polio will soon be.<br />
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Flu Treatment<br />
<br />
Antibiotics do not treat influenza or the cold, which are both viral infections. However, antibiotics can be used to treat flu complications such as <a href="https://en.wikipedia.org/wiki/Bacterial_pneumonia">bacterial pneumonia</a>, an ear infection (if it is caused by bacteria; <a href="https://www.sciencedaily.com/releases/2006/11/061106164651.htm">about half are viral</a> and sinusitis (if it is bacterial, <a href="https://health.clevelandclinic.org/2017/01/killer-sinus-infection-how-to-tell-if-yours-is-viral-or-bacterial/">most sinus infections are viral</a>).<br />
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All of the over-the-counter medications you find in pharmacies treat the symptoms of cold and flu viral infections rather then the virus itself. There is no treatment for the cold virus, but there are a few <a href="https://www.cdc.gov/flu/antivirals/whatyoushould.htm">antiviral medications</a> that can treat influenza, such as oral oseltamivir (Tamiflu), inhaled zanamivir (Relenza), or the intravenous drug peramivir (Rapivab). There are three main reasons why doctors don't tend to use these antivirals for a common bout of the flu without complications. First, even when they are started at the first appearance of symptoms, they tend to shorten the duration if the flu by only a day or two. Second, these drugs tend to cause nausea, vomiting and/or diarrhea (which may not be worth the trade-off), and they can interfere with other medications. Third, a concern more of the cost to the health system, is the fact that these drugs are expensive. A single adult course of <a href="https://www.drugs.com/tamiflu.html">Tamiflu</a> (75 mg twice a day for 5 days) costs about $100 in Canada. These antiviral drugs are, however, useful to treat those of us who are at risk of serious flu complications and those of us who have weakened immune systems. They are obtained through a doctor's prescription (in Canda).<br />
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What To Do When You Get The Flu<br />
<br />
Your body is busy fighting a battle that requires energy so it's best to give yourself a rest. Stay home. This will do not only yourself a favour but those around you as well by preventing its spread to others. Don't visit anyone in the hospital for this reason. Stay in bed or on the couch in a comfy blanket to ease those pesky chills. A fever (and the accompanying sweats) dries you out so drink plenty of fluids. Try to avoid the caffeine that might prevent you from napping and opt for herbal teas instead. Ginger teas help reduce nausea. If you can eat, do so but try to fuel your body with healthy choices. Homemade soups (no preservatives and usually much lower in salt) do triple duty by providing nutrition/fuel, hydration and comfort. Of course you don't want to cook when you're sick so it's not a bad idea to have a few containers made up in your freezer during flu season.<br />
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Do not work out, especially if you have a fever, any dizziness, a hacking cough or body aches. While regular day-to-day moderate exercise strengthens your immune system and offers increased protection against colds and flus, once you do get the flu, it is best to stop your routine especially if you have a moderate to severe case of flu. Intense exercise causes the body to release hormones, such as cortisol and adrenaline, both of which temporarily suppress the immune system. You don't want to fight against your body's natural defenses. A shift to gentle yoga (at home, not at the studio where you can spread it) and/or walking during a very mild flu not only can boost your mood but it eases some of the muscle stiffness associated with the flu, it helps stimulate the appetite, and it helps work up phlegm, easing breathing. If you have trouble getting out of bed, stay in bed and don't feel guilty (easier said than done especially for moms).<br />
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Over-The-Counter Flu Medications: Take With Caution<br />
<br />
Taking an over-the-counter remedy can also ease symptoms. Be careful to read the labels, especially for dosage and side affects. It is all too easy to unknowingly double or triple the dose of a powerful drug by combining different medications such as a pill with a flu drink, for example. Many flu remedies contain <a href="https://www.canada.ca/en/health-canada/services/drugs-medical-devices/acetaminophen.html">acetaminophen</a>, which treats pain and fever. An overdose of this drug can lead to liver damage or even acute liver failure.<br />
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It might be tempting to make up a hot toddy or some other boozy drink to deal with flu symptoms. There is some old mythology out there that alcohol sterilizes the virus somehow. When alcohol enters the body, it quickly enters the bloodstream (within minutes). It is then detoxified in the liver and excreted through the lungs, kidneys and in sweat, over a period of hours. So, yes, you will temporarily have alcohol in your blood but recall that even at the legal limit (here in Alberta) only 0.08% of your blood consists of alcohol, hardly a sterilizing strength. Alcohol dehydrates the body and it <a href="https://pubs.niaaa.nih.gov/publications/arh26-4/257-263.htm">has been shown to weaken the immune response if several drinks are consumed</a>.<br />
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Be warned, alcohol exacerbates the liver-damaging effects of acetaminophen. The evidence <a href="https://www.webmd.com/mental-health/addiction/news/20131104/tylenol-and-alcohol-a-bad-mix-study-suggests#1">is preliminary but disturbing</a>: combining even a small or moderate drink with a regular dose of acetaminophen can damage your liver. However, some alcoholic drinks such as wine appear to offer the body some immunoprotective antioxidants, a benefit, mind you, that is undone by consuming more than one drink. A glass of your favourite wine, if you can stomach it, can be a good strategy in the evening before sleep but NOT if you are going to take a nighttime flu medication.<br />
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Alcohol in the form of alcohol-based (70% alcohol) hand sanitizers and even ordinary hand soap <a href="https://academic.oup.com/cid/article/48/3/285/304169">are very effective in removing the virus from our hands</a>. This might be the most effective strategy of all to avoid getting the flu in the first place, and to avoid infecting others around you when you have it.<br />
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Keep in mind that <a href="https://en.wikipedia.org/wiki/Antihistamine">antihistamines</a> (for runny or itchy nose and sneezing) can make you drowsy so avoid driving. <a href="https://en.wikipedia.org/wiki/Decongestant">Decongestants</a> should be used with caution if you have hypertension (high blood pressure). These drugs enhance adrenaline and adrenaline-like hormones in your body, which restrict blood vessels in your nose and throat, reducing swelling and mucus formation. Vessel restriction also increases your blood pressure, which is a concern if you have heart disease or hypertension. There is new evidence as well that having the flu <a href="https://www.sciencedaily.com/releases/2018/01/180124172422.htm">increases the risk of a cardiovascular event such as a heart attack</a>, an additional reason to treat decongestants with caution. Finally, it appears that over-the-counter decongestant nasal sprays <a href="https://www.medicalnewstoday.com/releases/27323.php">can be physically addictive</a>. Keep in mind too that the adrenaline-like action of a decongestant can keep you from sleeping. That is why this ingredient is left out of nighttime store remedies.<br />
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A Few Safe Flu Remedies<br />
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Here are some safe flu remedies to consider. Nasal irrigation using saline solution (a <a href="https://www.webmd.com/allergies/neti-pots#1">neti pot</a>) is a natural and proven method to ease stuffiness. Over-the-counter lozenges, especially those that contain menthol (found in peppermint, eucalyptus and other mints) act to coat and soothe a sore itchy throat and may reduce coughing. A humidifier in the room can help you breathe more easily, especially at night. An old-fashioned saltwater gargle also helps (1/4 to 1/2 teaspoon salt in one cup of warm water) to relieve symptoms and <a href="https://www.webmd.com/cold-and-flu/features/does-gargling-wlth-salt-water-ease-a-sore-throat#1">it might even help flush out the virus</a>.<br />
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There is much literature online touting the use of <a href="https://en.wikipedia.org/wiki/Vitamin_C">vitamin C</a> supplements to boost the immune system and to prevent and treat infections, particularly the common cold. The scientific literature is conflicting. A large analysis of previous scientific studies <a href="http://www.cochrane.org/CD000980/ARI_vitamin-c-for-preventing-and-treating-the-common-cold">done by the Cochrane Database of Systematic Reviews in 2013</a> reveals that those who experience extreme physical stress such as exertion or exposure to cold and who may be deficient in vitamin C can reduce their incidence of colds by about half if they take a daily supplement. Taking a daily supplement appeared in some studies to slightly shorten the duration of colds among adults and children. Like the glass of wine mentioned earlier, vitamin C is an antioxidant. <a href="https://en.wikipedia.org/wiki/Antioxidant">Antioxidants</a> protect cells against damage from <a href="https://en.wikipedia.org/wiki/Radical_(chemistry)">free radicals</a>. Both are present naturally in the body. Many foods contain high levels of antioxidants and many dieticians recommend a diet high in citrus fruits, berries and vegetables, which will supply more than enough vitamin C. However, some people believe that taking large daily doses of vitamin C (such as 1000 mg) is beneficial. It is <i>generally</i> harmless to take even large doses of vitamin C (under 2000 mg daily) because the excess is rapidly flushed out of the body by the kidneys. However, there are <a href="https://www.umm.edu/health/medical/altmed/supplement-interaction/possible-interactions-with-vitamin-c-ascorbic-acid">a number of possible interactions to be aware of</a>. Of particular concern for us are some studies that suggest that a large dose of vitamin C can lower the rate at which acetaminophen is passed from the body in the urine, which means that vitamin C may a dangerous mix with many over-the-counter flu remedies, and even more so if you consume any alcohol as well.<br />
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All this cautionary advice might encourage you to seek an herbal flu remedy. <a href="https://www.webmd.com/cold-and-flu/over-the-counter-flu-remedies#1">According to webmd.com</a>, there is no hard scientific proof that any herbal remedies work against the flu. Beware that many herbal remedies contain active ingredients and the strength varies from product to product. You should always tell your doctor which herbal remedies you are taking (at any time) because they can interact with prescription medications making them either less effective or too effective.<br />
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When to Call The Doctor<br />
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This is advice for adults. For children, click <a href="http://caringforkids.cps.ca/">caringforkids.cps.ca</a> for <a href="https://www.caringforkids.cps.ca/handouts/influenza_in_children">an excellent page on advice</a>. It is published by the Canadian Pediatric Society. Often, a typical run-in with the flu passes in one or two weeks and you don't need to contact your doctor, as long as you don't have symptoms of flu complications as described earlier. Stay home, rest and save others from your nasty germs. But, influenza can quickly turn dangerous, so it is important to monitor your symptoms and contact your doctor if:<br />
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1) You have a very high fever, over 39.4°C (103°F), or a moderate fever that doesn't go down after 3 days<br />
2) You feel unusually short of breath<br />
3) You start to cough up yellow, green or brown sputum<br />
4) You experience a sharp pain in your chest when you breathe in<br />
5) You have a severe ear ache<br />
6) You feel light-headed or faint<br />
7) You have any serious chronic disease (heart, lung, kidney disease or diabetes or you are on immunosuppressant drugs)<br />
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(These guidelines are from <a href="https://www.health.harvard.edu/staying-healthy/when-to-contact-your-doctor-about-flu-symptoms">Harvard Medical School's site</a>)<br />
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A Further Caution: Know Where Your Online Information Comes From<br />
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There are now some excellent online reference websites for you to explore for information on influenza and other diseases and conditions. However, when researching health-related information online, one should check to see if the website offers accurate information backed up by peer-reviewed research (there should be links to scientific research papers). This is a task that is difficult for anyone not in a medical field so always trust the advice of your doctor, nurse or nurse practitioner first. These professionals are the real experts and they have your best interest in mind. Most informational websites, even some put out by medical schools and universities, are for-profit. This means that they make money by encouraging readers to buy certain healthcare products or drugs. Even medical advice from webmd.com, one of the top healthcare websites in the world, and a website accredited by a Washington-based non-profit accrediting organization, should be taken as a supplement to doctor's advice. It is owned by a private equity company and it is publicly traded, which means that it is obligated to its shareholders to make a profit, partly from advertising and partly from sponsorship from private drug companies. The New York Observer and the New York Times each published articles critical of webmd's reliance on drug company sponsors and how those sponsors influence content (see the reference section on its Wikipedia page <a href="https://en.wikipedia.org/wiki/WebMD#cite_note-19">here</a>).<br />
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Types of Flu and How it is Transmitted<br />
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Influenza is transmitted by an extremely tiny microscopic particle called a virion. Each spherical or oblong virion is about 100 nanometres wide. To put this in perspective, a human hair is about 100,000 nanamotres wide. A microscopic image of several flu virions is shown below.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg_v-e7wVN18FWDI5-jXGek5ZmAgozuUHoca3g7pxFyLAfdZF6U0SBkH1pKiN8JIGggAYWf37CTIg0C80pA_7Upww_AxY7hGhvG0jwJ0G8EKACXt7l-9T6ekvZjhU3LoEvanrWIr11j2Tfn/s1600/EM_of_influenza_virus.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="743" data-original-width="700" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg_v-e7wVN18FWDI5-jXGek5ZmAgozuUHoca3g7pxFyLAfdZF6U0SBkH1pKiN8JIGggAYWf37CTIg0C80pA_7Upww_AxY7hGhvG0jwJ0G8EKACXt7l-9T6ekvZjhU3LoEvanrWIr11j2Tfn/s400/EM_of_influenza_virus.jpg" width="376" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">CDC/Dr. Terrence Tumpey;Wikipedia</span></span></td></tr>
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Like the cold virus, the flu virus is an <a href="https://en.wikipedia.org/wiki/RNA_virus">RNA virus</a>. This means that its genetic material is composed of <a href="https://en.wikipedia.org/wiki/RNA">RNA (ribonucleic acid)</a> rather than <a href="https://en.wikipedia.org/wiki/DNA">DNA (deoxyribonucleic acid)</a>. Human cells contain both RNA and DNA, following a general rule in genetics that DNA makes RNA that makes proteins. The influenza virus is an infectious agent that replicates only inside the living cells of organisms. It is composed of a strand of RNA (genetic code) housed inside a glycoprotein (a protein that has a carbohydrate attached to it) coat.<br />
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You may have heard about how fast flu viruses <a href="https://en.wikipedia.org/wiki/Mutation">mutate</a>. By the time a new vaccine is formulated, one or more of the target viruses may have already mutated into a different form, making that vaccine less effective or even ineffective. Their RNA is the reason flu viruses can do this. Influenza viral genomes, as a group, have the highest mutation rates of any genome. Compared to DNA viruses (they cause cold, warts, herpes, chicken pox, etc.), RNA viruses tend to have higher mutation rates, and single-stranded RNA viruses (such as flu viruses) have the highest mutation rates of all. Within DNA viruses, <a href="https://en.wikipedia.org/wiki/RNA_polymerase">DNA-directed RNA polymerase</a> (part of the cell's RNA-making machinery) can proofread and fix code errors in newly replicated RNA. <a href="https://en.wikipedia.org/wiki/RNA-dependent_RNA_polymerase">RNA polymerases in RNA viruses</a> lack this proofreading step. A lack of genetic proofreading would lead to life-threatening cancers in complex organisms like us but it is actually a boon to the flu virus. It allows constantly occurring minor <a href="https://en.wikipedia.org/wiki/Point_mutation">point mutations</a> in the genetic code to make coat proteins that subtly but constantly change, enough to fool antibodies, like a thief choosing a new mask all the time. This high mutation rate also allows it to stay one or more steps ahead of virologists desperately trying to predict each year's new vaccine cocktail.<br />
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There are three types of flu virus: Type A, Type B and Type C. These three types, or genera as they are called taxonomically, can cause influenza in many different classes of vertebrates, including humans and other mammals such as pigs, dogs, seals birds, etc. Influenza A is of most concern. This type of flu virus mutates the fastest and it is the only type associated with past flu epidemics, including the devastating <a href="https://en.wikipedia.org/wiki/1918_flu_pandemic">1918 Spanish flu pandemic</a>, the <a href="https://en.wikipedia.org/wiki/2009_flu_pandemic">2009 swine flu pandemic</a>, the <a href="https://en.wikipedia.org/wiki/Influenza_A_virus_subtype_H2N2">Asian flu of the 1950's</a> and various <a href="https://en.wikipedia.org/wiki/Avian_influenza">bird flu </a>outbreaks. Some of <a href="http://www.clinlabnavigator.com/influenza-virus-subtypes.html">subtypes</a> of influenza A, such as <a href="https://en.wikipedia.org/wiki/Influenza_A_virus_subtype_H1N1">H1N1</a>, can be highly pathogenic and/or highly virulent, which means they have a high ability to cause disease and they have a high ability to infect a host, respectively. In other words, they spread quickly and they have a high mortality rate. Not all viruses of subtype H1N1 are so dangerous. Some strains of this subtype cause only mild seasonal flu. Virus phylogeny consists of type, divided into subtypes, which are further divided into strains.<br />
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The H1N1subtype is of particular interest to virologists. It is one of three subtypes that are always part of the flu vaccine cocktail. It was responsible for the deadly "Spanish" 1918 flu and for the 2009 "swine" flu pandemics, as well as others. There is currently (January 2018) a deadly epidemic of a <a href="https://www.dawn.com/news/1383308">new H1N1 strain in Pakistan</a> that mutated from the sine flu. An epidemic is an outbreak of disease that attacks many people at the same time in the same general location. A pandemic occurs when an epidemic spreads throughout the world.<br />
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Other strains of the H1N1 subtype are commonly found in small numbers during every annual flu season. Each subtype comes in numerous different and always-evolving variants or strains. One strain of H1N1 might produce an average short-lived isolated flu outbreak. Another might only infect pigs and not humans at all, while a third could be as dangerous at the 1918 Type A H1N1 strain.<br />
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<a href="https://en.wikipedia.org/wiki/Influenzavirus_B">Type B influenza</a> only infects humans and seals. Fewer hosts (fewer animal reservoirs of the virus) and a mutation rate that is 2 to 3 times lower than Type A influenza means that Type B is less dangerous. Unlike influenza A, Type B is broken down directly into strains and lineages rather than subtypes. There are only two lineages currently in circulation in the world. Each year's flu vaccine contains Type B virus. Type B flu virus can cause flu epidemics as well, but sufferers tend to have less severe flu symptoms than those from Type A viruses. <a href="https://en.wikipedia.org/wiki/Influenzavirus_C">Type C influenza</a> infects humans and pigs. Outbreaks of type C are rare and they tend to cause only mild flu symptoms but there have been local epidemics. This type of virus is more difficult to isolate and study so much less is known about it than the other two types. The good news is that by the time we are 10 years old, most of us have been exposed to type C flu and have antibodies against it. It is the slowest virus type to evolve and it doesn't present a serious threat to humans.<br />
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Influenza Virion Structure<br />
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The influenza virus is crafty. It probably <a href="https://en.wikipedia.org/wiki/Evolution_of_influenza">evolved for many millennia</a> infecting humans and various animals, spreading back and forth between these <a href="https://en.wikipedia.org/wiki/Vector_(epidemiology)">vectors</a>, although the first reliable evidence of an influenza outbreak <a href="https://en.wikipedia.org/wiki/Timeline_of_influenza">was a pandemic in Asia</a><a href="https://en.wikipedia.org/wiki/Timeline_of_influenza">, Africa and Europe</a>, recorded in 1580. Technically the virus is not a living organism because it needs a living host to survive and reproduce, but has evolved many strategies to carry on its progeny from one host to the next over the millennia, adapting to new hosts and changing conditions during the process.<br />
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The diagram below helps to explain how a particular virion is labeled (for example, H1N1). A Fujian flu virus (a type A virus) is used as the example. Types A, B and C flu viruses are structurally and compositionally very similar to one another.<br />
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: left; margin-right: 1em; text-align: left;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjQ19FyEZKv_7f3pD2BscK3OU5qIXCNl7dSJoNbbjiuKiI4xdWH-010EACITARWF6lxuVqQhiybQLy7yWNEBNeIbq2GfUgjmMZCXPO_FohDXMJt2ZR9cUftRc-38OjlLg9d3yRvp0Np8-Cj/s1600/590px-Influenza_nomenclature.svg.png" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="380" data-original-width="590" height="257" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjQ19FyEZKv_7f3pD2BscK3OU5qIXCNl7dSJoNbbjiuKiI4xdWH-010EACITARWF6lxuVqQhiybQLy7yWNEBNeIbq2GfUgjmMZCXPO_FohDXMJt2ZR9cUftRc-38OjlLg9d3yRvp0Np8-Cj/s400/590px-Influenza_nomenclature.svg.png" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Burschik;Wikipedia</span></span></td></tr>
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The squiggly purple lines inside the circle represent enclosed RNA genetic material. The glycoprotein coat is shown in red. In flu viruses, this coat is composed of two large glycoproteins: hemagglutinin (the small red "lollipop" structures, H) and neuraminidase (the rectangle-shaped structures, N. The arrow is a bit off.). <a href="https://en.wikipedia.org/wiki/Hemagglutinin_(influenza)">Hemagglutinin</a>, denoted as H left but elsewhere in this article shortened to HA, allows the virus to recognize and bind to its target cell. <a href="https://en.wikipedia.org/wiki/Viral_neuraminidase">Neuraminidase</a> (N or NA) enables new viruses made inside the target cell (or host cell) to be released. Both HA and NA are viral sites that antiviral drugs target. HA and NA are also <a href="https://en.wikipedia.org/wiki/Antigen">antigens</a> that our <a href="https://en.wikipedia.org/wiki/Antibody">antibodie</a>s target during an immune response to the virus. Each antibody made by our immune system targets a specific antigen, like a lock and key mechanism, and binds to it (shown below left). Different antibodies have many different functions. Those that attack flu and other viruses usually block part of the (virus's) antigen's surface, rendering it ineffective.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Fvasconcellos;Wikipedia</span></span></td></tr>
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Two glycoproteins, HA and NA, distinguish which subtype the virus is. H3N2 is another particular subtype of flu virus Type A. The flu vaccine almost always contains a strain of this subtype as well.<br />
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HA molecules on the surface of the flu virus envelope identify and bind to corresponding receptor sites on the membrane of an epithelial cell in the host's respiratory system. Once attached, the viral envelope fuses with the host cell membrane. The viral RNA genome then enters the host cell and commandeers its RNA-making and protein-making machinery to make new virus proteins and RNA. This process gradually weakens or kills the host cell while it sheds multitudes of new viruses into the respiratory system. See the diagram below showing how a virus attaches to and enters a host cell and how it uses the host cell's machinery to make new viruses that bud off the host cell to infect new host cells.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi-tVDfau3PXxkauHwKL7GIliJDsNi7zRstGq_jQkZv-pVhrhOajcPO883o8gZikAyr_jTOYf3J1jyFnLzeSgW7zNP8mz3vR7cUS_QAcYsj46Hbs94Rb8fSm38riNfrDnQnhoODYz-HqARc/s1600/925px-Virus_Replication_large.svg.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="852" data-original-width="925" height="588" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi-tVDfau3PXxkauHwKL7GIliJDsNi7zRstGq_jQkZv-pVhrhOajcPO883o8gZikAyr_jTOYf3J1jyFnLzeSgW7zNP8mz3vR7cUS_QAcYsj46Hbs94Rb8fSm38riNfrDnQnhoODYz-HqARc/s640/925px-Virus_Replication_large.svg.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">User:YK Times;Wikipedia</span></span></td></tr>
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Glycoprotein coat-making machinery is on the left in the cell and RNA-making machinery is in the center, inside the cell nucleus. Steps 1 through 7 are described in detail <a href="https://commons.wikimedia.org/wiki/File:Virus_Replication.svg">here</a> (under "summary").<br />
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The flu virus can be transmitted in three main ways. First, transmission occurs when viruses within the saliva and mucus (such as in a sneeze by an infected person) land directly on a new victim's eyes, inside the nose or inside the mouth. This is direct transmission. It can also take an airborne route, where someone later inhales virus-laden air sneezed or coughed out by an infected person. Third, someone can pick up the virus by touching a surface that was infected by a sick person or through skin-skin contact such as shaking hands. The flu virus can live outside the body <a href="http://www.sciencefocus.com/qa/human-body/how-long-can-virus-live-outside-body">for up to 24 hours on a hard surface</a> and for <a href="https://en.wikipedia.org/wiki/Influenza#Transmission">more than a week in mucus</a>. The simple message to wash your hands often and well if you are sick and if you are around people who are sick is a very effective method to avoid the getting and spreading the flu. A single sneeze or cough can spray <a href="https://en.wikipedia.org/wiki/Influenza#Transmission">up to half a million viral particles into the air</a>. Two methods you can use to stop this transmission route are to sneeze or cough into the crook of your elbow or to sneeze or cough into a tissue, then throw that tissue into the garbage and then wash your hands.<br />
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Why Do I have These Symptoms?<br />
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During the first day or two after exposure to the virus, your immune system is already responding by churning out antibodies and <a href="https://en.wikipedia.org/wiki/T_cell">T cells</a> (the immune system's "soldiers"). If you got the flu shot and it matches the strain of flu you caught or if you've previously been exposed to this strain, your body already has a stockpile of antibodies. They provide an immune shortcut, a kind of a one-up on the virus. Antibodies will recognize that viral strain and stop it in its tracks, preventing illness. If your flu shot does not match but is similar to this strain or if you were already exposed to a similar but non-matching flu in your past, you likely still have a advantage; your flu will <a href="https://www.wired.com/2009/06/old-people-may-be-immune-to-swine-flu/">likely be milder than it would have been otherwise</a>.<br />
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By around day three or four after being infected with the flu virus, your immune system, good as it is, is no longer keeping up with the viral onslaught. You go from feeling normal to feeling like you've been hit by a train, often over a period of hours. You have a fever, chills, headache and all of your muscles feel like you just did some kind of beast race. The root cause of your exhaustion, fever, headache, chills and muscle aches is your immune system. It's gone into code red emergency mode, creating a <a href="http://primer.crohn.ie/the-inflammatory-response">body-wide inflammatory response</a>, with these unpleasant symptoms as side effects.<br />
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Your entire body is now in flu-fighting mode and that is why it is wise to rest and fuel it for the war it is waging against the viral invasion. Dead epithelial cell debris clogs up your breathing passageways - you develop a dry cough. Your throat is sore; your nose is red, itchy and runny. The flu essentially blows infected epithelial cells apart. These are the cells that normally protect your respiratory tract. The virus causes tissue damage, felt as soreness, swelling and inflammation. It is this damage to the lining of the respiratory tract, and the detritus left behind, that can set up the stage for complications from the flu such as a possible secondary bacterial infection such as bacterial bronchitis or bacterial pneumonia.<br />
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Although it isn't much of a concern for healthy adults infected with mild to moderate seasonal flu, you should seek medical help if you start to feel worse after you've already started feeling better. Bacterial pneumonia comes on slower than flu symptoms do. Serious chills, serious sweating, a high fever, trouble catching your breath, faster breathing and faster pulse are <a href="https://www.webmd.com/lung/understanding-pneumonia-symptoms#1">signs that you may have pneumonia</a> as a complication of the flu. Go to emergency because it is a potentially very dangerous and rapidly evolving situation.<br />
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It is going to take about a week before your immune system starts to get the upper hand. In the meantime, there is a risk that bacteria and other viruses can invade you in your weakened state. It usually takes about two weeks before you can confidently feel recovered, and during this whole time, your body continues to shed the flu virus, although at a constantly decreasing level after the first few days when viral shedding peaks. This means that you are contagious throughout the whole time you are sick, and you are particularly contagious even before your first symptoms.<br />
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How the Flu Virus Makes Us Sick (Its <a href="https://en.wikipedia.org/wiki/Pathophysiology">Pathophysiology</a>)<br />
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One factor that makes it hard to contain the transmission of flu is the simple fact that you are contagious before the symptoms hit you. The virus has entered your nose, throat and lungs. It is getting right to work taking over the cellular machinery in your epithelial cells lining these airways so that it can copy itself and multiply. At this point you are contagious but you don't know it yet. Every sneeze and cough hurls new viruses into the air, and onto surfaces around you, and onto your hands as you politely try to cover your mouth and nose. A deeply ingrained regular hand-washing habit can avoid having others around you come down with the flu too.<br />
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When a flu virion enters the respiratory tract, its hemagglutinin, the glycoprotein on its surface membrane, recognizes and binds to sialic acid-containing receptor proteins on the membranes of epithelial cells. Once the virus binds to the epithelial cell, the cell engulfs it as well as the bit of cell membrane that was stuck to it, to make an <a href="https://en.wikipedia.org/wiki/Endosome">endosome</a> inside the cell that is filled with the virus. An endosome is depicted in the right side of the cell in the cell diagram earlier. The cell then does what it is programmed to do when a foreign body enters it. It acidifies the inside of the endosome and then begins to digest the contents. However, the virus is quite ingenious. As soon as the pH falls below 6, the HA molecule partially unfolds, releasing a peptide that acts like a grappling hook. Then the HA molecule refolds into a new low-pH-stable structure. It uses the "grappling hook" peptide to come up to and fuse its endosome membrane with the epithelial cell's inner membrane. Once done, it spills its contents, including its RNA, into the cell's cytoplasm and gets to work using the cell's replication machinery.<br />
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There are at least 18 different subtypes of HA: H1 through H18. Several of these only infect specific animal species. H1, H2 and H3 are human viruses. Of each subtype there are numerous continually evolving strains. Antibodies made in the body usually attack the specific subtype/strain hemagglutinin that the virus presents on its surface. Hemagglutinin (HA) <a href="https://www.theglobeandmail.com/life/health-and-fitness/health/canadian-researchers-closer-to-making-universal-flu-vaccine-reality-health/article32952985/">is a lollipop-shaped structure</a>. It has a head, which binds to sialic acid on target epithelial cells. It also has a distinct stalk. The head structure changes subtly but continuously thanks to frequent mutations in the RNA coding for it. Most antibodies bind to the HA near its "lollipop head," preventing it from attaching to sialic acid receptors on the epithelial cells. To a lesser extent, antibodies are also made to recognize and attach to the stem part of the HA molecule instead. These antibodies stop the virus by inhibiting the membrane fusion machinery, most of which is located in the stem part. The stem will become important later on in this article.<br />
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Why Are Some Flu Epidemics So Deadly?<br />
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Each subtype of flu (such as H1N1, for example) can come in many different strains. A specific strain of H1N1 caused the deadly 1918 flu (<a href="https://www.ncbi.nlm.nih.gov/pubmed/11875246">about 50 million deaths</a>) pandemic while another strain of H1N1 is currently a mild seasonal flu. Some strains are <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4450310/">far more pathogenic than others</a>. Viruses that contain avian (bird) hemagglutinins, such as H1, H6, H7, H10 or H15, appear to cause low-pathogenicity illnesses in birds but when these particular genes for the HA glycoprotein cross over into human strains of flu, they can potentially do far more damage in human lungs and they can be <a href="https://www.sciencedaily.com/releases/2014/11/141118105412.htm">far more deadly than they are in birds</a>. Avian subtype flu viruses also seem to cause a far more intense inflammatory immune response in humans, a physiological response that, in itself, can be deadly. In a <a href="http://mbio.asm.org/content/5/6/e02116-14.full">2014 study by Li Qi et al</a>., mice (with respiratory systems and epithelial cell receptors very similar to humans) injected with H1, H6, H7, H10 or H15 avian HA viral subtypes rapidly lost weight and some died from primary viral pneumonia (pneumonia caused by the flu virus itself) within a week. Other (non-avian) subtypes (H2, H3, H5, H9, H11, H13, H14 and H16) caused no significant disease in the rodents.<br />
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The 1918 flu appears to have been one of these avian/human crossovers. Normally, the immune system is immediately activated after exposure to a mild seasonal flu virus. Cells of the <a href="https://en.wikipedia.org/wiki/Immune_system">immune system</a> (<a href="https://en.wikipedia.org/wiki/White_blood_cell">white blood cells</a>) such as <a href="https://en.wikipedia.org/wiki/Macrophage">macrophages</a>, <a href="https://en.wikipedia.org/wiki/Cytotoxic_T_cell">cytotoxic T cells</a> and <a href="https://en.wikipedia.org/wiki/Neutrophil">neutrophils</a> recognize, target and kill virus-infected cells. This 3-minute video animation describes how these and other immune cells carry out an immune response:<br />
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<iframe allow="autoplay; encrypted-media" allowfullscreen="" frameborder="0" height="360" src="https://www.youtube.com/embed/AucZlvEv29Y" width="640"></iframe>
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However, for <a href="https://en.wikipedia.org/wiki/Spanish_flu_research">not entirely understood reasons</a>, avian-like flu viruses, such as the 1918 epidemic virus, <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2605683/">stimulate an exaggerated immune response</a> that can be as damaging as the virus itself. Even healthy cells of the respiratory tract are targeted and killed, leading to life-threatening events such as <a href="https://en.wikipedia.org/wiki/Acute_respiratory_distress_syndrome">acute respiratory distress syndrome</a> and <a href="https://en.wikipedia.org/wiki/Multiple_organ_dysfunction_syndrome">multiple organ dysfunction syndrome</a>. These events can kill a person within hours of infection with the virus. The 1918 flu virus, and other avian-type flu viruses appear to target not just pulmonary epithelial cells but <a href="https://www.clinicalcorrelations.org/?p=1862">also the cells lining the alveoli deep in the lungs</a>. The body has epithelial receptors for this particular avian-like strain of HA glycoprotein not only in the nose, throat, and upper respiratory tract but the lower tract as well.<br />
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By attacking not only the throat and nose but also deep inside the lungs, infection with the 1918 viral strain led to serious consequences such as rapid fluid and dead cell detritus buildup in the lungs. Unlike mild to moderate seasonal outbreaks, the 1918 flu targeted young healthy 20-35 year olds (although illness rates were highest among school age kids). This flu was contagious <a href="https://wwwnc.cdc.gov/eid/article/18/2/10-2042_article">but not unusually so</a>. Still, those who were infected with it <a href="https://www.thoughtco.com/1918-spanish-flu-pandemic-1779224">suffered greatly</a>. Within hours some victims experienced intense fatigue, and a cough violent enough to tear abdominal muscles. They turned blue as they coughed up foamy blood and many victims suffocated to death within two to three days of getting sick. It is important to note, however, that most victims died later on, approximately a week after getting sick and they died from secondary bacterial pneumonia, rather than the deadly acute immune reaction just described.<br />
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The types of casualties revealed a puzzle. Previously healthy victims with robust immune systems died partly because their immune system turned against them. In most 40 year olds, the immune response begins to weaken and it is simply unable to match such deadly intensity, and in the very young the immune system is not yet completely developed. Careful studies of the victim's histories also revealed another clue. These young healthy victims were not exposed to a similar flu in their youth, but older people living then were exposed, and some researchers suspect that this prior exposure is what saved many in the older group. Antibodies to a similar flu virus will lessen the severity of a flu infection. People born before 1875 were around 43 when the epidemic hit. These people had been exposed to a variety of subtypes of influenza A that some researchers suspect that exposure led to partial immunity against the 1918 flu strain. This, in addition to a less robust immune system that cannot run amok, might have offered older people some protection.<br />
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Understanding the unusual pathophysiology of the 1918 flu epidemic offers clues about how to avoid a similar flu deadly pandemic in the future. Knowing how our immune systems evolve over time, how different strains attack the body, where in the world and in what species new strains could originate from, and how previous exposure to past similar flu strains can moderate our immune response all help the world's health organizations zero in on what to watch out for as each new season takes shape.<br />
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Avian Flu Cross-Over: A Concern for the Future<br />
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H5N1<br />
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While the 1918 deadly H1N1 flu strain appears to have been a cross-over from an avian flu virus, this and similar strains may be much less likely to cause a future deadly pandemic. In recent years, outbreaks of H1N1 have been fairly mild and it makes the rounds often enough that most humans have antibodies against at least a few strains of this subtype. Those who get the flu shot will also have antibodies against various strains of the H1N1 subtype. Virologists now have their eye on a different avian flu subtype, <a href="https://en.wikipedia.org/wiki/Influenza_A_virus_subtype_H5N1">H5N1</a>, a subtype commonly called "bird flu" (even though there are many subtypes of avian influenza, see above). It is currently one of two most likely candidates for a future deadly flu pandemic. Even though mouse studies, exposing mice to avian H5N1, did not result in serious illness, it doesn't mean that this virus has not been deadly in humans.<br />
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Bird flu is largely a south Asian disease of birds but it <a href="https://news.nationalgeographic.com/news/2005/10/1005_051005_bird_flu.html">can infect a wide range of other hosts species as well such as pigs, cats and humans</a>. There are low pathogenic (LP) strains of H5N1 (these are also found in North America) and high pathogenic (HP) strains of H5N1. Virologists are particularly concerned with a high pathogenic strain called <a href="https://www.cdc.gov/flu/avianflu/h5n1-people.htm">HPAI H5N1</a>. It was discovered in China in 1996, isolated in a goose, and the first human outbreak of this strain was in 1997. The rate of infection has been increasing since then, with several hundred cases of this strain in humans now reported to the WHO (World Health Organization). WHO announced that between 2003 and 2013, 630 cases have been confirmed and of those, more than half, 375 people, have died. H5N1, at least currently, doesn't easily spread from birds to humans but when it does, the disease is often unusually pathogenic, and deadly. In a 2006 outbreak, limited human-to-human transmission was confirmed as well, which is even more worrying.<br />
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What makes all avian influenza subtypes a concern is where they tend to attack the body. As mentioned earlier, viral HA recognizes and attaches to sialic acid receptors in respiratory epithelium cells. There are two kinds of <a href="http://www.virology.ws/2009/05/05/influenza-virus-attachment-to-cells-role-of-different-sialic-acids/">sialic acid receptors</a>: 2,3 linked and 2,6 linked. Flu viruses that originate in birds tend to prefer the 2,3 linked type of sialic acid receptor, while flu viruses that originate in humans tend to prefer 2,6 linked receptors. 2,6 linked receptors are mostly found in the upper respiratory tract, such as in the throat, the nose, and in the upper trachea. Humans also have 2,3 linked receptors and these tend to be most numerous deep in the lungs. This means that when avian-type viruses infect humans, there is a higher risk that deadly primary viral pneumonia can set up deep in the lungs, and this accounts for its high mortality rate. An upside to this is that because the site of attack is deeper in the body it is more difficult to sneeze or cough out viruses, making these infections less likely to spread through airborne contact. So far, there has been no recorded instance of a highly pathogenic avian influenza outbreak that is transmitted through airborne contact. However, a <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2329826/"> 2008 study</a> found evidence that the H5N1 subtype, in addition to infecting deep lung tissues, can infect the gastrointestinal tract, the brain, the liver, the blood cells, and in one case it even crossed the placenta into the fetus of a pregnant woman, which means it could cause damage to and weaken various regions of the body.<br />
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H7N9<br />
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Another avian influenza Type A virus, <a href="https://en.wikipedia.org/wiki/Influenza_A_virus_subtype_H7N9">H79N</a>, has recently also appeared on WHO's radar, and this subtype might be considered even more worrisome than H5N1. There have been about 1200 confirmed cases of H7N9 so far, and about 40% of those have died. Not as much is known about the transmission and pathology of H7N9, but it also appears to attack the lower respiratory tract, leading to viral pneumonia. It also appears to overload the immune system causing a <a href="https://en.wikipedia.org/wiki/Cytokine_release_syndrome">cytokine storm</a>, which in some cases led to acute respiratory distress or multiple organ dysfunction syndrome. A cytokine storm acts like a dangerous positive feedback loop. It occurs when various immune cells are activated in large numbers. These cells release cytokines, which in turn activate even more white blood cells.<br />
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In 2013, virologists reported that it did not transmit easily from birds to humans and that person-to-person transmission was unlikely. Therefore it was unlikely to cause a pandemic. However, since then they walked that back. While H5N1 causes illness in birds, making it fairly easy to identify and monitor, H7N9 doesn't appear to cause any visible signs of disease in birds. This makes it virtually impossible to monitor in bird populations such as poultry farms. Since birds don't get sick, it also means that there could be a large sustained pool of the virus in the bird population. There is <a href="https://www.cdc.gov/flu/avianflu/h7n9-virus.htm">no evidence yet for person-to-person spread of this subtype</a>, but there is concern that the virus could mutate and gain that ability.<br />
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The Flu Shot: What Does It Do and Is It Worth It?<br />
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The annual flu vaccine typically reduces your risk of getting the flu by about 50% on average year over year, and if you do get the flu, the symptoms tend to be milder. Aside from taking antiviral drugs, it is the only action you can take to prevent the flu. Here in Alberta, the annual flu shot is provided free of charge through Albert Health Services. They provide it through immunization clinics or you can get it at your local pharmacy (like I do every year). It is generally available at the start of flu season, some time in October. Protection starts about two weeks after you get the shot. In the United States and In Canada, the flu shot is recommended for everyone aged 6 months and older. Vaccination rates in Canada have been steadily increasing over the past two decades. According to a <a href="http://www.statcan.gc.ca/pub/82-624-x/2015001/article/14218-eng.htm">Canadian report released four years ago (2014)</a>, about 30% of all Canadians got the flu shot annually, with a high of about 67% for seniors and a low of about 20% for people between 12 to 17 years old. By 2016, <a href="https://www.cdc.gov/flu/fluvaxview/coverage-1516estimates.htm">vaccination rates increased to about 42% for all Canadians</a>, 59% of which were children aged 6 months to 17 years. Every accredited medical website I checked online recommends that you get the annual flu shot.<br />
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A unique flu vaccine is formulated each year to protect against three or four of the most likely virus strains to show up. These strains are determined by the World Healthcare Organization (WHO), usually some time in February, for each upcoming year.<br />
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What's In the Flu Vaccine? Is it Safe?<br />
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I know a lot of friends and family who resist getting the annual flu shot, and the reason varies from a fear of needles to a belief it is ineffective to worries about the safety of the vaccine. Side effects from the flu shot can occur and that is why you are told to wait 15 minutes after your shot before leaving the pharmacy. The side effects are usually minor, with the most serious possibility being an allergic reaction, and that is primarily what the pharmacist watches for during those 15 minutes. Symptoms such as swelling around the eyes or lips, hives, a racing heart beat, dizziness or trouble breathing indicate that you are having <a href="https://www.cdc.gov/flu/protect/vaccine/general.htm">an allergic reaction to the flu shot</a>. Severe allergic reactions to the flu shot are very rare (there are just 1.31 reported cases of <a href="https://en.wikipedia.org/wiki/Anaphylaxis">anaphylaxis</a> per million doses given, <a href="https://www.cdc.gov/flu/protect/vaccine/egg-allergies.htm">according the CDC</a> in the United States). If you have a severe allergy to eggs you should talk to your doctor before getting the flu shot. However, the flu shot is recommended even for those with moderate egg allergies, provided they are monitored after the shot for symptoms. Most flu shots and the nasal spray are manufactured using chicken eggs so they <a href="https://www.healthline.com/health/cold-flu/flu-shot-ingredients">contain a small amount of egg protein such as albumin</a>.<br />
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Another possible avenue for allergic reaction is thimerosal, a preservative that is added to multi-use vials of flu vaccine. Prefilled syringes and the nasal spray do not contain it. Thimerosal exposure can <a href="http://allergy-symptoms.org/thimerosal-allergy/">trigger rare and mild allergic symptoms</a> such as itchiness, redness and swelling around the injection site. Thimersol is also present in make-up, soaps, some contact solutions and ointments. Thimerosal contains ethylmercury and some people worry about mercury exposure. Our body eliminates ethylmercury so it cannot build up in our tissues and cause damage. <a href="https://www.ncbi.nlm.nih.gov/pubmed/23401210">Methylmercury</a>, however, does build up in the body (it is the molecule that builds up in fish tissues and can be toxic). You can choose a thimerosal-free vaccine dose if you are concerned.<br />
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The vaccine also contains stabilizers such as sucrose (table sugar), sorbital (artificial sweetener) and monosodium glutamate (MSG). These additives prevent the vaccine from losing potency when exposed to heat and light. Even if you are diabetic or are sensitive to sorbital or MSG, the amount in your dose is far too small to cause any reaction. Antibiotics are also added to the vaccine, again in extremely small amounts. A small amount of emulsifier, polysorbate 80, is also added. This is the stuff in purchased salad dressings and sauces. The shot vaccine also contains formaldehyde, which is used to deactivate the virus. Formaldehyde, found in wood glues and adhesives, can cause eye and throat irritation and <a href="https://www.cancer.gov/about-cancer/causes-prevention/risk/substances/formaldehyde/formaldehyde-fact-sheet">it is a carcinogen with long-term large-dose exposure</a>. As a water-soluble gas, almost all of it is removed from the vaccine before packaging. The amount that is left in the vaccine is less than the amount found in your body naturally, and so is not a concern.<br />
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The flu vaccine causes approximately <a href="https://www.cdc.gov/vaccinesafety/concerns/guillain-barre-syndrome.html">one in one million elderly people to get Gullian-Barré syndrome</a>. This is a very rare disorder in which your immune system attacks your nerve cells, and it can occasionally lead to paralysis. You are more likely to get the syndrome after the suffering from the flu itself than from the flu vaccination. People with a history of Gullian-Barré syndrome after receiving a previous flu shot, however, should talk to their doctor before getting the current season shot.<br />
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If you do not feel well, you should talk to your doctor before you get the shot. You want to have a robust immune reaction to your flu shot to maximize antibody production. If your immune system is already taxed, your body is less likely to develop good immunity against the flu strains in it.<br />
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Shots and Nasal Sprays<br />
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As you suspect, the flu shot contains the flu virus, or viruses to be precise. Side effects from the flu shot include soreness, redness or swelling at the injection site, headache, mild fever, nausea and muscle aches (I usually experience a sore arm, the most common side effect, for a few days). You might experience a runny/stuffy nose for a few days after the nasal spray. These symptoms might sound familiar after reading this article. They are symptoms of the flu itself, albeit much milder. They are far easier to live with than the symptoms of the full-on flu itself. It is important to note that the vaccine viruses themselves DO NOT cause these symptoms. These are signs that your immune system is being activated. They are the immune response to the disease but not the disease. You CANNOT get the flu from the flu shot or the nasal spray.<br />
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The flu viruses in the flu shot are inactivated (dead). Formaldehyde inactivates the virus, while leaving the surface HA glycoproteins intact to trigger an immune response. The viruses in the nasal spray are <a href="https://en.wikipedia.org/wiki/Live_attenuated_influenza_vaccine">live but they are attenuated</a>, or weakened. First made available in 2003, some American studies have recently shown it to be less effective in reducing cases of the flu, and the reasons for that are not yet understood, which is unfortunate for children and others who fear needles. In the United States, the CDC did not recommend the nasal spray for this flu season (2017/2018) while Canada's National Advisory Committee on Immunization still recommends it, based on Canadian studies that show that it works. That being said, Alberta and Saskatchewan stopped offering the nasal spray for free last fall, although it is available at a cost.<br />
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Because the nasal spray contains a live (but weakened) virus, it can cause a mild flu infection. The virus in this case <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3595159/">is grown in a cold setting</a>, which means it can survive and reproduce in the cooler nasal passages but it cannot live elsewhere in the warmer (deeper) respiratory tract. Sniffles and a stuffy nose means that it is triggering an immune response and making antibodies. You DO NOT actually get the flu. It is not the cascade-like invasion of virus that is the hallmark of a bout of influenza. However, the nasal spray could lead to complications in people with already weakened immune systems. Wikipedia lists those who should not get the flu nasal spray <a href="https://en.wikipedia.org/wiki/Live_attenuated_influenza_vaccine#Not_recommended">here</a>. People who receive the nasal spray may also shed small amounts of live virus for about a week afterward, which means it could lead to transmission of the viruses in the vaccine, although it is a very minimal risk.<br />
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Soreness, redness or swelling at the injection site, headache, mild fever, nausea and muscle aches are all good signs that your immune system has recognized the viral invaders and is launching a counter-attack. Your immune system will remember those flu strains. If it encounters any of those strains in the future it will be able to attack the virus without delay.<br />
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How Long Does Immunity Last?<br />
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How long does immunity last against a particular flu strain? You need a flu shot every year not primarily because your immunity wears off but because flu viruses mutate into new strains every year. Each year, a new collection of three or four of the "worst and most likely" viruses is used to make vaccine. One or more of these viruses can and often will mutate before the vaccine is manufactured and distributed, which will make the vaccine less effective or even ineffective against that particular strain. It's a frustrating game of Russian roulette or maybe whack-a-mole.<br />
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Is there a side benefit from the yearly shot, such an ever-increasing arsenal of antibodies and <a href="https://en.wikipedia.org/wiki/Memory_B_cell">memory B cells</a> against various flu strains? I would like to think this is a bonus of getting the shot every year, but the evidence for this is not yet solid. It is unclear whether yearly vaccination produces a strong enough immune response to provide a lasting year-over-year memory B cell population that is large enough to provide strong immunity to each strain we receive. However, there are hints that this could be the case, at least for past exposures to wild strains of the flu. Evidence from studies on the 1918 flu pandemic suggests that antibodies to a similar viral strain can reduce the severity of a current infection. That work also suggests that memory B cells created in response to a flu infection, especially while the immune system is young and robust, can lead to decades and perhaps even a lifetime of immunity against that strain as well as similar future strains. <br />
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How long you retain antibodies and memory B cells depends on how powerfully your immune system reacted to the virus. For the best antibody production you need a healthy and mature immune system. A baby is born <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4707740/">with an immature but highly adaptive immune system</a>. It acquires immune memory in the form of long-term memory B-cells as the child grows and comes into contact with various antigens over the years. Memory B-cells migrate to the bone marrow after an immune response, where they live for up to several decades. These are the cells that produce (shorter-lived) antibodies in response to a repeat invasion of foreign material such as a virus. <a href="https://en.wikipedia.org/wiki/B_cell">B lymphocytes</a> make antibodies to an antigen (the naive B cell shown below). At the same time, they also make memory B cells, which remember that antigen and launch a faster antibody response the next time the body is infected with virus "A."<br />
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: left; margin-right: 1em; text-align: left;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg8a7ieXNBp8rXK4JbslCEV9Hrt4tJuCvgHPloidT4U31ZBBYJ1ENzVtoBMPZhL9vvwL1z8GVVSrn-nR3SyjopRFo3ZU9uya2iia7JTiTGIMKnQ05COkKRYCqkiTn2wGo3ZLBUEAxfsGY5C/s1600/240px-Original_antigenic_sin.svg.png" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="270" data-original-width="240" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg8a7ieXNBp8rXK4JbslCEV9Hrt4tJuCvgHPloidT4U31ZBBYJ1ENzVtoBMPZhL9vvwL1z8GVVSrn-nR3SyjopRFo3ZU9uya2iia7JTiTGIMKnQ05COkKRYCqkiTn2wGo3ZLBUEAxfsGY5C/s1600/240px-Original_antigenic_sin.svg.png" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: x-small;">Hazmat2;Wikipedia</span></td></tr>
</tbody></table>
As you get older you develop an expanding repertoire of memory B-cells to various antigens. Your immune system reaches its peak function at around age 30 and then goes into slow decline. At around 50, we have noticeably weaker immune systems in general but our overall health at this age makes a big difference. Although memory B-cells persist, the immune response in general declines with age as fewer immune cells are made after exposure. The equilibrium of the immune system is also weaker after around 50. Tolerance to self-antigens goes down, which means we experience more <a href="https://en.wikipedia.org/wiki/Autoimmune_disease">autoimmune diseases</a>, our bodies experience increased overall inflammation, and our systems no longer recognize and eliminate cancerous cells as efficiently.<br />
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All of this suggests that exposure to many strains of flu virus when we are young and healthy could build up a good arsenal of memory B-cells to help protect us against various strains of flu well into our declining years. It might mean that starting the yearly shot with its ever-changing cocktail of viruses as young as possible (at six months for the shot) and getting immunized every year might be the best long-term strategy against the flu. A couple of studies, however, show just the opposite - that getting the yearly shot <a href="https://www.statnews.com/2015/11/11/flu-shots-reduce-effectiveness/">might actually diminish one's immunity against the flu</a>, a perplexing finding. One possibility for this is negative interference. For example, if identical or very similar viral strains are present in the shot two years in a row, the antibodies produced in year one might neutralize the virus in the year-two vaccine before it can trigger a full immune response. In this case, infection with the strain in year two must rely on a two-year-old repository of antibodies/memory B cells to attack it. While the presence of antibodies should lessen the severity of the flu, they might not be as effective as antibodies made more recently. In other years, the opposite (positive interference) might occur, which could provide enhanced protection instead. Researchers need to determine if negative interference exists and what to do about it. One possible solution is a higher dose vaccine in year two, which would elicit a stronger response.<br />
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Some of my friends/family claim it might be better for kids to actually contract the flu each year rather than to get the shot or spray. It is possible that a full-blown bout of the illness could elicit a more robust immune response than the shot or spray, making a better arsenal of antibodies for the future, and <a href="https://ww2.kqed.org/stateofhealth/2016/02/24/better-immunity-from-flu-vaccine-or-flu-itself/">some researchers suspect this is the case but there is a price</a>. Getting a full-blown flu is massively unpleasant and it has significant risk associated with it, and vaccination does not. The yearly shot, I think, is the more logical and kinder strategy for your kids, and it might add that bonus of a broadening immunity against the flu.<br />
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I have found conflicting evidence in various research papers about how long the flu shot affects future immunity against the flu. <a href="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0016809">One study</a>, based on the 2009 H1N1 outbreak, for example, suggests that after immunization, immunity against that strain was lost within a year. Another study suggests that we could gain <a href="https://www.nature.com/articles/nature07231">immunity against a strain of the flu that could last a lifetime</a>. Importantly I think, this was based on a prior infection with the flu, not exposure from vaccination. 90 years after the deadly 1918 flu epidemic, 32 elderly volunteers still contained memory B cells circulating in their blood that readily secreted antibodies after exposure to hemagglutinin (HA) glycoprotein from the same strain.<br />
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Even though questions remain about how long immunity from the flu shot or nasal spray lasts, evidence that the annual flu shot offers some protection against the flu is clear. Even during years when the shot's effectiveness is low, it may still offer protection by reducing the severity of the illness, and therefore reducing the risks associated with serious and sometimes deadly flu complications.<br />
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The first flu vaccines were <a href="https://en.wikipedia.org/wiki/Influenza_vaccine#History">developed in the 1930's</a>. Flu vaccines have been widely administered throughout the world for well over 50 years, but there is still much to learn about how the vaccine works and how to maximize its effectiveness. Researching the effectiveness of flu vaccines is very difficult. The pool of test subjects is almost impossible to control for. Individuals have unique and highly variable immune system function, which depends on health history, exposure history, sex and age. The effectiveness of the flu shot varies based on two central things: how closely it matches current circulating viral strains, which are always in the process of changing, and second, on the particular immunity of the person being vaccinated (which can often be a black box).<br />
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How The Flu Vaccine Is Made<br />
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Over a hundred national influenza centres across over more than a hundred countries collect flu data all year long. They monitor which strains are making people sick and how those strains are spreading, and then they pass that data along to the World Health Organization (WHO) and other centres. The data is gathered and analyzed to determine which strains are most likely to spread and cause illness during the year ahead. Usually <a href="http://www.who.int/influenza/vaccines/virus/candidates_reagents/201703_qanda_recommendation.pdf?ua=1">three strains and sometimes four are selected</a>: most often they are an H1N1 strain, an H3N2 strain and a B strain. <a href="http://www.who.int/influenza/vaccines/virus/recommendations/en/">This link</a> lists the viral compositions of past (back to 2010) and present WHO-recommended flu vaccines.<br />
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Egg-based Flu Vaccine<br />
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The egg method has been used since the first flu vaccines were made, and it is still the method almost exclusively used. Each strain in the flu vaccine is produced separately in fertilized chicken eggs that are 11 to 12 days old. The following brief 2-minute video from McMaster University in Hamilton, Canada illustrates the basic procedure:<br />
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<iframe allow="autoplay; encrypted-media" allowfullscreen="" frameborder="0" height="360" src="https://www.youtube.com/embed/Eq9JIq9HvMg" width="640"></iframe>
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This fairly low-tech method is currently how most live attenuated and inactivated vaccines are made. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4401370/">This 2015 article</a> outlines the protocol very clearly and is easy to follow. The candidate viruses are injected into eggs and incubated for two days so that the viruses can replicate. Then the virus-containing fluid is collected. The viruses are inactivated (for the shot), and then purified and tested before they are released. Attenuated viruses are also manufactured using eggs but the process is different and relies upon some modern genetic tools. In this case, a universal master donor virus is used. This master virus is made to be cold-adapted and temperature-sensitive by being cultured at progressively colder temperatures. It is then used as a vehicle to combine with the genes for the current virus, and then it is attenuated. Specifically, the genes used are those that encode the virus strain's unique hemagglutinin (HA) surface glycoprotein.<br />
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Although it has been used for many decades, the egg system in general is not a perfect system for several reasons. One problem with egg incubation is that it takes a long time. Even though the actual viral replication time is short (a few days) the entire process from start to end takes several weeks to obtain a sufficient amount of virus. Growing human viruses in an avian environment also presents a problem. Recent research reveals that it prompts the (human-adapted) virus to adapt to its foreign (avian) environment. This means that by the time the viruses are harvested, there is a chance that the immunologically important HA structure <a href="https://www.nbcnews.com/health/health-news/here-s-one-reason-flu-vaccines-are-so-lousy-they-n818046">has mutated away from that of the original virus</a>. The altered-HA virus in the vaccine now matches a different antigen, not the one causing people to get sick. This is a problem found <a href="http://www.pnas.org/content/early/2017/10/31/1712377114">especially with H3N2 strains of type A virus</a>, and there are some questions about this happening <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2888842/">with H5N1 as well</a>. These two subtypes are part of most yearly vaccines. The H3N2 subtype in particular, for unclear reasons, grows poorly in eggs. Some years the virus grows so poorly that egg incubation fails altogether. When it does grow, it often means that its HA glycoprotein has mutated to help it replicate better (another function of this HA molecule). In this case, the HA either no <a href="https://www.sciencedaily.com/releases/2017/11/171106152302.htm">longer matches the original antigen</a> or its mutation <a href="http://www.pnas.org/content/114/47/12578">reduces the ability of our antibodies to attach to it</a>. Either mutation makes the vaccine less effective against H3N2 viruses. A fourth problem is that the timeline between virus identification and vaccine availability is 4-6 months, plenty of time for the "wild" virus itself to mutate within the human population so that the vaccine no longer targets it. A fifth worry is that because these are avian-type viruses that could also make chickens sick and die, a sudden pandemic of a virulent H5N1, H3N2 or other avian-type virus could come on scene without a ready supply of eggs. These are the primary reasons why we sometimes get frustratingly low success rates with the yearly flu vaccine, and why we need a better method soon.<br />
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Cell-based Flu Vaccine<br />
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In 2012, Flucelvax was the first flu vaccine manufactured using <a href="https://en.wikipedia.org/wiki/Cell_culture">cell culture technology</a> to be approved by the FDA in the United States. In this case, the virus was cultured in mammalian (dog kidney) cells) <a href="https://www.cdc.gov/flu/protect/vaccine/cell-based.htm">instead of a chicken egg</a>, an environment that is more similar to a human host environment. Dog kidney cells (a cell line called <a href="https://en.wikipedia.org/wiki/Madin-Darby_Canine_Kidney_cells">MDCK</a>) are a uniquely suitable epithelial cell substrate for culturing the influenza virus. Not only are they very similar to human epithelial cells, avoiding mutation pressure, but the virus also replicates readily in these cells. You might wonder why human cells aren't used. The canine version of an interferon-induced protein <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4514150/">doesn't resist viral replication as it would in a human cell line</a>.<br />
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Using mammalian cell culture technologies has several advantages over the egg method. There is hope that viruses cultured in mammalian cells do not experience as much pressure to adapt and mutate during culture. Importantly, while egg technology depends on having an egg supply (millions upon millions of eggs) ready, these culture cells can be frozen and banked, immediately ready for use, when a vaccine is needed quickly such as during a pandemic of a new strain. It also avoids possible allergic reactions against egg proteins and, finally, cell lines such as MDCK can be grown in a (supplemented) synthetic medium rather than commonly used fetal bovine serum. By avoiding bovine serum, the accidental transmission of some diseases such as spongiform encephalitis can be avoided. Since this method has been established now for a few years, why aren't most or all vaccines made this way, I wonder. Pharmaceutical companies appear to be r<a href="https://www.popsci.com/flu-vaccine-egg-less-effective#page-3">eluctant to invest in switching over their technology</a>. <br />
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DNA-based Flu Vaccine<br />
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A new and exciting approach currently underway goes even farther. The idea here is to <a href="https://newatlas.com/dna-vaccine-flu-universal/52768/">isolate part of the particular virus's genetic code and inject that into the body</a>, rather than the virus. The code will also contain special DNA code that allows it to enter our cells and direct them to make a flu antigen. Rather than a whole virus, this antigen could be the HA receptor itself or another viral segment. This approach would mean isolating a particular segment of the viral RNA and replicating it in large numbers in a cell culture. Like the cell-based method, this high-tech process takes much less time than isolating the virus and growing it in chicken eggs. Because the isolated genetic code remains identical to the virus's original RNA code throughout the process, there is no longer any problem with <a href="https://en.wikipedia.org/wiki/Genetic_drift">genetic drift</a> (mutation away from the original). The vaccine is always an exact match to the virus. As long as the "wild" virus doesn't mutate in the human population during manufacturing time, it will match the virus making people sick. This shorter process reduces the window of time when that can happen as well. One problem encountered so far, however, seems to be getting the body to make a strong enough immune response. For unknown reasons, isolated parts of the flu virus (such as the HA receptor glycoprotein) <a href="https://www.sciencedirect.com/science/article/pii/S0939641115002556">do not stimulate as vigorous a response as an invasion of the whole virus does</a>.<br />
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A Universal Flu Vaccine<br />
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Flu viruses are notorious for their mutation rate. This is probably the biggest hurdle faced when each yearly vaccine is created. By the time the vaccine is made, the virus has changed again. Some parts of the virus mutate at faster rates than other parts, and this can be exploited to make a <a href="https://en.wikipedia.org/wiki/Universal_flu_vaccine">universal flu vaccin</a><a href="https://en.wikipedia.org/wiki/Universal_flu_vaccine">e</a>. Our immune system naturally recognizes the HA receptor glycoprotein as an antigen and makes antibodies against it. As mentioned earlier, the HA receptor has two parts - a head and a stalk. Both the head and the stalk contain antigenic proteins but the immune system prefers to focus on the <a href="https://en.wikipedia.org/wiki/Immunodominance">immunodominant</a> head and makes antibodies against it. The problem is that head part of HA receptor mutates very often, meaning that antibodies induced by a flu vaccination often miss the mark. The stalk part, however, doesn't change much over the years. These proteins are encoded by <a href="https://en.wikipedia.org/wiki/Conserved_sequence">conserved</a> components of the viral genetic code.<br />
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In 2009, researchers discovered that the body <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2692245/">also makes antibodies against the HA stem</a> but not in as high a titre. The stem contains most of the virus's membrane fusion machinery. An antibody that binds to it <a href="https://en.wikipedia.org/wiki/Hemagglutinin_(influenza)#Neutralizing_antibodies">blocks it and prevents the infection of epithelial cells</a>, stopping the flu infection in its tracks.<br />
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If one can make a DNA vaccine against this conserved part of the virus, one can target the flu virus <a href="http://jvi.asm.org/content/88/6/3432.full)">no matter what new strain it has mutated into</a>. That is how a universal vaccine can be made against all influenza strains. The trick is to get the body to make a lot of stalk antigen and then to make a lot of antibodies against it, which it doesn't naturally do. The immune system tends to go for the HA head and ignore the stalk. <a href="http://www.nature.com/scibx/journal/v6/n22/full/scibx.2013.538.html">One approach being investigated</a> is to attach part of the stem to another protein called ferritin. The ferritin serves as a kind of glue that sticks a bunch of stem parts together and highlights their presence to the immune system. <a href="http://mbio.asm.org/content/1/1/e00018-10.full">Another approach</a> is to chop off the heads of the HA molecules and modify the stem so antibodies can attach to it more effectively.<br />
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A robust immune response against the flu viral HA stalk means the body will launch a rapid offensive against any future flu virus it encounters. If the researchers are lucky, it could even be a one-time vaccination if the production of memory B cells against it is robust enough, doing away with the hassle, significant expense and uncertainty of coming up with new yearly vaccines. Perhaps most importantly, it could protect us from the next highly pathogenic "pandemic" virus to come along, saving countless lives. It is frustrating for us to sometimes come down with the flu even after we've been vaccinated and to worry about the next inevitable deadly flu epidemic. It may seem that too little is being done to improve things but there are many lines of research underway that are focused on a new and better flu vaccine. It takes time, however, because each promising approach must be tested in preclinical (test animal) trials and if they show promise, and are proven to be safe, they can move on to clinical (human) trials, a process that generally takes several years. Still, what I've read makes me hopeful. I can imagine a day when my personally dreaded "Christmas flu" (even more dreaded than fruitcake) will be just a story from the scary old days. More importantly for all of us, we may soon never have to worry about a horrifyingly deadly pandemic like the1918 Spanish flu ever again.Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-44887293538366591692018-01-04T09:28:00.000-07:002018-01-04T09:28:42.420-07:00The Laws of Thermodynamics PART 4The Laws of Thermodynamics PART 1 <a href="https://www.blogger.com/"><span id="goog_532689932"></span>c<span id="goog_532689933"></span></a><a href="http://sciexplorer.blogspot.ca/2017/12/the-laws-of-thermodynamics-part-1.html">lick here</a><br />
The Laws of Thermodynamics PART 2 <a href="http://sciexplorer.blogspot.ca/2018/01/the-laws-of-thermodynamics-part-2.html">c</a><a href="http://sciexplorer.blogspot.ca/2018/01/the-laws-of-thermodynamics-part-2.html">lick here</a><br />
The Laws of Thermodynamics PART 3 <a href="http://sciexplorer.blogspot.ca/2018/01/the-laws-of-thermodynamics-part-3.html">click here</a><br />
<br />
The Third Law of Thermodynamics<br />
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The last in this four-part series of articles explores the third law, which focuses on the properties of systems that approach <a href="https://en.wikipedia.org/wiki/Absolute_zero">absolute zero</a>. The eerie quantum behaviour of matter <a href="https://phys.org/news/2015-06-absolute-molecules-exotic-states.html">becomes observable in extremely cold systems</a>. At much warmer temperatures, molecules and atoms have so much kinetic energy they cannot form the chemical bonds that hold liquids or solids together. However, as absolute zero approaches, such random particle motion is quieted down enough to begin to reveal matter's underlying quantum nature. A tremendous amount of research is underway to explore how <a href="https://en.wikipedia.org/wiki/Ultracold_atom">how ultra cold matter behaves</a>. New and exotic behaviours, such as <a href="https://en.wikipedia.org/wiki/Superfluidity">superfluidity</a>, emerge as the system's <a href="https://en.wikipedia.org/wiki/Thermal_energy">thermal energy</a> approaches zero. The third thermodynamic law is all about the energy of matter as it approaches absolute zero.<br />
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This law can be stated a few different ways. Put most simply, the entropy of a pure substance approaches zero when its temperature approaches absolute zero (0 K, -273.15°C, -459.67°F). It is a statement about the limits of temperature and entropy in a system. What is absolute zero? A substance, if it could reach exactly 0 K, would have no thermal energy left at all. Although it is theoretically possible, 0 K doesn't exist in nature, and this law really explores the reason why that is so. The coldest measured object in the universe is the <a href="https://en.wikipedia.org/wiki/Boomerang_Nebula">Boomerang Nebula</a>, which contains gases cooled to just 1 K. These gases are even colder than the empty space around them and that initially presented a puzzle. Background microwave radiation warms empty space to 2.73 K, so how is it possible to cool an object down more than the space around it? This posed a mystery to scientists <a href="https://www.sciencealert.com/scientists-can-finally-explain-why-the-boomerang-nebula-is-colder-than-space-itself">until recently</a>. They think that the Boomerang Nebula was created when a dying red giant star exploded as a smaller-mass companion star crashed into it, creating an exceptionally powerful explosion that ejected stellar gases outward at such velocity (10 times faster than a single exploding red giant of comparable mass) that the gas adiabatically expanded into an ultra-cold gas. The gas cloud would then gradually absorb heat from the microwave radiation bathing the space it occupies, until it reaches equilibrium with it, at 2.73 K<br />
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As matter approaches 0 K, atoms lose thermal energy. The electrons in atoms eventually fall down into low energy states. Does zero thermal energy mean that a system has zero entropy as well? Not necessarily, and this is the key point. When many substances freeze, they settle into a three-dimensional lattice-like arrangement. <a href="https://en.wikipedia.org/wiki/Crystallography">Crystallography</a> is the science that explores those arrangements that atoms assume when a substance freezes into a crystal. Only a perfect crystal of a pure substance could actually reach absolute zero. Only in such a flawless atomic arrangement could atoms lock into such a perfect alignment that has zero thermal energy. Atoms in it would not have any <a href="https://en.wikipedia.org/wiki/Kinetic_energy">kinetic energy</a>. They would not move about slightly in the lattice or undergo any translational or rotational movements. Even at 0 K, however, each atom would still vibrate (in its lowest energy state) about its equilibrium position within the crystal, but this motion is <i>not</i> transferable as heat. Individual molecules and atoms can never be totally frozen. They are quantum clouds that are always in motion associated with the uncertainty principle, and electrons by their nature are never stationary. None of this motion is thermal motion.<br />
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A perfect crystal would have to contain only one kind of atom or molecule. Otherwise there would be entropy associated with the mixing of two or more microstates (you can also say there is more than one microscopic configuration possible). We are reminded that entropy is also about the multiplicity of possible states in a system.<br />
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A perfect crystal, though impossible to create, is useful because it offers a benchmark we can use to describe what happens to entropy as real substances are cooled to absolute zero. Microscopic imperfections always form when substances crystallize, no matter how controlled the procedure is. Any imperfection in a crystal lattice adds disorder and therefore entropy. Even the most flawless diamond, for example, is never perfect. Defects inevitably get frozen into a crystal that pull the lattice pattern out of alignment. If just a single boron atom replaces a carbon atom in a diamond crystal, for example, it will shift the whole alignment microscopically, reducing the microscopic order and increasing its entropy. All pure substances that in theory would condense into perfect crystals have, in reality, point defects. Such a defect could be a single replacement (like boron for carbon) or a hole created by a missing atom. It could be a linear defect or a planar defect. These are all called <a href="https://en.wikipedia.org/wiki/Crystallographic_defect">crystallographic defects</a>. Every crystallographic defect adds <a href="https://en.wikipedia.org/wiki/Residual_entropy">residual entropy</a> to a crystal, and would prevent it from ever reaching 0 K.<br />
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Some pure substances, even if they could be entirely defect-free, will still never settle into an absolutely perfect lattice formation. These substances cannot reach 0 K because of their atomic makeup. They will always retain a certain amount of residual entropy. A classic example is carbon monoxide (CO). A CO molecule has a small <a href="https://en.wikipedia.org/wiki/Dipole#Molecular_dipoles">dipole moment</a>, which means it has a slightly lopsided charge (it has "a left" and "a right"). When the gas is cooled down far enough it will eventually form crystals. Those crystals will not be perfect because each CO molecule can be oriented CO or OC. The dipole moment is too small to assure that all align in one direction so there is a chance that CO could crystallize in another pattern such as CO:OC:CO instead of CO:CO:CO, for example. We can estimate this contribution to the crystal's entropy by calculating how many possible microstates a crystal sample of a certain number of molecules can achieve. For carbon monoxide we know there are two ways each CO molecule can exist in the lattice so we can give a value w = 2. For an N number of CO molecules in the crystal, there are wN ways, or 2N ways in this case, that the CO molecules can be arranged. There are 2N different microstates possible, and entropy, we now know, is all about the number of possible states a system can be in.<br />
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When carbon monoxide freezes into a crystal, it becomes locked into one of many possible <a href="https://en.wikipedia.org/wiki/Ground_state">ground states</a>, each of which corresponds to a specific microscopic configuration. It therefore must have residual entropy even at 0 K. Put more scientifically, carbon monoxide has a <a href="https://en.wikipedia.org/wiki/Degenerate_energy_levels">degenerate, or asymmetrical, ground state</a>. Asymmetrical ground states are very interesting to study. Times crystals, mentioned earlier, show that an asymmetrical ground state can be one in space (as with the carbon monoxide crystal) or one in time <i>and</i> space (as in a <a href="https://en.wikipedia.org/wiki/Time_crystal">time crystal</a>). These crystals have a very unusual property. While ordinary crystals, such as carbon monoxide, have an atomic structure that repeats in space, time crystals repeat in time as well: they maintain a constant perfectly regular atomic-level oscillation while in their ground energy state. Researchers thought this was impossible - the atoms in substances at ground state should be locked in place and shouldn't change because there is no energy available to change. Yet a time crystal changes from moment to moment, leading researchers to wonder if it is doing work without any input of energy, a microscopic sort of perpetual motion machine that (gasp!) breaks the second law of thermodynamics.<br />
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Technically a time crystal is indeed a system with no thermal energy available to do work. It was also assumed to be an isolated system, where no thermal energy is transferred into the system. Upon close study, however, physicists discovered that a time crystal system is not closed. Even though it is in its ground state and remains in its ground state, it is actually an open non-equilibrium system (and the first non-equilibrium ground state system ever discovered).<br />
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Proposed theoretically in 2012, time crystals were recently created in the lab. One way <a href="https://www.sciencealert.com/it-s-official-time-crystals-are-a-new-crazy-state-of-matter-and-now-we-can-create-them">to create a time crystal</a> is to line up a set of ytterbium atoms, all of which have quantum-entangled electrons, that is laser-cooled to its ground state. Then it is deliberately kept out of equilibrium by hitting the atoms with two alternating lasers. It is open to the environment and energy is continuously being supplied to the crystal. If the lasers are turned off the crystal stops oscillating. The fine point here is that it's not a transfer of thermal energy because the atoms stay in ground state. If even very slight changes were made to the magnetic field or to the laser pulses, the ytterbium line "melted," changing its phase of matter, a clear signal that thermal energy was absorbed by the atoms. The time crystal therefore does not convert thermal energy into mechanical work. To make the time crystal "tick," one laser sets up a magnetic field that the electrons respond to and the other one flips the spins of some of the atoms. All the atoms are entangled so they all settle into a perfectly stable repetitive spin-flipping pattern that, strangely, is exactly half the rate of the laser pulses. There is no heat exchange or change in entropy because it is in only one ground state at a time. Rather than being a thermodynamic process, this motion, without thermal or kinetic energy, appears to be the first example of broken <a href="https://en.wikipedia.org/wiki/Time_translation_symmetry">time translation symmetry</a> observed in nature. Time translation symmetry is the fundamental assertion that the laws of physics don't change over time, and the conservation of energy in nature depends on it. Scientists make the distinction that if time symmetry is broken <i>explicitly</i>, then the laws of nature can no longer be counted on. In a time crystal, however, the symmetry is broken <i>spontaneously,</i> which means that, only in a specific case, nature chooses a state that doesn't obey time translation symmetry. This exception to the rule is analogous to the breaking of CP and time symmetry observed by the weak force mentioned earlier.<br />
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Conclusion<br />
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The laws of thermodynamics have an all-encompassing scope. They preside over every other science from biology, to geology, chemistry, astrophysics and quantum physics, to name just a few. They outline the basic rules that every process within the vast universe to the sub-atomic realm must follow, which means that these rules must work to explain the extremes of nature, such as black holes. Every scientific discipline from engineering to the applied chemistries to quantum computing must take thermodynamics into account when systems are designed. The laws can also be harnessed as a tool to probe into the mysteries of space-time and quantum behaviour.<br />
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Thermodynamics impacts us all personally. It underlies everything we do in life. It is essential to understanding how all life processes work, and how life evolved on this planet. It will be an essential component in our search for efficient energy sources of the future. It will even help us look for and recognize life on other worlds, as scientists imagine all the ways living systems could utilize exotic sources of energy. Anytime energy is converted from one form into another, or into work and vice versa, in any process, thermodynamics determines what can happen and what can't happen and why.Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-92047614516224331892018-01-03T11:00:00.000-07:002018-01-04T09:29:25.162-07:00The Laws of Thermodynamics PART 3The Laws of Thermodynamics PART 1 <a href="https://www.blogger.com/"><span id="goog_532689932"></span>c<span id="goog_532689933"></span></a><a href="http://sciexplorer.blogspot.ca/2017/12/the-laws-of-thermodynamics-part-1.html">lick here</a><br />
The Laws of Thermodynamics PART 2 <a href="http://sciexplorer.blogspot.ca/2018/01/the-laws-of-thermodynamics-part-2.html">c</a><a href="http://sciexplorer.blogspot.ca/2018/01/the-laws-of-thermodynamics-part-2.html">lick here</a><br />
<br />
The Second Law of Thermodynamics<br />
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The concepts of heat, internal energy and thermal energy we just explored can easily be confused. We still casually but incorrectly talk about heat as if it is something that an object contains, or as a property of that object. Internal energy and thermal energy are sometimes incorrectly used interchangeably even in textbooks (they are interchangeable only in a theoretical <a href="https://en.wikipedia.org/wiki/Ideal_gas">ideal gas</a>, where there is no potential energy - the particle-particle interactions considered are perfectly elastic collisions between atoms which are treated as small hard spheres). Despite these challenges, the second law is no doubt the most misunderstood law in thermodynamics. Even by experts. We could look at it as something to be feared, but we can also see this law as the source of high quality mental fun. Borrowing from physicist Lidia del Rio once again this is where the village witch lives and it is where she does her best magic.<br />
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The second law introduces another term called <a href="https://en.wikipedia.org/wiki/Entropy_(classical_thermodynamics)">entropy</a>. This law states that entropy can never decrease over time in an isolated system. Unlike energy, entropy is not conserved in an isolated system. Remember, an isolated system is one in which neither energy nor matter can ever enter or leave. As a possible consequence, the universe as an isolated system might eventually suffer an ultimate fate called <a href="https://en.wikipedia.org/wiki/Heat_death_of_the_universe">heat death</a>, which is based on the second law of thermodynamics. The universe's entropy will continue to increase until it reaches a state of maximum possible entropy, or thermal equilibrium, where heat exchange between molecules and atoms is no longer possible. All thermodynamic processes die as a result. Entropy is an interesting and far-reaching concept. It does not always relate specifically to the internal energy of a system. Sometimes, entropy is (too-broadly) defined as the level of disorder in a system. It can also be defined as the number of possible <a href="https://en.wikipedia.org/wiki/Microstate_(statistical_mechanics)">microstates</a> within a system. Often, entropy <a href="https://en.wikipedia.org/wiki/Entropy_(information_theory)">is a measure of the amount of information in a system</a>.<br />
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Is the universe actually hurtling toward heat death? Is it truly isolated? Considering that highly organized structures evolve in the universe over time, the notion of increasing entropy in the universe presents some controversy. Wikipedia's brief <a href="https://en.wikipedia.org/wiki/Heat_death_of_the_universe#Controversies">"controversies" entry</a> on heat death offers arguments against even assigning entropy to the universe as a whole, and there are many more arguments for and against to be found online. One question I find intriguing is whether a gravitational field itself has entropy. Gravitational fields have a way of keeping objects out of thermal equilibrium with the space around them. This idea is explored in the controversial theory of <a href="https://en.wikipedia.org/wiki/Entropic_gravity">entropic gravity</a>. Here, gravity is treated as an emergent and <a href="https://en.wikipedia.org/wiki/Entropic_force">entropic force</a>: at the macroscopic scale it is homogenous but at the quantum scale it is subject to quantum disorder, that is, from quantum entanglement of bits of space-time information. This disorder is expressed as a force we define as gravity. It agrees with both Newtonian gravity and general relativity and it offers an explanation for "<a href="https://en.wikipedia.org/wiki/Dark_energy">dark energy</a>," as a kind of positive vacuum energy within space-time.<br />
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A common way of phrasing the second law is to say that a system always tends toward disorder over order, but this statement can be misinterpreted. In physics, entropy is <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Therm/entrop2.html">closely related to the concept of multiplicity</a>. I think it is probably the easiest way to think of entropy. Take as an example a system of 20 books. There are many more ways (a multiplicity of ways) to accomplish a jumbled pile (no rules) than to stack them up neatly (specific rules). We can say that the randomly jumbled mess of books has higher entropy than the neat stack does. What happens when we come along and straighten up the books? Does the book "system" go against entropy's natural tendency toward disorder? No, because we were involved in the process we became part of the system. We did work on the system to decrease its entropy. Overall, the entropy of the "us plus the books" system increased.<br />
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The second law also states that systems <a href="https://en.wikipedia.org/wiki/Thermodynamic_equilibrium">tend toward a state of equilibrium</a>. To explain this, let's take a different example. We have two volumes of gas separated by a movable wall inside a really good Thermos bottle, a bottle so well made we can consider the system isolated. The wall itself is also made of a perfect insulating material so no heat transfer can happen. We can think of this arrangement as two systems in mechanical contact with each other. To start with, we have one gas that is hot and the other one is cold. The hot gas will exert thermal pressure and push the wall into the cold side. We will assume that the wall movement is frictionless. The hot gas will expand and cool and the cold gas will compress and warm up (this is another example of an adiabatic process). The system does mechanical work (the expansion and compression of gases as well as the movement of the wall). The wall will stop moving when both gases reach the same pressure. If it is the same type of gas on each side, the compartments will also reach the same temperature. The two systems will come to rest at a state of thermodynamic equilibrium, and in this state, no part of the two-gas system can do any more work on the other part. This system still has thermal energy but that energy can't be used to do any work (within the system). This state has the highest entropy it can have under these circumstances. In this case, it is not self-evident that the system has reached a state of maximum disorder, or even that it has achieved the greatest multiplicity of possible states. We do know, however, that it has evolved toward an end state of thermodynamic equilibrium, which is also an evolution toward maximum achievable entropy.<br />
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As we can see, there is more than one way to look at entropy, like looking at a magic trick from different angles. No one angle gives it completely away.<br />
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The system's movement toward equilibrium is an <a href="https://en.wikipedia.org/wiki/Irreversible_process">irreversible process</a>. It can't return to its original state of disequilibrium unless work is done on it. We could set up a real system of two gases as close as we can to the one just described and we would discover why this move toward equilibrium is irreversible. It might seem that we could restart the experiment over and over forever. Each time we would do exactly the same mount of work to heat and cool our gases to their starting temperatures. But even the best insulating material is imperfect and only <a href="https://en.wikipedia.org/wiki/Superfluidity">superfluids</a> are truly frictionless. Some heat will be lost to the outside of the Thermos, and some energy will be lost as heat loss through friction by the wall as it moves. If the experiment was repeated over and over using the same Thermos and the same initial work input, the overall thermal energy available to the system would eventually decrease and the entropy of the system (which now includes the room into which the heat dissipates and the mechanism we use to heat/cool the gases) would increase.<br />
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Another way of defining an irreversible process considers the role of chaos in systems. Any system of interacting molecules will include interactions between them that are <a href="https://en.wikipedia.org/wiki/Molecular_chaos">chaotic</a> in nature. We often think of chaos as what our kids do to their rooms but it is also a scientific theory. Systems are very sensitive to initial conditions, so even very tiny deviations in conditions at the beginning of a process can result in significant differences in how that system progresses over time. A hurricane is a chaotic system and that's why its strength and path are impossible to predict with certainty even a few days out. Almost every real process contains one or more chaotic elements. If we try to reverse a process back to its initial conditions we cannot rely on a series of predictable step-by-step transformations to arrive at exactly the same starting state. The specific process is irreversible and the outcome is not repeatable.<br />
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Is there any truly reversible process in thermodynamics? Even an atomic clock, which relies on extremely stable microwave cavity oscillations, <a href="https://en.wikipedia.org/wiki/Atomic_clock#Evaluated_accuracy">shows minute frequency drift over time</a>. In these clocks, laser-cooled atoms "tick" back and forth between an excited state and ground state. The energy difference between the states is always perfectly precise because it is quantum in nature. However, even the <a href="https://en.wikipedia.org/wiki/NIST-F1">NIST-F1 cesium clock</a> loses a second every 300 million years. Rather than heat loss per se, this system loses energy because the field transitions dissipate energy (they do work at the quantum scale just to keep going). The clock's entropy increases, albeit very slowly.<br />
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A process that doesn't generate entropy is a reversible process. The second law of thermodynamics (that entropy tends to increase in systems) is a consequence of the irreversibility of processes. <a href="https://en.wikipedia.org/wiki/Maxwell%27s_demon">Maxwell's demon</a> was once held up as a theoretical system in which entropy could hold steady during a thermodynamic process. But under close scrutiny, even this famous thought experiment devised by <a href="https://en.wikipedia.org/wiki/James_Clerk_Maxwell">James Clerk Maxwell</a>, is not a reversible process. The idea itself though is intriguing: A microscopic demon guards the gate between two halves of a room. He lets only slow molecules into one half and fast molecules into the other half. Eventually you expect one half of the room to be warmer than the other half. This reduces the randomness of the molecular arrangement of gases and therefore reduces the entropy of the room (system). Looked at another way, it takes a system in equilibrium out of equilibrium. The argument seems rock-solid until you realize that the entropy of the demon itself (all that sorting work it is doing) actually increases the system's entropy overall more than it decreases it. In nature, many systems appear to have decreasing entropy. In all living systems, molecules are ordered into, and maintained as, intricate arrangements. Highly ordered galaxies appear to form from disordered clouds of gas. Disordered water vapour molecules flash-freeze into beautiful crystal patterns on a winter window. The key to all of these systems, living and non-living, is that they are open to some extent to their surroundings, like any real system is. In each case, surrounding entropy increases by an even greater amount, leading to a net entropy increase overall.<br />
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Scientists can only approximate a perfectly reversible process. An example is a process that is reversed by introducing an extremely tiny change to some property of a system that starts at equilibrium with its surroundings. If the tweak is small enough, the system remains so close to equilibrium that any deviation from it cannot be accurately measured yet the process itself does reverse.<br />
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In an ideal system designed to do work, such as a theoretical engine that has 100% efficiency, none of the work performed would be lost to heat transfer. The efficiency of a real engine, however, is always less than 100%, significantly less. If it is a piston engine or a steam engine, its efficiency can be analyzed quite easily by plotting a pressure-volume curve as it goes through a compression/expansion cycle. The area bound by the curve is the work done. The more efficient the engine is, the more closely that curve will follow an ideal equilibrium curve for that engine. An efficient engine never deviates far from its equilibrium state. A <a href="https://en.wikipedia.org/wiki/Carnot_cycle">Carnot heat engine</a>, mentioned earlier, is a theoretically ideal thermodynamic cycle, in which no energy is lost as "waste" heat transfer and there is no net increase in entropy. It represents an engine with 100% efficiency. An irreversible (real) process always strays away from that ideal equilibrium curve.<br />
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An example of a process that strays far from equilibrium is the carbon dioxide fire extinguisher once again. When you trigger the fire extinguisher, the carbon dioxide gas sprays out of the canister so fast that the air/carbon dioxide system has no time to reach equilibrium at first. The carbon dioxide cools adiabatically. The original amount of thermal energy is now spread over a large volume of gas cloud. Almost all of the potential energy of the pressurized system is lost through the work of adiabatic expansion. Even more work would be required to compress that gas back into the canister, much more than the work originally done because this process is now far from equilibrium. It is definitely not reversible and there is a significant increase in the system's entropy. However, entropy is also a statistical concept. Even in this case there is a chance, an infinitesimally small chance, that all the carbon dioxide molecules could spontaneously re-arrange themselves back into the canister (against their pressure gradient) through purely random molecular movements, reducing the system's entropy, and reminding us that the second law itself is statistical in nature. In a macroscopic system full of billions of atoms, it is vanishingly unlikely, but in a system of just a few atoms, the random chance of them all doing something together goes up. A triggered extinguisher, like all thermodynamic processes, proceeds in one direction only, and that is in the direction of increasing entropy. This direction, in turn, implies a forward arrow of time.<br />
<br />
Thermodynamic Arrow of Time<br />
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The universe follows the second law of thermodynamics, where all real processes are irreversible, with the consequence that <a href="https://en.wikipedia.org/wiki/Arrow_of_time">time must flow in one direction only</a>. The increasing entropy of an evolving system gives us an impression of time and means that we can distinguish past events from future ones. The exception is a system that is in prefect equilibrium. In this case, the entropy remains the same and it is impossible to distinguish a past state of that system from a present state. The arrow of time would have seemed self-evident to Rudolf Clausius, <a href="https://en.wikipedia.org/wiki/History_of_entropy">who coined the term "entropy" in the mid 1800's</a>, defining it as heat that incrementally dissipated from heat engines. The fact that time moves forward then seemed obvious but that confidence wasn't about to last. In just a couple of decades, <a href="https://en.wikipedia.org/wiki/Quantum_field_theory">quantum field theory</a> was formulated to allow for charge, parity and time reversal (<a href="https://en.wikipedia.org/wiki/CPT_symmetry">CPT symmetry</a>). At around the same time, the idea that <a href="https://en.wikipedia.org/wiki/Space-time">space and time are sewn together into a four-dimensional fabric</a> was quickly becoming accepted science. <a href="https://en.wikipedia.org/wiki/Introduction_to_general_relativity">General relativity</a> and <a href="https://en.wikipedia.org/wiki/Special_relativity">special relativity</a> (which preceded general relativity) both treat time as something that is malleable and part of the system rather than outside of it. These theoretical developments brought our assumptions about time, and the second law of thermodynamics itself, into question.<br />
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Time, as we experience it, is a broken symmetry; there is no mirror in which time flows backward. Broken eggs can't reassemble. Aging doesn't reverse itself except in The Curious Case of Benjamin Button. Why this is so is actually a deep mystery in physics. Charge, parity and time reversal (CPT symmetry) is a fundamental symmetry of physical laws (except thermodynamics). The implication of this symmetry is that there is no theoretical reason why a "mirror" universe, one with an <a href="https://en.wikipedia.org/wiki/Parity_(physics)">arbitrary plane of inversion</a> (which you could think of as a vast three-dimensional mirror), with reversed momenta (complete with time running backward) and populated with antimatter (carrying opposite charges), couldn't evolve under our very same physical laws. It might even start from a vast Big Bang and shrink in volume rather than expand as ours does. <a href="https://en.wikipedia.org/wiki/Entropy_(arrow_of_time)#Cosmology">Entropy in such a universe</a>, we might assume, would tend to decrease rather than increase. However, even this assumption might be too simple. A number of physicists speculate that even in a universe that oscillates - it expands and contracts over and over - entropy might continually increase.<br />
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We might think that CPT symmetry, a mathematical theorem particularly useful in quantum physics, is a violation of the laws of thermodynamics. It should dissolve the one-way arrow of time and the rule that entropy never decreases in an isolated system. For example, we know that antimatter particles exist in our universe and we might assume that they travel backward through time. This assumption is wrong but it is not so easy to understand why. We can take the <a href="https://en.wikipedia.org/wiki/Positron">positron</a>, the electron's antimatter twin particle, as an example. According to a theory called <a href="https://en.wikipedia.org/wiki/Quantum_electrodynamics">quantum electrodynamics</a> (QED), an antimatter particle, mathematically speaking, travels backwards in time. If you look at a <a href="https://en.wikipedia.org/wiki/Feynman_diagram">Feynman diagram</a> of particle interaction, backward time travel by particles is commonly depicted. Below, we can see that an e<sup>+</sup> (positron) and an antiquark (the q with the line over it) both travel backward in time in this depiction of electron/positron (e<sup>-</sup>/e<sup>+</sup>) annihilation (see the black arrows angled toward the left).<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Joel Holdsworth; Wikipedia</span></span></td></tr>
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Positrons are used everyday in large hospitals and we know that positrons are never emitted before a PET scanner is turned on in the hospital (thankfully). Why? A positron is exactly the same as an electron, but with positive charge. "Flip" time and you have an electron. A perhaps unsatisfying way to understand this problem is to treat time as a mirror-like parity that is flipped in a dimensional kind of way (think of four-dimensional space-time) rather than as the continuous forward flow we experience.<br />
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Even though all of our physical laws (except thermodynamics) display a fundamental CPT symmetry, it doesn't mean that all processes obey it. Three of the four fundamental forces of nature, the strong force, the electromagnetic force and gravitation (which, as general relativity, is not formulated in terms of quantum mechanics) obey CPT symmetry but the <a href="https://en.wikipedia.org/wiki/Weak_interaction#Violation_of_symmetry">weak fundamental force occasionally violates both parity and charge symmetry</a> at the quantum level. Recently, researchers discovered that this quantum process also sometimes violates time symmetry as well. Oscillations between different kinds of B-meson particles during the weak interaction happen in both directions of time but the rates are slightly different - this disparity doesn't have anything to do with the thermodynamic reason for time's broken symmetry however. An article from SLAC National Accelerator Laboratory at Stanford University <a href="https://www.slac.stanford.edu/pubs/slacpubs/13500/slac-pub-13518.pdf">offers an excellent technical comparison</a> between broken T-symmetry at the macroscopic and quantum scales. For a thorough discussion of what this means philosophically, try <a href="https://link.springer.com/article/10.1007/s10838-016-9342-z">this 2017 article</a> posted by the Journal of General Philosophy of Science. I found it a good read.<br />
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As we've seen, at the quantum scale, processes are time-reversible (with the weak force exception). Quantum processes, according to the <a href="https://en.wikipedia.org/wiki/Copenhagen_interpretation">Copenhagen interpretation</a>, are governed by the Schrodinger equation, which has T-symmetry built into it. <a href="https://en.wikipedia.org/wiki/Wave_function_collapse">Wave function collapse</a>, however, does not. This is a mathematical framework that links indeterminate ("fuzzy") quantum particle behaviours to the determinate macroscopic behaviours of substances. Therefore, it finds itself at the epicenter of how time symmetry in the quantum world breaks down at the larger scale we experience.<br />
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Mathematically, a quantum system is laid out as a superposition of several equally possible states (technically called eigenstates (https://en.wikipedia.org/wiki/Introduction_to_eigenstates)), which reduce to a single eigenstate when the system is observed or measured. How this happens, and even if this happens, physically, is up for debate. It is simply a mathematical description, which means that the process itself is a black box, but it does provide a link between <a href="https://en.wikipedia.org/wiki/Quantum_indeterminacy">quantum indeterminacy</a> and determinate macro-processes, thermodynamics being one of them. Somehow, inside the "black box," time switches from reversible to irreversible. How do two well established and experimentally proven but mutually exclusive theories (thermodynamics versus quantum mechanics) co-exist? This problem is encapsulated in <a href="https://en.wikipedia.org/wiki/Loschmidt%27s_paradox">Loschmidt's Paradox</a>. No one knows the solution to Loschmidt's Paradox, although theorists have been working on it for decades.<br />
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What is known is that the second law of thermodynamics (and its arrow of time) is a statistical principle based (somehow) on the behaviors of countless quantum time-symmetrical particles. If we keep the statistical nature of these descriptions in the front of our minds and go back to the simple example of a stack of books, we could add that, yes, there are countless more ways those books can fall into a messy pile than there are ways to stack them up neatly BUT there is no rule against the books spontaneously falling into a nice neat stack either. It's just extremely unlikely. If this happened, it would not mean that the second law of thermodynamics just broke. It means that the underlying statistical nature of the arrow of time is revealing itself. A puzzle appears, however, when we think again about the increasing entropy of the universe. We could assume from that that the universe initially had very low entropy. There were <a href="https://en.wikipedia.org/wiki/Planck_units#Cosmology">no indistinguishable particles or forces at the very beginning</a>. Through a series of <a href="http://abyss.uoregon.edu/~js/ast123/lectures/lec18.html">symmetry-breaking</a> processes, four distinct fundamental forces and all the myriad particles of matter emerged as the universe expanded and cooled. The question is, if the universe started with very low entropy wouldn't it have been extremely unlikely as well?<br />
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We might wonder if the arrow of time is an aspect of dynamics that <a href="https://en.wikipedia.org/wiki/Emergence">emerges at the macroscopic scale</a>. Emergence might not be the best description because at the subatomic particle level, time seems to exist but it runs in either direction. Time's one-way arrow only emerges within a large collection of molecules. Time's arrow might be better described as a symmetry that breaks at the macroscopic scale.<br />
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All these questions, I hope, point out that our understanding of time itself is a problem when we think about the second law of thermodynamics and entropy. Time is not a unified concept in physics. Depending on the theoretical framework you chose - quantum mechanics, classical dynamics or even general relativity - time can be reversible. Or it can be a one-way arrow. Or it can be illusion altogether, because general relativity treats time as one dimension in a four-dimensional stretchy fabric where all past and future times are equally present and our present time is non-important.<br />
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Black Hole Entropy: Testing Thermodynamics<br />
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A black hole tests all of our scientific laws and it is especially interesting when viewed as a thermodynamic object, which it surely is. A <a href="https://en.wikipedia.org/wiki/Black_hole">black hole</a> is a region of space-time bent so severely by gravity that nothing, not even electromagnetic radiation such as light, can escape. Once energy or matter crosses the black hole's <a href="https://en.wikipedia.org/wiki/Event_horizon">event horizon</a>, it is lost from our observations. Black holes can be directly observed when in-falling matter is heated by internal friction, creating an accretion disk external to the event horizon, and it can be extremely bright. A black hole also gradually emits radiation, called <a href="https://en.wikipedia.org/wiki/Hawking_radiation">Hawking radiation</a>, and it has a temperature, which means that these objects should be subject to the same laws of thermodynamics as any other object.<br />
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Entropy, as we've explored already, can be understood in several different ways. Some physicists might argue that entropy is best understood as a statement about information rather than about order or disorder. Generally the various descriptions of entropy agree with each other but under specific circumstances, individual weaknesses with each approach become evident. Theorists appear to be best equipped to tackle the question of black hole entropy by interpreting entropy as information. More information encoded in a system means it has higher entropy. As an isolated system's entropy can never decrease, the information encoded by an isolated system can never decrease. This is a slightly different take on the idea of entropy as a multiplicity of microstates.<br />
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Black holes are wonderfully mysterious objects. Inside a black hole, matter becomes inaccessible to our observations. But, the momentum and charge of that matter is conserved, and its mass remains to bend space-time around it. Most black holes, especially those formed by massive collapsing spinning stars, are expected to have not only charge but also significant angular momentum. According to the second law of thermodynamics, we expect that matter disappearing into a black hole should increase the black hole's entropy. This implies that a black hole has non-zero entropy. A number of theoretical physicists are currently working on how entropy works with black holes and how to measure it. The new methodologies they have come up with so far are surprising. We immediately realize how unusual a black hole is when we learn that instead of using volume to calculate the entropy of what we assume is a spinning spherical object, we must use the area bound by its event horizon instead. In 1973, Jakob Bekenstein calculated black hole entropy as proportional to the area of its event horizon divided by the <a href="http://scienceworld.wolfram.com/physics/PlanckArea.html">Planck area</a> (the area by which a black hole surface increases when it swallows one <a href="https://en.wikipedia.org/wiki/Bit">bit of information</a>). In 1974, Stephen Hawking backed up this work and additionally showed that black holes emit <a href="https://en.wikipedia.org/wiki/Thermal_radiation">thermal radiation</a>, which means they have a specific temperature. So how does the second law of thermodynamics enter? It has actually <a href="https://en.wikipedia.org/wiki/Black-hole_thermodynamics">been rewritten for black holes</a> to say that the total area of the event horizons of any two colliding black holes never decreases. This is part of the <a href="https://en.wikipedia.org/wiki/Black-hole_thermodynamics#The_second_law_2">closely analogous set of laws for black hole mechanics</a>.<br />
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How do we get an intuitive picture of how black hole entropy works? We can start by looking at a black hole's entropy statistically. Each of all the countless billions of particles that have fallen down the gravity well of a black hole will be in a specific <a href="https://en.wikipedia.org/wiki/Thermodynamic_state">thermodynamic state</a>. Each microstate will contribute to what should be an enormous number of <a href="https://en.wikipedia.org/wiki/Microstate_(statistical_mechanics)">possible microstate arrangements</a>. Microstates in a macroscopic system are all the different ways that the system can achieve its particular macro-state (which is defined by its density, pressure, volume, temperature, etc.).<br />
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By treating these microstates statistically, we can come up with an approximation of the black hole's overall entropy. This would be straightforward if black holes didn't present us with a unique and bedeviling twist: the <a href="https://en.wikipedia.org/wiki/No-hair_theorem">no-hair theorem</a>. The no-hair theorem argues that this approximation cannot be done. Aptly named, it tells us that a black hole can be described by only three classical parameters: its mass, its charge and its angular momentum. All the particles that fell into a black hole do not contribute to any kind of unique character to it. A black hole that swallowed up a cold hydrogen gas cloud looks the same as one that swallowed an iron-dense planet. Instead, the no-hair theorem treats a black hole as a ubiquitous and enormous single "homogenous" particle. A possible analogy might come from another housekeeping chore. The vacuum cleaner sucks up all the stuff off the floor. A CSI investigator could look in the canister afterward and determine, through skin flakes, hairs and other debris, who lived in the room, and perhaps even what they were doing over the week. In the case of black holes, the "canister contents" appear to become generic once they cross the event horizon. You can't tell what particular atoms fell in, when they fell in, and what their velocities, etc., were. The no-hair theorem is a mathematical theorem: it is the solution to <a href="https://en.wikipedia.org/wiki/Maxwell%27s_equations_in_curved_spacetime">Einstein-Maxwell equations of electromagnetism in curved space-time</a>. It turns all different forms of matter/energy into a generic <a href="https://en.wikipedia.org/wiki/Electromagnetic_stress-energy_tensor">electromagnetic stress-energy tensor</a>, which bends space-time. What this theorem implies is that all of the information encoded by the <a href="https://en.wikipedia.org/wiki/Quantum_number">special quantum characteristics</a> of each matter particle, such as baryon number, lepton number, colour charge, and even whether it is matter or antimatter, is annihilated when it falls into a black hole. More specifically, it implies that the black hole as a system will have much less entropy than the ordinary matter originally had. Matter and energy being lost in a black hole, if treated as an isolated system, appears to represent a process that decreases entropy, and that is a violation of the second law. <br />
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We can't just toss out the no-hair theorem as a mathematical curiosity that might well prove invalid in nature. d. Its validity is <a href="https://physics.aps.org/articles/v9/52">backed up by recent observations of black holes by LIGO</a>, a gravitational wave observatory. To add to this entropy problem we could draw the additional, and no less astounding, conclusion that the no-hair theorem suggests that a black hole is actually just one a single microstate (just one "particle"), which means it should have not just low entropy but zero entropy, if we interpret it as there's only one way to assemble a single microstate.<br />
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According to quantum mechanics, the quantum information encoded in a particle (its spin, angular momentum, energy, etc.) must be conserved during any process, which is a variation on the first law of thermodynamics, except that the focus here is on information rather than energy. Where does that information go inside a black hole? Hawking radiation, which leaves the black hole system, is a natural place to look, but Hawking's theory suggests that this radiation is "mixed," which means that it is generic; it doesn't contain any of the specific particle information that went into it. To make things even more interesting, there is some serious debate about <a href="https://en.wikipedia.org/wiki/Hawking_radiation#Overview">whether Hawking radiation, as described using quantum theory, actually is a form of thermal radiation</a>. The conclusion that a black hole has a temperature comes not from direct observation (which could be understood using classical statistics). It is based, instead, on quantum mechanics. Thermal radiation, emitted from a <a href="https://en.wikipedia.org/wiki/Black_body">black body</a> (a physical object that absorbs all incoming electromagnetic radiation), contains information about the body that emitted it. Hawking radiation contains no such information. It is based on the law of conservation of energy only. In space-time, <a href="https://en.wikipedia.org/wiki/Virtual_particle">virtual particles</a> pop in and out of existence all the time everywhere. They exist because space-time has a non-zero <a href="https://en.wikipedia.org/wiki/Vacuum_energy">vacuum energy</a>, which is a consequence of the <a href="https://en.wikipedia.org/wiki/Uncertainty_principle">uncertainty principle</a>. Close to the very high-energy space-time environment around a black hole, some theorists suspect that virtual particles, as particle-antiparticle pairs, have enough energy to become real particle pairs, with mass. If one of the pair falls in, it must have negative energy (according to an outside observer) in order to preserve the conservation of energy law. This also means it has negative mass (there is a <a href="https://en.wikipedia.org/wiki/Mass-energy_equivalence">mass-energy equivalence</a>), which means that the black hole itself loses mass and appears to emit a particle (again, as observed by an outside observer). Hawking radiation has not been observed yet, but by using an analogue acoustic black hole made in a lab, scientists <a href="https://www.sciencealert.com/a-lab-made-black-hole-might-have-finally-proved-stephen-hawking-right">have found strong evidence</a> that suggests Hawking radiation exists around real black holes. <br />
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Good evidence for the existence of black hole radiation, whether it is thermal or not, might solve the issue of conservation of energy but it doesn't appear to conserve information. Information can be lost by one of a pair of <a href="https://en.wikipedia.org/wiki/Quantum_entanglement">entangled particles</a> when it falls into a black hole and is lost. Entangled particles are thought to be very common in space-time and they can also be physically very far apart in space, across the universe in fact. Distance is irrelevant to quantum entanglement. Because of quantum mechanics, an entangled pair or a group of particles can be described by a singular unique quantum state (spin, angular momentum, energy, etc.) just as a single particle is. From a quantum mechanics point of view, the pair or group becomes a single particle (some theorists think that matter inside a black hole might be quantum-entangled). If one of a pair of quantum entangled particles falls into a black hole and loses its quantum signature, does its entangled partner pop out of existence somewhere else in the universe at the same time? It seems that this process would continuously decrease the entropy of the universe as a whole. The dilemma this presents is called the <a href="https://en.wikipedia.org/wiki/Black_hole_information_paradox">black hole information paradox</a>. One could argue that because time slows down to a stop in the infinite gravity well at the event horizon of a black hole, nothing really ever goes in. Its quantum information remains somehow encoded, smeared somehow across the event horizon, scrambled up and out of reach. Newer models of quantum gravity suggest that the particle left behind remains entangled by whatever form of matter/energy its partner is now in inside the black hole, thus dissolving the paradox. Another argument emerging among physicists is also exciting. It actually uses quantum entanglement to solve the black hole paradox. It uses wormholes to link the paradox phenomenon with the Einstein-Rosen Bridge (or wormhole), described by two previously unrelated theories. It is laid out in <a href="https://www.quantamagazine.org/wormhole-entanglement-and-the-firewall-paradox-20150424/"><span id="goog_292776609"></span>this Quantum magazine article<span id="goog_292776610"></span></a>. Entangled particles inside and outside a black hole could remain connected through the continuous space-time that would exist inside a wormhole, solving the information paradox. No one's sure yet if that idea holds up theoretically.<br />
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The <a href="https://en.wikipedia.org/wiki/Holographic_principle">holographic principle</a>, which is gaining momentum in theoretical physics, is yet another way to explore the information paradox. It suggests that a black hole encodes all of the particle information just outside the event horizon as <a href="https://en.wikipedia.org/wiki/Degrees_of_freedom_(statistics)">statistical degrees of freedom</a>. Degrees of freedom are a measure of information, closely related to the idea of multiple microstates. How this information is stored, and what form it is in, is impossible to visualize, however, because a black hole is treated theoretically (in almost all theories) as a four-dimensional (space-time) object. Although a black hole should appear as a sphere to an observer, it is not a three-dimensional sphere we can relate to. It is a singularity of mass. The event horizon, likewise, is not a physical two-dimensional barrier shell around a black hole. It is the last distance from which light can escape the gravitational well, measured as the <a href="https://en.wikipedia.org/wiki/Schwarzschild_radius">Schwartzchild radius</a>. For example, Earth has a Schwartzchild radius of about 2 centimetres, which means that if Earth's mass was compressed into a sphere of 2 cm radius, it would be so dense that it would spontaneously collapse into a black hole <a href="https://en.wikipedia.org/wiki/Gravitational_singularity">singularity</a>.<br />
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From our perspective, information is last observed at the event horizon of a black hole. This approach helps us understand why entropy is a measure of event horizon area, rather than volume. It also implies that a black hole, rather than being a zero-entropy object, is a maximum entropy object. Depending on how we look at it, generic information can be thought of as equivalent to maximally mixed information, an equilibrium state.<br />
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No matter how we look at the information paradox, information inside a black hole seemingly must get back out through black hole evaporation (as Hawking radiation). A black hole that doesn't feed on matter should gradually shrink and eventually disappear, meaning that somehow, the quantum information of all that matter must get back out. If the information is irretrievably lost from our universe, then black holes either do no obey thermodynamics or they represent some kind of door into some other entity and the universe is not an isolated system after all. It could even mean that ordinary matter and energy as we know it is actually an illusion. It could be information encoded on a surface area, making the universe, by extension, <a href="https://en.wikipedia.org/wiki/Holographic_principle">a hologram of that data</a>.<br />
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Black hole entropy has also recently been calculated based on a <a href="https://en.wikipedia.org/wiki/Supersymmetry">supersymmetric</a> black hole in <a href="http://superstringtheory.com/">string theory</a>. This <a href="https://arxiv.org/abs/hep-th/0703035">technical 2007 review by T. Mohaupt</a> describes the reasoning and process. This solution ties in with the holographic principle (which is also based on string theory) and its solution closely matches that of Jakob Bekenstein (who based black hole entropy on the area of the event horizon). The fact that the two calculations match up closely gives a number of theorists hope that string theory could be a route to ultimately solving the information paradox.<br />
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Based on all of these and other developments, physicists Brandon Carter, Stephen Hawking and James Bardeen have formulated <a href="https://en.wikipedia.org/wiki/Black-hole_thermodynamics#The_laws_of_black-hole_mechanics">a series of black hole mechanics laws</a>, which are analogous to the laws of thermodynamics. While thermodynamics is a classical science, these laws attempt to integrate general relativity, quantum mechanics and thermodynamics together. As I hinted at earlier, these mechanical laws offer up some seemingly odd conclusions. Uniform gravity at the event horizon is analogous to thermal equilibrium (the first law of thermodynamics). Entropy is analogous to the increasing surface area of the event horizon (the second law of thermodynamics). New theories about black hole entropy offer some serious food for thought because we approach entropy not just through the lens of classical mechanics but through the lenses of general relativity and quantum mechanics as well. Being a universal rule of nature, shouldn't entropy find its expression there too?<br />
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One of the most interesting approaches to black hole entropy was done under a specialized mathematical framework. It is <a href="http://www.preposterousuniverse.com/blog/2009/01/12/where-does-the-entropy-go/">laid out by physics theorist Sean Carroll</a> in one of his blog entries from 2009 (I wholeheartedly recommend his blog). As a black hole's rotation and charge increases, its entropy approaches zero. This statement is analogous to the third law of thermodynamics, which will be explored next. According to this law, no system can have exactly zero entropy, so this means there is a limit to the spin and charge of a black hole. The third black hole law of mechanics might represent the deepest puzzle yet for theorists.<br />
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A black hole at the spin/charge limit is called an <a href="https://en.wikipedia.org/wiki/Extremal_black_hole">extremal black hole</a>. It is a black hole that has the smallest possible mass at a given charge and angular momentum. It is therefore the smallest possible mass black hole that could theoretically exist while rotating at a constant speed. If these objects existed, they would be microscopic, but they are only theoretical. In theory, an extremal black hole could be created in which all of its energy comes from the charge, or electrical field, and none from matter. Such a black hole is a product of <a href="https://en.wikipedia.org/wiki/Euclidean_quantum_gravity">Euclidean quantum gravity</a>, a theory of space-time in which time is treated exactly like another spatial dimension. The entropy of such a black hole can be calculated by using string theory, and it comes out to be exactly zero, which is forbidden by the third law of thermodynamics. A first reaction could be, well, that's the end of that thought-stream, and Dr. Carroll suggests that this is exactly what the authors of the original paper thought. But then the idea was revisited because it seemed to hint at something very interesting.<br />
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In an extremal black hole, entropy discontinuously drops as charge is increased, and eventually its hits a limit where the mathematical solution the researchers used splits into two different space-times! Part of this mystery includes the fact that the mathematics of all charged black holes gives them not one but two event horizons. The outer event horizon is the one you expect - a point of no return. The inner event horizon is located between that point of no return and the singularity itself. What's unique is that an object between the two horizons isn't forced to crash into the singularity. Inside the black hole, moving forward in time means moving inward toward the inner event horizon. That part is inevitable. However, outside the black hole and inside the inner event horizon, time moving forward looks normal and an object in either of those spaces isn't forced anywhere.<br />
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As you increase the charge and keep the mass the same, the two horizons come together. You are moving toward an extremal black hole. You would expect that the region of space-time between the horizons will eventually disappear, but it doesn't. It reaches a finite size and stays there, until you reach an exact extremal black hole, and which point it suddenly and discontinuously disappears. The entropy, when calculated, decreases smoothly alongside the increasing charge until exactly when an extremal black hole is reached and at that point it suddenly slips to zero. This asks the question: is there a theoretical problem here or does the entropy suddenly escape into some new and different space-time (as space-time itself appears to split as well)? Does it offer a clue about where matter (and all that missing entropy) goes inside a real black hole? This new space-time is a mathematical solution called two-dimensional <a href="https://en.wikipedia.org/wiki/Anti-de_Sitter_space">anti-de-Sitter space-time</a> on a two-dimensional sphere. Dr. Carroll himself wonderfully refers to this hidden space-time as "Whoville" In his fascinating post on this mysterious theoretical journey. You can download a pdf of the scientific paper he and the original authors written on it <a href="https://arxiv.org/abs/0901.0931">here</a>.<br />
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Although we might not know exactly what kind of space-time black hole particles find themselves in if that's what they finds themselves in, black hole physics seems to be a great tool to use to search out the limits of the second law of thermodynamics. Black holes go an additional step by demanding that all of our disparate laws of nature come together to describe them. Dr. Carroll puts it well: black holes are fertile "thought-experiment laboratories" to test our understanding of thermodynamics, especially the second law.<br />
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For The Laws of Thermodynamics PART 4 <a href="http://sciexplorer.blogspot.ca/2018/01/the-laws-of-thermodynamics-part-4.html">click here</a>Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-65259408054937315112018-01-01T13:34:00.001-07:002018-01-03T11:01:08.762-07:00The Laws of Thermodynamics PART 2The Laws Of Thermodynamics PART 1 <a href="https://sciexplorer.blogspot.ca/2017/12/the-laws-of-thermodynamics-part-1.html">click here</a>.<br />
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First Law Of Thermodynamics<br />
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This law states that the total energy of an isolated system is constant. This energy is generally the sum of its kinetic, potential and chemical energies. In some systems, nuclear, magnetic potential or electrical energy can be considered as well. While the Zeroth Law defines the temperature of a system, this law defines the energy of the system. It also states that energy cannot be created nor destroyed. It can only transformed from one form to another. The idea is the same as the broader <a href="https://en.wikipedia.org/wiki/Conservation_of_energy">conservation of energy law</a> in physics, except that here we focus on internal energy. Although not a new idea in the mid 1800's, this law took time to hammer out empirically as a mathematical statement, which can be written as <span style="background-color: white; color: #222222; font-family: "times new roman"; font-size: 12.5pt;">Δ</span>U = Q + W. Yet another way to express this law is dU = -PdV. "d" is used because this formula for change is written as a <a href="https://en.wikipedia.org/wiki/Differential_of_a_function">differential </a>to describe a changing system. The change in internal energy of a system, in this case, is equal to the inverse of the pressure times the change in volume. There is no heat term. Sometimes, the internal energy of a system can change without any heat exchange. I mentioned a phase change as an example earlier. This can also happen when work is done at such a slow pace that heat dissipation in the form of friction approaches zero or when the system is adiabatic in nature (I will describe this kind of system in a bit). However it is written, the first law links internal energy, heat and work. More specifically, a change in internal energy can be achieved by countless different combinations of heat and/or work added or removed from a system.<br />
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Even though caloric was incorrectly thought to be a substance bearing heat the now-obsolete <a href="https://en.wikipedia.org/wiki/Caloric_theory">caloric theory</a> worked seamlessly with the current laws of conservation of energy and matter. It worked because it was assumed that caloric could never be created or destroyed in an isolated system. Thought of as a weightless self-repelling gas, caloric could pass through the pores in a liquid or solid from hotter areas into cooler areas. The coffee in our example cools, this theory would argue, because caloric suffused from the coffee into the surrounding air, warming it. We now know that the coffee cools because heat is transferred from the coffee to the countertop and the air through the processes of <a href="https://en.wikipedia.org/wiki/Thermal_conduction">thermal conduction</a> and <a href="https://en.wikipedia.org/wiki/Thermal_radiation">thermal (infrared on our case) radiation</a> respectively.<br />
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According to the first law, the internal energy of a system will decrease if it loses heat through thermal conduction, radiation or convection, or if it does work. The energy lost is transferred into the system's surroundings. As mentioned earlier, the total energy of the larger isolated system does not change. The internal energy of the coffee decreases as it cools but the internal energy of the room in which the coffee sits doesn?t change IF it is perfectly sealed off from any energy or material transfer, which nothing ever is in reality. In contrast, any work or heat that goes into a system increases its energy. For example, if we wind up an old-fashioned mechanical watch, we are applying potential mechanical energy to its spring mechanism. It will begin to tick and it will eventually wind down to a stop, when the potential spring energy is depleted. The energy hasn't disappeared, however. It was transferred into work ? the movements of the gears inside the watch and of the arms around the face. Some energy was transferred into sound energy as the ticking you hear. Energy was also transformed into internal friction in the spring and of the tiny gears against one another. The watch is a closed system. No matter is transferred, but heat from friction, though an immeasurably small amount, escapes the watch system into the surroundings. If we wind it up again, we have restored the system's energy by doing work on the system. The source of energy in the watch (which comes from our muscles winding it up) is transferred into work and waste heat each time. No real system has an infinite source of energy. Even the orbits of planets, etc., which seem to be eternal, are part of a system that loses energy through solar wind, resistance to the particles that make up interstellar space, and as gravitational and thermal radiation from the solar system. There is no such thing as <a href="https://en.wikipedia.org/wiki/Perpetual_motion">perpetual motion</a>. Both the first and second (as we will see) laws of thermodynamics forbid it. Even recently observed <a href="https://www.sciencealert.com/it-s-official-time-crystals-are-a-new-crazy-state-of-matter-and-now-we-can-create-them">time crystals</a> do not violate thermodynamics. These crystals spontaneously change from moment to moment. We will explore this fascinating new state of matter later on.<br />
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Adiabatic Process<br />
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Thermodynamics got its start with the study of a water vapour under pressure and heat. The behaviour of fluids (gases, liquids and plasma) under pressure and heat is still one of the main focuses in this field of science. Think of air and gasoline confined in the piston of an engine cylinder. If the pressure is kept constant in this system (the piston can easily move), the work done when heat is applied to it is equivalent to an increase in volume. After ignition of the gas/air, the piston is explosively pushed up. Heat is applied and the system does work. This is called an <a href="https://en.wikipedia.org/wiki/Isobaric_process">isobaric</a> system. If temperature is kept constant instead, it is called an <a href="https://en.wikipedia.org/wiki/Isothermal_process">isothermal</a> system. This time, the piston is bathed in constant-temperature reservoir such as a large water bath that absorbs and dissipates heat. The piston is pushed down to compress the air. Work is done to the system and its potential energy increases. Again, pressure and volume are directly and inversely related to one another. A third kind of process, called an <a href="https://en.wikipedia.org/wiki/Adiabatic_process">adiabatic process</a>, occurs when no heat is added to or removed from the system. It differs from the isothermal process just described because in that case heat was removed from the system into the water bath. Because no heat leaves or enters the system, the change in energy depends only on work that is either done to the system or done by the system. Often, an adiabatic process is one that is too rapid for any heat exchange to take place. An example is a carbon dioxide fire extinguisher. The gas is under pressure so when it is triggered, it expands very quickly. The compressed gas is at room temperature in the canister but when it expands its becomes cold. The same amount of thermal energy is spread over a much larger volume because it is conserved. The expanded gas stays cold at first because it doesn't have a chance to exchange heat from the room-temperature air around it. No heat is added to or removed from the system. Work is done through the expansion of the carbon dioxide. It might seem confusing that the expansion of the gas in this case has nothing to do with heating it. That phenomenon is <a href="https://en.wikipedia.org/wiki/Thermal_expansion">thermal expansion</a>, in which a substance experiences outward pressure as it is heated, and it is caused by the increasing kinetic energy of its molecules exerting outward pressure. <br />
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Heat of Fusion and Heat of Vapourization<br />
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Another example of an adiabatic process is our typical winter weather here in Alberta. Warm Pacific air streaming from the southwestern coast of British Columbia hits the Rocky Mountains and is forced up the mountainside, which means a climb from sea level to about 4000 metres. For every 1000 metres the air climbs, its temperature drops almost 10°C. Air that starts out at +10°C in Vancouver (at sea level) can drop to a face-numbing -30°C at the top of Mount Robson. Then the cold air slides down the Alberta side of the mountains and this time it warms up at the same rate. By the time it flows past the town of Cochrane in the foothills (about 1000 m altitude), for example, it has warmed up a balmy (for us!) 0°C. The air cools in the mountain pass because it has moved into a lower-pressure zone so it expands (to an average of about 0.60 atmosphere (atm) pressure compared to 1 atm at sea level). No heat actually leaves the air system. The expansion itself (work done by the system) lowers the temperature, because the same amount of thermal energy is spread over a larger volume. The thermal energy is conserved, obeying the first law of thermodynamics. As the air comes down the leeward side of the mountains, it is compressed once again to about 0.87 atm at Cochrane's altitude, which warms it to about 0°C.<br />
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This is not quite the famous <a href="https://en.wikipedia.org/wiki/Chinook_wind">Chinook</a>, however. Often, the air from the Pacific is laden with moisture, especially in winter. As it makes its way to the Rocky Mountains and it begins to climb, the moisture in the air, as liquid water, cools along with the air itself and eventually it reaches its freezing point and starts to switch over to (usually heavy) snow. When a substance freezes into a solid, it releases stored potential energy as a heat transfer. This is called either latent heat or <a href="https://en.wikipedia.org/wiki/Enthalpy_of_fusion">enthalpy of fusion</a> (the technical words for melting/freezing). The solid phase of a substance has a lower internal energy than the liquid phase because the inter-molecular bonds are stronger, providing more order. This means that the solid phase has lower potential energy. It is called latent heat because the heat lost from the substance as it freezes can't be measured as a change in temperature in the substance. A transfer of heat doesn't always coincide with a change in temperature. Latent heat is often referenced to a unit of mass. In this case it is called <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Class/PhSciLab/heati.html">specific heat of fusion</a>. The release of potential energy (as thermal energy) into the mountain air means that the air cools at a slower rate when it expands. Instead of cooling at a rate of 10°C per 1000 m, for example, it cools at a rate of about 8°C/1000 m. At the mountain peak, the temperature will be about -22°C (rather than -30°C) and about 8°C (rather than 0°C) when it reaches Cochrane, a typical Chinook day.<br />
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What exactly is happening at the molecular level? When water freezes (a <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html">phase change</a>), the molecules "lock" into place. They lose some freedom of movement compared to liquid water, which can flow. The solidified ice has less potential energy than liquid water so it must release that energy into the system, as a transfer of heat. Water releases about 6 kJ of energy per mole (called molar heat of fusion in this case) as it freezes into ice (or snow). For this reason, snowy days tend to be warmer than clear ones. The change from steam into liquid water represents an even larger release of energy. Water releases about 41 kJ/mol as it condenses from steam into liquid, an indication of how much more potential energy the gaseous state has over the liquid state. To vapourize, water needs to absorb 41 kJ/mol from its surroundings, one reason why a warm dry breezy day is perfect to put laundry out on the line. The moist clothes have a constant supply of warm dry air to evaporate their moisture into as the water absorbs energy.<br />
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Each substance has specific values for its heat of fusion and its heat of vapourization, which depend on its unique inter-molecular bonding. In other words, each substance has <a href="https://en.wikipedia.org/wiki/Latent_heat#Table_of_specific_latent_heats">its own </a><a href="https://en.wikipedia.org/wiki/Latent_heat#Table_of_specific_latent_heats">specific latent heat</a>. These values, in turn, contribute to the unique melting and boiling points of each substance. <a href="http://www.splung.com/content/sid/6/page/latentheat">Specific latent heat</a> is the amount of energy required to cause a phase change in a specific amount of a substance. If you would like to explore phase change more deeply, try my previous article "<a href="http://sciexplorer.blogspot.ca/2017/11/plasma-fourth-state-of-matter.html">Plasma: The Fourth State of Matter</a>." In it, I concentrate on the plasma state but in dong so I also explore what a phase change means in depth.<br />
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When a substance freezes or condenses, it releases heat into its surroundings. When a process releases heat, it is called an <a href="https://en.wikipedia.org/wiki/Exothermic_process">exothermic process</a>. The reverse processes - melting and evaporating - require heat to move forward. They absorb heat from the surroundings, and are called <a href="https://en.wikipedia.org/wiki/Endothermic_process">endothermic processes</a>. Water evaporating off clothes is an endothermic process.<br />
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This subject reminds us once again of the important but subtle distinction between temperature and internal energy. Temperature, explored earlier in this article, is the average kinetic energy of the molecules in a system. Internal energy, however, is the total energy of that system. It includes the potential energies of the molecules, in addition to their kinetic energies. Water vapour (at just over 100°C), for example, has more internal energy than liquid water (at 20°C) for two reasons. First, its molecules have more kinetic energy, which means it has more thermal energy (this can be measured as temperature). Second, its random molecular arrangement has much more potential energy than the more orderly arrangement of molecules in the liquid state (this can't be measured directly).<br />
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A related term is <a href="https://en.wikipedia.org/wiki/Heat_capacity">specific heat capacity</a>. Again using water as an example, a certain amount of thermal energy must be added to a volume of water in order to raise its temperature. Specifically, it takes 4200 J of energy to raise the temperature of 1 kg of water by 1°C. Every substance has its own specific heat capacity (SHC). The SHC of steel, by comparison, is 490 J/kg/°C. What does this mean? Water can hold a lot more heat than an equivalent amount of steel can. It also requires a lot more energy to warm up than the same mass of steel does. Water, in fact, has an unusually high SHC. That is why a warm water bottle makes such an excellent foot warmer in a cold bed. A steel disk of the same mass warmed to the same temperature would not only stub your toes. It would have roughly only a tenth of the heat available to warm them up. See <a href="https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html">this Engineering Toolbox table</a> to compare the SHC values of some other everyday substances.<br />
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Next we will explore the often misunderstood second law of thermodynamics and its implications in "The Laws Of Thermodynamics PART 3" <a href="http://sciexplorer.blogspot.ca/2018/01/the-laws-of-thermodynamics-part-3.html">click here</a>.Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-47644856391229461952017-12-31T12:30:00.000-07:002018-01-01T13:53:14.174-07:00The Laws of Thermodynamics PART 1The Laws of Thermodynamics
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Thermodynamics is the branch of physics that explores how heat and temperature relate to the energy of a system and its ability to do work. Almost every field of science, especially those in chemistry and engineering, relies heavily on an understanding of the principles of thermodynamics. Words such as internal energy, heat, temperature and entropy signal that a thermodynamic process is being described. Every process in the universe is a thermodynamic process.
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A Brief History
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Even the ancient Egyptians were curious about heat. In their time, free from any scientific encumbrances, they thought of heat as one of four essential or basic elements of matter, fire, along with water, earth and air. Much later, in the 17th and 18th centuries, investigators began to wonder if heat was a kind of physical substance, something that flows such as <a href="https://en.wikipedia.org/wiki/Phlogiston_theory">phlogiston</a> or <a href="https://en.wikipedia.org/wiki/Caloric_theory">caloric</a>. They imagined that heat physically flowed from one substance or object into another. In the early and mid-1800's, scientists such as <a href="https://en.wikipedia.org/wiki/Joseph_Black">Joseph Black</a>, <a href="https://en.wikipedia.org/wiki/Antoine_Lavoisier">Antoine Lavoisier</a> and <a href="https://en.wikipedia.org/wiki/James_Prescott_Joule">James Prescott Joule</a>, utilized the rigour of the scientific method to study heat scientifically and quantitatively. The study of thermodynamics as a science took off when engineers used these new concepts about heat transfer to maximize the efficiency of the first steam engines.
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The first steam engine, the <a href="https://www.egr.msu.edu/~lira/supp/steam/savery.htm">Thomas Savory steam pump</a>, patented in 1698, was a highly inefficient but inexpensive coal-chugging device designed to pump water out of coalmines. It wasn't elegant but in short order people realized that they could use the power of steam to manufacture machinery and transport goods and people. Besides leading to the first railway locomotives, the improved <a href="https://en.wikipedia.org/wiki/Steam_engine">stationary piston steam engine</a> revolutionized manufacturing in general. It was a crucial development that ushered in the Industrial Revolution across Europe and North America. The first stream engine, as we know it, had a piston that could generate and transfer power to a machine. It was invented in 1712 by <a href="https://en.wikipedia.org/wiki/Thomas_Newcomen">Thomas Newcomen</a>. <a href="https://en.wikipedia.org/wiki/James_Watt">James Watt</a> later in 1781 revolutionized the steam engine into a rotary motion engine that could drive factory machinery. He also introduced a condenser, which greatly improved its efficiency.<br />
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About a century after Watt's revolutionary work (in 1894), another Scotsman, William Thomson (<a href="https://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin">Lord Kelvin</a>), came up with the first precise definition of this new field of steam-engine thermodynamics, as well as the word itself: "Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency."<br />
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If we look back on all the frenetic activity of the 18th and 19th centuries that contributed to numerous branches of thermodynamics we have today, the birth of thermodynamics as a modern science is actually best attributed to a less well-known man, <a href="https://en.wikipedia.org/wiki/Nicolas_Leonard_Sadi_Carnot">Sadi Carnot</a>. Thirty years prior to Lord Kelvin's work, he published "Reflections of the Motive Power of Fire," an examination of heat, power and engine efficiency. In this treatise, he introduced the concept of work along with its connection to thermal energy (which is thermodynamics in a nutshell). He abstracted the steam engine so that, as an engineer and physicist, he could focus it into an idealized <a href="https://en.wikipedia.org/wiki/Heat_engine">heat engine</a>. By doing so, he developed the first model thermodynamic system, a concept that is still widely used today, especially among engineers who need to understand the often-nuanced differences between a real mechanical engine and the simpler idealized theoretical model. Most physicists today consider Sadi Carnot to be the father of thermodynamics. Yet, sadly, almost no one paid attention to his book during his lifetime, cut short at the age of 36 by cholera. Because cholera is so contagious, all of his belongings and almost all of his writings were buried along with him when he died. Miraculously, the book survived and Lord Kelvin, with much effort, was able to get a copy of it to use as part of the basis for his own famous work.<br />
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In A Nutshell<br />
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So what exactly is thermodynamics? You could think of it as the puzzle that connects four concepts: heat, temperature, energy and work. The box the puzzle comes in is the <a href="https://en.wikipedia.org/wiki/Thermodynamic_system">thermodynamic system</a>. There are <a href="https://en.wikipedia.org/wiki/Laws_of_thermodynamics">four basic laws</a> that tell us how the puzzle fits together: These are the zeroth law, the first law, the second law and the third law.<br />
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Physicist Lidia del Rio and co-authors recently wrote in the Journal of Physics A that "If physical theories were people, thermodynamics would be the village witch. The other theories find her somewhat odd, somehow different in nature from the rest, yet everyone comes to her for advice, and no one dares to contradict her."<br />
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What is a Thermodynamic System?<br />
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The concept of a thermodynamic system began with Sadi Carnot, who thought about the "working substance" in a heat engine. In his case, this working substance was a volume of water vapour. His working substance was, in essence, a thermodynamic system in itself. It was aptly named a "working" substance because, as a system, it can do work when heat is transferred into it. It can be put in contact with a piston or a boiler, for example. Below is a modern version of Carnot's engine diagram. It gives us an idea of how his working substance works in a simple heat engine.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEioHLPa47oSQId3mJH8-zz9tq07IpdQcopTF-YL9dbHS8zQUo2gn8yAuv-Q-lfwYA51IoUONs_RZXZyyiRVsWkbPgqVFRRuBfSP6uLRRa-EwZax2PCcDp-cS3bFuI4DHK7LNtLMnbaxc1WA/s1600/Carnot_heat_engine_2.svg.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="370" data-original-width="840" height="175" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEioHLPa47oSQId3mJH8-zz9tq07IpdQcopTF-YL9dbHS8zQUo2gn8yAuv-Q-lfwYA51IoUONs_RZXZyyiRVsWkbPgqVFRRuBfSP6uLRRa-EwZax2PCcDp-cS3bFuI4DHK7LNtLMnbaxc1WA/s400/Carnot_heat_engine_2.svg.png" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Eric Gaba; Wikipedia</td></tr>
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Thermal energy as transferred as heat from a hot furnace TH (left) through the fluid of a working substance such as water vapour (the circle) into a cold heat sink, TC (right). As the heat is transferred, it forces the working substance to do mechanical work on its surroundings (W). This work could be then be transferred to the cycles of expansion/contraction that turn a piston, for example.<br />
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This diagram is a simple theoretical thermodynamic system. Carnot thought of it as an isolated system. That means there would be no interaction whatsoever with the surrounding air, the walls of the containers used, etc. There would be no interactions that, in real life, make all thermodynamic systems, at least to some degree, open in terms of energy transfer. In a real steam engine, for example, there are numerous ways in which energy is lost to the surrounding materials and to the air, particularly as heat loss and as friction, a loss of usable work. In a real engine, there are always many ways in which energy that could theoretically do work is permanently lost instead and those losses are substantial. No real mechanical engine can be 100% efficient. Most <a href="https://en.wikipedia.org/wiki/Engine_efficiency">are far from it</a>. The latest gasoline fuel-injection combustion engine technology is up to about 35% efficiency. The latest stream turbine power generator technology <a href="http://www.power-eng.com/articles/print/volume-106/issue-8/features/new-benchmarks-for-steam-turbine-efficiency.html)">is about 42% efficient</a> which means that in both cases the majority of the energy generated is lost from the system and unavailable to do useful work.<br />
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Everything that has a temperature is a thermodynamic system, and in reality all such systems interact with other surrounding systems. The only truly <a href="https://en.wikipedia.org/wiki/Thermodynamic_system#Isolated_system">isolated system</a> is perhaps the universe itself (at least according to non-multiverse theories). These interactions mean that real thermodynamic systems are complicated, so when we want to study how a system works, we want to simplify it into a theoretically isolated system where we can limit its interactions with the world around it. We impose theoretical impermeable walls around it. "Isolated" is different from "closed." A <a href="https://en.wikipedia.org/wiki/Thermodynamic_system#Closed_system">closed thermodynamic system</a> is one in which energy, but not matter, can be exchanged with its surroundings. In an <a href="https://www.bluffton.edu/homepages/facstaff/bergerd/NSC_111/thermo2.html">open system</a> both energy and matter can be exchanged.<br />
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A theoretical example of an isolated system is hot coffee enclosed in an impossibly perfectly insulated Thermos bottle. It is never going to cool off (there is no energy transfer) and you can't smell its aroma (there is no transfer of matter). An example of a closed system would be hot coffee in a very well sealed plastic container. It's going to cool off but you can't smell it. An example of an open system could be the hot coffee in an open mug. You can smell its aroma because its molecules are mixing into the air you are breathing. There is an exchange of matter. It is going to eventually reach room temperature because the cup is transferring heat to the countertop it rests on and to the air around it. As an open system, it is going to change until it reaches a state of equilibrium with the room around it. The hot coffee will warm the room it is in but the room is likely to be so large compared to the coffee that the change would be undetectable. The room with the coffee in it, taken as an approximately isolated system, will have the same total energy when the coffee is hot as when it has cooled to room temperature because the energy is conserved.<br />
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We have described the "<a href="https://en.wikipedia.org/wiki/Thermodynamic_state">state</a>" of our coffee "system" as hot. The transfer of heat from the coffee to the air around it is an example of a <a href="https://en.wikipedia.org/wiki/Thermodynamic_process">thermodynamic process</a>. The coffee eventually cools from hot to room temperature. A thermodynamic variable or property is also called a <a href="https://en.wikipedia.org/wiki/State_function">state function</a>. A "state function" of this system is temperature. Internal energy, mass, pressure, volume, enthalpy and entropy (all concepts we will explore) are examples of state functions in thermodynamics.<br />
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The thermodynamic state of our coffee system is described by its properties such as temperature, volume, and pressure. As the coffee cools, its state changes. It will continue to cool until the coffee reaches a state of <a href="http://www.brighthubengineering.com/thermodynamics/4720-what-is-thermodynamic-equilibrium-part-one/">thermodynamic equilibrium</a> with its surroundings. Equilibrium means that the system is in balance with its surroundings. The coffee eventually reaches the same temperature as the air and countertop upon which it sits. This is <a href="https://en.wikipedia.org/wiki/Thermal_equilibrium">thermal equilibrium</a>. A system will also spontaneously move toward other kinds of equilibrium as well, such as mechanical equilibrium (toward equal pressure for example) and chemical equilibrium (toward minimal <a href="https://en.wikipedia.org/wiki/Gibbs_free_energy">Gibbs energy</a>, a form of potential energy that is widely used in chemical thermodynamics). Thermodynamic equilibrium is a state of complete equilibrium where all these kinds of equilibrium are in place. If the coffee was sealed in a pressurized but heat-permeable container, it would eventually reach thermal equilibrium but not complete thermodynamic equilibrium because its pressure cannot equalize with the air around it. When a system is in complete thermodynamic equilibrium, it will not change spontaneously. It will remain in the same state indefinitely, unless work is done to it. It cannot do work because it does not have energy available to do work.<br />
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The description of our coffee is a classical thermodynamic description. It doesn't take into account the states of the molecules or atoms or subatomic particles in the coffee and it makes the assumption that the dynamic process (the change that occurs) is continuous or smooth. The coffee cools gradually, not in jumps, for example. If we want to look at the individual dynamics of the atoms in our coffee, we need to deal with <a href="https://en.wikipedia.org/wiki/Quantum_thermodynamics">quantum thermodynamics</a>. This field of study explores the relationship between two independent theories: (classical) thermodynamics and <a href="https://en.wikipedia.org/wiki/Introduction_to_quantum_mechanics">quantum mechanics</a>.<br />
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A Glimpse Into The Future<br />
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Quantum thermodynamics attempts to describe thermodynamic changes that occur at the atomic scale. Analogous to how thermodynamics developed from trying to improve steam engine technology, quantum thermodynamics, a very young field, is growing out of our desire to shrink technologies into a variety of quantum machines. A significant challenge here is that classic concepts such as temperature and work need to find some kind of quantum counterpart. One breakthrough is that we can now equate a system's energy with its information, an idea that we will revisit later in this article. Internal energy doesn't equate to thermal energy in a system.<br />
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Changes at the quantum level are not continuous because particle properties are quantized. They come in discrete packets. If you recall the <a href="https://en.wikipedia.org/wiki/Matter_wave">de Broglie matter wave</a>, all matter at the atomic scale is both particle and wave in nature, and changes in energy are limited to specific jumps in energy levels. Subatomic behaviours belong to the realm of quantum mechanics. A quantum description of a system can give us an idea of how atoms behave but it can't tell us anything about the macroscopic system as a whole. We can't add up de Broglie waves to get a complete thermodynamic picture of the coffee in our earlier example. Technically, we can't study each individual particle in a system. In order to describe the internal energy of the coffee at the quantum level, we would need to know, at the same time, all the trajectories and kinetic energies, all particle masses, magnetic moments and angular momenta of billions of particles. Even if we could manage such a feat, we would still lack a complete thermodynamic description of the coffee because it also possesses <a href="https://en.wikipedia.org/wiki/Emergence#Emergent_properties_and_processes">emergent properties</a>. These are properties that emerge when a collection of many atoms behave together in a macroscopic system, such as temperature, volume and even pressure, which cannot be described at the level of individual atoms. Even if we could know all the quantum information of every particle in the coffee, we still couldn't know what its temperature is, and how fast it is cooling.<br />
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Complicating our effort further, all quantum systems have <a href="https://en.wikipedia.org/wiki/Uncertainty_principle">uncertainty built into them</a>. Quantum systems have a unique and intriguing property: you can't know both the energy and the position (or the momentum and the velocity) of any particle at the same time. This means that an atom, for example, cannot be treated like a tiny sphere of matter. It is more accurately a statistical cloud of where it might be and how fast it is going, which has uncertainty built into it.<br />
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This seemingly impossible problem does have a satisfactory solution however: we can use <a href="https://en.wikipedia.org/wiki/Statistics">statistics</a>. It is a mathematical bridge between quantum thermodynamics and classical thermodynamics. It deals with individual particles or atoms or molecules by taking averages of their dynamic properties, and using those average values to describe a system.<br />
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Thanks to advances in technology, even a system containing just a few molecules or even atoms can now be harnessed to do microscopic work. Last year three scientists won the Nobel Prize in chemistry <a href="https://phys.org/news/2016-10-nobel-chemistry-prize-world-tiniest.html">for developing the world's tiniest machines</a>, with motors smaller than the width of a human hair. Fraser Stoddart developed a molecular computer chip that can store 20 kB of memory. Bernard Feringa built a nanocar with four molecular motors as wheels. John-Pierre Sauvage inspired other researchers to develop machines such as microscopic robots that can grasp and collect amino acids. Work is underway to develop a fast-charging <a href="https://phys.org/news/2015-08-faster-battery-quantum.html">quantum battery</a> in which energy can be stored and released on demand <a href="https://www.technologyreview.com/s/507176/entanglement-makes-quantum-batteries-almost-perfect-say-physicists/">from a quantum system</a>.<br />
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As the wave of new micro-technologies gains momentum, the field of thermodynamics needed to describe how they work is having trouble keeping up. How do concepts of heat and efficiency translate into this new realm of tiny machines? While statistics can bridge the gap between the quantum realm and classical realm of a (large) macroscopic system, how can it bridge the gap between quantum-scale process in a quantum-scale (tiny) system and a macroscopic-scale (large) observer? Statistics, so useful when describing processes and states of macroscopic systems, finds little use here. In quantum machines, every quantum bit of information has real meaning and cannot be treated as a statistical average. Is irreversible heat loss in a macroscopic system equivalent to <a href="https://en.wikipedia.org/wiki/Quantum_dissipation">quantum dissipation</a> in a quantum system? There is a lot of heated debate about how to link quantum and macroscopic thermodynamic processes because all of these new technologies must obey the same thermodynamic principles as the original heat engine.<br />
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The Four Laws Of Thermodynamics<br />
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Zeroth Law<br />
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Though this law might not be familiar to a lot of us, its impact is quite enormous. It can be summed up in a simple statement: If two systems are in thermal equilibrium with a third system then they must also be in thermal equilibrium with each other. This implies that the sizes of the systems and what kinds of molecules they are composed of don't matter. As James Clerk Maxwell famously said, "all heat is of the same kind." It is a deceptively simple observation of equivalence, which underpins all the thermodynamic laws, and which establishes that temperature as a universal property of matter, as explained in this 4-minute video by The Royal Institution.<br />
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By being able to define temperature, we can measure the thermal energy in any system. We can use the scale of Fahrenheit, Celsius or Kelvin, but the Kelvin scale is the thermodynamic scale used by scientists. It starts at absolute zero, where there is no thermal energy in a system. A thermometer can be used to verify that systems in thermal equilibrium are at the same temperature.<br />
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If we define the barrier between two systems as a kind of wall permeable only to heat transfer, such as the wall of a coffee cup between hot coffee and our hands, we realize that only energy is transferred between the two systems, not matter. The zeroth law, though simple, shuts the door on earlier theories that treated heat as a physical entity, such as phlogiston or caloric, as types of matter thought to flow between objects.<br />
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Thermal Energy and Internal Energy<br />
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<a href="https://en.wikipedia.org/wiki/Temperature">Temperature</a> is the measure of the <a href="https://en.wikipedia.org/wiki/Thermal_energy">thermal energy</a> of a system. As a statistical measurement, it measures an average of all the individual kinetic energies of the atoms and molecules in the system, which cannot be individually measured. Individual molecules do not have thermal energy or a temperature. They have only kinetic energy from which thermal energy emerges at the macroscopic scale. Temperature offers us a hint of what is happening at the molecular scale. Every molecule in a system, such as our coffee, has three <a href="https://en.wikipedia.org/wiki/Degrees_of_freedom_(physics_and_chemistry)#Degrees_of_freedom_of_gas_molecules">physical degrees of freedom</a>. It can move in three dimensions in space, and each degree of freedom has <a href="http://www.physicsclassroom.com/class/energy/Lesson-1/Kinetic-Energy">kinetic energy</a> associated with it, which varies from molecule to molecule within the system. Some kinds of molecules (such as binary-shaped oxygen, O<sub>2</sub>) also have rotational motion about an axis (an additional degree of freedom). Thermal energy is the result of all of these molecular motions within a system. It is measured as temperature, which is their average kinetic energy. We might think of kinetic energy as unidirectional, such as an object rolling downhill gaining kinetic energy. Here, the important distinction is that the kinetic energies of the molecules are random and in all directions. There are cases where thermal energy cannot be measured as a temperature change, for example, during a phase change, which we will explore.<br />
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Work and Heat<br />
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Every thermodynamic system also has <a href="https://en.wikipedia.org/wiki/Internal_energy">internal energy</a>, which is a more encompassing form of energy, and which can be used to do <a href="https://en.wikipedia.org/wiki/Work_(thermodynamics)">work</a>. Work, conversely, can also be done on a system to increase its internal energy. Unlike thermal energy, internal energy cannot be directly measured. It is the sum of the system's thermal energy plus its potential energy. The internal energy of a system, the sum of the kinetic and potential energies of the molecules and atoms, does not include energy due to the motion or location of the system as a whole relative to its surroundings. The system's gravitational potential energy or its motion through space is not included in other words. However, chemical bond energy, magnetic moment, internal electrical field, nuclear potential energy, and internal stress can all be included as contributions to the internal energy of a system. The internal energy of a system can also be changed by adding or subtracting matter from the system, transferring heat into or out of the system, or by doing work on the system or by the system itself dong work. The internal energy of a system also changes if thermal energy is added or removed.<br />
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Work performed by a system is energy transferred by the system to its surroundings. Unlike internal energy or thermal energy, which are properties of a system, work is energy-in-transit. A system doesn't contain work; it is a process of energy transfer. Likewise, heat (though we commonly say that an object contains heat or an object is hot) is not a property of a system. It is better understood, like work, as energy in transit. The kinetic motion of molecules in a system can be the source of, and the effect of, the transfer of heat from another system. The term "latent heat" (which we will explore) is also not a property of a system. It is best understood as internal energy released from, or absorbed into, a system, as internal energy transferred without a change in temperature.<br />
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We can't measure internal energy directly but we can get an idea of the change in internal energy in a system by using the concept of <a href="https://en.wikipedia.org/wiki/Enthalpy">enthalpy</a>. As described nicely in Wikipedia, enthalpy describes the internal energy of a system plus the energy "needed to make room for it," which is measured as pressure and volume. Like internal energy, we can't measure enthalpy directly but we can measure the change in enthalpy in a system. The concept of enthalpy is a convenient way to link internal energy (thermal energy plus potential energy) with work in a system. Enthalpy (H) is essentially (internal energy (U)) + ("work" energy, or pressure x volume). It is useful for reactions that involve gases, where changes in pressure and/or volume tend to be significant and easy to measure. For reactions that only involve liquids or solids, the change in enthalpy will pretty much equal the change in internal energy because there will be hardly, if any, change in volume or pressure in the system. Solids and liquids resist compression and expansion.<br />
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This brings us the first law of thermodynamics, which establishes the existence of internal energy in a system, next.<br />
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The Laws of Thermodynamics PART 2 <a href="http://sciexplorer.blogspot.ca/2018/01/the-laws-of-thermodynamics-part-2.html">CLICK HERE</a>Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-50396420658248033032017-11-01T14:11:00.002-06:002017-11-01T14:11:44.037-06:00Plasma: The Fourth State Of MatterUnderstanding what plasma is can be confusing, even though plasmas are part of our everyday lives. The Northern Lights, lightning, flames, fluorescent bulbs, neon signs and even the Sun are all examples of matter in this particular physical state. Although most of us are well acquainted with at least three states of matter - solid, liquid and gas - getting to know plasma, and how substances become plasma, can be daunting, but also fun and very useful.<br />
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Plasma Is Everywhere<br />
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It might surprise us to learn that plasma is by far the most abundant physical state of matter in the universe, much more so than our familiar three states of solid, liquid and gas. Perhaps even more surprising is the fact that plasma is just one of <a href="https://en.wikipedia.org/wiki/List_of_states_of_matter">over 30 different states of matter currently listed on Wikipedia</a>. More yet will undoubtedly be discovered in laboratories. Most of these exotic states are unknown beyond research circles because they tend not to be observed except under extraordinary conditions. Plasma, in contrast, is the most common state of ordinary matter under natural conditions in our universe.<br />
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Most of our visible universe <a href="https://en.wikipedia.org/wiki/Astrophysical_plasma">consists of plasma</a>, and it is a fortunate thing because that is the key reason why we can observe it with our telescopes. Stars are entirely made up of it. Most interstellar gas is plasma. Some sources estimate as much as <a href="https://www.plasma-universe.com/99.999%25_plasma">99% of visible matter in the universe is in a plasma state</a>. By the end of this article we will understand how plasma can emit light. Even in our own solar system, over 99% of its mass is in a plasma state, thanks to the overwhelming mass contribution of our Sun. In our everyday life we regularly, but temporarily, witness plasma as lightning, fires, and the auroras, when energy is applied to matter, usually in its gaseous state. I did not explore plasma in high school and I had few occasions to explore it in my first years as an undergrad. One reason why plasma is not more thoroughly studied at these basic levels is that it can be difficult to jump from the fairly straightforward molecular theory needed to understand solids, liquids and gases into atomic and sub-atomic particle theories required to get a handle on the plasma state. Yet it's not only do-able; it's fascinating.<br />
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What Is a Phase Change?<br />
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A common theoretical thread to solids, liquids and gases is temperature. We can extend this temperature spectrum at the cold end by exploring the exotic-sounding Bose Einstein Condensate (BEC). In this state, mysterious quantum behaviours of matter are on a scale that we can observe. I wrote an article a few years ago that explores what happens when matter gets VERY cold and transforms into <a href="http://sciexplorer.blogspot.ca/2017/02/bose-einstein-condensate.html">a BEC</a>. Because stars are very hot, it is tempting to add plasma to the upper temperature end of this physical state spectrum. That, however, would be misleading and incorrect. Gases, when extremely hot, can break down into plasma but we do not need to heat a gas to transform it into plasma. Applying an electrical charge alone to a gas can do this. Think of a glowing neon tube light, for example. It is filled with gas in a plasma state but you can comfortably touch it with your hands.<br />
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There is an additional reason why temperature alone does not necessarily dictate the phase of a substance. We often first learn about phase changes by exploring water as a gas, liquid and solid, but when we talk about solids, liquids and gases, we really need to consider pressure in addition to temperature, which together determine what state matter is in. Under constant atmospheric pressure, temperature alone determines which state water will be in. This is how we encounter water in everyday life. We can also keep water at a constant temperature and change the pressure. Under enough pressure water will condense into liquid and even into exotic forms of ice. In a near-vacuum, water ice can sublimate directly into vapour.<br />
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Temperature, pressure and electrical potential are the three changing factors behind the phase changes we will focus on here. A phase change of any type relies on a boost or withdrawal of energy into/from matter. These aren't the only kinds of phase change that exist. The application of a magnetic field can change the physical state of magnetic material, for example. For all phase transitions (which is also called a change in physical state), the physical arrangement or ordering of atoms changes. In this article I'd like to start by exploring transitions between solids, liquids and gases in depth. Then I will compare those changes to transitions into a plasma state and explore what plasma is as a physical phase of matter.<br />
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Exploring Solids, Liquids and Gases Using Water as an Example<br />
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Matter changes from one physical state to another through a process called a <a href="https://en.wikipedia.org/wiki/Phase_transition#Types_of_phase_transition">phase transition</a>. Every chemical compound and element undergoes a phase transition under a specific combination of temperature and pressure. The periodic stable below lists all the known elements. At 0°C and 1 atmospheric pressure, all elements in red <i>type</i> are gases, elements in green type are liquids, elements in black type are solids, and it is unknown what physical state those in grey type are. These "mystery" elements start at atomic number 100, <a href="https://en.wikipedia.org/wiki/Fermium">fermium</a>. These elements, plus several other smaller atomic number elements <a href="https://en.wikipedia.org/wiki/Synthetic_element">are synthetic</a> or man-made. This is the highest atomic number element that can be created in a macroscopic amount, although a pure sample of this metal has not been created yet.<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiCFBcqq-hD08owvx8X_DKulBPARzsArsk6s8DSqj7a0wabxv-7jwvY-3MjSU7MhsZyxjGkeJa55b-Fy_3OIsxDw2f2orEeMirw_etnWJ4Hov0wpI2sBNp8wodDw_I5Kw3l2o2T3Hz8u2tL/s1600/1205px-Simple_Periodic_Table_Chart-en.svg.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="638" data-original-width="1205" height="338" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiCFBcqq-hD08owvx8X_DKulBPARzsArsk6s8DSqj7a0wabxv-7jwvY-3MjSU7MhsZyxjGkeJa55b-Fy_3OIsxDw2f2orEeMirw_etnWJ4Hov0wpI2sBNp8wodDw_I5Kw3l2o2T3Hz8u2tL/s640/1205px-Simple_Periodic_Table_Chart-en.svg.png" width="640" /></a></div>
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We can compare the melting and boiling points of most of the elements above but we are all most familiar with <a href="https://en.wikipedia.org/wiki/Properties_of_water">water</a>. Not only is water familiar but it is also exhibits some unique and fascinating properties unlike most other substances, and this will allow us to explore change of state in greater depth. It is a chemical compound of hydrogen and oxygen (H<sub>2</sub>O). We drink it in a liquid state. At 0°C, water freezes into ice, a solid state, and at 100°C it evaporates into a gas, water vapour. These transitions assume a pressure of one atmosphere (atm; the average air pressure at sea level)). To understand how pressure plays a role in the physical transition of a substance, imagine water vapour in a container fitted with a gas-tight plunger. The temperature is carefully maintained throughout the experiment at 130°C. The starting pressure is 1 atm. As the plunger lowers down into the container, the volume of gas decreases and the gas pressure increases. Soon the water vapour will be compressed into liquid water. 130°C is the boiling point at around 5 atm. Above 5 atm, water at 130°C will liquid. It will have to be compressed much further, to about 100,000 atm (because liquid water strongly resists compression) to solidify into scalding hot 130°C ice in the container.<br />
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Researchers at Sandia National Laboratories performed <a href="https://www.livescience.com/1385-scientists-ice-hotter-boiling-water.html">a similar experiment</a> several years ago. They subjected liquid water (starting at 1 atm) to extremely rapid compression (to 70,000 atm). The water shrank abruptly into a dense phase of solid ice. When the pressure was relieved, it melted (and expanded) back into liquid water. The ice formed under these conditions is not the everyday ice we make in our freezers. Ordinary freezer ice, as most of us know, is less dense than liquid water. At pressures above 100,000 atm, however, water only exists as different kinds of very dense ice, even at temperatures of hundreds of degrees centigrade. Scientists think such <a href="https://www.reuters.com/article/us-space-planet/hot-ice-may-cover-recently-discovered-planet-idUSN1621607620070516">hot ice exists in the deep interior of exoplanet GJ 436 b</a>, a Neptune-size ice giant that orbits very close to its parent red dwarf star, GJ 436, which is 33 light-years away from Earth. <br />
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A Phase Change is a Molecular Rearrangement<br />
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Chemically, water in any physical state remains water. That is, it retains its <a href="http://www.iun.edu/~cpanhd/C101webnotes/matter-and-energy/properties.html">chemical properties</a>. Any matter undergoing a phase change from solid to liquid to gas and vice versa retains its chemical properties. Each water molecule consists of two hydrogen atoms <a href="https://en.wikipedia.org/wiki/Covalent_bond">covalently bonded</a> to an oxygen atom. What changes during a phase transition is the molecular arrangement of these molecules (see below). At any particular temperature and pressure, water molecules will adopt the most <a href="https://en.wikipedia.org/wiki/Chemical_stability">thermodynamically stable</a> arrangement possible for that environment. In order to transition into a new arrangement, the molecule-molecule interactions must change. Molecule-molecule, or in the case of elements, atom-atom interactions can shift and reorganize a substance, changing its physical properties in the process. The interactions between molecules in a substance tend to be attractive. In solids, these attractive forces are strong enough to be called chemical bonds, but these bonds are still much weaker than the chemical bonds that bind each molecule itself together (such as the two covalent hydrogen bonds in water). The simple images below highlight the general differences in atomic/molecular arrangement between solids, liquids and gases.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhuVn8bsKZFJ3P_zNrisqZxJMWrv6v66fz9_mWyJwiyh57gDUWKUnrI12uSjvLwnRMx26UHSdKvcyA4_C9ztIGNISnuRw6kwOSJdzKurTwUlHxBY6AgBvwHw-7SOQrybR850_UB_75F05ok/s1600/Stohrem.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="242" data-original-width="473" height="326" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhuVn8bsKZFJ3P_zNrisqZxJMWrv6v66fz9_mWyJwiyh57gDUWKUnrI12uSjvLwnRMx26UHSdKvcyA4_C9ztIGNISnuRw6kwOSJdzKurTwUlHxBY6AgBvwHw-7SOQrybR850_UB_75F05ok/s640/Stohrem.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Materialscientist;Wikipedia</span></span></td></tr>
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This is an actual atomic resolution image of the lattice-like arrangement of molecules in solid strontium titanate. Chemical bonds between strontium titanate molecules hold them close together in a tight regular arrangement. Above about 2000°C, strontium titanate crystals will melt, breaking these relatively weak intermolecular chemical bonds.<br />
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: left; margin-right: 1em; text-align: left;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh6otg9MAOqjFQyW0W66AJccVlfHA2V5KlvNpHdDbS0Sb_6Kkluf5wx8eHNDZw6nuEoQu6fQ7cGnt2QX4JyRhNOl7FBzLdlmcYH_46GZ9aP37nxIYOP6MxcRhIb6Nfh8ddHx8uXLDETcfHQ/s1600/Teilchenmodell_Flu%25CC%2588ssigkeit.svg.png" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="301" data-original-width="440" height="136" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh6otg9MAOqjFQyW0W66AJccVlfHA2V5KlvNpHdDbS0Sb_6Kkluf5wx8eHNDZw6nuEoQu6fQ7cGnt2QX4JyRhNOl7FBzLdlmcYH_46GZ9aP37nxIYOP6MxcRhIb6Nfh8ddHx8uXLDETcfHQ/s200/Teilchenmodell_Flu%25CC%2588ssigkeit.svg.png" width="200" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Kaneiderdaniel;German-language Wikipedia</span></span></td></tr>
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This highly simplified two-dimensional representation (left) shows how atoms or molecules might be arranged in a typical liquid in a beaker. They have contact with their closest neighbours, where they experience very weak attractive inter-molecular forces, but there is no overall order to their arrangement, and the atoms/molecules can slide freely past one another.<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhYTLYCzLzeRMJdUqFUmeZomOiTms8BbVKL7PhY2R_Hb1TM9qVTQt1q2TIJ6perTVWWP9ngd-pG6oDxInnmPQXPm4oR7AFYzjVnTS8Bdvi7hCVgn9IDPlfRfQrg5UdlQ0et9q28bTB13bkH/s1600/Gas_molecules.gif" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" data-original-height="143" data-original-width="225" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhYTLYCzLzeRMJdUqFUmeZomOiTms8BbVKL7PhY2R_Hb1TM9qVTQt1q2TIJ6perTVWWP9ngd-pG6oDxInnmPQXPm4oR7AFYzjVnTS8Bdvi7hCVgn9IDPlfRfQrg5UdlQ0et9q28bTB13bkH/s1600/Gas_molecules.gif" /></a></div>
As the diagram right simplistically shows, the spaces between molecules/atoms in a gas are vast compared to the size of the atoms or molecules themselves, many magnitudes greater than this simple diagram suggests. This is true even for a highly pressurized gas. The atoms/molecules can move freely in all directions. They experience no appreciable attractive forces between them so they move independently of one another. The <a href="https://en.wikipedia.org/wiki/Ideal_gas_law">ideal gas law</a> assumes negligible molecular volume relative to gas volume and it assumes the absence of any molecular attraction. It quite accurately predicts how volume, temperature and pressure relate to one another in a gas. This means that most gases in reality act much like a theoretical ideal gas.<br />
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Water molecules bond with each other in a solid phase according to the <a href="https://en.wikipedia.org/wiki/Ice_rules">Bernal-Fowler ice rules</a>. This is where water gets quite interesting. Every oxygen atom can bond to 4 hydrogen atoms. Two bonds are strong. These are the covalent chemical bonds that hold each water molecule together, and they don't change during a phase transition. However, oxygen has 6 <a href="https://en.wikipedia.org/wiki/Valence_electron">valence</a> (or outer, chemically available) electrons. In water vapour, four electrons remain unbound as two <a href="https://en.wikipedia.org/wiki/Lone_pair">lone pairs</a>. As water vapour condenses into liquid water, some of the four unbound electrons take part in very weak molecule-molecule bonds with hydrogen atoms in adjacent molecules. These are called <a href="http://www1.lsbu.ac.uk/water/water_hydrogen_bonding.html">hydrogen bonds</a>. Unlike the strong covalent hydrogen-oxygen bonds holding each molecule together, these bonds are weak and in the liquid state, they are transitory. The (proton) positive charges of hydrogen atoms are attracted to the negative charge zones of nearby oxygen lone electron pairs.<br />
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The attractive force behind hydrogen bonding is about 90% due to the attraction between opposite charges. This is an <a href="https://en.wikipedia.org/wiki/Non-covalent_interactions">electrostatic phenomenon</a>. 10% of the bonding force is due to electron sharing. This is a quantum mechanical phenomenon and it is also responsible for (much stronger) <a href="https://en.wikipedia.org/wiki/Covalent_bond">covalent bonding</a>. In liquid water, the weak hydrogen attractions form and break very easily, allowing water molecules to slip past one another. The much stronger covalent intra-molecular bonds holding each water molecule together only break when water undergoes a chemical decomposition reaction called <a href="https://en.wikipedia.org/wiki/Electrolysis_of_water">electrolysis</a>, producing hydrogen and oxygen gases as products.<br />
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The diagram below models the hydrogen bonds between water molecules. Two lone electron pairs force every water molecule into a triangle shape, which makes it <a href="https://en.wikipedia.org/wiki/Chemical_polarity">polar</a> - each molecule has a positively charged region and a negatively charged region. The positive charge of hydrogen atoms is attracted to the negative charge zone of oxygen atoms. Hydrogen bonds are indicated by dotted lines.<br />
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<tr><td class="tr-caption" style="text-align: center;"><!--StartFragment--><span style="font-size: x-small;"><span style="background-color: #f8f9fa; background-position: initial initial; background-repeat: initial initial; color: #222222; font-family: "helvetica";">User Qwerter at Czech Wikipedia</span><!--EndFragment--> </span> </td></tr>
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The length and strength of hydrogen bonds is strongly dependent on temperature. As liquid water freezes, additional hydrogen bonds form between molecules. Every hydrogen atom is, in effect, bonded to two oxygen atoms - one bond is strong and one bond is weak. Under sufficient pressure and/or cold, water molecules come close enough together to bond into regular and stable lattice-like arrangements. As ordinary ice, the molecules form bonds that result in a hexagonal crystalline lattice arrangement. Under more extreme pressure/temperature regimes, water ice can transition into a variety of different lattice structures such as cubes, rhomboids and tetragons, which allow for denser molecular arrangements. Each transition into a denser crystalline lattice is a phase change. There are <a href="http://www1.lsbu.ac.uk/water/ice_phases.html">at least 17 known physical states of water ice</a> alone. Changes in the number, length and strength of hydrogen bonds underlie each of these additional solid phase changes.<br />
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Most liquid substances solidify into denser molecular arrangements but there are a few exceptions, and they are always due to unusual bonding between the molecules. Water is an example. Ordinary water ice is actually less dense than liquid water. That is why it floats. This phase is called <a href="https://en.wikipedia.org/wiki/Ice_Ih">hexagonal ice, or ice lh</a>. It is the only solid water phase encountered on Earth. Due to the unusual chemical bonds of water (responsible for its molecular shape and polarity), the hexagonal lattice arrangement of lh ice keeps molecules a bit further apart from one another than they are in the liquid state (yet the bonds themselves are stronger and more permanent). Ih ice has a density of 0.9167 g/cm<sup>3</sup> compared to liquid water with a density of 1.18 g/cm<sup>3</sup>. The diagram below shows what the hexagonal lattice of Ih ice looks like. Gray dashed lines indicate hydrogen bonds.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjqSmhnuX202LC8O5ZII5IEyZjGA5RhJZUqp83lkTKYbsujYXgE4NeOHJA4kcx8i5upM2FAkGFDw_BJUIjH7rg435EQ7ovGx-j4D5XKGOjGGVNzXSiYIICCQ59sXuJ8ZMxAOg58myGIw30Z/s1600/Hex_ice.GIF" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="765" data-original-width="900" height="544" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjqSmhnuX202LC8O5ZII5IEyZjGA5RhJZUqp83lkTKYbsujYXgE4NeOHJA4kcx8i5upM2FAkGFDw_BJUIjH7rg435EQ7ovGx-j4D5XKGOjGGVNzXSiYIICCQ59sXuJ8ZMxAOg58myGIw30Z/s640/Hex_ice.GIF" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">NIMSoffice;Wikipedia</span></span></td></tr>
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In addition to a number of solid crystalline lattice states, water can also exist as a solid lacking any crystalline structure, called <a href="https://en.wikipedia.org/wiki/Amorphous_ice">amorphous ice</a>. On Saturn's icy moon, <a href="https://en.wikipedia.org/wiki/Enceladus">Enceladus</a>, water ice strewn onto the surface from its many <a href="https://en.wikipedia.org/wiki/Cryovolcano">cryovolcanoes</a> is amorphous. Liquid water flash-freezes in the vacuum of space (zero pressure) before it can organize into any structure. In contrast, along Enceladus's unique "tiger stripes," the ice is crystalline because here it is kept warm enough long enough from the heat of geothermal activity to arrange into a more thermodynamically favourable crystalline structure.<br />
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Below is Cassini's view of Enceladus's south pole, showing 4-5 "tiger stripes," which are <a href="https://en.wikipedia.org/wiki/Fault_(geology)">tectonic fractures</a>.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiQs7hdOJO0geeaTDTGUPbFK75bAKqmI6OOAJxevWtR2xRi_d_IxbOjgLBUWv6tL_8T00vROjQJyqQkxzoiV6uju3S8WYlMQdEpy21zPK-BFuwZAuAz2dXRuDPqMgnmrLE9PABg08QWXAqg/s1600/Enceladus_Tiger_Stripes_Up_Close_PIA06247.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1024" data-original-width="1024" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiQs7hdOJO0geeaTDTGUPbFK75bAKqmI6OOAJxevWtR2xRi_d_IxbOjgLBUWv6tL_8T00vROjQJyqQkxzoiV6uju3S8WYlMQdEpy21zPK-BFuwZAuAz2dXRuDPqMgnmrLE9PABg08QWXAqg/s640/Enceladus_Tiger_Stripes_Up_Close_PIA06247.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">NASA/JPL/Space Science Institute</span></span></td></tr>
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Critical Point and Triple Point<br />
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The temperature and pressure at which a phase change occurs is called a <a href="https://en.wikipedia.org/wiki/Critical_point_(thermodynamics)">critical point</a>. The temperature of the critical point depends on the pressure of the system and vice versa, so on a graph you have a line rather than a single point (see the phase graph below for water). However, there is a single point, a specific temperature and pressure, at which gas, liquid and solid states of water all have an identical <a href="https://en.wikipedia.org/wiki/Thermodynamic_free_energy">free energy</a> (or internal molecular energy). All three phases coexist at this point, which is called the <a href="https://en.wikipedia.org/wiki/Triple_point">triple point</a>. A critical point is a line and a triple point is a point. In the graph below, two black lines represent the solid/liquid critical point and the liquid/gas critical point of water. In the lower center of the graph you can see the single triple for water. It is a single point on the graph found at 0.01°C (273.6°K) and 0.006 atm (611.657 pascals).<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjmSvbvT7j9x4MjWqLY_4_O2Edu51sl2IVdOLYZqhbiWpJiVwGaXWbMLSH41edDvLSGXNn6oFmXCIZvKzF1VxwbWodBAwG_oQoqI77jRjO9SjMYuXGSo8QYa25XpPBBYnQS0XS6UItRnN0Y/s1600/512px-Phase_diagram_of_water.svg.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="432" data-original-width="512" height="540" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjmSvbvT7j9x4MjWqLY_4_O2Edu51sl2IVdOLYZqhbiWpJiVwGaXWbMLSH41edDvLSGXNn6oFmXCIZvKzF1VxwbWodBAwG_oQoqI77jRjO9SjMYuXGSo8QYa25XpPBBYnQS0XS6UItRnN0Y/s640/512px-Phase_diagram_of_water.svg.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">mglee;Wikipedia</span></span></td></tr>
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<i>A note about pressure units on the above graph: 1 atm pressure is roughly equal to 1 bar pressure. 1 bar is equal to 100 kilopascals (kPa). The pascal is the internationally recognized SI unit for pressure. The bar and the atmosphere are older non-Si units. I am in the old habit of using atm units, where 1 atm is standard air pressure at sea level.</i><br />
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This 6-minute video from Bergen University in Norway demonstrates how water behaves as it approaches its triple point.<br />
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<iframe allowfullscreen="" frameborder="0" gesture="media" height="360" src="https://www.youtube.com/embed/HEzkHqWIiKM" width="640"></iframe>
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<br />
At the triple point, all of the water in a system can be changed into vapour, liquid or solid just by tweaking the pressure or temperature a tiny bit. The triple point is also the lowest pressure at which liquid water can exist. At lower pressures, as on the surface of an icy moon (at near vacuum), water ice, perhaps warmed when facing its star, will sublimate directly into vapour, bypassing the liquid state altogether. For most substances, the triple point is also the lowest temperature at which the liquid state can exist but water, due to its unusual hydrogen bonds, is an exception. Notice the odd little horn on the green section in the graph above. This is an anomaly of water. If the temperature is just below the triple point, a steadily increasing pressure will transform ordinary solid water ice (Ih) into denser liquid water (at around 1000 atm) and then back into an even denser solid (now as ice VI) at around 10,000 atm.<br />
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A Brief Look at The Thermodynamics of a Phase Change<br />
<br />
For the phase changes I've described so far, the transition process is abrupt, or more scientifically put, discontinuous. At a phase transition point (a critical point), such as the boiling point for water for example, two phases - liquid water and water vapour - co-exist at one specific temperature. In thermodynamic terms, they have the same average <a href="https://en.wikipedia.org/wiki/Gibbs_free_energy">Gibbs free energy</a>. In any system some molecules will randomly have slightly more energy and some will have slightly less. However, at critical point the substance cannot be distinguished as either gas or liquid.<br />
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All thermodynamic systems tend to assume the lowest possible Gibbs free energy. Systems also tend toward increasing <a href="https://en.wikipedia.org/wiki/Entropy">entropy</a>. Gibbs free energy can be looked at as a measure of potential chemical energy in a system that is available to do work in the system, while entropy can be thought of as a measure of the orderliness of a system. Highly ordered crystalline water ice has very low entropy compared to the high entropy of water vapour. Both states, however, are stable within their own temperature/pressure regime because each state has maximized its entropy according to the Gibbs free energy available to it.<br />
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Below the boiling point, the liquid phase is more thermodynamically stable (which means it has reached maximum possible entropy at a lower Gibbs free energy). As water vapour condenses into liquid water, it enters a more highly ordered state (lower entropy). As this phase transition occurs, an energy exchange takes place. As the water enters a state of lower Gibbs free energy, it releases the extra free energy as <a href="https://en.wikipedia.org/wiki/Latent_heat">latent heat</a>. Conversely, if we wanted to melt ice into water we would have to apply heat to it. In other words we would have to add latent heat to the system so that we can increase the entropy of the water molecules.<br />
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So far we've focused on water as our example, but every chemical substance and element can exist as a solid, liquid and gas, depending on its temperature and pressure. Each substance has its own unique critical points and triple point, and it may have additional solid lattice states as well. These critical points are unique to each substance and depend on the substance's unique molecular bonding.<br />
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The Relationship Between Pressure and Phase Change<br />
<br />
We've explored how substances and elements transition from solids to liquids to gases and vice versa. Increasing the pressure on a substance or increasing its temperature increases the average kinetic energy of that substance. Pressure and temperature determine which physical state a particular substance is most thermodynamically stable. The relationship between temperature and physical state is fairly straightforward as we've seen. However, the relationship between pressure and physical state depends on the starting physical state of the substance. A gas responds to pressure by shrinking in volume and increasing kinetic energy. A gas resists compression through thermal pressure: Even though a container full of gas appears static, the molecules in it are in constant motion. Collisions between those molecules and the sides of the container are detected as a force per unit area, or <a href="https://en.wikipedia.org/wiki/Pressure">pressure</a>. If a gas is compressed, it will heat up because the total thermal energy of that gas, which doesn't change, is now concentrated into a smaller volume. If heat is continually removed from the system as it's compressed, the gas will remain at the original temperature it was even though it will eventually condense into a liquid and then further solidify into a solid.<br />
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Liquids, unlike gases, tend to be fairly incompressible. This means that liquids don't experience much increase in kinetic energy when they are compressed, but they will compress under extreme pressure. The pressure that resists compression is not thermal pressure, as with gases, but another kind of outward pressure (which gases also express but is masked by thermal pressure). This pressure is the result of the <a href="https://en.wikipedia.org/wiki/Pauli_exclusion_principle">Pauli exclusion principle</a>. This quantum mechanical principle means that when electron orbitals in adjacent atoms are forced into very close proximity, they will powerfully resist overlapping one another into the same atomic orbital. To note, this is not the same thing as orbital sharing, in which an electron will occupy an empty orbital in an adjacent atom, and through such sharing create a chemical bond. I will explain this more accurately in a moment.<br />
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Under increasing pressure, the liquid will likely first become denser and more viscous and then it will transition into a solid, in which molecules will pack more closely together but in a very ordered arrangement that respects the exclusion principle. If you recall my earlier example of liquid water being subjected to sudden extreme pressure and condensing into a dense form of ice, in both cases it eventually becomes more thermodynamically favourable for the water molecules to transition into a high-density highly ordered form of ice than to remain as a highly pressurized liquid. That "decision" made by a substance is abrupt, which means it happens all at once.<br />
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Water Under Very Intense Pressure<br />
<br />
What happens if you continue to increase the pressure on a solid such as water ice? Solids, like liquids, resist compression. If we take a look at the water phase graph above once again, we see that above 100,000 atm (or bars), water exists only as a solid no matter what the temperature is, at least up to 350°C. Cubic ice VII will transform into an even denser cubic lattice called <a href="http://www1.lsbu.ac.uk/water/ice_x.html">ice X</a> as the pressure is increased. Its crystalline lattice will transition into increasingly denser molecular arrangements, each transition being a phase change. At the highest pressure on the graph (around 6-10 million bars) we see a phase of ice called <a href="http://ergodic.ugr.es/termo/lecciones/water1.html">high-pressure ice XI</a>. This extremely dense XI hexagonal ice is hypothetical at this time and should not be confused with <a href="https://en.wikipedia.org/wiki/Ice_XI">ice XI orthorhombic</a>, which likely forms at temperatures below -200°C at zero pressure, such as on the surface of Pluto's moon, <a href="https://en.wikipedia.org/wiki/Hydra_(moon)">Hydra</a>, for example.<br />
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A <a href="https://arxiv.org/abs/1003.5940">2010 article by Burkhard Militzer and Hugh Wilson</a> explains what researchers think happens when the pressure on ice increases even further than our graph goes. In this theoretical case, water bypasses hypothetical high-pressure ice XI. They suggest instead that above about 3 million bar, Ice X could transition into a different more complex lattice that contains 12 hydrogen atoms per unit. If the temperature is also increased to about 2700°C, they think the lattice might transition into yet another new arrangement. The hydrogen atoms may become mobile within a stable lattice consisting only of oxygen atoms. This would create a <a href="https://en.wikipedia.org/wiki/Superionic_water">super-ionic phase</a>. If the temperature is increased even further under this intense pressure regime, the oxygen atoms themselves may also become mobile and the lattice itself might melt into a new extremely dense unstructured phase of matter that no longer consists of water as we know it. The atoms are no longer chemically bound to each other. This is an exception to the simpler rule we started with - that the chemistry of substances does not change during a phase transition. As one goes deeper into almost any theory, rules based on simpler understandings tend to hit the roadside.<br />
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At temperatures below 2700°C and pressures above 48 million bar, a transition to a metallic ice phase is possible. Such intense pressure is expected to exist deep within icy Uranus or Neptune. In this phase, the structure of the ice would <a href="http://news.cornell.edu/stories/2012/01/scientists-predict-out-world-kind-ice">resemble stacked corrugated sheets of oxygen and hydrogen atoms</a>, which would be electrically conductive.<br />
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Even more extreme pressures can be forced on matter, for example, inside the core of a rapidly collapsing star. What would happen to water under such conditions? As pressure is increased within a solid, individual atoms resist being pressed closer together. To explain this, we need to revisit the <a href="https://en.wikipedia.org/wiki/Atomic_orbital">electron orbital</a>. Hydrogen is a simple example to illustrate this. This atom (at <a href="https://en.wikipedia.org/wiki/Ground_state">ground state</a>) has one electron <a href="https://socratic.org/chemistry/the-electron-configuration-of-atoms/arrangement-of-electrons-in-orbitals-spd-and-f">in its 1s electron orbital</a>. Any orbital can hold a maximum of two electrons, so 1s could also accommodate a second electron if there was one. An electron in another atom's orbital can temporarily occupy that role. If it is another hydrogen atom, then a covalent electron-sharing chemical bond is created, turning atomic hydrogen into hydrogen gas, H<sub>2</sub>.<br />
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An atom is <a href="https://en.wikipedia.org/wiki/Thermodynamic_system">a system</a>, and like all physical systems, it tends toward the lowest free energy state possible. There are many forces going on in a hydrogen molecule. The two electrons repulse each other. The two protons repulse each other. Each electron is attracted to both proton nuclei and vice versa. There is an optimum distance between electrons and protons where all of these electrostatic forces add up to a lowest possible energy state. This is the most stable state. When atoms are squeezed closer together (or pulled further part), they resist and a force must be applied. Although perhaps easier to visualize, the electrostatic interactions that I've just described here are insignificant compared to much more powerful quantum interactions that respect the exclusion principle. Two or more electrons cannot share identical <a href="https://en.wikipedia.org/wiki/Quantum_number">quantum numbers</a>. If two electrons share an orbital they must be of opposite spin (spin is a quantum number). This exclusion principle translates into <a href="https://en.wikipedia.org/wiki/Electron_degeneracy_pressure">a repulsive quantum force</a> that very powerfully resists additional pressure put onto an already extremely dense solid phase.<br />
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As pressure increases, the atomic orbital structure itself is forced to shift. Normally, electrons fill only a few energy levels in an atom. Many energy levels are unoccupied. Under extreme pressure, all the electrons are forced into the lowest energy level orbitals, as close to the nucleus as possible. This is ultra-dense <a href="https://en.wikipedia.org/wiki/Degenerate_matter#Electron_degeneracy">electron-degenerate</a> matter that is expected to exist in the core of a <a href="https://en.wikipedia.org/wiki/White_dwarf">white dwarf</a> stellar remnant. The exclusion principle means that two same-spin electrons won't share an orbital no matter how strongly they are forced together. Electrons, experiencing such intense repulsive quantum forces, respond by moving faster and faster in these lowest orbitals. Under enough pressure, they approach the speed of light, which is the limit of this arrangement. Protons begin <a href="https://en.wikipedia.org/wiki/Electron_capture">to absorb electrons</a>, creating a degenerate phase of matter, called <a href="https://en.wikipedia.org/wiki/Degenerate_matter#Neutron_degeneracy">neutron matter</a> which is the densest state of matter possible. It is thought to exist in super nova stellar core fragments called <a href="https://en.wikipedia.org/wiki/Neutron_star">neutron stars</a>. Such transitions into extremely dense matter are technically phase changes as well, as is the final transition of matter into a <a href="https://en.wikipedia.org/wiki/Black_hole">black hole</a>, in which matter as we know it collapses entirely.<br />
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Where Plasma Fits In As A Phase Transition<br />
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Where does plasma fit into this progression of phase transitions? The temperature of a substance or element is the measure of the average vibrational energy of its constituent molecules or atoms. Gas molecules generally have a great deal of vibrational energy as well as kinetic energy. They move around in all directions at great velocities, widely separated from each other. They occasionally bump into one another, and the force of these collisions translates into gas pressure. If a gas is cooled, it will eventually condense into a liquid at its critical point. Pressurizing a gas adjusts that critical point to a higher temperature. If you take another look at the phase diagram of water, you notice that water will boil into water vapour at just 50°C, rather than at 100°C if the pressure is 1/10th that of atmospheric pressure. At the other extreme, the temperature of water streaming from a deep sea hydrothermal vent <a href="https://en.wikipedia.org/wiki/Hydrothermal_vent#Physical_properties">can reach over 400°C</a> but it does not boil because the pressure is over 300 times atmospheric pressure at depths over 3000 m, where most hydrothermal vents are located.<br />
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One way to make plasma is to heat gas. What happens if we heat a container of gas rather than compress it? We increase the total kinetic energy of the molecules. If we heat a sample of hydrogen gas enough, we will eventually supply enough energy to break the chemical bonds and dissociate the gas into hydrogen atoms. The <a href="https://en.wikipedia.org/wiki/Bond-dissociation_energy">bond dissociation energy</a> of H<sub>2</sub> is about 50,000°C but this temperature depends on the density of the gas. As density increases, the bond dissociation energy increases (due to two particles taking up more space than one). Now a mono-atomic gas, the hydrogen atoms can be further energized into ions. The <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/ionize.html">ionization </a><a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/ionize.html">energy</a> of hydrogen is about 150,000°C at 1 atm. This is also pressure-dependent. As density increases, the ionization energy decreases. As atoms are forced together, the gaps between electrons energy levels get shorter, which means less energy is required to <a href="https://www.nde-ed.org/EducationResources/HighSchool/Radiography/ionization.htm">ionize</a> an atom.<br />
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Ionization means that the atom loses one or more electrons to create a gas that contains positive and negative ions. If we ionize hydrogen gas, we create a cloud of electrons and protons, called plasma. A very dense liquid-like plasma state in which protons are surrounded by a sea of mobile electrons, called <a href="https://en.wikipedia.org/wiki/Metallic_hydrogen">metallic hydrogen</a>, might exist deep inside Jupiter and Saturn. Because electrons are not confined to orbitals, this is also called a degenerate state of matter.<br />
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The process of ionization from a neutral gas into charged plasma is considered by many researchers to be the fourth physical phase after gases, liquids and solids. This phase transition, however, is a different and gradual, rather than abrupt, process. Atoms with more than one electron tend to lose electrons gradually as increasing energy is applied to a gas. The other three transitions - solid to liquid to gas - are each marked by an abrupt change in the arrangement of the molecules in a substance. In a transition from a gas into plasma, unlike the other three phase transitions, the substance also chemically changes because at least some molecular bonds are broken. For example, water vapour can ionize into hydrogen/oxygen plasma within a lightning bolt. The process of ionization into plasma is reversible, a characteristic in common with the three other changes of state. If energy is removed from the plasma, the ions will recombine into neutral atoms and molecules. However, in a complex mixture of gases, some new molecules might form as different ions react with one another.<br />
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The process of ionization takes place over a series of steps. We will go back to hydrogen as our example. As energy is pumped into atomic hydrogen gas, the kinetic energy of the atoms increases and some atoms become <a href="https://en.wikipedia.org/wiki/Excited_state">excited</a>. An atom is excited through two possible processes: collisions with other atoms and the absorption of electromagnetic energy (light).<br />
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The electron in a hydrogen atom at ground state is located near the nucleus in a lowest possible energy orbital, which is actually <a href="https://en.wikipedia.org/wiki/Atomic_orbital#Types_of_orbitals">a cloud of possible locations</a> rather than a defined circular orbit. The electron in each hydrogen atom can absorb either kinetic or photon energy and move to a higher energy orbital. Electrons can even move into higher energy orbitals that are not ordinarily occupied by electrons. The atom is now in an excited state. If energy is removed from this system, the electron will return to its lowest energy (ground) state by releasing exactly the same energy as the energy difference between the orbitals. The energy is quantized. It is released as photons of specific wavelengths. For hydrogen, some of these photons are in the visible range. An excited hydrogen atom commonly emits red or aqua blue photons depending on the orbital drop, but it can also emit higher energy ultraviolet photons if it absorbs and releases more energy. As energy is added to the system, the electron continues to move up into higher energy orbitals until it eventually breaks free from the atom altogether. The proton nucleus can no longer hold onto the electron so it is now a completely ionized nucleus without any electrons. If an atom with more than one electron loses some of its electrons, but not all, it is partially ionized. Our hydrogen atom is now a dissociated electron and proton. A gas of these ions is called completely ionized plasma, shown in the simple diagram below.<br />
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: left; margin-right: 1em; text-align: left;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhICW_Z56jZfT8HM1EccXHEdIyjv4Zar1Mfk43sxXtMEOE53PknwXbo4wiqErLf4XU61br9hIfSZzuZHh2lGbq8xdR4RIKeP4bELbkepunxB2bNgBR_2p66AJcBJ1tnEdpOTkHw-pmxpeCY/s1600/440px-Electron_Sea_%2528Plasma%2529.jpg" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="267" data-original-width="440" height="194" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhICW_Z56jZfT8HM1EccXHEdIyjv4Zar1Mfk43sxXtMEOE53PknwXbo4wiqErLf4XU61br9hIfSZzuZHh2lGbq8xdR4RIKeP4bELbkepunxB2bNgBR_2p66AJcBJ1tnEdpOTkHw-pmxpeCY/s320/440px-Electron_Sea_%2528Plasma%2529.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Spirit469;Wikpedia</span></span></td></tr>
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In this simple diagram (left) of completely ionized plasma, all electrons have been stripped from their nuclei, creating an electrically conductive "electron sea."<br />
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Under real conditions, low-energy plasmas often contain a mixture of excited, partially ionized and neutral atoms, which can be used to emit a beautiful glow in a <a href="https://en.wikipedia.org/wiki/Gas-discharge_lamp">gas-discharge lamp</a>, for example. The tube below, filled with diffuse low-energy hydrogen plasma, glows pink - a mixture of red and aqua blue photon emissions.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEib0Dd-waEHd2NidzGLNdHcjPFy_ApxeirgOmkbRkJ7gCDFIrJSttaV42OmOsoTrn22mePDJPT_xzustuaxrDQxObawZPLR87bdSV_Xex-rkHBTkZsniIE5OyEaWw_hkBUj-5m5qXQYWNlK/s1600/Hydrogen_discharge_tube.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="534" data-original-width="1600" height="212" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEib0Dd-waEHd2NidzGLNdHcjPFy_ApxeirgOmkbRkJ7gCDFIrJSttaV42OmOsoTrn22mePDJPT_xzustuaxrDQxObawZPLR87bdSV_Xex-rkHBTkZsniIE5OyEaWw_hkBUj-5m5qXQYWNlK/s640/Hydrogen_discharge_tube.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">www.pse-mendelejew.de; Wikipedia</span></span></td></tr>
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The plasma in the tube above contains mostly neutral and excited atoms and only a few ionized atoms. The degree of ionization in plasma, sometimes called plasma density or electron density, is the number of free electrons in a volume of plasma. Even a partially ionized gas which contains only 1% ionized atoms, can be considered plasma if it exhibits plasma behaviours such as responding to magnetic fields and conducting electricity. <br />
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The plasma in the tube above was not created by heating hydrogen gas until it ionized; this tube is at room temperature. Extreme heat is one way to create plasma. Hydrogen in the interior of the Sun and other stars is extremely high-energy completely ionized plasma because it is an incredibly hot environment. Any substance, if hot enough, will transition into plasma. Even the exotic solid states of water, which remain solid even up to 400°C, would eventually break down into plasma as temperature is increased. I should note here that this process is not chemical ionization of water, in which water (H<sub>2</sub>0) <a href="https://en.wikipedia.org/wiki/Self-ionization_of_water">self-ionizes</a> into hydroxide (0H<sup>-</sup>) and hydronium (H<sub>3</sub>0<sup>+</sup>) ions. That is a chemical equilibrium reaction, and no change of phase occurs.<br />
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Rather than through heat, the hydrogen plasma in the tube above was created by applying an electric potential to hydrogen gas. This is another way that energy is applied to a gas in order to ionize it. Instead of electrons being "shaken off" of an atom with high kinetic energy, electrons (which, remember, are charge-carrying particles) are drawn off the atom by a powerful electrical force. It is maybe analogous to ducklings being swept away from their mom down a stream with a powerful current. It can take tremendous energy to completely ionize a gas. It all depends on which molecules and elements the gas consists of and what kind of energy is applied. When we focus in on what happens to an ionizing atom, it is more accurate to talk about energy in term s of electron volts (eV) than in temperature, which is an average of particle energies. The most tightly bound (and stable) atom is helium. It has two electrons, and therefore two ionization energies - it takes 25 eV to remove just one electron (this partially ionizes the atom). It takes much more energy (about 54 eV) to strip both electrons off the helium nucleus (now the atom is completely ionized). If a collision with another atom strikes with over 54 eV of energy, the target atom can be completely ionized.<br />
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It takes far less energy to excite atoms rather than ionize them. Even a relatively small 120V household circuit can light up a small neon lamp, which is a vacuum tube filled with mostly neon gas and a little argon gas.<br />
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In gases, ionization is reversible. If the energy is removed from the system, such as turning off the applied voltage gradient (unplugging the hydrogen tube above), the protons recombine with free electrons, the atoms return to ground state, and molecules recombine into neutral gas (H<sub>2</sub>). In such a closed system of only a single atomic gas, the gas can be ionized and recombined, transitioned into liquid and solid states and back again, over and over, demonstrating an entirely reversible process, but under real conditions, the extreme energy of some plasmas can trigger various chemical reactions as well, particularly combustion reactions. These reactions are non-reversible and add a non-reversible component to the phase change.<br />
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In solids, in particular, the process is not easy to describe in terms of a phase change. An example here might be a <a href="https://en.wikipedia.org/wiki/Fulgurite">fulgurite</a>. Lightning strikes sand and leaves behind a hollow tube of glass buried in the ground. Lightning is an extremely powerful electric potential, of up to 500,000 volts. The melting point of <a href="https://en.wikipedia.org/wiki/Silicon_dioxide">silicon dioxide</a> (pure sand) is 1710°C and the boiling point is 2230°C. The interior of a lightning bolt <a href="http://www.lightningsafety.noaa.gov/temperature.shtml">can reach a temperature of 28,000°C</a>, far higher than the boiling point of sand, enough to chemically break it apart and at least partially ionize its silicon and oxygen atoms. When lightning strikes sand, some sand is explosively vapourized into gas and plasma, leaving a hollow tube, where the bolt struck, surrounded by a layer of molten sand that quickly solidifies into glass. If there were impurities in the sand, such as soil and plant debris, these components would have combusted and reacted with each other, resulting in new chemical compounds in the fulgurite's glass. Air molecules in the vicinity are also broken down into plasma temporarily. In this case, a series of very rapid phase changes has occurred from solid to liquid to gas to plasma, but there was also opportunity for chemical reactions to take place, which are irreversible. Even the air itself, which is a mixture of gases, does not transition to a plasma state and then back into neutral gases without some chemical changes taking place. Some highly reactive oxygen ions and excited oxygen molecules will recombine into ozone molecules, for example, creating the fresh-air smell after a thunderstorm.<br />
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Why isn't a neon sign or lamp hot? Neon lights might get warm to the touch but they never get hot. Yet we can think of the ionized atoms in plasma as hot. The atoms have at least enough energy to become excited and lose some outermost electrons. The plasma itself contains some free fast moving electrons with significant kinetic energy, but in the case of the low-density plasma in a neon lamp, it also contains mostly neutral atoms with far less kinetic and thermal energy. This means that its average temperature will be simply warm to the touch. A neon light contains very diffuse plasma and most of the neon gas remains in a neutral gas state that absorbs the excess energy of the electrons that collide with them. It doesn't take much energy to create glowing plasma in which only a few outermost electrons of atoms are excited and fewer still are stripped off, creating a small electrical current in the tube that sustains the excited-atom glow. Ordinary air is an exception. It is actually a powerful electrical insulator and would make a lousy plasma light. It will ionize and glow (this is a lightning bolt) only when it is subjected to a very powerful electrical potential of <a href="http://www.windpowerengineering.com/featured/business-news-projects/how-much-power-in-a-bolt-of-lightning">more than approximately 100,000V</a>. The <a href="https://en.wikipedia.org/wiki/Dielectric_strength">dielectric strength</a> of air, its ability to withstand a potential gradient before breaking down or ionizing, is 3.0 MV/m (million volts/metre), which is much higher than neon's, which is 0.02 MV/m. Dielectric strength is an intrinsic property of a material.<br />
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Understanding Plasma is Essential To Understanding Our Universe<br />
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Plasmas, no matter how they are created, have unique physical properties. They act quite differently from neutral gases. Like gases, plasmas do not have a definite shape or volume. They can be compressed fairly easily. Unlike gases however, which are electrically neutral, plasmas respond to electric and magnetic fields. Even though the charges are usually balanced overall in plasma, they are separate and free to move. This means they can produce electric currents and magnetic fields and they respond to them as well. Electromagnetic forces exerted on plasmas act on them across very long distances. This gives plasma special coherent behaviours that gases never display. The Sun and other stars are examples of plasma created under extreme heat. Their interiors consist of extremely dense, energetic, and completely ionized plasma. Powerful plasma currents exchange heat released from ongoing nuclear fusion in the stellar core. These physical currents are also powerful electrical currents because they are moving charges. The moving charges set up incredibly intense magnetic fields that can interfere with one another and snap. These are the mechanisms behind the <a href="https://en.wikipedia.org/wiki/Solar_flare">violent solar weather</a> that can damage communications, satellites and electrical systems 150 million miles away on Earth. While stars are made of dense plasma, most of the visible matter in the universe consists of very diffuse highly ionized plasma. Both kinds of plasma are the key reason why we can see distant stars and various gas clouds. They glow because they are plasmas that contain atoms that are continually being excited and returning to ground state, emitting photons of light in the process. Intergalactic space, interstellar space and interplanetary space are also all (extremely diffuse) plasma. Solar wind is plasma and Earth's ionosphere in the upper atmosphere consists of diffuse atmospheric gases ionized by solar radiation, or plasma in other words. At one point, prior to recombination, the entire universe consisted of completely ionized plasma. It was too energetic to support intact atoms. Expansion (and therefore cooling) allowed that primordial plasma to settle into the first and most abundant simple atoms such as hydrogen and helium. For these reasons, some theorists think that plasma theory should play a more dominant role <a href="https://en.wikipedia.org/wiki/Plasma_cosmology">in understanding the cosmology of the universe</a>, alongside general relativity, high-energy astronomy, mechanics and dynamics. <br />
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Plasma behaviour can be extremely complex. Understanding the complicated dynamics of stars, for example, requires computer modeling based on plasma theory, much of that theory belonging to the field of <a href="https://en.wikipedia.org/wiki/Magnetohydrodynamics">magnetohydrodynamics</a>, a field of study that only got under way in last few decades. Understanding the electrodynamics of plasma (the behaviour of an electrical fluid) on the largest scales might also help to explain the evolution of galaxies as well as their birth from the collapse of interstellar clouds into stars, which is not yet completely understood. Plasma theory might also shine some light on the mysterious nature of dark matter, required to explain the rotation curves of galaxies. Black holes, quasars and active galactic nuclei must all incorporate plasma dynamics in order for us to fully understand how they work. Extreme plasma dynamics could even represent missing pieces in our understanding of cosmic inflation and the accelerating expansion of the universe, the latter of which is now called <a href="https://en.wikipedia.org/wiki/Dark_energy">dark energy</a>. An intuitive understanding of plasma could be a useful key to understanding the way the universe works.<br />
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<br />Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-25725775584102227482017-08-17T11:09:00.001-06:002017-08-17T11:09:49.903-06:00The Perils of Cosmic Radiation: Are We Made for Deep Space Travel?We know we are intrepid explorers by
nature. Our pop culture reflects that fact. We believe that someday we will overcome
all technological odds to travel to, and set foot on, distant but promising <a href="https://en.wikipedia.org/wiki/Exoplanet">exoplanets</a>, perhaps somewhere where an alien
ecosystem has taken root. Maybe that life is biochemically similar to us. It's only a matter of time.
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There is a problem, however, one that has not
been given much play in our sci-fi movies and books. We are not made for outer
space. We are extremely delicate organisms that have evolved within a pocket
shielded from deadly <a href="https://en.wikipedia.org/wiki/Solar_wind">solar wind</a> and
even more violent <a href="https://en.wikipedia.org/wiki/Cosmic_ray">cosmic radiation</a>.
We live inside a thick envelope of gas surrounded by a powerful <a href="https://en.wikipedia.org/wiki/Earth%27s_magnetic_field#Magnetosphere">planetary magnetosphere</a>,
which in turn is enveloped in an even more powerful and far-reaching <a href="https://en.wikipedia.org/wiki/Heliosphere#Space_beyond_the_heliosphere">stellar magnetosphere</a>.
These powerful magnetic envelopes deflect most harmful radiation away and our
atmosphere does a thorough job of absorbing what still makes it through. As a
result, our bodies are exposed over our lifetimes to just a miniscule fraction
of the fast atomic fragments that bombard every square meter of deep space.</div>
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<span lang="EN-US">Space is not the empty, cold, benign
backdrop portrayed in most movies. It is a nuclear blast zone. Stars like our
Sun are ongoing fusion reactions sloughing off electromagnetic radiation, protons,
electrons, neutrinos, and small atomic nuclei in every direction. Almost
infinitely more powerful stellar explosions and collisions, happening all the
time across the universe, blast matter away at almost the speed of light.
Nothing slows these deadly particles down as they fly across the light-years.
An astronaut's suit or spaceship hull stands little chance against this
constant cosmic onslaught. No currently available material <a href="https://www.nasa.gov/feature/goddard/real-martians-how-to-protect-astronauts-from-space-radiation-on-mars">is strong or dense enough to absorb or deflect cosmic radiation</a> while also being lightweight enough to be
launch-able. Any ship-wide magnetic deflection shield powerful enough to work will require an enormous supply of energy during the decades it will take to get to even a relatively nearby exoplanet is also a
distant, if not impossible, technological dream. <o:p></o:p></span></div>
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<span lang="EN-US">Despite the inhospitable nature of space, we
have inhabited it, at least close to home where we have some protection from
cosmic radiation. The <a href="https://www.nasa.gov/mission_pages/station/main/index.html">International Space Station (ISS)</a> is an artificial biosphere that takes care of the physical needs of a few
humans at least on the scale of many months. On its predecessor, <a href="https://en.wikipedia.org/wiki/Mir">Mir</a>,
Valery Polyakov, a Russian astronaut, <a href="https://arstechnica.com/science/2016/03/meet-the-real-ironman-of-spaceflight-valery-polyakov/">spent 437.7 consecutive days in space</a> during 1994/1995. What we might forget is that this is time spent in space orbiting
close to Earth. Mir, for example, maintained a near-spherical orbit of between
352 km and 374 km above the surface of Earth, within the second most outer
layer of Earth's atmosphere called the <a href="https://en.wikipedia.org/wiki/Thermosphere"><span id="goog_1367724057"></span>thermosphere<span id="goog_1367724058"></span></a>,
which is just above the <a href="https://en.wikipedia.org/wiki/Ionosphere">ionosphere</a>,
which forms the inner edge of Earth's magnetosphere. At this altitude the
density of air is extremely low so there is no atmospheric protection from
solar or cosmic radiation. Air here is so diffuse that a single molecule of
oxygen would have to travel on average one kilometer before it collided with
another molecule. However, Mir was well within Earth's thick protective
magnetosphere. Even at its narrowest region, where it is compressed by incoming
solar wind on the side of Earth facing the Sun, the magnetosphere is about
60,000 km thick. <o:p></o:p></span></div>
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<span lang="EN-US">How does atmospheric cosmic and solar
radiation protection work? Our atmosphere is transparent to low frequency electromagnetic (EM) radiation
emitted from the Sun. Sunlight, for example travels through air to bathe us on the
surface. High-frequency EM radiation from the Sun, such as X-rays and gamma
rays, is absorbed by the plasma (consisting of electrons and electrically charged atoms and molecules) in Earth's ionosphere. In addition to EM
radiation, the Sun also emits particles in all directions, most of which are
protons, and they are traveling very fast, about 400 km/s. Earth's rotating
metallic core generates a powerful magnetic field that deflects most of these
charged particles away from the surface. Our atmosphere is also opaque to this
radiation. It absorbs what isn't deflected. Consider
<a href="https://en.wikipedia.org/wiki/Mars">Mars</a> for a moment. It possesses neither a deflecting planetary magnetosphere
nor a thick highly absorptive atmosphere. Its thin carbon dioxide rich
atmosphere provides some radiation protection, but not much. It is roughly <a href="https://phys.org/news/2016-11-bad-mars.html">2.5times less protective</a> than Earth's very thin thermosphere through which ISS orbits. Although its surface at the equator approaches a livable temperature, the continuous bombardment of radiation makes it more inhospitable than you might think.<o:p></o:p></span></div>
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<span lang="EN-US">While astronauts on Mir and now the ISS have
very little atmospheric radiation protection, they are protected from solar
wind by Earth's magnetosphere. Thanks to the Sun's magnetosphere they are also <i style="mso-bidi-font-style: normal;">mostly</i> protected from far more powerful radiation
- gamma rays and particles with far higher kinetic energy than any solar wind
particle. Yet even here close to Earth, astronauts can only stay on ISS for a
limited time. A very small amount of cosmic radiation makes it through our
solar magnetosphere, meaning that the astronauts do receive a low but cumulative
dose of cosmic radiation. A <a href="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0096099">2014 study by radiation expert Francis Cucinotta</a> indicates that ISS astronauts exceed their lifetime safe limit due to cosmic
radiation in just 18 months for women and two years for men.<span style="mso-spacerun: yes;"> </span><o:p></o:p></span></div>
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<span lang="EN-US">Protons (hydrogen nuclei) and, to a lesser
extent, larger atomic nuclei (such as alpha particles and helium nuclei), traveling
near the speed of light which is 300,000 km/s, pervade every square centimeter
of deep space. The ISS would never be feasible in this constant blast zone.
However, protected within the magnetospheres of the Sun and the Earth, an
aluminum hull just a few millimetres thick shields about 95% of the radiation
that strikes it. Even thick plastic stops this radiation, which consists mostly
of EM radiation along with relatively low-energy solar protons (averaging 400
km/s) that manage to pass through Earth's magnetosphere. We have to say <i style="mso-bidi-font-style: normal;">average </i>velocity here because the Sun
isn't static. <a href="https://en.wikipedia.org/wiki/Geomagnetic_storm">Magnetic storms</a> sometimes rage across its surface, accelerating particles up to 3200 km/s, in
the case of exceptionally violent storms called <a href="https://en.wikipedia.org/wiki/Coronal_mass_ejection">coronal mass ejections</a>.
A coronal mass ejection is a mass of highly magnetized plasma, a chunk of the
Sun, hurled off into space when a magnetic field snaps. Velocities up to 3200
km/s have been recorded by the <a href="https://lasco-www.nrl.navy.mil/">LASCO</a> onboard the <a href="https://sohowww.nascom.nasa.gov/">SOHO</a> satellite orbiting between
the Sun and Earth. Even this accelerated plasma, which <a href="http://www.businessinsider.com/solar-storm-effects-electronics-energy-grid-2016-3">would devastate Earth's electrical and communications systems</a>,
has nowhere the particle-to-particle punch of cosmic radiation, with velocities close
to 300,000 km/s.<o:p></o:p></span></div>
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<span lang="EN-US">As mentioned, materials are still in the process of <a href="http://newatlas.com/cosmic-ray-radiation-protection/24511/">being developed</a> that are both light and effective at shielding protons traveling close to light
speed. Most cosmic radiation comes from supernovae. It consists of stellar particles
violently spewed in every direction when a high-mass star reaches the end of
its life. The shock wave of the explosion is powerful enough to accelerate
material to near light speed. Only in extremely powerful accelerators can
particles on Earth approach such velocities.<o:p></o:p></span></div>
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<span lang="EN-US">Radiation itself can be a confusing
subject but at its core the concept of radiation is simple: it is an emission or
transmission of energy. It comes in four basic kinds: <a href="https://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html">electromagnetic (EM) radiation</a> (radio waves,
visible light, gamma rays etc.), <a href="https://en.wikipedia.org/wiki/Mechanical_wave">acoustic radiation</a> (sound waves, seismic waves), <a href="https://en.wikipedia.org/wiki/Gravitational_wave">gravitational radiation</a> (gravitational waves) and <a href="https://en.wikipedia.org/wiki/Particle_radiation">particle radiation</a> (on Earth we typically deal with alpha radiation – alpha particles, beta radiation – electrons, and neutron
radiation – neutrons). In terms of cosmic radiation, we are especially interested in protons, alpha particles and, to a much lesser extent, larger atomic nuclei. <o:p></o:p></span></div>
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<span lang="EN-US">Radiation is either <a href="https://en.wikipedia.org/wiki/Ionizing_radiation">ionizing</a> or <a href="https://en.wikipedia.org/wiki/Non-ionizing_radiation">non-ionizing</a>. With
the exception of microwaves, sonic devices and intense or prolonged light exposure that can cause photochemical burns, non-ionizing
radiation tends to present a minimal hazard to human health. This radiation simply doesn't have
enough energy to ionize atoms in living tissue. I will explain what "ionize" means in a moment. </span>Generally, a particle or wave must carry
more than 10 eV (<a href="http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ev.html">electron volts</a>) of energy to ionize atoms and therefore damage
biochemical bonds in molecules, but the line is blurry because some atoms
ionize more easily than others do and some chemical bonds break more easily than others do. Consider
the energy of a visible green light photon of about 2 eV. 2 eV is harmless (unless those 2 eV photons are concentrated into a laser. Then they can burn
your retinas and blind you). Radiation of 10 eV or more, however, has enough energy to strip
electrons off of (<a href="https://en.wikipedia.org/wiki/Ionization">ionize</a>) atoms and
molecules, breaking chemical bonds between them in the process. This radiation
<a href="https://en.wikipedia.org/wiki/Radiobiology">can be harmful and even lethal to humans</a>.
Short wavelength (very energetic) EM radiation such as X-rays and gamma rays
can break chemical bonds in DNA for example, creating genetic damage that can
eventually lead to cancer. It can also damage biological proteins and enzymes,
impairing cell function. All particle radiation from radioactive materials or
in the form of solar radiation and cosmic radiation is ionizing. It causes biological
damage at the microscopic level in our bodies. That said, The National Cancer Institute in the United States explains why radiation <a href="https://www.cancer.gov/about-cancer/treatment/types/radiation-therapy/radiation-fact-sheet">can sometimes be good thing</a>. Targeted at cancer cells, which are dividing uncontrollably, the ionizing radiation damages their cellular DNA so severely that the cells program themselves to die, shrinking the tumour.</div>
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<span lang="EN-US">Most cosmic radiation consists of
ultra-fast protons, and most of them are blasted out of exploding stars. A
proton, a particle normally confined inside an atomic nucleus, is an
exceptionally tiny object, weighing less than 2 x 10<sup>-27</sup> kg. Despite
its miniscule mass, a proton blasted from its atom and accelerated to near
light-speed (now we call it a relativistic proton) packs a catastrophic punch,
around 1 GeV (G means giga or billion electron volts). Compare this to the low
end of ionizing radiation at 10 eV. This force is roughly equivalent to a
baseball being thrown at you hard but with all of that impact condensed into an
area about one millionth of a nanometre wide (a proton is about 10<sup>-15</sup>
m in diameter). <o:p></o:p></span></div>
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<span lang="EN-US">You might expect a single proton at such
high velocity to fly right through your body, causing minimal damage along its
sub-microscopic course. After all, a general rule in physics is that the higher
the kinetic energy of a particle, the smaller the fraction of its kinetic
energy tends to get deposited in the material. The problem is that the proton
will, more than likely, glance off atoms in your body along the way. A homemade analogy here might be that while a massless "slender" gamma photon elegantly dances through the atoms, a massive proton acts like a bull in a china shop. </span>Those
atoms in the proton's path will be ionized, and they will ionize other atoms and so on, depositing
energy along an ever-widening path of damage in the body. This process is
technically called linear energy transfer (LET), which I will explain in a moment. An astronaut onboard the ISS
might by hit by only a few cosmic protons during a months-long mission but new
research indicates that even low-dose infrequent cosmic radiation exposures can
cause significant long-lasting damage, particularly to the brain. An astronaut
on route to Mars or on the surface of Mars, with only the Sun's magnetosphere
as protection (Mars doesn't have one), will receive a much higher exposure to
cosmic radiation than an ISS astronaut. Of most concern is the cumulative
damage to our fragile and very slow to heal brain.</div>
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<span lang="EN-US">Cosmic radiation <a href="https://en.wikipedia.org/wiki/Cosmic_ray#Discovery">is not a new discovery</a>.
Scientists have known about it for many decades. In fact, in 1909, Theodor Wulf
discovered that the rate of ion production inside a sealed container of air <a href="https://home.cern/about/updates/2012/08/cosmic-rays-discovered-100-years-ago">was higher at the top of the Eiffel Tower than at its base</a>,
refuting the then-current theory that radiation originated from radioactive
elements in the ground. A few years later Victor Hess launched ionization-measuring electrometers on a balloon during a near-total
solar eclipse. His results <a href="https://timeline.web.cern.ch/events/victor-hess-discovers-cosmic-rays">ruled out the Sun as the source of this radiation and confirmed Wulf's data</a>. <o:p></o:p></span></div>
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<span lang="EN-US"><a href="https://en.wikipedia.org/wiki/Cosmic_ray#Primary_cosmic_rays">Primary </a><a href="https://en.wikipedia.org/wiki/Cosmic_ray#Primary_cosmic_rays">cosmic radiation</a>,
originates outside the solar system and some of it comes from outside the Milky
Way galaxy. About ¾ of cosmic radiation consists of protons (hydrogen nuclei).
About a quarter consists of heavier alpha particles (helium nuclei) and about 1%
consists of still heavier nuclei, mostly lithium, beryllium and boron nuclei. These
heavier so-called <a href="https://en.wikipedia.org/wiki/HZE_ions">HZE (high atomic number and energy) ions</a>,
though far less abundant, are especially damaging. Though very sparse even in
deep space where there is no magnetic shielding, these infrequent impacts would
contribute significantly to an astronaut's overall radiation dose.<span style="mso-spacerun: yes;"> </span><o:p></o:p></span></div>
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<span lang="EN-US">A radiation dose is measured as an absorbed
dose. The SI unit is the gray (Gy). It measures the radiation's energy
deposited in the body. Radiation is also measured by the action upon the matter
of the body, by its <a href="https://en.wikipedia.org/wiki/Linear_energy_transfer">linear energy transfer (LET)</a>.
This means it measures the amount of energy lost per distance traveled. Two
equivalent absorbed doses can do vastly different amounts of damage in the body
depending on their LET values. A high LET means that the radiation leaves
behind more energy and therefore causes more atoms to ionize in its wake. A
higher mass particle, such as an alpha particle, will leave a track of higher
ionization density than a proton, if both are going the same velocity when they
strike the body. A gamma photon will have a far lower LET value yet. The chemical make-up of our cells <a href="https://www.mun.ca/biology/scarr/Specific_Ionization_&_LET.html">also plays a significant role determining ionization damage</a>. Low LET radiation, such as X-rays or gamma rays, ionizes
water molecules inside cells. It breaks them up into H<sup>+</sup> and OH<sup>-</sup>
ions (also called radicals). It does this damage over a long track through the
tissue so it tends to leave one event per cell. The often single H<sup>+</sup> and OH<sup>-</sup>
ion pair simply recombines to form water once again, releasing some energy in the process.
When ionization occurs over a shorter wider track, many H<sup>+</sup> and OH<sup>-</sup>
ions can form within each cell. A pair of OH<sup>-</sup> ions near each other
can recombine into H<sub>2</sub>O<sub>2</sub> (peroxide) instead. This molecule
causes additional <a href="https://en.wikipedia.org/wiki/Oxidative_stress">oxidative damage</a> to proteins, lipids and to DNA in the cell on top of the ionization damage to various biological molecules. </span>Dense clusters of high LET ionization
damage in the body means that cosmic particle radiation is extremely dangerous to astronauts,
especially to their brains as we will discuss below in more detail.</div>
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Radiation Units: A Frustrating Labyrinth</div>
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<span lang="EN-US">Articles about radiation can be very
difficult to grasp because <a href="http://www.sprawls.org/ppmi2/RADQU/">so many unfamiliar units</a> are thrown around seemingly
at random and sometimes interchangeably. A large part of this confusion stems
from the use of both "old" American units and SI (standard or metric
international) units. Here in North America the switch over to SI has been particularly
slow in the field of nuclear science in part because the stakes are so high. Even
a small conversion error can lead to dangerous radiation exposures. In addition, <a href="https://ieer.org/resource/classroom/measuring-radiation-terminology/">a number of different units must be used</a> to accurately describe radiation as it
travels from its source, through the atmosphere or through space and then into
our bodies. The rate of emission of radiation from its source is measured in
curies (old) or becquerels (SI). Sometimes radiation emission is measured in
terms of emission energy instead of rate. In this case it is measured in
electrovolts (eV) or joules (J, an SI-derived unit). I used eV earlier to
compare ionizing to non-ionizing radiation energy. Once the radiation is
emitted and is now ambient, its ambient concentration is measured in roentgens
(old) or coulombs/kg (SI). Once ambient radiation strikes a living body or
other object, the raw amount that object absorbs is measured in eitherrads (or
just rads; old) or grays (SI). I gray is equivalent to 100 rads. Rems (old) and
Sieverts further complicate the picture. These units measure the effective
dose, or dose equivalent, in other words the biological harm caused by radiation
in living tissue. Rather than describing a radiation disaster in terms of rads, for example, seiverts or rems can offer a better description of how the
radiation exposure will affect human health. This measurement reflects the
different LET values of different kinds of radiation as well as the kind of
tissue receiving the dose. Some body tissues <a href="https://www.ncbi.nlm.nih.gov/pubmed/22514910">are more sensitive to radiation than others</a>.
This measurement of dose equivalent is still an inexact science, however, as
there are so many not entirely known biological factors to consider. Randomized
experiments that test radiation damage to living human tissues are, of course
unethical. Animal study results are sometimes difficult to extrapolate to
humans because their bodies, tissues, and physiologies are different. We also
still don't know much about how our human tissues react to various kinds of radiation
exposures, especially from cosmic radiation.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
The Edge of a Mystery: The Brain and It's response To Radiation<br />
<span lang="EN-US"><br /></span>
<span lang="EN-US">Scientists once assumed that brain tissue
is less sensitive to radiation damage than other tissues because brain cells
tend to multiply at a much slower rate than other cells in the body, such as
gut and skin cells do, for example. Cells that divide less often spend less of
their time in the process of division. The DNA in a quiescent cell is tightly
coiled into a dense structure called <a href="https://en.wikipedia.org/wiki/Chromatin">chromatin</a>. This structure is very stable
and resistant to radiation damage and it offers a very small target as well.
When a cell starts the process of dividing, its DNA must unravel so the
machinery of DNA synthesis can replicate the entire genome. The DNA in this
state is highly susceptible to damage. Luckily, cells have mechanisms that
detect and repair DNA damage when it occurs, but if the radiation dose is high
enough, the dividing cell can't fix all the damage before it completes replication. It will then either program
itself to die off or it will pass on the DNA damage as mutations.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Based on instances of acute radiation
exposure studied after radiation accidents and war, tissues in which cells
multiply most rapidly such as those in the gastrointestinal tract, the spleen
and bone marrow, tend to present symptoms of radiation damage (<a href="https://en.wikipedia.org/wiki/Acute_radiation_syndrome">radiation sickness</a>) first. These
assumptions are <a href="https://en.wikipedia.org/wiki/Acute_radiation_syndrome#Signs_and_symptoms">based on whole-body one-time exposures to an absorbed dose of between 6 and 30 Gy</a>.
Neurological symptoms (including cognitive defects) typically only manifest
after a dose higher than 30 Gy occurs. This is a catastrophic dose; the victim will die within two days. There is a fascinating, if sobering,
chart of whole-body dose effects <a href="https://en.wikipedia.org/wiki/Acute_radiation_syndrome#Whole-body_absorbed_dose_effect">here</a>.
Scientists are discovering that the effects of acute exposure cannot be
extrapolated to the effects of sustained low-dose exposure, especially
long-term effects. The results don't take into account cellular damage that
takes weeks, months or years to manifest. More importantly, different tissues
in the body deal uniquely to long-term radiation exposures with different LET values
in complex ways that are still poorly known. The study we are most interested in
here (explored in detail below) considers both Gy and LET values as indicators of damage, in this case, to
brain cells. The results are based on both behavioural changes and structural
tissue changes in mice exposed to radiation that mimics cosmic radiation. <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Until a few years ago, NASA and other space
agencies only suspected that long-term cosmic radiation caused cognitive
impairment in astronauts. This suspicion was <a href="https://en.wikipedia.org/wiki/Radiation-induced_cognitive_decline">largely based on clinical data</a> from cranial radiotherapy and radiation treatment for brain cancer and then comparing
that data to before/after cognitive test results of astronauts. The
extrapolation from clinical data to cosmic radiation exposure, as they knew,
was problematic, much like comparing apples to oranges. A typical daily dose
during cranial radiotherapy <a href="https://en.wikipedia.org/wiki/Radiation_therapy#Dose">is about 2 Gy</a>.
Compare this to a far lower roughly 1/5000<sup>th</sup> daily dose of radiation
<a href="http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf">of around 0.48 mGy expected</a> for an astronaut during a round-trip and stay on Mars. Adding that exposure up
for a 300-day trip, for example, still adds up to just 1.44 Gy, less than the
dose of a single cranial radiotherapy treatment. Travel to Mars shouldn't be a
problem right? The trouble with comparing the two doses is that they have
vastly different LET values. In the clinic, X-rays and gamma rays are most often used.
These energetic EM photons are not nearly as densely ionizing as cosmic
particle radiation because a massless photon particle has far less momentum
than a particle of mass traveling at nearly the same velocity. <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">To really understand how long-term cosmic
radiation will affect astronauts during trips to Mars or on possible future
deep space missions, you need direct experimentation, using relativistic
particles. How would you design an experiment to do this? Charles Limoli, a
neuroscientist and radiation biologist at the University of California School
of Medicine, has taken some of the first steps in answering the question. His research
findings to date can be found in the February 2017 issue of Scientific
American. It's an excellent read that inspired me to write this article. He explains not only the radiation problems that current astronauts face based on current research findings, but he also outlines the challenges of getting the data
scientists will need to plan future deep space travel. A reference paper for
this article called <a href="http://advances.sciencemag.org/content/1/4/e1400256">What Happens to Your Brain on the Way to Mars</a> by Vipan Parihar et al. provides a good background for this research. Here I try to provide a glimpse into his work, and offer some insight into the challenges and excitement of
designing new experiments, into the process itself. The findings are preliminary so far but they are
quite stunning. Undoubtedly a lot of future work will build upon it. <o:p></o:p></span></div>
<div class="MsoNormal">
<br />
In this case, mouse brains are used as
living models for astronaut brains in deep space. There are three significant
challenges to this approach. First we have to ask if a mouse brain is similar
enough to a human brain in terms of structure and function to make a valid
model. We also have to ask if a mouse brain reacts to radiation the same way a
human brain does. Rats are used as extensively as mice in neurological studies.
There is a mountain of neurological data using either species, suggesting that
they are reliable and useful models. I wondered, though, if rats would make a
better model in this case. That turned out to be an interesting question. A
<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5087838/">2016 research paper</a> by Bart Ellenbroek and Jiun Youn explores the differences between rats and mice and how reliable they are as models
for human brain behaviour. The species differ in their behaviour, as anyone who has worked with both species knows. For example, rats
tend to be more comfortable with lots of human physical contact while mice can
be stressed by similar attention. Unexpected stress on an animal could affect
the results of behavioural tests. The researchers also point out that, while both
rodent brains are very similar to human brains, there are fundamental
differences between them that could affect the reliability of test results. Limoli's
research presents a unique physical limitation on what kind of animal model you
can use – it has to fit into the accelerator target area.</div>
<div class="MsoNormal">
<span lang="EN-US"><br /></span>
<span lang="EN-US">An additional thing to keep in mind is that the brain is still not fully understood. Neurology, neuroscience and psychiatry are very active fields of research. Still, at least one basic fact about the brain seemed to be firmly established: the brain contains two basic types of cells with neutrons performing the twin star roles of function and structure and with glial cells playing a supporting role. This is called the</span> <a href="https://en.wikipedia.org/wiki/Neuron#Neuron_doctrine">neuron doctrine</a>. Research over the last few years calls these assumptions into question. For example, a <a href="https://blogs.scientificamerican.com/guest-blog/human-brain-cells-make-mice-smart/">2013 Scientific American blog post</a> by Douglas Fields points out an unexpected but key difference between mouse
brains and human brains that suggests that glial cells are far more involved in function than researchers realized. It was assumed that glial cells can't do any electrical signalling. Until now they've
been thought of simply as physical and physiological support cells for neurons.
The researchers transplanted human glial cells into mouse brains and discovered
that these mice soon significantly surpassed their untreated siblings in both memory and
learning. Somehow human glial cells imparted an improved, perhaps more humanlike, cognitive ability into
a mouse mind. A specific type of cultured human glial cells called <a href="https://en.wikipedia.org/wiki/Astrocyte">astrocytes</a>
are in fact much larger and have a more variable morphology than mouse
astrocytes. It is a clue that these cells might be involved in the evolution of
human intellect. Researchers now know that glial cells not only propagate calcium signals over long distances but they also form electrically coupled synchronized units through gap junctions (similar to how heart cells are synchronized to contract during a heartbeat).</div>
<div class="MsoNormal">
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<div class="separator" style="clear: both; text-align: center;">
</div>
</div>
<div class="MsoNormal">
<span lang="EN-US">The second question is how do we reliably test
cognitive function in mice before and after radiation exposure? Fortunately
mice have been bred and used extensively for various kinds of cognition and
memory research for decades. There is lots of data to draw from as well as a
large collection of reliable cognitive and memory test protocols available to
use. This <a href="https://www.ncbi.nlm.nih.gov/pubmed/26629775">2015 compilation paper</a> by SM Holter et al. provides an overview of those tests. Third, how do we
expose our test animals to specific doses of relativistic particles? A tremendous
amount of energy must be used to accelerate particles to nearly the speed of
light. Few natural mechanisms outside of stellar explosions can do the job. The
only way to do this in a lab is to expose the mice to radiation inside a
particle accelerator. Fortunately, <a href="https://www.bnl.gov/nsrl/">NASA Space Radiation Laboratory</a>,
commissioned in 2003, is designed for exactly these kinds of experiments.
Radiation consisting of a variety of particles with a range of very high
energies is specifically designed to resemble cosmic radiation. <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Out of practical time constraint
limitations, the mice received a single dose of radiation equivalent to many
months to years of actual cosmic radiation exposure according to Limoli's
article. The article states a dosage of either 30 cGys or 0.3 Gy was used, which is
very low, approximately 50 times lower than an expected round trip to Mars
radiation dose. Would an astronaut's brain, exposed to the same amount of
radiation but spread out over many months, have enough time to repair the
damage between intermittent particle exposures? Researchers are not sure but the results of this experiment are not promising as we will see in a moment. </span>This research is an essential first step to
find out just how big a problem long-term cosmic radiation exposure could be for
future astronauts.</div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">A healthy brain <a href="https://en.wikipedia.org/wiki/Neuron">neuron</a> consists of a <a href="https://en.wikipedia.org/wiki/Soma_(biology)">soma </a>(cell body) which contains the nucleus and other organelles, <a href="https://en.wikipedia.org/wiki/Dendrite">dendrites</a> (branched projections) and an <a href="https://en.wikipedia.org/wiki/Axon">axon</a> (a long
slender electrically conductive projection). See the diagram below. It connects
to other neurons through specialized connections called synapses. </span><br />
<span lang="EN-US"><br /></span>
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjan1stiX08AUOVir9MWJEfx8exVAW6pwaYIQS8uTztEuV7BKVEN64KiuCVKwbJXMEh5L3IuAk2PYlesWScWOVn4QTGbt0SyQWb8I6HRfP5okMG8aV2qZfawoRZPmsHfuhR0LXMDPytCQ5K/s1600/Chemical_synapse_schema_cropped.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="678" data-original-width="544" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjan1stiX08AUOVir9MWJEfx8exVAW6pwaYIQS8uTztEuV7BKVEN64KiuCVKwbJXMEh5L3IuAk2PYlesWScWOVn4QTGbt0SyQWb8I6HRfP5okMG8aV2qZfawoRZPmsHfuhR0LXMDPytCQ5K/s640/Chemical_synapse_schema_cropped.jpg" width="512" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">(Wikipedia public domain)</span></td></tr>
</tbody></table>
<span lang="EN-US">One neuron
can contact another neuron's dendrite, soma or, less commonly, its axon via a
synapse. At the synapse an electrical signal is converted into a chemical
signal by the release of <a href="https://en.wikipedia.org/wiki/Neurotransmitter">neurotransmitter</a> molecules into the gap between two cells. </span>The neurotransmitter initiates an
<a href="https://en.wikipedia.org/wiki/Action_potential">action potential</a> in the
connecting neuron. See the enlarged box in the diagram above. One neuron can
connect to many other neurons to form complex neural networks in the brain.</div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">To start, a healthy mouse explores toys
in a box. Over a period of hours and days, its brain physically changes as it learns and forms
memories. The neurons in its brain <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3079328/">form new dendritic branches and trees</a> and create new synaptic connections. Dendritic branching can be very extensive
and complex. A single neuron can receive as many as 10,000 dendritic inputs
from other neurons.</span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">The toys in the box form part of a task that evaluates a mouse's cognition and memory abilities. In this task, called the <a href="http://www.nature.com/nprot/journal/v8/n12/abs/nprot.2013.155.html">novel object recognition task</a>, a mouse is placed in a box containing toys to explore. After
exploration for a fixed amount of time, the mouse was then removed from the box
and the locations of the toys are changed and some are replaced with new
toys. The mouse is returned minutes, hours or days later to the box to explore
the novel landscape. A healthy mouse is curious - it will quickly notice changes
and spend extra time exploring those changes. The time spent on checking out
new things compared to overall time present in the box is that mouse's
discrimination index, a reliable measure of its memory and learning ability.
Memory and learning takes place primarily within the brain's <a href="https://en.wikipedia.org/wiki/Prefrontal_cortex">prefrontal cortex</a> and <a href="https://en.wikipedia.org/wiki/Hippocampus">hippocampus</a>.
Limoli and his group discovered that a single dose of low-level cosmic-like
radiation exposure greatly reduced the mice's discrimination index values. The
deficit stood out in stark relief when irradiated mice were given the same follow-up tasks as
healthy mice. Their curiosity was greatly diminished. They didn't seem to
recognize changes made to their environment. <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<a href="https://www.blogger.com/blogger.g?blogID=5313396495335219568" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"></a><span lang="EN-US">Six weeks after a single exposure of either
5 or 30 cGy of radiation, both very low doses representing sparse particle
impacts, the performance of the mice dropped on average by a whopping 90%,
regardless of dose. Furthermore, they also found that these impairments lasted
12, 24 and even 52 weeks after the exposure, suggesting that the damage to the
mouse's brains didn't heal, at least within a year after being damaged.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">The researchers confirmed the physical
damage to the brain by imaging sections of the medial prefrontal cortex of healthy
non-radiated mouse brains and of irradiated mouse brains. The imaging data
revealed significant reduction in dendritic branching as well as a significant
loss of dendritic spines. A <a href="https://en.wikipedia.org/wiki/Dendritic_spine">dendritic spine</a> is a tiny protrusion from the main shaft of the dendrite that contains the
synapse that allows the dendrite to receive signals. If dendrites are the
branches on the brain "tree," then spines are the tree's leaves, as Limoli
describes it in his article. These structures <a href="https://en.wikipedia.org/wiki/Dendritic_spine#Importance_to_learning_and_memory">are very plastic</a>.
They undergo constant turnover in a healthy brain. The growth of dendritic
spines reinforces new neural pathways as an anatomical analogue of learning.
They also maintain memories. Environmental enrichment (providing lots of
learning opportunities) leads to <a href="http://www.annualreviews.org/doi/10.1146/annurev.neuro.30.051606.094222">increased dendritic branching, increased spine density and an increase in the number of synapses in the brain</a>. Cosmic radiation exposure not only undid the changes associated with learning
and new memory formation. It severely impaired the normal (baseline) cognitive
function of the brain as well.</span><br />
<br />
Only the medial prefrontal cortex, a region
known to be associated with learning and memory, was imaged but it seems
reasonable to assume that the radiation attacks the physical and functional
integrity of synaptic connections across the entire brain.<br />
<br />
Conclusion</div>
<div class="MsoNormal">
<span lang="EN-US"><br /></span>
<span lang="EN-US">This is disconcerting news to those of us
who dream of mankind eventually traversing the universe to explore other
planets and moons in person. Imagine the spectre of our best and most brilliant
men and women gradually losing their cognitive abilities, losing their memories
and <i style="mso-bidi-font-style: normal;">themselves</i> as they travel through
deep space. Cosmic radiation would impair astronauts in the most critical way.
The skills and mental acuity required to deal with maintenance issues and sudden
problems as they arise during a long-term space flight are what set astronauts
apart from the rest of us. <span style="mso-spacerun: yes;"> </span>Stasis would
be no solution, unless it is inside some as of yet undiscovered material that
can block out cosmic radiation. The fact is, we evolved inside a layered
magnetic cocoon where the threat of cosmic radiation doesn't exist. We'll have
to rely on another facet of our evolution, our ingeniousness, to get past this
hurtle. <o:p></o:p></span></div>
Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-15581791954723159392017-02-15T15:09:00.001-07:002017-02-15T15:09:34.626-07:00BOSE EINSTEIN CONDENSATEThis amazing state of matter was mentioned years ago in my <a href="http://sciexplorer.blogspot.ca/2011/03/very-hot-very-cold-superfluids.html">Very Hot, Very Cold</a> article (2011). At first a curiosity, now scientists are starting to see that these condensates may not only pry open the inaccessible quantum world for us but they might offer us a wide variety of new breakthrough technologies as well. Meanwhile the techniques used to create and maintain matter in this ultra-cold condensed quantum state have steadily improved. Researchers at the University of Alberta (my old "home") just created the <a href="https://www.ualberta.ca/science/science-news/2017/january/coldest-city-with-the-coldest-gas-in-the-world">coldest Bose Einstein Condensate (BEC) ever</a>, at just 40 billionths of a degree above <a href="https://en.wikipedia.org/wiki/Absolute_zero">absolute zero</a>! What happens to atoms when their motion literally begins to freeze? This great little 1-minute video offers a glimpse into what a BEC is:<br />
<br />
<iframe allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/shdLjIkRaS8" width="560"></iframe>
<br />
<br />
WHAT IS A BOSE EINSTEIN CONDENSATE (BEC)?<br />
<br />
Some Background First<br />
<br />
Just like a solid, liquid, gas or plasma, a BEC is a <a href="https://www.chem.purdue.edu/gchelp/atoms/states.html">state of matter</a>. We come across three states of matter - solids, liquids and gases - every day. Life on Earth relies on the fact that one substance in particular, water, exists in these three states within a relatively narrow temperature range. What determines the state of matter is the <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/molke.html">average kinetic energy</a> of the atoms that make it up. Generally speaking, atoms are most tightly packed within solids, less so in liquids and farther apart still in gases, but when collections of atoms are subjected to pressure this trend can reverse. As you might know, increasing the pressure of a gas, liquid or solid will also increase its temperature. Solids, depending on the kinds of atoms making them up, are not very compressible but they will eventually compress under sufficient pressure into a dense liquid state when the internal energy increases enough to break apart the inter-molecular bonds that make the solid stiff. The resulting very dense liquid will compress into a very dense gas and the gas, a much more compressible state, will compress further into an extremely dense example of a fourth state called <a href="http://education.jlab.org/qa/plasma_01.html">plasma</a>. Not all plasmas are hot and dense like this, but the Sun's hydrogen/helium plasma is a good example. Plasmas can also be diffuse cold ionized gases such the those that populate interstellar nebulae. Excited atoms in plasmas emit electromagnetic radiation. You can witness the light emitted by plasmas when you see sunlight or a lit neon sign. In the case of sunlight, pressure and heat are both at work. In the case of the neon sign, atoms in the plasma are excited by an electrical current. Atoms in a plasma state have so much energy they can no longer hold onto their outermost electrons. They are in an excited state, creating a separation of electrical charge.<br />
<br />
Under even more extreme pressure and heat, additional exotic physical states are possible, such as <a href="http://www.astronomynotes.com/evolutn/s10.htm">electron degenerate matter</a> inside white dwarf star remnants (our Sun is destined to become one eventually). Atoms in this exotic state are crushed by pressure. Nuclei have lost all of their electrons and a high-energy dense sea of negative charge surrounds them. As pressure is increased further, <a href="https://en.wikipedia.org/wiki/Degenerate_matter#Neutron_degeneracy">neutron degenerate matter</a> forms. In this case atoms are crushed into an ultra-dense neutron sea, the strange stuff of neutron stars, pulsars and magnetars. Electrons are so energetic that they combine with free protons to create additional neutrons. Increasing pressure further theoretically produces the densest state of matter possible - <a href="http://cerncourier.com/cws/article/cern/27997">quark matter</a>. In this state, neutrons are crushed into their normally confined component particles - quarks. Above this pressure, matter is completely crushed in the infinite gravity well of a <a href="https://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html">black hole</a> - a state, in which matter at the atomic scale can no longer be described using our current <a href="https://en.wikipedia.org/wiki/Introduction_to_quantum_mechanics">theory of quantum mechanics</a>.<br />
<br />
Just as there is a maximum threshold of energy above which atomic matter as we know it can't exist, there is a minimum threshold of energy below which atoms no longer behave in the ways we expect. Atoms become excited when their energy increases. In an analogous way, atoms become "de-excited" when their energy decreases. As atoms approach absolute zero, they become sluggish and ultimately condense into an additional physical state called a <a href="https://en.wikipedia.org/wiki/Bose-Einstein_condensate">Bose Einstein Condensate (BEC)</a>. Consider a balloon filled with steam, the gaseous state of water. Cool it down to room temperature and it will contain a small puddle of water inside it. Put it in a freezer and that water will solidify into ice. It's easy to visualize the atoms in steam moving around fast and bumping into each other, or atoms sliding around one another in liquid water, but there is no visible motion within a block of ice. Yet, undetectable to our eyes, there is. As the water freezes into ice, the atoms get close enough and slow down enough to form attractive chemical bonds with each other. In the case of water ice, they create a three-dimensional lattice. The type of bond arrangement depends on the kinds of atoms involved. No matter what the solid material is, the bonds hold the atoms more or less in place but they don't stop the atoms from jiggling about, like a runner jogging in place. This jiggling or oscillating motion, averaged over the material, is what we perceive as its heat or temperature. Lets say we cool our balloon down much further inside a special box that removes energy. The oscillations, on average, will slow down. Eventually, the ice will theoretically get so cold that the atoms no longer oscillate at all. At this point it has reached <a href="https://en.wikipedia.org/wiki/Absolute_zero">absolute zero</a>, a temperature measured as - 273.15°C or - 459°F or 0K. I say theoretically because it is not possible in practice to remove all the energy from a system (we are treating our balloon as a physical system). Scientists are finding ways to get very close to absolute zero, and as they do, matter begins to act very strangely. In theory, the ice in our balloon could transform from a solid into a BEC, and when it does it will exhibit some very interesting properties.<br />
<br />
Now To the BEC Itself<br />
<br />
Exotic states such as degenerate matter, which is thought to exist inside stellar remnants, represent the highest energy extreme of atomic matter, while BEC's represent matter's lowest energy extreme. Degenerate matter cannot be directly studied in the lab. Its creation would require an enormous input of energy that would be impossible to achieve and maintain. The gravitational pressure of an entire star is required. Unlike degenerate matter, which cannot be lab-created, no BEC should exist naturally at all. This state of matter is made in-lab only. Even the coldest atomic gas clouds in deep space are a million times too warm, at about 3K, to harbour any BEC matter. A temperature above even just a few microkelvins will disrupt the quantum mechanical quiescence of a BEC and transform it back into its original gas state. But why would such a cold state be a gas and not a solid? More on this to come.<br />
<br />
Each particle in matter is described mathematically as a <a href="https://en.wikipedia.org/wiki/Wave_function">quantum wave function</a>. Click on this <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/Scheq.html#c1">HyperPhysics link</a> to get an idea of what these kinds of equations looks like. Any matter particle exhibits wave/particle duality. It can <a href="https://en.wikipedia.org/wiki/Wave-particle_duality">act like a particle and like a wave</a>. In a BEC, particles act completely like waves. These <a href="https://en.wikipedia.org/wiki/Matter_wave">matter waves</a>, as they are called, stretch out as the atoms slow down. They start to overlap one another, and eventually they completely overlap into one large matter wave. At this point the gas is condensed into a BEC state.<br />
<br />
Though predicted decades earlier <a href="http://www.nature.com/nature/journal/v141/n3571/pdf/141643a0.pdf">by Albert Einstein and Satyendra Nath Bose</a>, the first BEC wasn't created until Eric Cornell and Carl Wieman <a href="https://www.nist.gov/news-events/news/2001/10/bose-einstein-condensate-new-form-matter">succeeded in doing so in 1995</a>. Hydrogen atoms were the logical first choice but providing the right conditions for BEC formation proved too difficult. Helium atoms were turned to next but they presented too many challenges as well. Instead, rubidium was the first successful BEC. Two techniques, lasers and evaporative cooling, which will be described in detail later on, were used to cool a diffuse cloud of rubidium atoms into a BEC state. The two men <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/2001/popular.html">received the 2001 Nobel Prize in physics</a> for their success in creating this new state of matter.<br />
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I mentioned that the universe is far too warm for natural BEC formation. It is possible, however, that all of the matter in the universe could very slowly transform into a BEC state as the universe continues to expand, and therefore, cool. Right now the universe is still "warmed" by radiation from the Big Bang. What were once the highest energy gamma rays possible are now stretched across the expanding universe <a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background">into low energy microwaves</a>, and they will continue to stretch into extremely long and imperceptibly weak radio waves. At this point, interstellar atomic gas clouds in the universe might be so cold that those atoms will begin to condense into BEC's.<br />
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We live within a very narrow range of temperatures, from about -50°C (for a few minutes) to about +50°C (for a few hours) with about 20°C being most comfortable. In this range, hydrogen is always a gas; iron is always a solid and so on, but every element can exist as a solid, liquid, gas and plasma, with each state having its own unique physical and chemical properties. As atoms are energized further, they lose their chemical and physical identities altogether when the nuclei themselves are torn apart. Although practical research is still at a young stage, BEC's also lose some of their original classical chemical and physical properties. In this case, a group of atoms takes on the properties of one single atom, and their normally hidden quantum nature reveals itself.<br />
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So, what does this look like? We can visualize what's going on by looking at how the atoms fall into a single lowest-possible energy state. The graphs below plot the energy distribution of a gas of rubidium atoms. At room temperature, the energy levels of the atoms (measured as their velocities) would be spread across a wide range. They would be evenly distributed across a grid like the ones shown below and it would be entirely red. Energy density increases from red, yellow, green, blue to white, the highest density. The three graphs below illustrate, left to right, an already very cold gas cooling into a BEC state. The change in energy distribution below is calculated using <a href="https://en.wikipedia.org/wiki/Bose-Einstein_statistics">Bose Einstein statistics</a>, which we will investigate further later on. The middle graph shows the energy distribution just before the appearance of a BEC and the graph on the right is of a nearly pure concentrate (notice the reduction in yellow around the peak). At this point, nearly all the atoms have condensed into identical lowest accessible (ground) quantum energy states, contributing to the peak density at the centre. Some researchers call this state a super atom, not to be confused with "<a href="https://en.wikipedia.org/wiki/Superatom">superatom</a>" which not a BEC but a cluster of distinct atoms.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhu0U1E_jK7H8pxHQnqQz94DbLwr1CyIXQvkdMIwwe92otXi5yxHSVIHSn49HBls79JTyAQX-Yq4xBxRMx0uecayUVvg96ZNIOHskrDUTj6kgg_ZRjAQ-VHOPeBqD8_RTPmd2ZT83qiCQtB/s1600/Bose_Einstein_condensate.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="420" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhu0U1E_jK7H8pxHQnqQz94DbLwr1CyIXQvkdMIwwe92otXi5yxHSVIHSn49HBls79JTyAQX-Yq4xBxRMx0uecayUVvg96ZNIOHskrDUTj6kgg_ZRjAQ-VHOPeBqD8_RTPmd2ZT83qiCQtB/s640/Bose_Einstein_condensate.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Image created by NIST/JILA/CU-Boulder</span></td></tr>
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A VARIETY OF BEC's HAVE BEEN MADE<br />
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The BEC state has been observed in very cold gases, in very cold liquids such as helium, and even within solid materials, in a special form. In addition to rubidium, lithium and other elements have been used, as well as a variety of molecules. Non-matter particles that have unique properties (these are boson particles such as <a href="https://en.wikipedia.org/wiki/Quasiparticle">quasiparticles</a>) <a href="https://en.wikipedia.org/wiki/Bose-Einstein_condensation_of_quasiparticles">can also condense into BEC's</a>. These particles do not form atoms. They can be thought of as localized excitations in an energy field. They include <a href="https://en.wikipedia.org/wiki/Polariton">polaritons</a>. A type of quasiparticle, polaritions are half matter/half light particles that will <a href="http://www.cbc.ca/news/technology/physicists-create-new-state-of-matter-in-a-solid-1.618603">condense into a BEC state inside a solid semiconductor</a> under the right conditions. Quasiparticle BEC's, like this example, can form at a much higher temperature than other BEC's do, at about 19 degrees above absolute zero. This makes them an interesting research focus because their unique BEC properties might have practical applications at temperatures that aren't too hard to achieve. Polaritons can also be created in gas BEC's, and as we will see later on, they play a key role in how light interacts with atoms in the BEC state. In 2010, researchers reported in Nature that they were even able to <a href="http://www.nature.com/nature/journal/v468/n7323/full/nature09567.html">confine and condense photons (particles of light) into a BEC</a>. Trapped in a "white box," <a href="https://en.wikipedia.org/wiki/Black-body_radiation">blackbody radiation</a> photons begin to act like a two-dimensional gas of massive bosons in a BEC state (photons are massless force-carrier <a href="https://en.wikipedia.org/wiki/Boson">bosons</a>). As we will see, atoms also act like massive bosons in a BEC (in a three dimensional gas). It is fascinating that massless force-carrying bosons (photons) and matter particles (atoms) can be nudged into an identical quantum state. It is yet another hint at how closely light and matter interact with one another.<br />
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BEC's ARE RULE-BREAKERS<br />
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Normally, atomic matter doesn't act like boson particles of force. The particles follow very different rules. Atoms won't overlap in one spot. That's why even matter crushed down to a neutron star still takes up space. But photons, for example, can and do (think of constructive and destructive interference of light). Only when atomic matter is very cold will it fall into a single ground energy state (into a single matter wave), and when it does, it breaks a fundamental rule of matter called the <a href="https://en.wikipedia.org/wiki/Pauli_exclusion_principle">Pauli Exclusion Principle (PEP)</a>. Matter particles are called <a href="https://www.pa.msu.edu/courses/1997spring/PHY232/lectures/atomic/bosons.html">fermions</a>. Two or more identical fermions (electrons, protons, neutrons or composite particles such as atoms) cannot share the same quantum state at the same time. This energy distribution rule of atoms is laid out by <a href="https://en.wikipedia.org/wiki/Fermi-Dirac_statistics">Fermi-Dirac statistics</a> from which the PEP is derived. It means that at most just one particle can occupy one quantum state in a system. Notice that this is NOT what you see happening in the graphs above. The atoms as matter waves are falling into the same energy state at the same place at the same time. The PEP is being broken.<br />
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This rule holds until the atoms approach BEC critical temperature. At this point, the atoms are quiet enough to fall into a single lowest possible energy state, breaking the PEP and now obeying Bose-Einstein statistics instead of Fermi-Dirac statistics. Bose-Einstein statistics describe the energy distribution of bosons (particles of force such as photons, and W and Z bosons of the weak force). Under these statistical rules, particles are described mathematically as symmetric wave functions (rather than fermionic asymmetric wave functions) and that means they can overlap. In a BEC, matter particles act like bosons.<br />
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They become one big single wave function. It is still atomic matter, but it is overlapped in one location. A BEC trapped in a magnetic bowl looks like a tiny spherical cloud with a dark dot in its centre. A gas cloud of atoms surrounds the actual BEC (the dark dot, which corresponds to the white peak in the graphs above). BEC's created from normally bosonic particles like photons and polaritons don't break the PEP because they don't fall under Fermi-Dirac statistics.<br />
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As an interesting aside, the Pauli Exclusion Principle (PEP) plays an essential role in atomic matter at the highest energy scales too. In these cases, rather than the principle being broken, it takes on a hero-like role that proves just how strong quantum forces within matter are. It explains <a href="http://astronomy.swin.edu.au/cosmos/E/Electron+Degeneracy+Pressure">degeneracy pressure</a>. Ordinary atoms take up space because electron degeneracy pressure <a href="https://en.wikipedia.org/wiki/Spin-statistics_theorem">keeps electrons with the same quantum spin apart</a>. Electrons also repel one another through <a href="http://www.physicsclassroom.com/class/estatics/Lesson-3/Coulomb-s-Law">electrostatic (same-charge) repulsion</a>. That's a different force that's also at play here. Because of charge repulsion, electrons in atoms tend to spread out and partially occupy several orbitals, like passengers on a jet tend to do when it's half full. When outward nuclear fusion pressure no longer counteracts inward gravitational pressure, atoms are squeezed together and the electrons are forced to fill up the lower energy quantum states. (There's a bunch of cargo in the back of the jet so everyone has to fill up the first few rows). Electrons are forced to get close despite electrostatic repulsion, <a href="https://en.wikipedia.org/wiki/Electrostatics#Electrostatic_pressure">a classically described force</a>. They refuse, however, to overlap into identical quantum states (two people can't sit in the same spot).<br />
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This much more powerful quantum, rather than classical, resistance is called <a href="https://universe-review.ca/R08-04-degeneracy.htm">quantum degeneracy pressure</a>. Atomic matter in this state, with electrons forced into all the lowest but not identical orbitals, is found in the extremely dense electron degenerate matter of white dwarfs. In a more massive and even denser stellar remnant such as a neutron star, where gravitational pressure is much higher, the electrons, forced against the electrostatically repulsive (positively charged) nucleus are so energized they approach the speed of light. At this point it is more energetically favourable for them to undergo <a href="https://en.wikipedia.org/wiki/Electron_capture">electron capture</a> through <a href="https://en.wikipedia.org/wiki/Inverse_beta_decay">inverse beta decay</a> than to remain traveling near light speed because, at this velocity, their <a href="https://en.wikipedia.org/wiki/Relative_atomic_mass">relative masses</a> are approaching infinity, a prediction <a href="https://en.wikipedia.org/wiki/Mass_in_special_relativity">made by special relativity</a>. Electrons combine with protons in the nucleus and transform them into neutrons (therefore the name "neutron" star). Unlike the white dwarf, just one kind of pressure prevents the neutron star from collapsing. This is the quantum degeneracy pressure of neutrons. These particles don't repel one another electrostatically because they are electrically neutral. In fact, they bind strongly to one another through the <a href="https://en.wikipedia.org/wiki/Nuclear_force">strong force</a>, which operates at very short (intra-atomic) distances. As outlined by the PEP, neutrons, like electrons and protons, are matter particles whose wave functions cannot overlap. If the stellar remnant is more massive than a neutron star, matter collapses altogether into a black hole (of which a quantum mechanical description is not yet available). Are the neutron wave functions forced to overlap inside a black hole? Is it a BEC? There is currently no way to know. Isn't it fascinating? Even under the pressure of a massive collapsing star matter waves refuse to overlap one another. Yet, when energy approaches zero, matter waves expand and smear across one another of their own accord.<br />
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HOW A BEC IS MADE<br />
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The first rubidium BEC was made by trapping a tiny ball of a few rubidium atoms using lasers and magnetic fields. This process is tricky. If the atoms get too close to each other they will form Rb<sub>2</sub> molecules. A gas at an ultra-cold temperature will condense into a liquid and then into a solid if the atoms are allowed to interact with each other. To cool the gas, infrared lasers bombard the atoms from every direction. One would think this should have the opposite effect of cooling. The intense photon energy should excite the atoms and add momentum to them, heating them up.<br />
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One trick to <a href="https://en.wikipedia.org/wiki/Laser_cooling">laser cooling</a> is to understand <a href="https://en.wikipedia.org/wiki/Temperature">temperature</a> as the average random kinetic energy of a group of atoms. By making the atomic motions less random, lasers narrow the energy distribution of the group of atoms (and create the single sharp peak in the series of graphs above.). When a photon strikes an outer electron in an atom, it can be absorbed, exciting the atom, and then be re-emitted or it can be reflected. An electron will only absorb a photon that matches its orbital energy (called <a href="http://www.kentchemistry.com/links/AtomicStructure/waveenergy.htm">transition energy</a>). This is where the word quantum derives its meaning; energy can only transition in discrete packets. In this case, a laser with a frequency just below that of the transition energy is used. A stationary atom in the cloud won't even "see" the photons. It won't absorb them because they aren't the right energy. An atom moving away from the laser also won't absorb a photon. It will "see" it as <a href="http://www.space.com/25732-redshift-blueshift.html">red-shifted</a>, having even lower energy in other words. An atom moving toward the laser, however, will "see" the photon as blue-shifted, and therefore at just the right energy to absorb. The atom is excited and then re-emits the photon, in a random direction. Now statistics comes into play: the photon hits the atom coming toward it. The photon slows it. That's a pure loss of momentum. But the atom then re-emits a photon (with the same energy) in a random direction. This time the direction is random so the change in momentum is not a pure gain. When absorption and emission are repeated many times over a group of atoms, the average random kinetic energy of the group decreases (pure losses and not-pure gains) and that means the temperature decreases. In other words, the lasers eventually align all the velocity vectors of the atoms, and when that happens they are close to transforming into a BEC. Just like the laser light (which is made of aligned in-phase photons) used to make a BEC, the BEC itself is made of aligned particles. They act like waves in phase with one another rather than as discrete particles of matter.<br />
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Atoms are also tiny magnets because their spinning electrons <a href="http://electron6.phys.utk.edu/phys250/modules/module%203/atoms_in_magnetic_fields.htm">create magnetic fields</a>. By applying a carefully aligned magnetic field to the group of atoms, they can be held in one place once they are cooled. The lasers can be turned off at that point. The lasers aren't perfect; some atoms will still have higher kinetic energy than others. They are still "hot." These atoms simply jump out of the magnetic trap, and are eliminated, leaving only the coldest atoms, the ones that can't jump out, inside. This is <a href="http://cold-atoms.physics.lsa.umich.edu/projects/bec/evaporation.html">evaporative cooling</a>. Heisenberg's <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/uncer.html">uncertainty principle</a> ensures that even in a quiet state such as this, there are tiny random movements in the system that eventually destroy the perfectly aligned quantum state. The original group of about 2000 rubidium atoms, that formed a BEC in 1995, lasted for <a href="https://www.nist.gov/news-events/news/2001/10/bose-einstein-condensate-new-form-matter">about 20 seconds</a> before it lost its coherence and dissipated back into ordinary gas.<br />
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This 6-minute video describes how a Bose Einstein Condensate is made:<br />
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<iframe allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/1RpLOKqTcSk" width="560"></iframe>
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As mentioned earlier, the technology has steadily improved since 1995, creating new kinds of BEC's that last longer, as well as solid-state quasiparticle BECs that can exist at higher temperatures.<br />
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WHY RUBIDIUM?<br />
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Any element's physical and chemical properties change as they transform from one physical state to another. For example, rubidium, the first element made into a BEC, is a very soft silvery white metal at room temperature. Rubidium gets its name from the dark purplish red colour of its flame. With a melting point of just 39.3°C, it is partly melted in the vacuum sample tube shown below.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Dnn87;Wikipedia</span></td></tr>
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A very reactive alkali metal, rubidium can explosively react with just the moisture in the air. As oxidation starts, it warms the solid into a liquid state, which is even more reactive. If this vacuum sample tube were broken, the rubidium would quickly explode. Rubidium readily vapourizes under ordinary pressure. Even an ordinary heating element (at around 700°C) will boil rubidium into a fairly colourless gas, which is one reason why it made the prefect first candidate for a BEC. How do we keep the rubidium gas in a gas state when we expect it to condense and then to freeze into a solid, and an exceptionally hard one at close to absolute zero? The answer is <a href="https://www.learner.org/courses/physics/unit/text.html?unit=6&secNum=6">to start with a very hot</a> (hundreds of degrees Celsius), therefore very diffuse gas. As it is cooled, conditions are carefully monitored to keep its density very low - about a million times less dense than air at Earth's surface. This prevents the atoms from condensing as they normally would.<br />
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The most important reason why rubidium works as a BEC is that these atoms are bosonic atoms. They are not actually bosons as we've previously learned, but under the right conditions they can display a bosonic nature. The general rule for bosonic atoms is simple (but the actual calculations are usually very complex). If an atom contains an even number of subatomic particles, it's a "boson." All neutral atoms have an equal number of electrons and protons so it's the neutron number that matters. Rubidium-87 is an <a href="https://en.wikipedia.org/wiki/Isotope">isotope</a> of rubidium with 50 neutrons, an even number, so it is a boson. From a quantum mechanical point of view it means that all the subatomic particles in rubidium can be spin-matched or polarized. This doesn't mean that all bosonic atoms make good BEC's (again, it's complicated). It also doesn't mean that a fermionic atom can't make a BEC. Helium-3, for example, is a fermionic atom - it has an uneven number of neutrons (one). It will, however, under extreme cooling, form a pair with another helium-3 atom (a <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/coop.html">Cooper pair</a> like those in superconductors), which in effect creates a bosonic composite particle, <a href="https://en.wikipedia.org/wiki/Helium-3#Cryogenics">which will condense into a BEC</a>. Because it is fermionic, helium-3 must be much colder than helium-4, a bosonic atom, to condense.<br />
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In addition to being plentiful, fairly easy to vapourize, and bosonic, rubidium-87 atoms have one lone electron outside a completely filled electron shell and this lone (interactive) electron makes it ideal for magnetic trapping.<br />
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HOW DOES A BEC BEHAVE?<br />
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BEC's act a lot like <a href="http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Rodney_Guritz%20Folder/properties.htm">superfluids</a>. Superfluid helium, for example, is created when liquid helium is cooled to almost absolute zero. Helium happens to be the only element that will remain liquid under normal pressure right down to absolute zero. Interactions between helium atoms are so weak that its ground state energy stays too high to allow it to condense into a solid, unless additional pressure forces the atoms closer together. Helium will, however, transition into a superfluid state. In this state, these atoms, like those in a gaseous BEC, no longer vibrate much at all with heat energy. Instead, they enter a calm state where many atoms begin to vibrate in unison like a single particle. This sounds like a BEC but there are key differences. Still, the two states are strongly linked and it is easy to see why many of the strange behaviours of superfluids also apply to BEC's.<br />
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Comparison to a Superfluid<br />
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A condensed gaseous atomic BEC is not a liquid. Nor does it follow the laws of <a href="https://en.wikipedia.org/wiki/Gas_laws">ordinary gas behaviour</a>. But is it an example of a superfluid? The little bead of rubidium BEC in the magnetic trap is a very rare case of a macroscopic fully coherent quantum object. Loss of phase coherence is the hallmark of the transition from quantum BEC into classical gas. The group of rubidium atoms will grow out of phase with each other in a matter of seconds and return to a classical object, an ordinary gas. Superfluids such as ultra-cold helium exhibit many of the same strange behaviours we explore here with BEC's and they are of quantum origin as well, but these behaviours are hidden in the dense liquid - you can't directly track the gradual loss of interference and other behaviours, as you can with a gaseous BEC. Furthermore, with a BEC, you can fine-tune experimental parameters such as the kinetic energy of the atoms, or its density (more about this in a bit), things you cannot readily do with a superfluid.<br />
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Even though the interactions between helium atoms are weak compared to other elements in a liquid state, helium atoms in a superfliud state interact quite strongly with each other compared to those in a diffuse gas BEC. These atom-atom interactions complicate the behaviour of a superfluid, complications generally not encountered in BEC's.<br />
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Both superfluids and BEC's rely on bosonic atoms. Cold liquid helium-4 transitions into a superfluid at about 2.17K. Below 1K (well below superfluid transition temperature), it exhibits zero viscosity even though only about <a href="https://www.researchgate.net/publication/238195557_Theoretical_Analysis_of_Neutron_and_X-ray_Scattering_Data_on_3_He">7% of the atoms are at ground state</a>. Compare this to a BEC, where all the atoms are at ground state. Helium-3 will not transition into a superfluid until temperatures dip to 2.5 mK (milli-Kelvins). At this point, helium-3 atoms pair up into bosonic Cooper pairs. A BEC must be much colder than a superfluid. A gas will not transition into a BEC until the temperature dips to just a few uK (micro-Kelvins). There is some disagreement among researchers whether a BEC is type of superfluid but most researchers agree that a superfluid is an example of partial Bose Einstein condensation.<br />
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Macroscopic Quantum Behaviour<br />
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You might be wondering at this point how you can observe a physical wave when what you seem to have is a coherent singular quantum wave, or mathematically put, a wave function. At this point quantum mechanics students get riled because they know a <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/wvfun.html">quantum wave function</a> is a purely mathematical construct made up of a real part and an imaginary (not physically possible) part. You can't see one: it doesn't exist in the real world. However, there is a way around this conundrum. The probability density of the wave, which is the absolute value of the wave function squared, always gives you a real and positive value, a value, which amounts to the real <a href="https://en.wikipedia.org/wiki/Standing_wave">standing wave</a> that you can observe. In other words the probability density is the mathematical description of the physical phenomenon. Can we ever "see" the quantum wave function in action? Yes, we can. We can generate and observe a BEC interference pattern. Rather than mixing together like two gases would, <a href="http://cua.mit.edu/ketterle_group/Projects_1997/Interference/Interference_BEC.htm">two BEC's in different phases will interfere</a> when they combine, setting up positive and negative lines of quantum interference, a quantum effect, which can be observed. In the negative lines there is only vacuum. Here, the matter waves of atoms interfere resulting in a space with no atoms. Although the total number of atoms in the mixture is conserved, the atoms simply disappear along lines of negative interference, a purely quantum effect that you can observe.<br />
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Quantum Vortices<br />
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Perhaps the most intriguing behaviour of BEC's is the <a href="https://en.wikipedia.org/wiki/Quantum_vortex">quantum vortex</a>, a behaviour that is already well studied in superfluids such as <a href="http://www.popsci.com/article/science/weird-ways-superfluid-helium">superfluid helium</a>. The classical analogue is stirring a cup of coffee. A little liquid tornado forms with a depression or hole in its centre. Internal friction eventually slows down the rotational motion and this classical vortex dissipates. Both superfluids and BEC's are frictionless. They exhibit zero viscosity and this means the fluid flows without any loss of kinetic energy. If you could stir a superfluid it would just flow around the spoon with no resistance. You can't stir a BEC with a spoon because all current BEC's are too small. The "dot" mentioned earlier tends to be a spherical or pancake shape that is around a millimetre across. But you can whirl it around by rotating the magnetic trap that contains it and, when you do, you create multiple tiny string-like whirlpools in it. Unlike any classical system, all the rotational motion of the BEC is sustained only by these quantized vortices because all the atoms in a BEC are in one quantum wave function. Because these vortices are quantum, their <a href="https://en.wikipedia.org/wiki/Angular_momentum">angular momenta</a> must be quantized. The angular momentum can only be expressed in whole integer packets. When a wave rotates (any wave, even a quantum wave), it forms a closed curve. In this case, those closed curves are confined to <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/debrog2.html">de Broglie wavelengths</a>. Like any standing wave, the wavelengths must be whole (an integer value). In a classical fluid like coffee the velocity of rotation increases smoothly from the spoon in the centre toward the walls of the cup. In a quantum fluid (superfluid or BEC), the velocity can only increase in packets like 0, 1, 2, . . . It takes less energy for the system to form lines rather than sheets, so you get a series of string-like vortices. In the BEC, the vortices tend to be multiple and they have very tiny holes, the width of which can vary depending on the atoms used. These holes, called filaments in a quantum system, don't decay by diffusion as they would in a classical system.<br />
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Put mathematically, a BEC quantum vortex is a direct result of the macroscopic wave function of the system. A BEC rotates by puncturing the condensate with filaments along which the quantum wave function vanishes to zero. These filaments are singularities in the wave function. A number in the wave equation, called the winding number, must be a whole integer (0,1,2, . . .). Mathematically this ensures that the wave function doesn't change value after each rotation. In physical terms this means that the velocity of the circulation has to be quantized. A quantum vortex can only spin at a discrete set of speeds and it can never die down smoothly like a classical vortex does. However, the motion around each vortex, called superflow velocity, acts classically like it does in the cup of coffee, and it can be described using ideal <a href="https://en.wikipedia.org/wiki/Fluid_dynamics">fluid dynamics</a>.<br />
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The study of <a href="https://en.wikipedia.org/wiki/Quantum_turbulence">quantum turbulence</a> (in which vortices are an example) began in the 1950's using superfluid helium. The availability of cold gas BEC's now offers a great advantage in this study because the turbulence can now be directly visualized in a BEC, rather than being hidden inside a dense liquid, which, depending on the temperature, can contain a complicating mixture of superfluid and classical viscous fluid. Even with this advantage, a lot of questions remain. It is not yet possible to predict where and how many vortices form and what their shapes will be. This problem isn't limited to the quantum vortex. <a href="https://en.wikipedia.org/wiki/Turbulence">Turbulence</a> itself is a very complicated phenomenon. It is strongly <a href="https://en.wikipedia.org/wiki/Nonlinear_system">nonlinear</a> and this means you can't input data into an equation and get a predictably straightforward answer. This is why weather forecasts are notoriously unpredictable.<br />
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The study of quantum vortices might make the study of turbulence easier. A classical vortex tends to be messy: it's unstable, it appears and disappears randomly and its circulation is not conserved. Quantum turbulence is a simpler system. It is composed <a href="https://arxiv.org/pdf/1004.5458.pdf">of a tangle of vortices that all have the same conserved circulation in the BEC</a>. Even so, quantum turbulence is complicated; it is still a system with many (albeit fewer) <a href="https://en.wikipedia.org/wiki/Degrees_of_freedom_(statistics)">degrees of freedom</a>.<br />
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In a perfectly condensed BEC, well below its critical temperature, you might expect vortices to be indefinitely stable because there is no friction to diffuse them, but they <a href="http://www.lps.ens.fr/~brachet/Publications_%26_Reprints_files/FDR_review_BEC.pdf">do decay over time</a>. Quantum vortices spontaneously and randomly reconnect, much like water spouts over an ocean do. This behaviour is analogous to eddies that form in a turbulent classical fluid. A quantum vortex can lose energy, dissipate over time in other words, by emitting sound. Sound appears to come from two processes. First, during the process of reconnection (depending on the angle of reconnection), vortex line length is destroyed. When this happens <a href="https://arxiv.org/pdf/cond-mat/0009060.pdf">a rarefaction pulse is emitted</a>, a sound in other words. A second source of sound emission may come from a cascade of <a href="http://faculty.washington.edu/luanne/pages/ocean420/notes/kelvin.pdf">Kelvin waves</a>, which are excitations in the BEC that result from vortex reconnections. These particular waves, which usually have very long wavelengths in nature, may be <a href="https://arxiv.org/pdf/cond-mat/0005280.pdf">short enough to cause sound radiation in this case</a>. By these mechanisms of energy dissipation (loses through sound energy), all quantum vortices eventually decay.<br />
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The Bosenova<br />
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Under the right conditions, a BEC made of rubidium-85 will explode in a manner that resembles a tiny supernova, a <a href="https://en.wikipedia.org/wiki/Bosenova">bosenova</a> (also less cutely called a BEC loss). Rubidium-85 is one of two naturally occurring isotopes of rubidium, the other being rubidium-87, the isotope which created the first BEC. Like rubidium-87, rubidium-85 is a bosonic atom, this time with 48 neutrons. One of the key differences between the two isotopes is that rubidium-87 has a positive <a href="https://en.wikipedia.org/wiki/Scattering_length">s-wave scattering length</a>. This means that the atoms naturally repel each other at low temperatures so it is easy to evaporatively cool the gas. Rubidium-85, in contrast, <a href="http://grizzly.colorado.edu/projects/burke99_rubidium_pra.pdf">has a negative scattering length</a>. This means that a condensate made of this isotope will tend to collapse in on itself, especially in a zero magnetic field. Because the atoms attract each other, it's also more difficult to cool it into a stable (non-interacting) BEC. It has to be just 3 billionths of a degree K above zero. Even with these challenges, rubidium-85 offers a unique bonus. In 2001, physicist Carl Wieman (half of the team that created the first BEC in 1995) <a href="http://www.nature.com/nature/journal/v412/n6844/abs/412295a0.html">adjusted the fine-tuning of a BEC droplet of rubidium-85</a> by changing the magnetic field in which the atoms are trapped. Doing this amounted to adjusting the self-interactions of the wave function (remember a BEC is a superimposed macroscopic wave function). In effect, he could dial between repulsion and attraction.<br />
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Dialed to repulsion, all the parts of the wave function push each other apart. The BEC droplet swells accordingly. Dialed to attraction, they pull together, and this is when unexpected dramatics begin. It starts to shrink gradually as expected, but then it shrinks suddenly, triggering an outward explosion that is tiny by everyday standards but with significant energy (about 100 nano Kelvins, nK) considering only a few thousand very low energy atoms are involved. After a few microseconds a much smaller remnant of BEC is left behind, surrounded by an expanding gas cloud of rubidium-85 atoms. This sudden collapse followed by an explosion that leaves behind a remnant and a gas cloud reminds one of a supernova, although the actual mechanisms involved are very different.<br />
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About <a href="http://www.colorado.edu/today/2001/07/17/form-matter-discovered-1995-shows-ability-collapse-explode">half the original atoms vanish during the explosion</a>. Researchers first thought that they might have either formed Rb<sub>2</sub> molecules in the explosion or that some atoms flew out of detector range before they were measured. Or (sharp breath in), they really disappeared. Perhaps an even deeper mystery presented here is how very cold BEC atoms with minimal energy available to them could explode in the first place. The thermal energy released is greater than the free energy of the original BEC. And, just to season the broth, why does some of the BEC survive?<br />
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In 2003, Masahito Ueda and Hiroki Saito explained theoretically <a href="https://arxiv.org/pdf/cond-mat/0305242.pdf">how a BEC collapses and explodes</a>, thus resolving some of the mystery. The idea is that even though the BEC state is a single matter wave, it still consists of a diffuse gas of atoms and they will interact when they are nudged closer together. When the magnetic field is tuned to just barely favour attractive atomic interactions, the number of <a href="https://en.wikipedia.org/wiki/Inelastic_collision">inelastic (interactive) collisions</a> between atoms remains negligible at first. But as the atomic density gradually increases, the density of the concentrate increases toward it centre. After a short period of time, the collision rate suddenly jumps and becomes significant, but just within a tiny localized central portion of the BEC (about a millionth of the volume). This triggers instability in the BEC. Not one but several intermittent explosions/implosions occur in rapid-fire, and each time several tens of atoms are lost from the condensate. These researchers determined that atoms are removed due to three-body loss. Three-body loss, or three-body decay, <a href="http://www.phys.ens.fr/~dalibard/publications/3bodyb.pdf">occurs when three rubidium atoms get very close to each other</a>, as they do in a shrinking condensate. Two of the atoms form a molecule (Rb<sub>2</sub>) in an excited state while the third atom carries away the energy released by the formation of the chemical bond. Each of these energies is much higher than the depth (energy confinement) of the magnetic trap, so three atoms (and their energies) fly off out of the system.<br />
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Although atoms are not lost irretrievably, the implosion viewed in terms of a closed system acts like a tiny atom drain, a tiny black hole. After the atoms are lost, outward kinetic pressure from the atoms minus the combined mass of the lost three atoms just surpasses the attractive force. This slight surplus in kinetic energy is enough to trigger a subsequent burst or explosion. Afterward, attractive energy then dominates and the BEC shrinks again. The cycle repeats until the number of atoms remaining is too small for attraction to overcome outward kinetic pressure. Because the collision-heavy region is confined to such a small portion of the BEC, some it survives the ensuing explosion.<br />
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BEC's Can Slow Down and Stop Light<br />
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The bosenova is a very interesting phenomenon but it seems to have few practical applications. The interaction between light and a BEC, on the other hand, is not only fascinating, it hints at new possibilities for storing and communicating information in the future.<br />
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The speed of light in a vacuum is an exact value, approximately 300,000,000 m/s (denoted <i>c</i>). In practice, however, a pulse of light contains many particles of light, or photons. Their average velocity is <i>c</i>. A pulse of light slows down (it <a href="http://www.physicsclassroom.com/class/refrn/Lesson-1/Optical-Density-and-Light-Speed">refracts</a>) when it travels from one medium such as air into another medium of different density such as water. If it strikes the new medium at an angle, the pulse of light appears to bend as its speed differs on different points along its wave front. The individual photons in that pulse do not slow down, however. They remain at vacuum light speed. What slows down is the pulse, because multiple interactions between the photons and the atoms in the medium take tiny amounts of time. As density increases, so does the number of photon-electron interactions, because there are more atoms for the photons to interact with. Photons may reflect off atoms and resume travel or they may be absorbed by an atom's outermost electron and then be re-emitted. These interactions mean it takes longer for the group of photons as a whole to get from point A to point B through the medium. The light pulse slows.<br />
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How about the photons themselves? Can anything slow a photon down? It turns out that a BEC can. In fact, it can even stop a photon (in effect) for a few seconds, then reconstitute it (with all the quantum information it contains still intact) and allow it to resume its course.<br />
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In 1999, Lene Hau and her associates <a href="http://www.nature.com/nature/journal/v397/n6720/abs/397594a0.html">slowed a light pulse down to just 17 m/s</a> (that's 20 million times slower than light traveling in a vacuum, <i>c</i>) in a BEC. A BEC, as we know now, is a condensate of atoms. The atoms are still a diffuse gas. As matter waves, however, they spread or smear out as their atomic vibrations slow down. Compared to an ordinary gas at room temperature, the matter waves (the de Broglie wavelengths) are about <a href="http://www.theory.caltech.edu/~preskill/ph12c/ketterle-physicsworld.pdf">10,000 times shorter than</a> the average distance between the atoms. These waves are longer than the distances between the atoms, so the matter waves overlap. It might be tempting to imagine a BEC as a clump of super-dense matter where the atoms themselves have all collapsed into one spot, but it is not. Only the matter waves superimpose.<br />
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Refraction, even at its extreme, slows light only by a small fraction (about a third). For example, gallium phosphide has one of the highest known refractive indexes of any material, and even through this, light slows only to about 86,000,000 m/s. A diffuse gas generally has a refractive index of just one, which means it does not slow light at all. To achieve slowing on the extreme scale here, something other than refraction must be involved. There are currently three theories used to explain how extreme slowing occurs. They are briefly described on the Wikipedia entry <a href="https://en.wikipedia.org/wiki/Electromagnetically_induced_transparency#Theory">here</a>. I will focus on the polariton (more precisely called microcavity <a href="https://en.wikipedia.org/wiki/Exciton-polaritons">exciton-polariton</a>) approach. It's the theory that best explains stopped light (which we will talk about next). A polariton is a not a physical particle in the same sense that a particle of matter is a physical particle. It is a quantum-only entity that is a hybrid light/matter <a href="https://en.wikipedia.org/wiki/Quasiparticle">quasiparticle</a>. It is an <a href="https://en.wikipedia.org/wiki/Emergence#Emergent_properties_and_processes">emergent</a> phenomenon called a collective electromagnetic excitation. It can take on particle characteristics, which, as a result, can have physical (measurable) effects on a system. These particular particles act like a gas, they can be trapped, and they can move just like real particles do.<br />
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Normally you can't shine light through a BEC gas condensate. As the gas cools, it changes from transparent to opaque. However, Lene Hau and her associates were able to electromagnetically induce transparency in a BEC within a very narrow spectral range. In this case, a cloud of sodium atoms is treated the same way as the rubidium BEC explored earlier. It is cooled by lasers and by evaporative cooling into a BEC state that is confined within a magnetic trap. The BEC is then made transparent by exposing it to a specific arrangement of laser beams. The lasers also allow photons traveling through the BEC to combine with atoms to create polariton quasiparticles. More technically put, the laser treatment induces strong light-matter coupling into a structure that combines <a href="http://www.physlink.com/education/askexperts/ae528.cfm">quantum wells</a> (think of an atom stuck in one spot as a standing wave) and <a href="https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.113.133601">photon cavities</a> (imagine a photon trapped between two very close tiny mirrors). This coupling is equivalent to a boson particle that is composed of a quantum well exciton and an optical cavity photon. Polaritons get mass from the atoms, so they travel slower than c. Remember that a BEC is, in effect, one big super-atom. It has the mass of all the atoms condensed into a single quantum state. It's mass is therefore the sum of the masses of all the atoms, so polaritons formed with incoming photons are likewise very massive and this means they must travel much slower than light speed. It is a quantum effect that significantly slows the photons traveling through the BEC because they enter a different particle state. This slowing effect is an entirely different mechanism from refraction, described earlier.<br />
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This 3-minute video describes how a BEC is used to slow down light.<br />
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<iframe allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/EK6HxdUQm5s" width="560"></iframe>
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In 2001, Ron Walsworth and his associates went one big step further. They stopped the propagation of light through a BEC altogether (and then restarted it). This time, using rubidium atoms once again, they gradually turned down the lasers once they had made the BEC transparent. The behaviour of the polaritons in the BEC shifted accordingly from photon toward atom. Eventually the polariton nature turned entirely into atom nature, and at this point the photons were effectively stopped in their tracks inside the BEC. Light stored in the polaritons as quantum information was now hidden in their atom-nature. Perhaps a better way of looking at it is to consider all the quantum information encoded in the light now trapped within the BEC gas, not in the atoms themselves but as quantum excitations within the gas. An exciton-polariton <a href="https://web.stanford.edu/group/yamamotogroup/research/EP/EP_main.html">is a localized quantum excitation</a>. Light in this way can be stored for up to seconds inside a BEC (and perhaps longer as the technique improves).<br />
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When the lasers were turned back up, the photon component of the polaritons increased and the light <a href="http://www.physicscentral.com/explore/action/light.cfm">resumed its travel</a>.<br />
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The ability to stop and restart photons could be very useful in a number of technologies. Imagine that instead of solid-state electronic qubits delivering information in a computer, photons could be used instead. They carry information faster, they don't heat up sensitive components and as we just saw their information storage can now be precisely controlled. One problem with using photons in communications has been how to stop them and decode their information. After flying down an optical cable they have to be stopped at your computer somehow. Nowadays, the information is transferred into an electronic system. The only way to stop a photon is to interact with it. A photon is absorbed by an ordinary atom. As it is absorbed it loses its quantum information, for example its polarization state, and an entirely new photon of equivalent energy is re-emitted in a random direction. In a laser-irradiated BEC, the laser is tuned in a way to prevent photons from being absorbed by the electrons in the gas atoms. The atoms instead form a cluster around each photon, creating a polariton quasiparticle. A BEC can stop and capture each photon, store its information intact (again, polarization could be used instead of 0's and 1's) for a specific period of tine and then send it on its way as a signal.<br />
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MANY POTENTIAL USES FOR BEC's<br />
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There is a potential goldmine of possibilities for BEC's both as research tools and in practical applications. This research is still in its early stages; it is less than two decades old, but as BEC-making technology improves and the list of BEC's with various properties grows, there is no doubt that some of their exciting quantum features will be exploited in new technologies. Because BEC's greatly magnify phenomena confined to the formerly inaccessible quantum world, BEC's might be manipulated as tools to directly observe and verify currently theoretical quantum behaviours such as the <a href="https://phys.org/news/2015-01-bose-einstein-condensate-quantum-mass-acquisition.html">mass acquisition of a normally massless particle called a quasi-Nambu-Goldstone boson</a>, which is thought to be the result of tiny quantum fluctuations. Until now, almost all investigations into the quantum world must be carried out in incredibly large and expensive particle accelerators. As well as expense, this limits the kinds of questions that can be asked. BEC's could be used as customizable quantum laboratories. They might also provide a direct window into the mysterious phenomenon of <a href="https://en.wikipedia.org/wiki/Quantum_entanglement">quantum entanglement</a>. BEC's don't normally contain entangled atoms but researchers have very recently discovered it might be possible to fine-tune the magnetic field around the condensate <a href="http://www.news.gatech.edu/2017/02/02/looking-entangled-atoms-bose-einstein-condensate">in such a way to entangle all the atoms in it</a>.<br />
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BEC's are already being exploited <a href="https://arxiv.org/pdf/0704.3011.pdf">to learn more about solid-state physics</a>. You can create an optical lattice in a BEC by using several lasers to make an interference pattern that looks (and acts) much like the crystal lattice patterns of atoms found in many solid materials. The big advantage here is that the same optical lattice can be repeatedly tuned and manipulated in different ways to see what happens. When using a solid, you have to regrow your sample every single time (and I assume you've got to be very consistent). The fine-tune-ability of a BEC also means it could be potentially used as a variety of high-precision measuring instruments. There is no obscuring noise to tune out in a purely quantum system. In quick succession science and technology have evolved through the golden age, the industrial age and the information age, Now, it seems that BEC's will help us usher in a quantum age.Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-47536783171679926962016-11-27T16:30:00.000-07:002016-11-27T16:43:26.210-07:00Is Scientific Thought in Danger?This morning I read a fascinating opinion <a href="http://www.cbc.ca/news/opinion/common-sense-under-trump-1.3863932">article</a> posted on CBC (Canadian Broadcasting Company, Canada's national news network) online called "How 'common sense' came to mean its opposite under Donald Trump" by Kate Heartfield. Here is a quote from the outset that encapsulates the popular, and worrisome, turn against scientific thought in North America and elsewhere:<br />
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Donald Trump's victory was the most dramatic demonstration yet that liars can win elections. All he had to do was demonize reason and fact as the province of hated "elites."<br />
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To a scientist, facts aren't taken lightly. It takes months, even years, of hard work gathering data and then running it through statistical analysis to make sure it offers a solid conclusion to a question posed in the form of an hypothesis. To get to a fact that describes a process, event or object requires a formal process of investigation that adheres to the scientific method and which is open to scrutiny, repetition, verification or refutation by one's peers. This standard ensures that fact is separated from guesswork, fiction, emotion and opinion. It is very important because scientific knowledge can only be built upon a solid foundation of factual knowledge and a strong theoretical base. Put in everyday language we in the sciences know that the facts we labour to gather are building blocks that can be used to invent new tools, approaches and methods that benefit all of humanity. Think of the many scientific facts you need to know in order to build a steam engine for example.<br />
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You might think the importance of facts is limited to the sciences but the utility of fact-checking goes far beyond. This past American election gave many people reason to distrust facts because statements passed off as facts were thrown about everywhere. Raw unexamined conclusions were drawn about people, their actions and their ideas so often that it was difficult to tell the difference between empty catch phrases and facts. Over several months of being saturated with out-and-out lies (tossed about even during televised debates!), you get used to the lying and it becomes normalized. You no longer gasp in horror. You think instead, well, that's Trump again. However, the war against facts wasn't intitiated by Donald Trump (our late Toronto mayor Rob Ford had the lie down to an art) and it didn't start with this election (think of the tobacco lobby hyping its own "studies" while suppressing unbiased independent research decades ago). It's ironic that this war against facts began in our current post-industrial era, where knowledge and expertise overtook physical labour as the most valuable asset of a society. The last 18 months in the U.S. saw the culture of lying come to its ugly head. The standard of telling the truth is gone and a number of economists, lawyers and constitutional experts are trying to figure out what repercussions will befall the U.S. as president-elect Donald Trump attempts to make real his shaky electoral platform built upon lies.<br />
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In her thoughtful article, Kate suggests that Trump's outrageous lies were effective at least in part because of a psychological phenomenon called the Dunning-Kruger effect. Quoting directly from her article, "People who know a very little about a subject - whether it's the stock market, the rules of grammar or a political policy - are more confident in their expertise than people who know a lot. 'The problem isn't that voters are too uninformed. It is that they don't know just how uninformed they are,' writes Dunning.<br />
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The cure for Dunning-Kruger is, paradoxically, more knowledge. But you can't convince someone to read a fact-check or an explainer with an open mind if they already think they know it all, and especially not if the person they trust is telling them everyone else is conspiring to trick them."<br />
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This describes a perfectly vicious circle. There seems to be nothing that even the most eloquent journalist could write at this point that could stop the war against facts. So many people have thrown up their arms and can no longer be reached. As she mentions, the popularity of trusting one's gut feelings over evidence and the widespread use of talking points play into our growing willingness to accept information point-blank, especially when it comes from someone we think of as familiar, famous or in a superior position of power. Trump, being familiar, rich and famous, seems to get away with conjuring up his own set of "facts."<br />
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I mentioned before that I believe democracy is North America's most precious core value. But when the majority of citizens accept what candidates and leaders say uncritically, what happens to democracy? People unknowingly throw away their chance to voice an informed opinion, one that reflects their OWN best interest not the candidate's, when they buy into emotionally charged fact-starved campaign rhetoric.<br />
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Trump essentially fashioned himself into a snake oil salesman, and he sold many snake oils: a budget that adds up, an "amazing" Mexican wall paid for and built by Mexicans, a thousand new coal mining jobs, a "great" new healthcare system, and the list goes on. Snake oils are appealing because they appear to be simple easy solutions to difficult problems. They are dangerous because they are, by their nature, too simplistic (do you wonder if Putin is going to take advantage of Trump's naivety, or if China will use the U.S.'s new protectionist stance to emerge as the world's new global trade leader? Will Trump influence negotiations between Germany's Deutsche Bank and U.S. federal regulators while his business owes that bank several hundred million dollars? What are the long-term and probably unintended consequences of a Trump presidency? To be able to foresee some of the problems, one has to dig into complex matters of constitutional and conflict of interest law, international and homeland economic policy and matters of national security, subjects that when explored in detail in journalistic articles are guaranteed to have far fewer readers than troll news articles armed with catchy emotionally triggering headlines, scandalous-looking photos and dumbed down downright false content. I feel so badly for those hard-working accredited journalists out there facing this onslaught of garbage.<br />
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We don't know it but they are trying to save us from ourselves. The stakes of whom we elect are high. Our choice affects our taxes, our healthcare, our environment, our kid's education and whether we have a job or not. We should owe it to ourselves to demand that every candidate's platform be based on fact. We should understand we have the right to immediately call out where things don't seem to add up and demand that we have the details explained to us in a satisfactory way. That is our foremost democratic right, and only serious fact-based journalism gives us the tools to exercise it. Otherwise we let our countries run without our consent; we open the door for them to run agendas no one asked for and which benefit no one but themselves. We let our system slide into a dictatorship.<br />
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Is it worth it to gravitate to the titillating Trump tweets or late night comedy shows to laugh at Trump's latest blunder? We all enjoy being entertained. The new populist politician doesn't talk down to us. He says it like it is. He doesn't overwhelm us with responsibility. He downplays the seriousness of his post. And he makes us laugh, either at him or with him. He gives us permission to pound our fists in righteous rage (also either at him or with him). Trump has brilliantly found our collective kryptonite and he knows how to use it.<br />
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This is the post-truth era. Maybe we have the luxury of choosing people who entertain us and talk like one of us. Maybe we also have the luxury of choosing pseudo-medicine, pseudo-science and pseudo-education for our kids. But those of us who still defend scientific rigour, who still uphold that there is a difference between facts and lies, they are looking around us these days and asking where everyone took off to. They are still at the worksite building away toward a solid future that cannot be easily torn down while the rest of us buggered off to get entertained, angered or appeased. Reading endless troll news and tweets became our soul-sucking addiction as the garbage piled up around us.<br />
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Maybe some of us left to build our own house of dreams. We will quickly discover that we are grossly unqualified. Built not from facts but from emotion, gut feelings and/or the easiest solution, our house is going to look like Ned Flanders' "Hurricane Neddy" house: a hallway shrinking down to nothing, a toilet in the kitchen, another room entirely electrified. Immediately upon inspection it falls down entirely.<br />
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Those of us who want to sift fact from fiction and who are exhausted of following circles of lies, have several tools at our disposal that will defend us against the snake oil. This is not a weapon only "the elite" can wield. It's right there online too and all we need to do is take some time to read and think. Soon we can be masters at critically evaluating any kind of information. Here are just a couple of non-partisan, non-affiliated articles designed to help us:<br />
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1) "<a href="http://science.howstuffworks.com/science-vs-myth/everyday-myths/10-tips-for-telling-fact-from-fiction.htm">10 Tips for Telling Fact From Fiction</a>" by <a href="http://howstuffworks.com/">Howstuffworks.com</a>. The website itself describes why it is a reliable source of information <a href="http://www.howstuffworks.com/faq.htm#U">here</a>.<br />
2) This brings me to my own hint: always look for the "about us" button and see what's there. If it is easy to find and the information there is clear and understandable, the site gets a nod for reliability. Google the author's name and the names of the website's creator or editor to find out what that person's affiliations are.<br />
3) "<a href="http://mediashift.org/2013/02/dont-be-fooled-use-the-smell-test-to-separate-fact-from-fiction-online038/">Don't Be Fooled: Use the SMELL Test To Separate Fact From Fiction Online</a>" by John McManus at <a href="http://mediashift.org/">Mediashift.org</a>. This article is especially useful for political content. The author is a former journalist, professor and author who has twice won the annual research award of the Society of Professional Journalists.Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-1369088169696667562016-11-15T13:42:00.001-07:002016-11-15T13:42:55.128-07:00To My American Readers<div class="MsoNormal">
<i>This is strictly my opinion. Feel free to
disagree with absolutely everything here but please don’t clog up my feedback
with hate mail.</i></div>
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<br /></div>
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<span lang="EN-US">When I look at my stats page for this blog
I always notice most of my readers are in the United States, and thank you so
much for that. Like my friends up here in Alberta, Canada, I watched your
election coverage with disbelief. I think almost no Democrat saw a Trump win
coming except maybe Michael Moore. I want to understand why half of you chose him.
Your choice hits us deeply because many of us see ourselves in you. I certainly
do; we are just not that different from each other. In fact, your 18-month-long
(?!) election process seems to have kicked off a parallel discussion up here. I
should warn you if you are thinking about moving to Canada it is not the
socialist Eden some of you might envision. We have a number of Trump-esque
political voices here too and by the time you go through the paperwork, we're
just as likely to have voted in someone far-right ourselves. The political
pendulum always swings back and forth. That said, come on up, welcome! At least
come visit and the first round's on me. <o:p></o:p></span></div>
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<br /></div>
<div class="MsoNormal">
<span lang="EN-US">If you've read some of my posts you'll know
I am unabashedly left-leaning, especially on social issues and our environment,
so I can relate to the policies of President Barrack Obama, as well as our
Liberal Prime Minister Justin Trudeau and my NDP Premier, Rachel Notley. But
here in Alberta we also have a healthy right-wing voting block, based both on
social issues and on our oilsands-enriched economy. About half my friends and
family would call themselves right wing so we have to find common ground, and
we do, and it has made for some of the most stimulating conversations I've
enjoyed. Yet, President-elect Donald Trump, according to his tweets and what
I've seen of his rallies, has gone far into new territory. It seems to be not just
a move back toward right-wing ideology, but toward a darker angrier incarnation
of it. Why?<o:p></o:p></span></div>
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<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Donald Trump won well over the 270
Electoral College votes needed to win. He was fairly and decisively voted into
power. The American people spoke very clearly it seems to me, and ongoing
anti-Trump demonstrations and riots won't change that. I admit that I tend to
read left-leaning news articles online so I see people trying to explain the
vote by conjuring up voter manipulation or even stupidity on the part of Trump
supporters. That guy's a moron, many are saying, and he hates women and that's
why Hillary lost – because too many Americans are backward anti-feminists. As a
woman it was pretty darn easy to agree with all that and leave it there. I
thought: I'm going to try and write something supportive to my fellow
democratic Americans in this dark time, and I assumed those reading my articles
are Democrats because I figured no Republican would read my articles. Then I
ended up writing four drafts of this article and none of them came out right. I
realized I didn't get it. Maybe I don't get Americans and I sure don't get
Trump winning.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">I gave up for a few days and this morning I
said to myself screw it and picked up an Edmonton Sun newspaper to read – a
right-wing paper that out of habit I normally avoid. Wow, do Trump supporters really
see people like me as smug left-wing elitists that are used to getting our own
way and cry like babies, like we are right now, when we don't? Are we
soft-minded proponents of a nanny state that is so over-regulated it stymies
any new ideas before they even have a chance? Do we elitists think only our
view is the correct one and refuse to look outside our bubble of
self-satisfaction? If this is true then it turns everything on its head. Democrats
are now the elitist slick well-financed too educated overlords and the Republicans
are the only people listening to the disavowed working class. Isn't that a 180
degree flip from just a few decades ago when democrats fought for unions and
working wages and healthcare for everyone? I think Americans are a smart bunch
of people much like us up here so I just can't believe this Trump victory came
about from ignorance or laziness or apathy, and I have to believe there is no
kernel of painful truth in what they say about us left-leaners. Looks like its
time to get out of my protective bubble and really hear what Trump voters are
telling Americans (and all of us). <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Wondering, then, how this could come about,
I find myself going back to where I felt cracks in my pro-left wing mindset and
that was the financial crisis of 2008. There is a distinct mistrust of
government and of the establishment in Trump's message and I felt that same
mistrust needling me back then. Many Americans suffered at the hands of
downright illegal manipulations by big banks. Hard-working Americans lost their
homes and then their jobs. And as I see it, there wasn't much of a recovery for
the working class afterward (the banks, large corporations and the stock market
seemed wonderfully resilient however). Just as that nightmare broke, Obama was
elected. I still remember the jubilant reaction to his moving speech. I
thought, and maybe others too, that he would clean up the corruption in the
financial sector and put some of these CEO's in jail. That he'd compensate the
people who lost so much. But it didn't happen. It led to a short-lived 99%
movement, especially among millennials, but no one seemed to listen to them and
it gradually disappeared away but was never resolved. There was never any
corporate fallout, why? Now I wonder if many Americans never forgot either. A
president can make slick and rousing speeches claiming to champion the working
class and then thoroughly ignore them while protecting the corporations who did
the damage instead. How much trust in Obama was lost over that?<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">I can also see why some people are not Hillary
Clinton fans – she is very well connected in Washington and supported by a
number of well-funded PAC's (political action committees). She could represent
more of the same, in which many would continue to feel disenfranchised and left
out. I can understand looking to an outsider businessman to run the country
from what should be a more practical economic outlook rather than someone who
might simply reward one's political allies and keep the status quo. <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">But here's what looks to me to be a nasty
kicker. From what I can gather, both Republicans (at least the establishment
ones) and Democrats have standing behind them a bunch of very wealthy
influential lobbies like the Koch brothers, banks, big oil companies, etc. and
these seem to be the real string-pullers. They are corporations making sure
that behind the scenes in congress things go their way, or their shareholders
way. Often that means tearing up unions, eliminating retirement funds, removing
healthcare and insurance plans and all those things that protect the working
class but cost money to the corporation. I think the Trump win (despite all the
odds and without nearly as much PAC backing) means it's time for Democrats and Republicans
to start talking to each other across the battlefront. It means we Canadians
need to step out of our trenches too. If there is one value that both sides
cherish above all it is democracy itself – the right of every citizen to
choose. I think it's high time to take a close look at democracy itself and ask
if it is being served the way our forefathers envisioned. I think PAC's and
anything like them need to go right now but is it too late? Someone's got to
vote them out and that's the president and congress (and here it's the PM and
the House calling to keep strict campaign donation limits). Trump actually has
the best mandate in a long time to make that happen as long as he can convince
congressmen to vote with their conscience instead of their pocketbooks (tall
order, maybe too tall). As a Canadian I am compelled to take a good hard look
at myself and revisit how my democratic process is protected.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">None of this makes me a Trump fan, however.
As I see him, he's woefully unprepared, he's ignorant because he doesn't read
and he's way too thin-skinned. He's a bigot, he's putting Stephen Bannon in a
top advisory position and he has never shown to my knowledge any concern for
working-class Americans before his campaign. How he's going to stand up for the
working class once elected is totally beyond me BUT he is indeed the political
outsider that so many Americans, I think, have been craving. And I think he's
the only choice that could put into stark relief something that might be very wrong
with our modern democratic systems around the world. I do think that we will
see Trump clones coming into power in other countries such as those coming up
for election in Europe.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">I don't think anti-establishment sentiment
is the only reason Trump the man has some allure. Consider for a moment that
Trump made most of his fortune in a country during which many of the rich got richer,
thanks to tax codes that benefit the wealthy, a healthy stock market, affordable
energy, a healthy job market and efficient international trade allowing abundant
raw materials to move to where you need them. Much of his success was the
result of being at the right place at the right time in history. He is very
much a product of the American Dream. He is a white North American male baby boomer,
and as a result he has enjoyed many advantages that I don't think will be there
for our millennials. He is a product of a temporary and unsustainable system.
And yet he didn't run on happiness and hope. He ran on fury and fear. His anger
has stirred up more rage among voters than I've ever seen – rage about losing
one's advantage in life to the elite, to over-regulation, to increasing numbers
of minorities, to terrorism, and I could go on.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">I see it as rage but I also think I sensed fear
right underneath it. Why would seemingly advantaged older white males be fearful
of anything? When I asked a few older male relatives questions along the same
vein over the years, I noticed that sometimes we started talking about a fear
that the world is changing too fast and they can't keep up with it and they
can't make sense of it. The world they grew up in does not work by the same
rules as it does now. We are facing globalization, a technological explosion,
an information explosion, an explosion of cultural exchange, and a rapidly
changing physical reality – our climate is changing and our resources are
dwindling – all at the same time. That is a completely legitimate fear. The
world in general is changing faster now than at any time in history. Some
people in the media called this election a vote for change but when I see it in
this way, it seems like a vote only for change back to the way things were. And
perhaps that's what fueled Trump most in his campaign – a powerful nostalgia
for his own personal best days when the world seemed predictable, something
that older white male voters, especially, would connect to on a deep level.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">If by making America great again, Trump
means to take everyone back to the prosperous 1950's I've got to say I think
that ship has long sailed. The States, from where I sit, is becoming this diverse,
influential, innovative country, as is Canada, and change will continue whether
we like it or not and whether we are ready or not. Our children's futures, I
suspect, will be very different from what we live right now. I really don't
want to see how the United States works with environmental laws, consumer protection
laws, government healthcare, immigrant protections, international trade and
women's rights rolled back. Instead of a return to the post-war 50's I'm afraid
it will be some twisted state bent toward isolationism and paranoia, something too
similar to creepy North Korea. In the end, I just don't think that's what
Americans, or even Trump himself, really have in mind for the future. If he ran
his campaign based on making life better for the working class then I think
working class voters should hold his feet right to the fire. It's time someone
put their concerns first.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
It would be easy to rail against Trump as a
woman, as a feminist, as an environmentalist, hell as someone who actually trusts
in scientific fact. It would feel much more comfortable to rally around
like-minded people and justify my own rage against "the other." But
after much thought it seems time to stop fighting, stop freaking out that the
world is going to hell inside my little echo chamber, take a breath, and listen
to others who have clearly told us all something important. Trump voter, I'll buy you a beer too :-)</div>
Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-53352725281635180612016-10-18T13:30:00.002-06:002016-10-18T13:31:06.598-06:00Supernovae PART 4: What Happens to Super-Massive Stars?<span style="color: purple;">For Supernovae PART 1: Introduction, click <a href="http://sciexplorer.blogspot.ca/2016/10/supernovae-part-1.html">here</a>.</span><br />
<span style="color: purple;">For Supernovae PART 2: Low Mass Stars, click <a href="http://sciexplorer.blogspot.ca/2016/10/supernovae-part-2-low-mass-stars.html">here</a>.</span><br />
<span style="color: purple;">For Supernovae PART 3: Massive Stars, click <a href="http://sciexplorer.blogspot.ca/2016/10/supernovae-part-3-massive-stars.html">here</a>.</span><br />
<br />
MASSIVE STARS BETWEEN 10 AND 150 M<br />
<br />
As we've seen in PART 3, stars with 10 - 50 solar masses tend to end their lives much more violently than less massive faint supernova stars do, but the process is essentially the same. In the cores of these stars, there is enough gravitational pressure for fusion to smoothly continue as neon, oxygen and eventually silicon fuse. If a core temperature of about 3 GK is reached, silicon and sulphur fuse with alpha particles released by the photodisintegration of these and other elements, <a href="https://en.wikipedia.org/wiki/Silicon-burning_process#Nuclear_fusion_sequence_and_silicon_photodisintegration">eventually creating, in a step-wise nucleus-building fusion process, nickel-56</a>. Of all the elements, iron-58 and nickel-62 have the highest <a href="https://en.wikipedia.org/wiki/Nuclear_binding_energy">binding energy per nucleus</a>. Fusion into the slightly smaller nucleus of nickel-56, produced when iron-52 fuses with an alpha particle, is the first reaction that consumes energy rather than releases it. This means that fusion stops at nickel-56. It is stellar ash. Nuclei larger than nickel-62 release energy when they split apart (nuclear fission) rather than when they fuse. Although the onion diagram we saw in PART 3 shows an innermost core of iron, nickel-56 is the last fusion product in any stellar core, regardless of stellar mass. Any further fusion consumes energy so fusion abruptly stops and the core collapses. Meteorites and rocky planets contain significant iron-56. This is the radioactive decay product of unstable nickel-56.<br />
<br />
In 10 - 50 M stars, like all stars, the outward pressure of nuclear fusion initially keeps matter in its ordinary atomic state (the main-sequence phase). As fuel is consumed and fusion eventually slows down, matter is squeezed into an electron degenerate state. Then, electron degeneracy pressure can no longer hold up the core, which exceeds the Chandrasekhar limit, a core mass of approximately 1.4 M. By this point, the star is on its way to an iron-core collapse. Electron degeneracy is overcome and the inner core implodes explosively, over a course of just a few seconds. The outer core follows inward, reaching a velocity of almost the speed of light. The core temperature spikes and electron capture proceeds rapidly, transforming the inner core into a neutron star. Neutron degeneracy pressure acts like a wall, as mentioned, creating a shock wave that redirects the implosion outward. In stars in this range the shockwave is more intense, resulting in a typical Type II-P supernova. Most of this energy comes from a 10-second blast of thermal neutrinos. These particles are much more abundant than the electron capture neutrinos formed earlier. Thermal neutrinos are formed at about 100 billion K as part of the pair production of neutrino/antineutrino pairs (in all flavours). With a whopping combined energy of about 10<sup>46</sup> joules, about 10% of the star's mass is carried off in a thermal neutrino blast. This is the explosive energy of the supernova. To get an idea of just how much energy 10<sup>44</sup> joules is, check out <a href="https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)">this energy scale page</a> and keep scrolling down. Of course, neutrinos are invisible. Through some process that isn't yet understood, most of the blast energy (10<sup>44</sup> joules) must be reabsorbed into the core somehow to produce the intensely bright EM explosion (a lot of which is in the visible range). During the brief microseconds before pressure and temperature dissipate, a cornucopia of elements heavier than iron, including heavy neutron-rich radioactive elements, <a href="https://en.wikipedia.org/wiki/Supernova_nucleosynthesis">are fused</a> and become part of the expanding cloud.<br />
<br />
Origins of the elements are shown in the helpful periodic table below.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhLBlIAhqGX_mJoTxHFlPmX_ULf5nlC2gh1twY9Aejfu47HyB_MfmqMHeEXO6J-9N91DnPHLGmbmIouUgsE4FGGhtUSZmpA_JP2rg6zrxhjhC9hpb3A2OthjVgokhiN6kgDuRGo6JRf8c-T/s1600/Nucleosynthesis_periodic_table.svg.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhLBlIAhqGX_mJoTxHFlPmX_ULf5nlC2gh1twY9Aejfu47HyB_MfmqMHeEXO6J-9N91DnPHLGmbmIouUgsE4FGGhtUSZmpA_JP2rg6zrxhjhC9hpb3A2OthjVgokhiN6kgDuRGo6JRf8c-T/s640/Nucleosynthesis_periodic_table.svg.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Cmglee;Wikipedia</span></span></td></tr>
</tbody></table>
For stars between 40 M and 90 M, some outer material might fall back onto the newly formed neutron star. If the star's core grows massive enough, estimated at between 2 and 3 M, it will overcome neutron degeneracy pressure (the TOV limit) and collapse into a black hole. The inward fallback of matter can reduce the outward kinetic energy of the explosion, so much so that in some cases there is no supernova at all. In some cases, the in-falling matter (particularly if it is rapidly spinning) generates two opposing jets of matter traveling outward at close to light speed, a brief event called a gamma ray burst (GRB) that can last from a few milliseconds to hours, or an exceptionally bright supernova (called a <a href="https://en.wikipedia.org/wiki/Hypernova">hypernova</a>) can result, or both.<br />
<br />
A massive star's mass, along with its metallicity, determines its eventual fate whether it will become a neutron star, black hole or leave behind nothing at all. Some stars will even collapse into a black hole with no accompanying supernova. The screen shot below from <a href="https://en.wikipedia.org/wiki/Supernova#Core_collapse">Wikipedia's Supernova entry</a> lists possible stellar fates.<br />
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZfpO3VwmDV4ehk0OWO5nvhhDkgDTXEpPWBglLr5U0XY0JSzPVb1bQ4plriB8xUIpsiXaw9YZteQKqAvCQARAYGVnb0fdSDbGOcwGQaQYxP578_WZ5Xnzn32H86tvxHTmsreNglo_4qhi5/s1600/Screen-Shot-2016-09-27-at-1.47.34-PM.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="352" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZfpO3VwmDV4ehk0OWO5nvhhDkgDTXEpPWBglLr5U0XY0JSzPVb1bQ4plriB8xUIpsiXaw9YZteQKqAvCQARAYGVnb0fdSDbGOcwGQaQYxP578_WZ5Xnzn32H86tvxHTmsreNglo_4qhi5/s640/Screen-Shot-2016-09-27-at-1.47.34-PM.png" width="640" /></a></div>
<div class="separator" style="clear: both; text-align: center;">
<br /></div>
This chart showcases the myriad ways in which massive stars can die. How a star dies depends primarily on two factors: how massive it is and of what it's made. Low-metal stars contain very few atoms more massive than helium as they start out on the main sequence of their lives; high-metal stars have a larger but still tiny percentage of heavier atomic nuclei. An enormous range of stars from 8 M up to 250 M can explode, and that explosion can be faint, typical or tremendously bright.<br />
<br />
A Note on Gamma Ray Bursts<br />
<br />
A <a href="https://en.wikipedia.org/wiki/Gamma-ray_burst">gamma ray burst (GRB)</a> is the most energetic event in the universe outside of the Big Bang itself. The most powerful one ever detected was the 2008 GRB (called <a href="https://en.wikipedia.org/wiki/GRB_080916C">GRB 080916C</a>). It was 2.5 million times brighter than the brightest supernova ever detected. To offer perspective, a typical Type II-P supernova usually outshines an entire galaxy. For a few seconds in 2008, its light, from 7.8 <i>billion light-years</i> away, was visible even to the naked eye.<br />
<br />
Gamma ray bursts appear to accompany the collapses of very massive rapidly spinning stars. They were first discovered in the 1960's when the U.S. detected what they suspected were flashes of radiation from secret Soviet nuclear tests. Researchers began to realize that these randomly located, powerful transient bursts did not accompany any obvious objects or stars in the Milky Way. In fact, none have ever been detected in our galaxy, though <a href="https://en.wikipedia.org/wiki/Gamma-ray_burst#GRB_candidates_in_the_Milky_Way">they may have occurred in its past</a>. Through a process not well understood, much of the energy (and a significant amount of the mass) of a collapsing massive star converts into gamma radiation released in two tightly focused bilateral jets that can travel across the universe. The 2008 GRB described had an estimated energy output of 9 x 10<sup>47</sup> J, about five times more energy than the equivalent of the entire Sun's mass.<br />
<br />
Sometimes described as a shot heard clear across the universe, these extremely energetic photons start out as gamma rays and stretch as the universe expands while they travel. Billions of years later, they are detected as much longer, less energetic, but still intense X-ray, radio or even infrared bursts. Although GRB's are fairly frequent events in the universe at large (about one per day is detected), Earth's detectors don't pick up the vast majority of them because we must be lined up with the narrow jet of the GRB in order to detect it.<br />
<br />
THE MOST MASSIVE STARS, OVER 140 M<br />
<br />
Pair Instability Supernova<br />
<br />
Very massive stars (over 140 M) may be completely destroyed by their powerful supernovae. A process called <a href="https://en.wikipedia.org/wiki/Pair-instability_supernova">pair instability</a> leaves nothing, not even a black hole, behind. In the core of such a star, collisions between atomic nuclei and gamma photons are so violent that the new gamma photons (shown as white squiggly lines below) created from those collisions produce new matter/antimatter pairs - electrons and positrons (black and white spheres below) - via a process called <a href="https://en.wikipedia.org/wiki/Pair_production">pair production</a>. The energy of the gamma photon must be extreme, equivalent to the rest mass of the particle pair (0.511 MeV x 2) in order for this to happen. Each particle pair created drains significant energy from the core as energy is converted into matter. As energy decreases, outward pressure drops and the core begins to collapse.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhuKtzBXxMBhKx5_NcacEjdrfav2iANLeGB0Qz2FvIBxovijZJLDf5f6pUGLPN5yFY8dDhkWpYeqfzXo8e5TuJWTCXbtn0XvpT-CLM-XD6BlwIoxfvDmvjvH_twfyaxnB0LmjQL4yBabCrr/s1600/Sn2006gy_collapse_ill.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="396" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhuKtzBXxMBhKx5_NcacEjdrfav2iANLeGB0Qz2FvIBxovijZJLDf5f6pUGLPN5yFY8dDhkWpYeqfzXo8e5TuJWTCXbtn0XvpT-CLM-XD6BlwIoxfvDmvjvH_twfyaxnB0LmjQL4yBabCrr/s640/Sn2006gy_collapse_ill.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">NASA/CXC/M. Weiss;Wikipedia</span></span></td></tr>
</tbody></table>
The temperature in the massive core is so high by the time it starts this final contraction that r<a href="https://en.wikipedia.org/wiki/Thermal_runaway">unaway fusion reactions</a> generate enough energy to blow entire star into space, sometimes accompanied with a particularly bright GRB. Only a nebula of ionized gases and heavy elements mark the original location of the star.<br />
<br />
Pair production reenacts matter creation that occurred just after the Big Bang. At a threshold temperature of about 10 GK, gamma photons convert back and forth into electron/positron pairs in an equilibrium state between energy and matter. Temperature is the average kinetic energy of particles so some gamma rays will be more energetic than average and initiate pair production well before this temperature is reached in the core. There is tremendous core energy in all extremely massive stars, but only stars within a specific stellar mass and metallicity range can undergo pair instability.<br />
<br />
Stars below 100 M aren't massive enough to trigger core pair production. They follow one of explosion pathways typical of stars between 40 M and 90 M. Stars between 100 and 140 M are large enough to trigger some pair production but it isn't energetic enough. There aren't enough pairs produced and photons taken out of the core to reduce the outward pressure enough to trigger runaway fusion. Instead the fusion rate is increased just enough to return the star to equilibrium. These stars go through several increased fusion/pair production cycles in a series of pulses, each time losing stellar gas, until the star mass falls below 100 M. Stars between about 140 and 250 M are true pair instability stars. In these stars, more thermonuclear energy is released than the entire star's gravitational binding energy. The entire star is ripped apart in what can be an exceptionally powerful supernova (a hypernova). Not all of its matter is converted into radiation, however. Up to dozens of solar masses of unstable nickel-56 can be blown away from the core, which decays into cobalt-56 and then into stable iron-56, contributing to an iron-rich stellar nebula. Solar material can also be sprayed across the universe in a powerful GRB as well. The radiation contributed by various heavy nuclei decay reactions could make such a supernova exceptionally luminous and long-lived as well.<br />
<br />
Stars Over 300 M<br />
<br />
The most massive known star is <a href="https://en.wikipedia.org/wiki/R136a1">R136a1</a>. Shown in the center of the 2010 near-infrared image below, Hubble Space Telescope resolved it from the <a href="https://en.wikipedia.org/wiki/R136">R136 concentration of stars</a>, located 156,000 light years away, in 1992.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj2m71IToSRCueRiJQAOIwDfRN0mkTW44FD9b1AET5viRiyUyrK_brMNip6zYzJr8LssJUF4pGVhP_xc_vvHZTTXUj7KgWcV1IiyGrFKh0ougZue8fKQ1ngrm9Tj47oUmnnHZ1lisPdH7wn/s1600/The_young_cluster_R136.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj2m71IToSRCueRiJQAOIwDfRN0mkTW44FD9b1AET5viRiyUyrK_brMNip6zYzJr8LssJUF4pGVhP_xc_vvHZTTXUj7KgWcV1IiyGrFKh0ougZue8fKQ1ngrm9Tj47oUmnnHZ1lisPdH7wn/s640/The_young_cluster_R136.jpg" width="636" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">ESO/P.Crowther/C.J. Evans;Wikipedia</span></span></td></tr>
</tbody></table>
There are many massive stars in this cluster. R136a1 is the most massive, hottest and most luminous of any known star. It would occupy the extreme top left corner of the Hertzsprung-Russell diagram. Formed from an initial mass of 325 M, it has a current mass of 315 M. Less than a million years old, intense EM radiation from this very hot star creates powerful solar winds that strip away its mass at a rate a billion times faster than the Sun. This star is close to the <a href="https://en.wikipedia.org/wiki/Eddington_luminosity">Eddington limit</a>. Calculated using hydrogen plasma, stellar winds from a star any more luminous would quickly strip away its mass before it could evolve further. GRB's and supernovae, by the way, greatly surpass the Eddington limit and that is why they are so brief and mass loss is so intense. R136a1 is still fusing hydrogen in its core, using the CNO cycle because its core is very hot. Significant levels of helium and nitrogen in its spectrum reveal that it is also strongly convective.<br />
<br />
Expected to stay on the main sequence for only 1.7 million years, stars such as this live hot and die young. By the time it uses up its hydrogen and evolves into a luminous blue variable star, it will have only about 80 M left. As carbon and oxygen accumulate in its helium core, core temperature will continue to rise and mass loss will increase further. After several hundreds of thousands of years the helium will be used up. Heavier element fusion will last only a few thousand years. Though estimates vary greatly its massive core will collapse and likely trigger a supernova that may or may not leave behind a black hole. Its supernova spectrum will likely classify it as a Type 1c explosion because by the time it explodes, its hydrogen and helium will have long been blown away by stellar winds.<br />
<br />
It is expected to have a metallicity similar to the Large Magellanic Cloud in which it exists, about 1/4 that of the Sun. The Large Magellanic Cloud is a gas-rich metal-poor neighbor galaxy full of star-forming nebulae and young populations of stars. Below, enormous R136a1 (bright medium blue) is compared a 0.1 M red dwarf, the Sun (shown incorrectly yellow) and an 8 M blue star (pale blue), all in the main-sequence phase of their lives.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhdDsj1B7BRkeqk8ezbunlqQ9yJ138juEF3NAOTJYvxN5YnHkgVypV6FfXg0FK7-0-X4nnFx4oUb65ECRU41KV6FCAcEM4GyDlFh49NjGHZMDhZiMr4araxN6JOm7iueYwLQHyqswiVSKOy/s1600/The_sizes_of_stars.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="360" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhdDsj1B7BRkeqk8ezbunlqQ9yJ138juEF3NAOTJYvxN5YnHkgVypV6FfXg0FK7-0-X4nnFx4oUb65ECRU41KV6FCAcEM4GyDlFh49NjGHZMDhZiMr4araxN6JOm7iueYwLQHyqswiVSKOy/s640/The_sizes_of_stars.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">ESO/M. Kornmesser;Wikipedia</span></span></td></tr>
</tbody></table>
Massive Metal-free Stars<br />
<br />
Despite the confirmation of R136a, <a href="https://en.wikipedia.org/wiki/Stellar_population#Population_III_stars">stars over 300 M</a> remain a bit of a mystery. Estimates of how these stars die vary greatly depending on the model used. While it is possible that they could be totally destroyed in pair-instability supernovae like slightly less massive stars, it is also possible that they could end as <a href="https://en.wikipedia.org/wiki/Photodisintegration">photodisintegration</a>, rather than pair production, triggers core collapse. This process can be modeled with a massive metal-free star, consisting of hydrogen, helium and a tiny percentage of lithium and beryllium - precisely the kind of first stars to form from the pristine gases and dust left over from the Big Bang. These stars fused the first heavier elements to exist in the universe.<br />
<br />
Unlike R136a1, no zero-metallicity stars, stars formed strictly from primordial material left over form the Big Bang, have been directly detected, but theory points to their once existence. Because we look back in time as we peer across the universe, these stars, if they exist, will be located extremely far away at the extreme edge of the visible universe. They may have shone just as the first light from the Big Bang itself was able to stream through space, and space itself would have been just 1/30th of its present size. This would result in extremely red-shifted extremely dim light from once exceptionally bright bluish white behemoths.<br />
<br />
This might be what those stars look like today. Not quite what one might expect at first thought.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhz8jjOyWlheKh-TEmdJCGxRVo5SzuCJq0mdH5fpiRpgtVfZ4YBKoufHBx2ldt1BH7tcjy29haph5tdI_OQbw5YLaCj_M5YjYYhC8ubdwmTtcP0Uxi0gOcPHMZZma_LujNVfXD8IdN2FxGK/s1600/Ssc2005-22a1.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="283" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhz8jjOyWlheKh-TEmdJCGxRVo5SzuCJq0mdH5fpiRpgtVfZ4YBKoufHBx2ldt1BH7tcjy29haph5tdI_OQbw5YLaCj_M5YjYYhC8ubdwmTtcP0Uxi0gOcPHMZZma_LujNVfXD8IdN2FxGK/s640/Ssc2005-22a1.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">NASA/GFSC/JP- Caltech</td></tr>
</tbody></table>
This infrared image taken by the Spitzer Space Telescope in 2005 has all the stars, galaxies and artifacts greyed out and the background enhanced to reveal a glow not attributed to present stars or galaxies and not attributed to cosmic background radiation. Researchers are uncertain but it might be the extremely red-shifted light from the first stars to shine in the universe.<br />
<br />
Below, an artist's impression shows what those first stars may have looked like at the time, just 400 million years after the Big Bang. Their light would have been the first light to shine from any object ever.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhTJMXwYQnz_l5K0DGbY3yW7A69zAKKfmg3Wu4oxYKuFI_FioXkGLmjPF6wFoajdWiMpvyaw-YzMhDrygeWIJbr3_IWD3cy10Gfuk0UlSzv-vCFB-GIq_cjWpjbzGMjC_7o5QKXDiPGG1js/s1600/NASA-WMAP-first-stars.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="360" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhTJMXwYQnz_l5K0DGbY3yW7A69zAKKfmg3Wu4oxYKuFI_FioXkGLmjPF6wFoajdWiMpvyaw-YzMhDrygeWIJbr3_IWD3cy10Gfuk0UlSzv-vCFB-GIq_cjWpjbzGMjC_7o5QKXDiPGG1js/s640/NASA-WMAP-first-stars.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">NASA/WMAP Science team</td></tr>
</tbody></table>
It is possible that such distinctly metal-free stars would not be visible at all to us today. If these stars were convective, they could dredge up their fusion products from their cores to their surfaces, making them indistinguishable from regular low-metal stars. These stars could have lived their short lives before EM radiation could escape (during the so-called the "<a href="http://www.pbs.org/wgbh/nova/next/physics/cosmic-dark-age/">dark ages</a>" prior to the <a href="https://en.wikipedia.org/wiki/Recombination_(cosmology)">recombination period</a>). Interstellar space then was hot dense plasma consisting of photons, electrons and protons. A process called <a href="https://en.wikipedia.org/wiki/Thomson_scattering">Thomson scattering</a> made it opaque to all EM radiation. Until the universe expanded and cooled enough for protons and electrons to recombine into neutral atoms, no light, including that from any embedded stars, could shine outward to be detected by us.<br />
<br />
Unlike the catastrophic runaway fusion reactions triggered during pair-instability, photodisintegration is an endothermic process for nuclei up to iron (the most tightly bound nucleus). It absorbs energy. And unlike the incomplete photodisintegration process in less massive stars, which knocks off one or two nuclear protons or neutrons, the disintegration in this case should be complete, down to alpha particles and protons. Based on computer modeling, this process should start at about 6 x 10<sup>9</sup> K.<br />
<br />
Most, but not all, modeling suggests that these no-metal stars could have been very massive, perhaps up to 1000 M. Star-forming molecular clouds then would have been much warmer, up to 800 K (525°C) as opposed to cold molecular clouds just a few degrees above absolute zero today. This means that in such an energetic environment a much larger minimum mass would be required to form a star and a much higher Eddington limit could be achieved. These stars would have appeared much like R136a1 - very massive, very hot, very bright and very short lived, burning through their fuel in just a few million years.<br />
<br />
Because these stars have no or almost no carbon in them to start with, there would be no carbon to trigger the <a href="https://en.wikipedia.org/wiki/CNO_cycle">CNO (carbon/nitrogen/oxygen) fusion cycle</a>. The only fusion reaction available during the main-sequence phase would be the p-p chain reaction of hydrogen fusion. This reaction is much less temperature sensitive than CNO fusion, and this means that it doesn't serve as the built-in thermostat that CNO fusion does. The star's core could therefore get much hotter than it would in stars with metallicity. Eventually it would get hot enough to trigger the triple alpha fusion process and from there fusion would proceed, as carbon is fused from beryllium and helium. Once sufficient carbon is present, the CNO cycle would be triggered as well.<br />
<br />
Though fusion would not be runaway, it would be at such a high rate under such enormous gravitational pressure that it would eventually trigger photodisintegration. Gamma photons would be powerful enough to rip apart even the smallest most tightly bound nuclei. In a matter of perhaps minutes, millions of years of core fusion would come undone as gamma photons, more energetic than those in pair instability cores, are absorbed by the core nuclei, causing them to split into alpha particles, free neutrons and free protons. The sudden absorption of free energy (the gamma photons) would trigger catastrophic complete collapse before the core had any opportunity to re-ignite fusion. The core, as a result, would continue to collapse into a massive black hole, with no counter-process to stop it.<br />
<br />
If massive low to no-metal stars ended their lives because of photodisintegration, there should be a number of massive black holes left from their destruction. If little or no explosion occurred after the stars collapsed, the black holes should have masses close to the original stars. Unexpectedly bright galaxies (suspected to contain clumps of very massive hot stars) from the period when light first began to shine in the universe are <a href="http://www.eso.org/public/news/eso1524/">now being detected using a variety of telescopes</a>. Some individual star candidates between 250 and 1000 M <a href="http://www.nasa.gov/vision/universe/starsgalaxies/Black_Hole.html">have also been indirectly detected</a>. This leaves the mystery of how the young universe seeded itself with nuclei larger than beryllium. A near complete collapse into a massive black hole takes any metals formed right along with it. It is possible that the first stars spewed out their matter in powerful GRB's as they collapsed into black holes (and these most ancient GRB's should be detectable). It is also possible that the first stars were slightly less massive than 250 M, and they might have blown up completely as extremely powerful supernovae instead, spewing out lots of fusion products and leaving behind no trace other than a large radioactive cloud destined to become a star nursery for stars with metallicity.<br />
<br />
Despite gas-rich low-metal regions that are still actively making huge R136a-like stars, ancient massive, luminous, hot, metal-free stars could represent the ancient beginnings of a global evolutionary trend toward decreasing stellar mass, as star nurseries in general cool and grow richer in massive elements. The most abundant type of star today is the red dwarf. These stars are so long-lived that if they formed early on in the universe they should still be burning. However, <a href="https://en.wikipedia.org/wiki/Red_dwarf#Description_and_characteristics">no metal-free red dwarf stars have been observed</a> (although they are dim and small so they would be very difficult to detect at such a great distance). All observed red dwarfs have metal content and almost all of them contain a significant percentage of larger elements, indicating that they were formed in molecular clouds thoroughly seeded with the residues of many previous supernovae. This, along with the fact they are found only in the spiral arms of young galaxies like our own, indicates that red dwarfs tend to be more recently formed than distant and long-dead metal-poor massive stars.<br />
<br />
CONCLUSION<br />
<br />
<div class="MsoNormal">
<span lang="EN-US">Stars dotting the night sky might seem
eternal, but if we could live for millions of years we would bear witness to
countless dramatic life and death events. After lives that last anywhere from
millions to trillions of years, stars die. Some die peacefully and gradually
while others blow up with unimaginable force. Understanding what happens to matter
under such atom-ripping conditions is a huge challenge because these forces
cannot be replicated on Earth. </span>Gravity crushes atoms in the cores of massive stars into mysteriously dense physical states where the rules of atomic behaviour no longer apply. During some supernovae, matter is crushed completely into a black hole where even the rules of space and time no longer apply. In both cases, new larger atomic nuclei are also created. Scientists are just now able to fuse the largest of these, such as ununoctium (which decays within a millisecond), in the
most powerful particle accelerators. It is a job requiring extreme energy, one that
is done naturally in microseconds during a supernova.</div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
The universe burst into being almost 14
billion years ago, laced with only the lightest elements, mostly hydrogen along
with helium and a tiny amount of lithium that were fused before expansion and
cooling halted the process. Gravity shaped that primordial material into stellar
fusion reactors that supply the conditions to fuse a far greater variety of
elements, up to iron. The synthesis of even heavier elements requires enough
energy to overcome a highly endothermic process that absorbs energy rather than
releases it. Only in the very brief, chaotic and extremely energetic
environment of a supernova can elements such zinc, silver, gold, mercury and lead,
as well as heavy unstable elements such as radium, uranium and plutonium come
into existence. Furthermore, the explosive process blasts out not only those
elements fused in the supernova but those fused earlier inside the star as
well. That debris cools into molecular clouds that later become new stars and their planets. Rocky planets like Earth can
only form from molecular clouds laced with elements forged in the bellies of
massive stars and forged in their violent deaths.</div>
Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-78781343339408592862016-10-17T14:06:00.001-06:002016-10-17T14:06:32.467-06:00Supernovae PART 3: Massive Stars<span style="color: purple;">For Supernovae PART 1: Introduction, click <a href="http://sciexplorer.blogspot.ca/2016/10/supernovae-part-1.html">here</a>.</span><br />
<span style="color: purple;">For Supernovae PART 2: Low Mass Stars, click <a href="http://sciexplorer.blogspot.ca/2016/10/supernovae-part-2-low-mass-stars.html">here</a>.</span><br />
<br />
MASSIVE STARS: SOME EXPLODE, SOME DON'T<br />
<br />
Stars with less than 8 M mass end up as white dwarfs (see PART 2) but those between 8 M and 10 M might explode as supernovae instead. Below is an artists' impression of supernova 1993J, observed 23 years ago. It blew up in Messier 81 Galaxy, 12 million light-years away. The original star was about 10 times more massive than the Sun (10 M) and about 1000 times brighter.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh2p5vhnICI2qgdgvEGJWtaLwCw0RLAcYPZEVXJeuka9LDuZjN8tYnLQpbKX20y9AjipgjH-htcnJYf8tNAY-IUzFkmTrX4gZL1bid7c7AWV5jQhNFVRMBt79snBsdviHC85Cqd7wl6osrf/s1600/Artist%2527s_impression_of_supernova_1993J.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh2p5vhnICI2qgdgvEGJWtaLwCw0RLAcYPZEVXJeuka9LDuZjN8tYnLQpbKX20y9AjipgjH-htcnJYf8tNAY-IUzFkmTrX4gZL1bid7c7AWV5jQhNFVRMBt79snBsdviHC85Cqd7wl6osrf/s640/Artist%2527s_impression_of_supernova_1993J.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: x-small;"><span style="background-color: #f9f9f9; color: #252525; font-family: sans-serif;">NASA, ESA, and G. Bacon (STScI);</span><span style="color: #252525; font-family: sans-serif;">Wikipedia</span></span></td></tr>
</tbody></table>
The tipping point here is not completely understood. The star mass range that will explode depends on the model used. While red dwarfs are by far the most common stars in the universe, these more massive 8 - 10 M stars, in turn, are much more common than supermassive stars over 12 M. This means that 8 - 10 M stars probably represent most of our observed supernovae. Those at the lowest mass limit seem to explode as faint II-P supernovae. These are faint relatively low-energy explosions in distant galaxies, so, though common, they are not as easy to observe as larger brighter supernovae.<br />
<br />
The current upper limit of stellar mass is at least 150 M and <a href="https://en.wikipedia.org/wiki/Stellar_mass#Range">might be up to 250 M</a>, although even more massive stars up to 1000 M might have lived (briefly) when the universe was very young. One might expect a direct relationship between stellar mass and the intensity of the eventual supernova, but the reality is far weirder. In some cases the supernova is unexpectedly faint or absent altogether.<br />
<br />
A Word on Supernova Classification<br />
<br />
The last two decades, once supernovae could be regularly observed and studied, have revealed an amazing variety of explosions. Attempts to classify all of them is challenging at best. Ultimately researchers would want to classify them based on the star's initial mass, its metallicity and the mechanics of the explosion. Because these parameters take time to figure out, supernovae, when they are first observed, are placed instead into an easier quicker scheme based on the spectra of electromagnetic (EM) radiation they emit. Based on this, five basic classes (Type 1, Type II, Type III, etc.) are often discussed, which are further divided up into subclasses (Type II-P, Type 1a). Instead of an exhaustive survey of them, we'll continue to go upward in star mass, comparing how the stars die and why.<br />
<br />
Mass-Exchange Type 1a Supernovae<br />
<br />
While supernovae in general vary greatly in terms of their underlying mechanisms, their output energies and their duration, <a href="https://en.wikipedia.org/wiki/Type_Ia_supernova">Type 1a supernovae</a> are a unique situation. They are so similar to one another by all accounts that they can be used as <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/astro/stdcand.html">standard candles</a>. Their light curves resemble the graph below left where luminosity is plotted against time. All Type 1a supernovae have the same peak luminosity (or absolute brightness). The radioactive decay of nickel (a product of the star's runaway fusion leading up to the explosion, as we will see) creates the peak in brightness. Then cobalt decays, emitting EM radiation.<br />
<br />
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: left; margin-right: 1em; text-align: left;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhc6Hc_nyuYnFW_eo5nyyUaj0JJSVWY-w-7iinr3LIx-1cy0fXvnvB603sqr_7F7r90Qw7YZuIUlp0pmHPeyyYl0Jg2sA5sJahOYPhtce5tYAXB_WOlWMJm6W6DP-kaZtgHFSOvuKbwjjg7/s1600/SNIacurva.png" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhc6Hc_nyuYnFW_eo5nyyUaj0JJSVWY-w-7iinr3LIx-1cy0fXvnvB603sqr_7F7r90Qw7YZuIUlp0pmHPeyyYl0Jg2sA5sJahOYPhtce5tYAXB_WOlWMJm6W6DP-kaZtgHFSOvuKbwjjg7/s1600/SNIacurva.png" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Xenoforme-commonswiki;Wikipedia</span></span></td></tr>
</tbody></table>
If you know that the absolute brightness of a Type 1a supernova is always the same, you can measure its observed brightness and then calculate its distance from you using the inverse square law, where light from an object decreases at a rate proportional to the inverse square of its distance from an observer. These supernovae occur all over the universe, so they can be used to estimate how far away various distant galaxies are. The discovery of these supernovae was also used to prove that the universe is <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/astro/univacc.html">undergoing accelerating expansion</a>. They are, however, rare - detected only about once every 100 years on average.<br />
<br />
Unlike other supernovae, Type 1a supernovae require more than one star. The explosion mechanism itself is well worked out but how events come about to trigger it is not entirely understood. One thing is certain. Individual stars never explode as Type 1a supernovae. At least a binary pair of stars is required. According to the current model, one of those stars must be a white dwarf. The other star can be any star or a stellar remnant, including another white dwarf, as shown below in this brief European Southern Observatory (ESO) animation.<br />
<br />
<iframe allowfullscreen="" frameborder="0" height="450" mozallowfullscreen="" src="//commons.wikimedia.org/wiki/File:Artist’s_impression_of_two_white_dwarf_stars_merging_and_creating_a_Type_Ia_supernova.ogg?embedplayer=yes" webkitallowfullscreen="" width="700"></iframe>
<br />
This is an animation of what is expected to happen to the planetary nebula <a href="https://en.wikipedia.org/wiki/Henize_2-428">Henize 2-248</a> when two white dwarfs orbiting each other at its centre merge in about 700,000 years. The ensuing Type 1a supernova will completely destroy both stars.<br />
<br />
The current Type 1a supernova model is based on a carbon/oxygen white dwarf in particular. The composition of white dwarfs can vary from helium (hydrogen fusion inside low-mass red dwarfs) to carbon/oxygen (helium fusion inside Sun-like stars) to oxygen/neon/ magnesium (carbon fusion inside 8 - 10 M stars). Most white dwarfs that exist today are carbon/oxygen white dwarfs.<br />
<br />
A slowly rotating carbon/oxygen white dwarf accretes matter from a companion star until it just about overcomes electron degeneracy pressure. This mass limit is called the <a href="https://en.wikipedia.org/wiki/Chandrasekhar_limit">Chandrasekhar limit</a> (1.44 M). Just as the star is about to reach it, increasing pressure within the star ignites carbon fusion. It will continue to fuse carbon over a period of about 1000 years until the fusion reaction ignites a flame front, which heats the star enough to trigger oxygen fusion. Although the flame front mechanism is up for debate, the star begins to heat up quickly once this point is reached, an event not unlike the helium flash described earlier. In just a few seconds, most of the carbon and oxygen are fused into various heavier elements. The star is degenerate so it can't expand to cool off and regulate its rate of fusion, so it evolves rapidly into a runaway reaction that blows it up in a supernova. The shockwave from the explosion is particularly violent. This is why they can be seen from even extremely distant galaxies. The material blowing up is intensely hot dense electron-degenerate matter undergoing runaway fusion. It is estimated to travel up to 20,000 m/s (compare this to typical supernova shockwave velocities of 40,000 km/h, equivalent to about 11,000 m/s). Because the starting mass is always the same, these supernovae have very similar <a href="http://astronomy.swin.edu.au/cosmos/A/Absolute+Magnitude">absolute magnitudes</a> of close to -19.3. That's about five billion times brighter than the Sun. In fact, a Type 1a supernova can outshine an entire galaxy. Although the theory is not worked out, rapidly spinning white dwarfs might be able to exceed the Chandrasekhar limit before going supernova, perhaps <a href="https://www.newscientist.com/article/dn23282-astrophile-dizzy-dwarf-star-will-spin-itself-to-death/">into an even brighter and more powerful e</a><a href="https://www.newscientist.com/article/dn23282-astrophile-dizzy-dwarf-star-will-spin-itself-to-death/">xplosion</a>. Of course, these supernovae would not be typical Type 1a explosions.<br />
<br />
Current theory suggests that binaries containing oxygen/neon/ magnesium white dwarfs won't go supernova when the white dwarf accretes mass. Carbon fusion can't be triggered because the star is already full of carbon fusion products. Like carbon/oxygen dwarfs, these stars heat up under increasing pressure, but before higher-level fusion temperatures are reached, the star exceeds the Chandrasekhar limit and collapses further into a neutron star, an even denser state of matter in which the only outward pressure preventing total collapse into a black hole is the strong force.<br />
<br />
Although there is not much theoretical knowledge to work from, a helium white dwarf accreting mass in a binary system might not go supernova either. Helium white dwarfs are extremely rare, although they will be common in about a trillion years. No red dwarfs have lived long enough to evolve into one, so observed (all extremely low mass) helium dwarfs are thought to have formed <a href="http://iopscience.iop.org/article/10.1086/303686/fulltext/34537.text.html">during the evolution of certain binary pairs</a>. Helium fusion could ignite as the helium white dwarf reaches sufficient pressure but it might release that energy in a series of helium flashes rather than an explosion or further collapse. A helium flash is energetic enough to relieve the star of its degeneracy state but not so energetic that it blows the star apart. The star could continue to accrete mass in this fashion until it is massive enough to explode as a mid-mass star core collapse supernova (which will be discussed next).<br />
<br />
Type II Supernovae<br />
<br />
A <a href="https://en.wikipedia.org/wiki/Type_II_supernova">Type II supernova</a>, like the faint II-P mentioned, is distinguished by the presence of hydrogen lines in its spectrum, which means that these stars still have a significant outer hydrogen shell when they blow up. Stars between 8 and 50 M typically explode as Type II supernovae. These stars are usually found in the arms of spiral galaxies like our own. They end their lives in rapid core collapse leading to a violent explosion, like the one in the centre of the image below left.<br />
<br />
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: left; margin-right: 1em; text-align: left;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgDADxcwNfd2smONxh8vANf606mUdIAEbWq7QuGU2fNHCNGmXAL7SNHY6_9kvmWy0m0ESeoDDVJOI5HZWG_cHSsjELEvi3n8Nm9N1M8Or40aYNi0F1jLwqdK6zDtvfWZDmPAYdEKAx_nm_L/s1600/HST_SN_1987A_20th_anniversary.jpg" imageanchor="1" style="clear: left; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" height="312" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgDADxcwNfd2smONxh8vANf606mUdIAEbWq7QuGU2fNHCNGmXAL7SNHY6_9kvmWy0m0ESeoDDVJOI5HZWG_cHSsjELEvi3n8Nm9N1M8Or40aYNi0F1jLwqdK6zDtvfWZDmPAYdEKAx_nm_L/s400/HST_SN_1987A_20th_anniversary.jpg" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: x-small;"><span style="background-color: #f9f9f9; color: #252525; font-family: sans-serif;">NASA, ESA, P. Challis, and R. Kirshner (Harvard-Smithsonian <br />Center for Astrophysics);</span><span style="color: #252525; font-family: sans-serif;">Wikipedia</span></span></td></tr>
</tbody></table>
This Type II supernova called <a href="https://en.wikipedia.org/wiki/SN_1987A">SN 1987A</a>, was one of the brightest supernovae witnessed in modern times. It blazed as bright as 100 million Suns for months after the initial explosion. A pink ring, about 1 light-year across, glows brightly as a shockwave blasts against the ring of material shed by the star approximately 20,000 years before it exploded.<br />
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Not all 8 - 50 M supernovae are Type II, however. A number of Type 1 supernovae can also occur. Their spectra have no hydrogen lines. Some massive stars shed their outer shell of hydrogen (Type Ib) and some shed both hydrogen and helium (Type Ic) before their cores collapse and explode. These explosions seem to be limited to mid to high-metal stars that are at least 40 M. Many of these stars, once in the red giant phase, are unstable and they express it in as many ways as we middle-aged people express our mid-life crises. Some shed matter steadily, but others rhythmically pulsate, throwing off layer after layer. The least stable stars flare up in irregular violent bursts, some violent enough to shed their entire hydrogen/helium envelope altogether, leaving them "naked" as they rage into the ensuing supernova. White dwarfs destined to become Type 1a supernovae have no outer layers of hydrogen or helium to start with before they explode. This is what puts them in the Type 1 category.<br />
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Faint Type II-P supernovae, those often associated with the smallest mass stars to explode, were mentioned earlier. Not all Type II-P supernovae are faint, however. Some, such as <a href="https://en.wikipedia.org/wiki/SN_1987A">SN 1987A</a>, the explosion of a massive <a href="https://en.wikipedia.org/wiki/Sanduleak_-69%C2%B0_202">20 M star</a>, are exceptionally powerful. As a point of interest, <a href="https://arxiv.org/abs/1111.2509">SN 1987A was a peculiar Type II-P event</a>. The "P" in Type II-P refers to "plateau." Its EM spectrum more or less maintains its luminosity for a few months after the initial blast, whereas the light curve from a Type II-L (linear) supernova linearly drops off, as shown below right in a simplified graph. The comparison of these two Type II supernovae provide a good example of <a href="http://astronomy.swin.edu.au/cosmos/T/Type+II+supernova+light+curves">how light curves tell physicists what's going on</a> during these supernovae. Stars with a wide range of masses can end as Type II-P supernovae. Although all have a plateau, the luminosity of the explosion varies widely, depending on the star's mass.<br />
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhrlbluDPJ4eOJtvtwFinafVn6pdyLMQjoewBCRq6kv1e3KPEaXiYz6GuoLjw7Wl8hdtV0OPFbSEINyfyiyVtNLHxO20pLUPRUb7gucMEp7SFByjLO_q2sea0NopLbrP7FJmsLeTHwd26xt/s1600/SNIIcurva.svg.png" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" height="226" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhrlbluDPJ4eOJtvtwFinafVn6pdyLMQjoewBCRq6kv1e3KPEaXiYz6GuoLjw7Wl8hdtV0OPFbSEINyfyiyVtNLHxO20pLUPRUb7gucMEp7SFByjLO_q2sea0NopLbrP7FJmsLeTHwd26xt/s320/SNIIcurva.svg.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Xenoforme-commonswiki;Wikipedia</span></span></td></tr>
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Unlike Type 1a supernovae in which the entire star is a core remnant that heats up and blows itself apart, all Type II, Type 1b and Type 1c supernovae come about when the core starts to collapse. The first sign of a core collapse supernova is a burst of invisible and difficult to detect <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/neutrino.html">neutrinos</a>. A few hours later the shockwave itself breaks out of the star, releasing an intense burst of EM radiation, usually an ultraviolet flash. At first the photons can't escape. They are trapped in a thick envelope of ionized hydrogen around the star. Once the hydrogen cools enough to return to its neutral atomic state, the layer turns transparent. At this point the supernova becomes optically visible as it expands. A peak in the visible light curve occurs when the surface area of the star is increasing while its temperature has not had a chance yet to decrease. The time spent in the plateau phase for SNII-P depends on the thickness of the hydrogen shell. A thicker shell means a longer plateau. SNII-L stars have a much smaller hydrogen shell to start so light leaves in a sharper single burst. In both cases, visible light drops off to a radioactive tail, where light is emitted from the conversion of unstable cobalt-56 into stable iron-56. <br />
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Before the blast, these stars started to fuse elements heavier than helium in their cores. If the core contracts sufficiently to reach a temperature of about 1.1 GK (gigakelvins or 10<sup>9</sup> Kelvins), nuclei such as neon, created by carbon fusion, will begin <a href="https://en.wikipedia.org/wiki/Neon-burning_process">to partially disintegrate into alpha particles</a> (helium nuclei) and gamma radiation. Other neon nuclei capture these alpha particles to create magnesium, while still others absorb gamma photons to create oxygen. Oxygen then fuses to form sulphur, silicon and smaller amounts of various larger elements in the core.<br />
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This decomposition process depends on the mass of the star. Only stars of about 8 - 10 M and more undergo this process and blow up as supernovae. While the cores of lower mass stars like the Sun collapse too (into white dwarfs), there is no decomposition and no electron capture, an additional process that is described next. For these low mass stars, the outer layers blow away violently but not explosively, leaving a degenerate inert core behind. If the star is over 8 - 10 M but its core is not massive enough to convert all the neon into oxygen and magnesium, fusion will eventually slow down and it will begin to collapse. The matter in the core by this time is already in an electron-degenerate state. As gravity becomes the dominant force, electron degeneracy pressure can no longer prevent further collapse. Electrons, already squeezed into lowest energy orbitals are now squeezed into the nuclei themselves, a process that is called <a href="https://en.wikipedia.org/wiki/Electron_capture">electron capture</a>. Nuclei capture electrons from their innermost shells. When an electron is absorbed, it changes a proton into a neutron while emitting an electron neutrino (an example of a <a href="https://en.wikipedia.org/wiki/Weak_interaction">weak interaction</a>). This process creates smaller amounts of additional elements such as aluminum and sodium in the core. Eventually, the star becomes layered like an onion with elements that fuse at lower temperatures (starting with hydrogen and then helium) occupying the more outermost shells. The diagram below left is a simplified not-to-scale cross-section of a massive star. All the elements are in a plasma state (nuclei are free in a now-diminishing sea of electrons), where the largest nuclei "rain down" through inner levels.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">User:Rursus;Wikipedia</span></span></td></tr>
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Although this diagram, like most, shows an iron core, some supernova-destined stars are not massive enough to fuse iron. The deepest core elements in the least massive supernova stars (those 8 M to 10 M) are more likely to be oxygen, neon and magnesium. In all cases, however, the degenerate core is receiving a continuous injection of energy from gamma radiation as well as an energetic burst of electron neutrinos from the electron capture processes. In this extreme energy environment, the atomic nuclei themselves begin to <a href="https://en.wikipedia.org/wiki/Photodisintegration">photodisintegrate</a>. High-energy gamma photons begin to break up the nuclei into alpha particles.<br />
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The core is degenerate so it cannot exert fusion pressure and expand to release the heat. Instead, gravity remains dominant. It pushes on the core until it becomes so dense that even neutrinos, for which all atomic matter is normally invisible, are trapped. The additional energy of the trapped neutrinos leads to a massive spike in energy. The core suddenly and violently implodes into itself. What remains of the core is destined to be a <a href="https://en.wikipedia.org/wiki/Neutron_star">neutron star</a> unless the original star was very massive. In that case the core's matter will collapse completely into a <a href="https://en.wikipedia.org/wiki/Stellar_black_hole">stellar black hole</a>. Stars with a wide range of masses end up as neutron stars but all neutron stars are the same diameter, roughly between 10 to 30 km, and the same mass, roughly between 1 and 3 M, depending on the theoretical model used. Think of the Sun's entire mass squeezed into a sphere the size of Earth. That's about a millionth of its original volume, giving you an idea of how much of an atom's volume is empty space and how much it can be squeezed. The rest of the star will be explosively blown away. There are approximately 100 million neutron stars in the Milky Way alone.<br />
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In neutron stars, the only force preventing total collapse into a black hole is <a href="https://en.wikipedia.org/wiki/Degenerate_matter#Neutron_degeneracy">neutron degeneracy</a> pressure, an expression of the fundamental strong force. As mentioned earlier, the electron degenerate matter of a white dwarf is matter that is prevented from further collapse by the counterforce described by the Pauli exclusion principle. The matter cannot contract or expand in response to changes in temperature, but its electrons can move faster or slower. In the hottest white dwarfs, electrons are so fast they escape their atoms to create a sea of nuclei (mostly alpha particles) within a sea of fast-moving free electrons. There is a limit to how fast the electrons can move and that is the speed of light. As this limit is approached, which is the Chandrasekhar limit attacked from a different angle, electron degeneracy pressure can no longer support the matter. The matter suddenly collapses into neutron degenerate matter.<br />
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A Note About Stellar Remnant Mass<br />
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There is some discrepancy between models at which core mass will trigger total collapse into a black hole. For white dwarfs, the current often-cited <a href="https://en.wikipedia.org/wiki/Chandrasekhar_limit">Chandrasekhar mass limit</a> for electron degenerate mass is 1.39 M, although a value of 1.44 M is also often cited. Other models suggest that within very powerful magnetic fields, super-white dwarfs, <a href="https://arxiv.org/pdf/1404.7627.pdf">with masses over 3 M might be stable</a>. As mentioned earlier, rapidly spinning white dwarfs might also have a higher mass limit (and these will likely also exhibit powerful magnetic fields). Generally, however, 1.39 M is the limit for electron degenerate mass. The analogous upper limit of neutron degenerate mass (a neutron star) is called the <a href="https://en.wikipedia.org/wiki/Tolman-Oppenheimer-Volkoff_limit">Tolman-Oppenheimer-Volkoff (TOV) limit</a>. It is between 2 and 3 M. The range in values for both mass limits points out where science continues to be a work in progress. The <a href="https://en.wikipedia.org/wiki/Equation_of_state">equations of state</a> used to calculate these masses, especially the TOV mass, are not well understood for matter that is no longer in its ordinary atomic state. No physical lab can supply the kind of energy required to directly observe matter in either degenerate state. For those of you interested in a more detailed exploration of the physics of neutron degenerate matter, try <a href="https://arxiv.org/pdf/1001.1272.pdf">this 2010 paper</a>, intended as an online course.<br />
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At around 2 to 3 M, some stellar cores might collapse into a hypothetical intermediate state of exotic quark matter, into a <a href="https://en.wikipedia.org/wiki/Quark_star">quark star</a> in other words. In this case, neutrons themselves can't hold up intact but matter doesn't completely collapse into a black hole. Instead neutrons break down into a sea of free quarks and gluons, the particles that make up neutrons and protons. The strong force, partially overcome here, normally confines quarks into neutrons and protons. Any further pressure (mass over an upper limit of 3 M) would result in total collapse into a black hole. Black holes in general have an <a href="https://arxiv.org/pdf/0808.2813v2.pdf">upper mass limit of a gargantuan 10<sup>10</sup> M</a> in theory but, based on observational data, <i>stellar</i> black holes range from five to dozens of solar masses.<br />
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If the core mass is less than the TOV limit, neutron degeneracy pressure will prevent further collapse. This pressure acts like a wall during core collapse. The imploding core slams inward, hits neutron degeneracy pressure with the power of the strong force, is immediately stopped in its tracks, and rebounds hard, producing a shockwave that expands outward in all directions. The shockwave explosively expels all the outer stellar material into space, leaving behind a neutron star.<br />
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A Mysterious Variety of Neutron Stars<br />
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All neutron stars spin very fast and have very powerful magnetic fields, particularly right after they form. The black sphere in the centre of the brief animation below is a rotating neutron star. The lines that curve around it are magnetic field lines and the pink cones emanating from it are EM emission zones.<br />
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(Jim Smits;Wikipedia)<br />
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Neutron stars emit EM radiation and that is how they are detected. Although neutron stars consist mostly of compact neutrons, the outermost layers, under less intense pressure, are thought to be composed of electrons and protons. The radiation, most often observed as radio waves, is the result of electrons accelerating along powerful magnetic field lines between the magnetic poles of the star and emitting curvature EM radiation. Photons interacting with the magnetic field can create electron-positron pairs that emit additional (gamma) radiation.<br />
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The angular momentum of the original spinning massive star is conserved. All stars rotate, and they vary greatly in diameter. If the original star was very large and rotating fast, the much tinier neutron star will rotate <i>very</i> fast. The fastest observed rate <a href="https://en.wikipedia.org/wiki/Neutron_star#Spin_up">is 716 rotations per second</a>. Neutron stars also have very strong magnetic fields and this is mysterious because neutrons are electrically neutral particles. There are several possible explanations for it. First off, as mentioned, most researchers agree that some electrons and protons <a href="http://www.astronomycafe.net/qadir/q250.html">remain in the uppermost portion of the neutron star's crust</a> where there is insufficient pressure to maintain the matter in a neutron degenerate state. There may be enough of them, and they may be moving fast enough, to maintain the original magnetic field of the star, perhaps acting as a kind of <a href="https://en.wikipedia.org/wiki/Magnetohydrodynamic_generator">magnetic dynamo</a>. Some experts suspect that the <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/fluxmg.html">magnetic flux</a> of the field itself is conserved and compressed into the much smaller neutron star. Others suggest that the remaining protons exist in an exotic superconducting state, which can multiply the magnetic field. Still others think that <a href="http://irfu.cea.fr/Sap/en/Phocea/Vie_des_labos/Ast/ast.php?id_ast=2933">a fossil magnetic field</a><a href="http://irfu.cea.fr/Sap/en/Phocea/Vie_des_labos/Ast/ast.php?id_ast=2933"> remains frozen</a> in the collapsing plasma that formed the neutron star. You might find <a href="https://www.researchgate.net/post/What_is_the_origin_of_the_magnetic_field_of_a_neutron_star">this conversation</a> at <a href="http://researchgate.net/">researchgate.net</a> especially interesting as experts wrestle with this mystery.<br />
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Some neutron star remnants can have an even more exceptionally powerful magnetic field. These mysterious variants are called <a href="https://en.wikipedia.org/wiki/Magnetar">magnetars</a>. Generating the most powerful magnetic field known, as much as 10<sup>11</sup> tesla, it would distort the electron clouds in the atoms of your body and kill you instantly from 1000 km away.<br />
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Born from a very large star, a highly magnetized neutron star often rotates exceptionally fast as well. Electrons can be accelerated so violently they emit <a href="https://en.wikipedia.org/wiki/Pulsar#Formation.2C_mechanism.2C_turn_off">an intense binary jet of EM radiation</a> that, if directed at Earth, identifies it as a <a href="https://en.wikipedia.org/wiki/Pulsar">pulsar</a> because the radiation appears to rapidly pulse. Both magnetars and pulsars are believed to be very young neutron stars. Eventually their rotation rates wind down as energy is lost to the magnetic field and as EM radiation as well. Even an old neutron star is mysterious. Researchers don't really know if a core of even denser quark matter exists deep inside.<br />
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<tr><td class="tr-caption" style="text-align: center;">Robert Schulze;Wikpedia</td></tr>
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An apple-sized inner core of matter could be squeezed so hard it overcomes quark degeneracy pressure to become even denser <a href="https://en.wikipedia.org/wiki/Electroweak_star">electroweak matter</a>. These hypothetical physical states could influence the behaviours of neutron stars.<br />
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Next we explore the sometimes unexpected ways in which massive stars of between 10 and 150 M end their lives.<br />
<br />Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-31498055955130442752016-10-13T14:18:00.001-06:002016-10-13T14:18:45.658-06:00Supernovae PART 2: Low Mass Stars<span style="color: purple;">For Supernovae PART 1: Introduction, click <a href="http://sciexplorer.blogspot.ca/2016/10/supernovae-part-1.html">here</a>.</span><br />
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Brown Dwarf Stars<br />
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The smallest mass that can form into a star is a brown dwarf.<br />
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A <a href="https://en.wikipedia.org/wiki/Brown_dwarf">brown dwarf</a> forms from a protostar mass less than 0.08 M (M = one solar mass or 2 x 10<sup>30 </sup>kg). This mass is <a href="https://en.wikipedia.org/wiki/Sub-brown_dwarf">on the edge between being a star and being a planet</a>.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="background-color: whitesmoke; color: #555555; font-family: sans-serif;"><span style="font-size: x-small;">NASA/JPL-Caltech/UCB</span></span></td></tr>
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What distinguishes a brown dwarf as a star is its formation. Unlike a <a href="http://phys.org/news/2015-01-planets.html">planet formed from the protoplanetary disk material around a star</a>, a brown dwarf is formed in the manner described in PART 1. In this case, however, the star's core is too small to have sufficient pressure and heat to ignite the fusion of hydrogen into helium. Fusion does ignite, though. If the mass is at least 0.0125 M (about 13 times the mass of Jupiter), it will trigger the fusion of <a href="http://chemistry.about.com/od/hydrogen/a/Deuterium-Facts.htm">deuterium</a> into helium, at least temporarily. Deuterium is a stable less abundant isotope of hydrogen that contains a proton as well as a neutron in its nucleus. <a href="https://en.wikipedia.org/wiki/Deuterium_fusion">This fusion reaction</a> requires a lower ignition temperature - roughly 1 x 10<sup>6</sup> K compared to 4 x 10<sup>6</sup> K required for hydrogen fusion. This means that 0.0125 M is the smallest possible protostar mass that can evolve into a main-sequence star. Even a large-end brown dwarf will only shine very briefly and dimly. It will eventually cool and die gradually and peacefully into an inert mass. At first only theoretical objects, since the 1990's hundreds of brown dwarfs have been identified by infrared surveys. Almost impossible to see in visible light, the cores of brown dwarfs are compressed enough to emit heat, and that heat gives them away even long after deuterium fusion has fizzled out.<br />
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Red Dwarf Stars<br />
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A slightly higher mass protostar evolves into a <a href="https://en.wikipedia.org/wiki/Red_dwarf">red dwarf star</a>. With a mass between about 0.08 M and 0.50 M, these stars are massive enough to fuse hydrogen into helium through the <a href="https://en.wikipedia.org/wiki/Proton-proton_chain_reaction">proton-proton chain reaction</a>. An artist's conception of a typical red dwarf star is shown below right. Though called "red" dwarfs, the surface temperature of these stars means that they would look orange at close range.<br />
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<tr><td class="tr-caption" style="text-align: center;">Courtesy NASA</td></tr>
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Red dwarfs with a mass less than 0.35 M are fully <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html">convective</a>, and this makes them uniquely long-lived stars. The helium produced during fusion mixes throughout the star rather than building up in the core as it does with most larger stars. This means that a red dwarf makes thorough use of its hydrogen fuel. It evolves very slowly and maintains a constant luminosity for up to trillions of years before it leaves the <a href="https://en.wikipedia.org/wiki/Main_sequence">main-sequence stage</a> of its life. In main-sequence phase these stars are very dim, emitting between 1/10,000th up to 1/10th of the Sun's luminosity. Although red dwarfs are by far the most common stars in the universe, making up about 75% of all stars, they are difficult to observe. Only very young red dwarfs currently exist. After all, the universe is only 13.8 billion years old. Eventually a red dwarf will fuse all of its hydrogen into helium, end its main-sequence phase, and evolve into a helium white dwarf.<br />
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White Dwarf Stellar Remnants<br />
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A <a href="https://en.wikipedia.org/wiki/White_dwarf">white dwarf</a> is a stellar remnant. These stars cannot form directly in a nebular cloud. They are denser than any main-sequence star because they are composed of atomic nuclei packed together in a sea of free electrons rather than ordinary atomic matter. Intense force is needed to squeeze atomic matter into such a state. About 97% of the stars in the Milky Way, all stars too small to end up as neutron stars or black holes - from red dwarfs to stars ten times more massive than our Sun - will end up as white dwarfs. When a red dwarf burns up most of its hydrogen fuel, fusion begins to taper off. As the outward pressure from fusion decreases, the star (almost all helium by this time) is overcome by gravitational pressure. Electrons are forced down into orbitals closer and closer to the nuclei, similar to the kind of compression that happens when matter cools off but this case is unique. Even though the matter is compressed, thermal (outward) pressure decreases rather than increases as it normally would because atoms can't move around freely. This means that the core continues to contract. There is a limit to how close electrons can get, however. More than one electron cannot occupy the same quantum state in matter, according to the <a href="https://en.wikipedia.org/wiki/Pauli_exclusion_principle">Pauli exclusion principle</a>. This generates a counter pressure against further collapse. It is called <a href="https://en.wikipedia.org/wiki/Degenerate_matter#Electron_degeneracy">electron degeneracy</a><a href="https://en.wikipedia.org/wiki/Degenerate_matter#Electron_degeneracy"> pressure</a> because, as electrons are forced into lowest possible orbitals, they themselves move faster and faster, generating their own pressure. The helium nuclei at this point can no longer hold onto their electrons so a sea of free fast electrons forms, embedded with helium nuclei. Because it is not in an ordinary atomic state, electron degenerate matter cannot cool off like ordinary matter does. The star will continue to shine white-hot for billions of years and it will only very gradually cool to an inert <a href="https://en.wikipedia.org/wiki/Black_dwarf">black dwarf</a>. Heat slowly diffuses outward from its degenerate inner core to a thin outer layer of non-degenerate atomic matter. This thin layer of matter is able to radiate the heat into space, emitted as light, first blue-white, then yellow, orange, and finally red. The brief animation below shows what this process would look like over time.<br />
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The heat loss is very inefficient and that is why white dwarfs cool very slowly. Although white dwarf stars on their own die very slowly and peacefully like this, white dwarf relationships with other stars or stellar remnants end badly in explosions called Type 1a Supernovae. They will be explored in PART 3.<br />
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MID-SIZE STARS INCLUDING THE SUN<br />
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Protostars with masses between 0.50 M and 10 M are mid-size stars. These stars are destined to undergo a <a href="https://en.wikipedia.org/wiki/Giant_star#Intermediate-mass_stars">red giant phase</a> at the end of their main-sequence phase, before they end as white dwarfs. Unlike smaller red dwarf stars, these stars are not significantly convective, The plasma doesn't mix much so a shell of hydrogen remains unburned around the core as it undergoes fusion. Smaller mid-size stars like the Sun utilize the proton-proton chain reaction to fuse hydrogen into helium. With increasing stellar mass, another fusion chain reaction called the <a href="https://en.wikipedia.org/wiki/CNO_cycle">CNO cycle</a> becomes the more efficient reaction, fusing hydrogen into helium at higher core temperatures. Whereas proton-proton chain fusion ignites at about 4 x 10<sup>6</sup> K, the CNO fusion reaction becomes self-maintaining at about 15 x 10<sup>6</sup> K. There are various reaction paths available, where carbon, oxygen and nitrogen (products of helium fusion) function as catalysts. Stars over 1.8 M almost exclusively utilize to the CNO cycle.<br />
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Mid-size stars go through a more complex evolution than smaller red dwarfs. During any star's main-sequence phase, the core is a spherical fusion reaction that maintains hydrostatic equilibrium between outward radiation pressure and inward gravitational pressure. The smaller the star's mass, the longer it will stay in equilibrium as a main-sequence star. Fusion reactions take place at a slower pace in general and increased convection stirs new reactants into the core. The <a href="https://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram">Hertzsprung-Russell diagram</a> (below) plots stars based on luminosity versus surface temperature. This diagram, created in 1910, led to major theoretical developments in stellar physics, well before stellar fusion was understood to be the reaction mechanism in stars (discovered in the 1930's by <a href="https://en.wikipedia.org/wiki/Hans_Bethe">Hans Bethe</a>). The diagram has evolved since then as theory evolved. It is still one of the most useful tools astronomers have to study stellar evolution. It highlights a number of important trends, the most obvious one being that luminosity tends to increase with surface temperature. You can also see that a star's colour is <a href="http://www.universetoday.com/24640/color-of-stars/">an indication of its surface temperature</a>. A star's mass determines its position on the main sequence. More massive stars are hotter and brighter than less massive stars. Though shown incorrectly as pale yellow in this case, the Sun is a white (white-hot) star of luminosity 1 (solar unit). It is located in the middle of the down-sloping central spine of the main sequence.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Image created by the European Southern Observatory; original source <a href="https://www.eso.org/public/images/eso0728c/">here</a>.</span></td></tr>
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Our Sun, at 1 M (solar mass), <a href="https://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System#The_Sun_and_planetary_environments">is an example of a mid-size star</a>. The photo below left shows you what the Sun looks like in visible light through a filter.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="background-color: #f9f9f9; color: #252525; font-family: sans-serif; font-size: 13.300000190734863px;">Geoff Elston; Society for Popular </span><span style="color: #252525; font-family: sans-serif; font-size: x-small;">Astronomy:Wikipedia</span></td></tr>
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Its position is currently on the main sequence but it will move across the diagram as it evolves. The Sun will exist as a main-sequence star, fusing hydrogen into helium for a total of 10 billion years. In another 4.5 billion years, its core's hydrogen fuel will be almost entirely fused. At that point, like a victim shot in the saloon of an over-acted Western, the Sun will begin a dramatic and complex series of death throes. The core will start to collapse under its own weight as dwindling outward pressure from hydrogen fusion no longer sustains its volume. The core will heat up under increasing pressure and eventually this process will cause the Sun to expand into a red giant (see the bubble attached to the right of the main sequence in the Hertzsprung-Russell diagram). This change happens because the core has gotten so hot that hydrogen in the shell around the core starts fuse causing the shell to expand. The Sun's surface temperature will cool from white to reddish-orange as it balloons outward to a radius about 200 times its current size (the H-R diagram is not to scale). Despite its cooler surface temperature, it will be almost 3000 times more luminous than it is now because it will be so huge. Its luminosity will increase as it ascends the short red giant branch as the rate of hydrogen fusion in the outer core shell layer increases. The diagram below puts the change in size (200X) into perspective.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="background-color: #f9f9f9; color: #252525; font-family: sans-serif; font-size: 13.300000190734863px;">Oona Räisänen (</span><a class="extiw" href="https://en.wikipedia.org/wiki/User:Mysid" style="background-image: none; color: #663366; cursor: pointer; font-family: sans-serif; font-size: 13.300000190734863px; text-align: start; text-decoration: none;" title="w:User:Mysid">User:Mysid</a><span style="background-color: #f9f9f9; color: #252525; font-family: sans-serif; font-size: 13.300000190734863px;">), </span><a class="extiw" href="https://en.wikipedia.org/wiki/User:Mrsanitazier" style="background-image: none; color: #663366; cursor: pointer; font-family: sans-serif; font-size: 13.300000190734863px; text-align: start; text-decoration: none;" title="w:User:Mrsanitazier">User:Mrsanitazier</a><span style="color: #252525; font-family: sans-serif; font-size: x-small;"><span style="background-color: #f9f9f9;">;Wikipedia </span></span></td></tr>
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Having left its main-sequence phase, the Sun will arrive at a new but shorter-lived equilibrium, perhaps lasting hundreds of million of years. It will now occupy the tip of the <a href="https://en.wikipedia.org/wiki/Red-giant_branch">red giant branch (RGB)</a>, the bubble on the Hertzsprung-Russell (H-R) diagram above. As the Sun continues to fuse hydrogen in the thick shell around its core via the CNO cycle, the helium in the core builds up mass. During the last million years or so of the RGB phase, the Sun will lose about 20% of its mass as rapid hydrogen fusion ejects gas from the outermost layers. As hydrogen is used up and fusion slows down, gravity dominates over fusion pressure. Core pressure meanwhile builds as new helium is deposited around it. Gravity squeezes the helium atoms into an electron degenerate state. In stars up to about 2 M, the RGB phase will end abruptly when the inert degenerate helium core reaches the ignition temperature (about 10<sup>8</sup> K) for fusion into carbon and oxygen using the <a href="https://en.wikipedia.org/wiki/Triple-alpha_process">triple alpha process</a>. Much of the core helium will fuse simultaneously in a violent but invisible process called a <a href="https://en.wikipedia.org/wiki/Helium_flash">helium flash</a>. It will all take only a few minutes because the core is mostly degenerate. Degenerate matter has no opportunity to expand and dissipate the fusion heat. The reaction rate increases into a runaway state, in which a positive feedback loop further increases the reaction rate. All this takes place in an electron sea that is a perfect conductor of heat. The electrons transfer the energy of the fusion reaction throughout the core almost at once, so runaway fusion takes place across it simultaneously. If light could stream away from the Sun at this point, it would exhibit a flash so bright it would be about 10<sup>11</sup> times brighter than it is now, about as bright as the entire Milky Way, but it won't be able to. Energetic photons produced by the flash will be absorbed by the Sun's now extremely thick and dense outer envelope of plasma instead.<br />
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The force of the run-away helium flash will blast the core nuclei and electrons apart, allowing them to reorganize as ordinary atoms. The core will quickly expand to dissipate the reaction heat. Triple alpha fusion will continue at a now steady rate as the Sun <a href="http://www.universetoday.com/18847/life-of-the-sun/">enters an even briefer third stable period</a>, lasting about 100 million years, on the <a href="https://en.wikipedia.org/wiki/Horizontal_branch">horizontal branch</a>. This branch is not shown in the H-R diagram above. If you scroll downward you will see an evolution graph specific for the Sun. The horizontal branch is a dotted line band. By this time, about 40% of the Sun's total mass is helium and 6% is carbon.<br />
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When the helium starts to dwindle, the core will contract further under gravitational pressure and the Sun will expand once again as it resorts to fusing the remaining hydrogen and helium in its outer layers. It will now reach a new stability as an <a href="https://en.wikipedia.org/wiki/Asymptotic_giant_branch#AGB_stage">asymptotic giant star</a>, reddish orange, and about 2000 times more luminous than it is now. In another 30 million years, it will begin to eject its outer layers of mass through a series of <a href="http://blogs.discovermagazine.com/badastronomy/tag/thermal-pulse/#.V-QM-junP0c">thermal pulses</a>, each one lasting about 100,000 years, where the core temperature wobbles slightly, causing significant changes in the fusion rate. The ejected outer envelopes of gases will form a planetary nebula around a core remnant that has recompressed into the degenerate matter of a carbon/oxygen white dwarf, containing about half the Sun's current mass squeezed down to the size of Earth. Starting out bluish white hot and about 100 times more luminous than the Sun is now, it will very, very gradually fade into a black dwarf.<br />
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The life of the Sun is illustrated in more general terms below, where changes in size are shown to scale. The Sun started out as a relatively small star formed from a molecular cloud. It will spend most of its life as a stable white star until its core begins to run out of hydrogen fuel. It will expand to 200 times its former size into a red giant star. Eventually its outer layers of material will blow away leaving behind an Earth-sized white dwarf (tiny white dot) in the center of a planetary nebula.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">ESO/S. Steinhofel;Wikipedia</span></span></td></tr>
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The graph below tracks the evolution of a 1M star like the Sun.<br />
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<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">Lithopsian;Wikipedia</span></span></td></tr>
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Though complex and dramatic, the death of a star like our Sun is a mild event by astronomical standards. When our Sun leaves the main sequence, planetary orbits will probably be disturbed by powerful solar wind gusts and the innermost rocky planets will be engulfed as the Sun expands to a red giant. Still, there is some evidence that Earth, though scorched, sterilized and possibly engulfed by hot plasma at some point, <a href="http://physicsworld.com/cws/article/news/2007/sep/12/earth-could-survive-a-red-giant-sun">might survive physically intact</a> to eventually orbit the Sun's white dwarf remnant.<br />
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For all mid-sized stars the evolutionary scenario is similar with some exceptions. Stars slightly smaller than our Sun also evolve into red giants but their cores don't have enough mass to ignite helium fusion. They don't reach the tip of the red giant branch shown above, and there is no helium flash. Instead, these stars move directly into a state that more resembles the post- AGB (asymptotic giant branch) top horizontal line in the graph above, except that they are less luminous. Like the Sun, they eventually blow off their outer layers to form a planetary nebula, leaving behind, in this case, a helium white dwarf. In stars more massive than our Sun, between 2.25 M and 10 M, the core is large enough and dense enough to reach the fusion ignition temperature of helium before it has a chance to contract into a degenerate state. This eliminates the helium flash from their life cycle for a different reason. Like the Sun, these more massive stars expand into red giants as the outer layers of the star cool and become opaque to radiation. However, those layers of gases are even thicker in these more massive stars. They effectively hold in the heat and cause the hydrogen shell around the core to further heat up and expand to form much larger stars, often called <a href="https://en.wikipedia.org/wiki/Asymptotic_giant_branch#Super-AGB_stars">super-AGB stars</a>, which tend to evolve further along the horizontal branch than Sun-like stars do. They can become very unstable pulsating stars that can shine yellow, white or even blue-hot. The particular evolutionary path of the star depends on its heavy element content (called <a href="https://en.wikipedia.org/wiki/Metallicity">metallicity</a> in astronomy) and helium content to start with. The contribution of these elements, in turn, depends on the composition of the molecular cloud in which the star formed. The lines are dotted in the graph above because these horizontal evolutionary paths are still being modeled. The cores of these more massive stars eventually become degenerate and hot enough to fuse carbon into larger nuclei. Some of these stars might ignite a carbon flash analogous to the helium flash of smaller stars. If the original star is between 8 M and 10 M, it will most likely form an initially blue-hot white dwarf surrounded by a planetary nebula, much like less massive mid-range stars, with the exception that the white dwarf will be made of oxygen, neon and magnesium, all of which are products of <a href="https://en.wikipedia.org/wiki/Carbon-burning_process">carbon fusion</a>.<br />
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As we've seen here, low mass (small and medium sized) stars die in a step-by-step process that, though violent, is not explosive. At around 8 M in some cases, stars have enough mass to go out spectacularly and these supernovae can vary considerably. Some massive stars buck the trend and don't explode at at all. It all depends on the high-energy physics of matter deep within their cores, next.<br />
<br />Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-58305756799324493702016-10-11T14:41:00.000-06:002016-10-16T13:06:15.673-06:00Supernovae PART 1: Introduction<div class="MsoNormal">
The <a href="https://en.wikipedia.org/wiki/Milky_Way">Milky Way</a> is full of stars, gas and dust. The bright white dot in the centre of the photo
below is Jupiter. The red laser from one of the three observatories in the photo points directly at the heart of our galaxy.</div>
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<tr><td class="tr-caption" style="text-align: center;"><span style="text-align: start;"><span style="font-size: x-small;">ESO/Y.Beletsky;Wikipedia</span></span></td></tr>
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<!--StartFragment--><span style="font-size: x-small;"><span lang="EN-US" style="font-family: "geneva";">A false-colour image of the Sun taken in the <br />extreme
ultraviolet range of the electromagnetic <br />spectrum taken by NASA/SDO (AIA)</span><!--EndFragment--> </span> </td></tr>
</tbody></table>
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The photo above of the Milky Way shows us only a
small percentage of all stars in the night sky above us. Most of them are
invisible to our naked eye from Earth, even those within our own Milky Way.</div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US"><a href="https://en.wikipedia.org/wiki/Star">Stars</a>, like our Sun (right), are gigantic spherical nuclear fusion reactions.</span></div>
<div class="MsoNormal">
<span lang="EN-US"><br /></span></div>
<div class="MsoNormal">
<span lang="EN-US">For most of a star's life, the outward
blast of nuclear fusion is balanced by the force of gravity squeezing down on
the star and it burns steadily. That balance eventually ends when the star runs
out of fuel, and when it does it can (but not always!) produce the most violent
explosion observed in the universe, a <a href="https://en.wikipedia.org/wiki/Supernova">supernova</a>.
The <a href="https://en.wikipedia.org/wiki/Crab_Nebula">Crab Nebula</a>, below, is a six
light-year wide remnant of a star that once existed about 6500 light-years away. It exploded in
1054. Not visible is a stellar remnant called a <a href="http://imagine.gsfc.nasa.gov/science/objects/pulsars1.html">neutron star</a> in the centre. Its powerful magnetic field whips up electrons in the cloud
around it almost to the speed of light. They emit the eerie bluish glow.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjlj3hq94EJi-lmDq5z9tH0_i8mL6cUeIWoSd3V_bO4QMNYebzEpIJd8QALKU98gBNzHyQPta9GwxzaKTD0hq3EWW-zsqb7pXl3nZJ9need7Sv_OfiPVLraVPimtNHLl79lzh7dn74aybI4/s1600/Crab_Nebula.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjlj3hq94EJi-lmDq5z9tH0_i8mL6cUeIWoSd3V_bO4QMNYebzEpIJd8QALKU98gBNzHyQPta9GwxzaKTD0hq3EWW-zsqb7pXl3nZJ9need7Sv_OfiPVLraVPimtNHLl79lzh7dn74aybI4/s640/Crab_Nebula.jpg" width="640" /></a></div>
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<br /></div>
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<span lang="EN-US">The image of the Crab Nebula is a mo taken by the <a href="http://hubblesite.org/the_telescope/">Hubble SpaceTelescope</a>. The various glowing colours are different excited ionized elements
blown out in the blast. <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">On Earth, we observe a distant supernova as
the sudden appearance of a bright "new" star that fades over a period
of weeks or months. A massive star can explode and then disappear into a <a href="https://en.wikipedia.org/wiki/Black_hole">black hole</a>. Or it can leave absolutely no
trace whatsoever of its existence. Often, <a href="https://en.wikipedia.org/wiki/Compact_star">a remnant remains</a> after the original star dies, composed of the strangest densest matter
known. By exploring how stars live and die, we are exploring how matter works at
its limits and beyond its limits.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Like us, <a href="https://en.wikipedia.org/wiki/Stellar_evolution">stars have life cycles</a>.
They are born, they change throughout their lives, and eventually they die.
While some stars simply fade away, others go out spectacularly as supernovae.
The science behind star death <a href="https://en.wikipedia.org/wiki/History_of_supernova_observation">is evolving quickly</a> thanks to new robotic high-resolution telescopes that can quickly cover large
regions of the night sky and which can recognize even very minute changes in
stellar luminosity. Supernovae happen suddenly and fade quickly. Now
astronomers can catch one as it unfolds. By continuously scanning across
hundreds of galaxies every 30 minutes, the <a href="https://en.wikipedia.org/wiki/Kepler_(spacecraft)">Kepler Space Telescope</a> designed primarily to detect extrasolar planets, for example, can also catch
the first minutes of a supernova. In 2011 <a href="http://www.nasa.gov/feature/ames/Kepler/caught-for-the-first-time-the-early-flash-of-an-exploding-star">it caught the initial shockwaves of two brilliant distant supernovae</a>,
as two massive red supergiant stars exploded in separate incidents. Watch this brief
animation of a supernova shockwave flash. In reality the flash lasts for about
an hour. The animation, based on Kepler's observations, was created by NASA's
<a href="https://en.wikipedia.org/wiki/Ames_Research_Center">Ames Research Center</a>.</span><br />
<br />
<iframe allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/UpMXTM8OYT8" width="560"></iframe>
</div>
<div class="MsoNormal">
<span lang="EN-US"><br /></span>
<span lang="EN-US">Supernovae are not only fascinating in
themselves. They are essential to the creation of all the planets, moons,
asteroids and life in the universe. While stars <a href="http://physics.about.com/od/physicsqtot/g/StellarNucleosynthesis.htm">fuse together elements from neon to nickel in their cores</a>,
elements <a href="http://www.rsc.org/periodic-table">with nuclei larger than nickel</a> can
only be created <a href="https://www.blogger.com/"><span id="goog_1347884627"></span>in the intense furnace of a supernova explosion<span id="goog_1347884628"></span></a>.
By understanding the physics of supernovae explosions, physicists can understand
how they seeded the universe with these heavy elements over time. Using mathematical
computer modeling and observational data, they are discovering a surprising
myriad of ways in which stars violently end their lives, some of which test the
limits of current theory. As the most energetic events in the universe, they
offer clues about how the universe is evolving over time.<o:p></o:p></span></div>
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<br /></div>
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<span lang="EN-US">STAR FORMATION<o:p></o:p></span></div>
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<br /></div>
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<span lang="EN-US">All stars start out the same basic way. A
star's life begins when a cloud of dust and gas in space collapses under its
own gravitational attraction. These clouds are called <a href="http://www.universetoday.com/61103/what-is-a-nebula/">nebulae</a>.
A nebula consists of various molecules, neutral atoms of hydrogen and helium, and
<a href="https://en.wikipedia.org/wiki/Ionization">ionized</a> gases. It can be vast, up to
millions of <a href="https://en.wikipedia.org/wiki/Light-year">light-years</a> in diameter.
<o:p></o:p></span></div>
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<br /></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhG8FFoEbcfK9-DlWAzyYKQkMvmKM9kDLNW765IHvbqsMEklkJuNFc4mOAr2K6SpRUmIWpSlZU2Oae6bq7Tnyd7RiY-ZTFwlnR_J3tXrRe4-rKRSX-PEJjzerrBzDMBbfjgLmvdPdGk24ec/s1600/Eagle_nebula_pillars.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="630" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhG8FFoEbcfK9-DlWAzyYKQkMvmKM9kDLNW765IHvbqsMEklkJuNFc4mOAr2K6SpRUmIWpSlZU2Oae6bq7Tnyd7RiY-ZTFwlnR_J3tXrRe4-rKRSX-PEJjzerrBzDMBbfjgLmvdPdGk24ec/s640/Eagle_nebula_pillars.jpg" width="640" /></a></div>
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<br /></div>
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<span lang="EN-US">This iconic composite photograph taken by
the Hubble Space Telescope is of the "<a href="https://en.wikipedia.org/wiki/Pillars_of_Creation">Pillars of Creation</a>," part of the
<a href="https://en.wikipedia.org/wiki/Eagle_Nebula">Eagle Nebula</a>. These pillars,
composed mostly of molecular hydrogen gas and dust, are star nurseries.</span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjTaNmwaSrA_rYs0drhrJajL36eX8xBze2i_jnw5SQEG3cZVXCiHxU_-1DREaIpzel9S9Q1ro0iqFh1oAanArUqayKla48TCSK8ieSsvzTzD3JEM2MvwLEzbJ-6RNPGVikweOxD2Kqldo_e/s1600/PIA19872-12thAnniversarySpaceCalendar-SpitzerST-20150820.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="360" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjTaNmwaSrA_rYs0drhrJajL36eX8xBze2i_jnw5SQEG3cZVXCiHxU_-1DREaIpzel9S9Q1ro0iqFh1oAanArUqayKla48TCSK8ieSsvzTzD3JEM2MvwLEzbJ-6RNPGVikweOxD2Kqldo_e/s640/PIA19872-12thAnniversarySpaceCalendar-SpitzerST-20150820.jpg" width="640" /></a></div>
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<span lang="EN-US">This mosaic photograph of stunning nebulae
shows off the <a href="http://www.spitzer.caltech.edu/mission">Spitzer Space Telescope's</a> capability. These infrared images were used for its 12<sup>th</sup>
Anniversary calendar. <o:p></o:p></span>As beautiful as they are, nebulae don't last.</div>
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<span lang="EN-US">Nebulae are transient structures in the
universe. The Eagle Nebula, which is about 7000 light-years from Earth (shown
below), <a href="http://www.universetoday.com/1196/eagle-nebulas-pillars-were-wiped-out-thousands-of-years-ago/">may not even exist anymore</a>.
You can see the Pillars of Creation in the bright white center of the much
larger Eagle Nebula below.<o:p></o:p></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjbPokJAfnRc9eIzD2PoNdFEUofW5tAcfNzbsi29Rd3M6PJn_b6j26MH34GATse29QaXYN18gT200AiXUq26oHClO2SmA4kiURHTUowpbDp8lBG0hlYFpafDN6-B-oWXFRDzoThO4aebrlD/s1600/Eagle_Nebula_from_ESO.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjbPokJAfnRc9eIzD2PoNdFEUofW5tAcfNzbsi29Rd3M6PJn_b6j26MH34GATse29QaXYN18gT200AiXUq26oHClO2SmA4kiURHTUowpbDp8lBG0hlYFpafDN6-B-oWXFRDzoThO4aebrlD/s640/Eagle_Nebula_from_ESO.jpg" width="640" /></a></div>
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<span lang="EN-US">This is a three-colour mosaic image of
Eagle Nebula taken by the Wide-Field Imager at <a href="https://en.wikipedia.org/wiki/La_Silla_Observatory">La Silla Observatory</a> in Chile.<o:p></o:p></span></div>
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<span lang="EN-US">The Spitzer Space Telescope recently imaged a rapidly expanding cloud of hot dust in this region of space.
It might be the signature of an intense shockwave produced by a nearby supernova.
Knowing the velocity of the shock wave, researchers estimated it will reach the
nebula in about 1000 years. We will be able to witness that carnage from Earth
about 1000 years from now, not because we will be seeing it in real time but
because we are seeing the nebula from 7000 light-years away. When we look up at
space, we are looking back in time. We see the Pillars as they were 7000 years
ago. If the hot cloud is indeed a shockwave, the nebula was actually destroyed
6000 years ago. <o:p></o:p></span></div>
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<i style="mso-bidi-font-style: normal;"><span lang="EN-US" style="background-color: #fff2cc;">A
Refresher Note On Space, Time and Light-speed<o:p></o:p></span></i></div>
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<span style="background-color: #fff2cc;"><br /></span></div>
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<span style="background-color: #fff2cc;"><i style="mso-bidi-font-style: normal;"><span lang="EN-US">I
find it is always useful to mentally refresh myself about how space works
because it can seem counter-intuitive.</span></i><span lang="EN-US"> <i style="mso-bidi-font-style: normal;">ALL electromagnetic (EM) radiation travels
at light speed. This includes visible light, gamma rays, X-rays, radio waves,
etc. Distant supernovae we observe from Earth exploded hundreds of millions up
to billions of years ago. The <a href="http://news.nationalgeographic.com/news/2013/04/130405-supernova-farthest-science-space-universe-hubble/">most distant supernova observed</a> so far (by Hubble) is called UDD10Wil and it is about 10 billion light-years away.
This means its star exploded 10 billion years ago when the universe was only
about 4 billion years old. The environment then and the star itself were probably
very different from our modern universe today. When the <a href="https://en.wikipedia.org/wiki/James_Webb_Space_Telescope">James Webb Space Telescope</a> starts operating in
2018, astronomers will be able to routinely observe the supernovae of the very
first stars to form just hundreds of millions of years after the <a href="https://en.wikipedia.org/wiki/Big_Bang">Big Bang</a>.
<o:p></o:p></i></span></span></div>
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<i style="mso-bidi-font-style: normal;"><span lang="EN-US"><span style="background-color: #fff2cc;">Supernova
shockwaves, although they travel very fast, <a href="http://www.space.com/22446-supernova-shockwave-speed.html">about 40,000 km/</a><a href="http://www.space.com/22446-supernova-shockwave-speed.html">h</a>,
they do not approach light-speed. A supernova shockwave <a href="http://phys.org/news/2016-03-astronomers-glimpse-supernova-shockwave.html">was captured during its first minutes</a> for the first time this year. Just before all or most of the star blows apart, a shockwave starting
in the center of the star reaches its surface and expands outward into space,
accompanied by a flash of light. It works much like a mechanical shockwave such
as thunder rumbling through Earth's atmosphere, except that a supernova
shockwave travels through the much more disperse and often ionized gases of
interstellar space. To compare medium densities, Earth's atmosphere contains
about 3 x 10<sup>18</sup> molecules per cubic centimeter (cm<sup>3</sup>),
while interstellar gas contains just 1 atom per cm<sup>3</sup>. The densest
nebula <a href="http://www.astronomy.ohio-state.edu/~ryden/ast162_3/notes11.html">contains about 10,000 molecules per cm<sup>3</sup></a>.
What makes the shockwave visible are gamma photons accompanying the breakout
flash, speeding out of the stars collapsing core and out through the star's
surface (traveling at light-speed). By the time one of Earth's telescopes "sees"
the distant EM flash, very short wavelength (invisible) gamma photons <a href="http://astronomy.swin.edu.au/cosmos/c/cosmological+redshift">have stretched</a> into intense visible light.</span><o:p></o:p></span></i></div>
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<span lang="EN-US">Why and when does a nebula collapse into a
star? A gas/dust cloud exists in a delicate state of <a href="https://en.wikipedia.org/wiki/Hydrostatic_equilibrium">hydrostatic equilibrium</a>.
It is balanced between two opposing forces: Internal (outward) pressure is
exerted <a href="https://en.wikipedia.org/wiki/Kinetic_theory_of_gases#Temperature_and_kinetic_energy">by the thermal (colliding) motion of the molecules and atoms themselves</a> and by the <a href="https://en.wikipedia.org/wiki/Magnetic_pressure">interactions between (repulsive) magnetic fields</a>.
Ionized gases in the cloud are charged objects. As they move about randomly in
the cloud they create magnetic fields, which exert outward pressure. Meanwhile,
the cloud particles experience the attractive (inward) force of gravity between
them. A balance between these forces is reached, the cloud remains stable, until an outside force applied
to the cloud disturbs it. Even a relatively mild disturbance from a
gravitational collapse into a star nearby <a href="https://en.wikipedia.org/wiki/Jeans_instability">can trigger a local collapse</a>.
Regions of dust and gas can also simply collapse under their own increasing
density. A typical nebular cloud can be a very productive birthing centre.
Averaging about 100 light-years across and containing up to 6 million solar
masses (M) of matter, a single nebula can give birth to millions of new stars.<o:p></o:p></span></div>
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<span lang="EN-US">A typical nebular cloud consists mostly of
hydrogen. Depending on when and where a nebula exists, it will also contain
traces of other larger atoms, molecules and different ionized gases as well,
blown in from past supernovae. All the material of our solar system originally
collapsed from a nebular nursery cloud much like the Eagle Nebula. <o:p></o:p></span></div>
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<span lang="EN-US">The parts of a nebula that are active star
nurseries tend to be dense cold darkly opaque molecular clouds. In these
regions, hydrogen exists as an H<sub>2</sub> molecule rather than as an ionized
atomic gas (a proton). Here, thermal (outward) pressure exerted by the cold hydrogen
is minimal, which gives gravitational pressure an edge. It makes local
collapses into denser regions possible. Gas and dust collapse inward to an
increasingly dense local core that eventually evolves into a dense spinning mass
called a <a href="https://en.wikipedia.org/wiki/Protostar">protostar</a>.
It spins because all the <a href="https://en.wikipedia.org/wiki/Angular_momentum">angular momenta</a> of the particles are conserved.<o:p></o:p></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjot-HcfZPJhfUHXyi49rDATKbR3XYEp7FfXHcfO99iu5kV9YcfLeZWCvy533HgUBS_UacbgU-RSy954i8eo-iwDBQztehMQFCKZK-KmdetCFo_43uKMVdRTGt9hyphenhyphen7lqw1ERHUWMTBWSS2o/s1600/Witness_the_Birth_of_a_Star.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="300" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjot-HcfZPJhfUHXyi49rDATKbR3XYEp7FfXHcfO99iu5kV9YcfLeZWCvy533HgUBS_UacbgU-RSy954i8eo-iwDBQztehMQFCKZK-KmdetCFo_43uKMVdRTGt9hyphenhyphen7lqw1ERHUWMTBWSS2o/s400/Witness_the_Birth_of_a_Star.jpg" width="400" /></a></div>
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<span lang="EN-US">A typical protostar forming within a dense
molecular cloud is illustrated left. Two bipolar stellar jets of material are
likely the result of interactions between powerful magnetic fields wrapping
around the forming star. They spin the material round and round and eject it
from the star's magnetic poles.<o:p></o:p></span></div>
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<span lang="EN-US">In 2004, NASA <a href="http://www.nasa.gov/topics/universe/features/xray-flaunt.html">directly glimpsed a protostar (V1647) for the first time</a>. Still swathed in its birth blanket of gas and dust, it was actively
accreting mass, a process animated in this brief video clip.<o:p></o:p></span></div>
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<div class="MsoNormal">
<span lang="EN-US"><br /></span>
<span lang="EN-US">The protostar continues to accrete gas and
dust from the surrounding cloud for millions of years until it has enough mass
to evolve into a <a href="https://en.wikipedia.org/wiki/Pre-main-sequence_star">pre-main-sequence star</a>.
When it reaches this point it has gained its final mass. The star's mass will
determine how it will then evolve over millions to trillions of years to come, as
a <a href="https://en.wikipedia.org/wiki/Main_sequence">main-sequence</a> star. It will also determine whether or not it will eventually explode as a
supernova. The larger the initial mass of dense gas and dust, the larger the
star will be. Because stars vary widely in mass, what defines the birth of a main-sequence
star is its core temperature rather than its final mass. Fusion is a very
temperature-sensitive reaction. When the core is hot enough to trigger the
fusion of hydrogen into helium, the star proper is born and it begins to shine
brightly. More massive stars fuse hydrogen at a higher rate than smaller stars
do. They go through their fuel faster, which means they don't last as long. The
most massive stars last for just a few million years while the smallest ones
last for trillions of years.<o:p></o:p></span><br />
<span lang="EN-US"><br /></span>
<span lang="EN-US">Next, we will start with the life cycles of the least massive stars - brown dwarf stars, red dwarf stars and small to mid-sized stars like our Sun - and see how they end up. Will our Sun explode into a supernova one day?</span></div>
Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-3517308015923372602016-09-09T11:35:00.004-06:002016-09-09T11:44:18.313-06:00Hello? Earth Calling . . . PART 6<div class="MsoNormal">
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 1</b> <a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-1.html">CLICK HERE</a></span><br />
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 2</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-2.html">CLICK HERE</a><br />
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 3</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-3.html">CLICK HERE</a><br />
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 4</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earthcalling-part-4.html">CLICK HERE</a><br />
<span lang="EN-US"><span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 5</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-5.html">CLICK HERE</a> </span><br />
<span lang="EN-US"><br /></span><span lang="EN-US">(This is the final post in this series.)</span><br />
<span lang="EN-US"><br /></span>
<span lang="EN-US">What's Next in Exoplanet Hunting?<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">We might never be able to set foot on an
exoplanet ourselves but don't let that get you down. While sci-fi fantasy is
always tantalizing, our scientific reality is evolving so fast that I think we
can soon reach the real answer to our most basic question, "Are we
alone?" We might soon be able to "see" an exoplanet directly,
and learn much about it. NASA's <a href="https://en.wikipedia.org/wiki/James_Webb_Space_Telescope">James Webb Telescope</a> (see below) is set to launch in 2018. <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgDC30Bahl6Vg8X7CzIvNduamoFeqJZ7Tdpc83vR9vSK6COTGwaivdWVMXAI-kBB4d-qifprGdy3vSG_auy8tnz7BtBIrOJfjQckAPXTUIdC7Rtesr0VzA_ybyphkaUb8iGK7H2C_G6E6C-/s1600/James_Webb_Space_Telescope_2009_top.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="512" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgDC30Bahl6Vg8X7CzIvNduamoFeqJZ7Tdpc83vR9vSK6COTGwaivdWVMXAI-kBB4d-qifprGdy3vSG_auy8tnz7BtBIrOJfjQckAPXTUIdC7Rtesr0VzA_ybyphkaUb8iGK7H2C_G6E6C-/s640/James_Webb_Space_Telescope_2009_top.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Illustration of the James Webb Space Telescope, as of 2009. Launch date is expected to be 2018. Credit NASA</span></td></tr>
</tbody></table>
<div class="MsoNormal">
<span lang="EN-US">Its main mission is to study the formation
of distant stars and galaxies but it is also designed to directly observe exoplanets
and study their atmospheres for biosignatures of life. It will be super-stable
with optical components that distort the image less than a nanometer, about the
size of a few atoms. It will be equipped with a honeycomb-like multi-piece mirror
that is about four times larger than Hubble's mirror (Hubble Telescope is shown below).<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj7NBN9A5A4KLTcDEtyS_QCI46r0EVAPS2yMFjU-YXzgbV853tlUr0sSqrVkXZGuKQ5DYZRaumyA0U8-_7HCMWkovTJX72jtU8XONkzBjBk0DZUAQeadoLqSA10MSQ031-s5EeWxT5xczhJ/s1600/HST-SM4.jpeg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj7NBN9A5A4KLTcDEtyS_QCI46r0EVAPS2yMFjU-YXzgbV853tlUr0sSqrVkXZGuKQ5DYZRaumyA0U8-_7HCMWkovTJX72jtU8XONkzBjBk0DZUAQeadoLqSA10MSQ031-s5EeWxT5xczhJ/s640/HST-SM4.jpeg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Hubble Space Telescope departing from the space shuttle Atlantis in 1990. It is still in operation. Credit NASA</span></td></tr>
</tbody></table>
<div class="MsoNormal">
<span lang="EN-US">James Webb telescope will observe in
near-infrared and no doubt it will deliver good exoplanet data. But its development began in 2002, and since then exoplanet
astronomy has literally exploded. New technologies specific to viewing exoplanets is what we will need very soon. The history of exoplanet exploration, though short, is interesting. It started out with much speculation and many
false discoveries. The first exoplanet (<a href="http://www.openexoplanetcatalogue.com/planet/Gamma%20Cephei%20b/">gamma Cephei b</a>)
was <a href="http://adsabs.harvard.edu/doi/10.1086/166608">detected in 1988</a> by Canadian
astronomers Bruce Campbell, G.A.H Walker and Stephenson Yang at the University
of Victoria. This planet, a huge gas giant <a href="http://planet.wikia.com/wiki/Gamma_Cephei_Ab">1.6 times larger than Jupiter</a>, was at the limit of detection then so the discoverers and other astronomers
remained skeptical of it for years (solid confirmation of it didn't come until
2002). Some astronomers consider this team to be the true pioneers of exoplanet
exploration but if you look you will notice that it is downplayed even on
Wikipedia (and fair enough - they did retract their discovery). The story of their discovery, well told in <a href="http://www.theglobeandmail.com/technology/science/lost-world-how-canada-missed-its-moment-of-glory/article4290133/?page=all">this Globe and Mail article</a>, is an interesting one full of self-doubt and heartbreak. I think it is
evidence that we Canadians need to toot our own horn a bit more. <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">After that other confirmations of
exoplanets trickled in. As astronomers began to turn their attention to
exoplanets, indirect detection methods improved, and the floodgates opened. Now
garnering much interest from many scientific fields and excitement from
everyone around the world, scientists and engineers are already at work on
ambitious next-generation space telescopes such as (NASA's) <a href="http://www.space.com/31778-nasa-next-great-space-telescope.html">HabEx</a>.
These telescopes will be devoted to direct exoplanet imaging. They aim to
achieve an optical stability in the picometer range, less than the diameter of
an atom. They also plan to use even larger mirrors and observe across the
visual, near-infrared and ultraviolet spectra, which will offer even more detailed
exoplanet surface/atmospheric data. It might take up to 30 years before one of these
telescopes is a reality, however. A mission needs to be finalized and the
technology required has to be developed first.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">HabEx (<a href="http://www.jpl.nasa.gov/habex/">Habitable Exoplanet Imaging Mission</a>) would allow us to directly observe
exoplanets within about a 30 light-year radius from Earth. Another NASA mission
just initiated and in concept stage is the <a href="http://asd.gsfc.nasa.gov/luvoir/">Large Ultraviolet/Optical/Infrared(Luvoir) Surveyor</a>, which is an even more
ambitious telescope with a larger mirror. Either mission will provide
groundbreaking new data by directly imaging exoplanets with incredible
precision across different spectra for the first time. Direct imaging, through
the James Webb telescope coming soon, and then hopefully to be followed up with
one of these next-generation telescopes, is the only way to really know if a
planet is potentially habitable. The light absorbed by its atmosphere must be thoroughly
analyzed to know what it is composed of, what the climate and weather are like,
whether the surface is protected from radiation and temperature extremes, and
whether liquid water is likely to be present. Besides determining whether a
planet is potentially habitable, these missions have the ultimate goal of
discovering chemical evidence for extraterrestrial life.<o:p></o:p></span></div>
<div class="MsoNormal">
<div class="MsoNormal">
<span lang="EN-US"><br /></span>
<span lang="EN-US">Still in conceptual stage, HabEx will be a
large space telescope that will focus first on planets that are the right
distance from their stars to have liquid surface water. I suspect Proxima b will
be at the top of the list. The ability to directly image this tiny faint distant
planet in superb detail will be amazing.</span></div>
<span lang="EN-US"><br /></span><span lang="EN-US">Even with this most advanced technology, astronomers won't be able to see a round defined planet image. Instead they
will see an image that is less than one pixel. That might not sound impressive
but the information in that pixel is astounding. That tiny dot can be analyzed
as an absorption spectrum across various wavelengths in the visible,
near-infrared and UV spectra. This EM radiation is starlight that passes
through the exoplanet's atmosphere twice. It is absorbed by the atmosphere and
then reflected from the planet back out into space and to the telescope. The
absorption bands in the spectra will tell the astronomers which elements are
present in the exoplanet's atmosphere and possibly on its surface as well.
Those signatures will give clues to the planet's surface temperature and
pressure, overall habitability, and even signs of life that might be present if
atmospheric oxygen, ozone or methane is found. <o:p></o:p></span>
<br />
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">The real challenge for this technology will
not be trying to capture the planet's faint image. The <a href="http://hubblesite.org/">Hubble Space Telescope</a> can capture images even fainter. The problem is that planets are always right
next to very bright stars. As astronomer Scott Gaudi explains, there is a
common analogy used to explain how much brighter stars are than their planets:
its like trying to see a firefly against a searchlight, except the firefly is a
thousand times fainter. The James Webb telescope will be outfitted with
<a href="https://en.wikipedia.org/wiki/Coronagraph">coronagraphs</a> attached to it. The coronagraph is an established technology that has been used
since the 1930's to block out the Sun's central disk so that we can view its
coronasphere. A coronagraph works but not perfectly. HAbEx will go one step
further by using two technologies. It <a href="http://www.universetoday.com/128664/starshade-prepares-image-new-earths/">will have a star shade as well</a>, which is unfurled in front of the telescope and which must be
perfectly matched to the star and the telescope. Watch <a href="http://www.jpl.nasa.gov/video/details.php?id=1284">this NASA/JPL video</a> of how this giant sun-flower shaped shade will unfurl in space.</span><br />
<span lang="EN-US"><br /></span>
<span lang="EN-US">This star shade will be a new technology.
Coronagraphs will be used while the telescope scans for suitable planets and
the star shade could be unfurled for much better starlight blockage and more
in-depth imaging once a candidate is found. HabEx (or the Luvoir Surveyor) could
be online within a few decades. When it is, we will have not just a growing
list of established exoplanets to mull over but we will obtain unique descriptions
of them as well, something to really pique our imaginations! It will be very
interesting to be able to directly image Proxima b, for example, and see what
kind of atmosphere, if any, this rocky planet has. The question of whether it
has liquid surface water might be answered as well. Meanwhile, there is a tantalizing chance that we might not have to wait decades to know this answer. Astronomers speculate that
there is a <a href="http://www.torontosun.com/2016/08/24/new-planet-discovered-near-solar-system">1.5 % chance that Proxima b transits in front of its star</a> from Earth's perspective. If it
does, we might have more information about it even sooner. Astronomers might be
able to discern some data about its atmosphere by comparing chemical analysis
of the absorption spectrum of Proxima Centauri during transit and when the
planet is away from the star.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">If and when HabEx, or a similar telescope, offers
promising data that doesn't rule out carbon-based life on Proxima b or another
nearby Earth-like planet, I suspect projects like StarChip will follow, sending
a probe there to investigate further. All of this is still decades away but it
is tantalizingly feasible, and we should then have our best chance yet to find solid
evidence for extraterrestrial life in our universe.<o:p></o:p></span></div>
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Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-90259636084162346202016-09-07T14:38:00.000-06:002016-09-09T11:04:24.392-06:00Hello? Earth Calling . . . PART 5<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 1</b> <a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-1.html">CLICK HERE</a></span><br />
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 2</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-2.html">CLICK HERE</a><br />
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 3</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-3.html">CLICK HERE</a><br />
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 4</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earthcalling-part-4.html">CLICK HERE</a><br />
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<span lang="EN-US">The Appearance of The First Cell<o:p></o:p></span></div>
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<span lang="EN-US">It is not too far of a theoretical jump from the protein/RNA co-evolution explored in the previous article to a circular self-replicating
single-stranded RNA sequence a few hundred nucleotides long. This is, in
essence, a viroid in our modern ecosystem. But it is not a cell and according to most experts it is not alive. Life makes its indisputable appearance when the first cells appear on Earth. Biochemical activity is now confined and protected from the outside elements by a membrane and/or cell wall. The first simple cells would have evolved in a world where viroids (open genetic material) and virus-like entities (genetic material enclosed in a protein coat) exchanged genetic material between themselves and between the first simple cells to evolve, such as <a href="https://en.wikipedia.org/wiki/Archaea">archaea</a> and <a href="https://en.wikipedia.org/wiki/Bacteria">bacteria</a>. This process is called <a href="http://rspb.royalsocietypublishing.org/content/277/1683/819">lateral gene transfer</a> and it could account for the acquisition of new biochemical pathways in microbes. The opportunity to acquire various new defensive chemical arsenals might also have allowed these first simple cells to survive the rapidly changing harsh conditions prevalent on our young planet at the time.</span></div>
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<span lang="EN-US">The first simple cell membrane <a href="https://en.wikipedia.org/wiki/Protocell">might have been a simple bilayer phospholipid vesicle</a>, a hollow spherical shell structure. Chemical pathways responsible for the pre-biotic formation of phospholipids are <a href="http://www.ncbi.nlm.nih.gov/pubmed/27043635">fairly well understood</a>. When <a href="https://en.wikipedia.org/wiki/Phospholipid">phospholipids</a> (which have hydrophobic or water-hating tails) are placed in water they spontaneously form vesicles where the tails face inward. The gradual
evolution of a more complex <a href="https://en.wikipedia.org/wiki/Cell_membrane">cell membrane</a> (equipped with <a href="http://www.ncbi.nlm.nih.gov/books/NBK21140/">channels and cell pumps</a>) and, for some cells, an even more protective <a href="https://en.wikipedia.org/wiki/Cell_wall">cell wall</a> would follow. As archaea and bacteria make their first appearance, <a href="https://simple.wikipedia.org/wiki/Origin_of_life#Current_models">Benal's third stage of evolution</a><a href="https://simple.wikipedia.org/wiki/Origin_of_life#Current_models"> toward life </a>- from complex biomolecules, like proteins and RNA, to cells - would be achieved. Modern
archaea micro-organisms such as the simple unicellular organisms that <a href="https://en.wikipedia.org/wiki/Thermophile">live around hydrothermal vents</a> and provide part of the food chain base there, might
resemble what the first forms of life on Earth looked like. Archaea have the
simplest life plan on this planet. They look like bacteria but they are
biochemically very different. The chemistry of their bilayer phospholipid
membranes is unique. It contains ether bonds that are more chemically resistant
and heat-stable than those in either bacterial or eukaryotic cells. <a href="https://en.wikipedia.org/wiki/Eukaryote">Eukaryotic cells</a> are the kind of cells that make us up – with the exception of the extensive <a href="https://en.wikipedia.org/wiki/Gut_flora">microbial flora in our guts</a>, which consist of
bacteria, fungi and archaea. Our relationship with unicellular life is even
more intimate and intermingled than hosting microbes in our guts. Extensive evidence suggests that genetic vestiges of
ancient unicellular microbes are present in each and every eukaryotic cell in
our bodies as well. Our cells contain a complex mosaic of genetic material that was obtained by genetic exchanges between ancient eukaryotic micro-organisms, bacteria and archaea. In addition to lateral gene transfer, ancient cells also likely went one step further by simply engulfing other cells and eventually utilizing their unique cellular machinery as <a href="https://en.wikipedia.org/wiki/Organelle">organelles</a>. This evolutionary process, <a href="http://www.biology-pages.info/E/Endosymbiosis.html">called endosymbiosis</a>, may be responsible for the appearance of the first (organelle-containing) eukaryotic micro-organism. <a href="https://en.wikipedia.org/wiki/Symbiogenesis">Symbiogenesis</a> is the theory that various organelles inside our eukaryotic cells originated from
symbiotic (cooperative) relationships between different strains of ancient
archaea and bacteria. There is strong evidence that mitochondria,
the "powerhouses" of our cells where ATP is produced, <a href="https://en.wikipedia.org/wiki/Mitochondrion#Origin_and_evolution">are of bacterial origin</a>. Those ancient bacteria were likely engulfed and incorporated into a eukaryotic predecessor.<o:p></o:p></span></div>
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<span lang="EN-US">Tough Intrepid Archaea<o:p></o:p></span></div>
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<span lang="EN-US"><a href="https://en.wikipedia.org/wiki/Archaea">Archaea</a> are especially interesting from a
life origin point of view because they are the most likely candidates to handle
the extreme conditions on our young planet. Tough ether membrane bonds explain
why many archaea are extremophiles, able to live in environments far too harsh
for other organisms. Most archaea also possess a unique protein cell wall,
which makes them even tougher and which further differentiates them from
bacteria. Archaea possess genes and metabolic pathways that closely resemble
those of eukaryotes (again suggesting that eukaryotes borrowed these useful
traits from archaea) but, unlike eukaryotes and like bacteria, archaea don't
have any internal structure such as organelles. A single circular strand of DNA
and a few independent DNA pieces called plasmids float inside an amorphous
cytoplasm. DNA transfer between cells is common and viruses can infect them as well.
These sources of new DNA promote rapid evolution in times of hardship, and make
ancient symbiotic relationships easy to visualize. The wide variety of chemical
reactions that take place inside these tiny cells is really what sets them
apart and this is what ultimately made them so wildly successful, allowing them
to inhabit virtually every possible location on Earth and is what allowed this <a href="https://en.wikipedia.org/wiki/Domain_(biology)">life domain</a> to exist longer than any known living organism. This chemical variety also enables archaea to utilize many
different sources of energy. This makes them a prime candidate to look for on
other planets and moons where a carbon-based biochemistry could also evolve.<o:p></o:p></span></div>
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<span lang="EN-US">Archaea's unique biochemistry suggests that
these organisms evolved independently from bacteria, even though they share the
same basic genetic structure – a single circular strand of DNA and possibly
plasmids as well. The shared structure of circular DNA means that the
origin of DNA probably predates the separation of archaea and bacteria into two significantly
different evolutionary domains. <a href="https://en.wikipedia.org/wiki/Archaea#Origin_and_evolution">Chemical fossils of archaea's unique lipids</a> were found in some of Earth's oldest known
sedimentary rock in Greenland, which is dated to 3.8 billion years old. This supports
increasing evidence that archaea was Earth's first living organism. Archaea
might also be responsible for 4.1 billion year old carbon isotope chemical
fossils indicating a life process, mentioned in a previous article in this series. However, this is evidence only for carbon-based life, not for any specific life domain.<o:p></o:p></span></div>
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<br /></div>
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<span lang="EN-US">Archaea, Bacteria and Eukaryotes: A Rich
Tapestry of Earth Biochemistry<o:p></o:p></span></div>
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<br /></div>
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<span lang="EN-US">While archaea stands out as being the best
candidate for surviving deep underground when Earth's surface was far too hot
and violent for life, aggregates of both <a href="https://en.wikipedia.org/wiki/Archaea#Genetics">modern archaea</a> and <a href="https://en.wikipedia.org/wiki/Archaea#Genetics">modern bacteria</a> behave in additional ways that make them both ultimate survivors. They can transfer
genes laterally among one another and they can undergo <a href="https://en.wikipedia.org/wiki/Genetic_recombination">recombination</a> (gene
mixing) at rates far higher than more complex eukaryotic unicellular organisms
can. One can guess, with so much genetic variation available, that these
organisms could adapt remarkably well and quickly to the dramatically
ever-changing conditions on early Earth, especially on the surface. As
conditions moderated over millions of years, variants that could utilize the
Sun's ultraviolet light for energy evolved, leading eventually to the first
simple photosynthetic biochemical pathways, <a href="http://www.ucmp.berkeley.edu/bacteria/cyanolh.html">probably in cyanobacteria</a>.
Oxygen, the waste gas of photosynthesis, oxidized iron in rock and was absorbed
by organic material. Eventually, it built up in the atmosphere. Toxic to
<a href="https://en.wikipedia.org/wiki/Anaerobic_organism">anaerobes</a> (which
includes many archaea and bacteria), <a href="https://en.wikipedia.org/wiki/Great_Oxygenation_Event">atmospheric oxyge</a><a href="https://en.wikipedia.org/wiki/Great_Oxygenation_Event">nation</a> not only kicked off one
of Earth's most significant extinction events, it reacted with atmospheric
methane, a potent greenhouse gas, triggering the <a href="https://en.wikipedia.org/wiki/Huronian_glaciation">longest global glaciation period in Earth's history</a>.
Despite the catastrophe, life persisted and <a href="https://en.wikipedia.org/wiki/Cellular_respiration#Aerobic_respiration">aerobic</a> organisms (those that
require oxygen to live) evolved. Oxygen made it energetically possible for complex multicellular highly mobile
organisms such as us to evolve. The <a href="https://en.wikipedia.org/wiki/Electrochemical_gradient">electrochemical transport chain</a> of <a href="https://en.wikipedia.org/wiki/Cellular_respiration">cellular espiration</a> in our cells uses oxygen to metabolize molecules such as
high-energy sugars, a process which yields more energy than <a href="https://en.wikipedia.org/wiki/Fermentation">fermentation</a> or anaerobic respiration. The downside of oxygen-based metabolism is the
<a href="https://en.wikipedia.org/wiki/Oxidative_stress">oxidative stress</a> placed on cells. Oxygen is a very reactive molecule so peroxides and free
radicals, which damage proteins and DNA, build up in cells. Cells have evolved
various defense mechanisms to eliminate the destructive molecules and DNA and
proteins are constantly repaired, at some metabolic cost (this is one reason why
we age and die).<o:p></o:p></span></div>
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<span lang="EN-US">Obtaining Energy: Survivors Versus
Specialists <o:p></o:p></span></div>
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<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Earth's biosphere boasts three different
methods for carbon-based organisms to obtain energy (cellular respiration), a
key and universal requirement of life: anaerobic respiration, fermentation and
aerobic respiration. Each has its own advantages and disadvantages, and all
three are required for complex organisms like us to survive. Many unicellular
fungi (yeasts) and bacteria utilize the simple process of fermentation to
obtain energy. There is no complex electrochemical gradient involved. The
simplest reactions turn sugars into alcohols. The production of bread, beer,
wine and cheese all require fermentation. <a href="https://en.wikipedia.org/wiki/Ruminant">Ruminants</a> such as cattle, goats and deer have evolved long guts full of bacteria optimized
to ferment the otherwise indigestible cellulose in grasses, bark and twigs.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Fermentation also functions as a "plan
B" in the metabolism of some of our mammalian tissues. For example, our
muscle cells <a href="https://en.wikipedia.org/wiki/Lactic_acid_fermentation">turn to fermentation</a> when they are not getting enough oxygen to function, as during a long strenuous
workout when glucose stored in the muscle cells is used up. Fermentation produces
lactic acid as a cellular waste product and that makes our muscles feel sore
and stiff afterward. Our bodies are specialized for optimal performance over a
narrow range of conditions, such as temperature, food, and the right mixture of
gases to breathe. This energy efficiency has allowed our large curious
energy-draining brains to evolve. Microbes such as archaea trade efficiency for
survivability under great and unpredictable environmental stresses. For example, bacteria and archaea survived for millions of years tucked away in areas devoid of oxygen while
Earth's surface remained frozen solid. These organisms, though tough and
versatile, have a much less efficient electrochemical gradient than
oxygen-using aerobes. In the anaerobic electrochemical transport chain, less
oxidizing substances such as sulphates, nitrates and sulphur are used instead
of oxygen. Less oxidation = less available metabolic energy. On a very stable planetary environment complex organisms like us could
excel but in unstable conditions, microbes will likely win the life game. These
differences on Earth offer clues to what kinds of life we could expect to
detect on various exoplanets based on their geology and climate.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Unique Geological History = Unique
Planetary Biosphere <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Simple unicellular organisms, though not
winners in the energy game, are winners at long-term survival. Dwelling in soil
and under water, they make life for multicellular organisms such as us
possible. They are key drivers of the carbon and nitrogen cycles, and they
break down dead organic matter and remove heavy metals from solution in water.
Life evolved under a great variety of environmental pressures, creating a great
range of biochemical adaptations. This rich variety is what our complex modern
biosphere is based upon. How likely is it that such a variety of unicellular
life evolved under harsh and rapidly changing conditions on another world? A
planet's unique biochemical variety might depend on the changing conditions in
which it evolved geologically. Is a wide variety of unicellular life necessary
for more complex multicellular life, intelligent technology-bearing life like
us, to evolve? How many worlds have life that is restricted to a simple palette
of a few different but tough unicellular plans? <o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Unicellular Life Might Be Plentiful In the
Universe<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Although knowledge about our evolutionary
progression from pre-life chemistry to simple unicellular life is not yet
seamless, research in many areas is beginning to fit enough pieces on the table
to glimpse what life's beginnings might look like. Any of the three
carbon-based biochemical energy pathways described above (and more) could
evolve on other worlds if a variety of organic molecules are present along with
liquid water and available energy. By looking closely at the evolution of
archaea and bacteria on Earth, we get the sense that at least simple
unicellular carbon-based life could evolve even in very different and very
hostile environments.</span></div>
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<div class="MsoNormal">
<span lang="EN-US">The origin of proteins, RNA and DNA
explored in the previous article in this series does not mean that other completely unique kinds of biochemistry
couldn't develop into possibly very complex living organisms on other planets. It
only means that Earth's general biochemistry is the one that won out over time
here. Exotic biochemistries might exist on exoplanets, perhaps under
temperature or pressure extremes not encountered on Earth, utilizing biological
solvents other than water and deriving energy from a star unlike the Sun. However, based
on what we know about Earth's history, archaea-like carbon-based life, and life
evolved from archaea-like ancestors seems to be a good bet, at least on planets with
liquid water. One can argue that an archaea-like <a href="https://en.wikipedia.org/wiki/Last_universal_ancestor">last universal ancestor</a> evolved into our unique complex
multicellular life as a response to Earth's unique geological evolution. Who
knows how a similar unicellular ancestor might evolve on an exoplanet where
geological evolution veered off in another direction?<o:p></o:p></span></div>
<div class="MsoNormal">
<span lang="EN-US"><br /></span></div>
<div class="MsoNormal">
<span lang="EN-US">We've explored the past - how life came about on our once young and very violent planet. Next we will look at what the future holds. Scientists are working on sophisticated technologies that will get us a closer and more intimate look at the exoplanets we are discovering on almost a daily basis.</span></div>
Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-50534820872266636552016-09-06T13:18:00.000-06:002016-09-07T10:41:50.128-06:00Hello? Earth Calling . . . PART 4<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 1</b> <a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-1.html">CLICK HERE</a></span><br />
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 2</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-2.html">CLICK HERE</a><br />
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 3</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-3.html">CLICK HERE</a><br />
<br />
How did Life Arise On Earth?<br />
<br />
No one knows exactly how life arose from non-living matter on Earth, but many researchers focus on the kinds of chemical reactions that could have given rise to the first living system. This is the field of <a href="https://en.wikipedia.org/wiki/Abiogenesis">abiogenesis</a>. It is a multidisciplinary study that relies heavily on geology, chemistry and biology.<br />
<br />
It is now well known that the starting material for life on Earth consisted of complex <a href="https://en.wikipedia.org/wiki/Organic_compound">organic molecules</a>. These are molecules that contain carbon, and they are not just found on Earth. They are present throughout our solar system and even in interstellar space. There, many complex molecules are formed on the surfaces and inside the water ice mantles of dust grains in the star-forming cores of giant gas clouds. Simple molecules like hydrogen and carbon monoxide are <a href="http://www.astro.cornell.edu/~rgarrod/research/complex-organic-molecules/">excited under gravitational pressure as they are bombarded with ultraviolet (UV) radiation</a>. In this environment they break apart into reactive radicals. These charged molecular fragments reform over and over, gradually building new increasingly complex molecules. A <a href="http://www.bbc.com/news/science-environment-29368984">tantalizing recent discovery</a> was the detection of iso-propyl cyanide in a star-forming cloud 27,000 light years from Earth. <a href="https://en.wikipedia.org/wiki/Isopropyl_cyanide">Iso-propyl cyanide</a> (C<sub>4</sub>H<sub>7</sub>N; shown below left as a simplified molecular diagram) is a fairly complex organic molecule composed of nitrogen, carbon and hydrogen and, like proteins, it has a branched carbon backbone.<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi5hAlSoiRObJBo1G5EmHAfDEcckKnhK-AWh7KpF75uVk3Qqch1VTa9XQi667V22kCpaoRSB6caJwxK-goj7smwM8NaDoKJVq3cY7bng7vTW8rWGaB_d9uzdrET04Pbvvc44iWWagGBgwgt/s1600/Isobutyronitrile.svg.png" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="200" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi5hAlSoiRObJBo1G5EmHAfDEcckKnhK-AWh7KpF75uVk3Qqch1VTa9XQi667V22kCpaoRSB6caJwxK-goj7smwM8NaDoKJVq3cY7bng7vTW8rWGaB_d9uzdrET04Pbvvc44iWWagGBgwgt/s200/Isobutyronitrile.svg.png" width="197" /></a></div>
Perhaps most intriguing is its carbon-nitrogen triple bond (triple lines, left), one of the most abundant bonds in biochemistry. This high-energy chemical bond participates in amino acid synthesis (amino acids bond together to form proteins). Its presence along with various other organics already detected in interstellar space suggests that life's potential building blocks are widespread in our galaxy. <a href="http://www.space.com/15089-life-building-blocks-young-sun-dust.html">Computer models suggest</a> that complex organic molecules also formed in our Sun's protoplanetary disc, a dense cloud of cosmic dust that would later form the Sun, Earth and all the other planets, moons and asteriods.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFOh18XYH62tBa-CmQxsfUOke2zzOS3fSaWeBcy1QqtT_kjBJIOt_CSCT7LGQgiGegMbLahZbMMzNpc4iijw6lvi2cNv1Q4NpLKVWPxts6Usyfod6z6JT6OMZ94Q_a47APsY8ZDwbC9Fjj/s1600/Artist%25E2%2580%2599s_Impression_of_a_Baby_Star_Still_Surrounded_by_a_Protoplanetary_Disc.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="426" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFOh18XYH62tBa-CmQxsfUOke2zzOS3fSaWeBcy1QqtT_kjBJIOt_CSCT7LGQgiGegMbLahZbMMzNpc4iijw6lvi2cNv1Q4NpLKVWPxts6Usyfod6z6JT6OMZ94Q_a47APsY8ZDwbC9Fjj/s640/Artist%25E2%2580%2599s_Impression_of_a_Baby_Star_Still_Surrounded_by_a_Protoplanetary_Disc.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">European Space Agency;Wikipedia</span></td></tr>
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Like other star-forming clouds, UV photons from the forming Sun would have broken down molecular bonds in the protoplanetary dust and allowed those short simple fragments to recombine into more complex molecules. Earth, however, formed as a molten ball of material so hot that most of the chemical bonds in those complex organic molecules would have been destroyed, leaving behind much simpler molecules like hydrogen, water and carbon monoxide once again.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgFI71YZzXPruzaneNVt4pGHQccgiwDKfOEBWbYgpcRGDyVtKZk3TXATExZykfZ0DELNiMmwUhBBv0EQMBN75hbe4NDAB4Dj_-IRzPDR-EcDT6vKmW7_nWRJdRANeyFSTEKTVTARPMiwS5e/s1600/Hadean.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="344" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgFI71YZzXPruzaneNVt4pGHQccgiwDKfOEBWbYgpcRGDyVtKZk3TXATExZykfZ0DELNiMmwUhBBv0EQMBN75hbe4NDAB4Dj_-IRzPDR-EcDT6vKmW7_nWRJdRANeyFSTEKTVTARPMiwS5e/s640/Hadean.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Tim Bertelink;Wikipedia</span></td></tr>
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Once Earth cooled even slightly, in the presence of energy and water, the processes of increasingly complex organic chemistry would once again be underway. At the same time, as much as 40,000 tons of <a href="https://en.wikipedia.org/wiki/Cosmic_dust">cosmic dust</a> (which consists of both solar and interstellar molecules) rained down on the surface of Earth every year, offering its supply of complex organic building blocks.<br />
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Besides active complex organic prebiotic chemistry on Earth, other prerequisites scientists think are essential to life were present as well, including abundant water, terrestrial life's solvent. When Earth was just 500 million years old, it already contained oceans. Where the water originated remains somewhat mysterious but the picture is becoming clearer. Some researchers believed that comet impacts were a major contributor, but isotope analysis <a href="https://en.wikipedia.org/wiki/Origin_of_water_on_Earth">argues against this</a>. Newer data suggests that the protoplanetary material itself, full of water-rich carbonaceous chondrites that aggregated to form Earth, <a href="http://www.nature.com/news/common-source-for-earth-and-moon-water-1.12963">contained sufficient water to form oceans</a>. When the young Earth collided with another body and gave rise to the Moon, according to the <a href="https://en.wikipedia.org/wiki/Giant-impact_hypothesis">giant impact hypothesis</a>, the impact would have resulted in a primordial rock-water vapour atmosphere that would quickly condense into oceans.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgwiT2rax_-H6pWVufep2ZtUmX6edlaBJK_HPpmmlmLxZLiK1aMvtoi2-vcjgSNKJrZFeCG8XRW_aOrtPjcVE23sI5_x-FLCZNEoqmtdygSk40D0WgAhG1pS_v0aim8-sOUGztUI1L9lZ3m/s1600/Artist%2527s_concept_of_collision_at_HD_172555.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="512" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgwiT2rax_-H6pWVufep2ZtUmX6edlaBJK_HPpmmlmLxZLiK1aMvtoi2-vcjgSNKJrZFeCG8XRW_aOrtPjcVE23sI5_x-FLCZNEoqmtdygSk40D0WgAhG1pS_v0aim8-sOUGztUI1L9lZ3m/s640/Artist%2527s_concept_of_collision_at_HD_172555.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Artist's rendering of the collision between Earth and a Mars-sized planet that formed our Moon approximately 4.5 billion years ago. Credit to NASA</span></td></tr>
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There is evidence as well that those oceans <a href="http://www.pnas.org/content/98/7/3666.full">could have been as hot as 230°C</a> at first. The water condensed instead of evaporating away to be eventually lost to space because the pressure of the, then dense carbon-rich, atmosphere would have been too high to allow the water to boil.<br />
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Earth at this time would also have been constantly bombarded with asteroids, creating significant geological stress on the thin young crust and <a href="https://en.wikipedia.org/wiki/Hadean#Atmosphere_and_oceans">giving rise to intense volcanic activity</a>. This violent period gave Earth all the conditions required to form life. The dense volcanic atmosphere itself also supplied abundant organic raw material, a chemical makeup that is <a href="https://en.wikipedia.org/wiki/Abiogenesis#Early_geophysical_conditions">thought to resemble gases released by volcanoes today</a>, such as water vapour, carbon dioxide, sulphur, methane, nitrogen and hydrogen gas. With energy supplied by intense volcanic lightning and UV radiation from the Sun, chemically active organic compounds such as hydrogen cyanide, formaldehyde, acetylene and ammonia were created. These conditions have been recreated by the famous <a href="http://www.windows2universe.org/earth/Life/miller_urey.html">Miller-Urey experiment</a>, which established that even many highly complex amino acids, the building blocks of proteins, <a href="https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment#Chemistry_of_experiment">can be created from simple inorganic compounds</a> under those conditions.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjSdWdq_UcrXfF4gWbhIu0ITobMdjJMO0YZBy42Cy9PYYw42YJ7VwNcXfb_ensQ95OO_p586BSfi1xvFGSBe9tgoqWMYbjnR7KPfXhi4GIsOfGnlJifiBZfq-j_Wyy9OgmHNwrXuk6DL0ZP/s1600/Miller-Urey_experiment-en.svg.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="594" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjSdWdq_UcrXfF4gWbhIu0ITobMdjJMO0YZBy42Cy9PYYw42YJ7VwNcXfb_ensQ95OO_p586BSfi1xvFGSBe9tgoqWMYbjnR7KPfXhi4GIsOfGnlJifiBZfq-j_Wyy9OgmHNwrXuk6DL0ZP/s640/Miller-Urey_experiment-en.svg.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">General set-up of the Miller-Urey experiment. Credit to YassineMrabet;Wikipedia</span></td></tr>
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These conditions were present for just 500 million years before signatures of life appeared in the (chemical) fossil record, suggesting that when conditions are right, the evolution from abiotic organic chemistry to biochemistry happens very rapidly from a geological standpoint.<br />
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As life arose, the environment on Earth would be deadly to most organisms today, including us. There was no oxygen in the atmosphere. There may have been very little or no land above water and the water itself was probably still around 100°C (boiling hot) when cellular life made its appearance. The young Sun's radiation output was also different than it is today. Even though the Sun was fainter in the visual spectrum, young Earth <a href="https://homepage.univie.ac.at/manuel.guedel/papers/guedelkasting/guedelkasting.pdf">would have been subjected to more intense UV and X-ray radiation</a>. Earth did not have a protective ozone layer until approximately 600 million years ago when plant life began to colonize land and release significant oxygen into the atmosphere (ozone is created when UV radiation splits oxygen (O<sub>2</sub>) molecules high up in the atmosphere, which then reform into ozone (O<sub>3</sub>)).<br />
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Life that managed to arise under these harsh conditions was also <a href="https://en.wikipedia.org/wiki/Late_Heavy_Bombardment">regularly bombarded by asteroid impacts</a>. This was likely the result of gravitational instability caused by giant planets such as Jupiter and Saturn moving outward to their present orbits. A large meteor would strike the young Earth and cause all the surface water to vapourize. Computer models suggest that it would take up to 3000 years for the cloud deck to settle and rain back down into oceans. At first, life would not have a chance to develop but eventually, as asteroid impacts became fewer and less severe, large mats of unicellular <a href="http://science.howstuffworks.com/life/cellular-microscopic/extremophile.htm">extremophile</a>-like microbes <a href="https://en.wikipedia.org/wiki/Microbial_mat#Role_in_the_history_of_life">formed</a>. Eventually, <a href="http://www.ucmp.berkeley.edu/bacteria/cyanolh.html">cyanobacteria</a> colonized much of young Earth, oxygenating the oceans and atmosphere.<br />
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjFKaYo9DUpd_X2tsPgTRGfATp3M88ly5SqSaAAgkX0E1xLWjbrIpgvWvlG5BvqzTP3SpQd1wkqG0JjlehyVjhREU9fqt39d6M8slbHVMB7iDUZsJ4oNkAO05IkKmwiUgMxBEljnKhTVg45/s1600/1200px-Tolypothrix_%2528Cyanobacteria%2529.JPG" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjFKaYo9DUpd_X2tsPgTRGfATp3M88ly5SqSaAAgkX0E1xLWjbrIpgvWvlG5BvqzTP3SpQd1wkqG0JjlehyVjhREU9fqt39d6M8slbHVMB7iDUZsJ4oNkAO05IkKmwiUgMxBEljnKhTVg45/s320/1200px-Tolypothrix_%2528Cyanobacteria%2529.JPG" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Microscopic colonial filaments of <br />cyanobacteria; Mathewjparker;Wikipedia</span></td></tr>
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Until last year, the first life on Earth was thought to be cyanobacteria. Australian microfossils of strands of these unicellular organisms <a href="http://www.americanscientist.org/issues/pub/the-beginnings-of-life-on-earth/1">date back to about 3.5 billion years ago</a>, although there is now some dispute about the identity of these fossils. 3.7 billion year old rock in Greenland more recently <a href="http://www.cbc.ca/news/technology/oldest-fossils-greenland-1.3743115">revealed 1-4 cm tall humps in the rock</a>, later confirmed as fossilized communities of <a href="https://en.wikipedia.org/wiki/Stromatolite">stromatolites</a>. These structures are a bit like tiny apartment complexes for bacteria, especially cyanobacteria. Though which kind of microbe the stromatolites housed is unknown, they are proof that unicellular life lived as long ago as 3.7 billion years.<br />
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In <a href="http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html">even more ancient Australian rock</a>, chemical fossils <a href="http://www.pnas.org/content/112/47/14518.full">containing a specific mix of carbon isotopes unique to life were found</a>. These chemical traces of life are dated even further back to 4.1 billion years, a period when hot pressurized oceans were still forming and Earth was decidedly violent. Because it is a chemical fossil rather than a physical fossil, the type of organism that lived then is unknown. It might be the chemical footprint of an organism similar to bacteria, archaea or even of early eukaryotic life. It could be of an <a href="https://en.wikipedia.org/wiki/Archaea#Origin_and_evolution">ancestral life form common to all three domains</a> before they diverged evolutionarily.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjXrDO4jMRW9t05bkFZDMQtVrKYhn-hGlaQGBTd_hntFjTJ8-XjPxyl-NxN1oRVh0m79-q4JnyfJEudENjMXYfuTXfla29_BbjgBooDzDbSmeTX-E3qywsKEMcWm29O_bqqpO2d7weJwg18/s1600/1600px-Phylogenetic_tree.svg.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="432" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjXrDO4jMRW9t05bkFZDMQtVrKYhn-hGlaQGBTd_hntFjTJ8-XjPxyl-NxN1oRVh0m79-q4JnyfJEudENjMXYfuTXfla29_BbjgBooDzDbSmeTX-E3qywsKEMcWm29O_bqqpO2d7weJwg18/s640/1600px-Phylogenetic_tree.svg.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Life on Earth is divided up into three domains: Bacteria, Archaea and Eukaryota. All three may have diverged from a common ancestor, <a href="https://en.wikipedia.org/wiki/Last_universal_ancestor">LUCA</a> (bottom vertical line). Image credit to Eric Gaba;Wikipedia.</span></td></tr>
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It could be the signature of carbon-based life that once existed and then was wiped out in the violence of a young unstable planet, to be replaced by our ancient microbial ancestors. The first Earth organisms to leave behind sufficient fossil evidence might be just one variation on a much larger theme. Several unique biochemistries could have evolved on Earth but only the ones that could adapt best to the rapidly changing conditions on early Earth survived and colonized. The general biochemistry that survives today might simply have been lucky enough to evolve in a relatively calm period between devastations or in a place that was sufficiently protected. Somehow, we know, life made its first appearance. At what point did a collection of chemical reactions become the biochemistry of a living organism?<br />
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Did Non-life Become Life In a Hydrothermal Vent?<br />
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When did the tipping point occur where non-life organic chemistry became life's biochemistry? In 1967, molecular biologist John Bernal <a href="https://en.wikipedia.org/wiki/Abiogenesis#Current_models">suggested that life forms in three stages</a>. First, biological monomers (such as amino acids) are created. This stage seems to have been <a href="http://www.universetoday.com/37630/amino-acid-found-in-stardust-comet-sample/">achieved throughout the universe</a> in interstellar space as well as on solid planets, moons and even on asteroids. The second stage is the origin of biological polymers, such as proteins, which consist of several amino acids linked together.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjy3gzRVOyvrEVbx8cvAVSpzUDeP9L789-5j5Chs8fhv0od0k0WNLmtBypPbD0a97sH30nsukdM9AVm92GK2cgveYEVVdPc0KXnnwtmtQQQEmVi68JMp2yuJh67eL-E8RHdytgOcUBaPVOi/s1600/Peptide-Figure-Revised.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="492" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjy3gzRVOyvrEVbx8cvAVSpzUDeP9L789-5j5Chs8fhv0od0k0WNLmtBypPbD0a97sH30nsukdM9AVm92GK2cgveYEVVdPc0KXnnwtmtQQQEmVi68JMp2yuJh67eL-E8RHdytgOcUBaPVOi/s640/Peptide-Figure-Revised.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Diagram of the chemical structure of the peptide bond (in the box) between two amino acids, creating a peptide chain, which is a protein. Credit to Chemistry-grad-student;Wikipedia.</span></td></tr>
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The achievement of this stage is less well understood. One big problem is that many organic chemicals produced under Miller-Urey conditions would react with amino acids that form as well, preventing them from bonding into peptide chains. Some work suggests that networks of reactions that begin with hydrogen cyanide and hydrogen sulfide (two molecules that should have been abundant on young Earth) found in shallow water irradiated by UV light <a href="http://www.sciencemag.org/news/2015/03/researchers-may-have-solved-origin-life-conundrum">could produce amino acids, as well as nucleic acids and lipids</a>, while producing few molecules that would inhibit protein synthesis.<br />
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Another theory suggests that intense UV radiation from our young Sun may not have been responsible for kick-starting life. In fact, the earliest organisms might have required protection from it instead. Many asteroid strikes during The <a href="https://en.wikipedia.org/wiki/Late_Heavy_Bombardment">Late Heavy Bombardment</a> would have involved giant asteroids far larger than the one <a href="https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event">that caused the extinction of the dinosaurs</a> and these strikes would repeatedly sterilize Earth down to a depth of at least tens of metres, dooming any bourgeoning life forms on the surface. Meanwhile, life could have emerged around deep-sea <a href="https://en.wikipedia.org/wiki/Hydrothermal_vent">hydrothermal vents</a> where conditions would be much more stable.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjrKrvyORw0fZ5uE_SrIGEciniPi5__hm2bEz8ydHPZHl7dWc3vZVAafEQbsrzOcQf8ySQPsI88wqp37RpDilrvOAFt4KCKBBV2r39xGDWMmoSwmWQk1-2IN0QkeFw7S4WmYRtGPZLh61dk/s1600/Champagne_vent_white_smokers.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjrKrvyORw0fZ5uE_SrIGEciniPi5__hm2bEz8ydHPZHl7dWc3vZVAafEQbsrzOcQf8ySQPsI88wqp37RpDilrvOAFt4KCKBBV2r39xGDWMmoSwmWQk1-2IN0QkeFw7S4WmYRtGPZLh61dk/s640/Champagne_vent_white_smokers.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">White smokers around Champagne Vent deep in the Marianna Trench. Credit:NOAA</span></td></tr>
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Deep under water, under intense pressure and heat and no sunlight, the <a href="https://en.wikipedia.org/wiki/Laws_of_thermodynamics">laws of thermodynamics</a> might have driven the synthesis of increasingly complex organic molecules and eventually lead to the first living cells. Computer simulations of geothermally heated ocean crust yield an even greater variety of amino acids and other organic molecules than any version of the Miler-Urey experiment has. The diagram below of a typical hydrothermal vent offers an idea of just how complex that chemistry can be.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj4ncmENTwGmbef94BoaqeJ-djGaunnHt3cBOL0mESbfIUTIyCRzJU09Pgzs8sQT475mPV8bQHHa5iRkpwyI5ancGySySk78Mk2-8OYZmYt_KneylsCDsGUkqcvGSPafaoVFZ43Site5szz/s1600/Deep_sea_vent_chemistry_diagram.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="372" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj4ncmENTwGmbef94BoaqeJ-djGaunnHt3cBOL0mESbfIUTIyCRzJU09Pgzs8sQT475mPV8bQHHa5iRkpwyI5ancGySySk78Mk2-8OYZmYt_KneylsCDsGUkqcvGSPafaoVFZ43Site5szz/s640/Deep_sea_vent_chemistry_diagram.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">This diagram of the biogeochemical cycle of a typical hydrothermal vent created by the U.S. government offers an idea of the complex chemistry at work.</span></td></tr>
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Everett Shock and Mitchell Schulte suggest that there is <a href="http://onlinelibrary.wiley.com/doi/10.1029/98JE02142/abstract">significant thermodynamic drive to create these organic compounds</a> as chemically unstable hydrothermal fluids are spontaneously driven toward a state of <a href="https://en.wikipedia.org/wiki/Chemical_equilibrium">chemical equilibrium</a>.<br />
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A similar thermodynamic disequilibrium drives the elegant process of photosynthesis, <a href="http://sciexplorer.blogspot.ca/2013/05/photosynthesis.html">explored in a previous article</a>. In that case, the photosynthetic production of energy-rich sugars in plants occurs down an <a href="https://en.wikipedia.org/wiki/Electrochemical_gradient">electrochemical gradient</a> across disc-like membranes that are located within cellular organelles called chloroplasts. A recent study done by Russell et al (2014) suggests that a similar membrane-spanning electrochemical gradient <a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3995032/">may have been a kind of prebiotic nano-engine</a>, a precursor to the <a href="https://en.wikipedia.org/wiki/Molecular_motor">molecular motors</a> across cell membranes and inside organelles today, which are driven by energy gradients moving toward chemical equilibrium. Examples of these very efficient motors are myosins that contract our muscles, the cilia that move mucus and dust out of our nasal passages, the tiny propeller-like flagella that propel some bacteria, protists and sperm, the proteins that condense long double strands of DNA into chromosomes, and even RNA polymerase, which transcribes new RNA from a DNA template, is a molecular motor.<br />
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Russell et al's modeling shows that the synthesis of various organic molecules (alkanes, alcohols, ketones, carboxylic acids, aldehydes, etc.) is favourable (and exothermic, which means it releases energy) when 2°C seawater containing bicarbonate is mixed into 350°C fluid water (water under enough pressure to stay liquid) containing dissolved carbon dioxide and hydrogen gas. This scenario is much like the conditions around a hydrothermal vent. The initial hydrothermal fluid becomes increasingly <a href="https://en.wikipedia.org/wiki/Redox">chemically reduced</a>. In chemistry, reduction means the acceptance of electrons. For example, when iron and oxygen react to make rust, the iron is oxidized and the oxygen is reduced – it accepts electrons from the iron atom. As the fluid becomes more and more reduced, the rate of synthesis of organic compounds increases because the reaction becomes increasingly thermodynamically driven by the increasing differences in oxidation state (as an example, Fe, Fe<sup>2+</sup> and Fe<sup>3+</sup> represent three increasing states of oxidation for iron as it rusts in the presence of water or moist air). They also note that Earth's oceanic crust is mostly basalt. This is the rock through which hydrothermal vent fluid travels upward. Early Earth's basalt would have had a mineral composition much different from modern basalt because it had not yet gone through billions of years of recycling through continental plate <a href="https://en.wikipedia.org/wiki/Subduction">subduction</a>. In the beginning basalt was more highly reducing because it contained more magnesium and iron, two elements with multiple oxidation states. All of this suggests that conditions around 4 billion years ago were ripe for maximizing complex organic molecule syntheses, with nearly complete conversion of inorganic carbon (in bicarbonate in the water) into a wide variety of complex organic compounds.<br />
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Additional evidence from microbiology supports the hydrotherrmal vent life origin hypothesis. The last universal common ancestor of all living organisms on Earth may have been a unicellular thermophilic (heat-loving) organism, the kind of simple unicellular life that could have evolved around hydrothermal vents. Akanuma et al., (2013), make the argument that a universally conserved enzyme in current (extant) archaea and bacteria species, called <a href="https://en.wikipedia.org/wiki/Nucleoside-diphosphate_kinase">nucleoside diphosphate kinase</a> (NDK), <a href="http://www.pnas.org/content/110/27/11067.abstract">must be an indispensible part of modern cell metabolism</a>. This enzyme is not only found in all simple unicellular organisms, it is also found in a very similar form inside the mitochondria and in the cytoplasm of every living cell on Earth. It is the source of both RNA and DNA (ribonucleic acid and deoxyribonucleic acid; genetic material) precursors, and every cell on Earth uses RNA, DNA or a combination of the two (like our cells do) to reproduce. Knowing this, the researchers surmised that NDK must have been inherited from a common ancestor. It is thought to be a highly conserved protein, meaning its amino acid sequence hasn't changed much over billions of years, but it has evolved somewhat. They attempted <a href="https://en.wikipedia.org/wiki/Ancestral_reconstruction#Molecular_Evolution">to reconstruct its ancestral protein sequence</a> by inferring it from NDK sequences of organisms along a reverse phylogenetic tree. They did this by obtaining the gene sequence for the proteins in NDK in extant life. Then, using reliable established methods from population genetics and probability theory, they came up with a series of possible ancestral sequences for the enzyme. After this step, they spliced those genes into very serviceable E. Coli bacteria, which then accurately translated the genetic code into the ancestral proteins. Finally, they exposed these proteins to various temperatures. They discovered that these enzyme proteins are extremely stable at very high (hydrothermal vent) temperatures, and they function optimally at around a hot 85°C. Compare this temperature to our human upper limit. Wet-bulb temperatures above 35°C for six hours or longer <a href="http://www.purdue.edu/newsroom/research/2010/100504HuberLimits.html">are fatal to humans and most other mammals</a> because our cell membranes become unstable and most of our enzymes (essential for life's functions) become denatured. We certainly die at 85°C, but the NDK in our cells, with its ancient lineage, likely keeps on working just fine. This research suggests that the ancient enzyme and therefore the organism itself that housed it, life's universal common ancestor (LUCA), was likely a thermophile. It was probably a simple bacteria-like or archaea-like microbe and it may have gotten its start in a hydrothermal vent, meaning some of our cell machinery did too.<br />
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The Possible Co-Evolution of RNA and Proteins<br />
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Benal's third stage is the evolution from molecules to cells. The drive-to-equilibrium theory and the NDK research we explored provide a possible chemical scenario where inorganic molecules evolve into increasingly complex organic molecules such as nucleotides, lipids and amino acids, paving the way toward proteins, and toward RNA and DNA synthesis. Before we get to stage 3, however, we need to examine protein synthesis a bit further. We still don't have a possible scenario for their synthesis from amino acids, even though we have established ways in which it is possible. We know that eventually proteins evolved. As shown in an earlier diagram, proteins are composed of amino acids held together by peptide bonds. The big stumbling point here is that peptide bonds do not form spontaneously. It takes a lot of energy to make that reaction favourable enough to proceed. Inside cells, this energy is supplied by an energy-storage molecule called <a href="https://en.wikipedia.org/wiki/Adenosine_triphosphate">adenosine triphosphate</a> (ATP) and cellular enzymes <a href="http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Kinetics/Modeling_Reaction_Kinetics/Temperature_Dependence_of_Reaction_Rates/The_Arrhenius_Law/The_Arrhenius_Law%3A_Activation_Energies">are used to lower the activation energy</a> (kind of like an energy hurdle that must be jumped over) of the synthesis reaction. Therefore, it is unlikely that proteins such as NDK arose unless there was a lot of free energy (perhaps in the form of heat) and a catalyst of some kind came on the scene. That catalyst could have been a simpler precursor to a modern RNA molecule and it suggests a possible scenario where RNA and proteins co-evolved. RNA is a fascinating molecule. It is self-replicating and it is an enzyme. Enzymes act as catalysts for biochemical reactions. They lower the activation energy and, by doing so, they allow reactions to proceed and they increase the reaction rate. According to the <a href="https://en.wikipedia.org/wiki/RNA_world">RNA world theory</a>, spontaneously self-replicating ribonucleic acid (RNA) molecules may have been the ultimate precursor to all life on Earth. Modern RNA <a href="https://en.wikipedia.org/wiki/RNA">is a cellular master of all trades</a>. Not only does it function as genetic material (which acts as an information storage molecule for cellular reproduction), it is also an essential non-protein enzyme. In cells, it catalyzes the formation of peptide bonds between amino acids to create protein polymers. In our bodies this happens inside the ribosomes in our cells. Inside the ribosome organelle, a complex composed of several RNA molecules and proteins carry out protein synthesis. If the first simple strands of protein polymerized thanks to the enzymatic boost from simple RNA-like molecules, we need to get to the synthesis of a simple RNA-like molecule. To evolutionarily get to the first RNA, we need to propose an abiotic (non-living) RNA synthesis pathway. The catch now is that the RNA molecule itself is very complex molecule.<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgJXLqRT0W4FWL6WbRDbI7syMlSvfJRqHECWFSnP0AtDDBhq5jIw3ciYc_8VVfgcq0Cyv6aLPLwOC3N7CBSmDdyi_rkN0Kw4NErcAlK29Lv70CaQDQ3A7sE1aFO4STKJ7yi33ZC80iVUZQI/s1600/Pre-mRNA-1ysv-tubes.png" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgJXLqRT0W4FWL6WbRDbI7syMlSvfJRqHECWFSnP0AtDDBhq5jIw3ciYc_8VVfgcq0Cyv6aLPLwOC3N7CBSmDdyi_rkN0Kw4NErcAlK29Lv70CaQDQ3A7sE1aFO4STKJ7yi33ZC80iVUZQI/s400/Pre-mRNA-1ysv-tubes.png" width="216" /></a></div>
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A hairpin loop of RNA is shown left. A single strand is folding back on itself. It can be extremely long, composed of up to hundreds of nucleobases along a ribose-phosphate backbone. Nucleobases are green and the ribose-phosphate backbone of the molecule is blue. Image credit to Vossman;Wikipedia.<br />
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Most researchers consider it <a href="http://adsabs.harvard.edu/abs/2009Natur.459..239P">unlikely that the ribonucleotides within this molecule would form non-enzymatically</a>. A nucleotide is a basic building block of RNA and DNA. Each nucleotide follows the same basic plan. It is composed of a nitrogenous base (a nucleobase as in above) plus a 5-carbon sugar that is either ribose (for RNA) or deoxyribose (for DNA) and at least one phosphate group (HPO<sub>4</sub><sup>2-</sup>).<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEieL-sd_WuZJ79ByMpg8Fgd2VwbcPxnJ8p1jsw1neBeeZqa1288w7faOvC-HsjLICxbl9rLuaXKEBh-vj6o4He-xcKWfciju58lYGP-90QpwR_AbvmXCLTOe7r5YM1GJ-WfykYhSTKh4dSA/s1600/Ribonucleotide_General.png" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="252" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEieL-sd_WuZJ79ByMpg8Fgd2VwbcPxnJ8p1jsw1neBeeZqa1288w7faOvC-HsjLICxbl9rLuaXKEBh-vj6o4He-xcKWfciju58lYGP-90QpwR_AbvmXCLTOe7r5YM1GJ-WfykYhSTKh4dSA/s320/Ribonucleotide_General.png" width="320" /></a></div>
The general structure of a ribonucleotide consists of a phosphate group (the left part of the diagram shown right), a ribose sugar group (the bottom right pentagonal ring) and a nucleobase (top right). The base, or nucleobase, can be adenine, guanine, cytosine or uracil in RNA. (A, G, C or U). Image credit to Binhtroung;Wikipedia.<br />
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Ribonucleotides, as you can see, are highly complex organic molecules. The biggest problem when faced with the question of how nucleotides were formed non-biologically is the sugar unit. Sugars are biological molecules common and essential to plants, animals and many unicellular organisms but the 5-carbon sugars present in genetic material are <a href="http://phys.org/news/2016-06-sugar-derivatives-meteorites-enantiomeric-excess.html">found only in very trace amounts on meteors and geologically</a> on Earth as sugar acids. Sugars are geologically rare and they also tend to be unstable. They decompose when exposed to heat (for example, making caramel, a partial decomposition of sucrose, requires less than 160°C) so if they were synthesized non-biologically they would not survive early Earth's violent conditions for long. However in 2011, British chemist John Sutherland <a href="http://www.ncbi.nlm.nih.gov/pubmed/21930577">discovered a non-biological pathway</a> to create pyrimidine nucleosides that bypasses free sugars altogether. These exciting results were confirmed and explored further in a subsequent series of papers. Instead of sugars, 2 and 3-carbon molecules (glycolaldehyde, glyceraldehyde cyanamide, etc.) could be used and these are all molecules that would have been much more commonly available on and in our young planet.<br />
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There is also evidence that RNA didn't need to start out as the long complex chain of ribonucleotides that most modern forms tend to be. Even quite short sequences of RNA can still be enzymatically active. Much shorter simpler RNA-like polymers have even been shown to catalyze the formation of peptide bonds. Rajamani et al (2013) showed that these polymers <a href="http://www.ncbi.nlm.nih.gov/pubmed/18008180">could be created non-biologically</a> (abiotically) in a lipid-rich environment. Lipids are naturally occurring molecules such as waxes, fats, phospholipids and sterols.<br />
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Once short RNA-like polymers arrive on the scene, what processes would abiotically drive them toward achieving the complexity of modern RNA? This part of our story gets quite interesting. Tracey Lincoln and Gerald Joyce (2009) suggest <a href="http://science.sciencemag.org/content/323/5918/1229.full">a form of chemical evolution might be responsible</a>. The genetic code in RNA is built as a chain of four different ribonucleotides. Each one – G, U, A and C – has a different nucleobase that makes it unique. The chemical bonds that hold these ribonucleotides together have a fairly low potential energy. This means that (unlike peptide bonds) these bonds would have formed and broken apart regularly. While this took place, some particular combinations would have catalytic properties that can lower the activation energy required for their particular sequence to be created. This means those sequences would stay together for longer periods than other random sequences. They could also grow longer and form faster before breaking down again. RNA is a self-replicating molecule and these sequences would be able to replicate more frequently, giving them a competitive edge. Some biologists consider this point <a href="http://www.la-press.com/the-dna-habitat-and-its-rna-inhabitants-at-the-dawn-of-rna-sociology-article-a3571">to be where life started</a>. There are no living cells yet, but evolving RNA, by using chemical bond energy to replicate its strands, fits some definitions of life. Eventually, sequences that catalyze peptide bonding would randomly be built and some of the proteins that formed as a result would be active enzymes, which in turn would assist in RNA synthesis. A positive feedback system would be set up. Even short 5-ribonucleotide strands have been shown <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2826339/">to catalyze protein synthesis</a>. This RNA-protein co-evolution scenario provides us with a way in which we can start to visualize how the cellular machinery of life got its start billions of years ago. Eventually that machinery became housed in a protective envelope to form the first simple cells.<br />
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Next we will explore what may have been the first living cells on Earth.<br />
<br />Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-26297088060444486772016-09-04T09:20:00.000-06:002016-09-05T11:56:02.657-06:00Hello? Earth Calling . . . PART 3<div class="MsoNormal">
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 1</b> <a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-1.html">CLICK HERE</a></span><br />
<span lang="EN-US"></span><br />
<span lang="EN-US"><i>For</i> <b>Hello? Earth Calling . . . PART 2</b> </span><a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-2.html">CLICK HERE</a><br />
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<span lang="EN-US">Where Could Life Evolve?</span></div>
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<span lang="EN-US">Defining what is alive is very difficult but
this question is of great importance to NASA and other space agencies around
the world because they need to know what possible forms life can take in order
to set up scanning parameters when they explore other planets and moons in our
system as well as exoplanets. Often asked when a new exoplanet is discovered is
whether the planet lies within its star's "<a href="https://en.wikipedia.org/wiki/Circumstellar_habitable_zone">Goldilocks zone</a>."
This is the distance from its star where the average atmospheric temperature (given
sufficient pressure) should allow liquid water to exist on the surface. <o:p></o:p></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiyiK6aGCBuxxJi8q0f6I6YDAodkysQCof4ce6RLNgTaNjVAibhJdDJd3qC-D03iHIW8pnJKr7vcNkRiJFd4jdy0U9gUtTSPSj2U3tNVvfDia300am3YgHqiWzno8ygIEnWgtVy-pQEd4ah/s1600/Habitable_zone_-_HZ.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="390" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiyiK6aGCBuxxJi8q0f6I6YDAodkysQCof4ce6RLNgTaNjVAibhJdDJd3qC-D03iHIW8pnJKr7vcNkRiJFd4jdy0U9gUtTSPSj2U3tNVvfDia300am3YgHqiWzno8ygIEnWgtVy-pQEd4ah/s640/Habitable_zone_-_HZ.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">The diagram above compares habitable zones based on star luminosity. Notice that Earth is right in the middle of our Sun's habitable zone. Mars would be in the habitable zone of a hotter, brighter star, and even Mercury would be in a habitable zone if it orbited a much dimmer red dwarf. This is not drawn to scale. Credit to Chewie;Wikipedia.</span></td></tr>
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<span lang="EN-US">The Goldilocks zone is a rough guideline
for where life could exist based on what was known about Earth life decades
ago. Astrobiologists now speculate that life could have evolved in subsurface
oceans of liquid water on moons in our solar system such as Titan, Enceladus,
Europa and Ganymede. <o:p></o:p></span></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhnGw5elmFLKE2jlkQAYbsahRl5HVjx_IXiNq5IeGk8LBeAdkwVrwjtNlnftxyoRZOekz1XKLtvfhi-auuEurggGvglhSGOWZsUupLp00sPUOzmobkX2VFBbgvpi36JyhXP8ijVKm0Nc215/s1600/Titan_poster.svg.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="334" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhnGw5elmFLKE2jlkQAYbsahRl5HVjx_IXiNq5IeGk8LBeAdkwVrwjtNlnftxyoRZOekz1XKLtvfhi-auuEurggGvglhSGOWZsUupLp00sPUOzmobkX2VFBbgvpi36JyhXP8ijVKm0Nc215/s640/Titan_poster.svg.png" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Saturn's moon Titan, not in our Sun's habitable zone, nevertheless could potentially harbour exotic methane-based life on its surface as well as carbon/water-based life in its subsurface ocean, which is shown here as a blue layer in the cut-away image above left. Credit to Kelvinsong;Wikipedia</span></td></tr>
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<span lang="EN-US">Discoveries of extremophilic organisms on Earth
and the modeling of potential non-water biochemistries have stretched what we
think of as necessary conditions for life. This progress has widened the
potential zone of habitability and even makes it irrelevant. Additionally we
are learning that life on Earth today is not what life was like on Earth billions
of years ago. Life arose when conditions on our planet were nothing like what
they are now. Should we be looking for planets that might provide those
conditions as well? For now, liquid surface water and atmospheric oxygen, two
features of Earth-based life, serve as logical starting points for the search
for extraterrestrial life.<o:p></o:p></span></div>
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<span lang="EN-US">How and When Does Chemistry Become Life?:
Definitions <o:p></o:p></span></div>
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<span lang="EN-US">NASA offers <a href="http://www.nasa.gov/vision/universe/starsgalaxies/life's_working_definition.html">a working definition of life</a> based on an entity's ability to take in energy from the environment and
transform it for growth and reproduction. Scientists use this definition as a
basic rationale to look for signs of possibly life-connected chemical transformations
in the atmospheres of other moons and planets such as Titan and Mars. For
example, trace amounts of methane found in the <a href="https://en.wikipedia.org/wiki/Atmosphere_of_Mars#Methane">atmosphere of Mars</a> between 2003 and 2006 by the European Space Agency's <a href="https://en.wikipedia.org/wiki/Mars_Express">Mars Express Orbiter</a>,
and several ground-based observations, excited scientists looking for signs of
life. Atmospheric methane is unstable. It quickly breaks down under the Sun's
UV radiation and it reacts chemically with other gases in the Martian
atmosphere. This means there must be an active source and it <a href="https://en.wikipedia.org/wiki/Life_on_Mars#Methane">produces approximately 270 tons per year</a>.
Volcanism and meteorite strikes can theoretically produce this much methane but
neither event has occurred recently enough. It is also possible that water-rock
reactions and pyrite formation produce methane abiotically (via non-living chemistry). Organic compounds
on meteorites could also be converted to methane through UV radiation. It is possible too (and this is what excites the researchers) that <a href="https://en.wikipedia.org/wiki/Methanogen">methanogenic microbes</a> could be responsible. These
microbes exist on Earth in oxygen-poor wetlands and inside our guts. <o:p></o:p></span></div>
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<span lang="EN-US">In the digestive tract of a termite's gut (termites shown left),
protozoa break down cellulose in wood so the animals can digest it. That
process releases hydrogen gas, which reduces energy available to the protozoa.
Methanogenic archaea in the gut help the protozoa out by consuming the hydrogen, which is a great example of
three-way mutualism between termites, archaea and protozoa. Photo is provided by Scott
Bauer;Wikipedia<o:p></o:p></span></div>
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<span lang="EN-US">Extremophile methanogenic species live in
hot springs and in hydrothermal vents. There they require no oxygen or organic
nutrients. They use hydrogen as their energy source, which they could obtain on
Mars just beneath the surface, perhaps where it is also warm enough for liquid
water to exist and where it is protected from sublimation into the thin
low-pressure Martian atmosphere. No direct evidence for any life has yet been
found but the possibility has not been ruled out either. <o:p></o:p></span></div>
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<span lang="EN-US">If microbes do exist on Mars, they could
have hitched a ride from ancient Earth. Tough spores of ancient Earth
methanogens could have even been blown into space by meteoric impacts to
eventually land on Mars and take up residence there (and vice versa) according
to a hypothesis called <a href="https://en.wikipedia.org/wiki/Panspermia">panspermia</a>.
This is also one reason why probes sent to other planets and moons must be
absolutely sterile, to <a href="https://en.wikipedia.org/wiki/Planetary_protection">avoid accidental contamination</a> and a false positive result for life. <o:p></o:p></span></div>
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<span lang="EN-US">NASA's definition of life (the ability to
take in energy from the environment and transform it for growth and
reproduction) works quite well but there remains <a href="https://en.wikipedia.org/wiki/Life#Definitions">no universally accepted definition of life</a>. Even NASA's
working definition is imperfect because it excludes worker bees and mules, for
example, as those individuals don't reproduce. However, life can be narrowed
down to traits shared by all known living organisms, even the exotic
extremophiles. Here on Earth two commonalities stand out - a carbon-based
chemistry and a dependence on water. Most experts also require living organisms
to have cells, while others do not. <a href="https://en.wikipedia.org/wiki/Virus">Viruses</a>,
<a href="https://en.wikipedia.org/wiki/Plasmid">plasmids</a> and <a href="https://en.wikipedia.org/wiki/Viroid">viroids</a> are nothing more than fragments of RNA or DNA (two kinds of genetic material).
In viruses the genetic material is contained in a protein sac or coat but it is
not a proper cell because it contains no cytoplasm and no biomolecules are enclosed
within a membrane. <o:p></o:p></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiL8fdgYJDpxn5lz6NlJEcvTE8_PUHIxybiHcpNs7iYB9aGW8zYd4BNf9vrdbQSz7W2keD6fPQAEbKHvyc60Fo8QoWTWWRfNc9IaWPJXZ_c1EehT90Qz7QPb8Tl0MMn47K2cVk6aMtsxYGQ/s1600/TMV_structure_simple.png" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="283" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiL8fdgYJDpxn5lz6NlJEcvTE8_PUHIxybiHcpNs7iYB9aGW8zYd4BNf9vrdbQSz7W2keD6fPQAEbKHvyc60Fo8QoWTWWRfNc9IaWPJXZ_c1EehT90Qz7QPb8Tl0MMn47K2cVk6aMtsxYGQ/s400/TMV_structure_simple.png" width="400" /></a></div>
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<span lang="EN-US">This diagram (right) of a tobacco mosaic virus
shows RNA (red) coiled within in a helix of external protein. Diagram by Thomas
Spiettstoesser;Wikipedia.<o:p></o:p></span></div>
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<span lang="EN-US">Despite their simplicity, Viruses are capable of reproducing as a
unit. In plasmids and viroids, the genetic material is loose. There is no
enclosure at all. A plasmid (see below left) is a small DNA strand that exists inside a cell
that is separate from its chromosomal DNA. Often found in simple unicellular
organisms, plasmids perform functions useful to the cell. For example, a plasmid
can become active in times of hardship to produce a protein toxin that it codes
for. The toxin can afford the organism a temporary protective advantage. <o:p></o:p></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFNTAHBO_CDt3gwxf-GEZBlLlg0d8mA87eUSJgRx9RLKozRW7trJT9a5SUZkbPmflEhA7CSU9MWNrrc243lyq77nlwXsMX1WnYN0MtJp4Wg4dHQjmH65lPdnyHB9D0FBWLEVkfUIgo-CWH/s1600/320px-Plasmid_%2528english%2529.svg.png" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFNTAHBO_CDt3gwxf-GEZBlLlg0d8mA87eUSJgRx9RLKozRW7trJT9a5SUZkbPmflEhA7CSU9MWNrrc243lyq77nlwXsMX1WnYN0MtJp4Wg4dHQjmH65lPdnyHB9D0FBWLEVkfUIgo-CWH/s1600/320px-Plasmid_%2528english%2529.svg.png" /></a></div>
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In this not-to-scale diagram (left), simple
circular double-stranded DNA strands of plasmids exist along with the host's
DNA inside a bacterium. Work by User:Spaully;Wikipedia.</div>
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<br /></div>
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<span lang="EN-US">Viroids, on the other hand, are pathogens.
Consisting of a single circular strand of RNA, they don't code for any protein.
Instead, once inside a host they simply replicate using the host's cell
replication enzymes. They infect the host's body and in doing so they are parasitic.
<o:p></o:p></span></div>
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<span lang="EN-US">An additional infectious agent is the <a href="https://en.wikipedia.org/wiki/Prion">prion</a>, which is composed entirely of protein.
When it infects, usually neural, tissue in a host, it induces proteins to fold
into the same misfolded shape as the prion. As this process continues,
eventually amyloid folds form, consisting of tightly packed aggregates of
misfolded proteins, which cause tissue damage and eventually death. <o:p></o:p></span></div>
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<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj60AcrbZy7iOO7FTheZH3aGu3aYdShmeDW8_2o4oN-wv8aifk6h3dno3mGD_9_6qQuF8uu77c4tNFGQVXic8xXeiDfrCRHvfC90WsOmAaufPm2oLTP1m09AwCnkBVhNQbzRZFdk1PWgfRZ/s1600/Histology_bse.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="318" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj60AcrbZy7iOO7FTheZH3aGu3aYdShmeDW8_2o4oN-wv8aifk6h3dno3mGD_9_6qQuF8uu77c4tNFGQVXic8xXeiDfrCRHvfC90WsOmAaufPm2oLTP1m09AwCnkBVhNQbzRZFdk1PWgfRZ/s400/Histology_bse.jpg" width="400" /></a></div>
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<span lang="EN-US">This micrograph of bovine brain tissue (right) is
full of vacuoles or holes giving it the telltale spongy appearance of the
prion-transmitted disease, spongiform encephalopathy. Credit to dr. Al
Jenny;Wikipedia.<o:p></o:p></span></div>
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<span lang="EN-US">Viruses, plasmids, prions and viroids, by
virtue of their simplicity, might represent relics from the time when life
began to evolve on Earth. Viroids, which consist of short circular strands of
RNA, may have been the first "proto-life" form to exist in a world
before DNA or even proteins evolved. Now functioning as pathogens, these simple
RNA strands may have once replicated in free form in ancient watery
environments on Earth. By NASA's definition they would be considered alive, but
most experts consider them, as well as plasmids, prions and viruses, to be nonliving
because outside of their intracellular environment they cannot do anything.<span style="color: red;"> </span>They rely on their host cell's replication machinery
to reproduce, which leaves the question of how an ancient proto-life viroid could
replicate outside of a cell. Instead of the cellular RNA polymerase used by a
modern viroid, an ancient viroid might have taken advantage of some other organic
molecule that catalyzed its replication reaction, a basic idea that we will expand on.</span></div>
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<span lang="EN-US">Potential Chemistries of Life<o:p></o:p></span></div>
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<span lang="EN-US">We know that carbon-based biochemistry in a
water solvent worked here on Earth, but it might not be the only potential life
chemistry. To explore what kinds of chemistry could work, we can start by
asking what makes carbon and water work. What other chemical compounds could
function in similar ways? As mentioned previously, a poly-lipid based
biochemistry using methane as a solvent might work in an environment like Titan.
In addition, chemists know several complex chemistries that use sulfuric acid
as a solvent for their reactions. Ethane-ammonia is another potential
biological solvent. These are <a href="https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-water_solvents">just a few possibilities</a>.
Water works well as the solvent for carbon-based biochemistry. It is liquid
over a wide range of temperatures. It has a high heat capacity so it can be
used to regulate an organism's temperature. It can dissolve many compounds. It can
act as an acid (H<sup>+</sup>) or a base (OH<sup>-</sup>), which makes it a useful reactant and a
product in many biochemical reactions. By itself water isn't a great <a href="http://scifun.chem.wisc.edu/chemweek/biobuff/biobuffers.html">biological buffer</a> but when phosphates or carbonates are dissolved in water it becomes a great buffer, keeping the internal pH environments of cells constant. Planets and moons where a compound tends
to be in a stable liquid state at that planet's/moon's average temperature and
pressure can potentially use that compound as a biological solvent. A planet (or
moon) could be very cold and use methane or ethane. A planet with extreme
surface pressure could use liquid nitrogen or even supercritical hydrogen as a
biological solvent. An extremely hot planet could use liquid sodium chloride as
a solvent. From a potential solvent perspective, the Goldilocks range of
exoplanets and moons is enormous. Could life using one of these exotic solvents
evolve into complex organisms as well? <o:p></o:p></span></div>
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<span lang="EN-US">More than a solvent is required for life,
at least on Earth. An entire suite of complex biochemistry is also essential. Carbon
takes on the central role in biochemistry. It is abundant. It is small and
light weight so it is easily manipulated by enzymes. Perhaps most usefully, it
has four valence electrons to bind to other elements such as hydrogen, oxygen
and nitrogen as well as with other carbon atoms, so it can form the long
complex carbon chains of proteins and DNA. However, carbon is not the only
element with these properties. Silicon also has four valence electrons and it
can bind to itself. Silicon-based life, though, would have to be quite exotic.
Unlike carbon, silicon tends to form crystal lattices rather than chains. The
big downside of silicon is that its lattices don't break apart and reform
easily into new compounds, so it lacks the chemical plasticity that makes
carbon so useful for organism growth and decay. Silicon would not be able to cycle through living and non-living systems <a href="https://en.wikipedia.org/wiki/Carbon_cycle">as carbon does</a>, which ensures that carbon-rich
nutrients are always available to organisms. Other elements besides carbon can
form long chains, such as chlorine, sulphur, nitrogen and phosphorus.
Phosphorus together with nitrogen can also form a wide variety of different
molecules, including some that are ring shaped, similar to many complex
carbon-based biomolecules. Several metal oxides <a href="https://www.newscientist.com/article/dn20906-life-like-cells-are-made-of-metal/">can form a variety of complex structures as well</a> and they offer the advantage of being more thermally stable at very
high temperatures than carbon compounds. <o:p></o:p></span></div>
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<span lang="EN-US">Defining Life Is Tricky<o:p></o:p></span></div>
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<span lang="EN-US">While NASA defines life as an entity's
ability to take in energy from the environment and transform it for growth and
reproduction, there are other ways of approaching the mystery of life. A
definition that defines life as any self-sustaining system that undergoes
Darwinian-like evolution can be useful but it too has problems. "Self-sustaining"
can be difficult to pin down. As mentioned, viruses, plasmids, prions and
viroids are clearly not self-sustaining. They can't live or evolve outside of a
living host. But even humans are not strictly self-sustaining. We need an
ecosystem that includes other organisms to supply our oxygen and food and
recycle our nutrients. Even our own bodies are more of an ecosystem than a
single self-sustaining species. Our guts, for example, are complex communities rich with
various bacteria, fungi and other simple organisms without which we could not
digest food and absorb its nutrients properly, and we would eventually die.
Evolution is problematic as well. How long of a time-scale does evolution
require? It could take place so slowly that it would be virtually impossible to
detect and measure.<o:p></o:p></span></div>
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<span lang="EN-US">The definition of life as any
self-sustaining system that undergoes Darwinian-like evolution, though
problematic, opens up some fascinating questions. The ultimate self-sustaining
system that evolves over time is Earth's biosphere, which includes obviously
living components but also non-living (abiotic) parts such as the atmosphere
and the water cycle. Is it as a whole a living system? This proposition is
called the <a href="https://en.wikipedia.org/wiki/Gaia_hypothesis">Gaia hypothesis</a>. According
to this hypothesis, organisms co-evolve with their abiotic environment. A good
example is Earth's stable oxygen-rich* atmosphere. The oxygen owes itself to
the process of photosynthesis carried out by living organisms – plants, algae
and cyanobacteria. Despite changing weather and a cycling climate, the oxygen
level currently maintains a very consistent 21% of the total gas by volume. It
<a href="https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Third_atmosphere">wasn't always like this</a>.
The equilibrium state of atmospheric oxygen has shifted, or evolved, over time.
Once oxygen began to accumulate on Earth, it reached an equilibrium state of
around 15%, which it maintained for billions of years. 280 million years ago
oxygen levels increased again and peaked at an equilibrium state of about 30%,
due to a wet warm climatic period filled with lush oxygen-releasing vegetation.
Our atmosphere is a self-regulating complex system with a stable but evolving
equilibrium, which under this definition could be considered alive. <o:p></o:p></span></div>
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<span lang="EN-US"><span style="background-color: #fff2cc;">*An interesting side story here is that
oxygen is <a href="http://www.dailygalaxy.com/my_weblog/2015/09/astrobiologists-oxygen-is-not-a-definitive-sign-of-exo-planet-life-the-earths-atmosphere-contains-oxygen-because-plants-co.html">no longer necessarily the clear biomarker of life</a> on a planet that
scientists once thought it was.
Atmospheric oxygen is very reactive, much like Martian methane gas. It reacts
with rocks to form oxides, taking it out of the atmosphere. It was thought that
in order to maintain atmospheric oxygen on a planet, biochemistry (such as photosynthesis)
carried out by some living organism must continuously replenish the atmospheric
gas. Now there is evidence that a planet with just 0.05% titanium oxide on its
surface (titanium oxide is known to be fairly abundant on the surfaces of rocky
planets in our solar system as well as on the Moon) could produce as much
atmospheric oxygen as Earth has, through an entirely abiotic photocatalytic
reaction, meaning that oxygen in an exoplanet's atmosphere is not necessarily,
but could be, a sign of life there.</span><o:p></o:p></span></div>
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<span lang="EN-US">Identifying a biochemical signature of life
on another world can be very tricky even for NASA, and the exploration of Mars
provides a good cautionary example. When the <a href="https://en.wikipedia.org/wiki/Viking_program">Viking Lander</a> landed on Mars in 1976, it looked for signs of Earth-like metabolism as a
possible indicator of life there. <o:p></o:p></span></div>
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<br /></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgSUg3c8zR1LmU44cIsTVGw6PvBq8myMLO1utW6VnLIyUWkp8NM2uS1ECq17Epz9s9JYxbLbu3Yt0j57rJ5DUD4IrzdwTWRbKEOs1vwhLHQPq79t9UGQCGe8tjfHdLmsdlHTZNZd6L7py2R/s1600/Viking_Lander_Model.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="516" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgSUg3c8zR1LmU44cIsTVGw6PvBq8myMLO1utW6VnLIyUWkp8NM2uS1ECq17Epz9s9JYxbLbu3Yt0j57rJ5DUD4IrzdwTWRbKEOs1vwhLHQPq79t9UGQCGe8tjfHdLmsdlHTZNZd6L7py2R/s640/Viking_Lander_Model.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">An artist's concept of a Viking lander sampling Martin soil. Image from NASA</span></td></tr>
</tbody></table>
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<span lang="EN-US">To test for this, it added radioactively
labeled liquid nutrients to a Martian soil sample. The premise is that if the
nutrients are consumed, a waste gas should be released and it should also be
radioactively labeled and therefore detectable. This should indicate that some
kind of metabolic reaction is consuming the nutrients, perhaps the work of a
simple unicellular life form such as bacteria or archaea. The test was positive
but after the initial euphoria died down a bit, subsequent tests revealed that
the reaction was instead a result of Mars's unexpectedly unique soil chemistry.
<o:p></o:p></span></div>
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<span lang="EN-US">An alternative definition of life is that
it possesses some kind of embedded instructions, not necessarily DNA or RNA but
something that works in a similar fashion. This definition covers all known
living organisms and rules out cases, which we know are not alive, such as a
wild fire, which grows, reproduces and uses energy and would be considered
alive by NASA's definition. Certain chemicals <a href="http://science.sciencemag.org/content/339/6122/936">can also act as if they are alive</a>.
Exposed to light and fed by chemicals, some compounds can form crystals that
move, break apart and form again. <o:p></o:p></span></div>
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<span lang="EN-US">The Wikipedia <a href="https://en.wikipedia.org/wiki/Life#Alternative_definitions">entry on alternative life definitions</a> describes and weighs several definitions that I don’t mention here,
and it is well worth a read. I was fascinated reading it and finding that as those definitions evolved over
time. There is gradual move away from Descartes-style <a href="https://en.wikipedia.org/wiki/Reductionism">reductionism</a> (living organisms are composed of parts that function together like a machine)
toward <a href="https://en.wikipedia.org/wiki/Systems_theory">systems theory</a> (the
complexity of a living organism – and life itself – <i style="mso-bidi-font-style: normal;"><a href="https://en.wikipedia.org/wiki/Emergence">emerges</a></i> from simpler organizations of non-living systems. This, as well as a move
toward a multidisciplinary approach to problems, is a very current <i>very big</i> trend
in science in general. As an example of this, last year NASA launched a
multidisciplinary approach called <a href="https://en.wikipedia.org/wiki/Nexus_for_Exoplanet_System_Science">T</a></span><span lang="EN-US" style="mso-bidi-font-family: Helvetica; mso-bidi-font-size: 12.0pt;"><a href="https://en.wikipedia.org/wiki/Nexus_for_Exoplanet_System_Science">he Nexus for Exoplanet <span style="text-underline: #128B02;">System</span> Science</a>, or NExSS,</span><span lang="EN-US"> to the search for extraterrestrial life, which will bring together
experts in many fields from universities and institutes across the U.S. They
will apply a systems analysis approach to existing and coming exoplanet data
that will help them interpret observations.<o:p></o:p></span></div>
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<span lang="EN-US">Within any of these life definitions, grey
areas exist but by trying to be specific, there is the danger that no single
definition will be broad enough to cover the vast array of permutations in
which life might exist in the universe. Science fiction writers are expert at testing
the boundaries of what could be life. Could <a href="https://en.wikipedia.org/wiki/Philosophy_of_artificial_intelligence">artificial intelligence be alive</a>?
What about non-physical sentience (see the <a href="https://en.wikipedia.org/wiki/Mind%E2%80%93body_problem">mind-body problem</a>)?
While these authors clearly deal in fiction, those same questions are also
seriously asked in the scientific community. The same kind of imagination sci-fi
writers use is needed to figure out what kinds of biochemistry are possible in
some of the extreme environments one might encounter on exoplanets. Honestly,
what could be more fun than that job?<o:p></o:p></span></div>
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<span lang="EN-US">Even when life is present on a planet, its
biosignature might not be readily detectable on its surface. To a distant alien
observer, Earth during various periods of its evolution could have been
mistaken for being completely lifeless. Life might have originated here well
hidden in deep ocean hydrothermal vents on what once appeared on the surface as
a forbiddingly hot, toxic and violent world. Later on, any biosignature would
have been hidden under ice that was kilometres thick when Earth <a href="https://en.wikipedia.org/wiki/Snowball_Earth">underwent planet-wide glaciation</a> a few billion years after life first appeared, a period that could have lasted for
millions of years.<o:p></o:p></span></div>
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<span lang="EN-US">Earth from afar then would look much like
<a href="https://en.wikipedia.org/wiki/Europa_(moon)">Europa</a> does now. Under -160°C ice
<a href="https://en.wikipedia.org/wiki/Europa_(moon)#Surface_features">as thick as 30 km and as hard as granit</a><a href="https://en.wikipedia.org/wiki/Europa_(moon)#Surface_features">e</a>,
life could exist <a href="http://www.nasa.gov/feature/jpl/europas-ocean-may-have-an-earthlike-chemical-balance">in Europa's dark deep subsurface ocean</a>.<o:p></o:p></span></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhxrs6_4Bzz-Ex2vAiEU20DnX-Yjk6ozmvzRHRTxqj6Q8ORGvEWVwL9Yvy7qdjQ1XHyDo87fnhSrwGoXhQM35cgQZei33k_Sd7Bs5xfyf9xQPXL0rs1UENJCSUO-aEO2vEOf1bzqyfyMNmE/s1600/PIA01295_Europa_Global_Views_in_Natural_and_Enhanced_Colors.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhxrs6_4Bzz-Ex2vAiEU20DnX-Yjk6ozmvzRHRTxqj6Q8ORGvEWVwL9Yvy7qdjQ1XHyDo87fnhSrwGoXhQM35cgQZei33k_Sd7Bs5xfyf9xQPXL0rs1UENJCSUO-aEO2vEOf1bzqyfyMNmE/s640/PIA01295_Europa_Global_Views_in_Natural_and_Enhanced_Colors.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Natural-colour image (left) and enhanced-colour image (right) of Europa taken by the Galileo spacecraft in 1997 from 1.25 million km away. Credit to NASA</span></td></tr>
</tbody></table>
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<span lang="EN-US">We could discover a similar exoplanet, with
no observable biosignature, and dismiss it even though life may be abundant.
This is one reason why astrobiologists do not discount the possibility of life
hidden deep under the ice layers of frozen moons and planets in our own solar
system, and why they try to keep an open mind with the discovery of each new exoplanet.<o:p></o:p></span></div>
<div class="MsoNormal">
<span lang="EN-US"><br /></span></div>
<div class="MsoNormal">
<span lang="EN-US">Next we will explore pathways from non-living organic chemistry to living biochemistry on early Earth, the only known planet or body where we know for certain this transition took place. Life from non-life is a huge mystery, one that determines how likely our universe is to have life on other planets.</span></div>
Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0tag:blogger.com,1999:blog-5313396495335219568.post-60072237870738537902016-09-03T12:25:00.001-06:002016-09-03T12:25:52.908-06:00Hello? Earth Calling . . . PART 2<div class="MsoNormal">
<span lang="EN-US">Are you looking for <b>Hello? Earth Calling . . . PART 1</b>? It's <a href="http://sciexplorer.blogspot.ca/2016/09/hello-earth-calling-part-1.html">HERE</a>.</span><br />
<span lang="EN-US"><br /></span>
<span lang="EN-US">Are Aliens Calling Us?<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">As our detailed exoplanet catalogue builds
up, there is always the possibility that we could pick up an alien signal. A
detectable electromagnetic (EM) signal would have to come from nearby because the signal (with no
loss due to reflection or diffraction by any close by particles or obstacles)
<a href="https://en.wikipedia.org/wiki/Free-space_path_loss">rapidly loses strength</a> according to the <a href="https://en.wikipedia.org/wiki/Inverse-square_law">inverse squar</a><a href="https://en.wikipedia.org/wiki/Inverse-square_law">e law</a>. Of the estimated 1400 stars within 50
light years of Earth, 40 have been confirmed to have planet systems. How many
of these might support intelligent life? How many of those might be using EM
radiation to send a signal? Would it be powerful enough to detect? </span><br />
<span lang="EN-US"><br /></span>
<span lang="EN-US">Earth is a
miniscule target from even 50 light-years away. A signal beam from this distance would have to be aimed right at Earth in order to be detected. Electromagnetic
waves might not be the only way to signal. Other potential methods that an
alien civilization could use to signal or communicate are discussed on <a href="https://en.wikipedia.org/wiki/Search_for_extraterrestrial_intelligence">this Wikipedia entry</a>.
Radio waves seem likely to be used because they are long enough to pass through
gas clouds and atmospheres without being reflected or diffracted. They can
travel unmolested a long way through interstellar space. Optical lasers have
also been proposed. Perhaps a signal could utilize a series of laser bursts,
but at least the ones we can make would be too
weak to detect over a nearby star's luminescence. If the aliens can manipulate
immense energies, they could use gamma ray bursts or even gravitational waves.
Alien civilizations could also inscribe a message in matter and send it over
long distances. NASA used this general approach (the <a href="https://en.wikipedia.org/wiki/Voyager_Golden_Record">Golden Record</a>) on the Voyager spacecraft.
An alien technology might be able to inscribe microprobes and program them to
travel to every habitable star system within a certain radius. Intelligent aliens
could also unintendedly leave technosignatures of their existence. For example,
light reflected from large <a href="https://en.wikipedia.org/wiki/Space_mirror_(geoengineering)">space mirrors</a> or <a href="https://en.wikipedia.org/wiki/Dyson_sphere">Dyson spheres</a> or the presence
of excess chemicals or radiation in one location could offer clues to their
presence. The technical difficulties of sending out a strong signature over
thousands of light-years doesn't rule out the possibility that millions of
intelligent alien civilizations are signaling within our galaxy but, depending
on how far away they are, it could take thousands to millions of years to
receive a signal (if it is powerful enough). By then, the signal would tell us
only that intelligent life existed long ago far away. Still, that discovery
alone would change how we view ourselves in this universe. We are making the assumption here that intelligent aliens would want to develop signaling technology and use it.
We have only our own species as a motivation reference.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Are We Calling Aliens?<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">How would an alien species know we are
here? Earth being monitored by intelligent technically superior aliens has been
a long running theme in science fiction. An unfriendly advanced alien species could
plunder Earth War-Of-The-Worlds-style for resources such as water, killing us
in the process. Or, aliens could be friendly. In the movie, Contact, aliens
bounced our first TV signals back to us as a "Hey, we hear you
Earth!" and, encoded inside them, they gifted us with wormhole spacecraft
technology. Signals from our radios* and TV's have been spreading out into
space around us for several decades, which could potentially alert aliens
within 60 or so light-years to our presence. Some wonder, as in Contact,
whether we should be advertising our existence to potential enemies. There are
several reasons why we probably have nothing to worry about. First, our sphere
of influence is tiny. As mentioned, broadcast signals grow increasingly weak as
they travel from their source. Second, while TV and radio signals were once
broadcast from huge ground stations at thousands of watts, almost all are now transmitted
via satellites at around 75 watts, and they are directed from aerials aimed down
at Earth rather than being shot out in all directions including space. There
may have been a window of a few decades when perhaps someone listening from as
far as the Gliese star system might have heard our broadcasts but they would
have needed sensitive technology to receive it because it would have been literally
billions of times weaker then what receivers on Earth picked up. More likely,
Earth would be discovered through indirect observation similar to what we are
using to detect exoplanets now, or through more advanced direct imaging
technologies. Here too the odds are not great because Earth is a very small
planet and would be difficult to detect from tens to hundreds of light-years
away. Still, aliens with biochemistries like ours might be excited to discover
us. Earth's liquid water and ice could be detected by their reflectivity and
the presence of our atmospheric (biologically produced) oxygen gas could be
detected by studying the absorption spectrum of sunlight reflected from Earth.<o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US"><span style="background-color: #fff2cc;">*Just a brief refresher on radio waves and
sound travel: The opener sequence of the Contact movie was misleading. Sound does not travel through
space because it is a mechanical wave that requires a medium (such as air or
water or even solid materials) to propagate. Radio waves travel through space
as <a href="https://en.wikipedia.org/wiki/Electromagnetic_spectrum">electromagnetic (EM) radiation</a>.
Shortwave radio and some AM radio frequencies are too short to pass through Earth's
<a href="https://en.wikipedia.org/wiki/Ionosphere">ionosphere</a>. Instead they bounce off the
ionosphere as if it were a solid barrier and right back to Earth. Longer
wavelength FM radio and TV frequencies travel right through the ionosphere into
space.</span><o:p></o:p></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span lang="EN-US">Our magnetosphere might be the first Earth
signal detected by correctly situated aliens. <o:p></o:p></span></div>
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<br /></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgl_1sQZaDq7lyziyLoorjte-0WFbIK25Ka0rUKd7Y3Fweito68y7xUW2oVk73f6-Yjnb8U_pH1TxnwA4pKgk3WvMvWi1KktvhZfdd5khMCYsCnZ0DEm5YuiF7AF3RRzq2cKzyRhm5NuMTS/s1600/Magnetosphere_rendition.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="347" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgl_1sQZaDq7lyziyLoorjte-0WFbIK25Ka0rUKd7Y3Fweito68y7xUW2oVk73f6-Yjnb8U_pH1TxnwA4pKgk3WvMvWi1KktvhZfdd5khMCYsCnZ0DEm5YuiF7AF3RRzq2cKzyRhm5NuMTS/s640/Magnetosphere_rendition.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">This is an artist's not-to-scale rendition of Earth's magnetosphere (with the purple outline) created for NASA.</span></td></tr>
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<span lang="EN-US">Recent data from the European Space
Agency's Cluster mission suggests that if aliens listened to Earth, <a href="http://www.space.com/5577-earth-cries-recorded-space.html">they might hear a loud and obnoxious series of chirps and whistles</a> if they were in the right location in space.
Intense radiation, called <a href="https://en.wikipedia.org/wiki/Auroral_kilometric_radiation">auroral kilometric radiation</a> emanates from our planet in a narrow beam of extremely intense (1 – 10 <i style="mso-bidi-font-style: normal;">million</i> watt) radio waves. The radiation
comes from solar wind particles striking Earth's magnetic field and
accelerating along it (called <a href="https://en.wikipedia.org/wiki/Cyclotron_radiation">cyclotron radiation</a>). Luckily for us our
ionosphere blocks it. Otherwise the noise would easily drown out all of our
broadcasts. Only satellites in high orbits above the ionosphere, such as the
<a href="https://en.wikipedia.org/wiki/Fast_Auroral_Snapshot_Explorer">FAST satellite</a>,
can detect them. On the flip side, similar magnetosphere radiation could be
used to detect exoplanets, at least those that are correctly oriented to Earth. It is not a biosignature (proof of life), however, but <a href="http://science.nasa.gov/heliophysics/focus-areas/magnetosphere-ionosphere/">magnetospheres</a> are protective bubbles that prevent planetary atmospheres from gradually blowing away in their star's <a href="https://en.wikipedia.org/wiki/Stellar_wind">stellar wind</a>. The presence of one around a planet is good sign, but not proof, that life could exist on it.<o:p></o:p></span></div>
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<span lang="EN-US">Picking The Right Planet: What Would Alien
Life Look Like?<o:p></o:p></span></div>
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<span lang="EN-US">If we want to undertake the long-term
mission of sending a probe to an exoplanet, the first step is to choose a
planet that has promising evidence of life. What are the signals of life on a
planet? To answer that question we must first figure out what life is. That
answer is nowhere near as easy as one might guess, but this is also where things
get very interesting. <a href="https://en.wikipedia.org/wiki/Astrobiology">Astrobiology</a> is a rapidly expanding interdisciplinary field that studies the origin and
evolution of life on Earth as well as the possibility of life on other worlds.
This field attempts to set the parameters of what life is and what it requires.
NASA offers <a href="http://www.nasa.gov/vision/universe/starsgalaxies/search_life_I.html">an online astrobiology magazine</a> that features many great articles worth a read.<o:p></o:p></span></div>
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<span lang="EN-US">Naturally, the science starts right here at
home on Earth, where life is abundant and comes in a stunning variety of forms.
Not long after NASA was established in 1958, the agency began to look for the presence
of life beyond Earth. It was a <a href="https://en.wikipedia.org/wiki/Viking_lander_biological_experiments">significant component</a> of the Viking missions to
Mars in
the 1970's. At the same time, biologists were beginning to discover fascinating
examples of microbial life on Earth that live without sunlight, in extreme cold
or heat, under intense pressure, in extremely acidic or salty environments and
even in highly radioactive environments. These microbes are now called
<a href="https://en.wikipedia.org/wiki/Extremophile">extremophiles</a>. These discoveries are
rapidly expanding the list of potential environments where life could arise.
The <a href="https://en.wikipedia.org/wiki/Curiosity_(rover)">Curiosity rover</a> has been
looking for environmental conditions favourable for microbial life since it
landed on Mars in 2012 as part of NASA's Mars Science Laboratory mission. <o:p></o:p></span></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhea6vOmeF1rF9s62F0FmeElB0wj7TPdhZhHmmAHUSNyyFd3Pzq1a54Lp3C2oeI762bgZz5y2C46Oj9I08NWzlThrxUp55lWwMddF9zBK0x0OZcuqfT0htZU_dHd436aqLv2kHAhZAlDxzL/s1600/PIA20316-MarsCuriosityRover-SelfPortrait-SandDune-20160119.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhea6vOmeF1rF9s62F0FmeElB0wj7TPdhZhHmmAHUSNyyFd3Pzq1a54Lp3C2oeI762bgZz5y2C46Oj9I08NWzlThrxUp55lWwMddF9zBK0x0OZcuqfT0htZU_dHd436aqLv2kHAhZAlDxzL/s640/PIA20316-MarsCuriosityRover-SelfPortrait-SandDune-20160119.jpg" width="460" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">NASA's Curiosity Rover took this selfie on Aeolis Mons, Mars, in January 2016</span></td></tr>
</tbody></table>
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<span lang="EN-US"><a href="http://astronomy.nmsu.edu/tharriso/ast105/making_sense.php.html">Chemical analysis of Titan's atmosphere and surface</a> by
the Cassini-Huygens mission to Titan, Saturn's largest moon, reveals that even
though Titan is far too cold to have surface liquid water, its atmosphere is
thick, chemically active, and filled with a diverse assortment of <a href="https://en.wikipedia.org/wiki/Organic_compound">organic compounds</a>. This list of
compounds includes complex chemicals such as polycyclic aromatic hydrocarbons
(PAH's), which are composed of two or more carbon-based benzene rings. Other
research suggests that PAH's <a href="https://en.wikipedia.org/wiki/PAH_world_hypothesis">could be possible starting materials</a> for the non-biological synthesis of compounds such as amino acids and nucleotides.
These are the raw materials for proteins and DNA (deoxyribonucleic acid), two
biomolecules that are essential even to the simplest forms of life on Earth. <o:p></o:p></span></div>
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<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGb5Igq97E0lom64VHUJQf9AnUxv5BEuCEX8Ul1TFDjAIn-NmuJV2h-jgNn1gQt-p2eSuwt5UdE-YZIGQ6wN6eSh0K8p3q9uC4p6GAFlcJ7uKX7Di4w98zUKmZ85Jme1IuyDRYxyQ3UZUa/s1600/Titan-Complex_%2527Anti-greenhouse%2527.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGb5Igq97E0lom64VHUJQf9AnUxv5BEuCEX8Ul1TFDjAIn-NmuJV2h-jgNn1gQt-p2eSuwt5UdE-YZIGQ6wN6eSh0K8p3q9uC4p6GAFlcJ7uKX7Di4w98zUKmZ85Jme1IuyDRYxyQ3UZUa/s400/Titan-Complex_%2527Anti-greenhouse%2527.jpg" width="386" /></a></div>
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Shown left is a true-colour image of Titan's hazy brown
hydrocarbon-rich atmosphere taken by the Cassini orbiting space probe. Arriving
at Saturn in 2004, the Cassini orbiter mission is still active.</div>
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<span lang="EN-US">Liquid surface water has long been thought
to be essential for life on a planet. It is life's solvent on Earth, the medium
essential for all known biochemical reactions. All of the reactions that take
place in our bodies, for example, occur in an aqueous (water) solution within cells and
between cells. Astrobiologists are now exploring the possibility that on other
worlds other molecules could take on a life solvent role. On Titan, hydrocarbons
such as methane or ethane <a href="https://en.wikipedia.org/wiki/Life_on_Titan">could act as a biological solvent</a>.
Unlike water, methane is nonpolar, and it is not as strong a solvent. This gives
it advantages and disadvantages. It would not transport substances through a
cell wall as easily as water does - disadvantage. It is less chemically reactive
than water – advantage (and perhaps a surprise to some when they consider its
flammability). Methane and ethane do not tend to break down large organic molecules
as quickly as water does (through hydrolysis). This means that complex
biomolecules would be more stable in such an environment, and its biochemistry
could take better advantage of <a href="https://en.wikipedia.org/wiki/Hydrogen_bond#Hydrogen_bonds_in_DNA_and_proteins">hydrogen bonding</a>,
a chemical reaction that is essential to building proteins, DNA and cellulose
in plants. In fact, in 1981 Isaac Asimov, a biochemist as well as famous
science fiction author, went further by suggesting that methane biochemistry
might do away with proteins altogether. Poly-lipids could <a href="https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Methane_and_other_hydrocarbons">instead fulfill protein's role</a> in a nonpolar solvent such as methane. A hypothetical cell membrane that could
function in liquid methane <a href="http://phys.org/news/2015-02-life-saturn-moon-titan.html">was computer-modeled in 2015</a>.
Instead of phospholipids (compounds of phosphorus, carbon, hydrogen and oxygen),
which build cell membranes on Earth, this membrane would be composed of carbon,
nitrogen and hydrogen. It would have the same stability and flexibility as the
phospholipid cell membrane used by life on Earth. <o:p></o:p></span></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh7kepIpeDZB6h50RSBqJjwON5ZLpI5aHCgZas4RMWjS_Y3B92hxLJl1y5J7IPRo7G6YYdCMw1PAy2tSTw0fk0Ym17jl1ktERZKECsgzGOugAITK6cNe9e8v2t0qHmMIoalDufeRIuDImqR/s1600/PIA10008_Seas_and_Lakes_on_Titan.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="518" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh7kepIpeDZB6h50RSBqJjwON5ZLpI5aHCgZas4RMWjS_Y3B92hxLJl1y5J7IPRo7G6YYdCMw1PAy2tSTw0fk0Ym17jl1ktERZKECsgzGOugAITK6cNe9e8v2t0qHmMIoalDufeRIuDImqR/s640/PIA10008_Seas_and_Lakes_on_Titan.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">False-colour radar mosaic of Titan's north pole region, showing dark blue methane/ethane/dissolved nitrogen lakes, oceans and rivers taken by the Cassini orbiter.</span></td></tr>
</tbody></table>
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<span lang="EN-US">Analogous to Earth's water cycle, Titan has
a methane cycle, which transports energy as it cycles between an atmospheric
gas and a liquid in its oceans, lakes and rivers. Imagine, under the gloomy
thick atmosphere of Titan, a -180°C hydrocarbon river is filled with
living organisms. Their biochemistry would be nothing like Earth life. Perhaps a cautious note to offer here is that because Titan is so cold far less free energy is available for biochemical reactions (life processes). Earth, in contrast, was downright hot when life first evolved here, as we will see later on. Still, as far as we know, life cannot be ruled out. The relatively low-cost <a href="https://en.wikipedia.org/wiki/Titan_Mare_Explorer">Mare Explorer</a> could be the first mission to directly explore Titan's hydrocarbon seas.
<o:p></o:p></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjNqfZ6loDy4juo4nSwTAWJB_zhHey8FdcRHaYI9v148ecFh9FQ2BNKlwefEQs0s3J7xvNQqCMRGdEpKuLlluxxy8a5L2GbyU7UAK-bVx-lSxNXhXEgqKPWSeCpjBDiKehOme7xMSHa9C9x/s1600/TSSM-TandEM-Lander.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjNqfZ6loDy4juo4nSwTAWJB_zhHey8FdcRHaYI9v148ecFh9FQ2BNKlwefEQs0s3J7xvNQqCMRGdEpKuLlluxxy8a5L2GbyU7UAK-bVx-lSxNXhXEgqKPWSeCpjBDiKehOme7xMSHa9C9x/s400/TSSM-TandEM-Lander.jpg" width="366" /></a></div>
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<span lang="EN-US">Right is an artists' concept of the Titan Mare
Explorer probe, provided by NASA.<o:p></o:p></span></div>
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<span lang="EN-US">A lander would splash down on one of the
planet's largest lakes, Ligeia Mare, and explore its chemistry and depth, as well as the surface
atmosphere and local weather, while taking photos. It was set to launch in 2016
but it remains at the conceptual stage. <a href="https://www.nasa.gov/content/titan-submarine-exploring-the-depths-of-kraken/#.V8iOfGWnP0f">Another proposed NASA mission</a> could follow up on the Mare Explorer mission by exploring its largest hydrocarbon
ocean, called Kraken Mare. While the Mare explorer would use a stationary
floating probe, this Titan Submarine mission, which could begin as soon as 2038, would be mobile. <o:p></o:p></span></div>
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<span lang="EN-US"><br /></span></div>
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<span lang="EN-US"><br /></span></div>
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<span lang="EN-US">This cute 2015 video below, showcasing the Titan Submarine, was created by the
NASA Glenn Research Centre:<o:p></o:p></span></div>
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<span lang="EN-US"><br /></span>
<span lang="EN-US">Still in its conceptual stage as well, this unmanned
submarine would investigate the ocean using a variety of probes. Let's hope that
something gets underway soon so we can begin to answer our questions about life
on this moon. Imagine the excitement if evidence for life with a biochemistry
different from Earth were discovered. It would completely change how
astobiologists approach the question of extraterrestrial life and it would
greatly broaden our hope that we are not alone in the universe. Finding no
signs of life there, though disappointing, would also be very useful data that
coud be used to refine future missions to other worlds.<o:p></o:p></span><br />
<span lang="EN-US"><br /></span>
<span lang="EN-US">Next we will delve deeper into what life is, its chemistry and where it could evolve.</span></div>
Gale Marthahttp://www.blogger.com/profile/16783635636114233136noreply@blogger.com0