All About the Particles in Physics

If you have read some of my other articles, you will have come across particles, many with strange names you may not have heard of before. In this article I try to show you what these particles are and how they fit into the physics of matter and energy.

What Exactly Are Particles?

When we say "particle," it's easy to assume we mean a tiny grain of matter, like a particle of dust. In physics, however, they have a technical definition:

Particles are excitations of quantum fields.

This sounds annoyingly complicated but we will need it later on as we explore. If you look up quantum field theory on Wikipedia, you may be further dismayed at the immediate plunge into advanced theory. What I hope to give you here is simply a feel for it, so I recommend clicking this link and going to the introduction. Even if you don't research quantum field theory on your own, I try to provide enough background so that you can see how it works to help us understand the physics definition of a particle.

The quantum field concept is fleshed out in what we call the Standard Model. This model is quickly becoming the go-to model for those of us trying to understand matter and energy. Everything in the universe is made up of 12 basic building blocks or particles, governed by 4 basic fundamental forces. These building blocks can be further divided into matter particles and force-carrier particles, and there are many of them. Since its development in the early 1970's this model has been able to explain a whole gamut of experimental results and has accurately predicted a wide variety of phenomena.

To understand particle physics, we will begin by figuring out what a quantum field is.

Excitation in a Quantum Field

A quantum field is a measurable physical property that is a part of every point in space-time. Space-time, you might remember, is a mathematical model that combines the three dimensions of space together with time. You can think of it as the ultimate backdrop of the universe. Everything else happens in space-time. It can move and it is stretchy - it can be distorted and even twisted. It is this kind of movability that requires us to use Einstein's theory of relativity. Phenomena can occur in different frames of reference and we must take this into account when we measure things like velocity and even time. Within this framework, one or more quantum fields may operate. Quantum field theory describes these operations, which we can measure as fundamental forces, such as the weak force, the strong force, electromagnetism and, possibly eventually, gravity.

Virtual Particles

Forces between particles are carried out by other particles. For example, the electromagnetic force between two electrons, perhaps during a chemical reaction or magnetic attraction, is carried out, or mediated by, an exchange of photons. The weak force, associated with radioactivity, is mediated by particles called W and Z bosons, and the strong force is mediated by particles called gluons. Each of these force-carrying particles, the photon, W and Z bosons and the gluon, all referred to as gauge bosons, is a virtual particle.

A virtual, or force-carrying, particle can be thought of as a quantized excitation in a field. It does not exist by itself; it cannot be measured except by measuring the force it is carrying. Real particles, on the other hand, can indeed by measured and they do exist by themselves.

Real (Elementary) Particles

All matter is made up of two kinds of elementary particles, leptons and quarks. Ordinary everyday matter consists of specific leptons called electrons as well as quarks, which make up the neutrons and protons within the atomic nucleus.

Ordinary Matter - The Atom

All matter, from helium to aluminum to uranium, is composed of atoms. The universe consists of at least 118 different kinds of atoms, which are differentiated from each other by the number of protons they have in their nuclei. All electrically neutral atoms have the same number of electrons as protons. If electrons are lost or added, these atoms become ions and they have a charge. For example an oxygen ion has a charge of -2, because it has two extra electrons associated with it, 10 electrons rather than 8. The number of neutrons can also vary among the same kind of atom. Most oxygen atoms have 8 protons and 8 neutrons, but some oxygen atoms may have as many as 9 or 10 or as few as 13 neutrons within their nuclei. These represent less stable nuclear arrangements, so these oxygen isotopes, as they are called, are unstable and radioactive as a result.

This is a pre-quantum simplified diagram of what an atom looks like:

Negatively charged electrons orbit around a nucleus composed of positively charged protons and neutrally charged neutrons. The modern quantum atomic model is far more complex but for our purposes right now, the simplistic Bohr-like model above works. Each neutron and proton consists of smaller elementary particles called quarks, which are bound to each other by gluons, virtual particles that carry out the strong force, shown by yellow lines below.

Categories of Particles - Fermions and Bosons

Protons and neutrons are called baryons. Ordinary matter, made up of protons and neutrons, is called baryonic matter. Quarks (baryonic) and electrons (leptons as we will see later) are all examples of fermions.

Particles can be placed in one of two broad categories based on their quantum state - either as fermions or bosons.

Fermions

Subatomic particles, both elementary such as electrons and composite such as protons, are examples of fermions. Fermions have a half-integer spin. What is this spin? The spin of an elementary particle, as far as we know, is an intrinsic physical property of it - the particle has no inner structure. Its spin is a specific angular momentum that can't be altered; it defines one kind of particle from another kind. The spin of composite particles, on the other hand, is usually understood as the total angular momentum of its constituent particles. Because fermions have half-integer spins (again, for reasons that are still not fully understood) they obey a law called the Pauli Exclusion Principle, which means that no two fermions in the same quantum state can exist in the same place at the same time. More than one boson, on the other can occupy the same quantum state at the same time. This difference in quantum state separates fermions (almost always associated with matter) from bosons (usually force carrier particles).

Every fermion has an antimatter twin particle. An electron's antimatter twin, for example, is a positron. These anti-particles can be fundamental or composite. There are antiquarks, anti-protons, etc., and they make up antimatter. An antimatter twin of you is theoretically possible but the jury is still out as to how much antimatter actually exists in the universe. At one point, shortly after the Big Bang, most researchers believe the universe was equally seeded with both kinds of matter. When a fermion and its antiparticle meet, they instantly annihilate into a burst of pure energy, which appears in the form of a fundamental boson, such as a photon. Bosons, force carrier particles, cannot be classified as antiparticles. They are simply their own antimatter twin, so there is no such thing as an anti-strong force or anti-electromagnetic force, for example.

Besides quarks and electrons, the building blocks of ordinary matter, there are many other elementary particles as well. We'll explore some of these next.

Different Kinds of Quarks; Different Kinds of Leptons

The quarks inside atomic nuclei are either up quarks or down quarks. There are four other kinds of quarks beside these, called bottom, top, strange and charm. These four all have higher masses and are unstable - they quickly decay into the stable up and down quarks of matter. They are created only in very high-energy collisions such as in cosmic rays or inside particle colliders. All quarks interact through the strong force.

Electrons are by far the most common kind of lepton but there are several other kinds of leptons as well. Electrons have the least mass and are the most stable of all the charged leptons. The charged leptons interact through the electromagnetic force as well as the weak force, but no leptons interact through the strong force. Only quarks do. The heavier charged leptons (the muon and the tau particles), like the exotic heavy quarks we just learned about, are made only in cosmic collisions and particle colliders. They quickly decay into ordinary electrons. In addition to charged leptons there are neutral leptons called neutrinos (which also come in different classes - electron neutrino, muon neutrino and tau neutrino). These particles are rarely observed because they rarely interact with anything. They have a very tiny mass and pass right through ordinary matter almost unaffected. They are produced during radioactive decay and fusion reactions (inside the Sun for example) and interact exclusively* through the weak force. Neutrinos have a very interesting story to tell, which we will read more about later on.

The diagram below lays out the 12 matter and 3 force-carrier particles we have explored so far. Fermions are either purple or green (notice that they all have a 1/2 integer spin). You might wonder what eV means. Instead of grams, the masses of subatomic particles are expressed in electron volts (eV), the energy equivalent of mass. To explain this, consider the annihilation of an electron and a positron. Both have an equivalent mass of 0.511 MeV. When they annihilate, 1.022 MeV of energy is produced, in this case in the form of a photon of that energy. This unit is a mass-energy equivalent unit. The bosons are orange, and we will explore these in more detail next. (Did you notice that the W and Z boson each have a mass value. Intriguing isn't it?)

copyright: MissMJ (Wikipedia)

All fermionic and bosonic elementary particles cannot be broken down into smaller substructures. Mathematically these particles are treated as single points, although some particle theories such as string theory give them a physical dimension. All elementary particles are thought to obey the laws of quantum mechanics. Remember that they are, strictly speaking, excitations of quantum fields. Within the quantum equations that describe these fields, particles are not points at all, but instead are wave functions. They express both particle and wave behaviours, depending on how they are being measured. This has been experimentally proven with both electrons and photons. A particle is also a probability amplitude of position and momentum. That means that its position and momentum (or energy or velocity) cannot both be measured at the same time. This follows the rules of the Uncertainty Principle. You will never be able to observe an electron or proton or gluon as some kind of moving particle under a powerful futuristic microscope. We cannot directly observe a wave function. However, this hasn't stopped physicists from being able to image individual atoms, or at least get a good approximation of them, using a new high-resolution electron microscope with a maximum resolution of less than 50 picometres. The diameter of a typical atom is about 100 picometres. Nanoparticles gave the scientists a great test material to use because they have a very small number of atoms spread out over a large surface area. This is a computerized image of a nanoparticle with each individual atom depicted as a yellow sphere:

Copyright: Swiss Federal Laboratories for Materials Science and Technology

It is important to keep in mind that this is an approximation. All atoms jostle around and vibrate, even as they approach absolute zero. They don't sit perfectly still in networks. However other weird things do start to happen to atoms when they get very cold. To find out, check out my article called "Very Hot, Very Cold."

Bosons

These are, for the most part, force-carrier particles. They all have an integer spin. There are composite boson particles such as mesons, superfluids and Bose-Einstein condensates, as well as fundamental boson particles. The fundamental bosons are all considered to be force-carriers. These are called gauge bosons. Their interactions are described by gauge theory.

The meson is made up of a quark and an antiquark. It is highly unstable and is formed only in cosmic ray collisions and colliders. These interesting particles interact with both the strong and the weak force, and there are many of them. Mesons, although they consist of quarks, are not fermions, nor are they constituents of baryonic matter. The quarks have their spins anti-aligned so together they make up composite particles of zero spin, and that makes them bosons. There are charged and uncharged mesons, and there are different mesons based on the types of quarks that make them up. Charged mesons are unique in that they interact with all four fundamental forces, weak, strong and electromagnetic, as well as, presumably, gravity, as they do have mass as well. Like the gauge bosons, the lighter mesons are indeed force carrier particles (the pion, for example, helps mediate the strong force) while the heavier ones may not be. Superfluids and Bose-Einstein condensates, on the other hand, are examples of atoms of ordinary matter acting like bosons under extreme conditions, and they are not force-carrier particles.

Here I will concentrate on elementary bosons - the gauge bosons (photons, W and Z bosons and gluons) as well as two theoretical gauge bosons - the Higgs boson thought to impart mass on other particles, and the graviton, the sought-after particle of gravity.

Gauge Bosons

These bosons carry out the fundamental forces, as virtual particles. W and Z bosons carry out the weak force. Photons carry out the electromagnetic force. And gluons carry out the strong force. What is interesting about these particles is that the W (2 kinds of these) and the Z bosons have mass while the photon and gluon do not. The reason for this is believed to be the result of symmetry breaking very early on as the universe evolved. This is a process through which scientists think our fundamental forces "settled out" of the universe as it expanded and cooled after the Big Bang (an explosion of tremendous pure energy from which the universe began). You can think of this process as analogous to ice "settling out" of water as it cools and freezes. If we run the universe's evolution backward through time, we see evidence that the separate forces and matter we have today recombine step by step as we move backward through an ever-increasing energy environment. We ultimately reach a point where all energy and matter are recombined into one universal "mother" energy, the grand unified force.

This simple diagram shows how the fundamental forces broke from a theoretical grand unified force soon after the Big Bang:

The image below displays the evolution of the fundamental forces as well as particles and structures such as stars:

(copyright: John von Neumann Institut fur Computing)

Each colour band is a phase transition, where symmetry is broken. Notice the energy decreasing over time.

The weak force and the electromagnetic force were coupled together into one force called the electroweak force when the universe was very young and filled with intense energy (in any environment where the energy is high enough, these forces would recombine, as in high energy colliders for example). This is the point where the Higgs boson comes into the story. The Higgs boson is a massive theoretical force-carrying particle that is thought to impart mass on other particles, through an interaction with its force field, called the Higgs field. According to the Higgs theory, at this point in the universe's evolution, the three weak force bosons and the Higgs boson were all coupled together to a Higgs field, gaining mass due to the Higgs mechanism. When the electroweak force broke into the weak force and the electromagnetic force (through symmetry breaking), the W and Z bosons remained coupled to the Higgs field (and a Higgs boson was released) while the photon did not, and so that is why the photon is massless. The W and Z bosons, though massive, do exist as virtual particles mediating the weak force. Their mass, in fact, explains why they have such a short range of influence. Like the photon, W and Z bosons also exist as real particles as well. Gluons are thought to exist de-confined, or free from quarks (which themselves are de-confined from nucleons), as real particles in quark-gluon plasma inside the incredibly hot dense cores of neutron stars.

Higgs theory predicts a massive Higgs boson, which has not yet been observed. A great deal of energy would be required to create it (equivalent to the energy of the very early universe) and physicists at the Large Hadron Collider at CERN are currently working on producing one. In fact, just today as I write this article (December 2010), scientists at CERN released an announcement that they may have already glimpsed the Higgs boson! They saw excess energy spikes corresponding to around 124 - 125 GeV during very powerful particle collisions, and these spikes correspond to a predicted Higgs energy signal. Now they need to rule out background noise. A more definitive announcement could come within the next year.

The strong force broke from the electroweak force even earlier on, mediated by gluons. One of the figures above shows gravity breaking the earliest of all the fundamental forces. It is a guess; no one knows if gravity, like the other three fundamental forces, came into existence with a symmetry-breaking event and an associated lowered energy state, releasing a field and an associated gauge boson particle, the graviton. There are many theorists currently working on this problem. Somehow the acquirement of mass is coupled to gravity, as if mass is equivalent to the gravitational charge of a field. Is there an interaction between the Higgs field and the gravitational field? We must also consider that gravity, according to general relativity, more accurately acts on the energy of a particle (it acts on the particle's stress-energy tensor). This means that gravity only looks like it's interacting just with the mass of an object when it is non-relativistic (when it is at rest). When that particle approaches light speed, gravity is interacting more accurately with its momentum, its total energy in other words. As well, some theorists think of gravity not as a fundamental force at all but an emergent property of the universe itself.

Higgs Boson

Fermions attain their mass through interaction with the Higgs field, but not in the same way as W and Z bosons do. We know that force particles arise from fields spread out over space and time. Parameters in the equations for the field associated with the Higgs boson can be chosen so that the lowest possible energy state of that field (the empty space of our universe today) is a non-zero value. The equations predict that all particles that can interact with the Higgs boson gain mass from the interaction (which comes from the non-zero energy). The mass of the particle in this sense comes from its inertia or resistance to motion when we try to move it while it is being "grabbed at" by the Higgs particle with which it is interacting. In terms of thinking of Higgs as a field, when a particle moves through the Higgs field, it distorts it - the field clusters around the particle, giving it mass. If the Higgs boson is discovered, it will fill in a major piece predicted by the Standard Model, giving us even more confidence in the validity of this model.

We now have an idea of what some of the various fermions and bosons are and how they interact with each other. The diagram below illustrates those interactions:

You might have some questions about this diagram. Quarks, for example, interact with the strong force through gluons, as we now know. But you will see here that they also interact with the weak force through the W and Z bosons. This occurs during a process called quark transmutation. During nuclear fusion and radioactive decay, up quarks decay into down quarks (fusion) and down quarks decay into up quarks (decay). You might also wonder how quarks interact with photons. It occurs through pair production, a process in which two very high energy gamma photons smash into each other, annihilating themselves and producing a proton/antiproton pair (other particle/antiparticle pairs can be produced, depending on the energy of the two photons). Remember that protons, like neutrons, are composed of quarks.

Next, we will move beyond the Standard Model to explore some particles that are more difficult to define.

Graviton

The Standard model does an impressive job of describing three of the four fundamental forces in terms of how they evolved, how they are related to each other, and how they are mediated by particles. Gravity, on the other hand, remains a mystery. Gravitons, theoretical spin-2 bosons, are unique in that they describe the behavior of space-time itself while the other forces play out "on the space-time stage." Any attempts to describe gravitons in the same way as other bosons are met with severe difficulties. And yet, it seems reasonable to expect a force-carrier particle that mediates the fourth fundamental force of gravity. Please take a look at my article called "Gravity" for a full exploration of gravity and the hypothetical graviton.

Dark Matter Particles?

In astronomy, dark matter is matter that neither emits nor scatters light (or any other electromagnetic radiation) so we cannot observe it. However, it is believed to make up about 83% of the matter in the universe. It is the "missing mass" that accounts for higher than expected orbital velocities of galaxies inside galaxy clusters and observations of gravitational lensing caused by galaxy clusters.

Some of this matter may just be ordinary matter that doesn't emit any electromagnetic radiation. These could be unassociated planets, brown dwarfs, dead carcasses of burnt out stars and possibly even lone black holes, but researchers think these together could only make up a small fraction of the total dark matter, and the rest, they believe, is nonbaryonic matter. Recall that a baryon is simply a composite particle made up of three quarks, protons and neutrons. Non-baryonic matter is matter that isn't made up of atoms, and in this case it also does not interact with ordinary matter through the electromagnetic spectrum. That means it has no electric charge, for example. The only way we can detect it is through its interaction with gravity. One of the leading theories suggests that this dark matter may be made up of neutrinos. These particles have no charge, and a very small (and still disputed) mass. They do however interact very occasionally with ordinary matter.

The Enigmatic Neutrino

Neutrinos are created during some kinds of radioactive decay, nuclear reactions and when cosmic rays hit atoms. The young universe may have been flooded with these particles. Today, we get most of our neutrinos from the Sun. About 65 billion of them pass through each square centimeter of Earth every second! The 1995 Nobel Prize in Physics went to a research group who managed to directly detect the neutrino, or at least its antimatter twin: Antineutrinos created in a nuclear reactor reacted with protons producing neutrons and positrons.

Neutrinos come in different flavours - electron neutrinos, tau neutrinos and muon neutrinos. Strangely enough, they can oscillate between these flavours as they propagate through matter (you need to detect all three when you are measuring them). We already know that neutrinos have no charge. However if they have even a tiny mass, they may also have a small magnetic moment and because of this moment they could theoretically interact weakly* with the electromagnetic spectrum, adding to the neutrino puzzle, which now has an even larger mysterious piece to contemplate: New research reveals that neutrinos may travel faster than light speed! If neutrinos are massless, as was first thought, there is no reason why they shouldn't be able to travel at light speed just as photons do. But if they do have even a tiny mass, Einstein's law of special relativity states that their mass would approach infinity at light speed (or put another way, it would take an infinite amount of energy to get a particle with mass to light speed). And yet, the OPERA collaboration experiment measured neutrinos exceeding light speed in September and again in November this year (2011). Obviously, researchers are wary about this new data and are busy trying to further confirm it. If it turns out to be true, we have a big problem on our hands: Einstein's theory of special relativity, upon which the cornerstone of all relativistic theory is based, states that nothing can travel faster than light speed. And yet, general relativity does not necessarily rule out all faster than light speed: If a distortion in space-time allows space-time itself to move faster than light speed (and there is no rule forbidding this), it can carry a particle (or anything else) along with it, at faster-than-light speed. In this case the particle, by way of the same theory, would travel backward through time, much like a theoretical particle called a tachyon does. Neutrino research is a very hot area right now, for good reason.

Besides neutrinos, dark matter could also be made up of exotic particles such as axions or supersymmetric particles.

Axions - Another Candidate for Dark Matter

Axions are theoretical particles that have no electric charge, a very small mass, on scale with neutrinos, and very low interaction with the strong and weak forces. As a result, they interact only very weakly with ordinary matter. They are predicted to change into and from photons in the presence of strong magnetic fields and this may be used to try to detect them. Axions may have been made en mass during the Big Bang and they should be very stable (like neutrinos) and so they may contribute significantly to the hidden mass of galaxy superclusters.

Supersymmetric Particles?

Finally, supersymmetric particles may contribute to the mass of dark matter. According to supersymmetry theory, every fermion should have a partner boson and every boson should have a partner fermion. These shadow particles are heavier, with masses up to a thousand times greater than the their corresponding real particle, so it will not be easy to create one in a collider. However, the Large Hadron Collider at CERN can now create the extreme high-energy environment needed to begin to look for these theoretical particles.

(From: The Particle Adventure (http://particleadventure.org/supersymmetry.html))

There is no direct evidence for the existence of supersymmetry. It is a possible solution to several theoretical problems in which the universe now exists as a broken symmetry in which supersymmetric particles are far heavier than their real twins and therefore only exist when an extreme high-energy environment is recreated. I find it intriguing to think of supersymmetry as a theoretical mirror in which you can see the force particle twin of a particle of matter and vice versa, and that the line we easily draw between matter and energy can be completely erased. In fact, the line didn't even exist right after the Big Bang, not until some expansion and cooling, some settling of energy, could allow for the first of many symmetries to be broken. There is snag here: Supersymmetric particles are all high-energy particles thought to exist only under the universe's extreme energy conditions. How could any particle exist today in our relatively quiescent universe and impart hidden mass? This is where a new particle, the neutralino, comes in. There are four theoretical neutralinos that are fermions with no charge, the lightest of which should be stable and could exist in the universe today. They are all supersymmetric partners of bosons. It won't be easy to find, however. In most models, all supersymmetric cascade decays (which can hopefully be produced soon at CERN) end up decaying into this particle, which then will leave the detector unnoticed, because these light neutralinos should only interact with W and Z bosons. The only way to infer their existence is to look for unbalanced momentum in the detector. Even so, if the neutralino could be detected, it would boost both supersymmetry theory and help us figure out what dark matter is. Other attempts at direct neutralino detection are now being considered as well.

Conclusion

We are all fairly familiar with the particle theory of matter. Molecules break down into atoms and atoms break down further into electrons, proton and neutrons, and these nucleons break down further still into quarks. It is quite easy to visualize this concept of matter. For us to move beyond this to understand both matter and energy as particles, however, we must get comfortable with a new way of approaching the problem. We must embrace the idea of a particle, either matter and energy (or force), as a quantum entity, a wave function. It isn't easy or even comfortable to revisit the atom as a cloud of probabilities. In our everyday experience, atoms seam so much more steadfast and reliable than that. How can the atoms in my cup of coffee be wave functions?

This new approach pays off for us because now we can conceptualize (almost) all the forces that operate in the universe as well as matter. We can begin to appreciate the idea that particles of force and particles of matter are actually very closely related to each other, perhaps they are even mirrors of each other. And with this quantum approach we can begin to peel away at the ultimate mystery of how forces and matter came to be in our universe.

For an extensive list of currently known particles, click on this link.

This 5-minute video from CERN explores the Standard Model of particle physics in an easy-to-follow format, nicely summing up this article:



I also recommend an excellent on-line resource called The Particle Adventure. I suggest starting with the Standard Model and going from there. It will help you understand much of what I've tried to cover and it's great for kids from age 12 to 112!

Gravity

Of all the four fundamental forces, the force of gravity is the one most familiar to our daily experience. We are even born with innate sense of this force. It is called the Moro reflex, demonstrated in this brief video:



This basic and primitive reflex is a response to a sudden loss of support, of falling in other words, and it is built into us in incomplete form as early as 28 weeks gestation. By week 34 it is fully developed, and by around 3 to 6 months of age it disappears once again. By then, we are well into exploring the force of gravity by dropping objects and watching them fall, and soon after, we are falling down ourselves as we learn to walk. Throughout our lives we experience the force of gravity and without it, we can quickly become disorientated, as test pilots and astronauts can attest.

And yet, gravity, for all its familiarity, is the least understood fundamental force. In this article we will explore how it works and how it fits into current theories about forces and the particles that mediate them. In the process we will acquaint ourselves with the mystery of gravity, a mystery that will inevitably lead us to a larger ultimate mystery.

You've Come a Long Way Baby: The Changing Face of Gravity

Newton's Gravity

In the late1600's we thought we had the force of gravity all wrapped up. Isaac Newton famously sat under an apple tree and when an apple fell on his head he had an epiphany, or so the legend goes: Apples fall to the ground because they are attracted to it. And the ground is also attracted to the apples. In 1687, he published this idea in his Theory of Universal Gravitation. Gravity, he discovered, is a predictable force of attraction that acts on all matter in the universe. It is directly proportional to the mass of the objects in question and is inversely proportional to their distance from each other. Newton's  law of universal gravitation looks like this:

where F is the gravitational force between the masses,
G is the gravitational constant,
m₁ and m₂ are the masses of the two objects,
And r is the distance between.

G, the gravitational constant, with a value of approximately 6.67 x 10^-11N (m/kg)², appeared in Newton's equation but it was not actually measured until 71 years after his death, by Henry Cavendish in 1798. Newton simply knew that the values were proportional to each other and so a constant of some kind was needed to express this relationship (and he was genius enough to know this would be a very small number).

Cavendish set out to confirm Newton's theory and to confirm his constant, G. It was no easy task because this constant, as you saw, is an extremely small value. He ingeniously created a torsion balance, shown in this diagram:

Two masses are suspended at either end of a bar, which is suspended from the ceiling by a thin wire. Attached to this wire is a mirror off which a beam of light is reflected. When he brought a third mass close to one of the suspended masses, it attracted one of the ends of the torsion balance causing the whole thing to rotate, slightly changing the direction of the beam of light. By carefully measuring the angle of deflection he could measure the extent to which two known masses attracted each other and he could get a value for G to within 1%. What's so cool about this experiment is that such a simple apparatus can measure a force so infinitesimally small. We can only feel Earth's gravitational attraction to us because Earth's mass is so big, and that is why in everyday life everything seems to fall to Earth. In fact, Newton's apple and the Earth fall toward each other and Newton understood that. He also understood that the distance of projectiles depended on both Earth's gravity and their velocity. If a projectile were shot out horizontally with increasing velocity its trajectory would become longer and longer and eventually, with enough velocity, it would never hit the ground. It would, in essence, be in orbit around Earth. And this is how the Moon orbits Earth. The Moon is constantly falling to Earth but its velocity keeps it in orbit. If the Moon's mass increased or it's orbit slowed down, it's orbit would decay and it would eventually slam into Earth. This brief 2-minute video explains this concept of orbit:



He quickly realized he could use this value, and the acceleration due to gravity worked out by Galileo in the 1600's, to accurately measure Earth's mass. Soon, scientists were using the formula to predict the behaviours of planets, comets and asteroids. It was used to predict the presence of the planet, Neptune, as well as the trajectory of Halley's comet. It was a brilliant step forward but it was not to be the final word on gravity as Albert Einstein, a little over 100 years later, would demonstrate.

Einstein's Gravity

For over 200 years, Newton's theory of universal gravitation remained unquestioned - it was very successful at describing the motion of objects. Gravity, according to scientists in this age, was an attractive force between masses that acted universally across the universe.

In the early 1900's, however, this nicely sewn up theory began to unravel when Albert Einstein, dealing with the ramifications of a universal speed limit on light, realized that if light speed could not change then space and time had to. He then set about attempting to describe this elastic space-time that this theory of special relativity required. He formulated a mathematical framework for space-time, an invisible framework that underlies the universe, which can be stretched, twisted and warped. If you would like to know more about this discovery, I explore Einstein's space-time in my article called "Time."

Einstein soon realized that not only were time and space themselves at the mercy of space-time stretching, so was gravity. In fact, gravity was the stretching, or the shape of space-time. This would become the seed of his theory of general relativity.

This clip from the NOVA series, The Elegant Universe, describes how our understanding of gravity evolved from Newton's time into Einstein's theory of gravity, with a slightly different twist than what I have outlined here:



We experience gravity as the motion of objects following the curvatures of space-time. This concept challenges the idea that gravity is a force at all, and we will explore this implication in greater detail soon.

You may have seen diagrams in which heavy objects, Earth for example, bend this space-time fabric. In fact, NASA physicists are measuring that curvature right now using the Gravity Probe B. This is a diagram of space-time curved around Earth and the probe:

They are finding that there is indeed a space-time vortex around Earth and its shape matches Einstein's theoretical predictions. As you think about space-time, please keep in mind that this diagram shows us a 3-dimensional curvature. In reality, the curvature is 4-dimensional, through the 3 dimensions of space and 1 dimension of time. This 4-dimensional shape is impossible for us to visualize. Its geometry is instead described using mathematical formulas.

Einstein expanded his theory of special relativity to describe the behavior of masses in space-time, and he called this new theory general relativity (the earlier link above brings you to an introduction, click here for a more in-depth discussion). In a nutshell, mass causes space-time to curve. Do you see how this new concept does away with Newton's idea of an attractive force between masses? For most of our everyday experience, Newton seems right - gravity appears to act as an attractive force - but that is only how it appears. The idea that outer space, seemingly an empty vacuum, has a built-in structure that can be altered according to certain rules is still very weird to us. If you have read some of my earlier articles, you might not be so shocked, however - remember how perfectly empty vacuums, at their tiniest fundamental level, teem with unpredictable quantum fluctuations? Einstein's bizarre idea of space-time broke the way for us to re-visit the mysteries of the universe from a new perspective. Weaving this new perspective together with a quantum mechanical description of the universe, which is, itself, utterly mysterious at its core, means that we must weave two very weird and mysterious concepts together in order to approach the heart of gravity. Let's take this one step at a time and begin where it all started, with general relativity.

General Relativity

To understand gravity we must learn the basic ideas behind general relativity, and in order to appreciate general relativity, we must abandon our old notions of space and time and embrace space-time. You may want to begin by thinking of space-time as an invisible stream flowing ever onward, bending in response to objects in its path, carrying everything in the universe along with it as it twists and turns, as was elegantly described by physicist Hans von Baeyer.

This theory can be daunting but I hope to guide you toward an intuitive feel for how it works. Einstein took his theory of special relativity and expanded it to take into account areas that are accelerating with respect to each other. Recall that relativity is all about which frame of reference you are measuring something from. In physics we call these areas non-inertial frames of reference. This theory is written out as a set of field equations that describe and relate the curvature and the distribution of matter within space-time. These equations can be used to represent the geometry of space-time, much like the diagram of Earth shown above. In developing general relativity, Einstein had to refine his concept of space-time into these much more mathematically precise field equations. He also developed the idea that all space-time coordinates, all parts of space-time, are treated equally by the laws of physics. Like the speed of light, the laws of physics are treated as universal throughout the universe. This means that, for example, a skydiver in free fall, experiencing an acceleration of 1 g (9.8 m/s²) is experiencing exactly the same thing as an astronaut in a spaceship that is accelerating at a rate of 1 g through empty space. As the skydiver falls toward Earth, he is falling, in a sense, toward a space-time "dent," Earth's dent. In the accelerating spaceship, where there are no dents, that is, no curvatures of space-time caused by nearby planets or stars, the geometry of space-time would appear to curve into exactly the same shape as it's curved around Earth. This is the essence of relativity.

Objects with mass curve space-time and they also follow the shortest path as they move through space-time. The orbits of planets around the Sun are examples of objects taking the shortest paths, the paths that require the least amount of energy. In physics, this path is called a geodesic.

Light also follows the curves of space-time. In 1919, not long after Einstein published his theory of general relativity, astronomer Arthur Eddington measured the deflection of light caused by the Sun during a solar eclipse, precisely matching Einstein's prediction.

Gravitational Time Dilation

Einstein's theory of special relativity predicts that time is relative - it appears to slow down at very high relative velocities. When this theory is expanded and refined into general relativity, gravity, or the curvature of space-time by mass, will also appear to slow down, or dilate, time. Gravity also stretches or shrinks distances perpendicular to the gravitational field. To outside observers, time would even appear to slow to a stop at the event horizon of a black hole, which curves space-time so intensely that the curvature becomes infinite. Nothing, not even light, can escape this maximum curvature of space-time. To learn more about black holes, please see my article called Stellar Objects Part 5: Black Holes. We should keep in mind that black holes are theoretical. No black holes have yet been directly observed but there is growing physical evidence for them based on their predicted effects on the motion of nearby stars and other bodies.

Gravitational Waves

Einstein's field equations also predict that the motion of a mass through space-time will cause a disturbance in it, an oscillation that travels at the speed of light. These waves travel right through matter, but their strength weakens proportionally to the their distance from their source. Such a wave will alternately stretch and shrink distances as it travels, but this is a very subtle effect. Even if a quite strong gravitational wave travelled through Earth, it would affect distances on the scale of the diameter of an atom. Gravitational waves also exhibit another strange quality: If an object itself emits gravitational waves, its mass is predicted to decrease. You might recall that, according to special relativity, mass is dependent on the velocity of the observer. This is called relativistic mass, and it stems from Einstein's concept of mass-energy equivalence. In contrast, an object's rest mass is its Newtonian, or invariant, mass. These two definitions of mass play a functional role in particle physics where particles are accelerated to enormous velocities. Gravitational waves have been documented. The 1993 Nobel Prize in Physics went to two researchers, Russell Hulse and Joseph Taylor who were able to prove that a binary system consisting of two neutron stars orbiting each other, called PSR1913+16, emits gravitational waves. As I mentioned, mass and energy are equivalent. The orbital period of one of the stars is decreasing at precisely the rate predicted if the system were losing mass/energy by radiating gravitational waves.

You may want to think of gravitational waves like this: An object with mass creates a curvature in space-time. If that object moves around through space-time, that curvature changes to reflect its changing location. Under certain circumstances, if that object is accelerated, it can generate a disturbance in space-time that spreads like ripples on a pond. These are gravitational waves, also called gravitational radiation. You might be wondering what these special circumstances are. Acceleration that is not perfectly symmetric (such as an expanding and contracting sphere) or cylindrically symmetric (like a spinning disc or sphere) can generate gravitational waves. I can use a spinning dumbbell as an analogy. If the dumbbell spins like wheels on an axis, it won't generate any gravitational waves but if it tumbles end over end (like the neutron star binary system), it will. The faster it tumbles, the greater the gravitational radiation it will give off. As you might imagine, astrophysicists are busy right now looking for gravitational radiation signatures of various objects such as supernovae, provided the explosion is not symmetrical. These signatures might also provide information about the mysterious black holes mentioned earlier, and about the Big Bang itself. Remember the example of the spaceship accelerating a 1 g, where space-time would appear to curve? That ship is creating gravitational waves but they would be extremely small and virtually undetectable, even right at the source, the ship itself.

An experiment under development to measure gravitational waves is a joint NASA/ESA effort called the Laser Interferometer Space Antenna. This is an artist's conception of how it will work:

Each of three spacecraft will be placed in an individual Earth-like orbit around the Sun, flying in a triangular formation with equal arms of 5 million km. The spacecraft will measure tiny warps in space-time caused by gravitational waves as they pass through the triangle, compressing and stretching it. The shape and timing of waves will be measured and they will hopefully help us learn about the systems that created them.

Gravitational Lensing

Light follows the curves of space-time. In 1919, Eddington proved it, as we learned earlier. This phenomenon is called gravitational lensing. Because light is deflected in a gravitational field, light from a very distant object can reach an observer along two or more paths. As a result, an observer could see the same object in two or more places in the night sky. A famous example is called the Einstein Cross, shown here:

This is a gravitationally lensed quasar, four images of it, around a foreground galaxy. The gravitational field of the galaxy (400 million light years away) bends the light from the more distant quasar (8 billion light years away). It is important to remember that light is simply following the space-time geometry. The photons themselves have no mass and they are not attracted or pulled in by large masses nearby. This is exactly why photons are lost in a black hole. The black hole doesn't suck them in - the photons are simply moving through the extreme space-time geometry that the black hole creates. From each photon's point of view it is traveling in a straight line. Again, this is the nature of relativity.

Quantum Gravity

General relativity, though modern, is an example of a classical theory - it does not take into account quantum mechanical effects. A quantum version of general relativity is modern physic's version of the Holy Grail. It has been an incredible ongoing challenge to formulate this quantum version. However, two theories, string theory and loop quantum gravity, show some promise toward developing a theory of quantum gravity. We will explore their promise and their pitfalls in a moment but first, I would like to stress the enormity that an ultimate break-through represents to the scientific community, and to all of our understanding of how the universe works.

The "Holy Grail" - a Word of Caution

If you have been reading my articles you will notice that I tend to come back again and again to the dream of reaching a single unified theory of "everything," as it is often called, that is, of combining relativity (the physics of the very large) and quantum mechanics (the physics of the very small) into a single theory that makes sense and gives us a smooth picture of phenomena across the entire spectrum of scale. Perhaps it would be appropriate for those of us yearning for such a breakthrough to wonder why we desire this so much. Physicists are human beings and, like most of us, have a strong desire to make sense of things. It seems logical that our universe should operate in a common-sense predictable manner. However, as some of my articles have hinted already (check out the article on the nature of light for example), there are inner workings that are simply mystifying. It's easy to forget that universe doesn't owe us a simple explanation and perhaps this yearning for meaning is more about our nature than about Mother Nature. I suggest that as you deepen as a scientific explorer, try not to leap to one theory or another for quick answers. In this article in particular you are about to come across a lot of theories and none of them will be totally satisfying. Explore and read everything (including this!) with a grain of salt, and try to find a tolerable space within the mystery. As you develop your taste for the unknown you may find that, at the point precisely where we don't have any answers, there is extremely fertile ground where our sense of wonder has roots, and where new scientific understanding has opportunities to grow.

Quantum Gravity, General Relativity and the Graviton

Physicists are in hot pursuit of a workable theory that describes gravity at the quantum level. The other three fundamental forces (strong, weak and electromagnetic) can all be described in the language of quantum mechanics. Gravity, for several reasons, is not nearly so easy to pin down. For example, these three forces are measurable at the level of the very small but gravity is so much weaker - for practical purposes, gravity can even be excluded from quantum mechanics equations, and it currently is. We have no experimental evidence for gravity operating at the quantum level. However, physical behaviours on the scale of the very large, such as how stars and galaxies work, depend almost entirely on the physics of gravity. Our ability to describe the behaviour of gravity breaks down when we talk about very large masses squeezed into very small volumes (black holes, the Big Bang). Neither general relativity nor quantum mechanics can be used alone to describe it - both are required together and we do not have the theoretical framework to do that. We have a problem of scale when we try to design experiments that test quantum gravity. We also have a problem of assumptions on how the universe works. For example, quantum mechanics depends on quantum field theory in which each fundamental force is mediated or carried out by a particle within the flat space-time described by special relativity. Gravity, on the other hand, is modeled on the curvature of space-time. According to general relativity, space-time geometry is dynamic - it can't be independently pinned down to any kind of fixed background (the consequences of this are profound and they still have yet to be worked out! - this is what makes general relativity so hard to grasp). Quantum mechanics, in contrast, depends on a fixed background (loop quantum theory attempts to come up with a quantum dynamics framework that is background-independent as we will soon see). When the equations for quantum mechanics are combined with those describing general relativity, you get infinite values that don't cancel out. Another way of putting this problem is that under low energies, quantum gravity reduces down to gravity as defined by general relativity but at very high energies, such as within black holes, gravity cannot be described because quantum effects tend to take over and those infinite values tend to take over the equations used to describe it. Physicists have been reworking these equations, some of which are very complex, for decades in an attempt to put them together in a sensible way with some limited success. For example, the structure of general relativity can be shown to follow, at least in an incomplete way, from the quantum mechanics of a theoretical massless spin-2 particle, and this has been named the graviton. It would be the force carrier or mediator particle for the gravitational field. Gravitons are special in that they play a role, unlike the particles mediating the other fundamental forces, in defining the space-time in which the other particles carry out their various functions. The existence of a graviton agrees with both string theory and loop quantum theory (in fact these two theories require it), which I will describe next, beginning with strings.

String Theory

What is gravity according to string theory and how does it propose to knit two mutually exclusive views of the universe together?

String theory originated as a theory that describes the strong force but researchers soon realized that the graviton, as well as all other mass and force particles, could be described mathematically as strings. Here, the condensation of certain vibration modes of a very tiny 1-dimensional string is equivalent to the modification of an original background, that being a field. It is a weak form of background dependence, which makes it not quite perfect for some theorists. A quantum theory of gravity should be background-independent. An exchange of gravitons should be equivalent to a change in background. This is a fancy way of saying that gravity itself is the curvature, the change, in space-time. You can't superimpose this on another field. Another pesky problem is that string theory comes in several equally workable forms, and no one knows what this means. Finally, the strings we are talking about are infinitesimally small, so small it is almost impossible to imagine how physicists could prove it experimentally. String theory itself, since its inception in 1969, has had its golden moments when everyone wanted to study it as well as its doldrums when no respectable researcher would touch it with a ten-foot pole. Right now, many prominent physicists, such as Stephen Hawking, Edward Witten and Leonard Susskind, believe it has some real potential toward combining quantum mechanics and general relativity into one grand scheme. Hawking goes as far as to say that M-theory, one kind of string theory, is the only plausible candidate for a unified theory. Richard Feynman and Sheldon Glashow, however, view it as more philosophy than science - what good is a theory that can never be verified through experiment? Is that not the heart of physics, that which distinguishes it from other "softer" sciences?

What exactly is string theory? Other models consider particles to be either zero-dimensional points or billiard-type balls. In string theory, particles are very tiny (Planck-length) 1-dimensional strings. This diagram gives you an idea of how these fundamental strings make up matter, for example:

Copyright:MissMJ (Wikipedia)

(1) denotes matter (diamond, made up of carbon atoms), (2) is the molecular structure composed of atoms, (3) is a single carbon atom. (4) is an electron, (5) are the 3 quarks within a single nucleon (either a proton or a neutron), and (6) shows us that quarks and electrons are ultimately made up of strings.

These strings can vibrate at various frequencies and it is the vibrational mode that characterizes the string as matter, light or energy. It gives the particle its charge, mass and spin. The universe, according to string theory, is a symphony. According to most string theories, the theoretical graviton is a closed string loop in an especially low-energy vibrational state, and this loop traces out the surface of a kind of graph called a worldsheet. In this case, because the graviton string is a closed loop, its worldsheet is a pipe shape with no boundaries. Gravitons are surface waves across this shape. Some theorists believe that we live in a three-dimensional subspace of space-time, which can consist of as many as 11 dimensions. Because gravitons have no boundaries, they can move freely between and around our brane, as it is called, and this "leakage" of gravitons from this brane into higher-dimensional space could explain why gravity is so much weaker than the other three fundamental forces. As well, gravitons from other branes adjacent to ours could help explain what we call dark matter. This idea requires us to re-visualize the universe as consisting of four unfolded dimensions of space-time as well as up to 7 tightly folded up dimensions. We do not experience these higher dimensions but they may play a vital role in quantum behaviours. An analogy is a garden hose. To us from a distance it appears as a 1-dimensional line. As we draw nearer, we realize it is two-dimensional; it has width and length. As we get even closer, we see that it clearly has three dimensions - length, width and depth, or a circumference. In a similar way, we do not see or experience all the extra dimensions in which a graviton string can potentially move across. Although string theory is far from being experimentally verified, it has a certain elegance that is hard to ignore.

Loop Quantum Gravity

If the notion of extra dimensions of space-time is not palatable to you, consider loop quantum gravity. First formulated in the 1980's and a major contender for quantum gravity, this theory incorporates general relativity and also quantizes space and time (helping it fit into quantum mechanics), without resorting to extra dimensions.

Loop quantum gravity attempts to formulate a quantum theory for gravity based directly on Einstein's geometrical description of space-time. Remember that Einstein did not think of gravity as a force, unlike the strong, weak and electromagnetic forces. He thought of gravity as a property of space-time itself. This theory has its problems just as string theory does, but it goes a considerable way toward describing space-time in the language of quantum mechanics. It is very appealing because it does not require any major reworking of either quantum mechanics or general relativity. It sort of takes them as they are and works from that - and we don't need to add extra dimensions or other exotica to this concept in order for it to work, with one strange little exception as we will soon find out.

Many people, including me, find this theory complex and difficult to understand but let's give it a try. In general relativity, space-time is a smooth continuum - you can theoretically divide it into smaller and smaller volumes ad infinitum. Loop quantum theory, because it is a quantum theory, breaks down this smooth space-time into a discrete basic structure, as it must, but this structure is a bit more complex than just sliced up single discrete units. This theory calls space-time fabric a spin network that is made up of lines and nodes. You can visualize it as a string of Christmas lights except that instead of a single string, each light (called a node) is connected to many other lights (other nodes), shown here:

(Courtesy: Einsteinonline.com)

These nodes carry numerical values, so, depending on the number, they stand for space-time volume building blocks of different sizes. The smallest possible volume, or unit, of space-time is a region containing only a single node of lowest possible value. Adding nodes or increasing their values increases the size of the building block. You can think of space-time a bit like a set of Lego blocks, and the smallest unit is like the tiny one-piecer. This one-piecer has a numerical value associated with a massless spin-2 particle, a graviton. And this graviton, not surprisingly, is roughly the cube of a Planck length (10^-33 cm).

This means that the smallest volume of space-time is this Planck volume and all theoretically measurable volumes are multiples of this volume. They are quantized, in other words, much like electron energies are quantized inside atoms. Long-wavelength gravitational waves in otherwise flat space-time can be described as excitations of these quantum states. It has the nice side effect of doing away with singularities such as the Big Bang and black holes, with infinite mass crammed into a point of zero volume. Because space-time is not localized to a single point, the spin network described here has been coined spin foam, analogous to but not quite the same as the quantum foam I have mentioned in other articles. I could say that string theory also does away with singularities, owing to its one-dimensional strings. The problem with this is that string theory is not entirely background-independent and so we can't make that claim so enthusiastically since it doesn't provide us with a complete picture of space-time itself. Loop quantum theory can also describe black hole radiation and the relationship between a black hole's entropy and its surface area.

Loop quantum gravity has a gigantic plus going for it - we can potentially obtain some direct physical evidence for this smallest unit of theoretical volume in space-time. The light from gamma ray bursts, for example, should scatter off the discrete structure of the quantum geometry of space-time, much like light scattering off molecules in the air  as it passes through it, but on a much smaller scale, the Planck scale, about 10^-20 the diameter of a single proton. This means that the effect we can expect is extremely tiny. Fortunately, we have gamma ray bursts that we can measure from great distances, billions of light-years away, and over these distances the tiny quantum effect is amplified to the point where it could be measured.

According to this theory, each gamma photon occupies a region of lines at each instant as it moves through the spin network. This discrete nature of space-time should cause higher energy photons to move through slightly faster than lower energy photons. There is a potential fly in this promising ointment, however, and it has to do with the speed of light. Einstein's theory of special relativity tells us that light has a speed limit. In technical terms we say it obeys Lorentz invariance - it is the same for all observers and for all energies of light photons. This has been experimentally verified with great precision. Loop quantum theory, on the other hand, predicts that this invariance breaks down as we approach Planck scale so that high-energy photons should travel faster than low-energy photons. This effect seems to bring special relativity into question. However, Richard Feynman, using quantum electrodynamics, showed us that, at the quantum level, photons could travel faster than the speed of light, as well as slower, and that these strange occurrences disappear as the scale is increased (see my article on the nature of light).

A gamma ray burst is perfect for amplifying these expected but tiny speed differences. Astrophysicists found just such a perfect burst in 2009 using the Fermi Gamma-ray Space Telescope. They focused on a single very energetic (31 GeV) photon from gamma ray burst GRB 090510. It was a brief burst but it should have given measurable results - and they were disappointing - all the photons (of varying energies) of the burst, including the very energetic one, arrived at exactly the same time. What this means is that they need more testing using data from other gamma bursts. It is also possible that the researchers timing assumptions need to be fine-tuned. The researchers, in their paper on this experiment, suggested various explanations for this result, one of which is that high-energy photons might be released earlier than lower-energy ones. I sense a hint of panic in these explanations. Every researcher involved in quantum gravity theory wants to be the one to find the Holy Grail and the competition is fierce. All is not lost: there may be yet another way to test loop quantum theory. Researchers in France and the US have come up with a theory that evaporating black holes, in addition to emitting Hawking radiation should emit one or two distinct loop quantum signatures as well, depending on the size of the black hole. The challenge now will be finding evaporating black holes to test this theory.

Negative Gravity?

The equations that describe general relativity cannot construct a negative geometry unless a theoretical negative mass, which the equations do not rule out, is introduced. This negative mass would produce a repulsive gravitational field. Do we have any contenders for negative mass?

Antimatter?

It is important to distinguish negative mass with antimatter. The rules of CPT symmetry tell us that antimatter, while opposite in charge and magnetic moment from ordinary matter, has positive energy content and reacts to gravity just like ordinary matter does.

Dark Matter?

Dark matter, which you may have heard about, is matter that neither emits nor scatters electromagnetic radiation. This matter does not appear to interact with itself or other forms of matter (with the exception of possible weakly interacting particles which physicists are now trying to detect). Its interaction with gravity gives us a way of indirectly detecting it, through its gravitational effects. The orbital velocities of galaxies within clusters are too high to account for the mass we can measure. Dark matter is believed to make up over 80% of all the matter in the universe. This matter, just like ordinary matter and antimatter, reacts positively to gravity.

Thus, we do not have any contenders for negative mass. This hasn't stopped many researchers over the decades from attempting to demonstrate the existence of negative or anti-gravity, but none have been successful. The idea of negative gravity was, in fact, dead until 1998, when a new form of energy was suddenly plunked down on the table for consideration. It began when astrophysicists were shocked to discover that the universe is expanding at an increasing rate. Only a force that opposes the force of gravity could cause masses such as galaxies, which are moving away from each other, to accelerate rather than eventually slow down and begin to collapse back in toward each other. All the mass in the universe should be self-attracting, slowing down the expansion. A force with negative pressure seems to be involved and it has been coined dark energy. This new energy presents physicists with an enormous new challenge that brings our current understanding of fundamental forces, and gravity itself, into question.

Dark Energy

In the 1930's, contrary to Einstein's view of a static universe, Edwin Hubble discovered that the universe is expanding. This expansion is now thought to be driven by vacuum energy. Even completely empty space contains a minimum energy, which exerts negative pressure. Only recently did we know that the rate of expansion is increasing. The question is whether vacuum energy contributes to accelerating expansion or not. According to one theory, called the cosmological constant theory, it does. The cosmological constant gives us a value for the universe's vacuum energy.

At this point, you might wonder how the notion of negative pressure fits into general relativity. This is explained by something called the Stress-energy tensor, which is the physical quantity that causes matter to generate a gravitational effect. The Stress-energy tensor contains not only the energy or matter density of a substance but its pressure and viscosity as well. It gives us a more refined description of how matter interacts with gravity. Using this approach, cosmologist Marco Spaans suggests that the cosmological constant (vacuum energy) could be increasing as the number of black holes in the universe increases. The quantum properties of their particular space-time could, in theory, squeeze out vacuum energy, contributing to ever-increasing vacuum energy, and therefore, accelerating expansion of the universe.

However, in order for the cosmological constant theory to explain accelerating expansion, it must predict a constant that describes the rate of acceleration we measure. Most quantum field theories predict a constant that is 100 orders of magnitude too big. Other theories predict as constant of zero. It is a very economical theory because one numerical value could explain many different kinds of data from the WMAP data to recent supernovae data but unless a better value can be found this theory has limited usefulness.

A competing theory of dark energy is that of quintessence. Here, the potential energy of a dynamic field, which acts like a perfect fluid, could account for the acceleration. The field can vary in space and time. Quintessence itself can be attractive or repulsive depending on the ratio of potential energy to kinetic energy in the universe. As this ratio changes, the expansion rate of the universe changes. The quintessence field has a density that closely tracks, but is slightly less than, the radiation density of the universe until a point in the universe's evolution when matter and radiation equal each other in total energy density. A this point quintessence begins to act more and more like the repulsive dark energy that seems to dominate the universe today (74% of the total mass/energy of the universe). Some theorists are currently interested in trying to link quintessence with inflation, the early extreme expansion of the universe, in the hope that one theory could ultimately explain both phenomena. Subtle measurable violations of Einstein's equivalence principle (this is the principle that?s says that a spaceship accelerating at 1 g is equivalent to a skydiver in free fall accelerating at 1 g) and measurable variation in fundamental constants in space or time could be used as evidence for this theory, but no evidence has yet been found.

Dark energy may be a property of space-time itself, as hinted at by the cosmological constant theory. It could be a new dynamic fluid of some unknown composition as postulated by the quintessence model. It could also be telling us that Einstein's theory of gravity is incomplete because it does not describe the changing behavior of the universe over time. As to the question of whether dark energy is actually negative gravity, we cannot really say until we understand either the forces and fields involved or the space-time fabric itself, or both, better. Obviously, dark energy is a giant hurdle we need to overcome before we can approach any universal theory of everything.

Conclusion

Over the past few years, the hard work of scientists has given us much food for thought, refining our concept of gravity toward a eventual understanding that may weave it into an elegant fabric of space-time as well as give it its home within the quantum mechanical model of particle physics. Researchers may indeed find the ultimate answer to the question of what this universe is ultimately made of, the "theory of everything." We seem to be close, and far away, at the same time.

It is still appropriate to learn first about Newtonian gravity in school science, to know its history and get a good theoretical background. It is also important that as scientific explorers we build our knowledge from a firm foundation. But I hope we appreciate too that Newtonian gravity represents a door just ajar onto great mysteries of gravity and ultimately the mystery of the universe itself. As we learn about gravity, we may begin to deepen our appreciation for how scientific understanding is refined and built up through hard work and imagination, and how fragile those theories we build can be.

Magnetism Explained: 1 An Introduction To Magnetism

This article covers a general overview of magnetism and how our understanding of this phenomenon has grown over the millennia since it was first discovered.

We all tend to think of magnetism as a mysterious phenomenon specific to specially treated iron or steel, like these steel fridge magnets.

Magnetism is actually a much more widespread phenomenon than this. In this article, we will explore how magnetism arises, how it is a part of every atom, and how magnetism fits into the world of forces around us.

The Discovery of Magnetism

As far back as 600 BC, ancient Greeks wondered about the strange properties of naturally occurring stones called lodestones  (Fe₃O₄), such as the one shown here, courtesy of Geology.com (an excellent resource for all kinds of rocks)

Notice all the iron filings attracted to it.

These stones were found near the ancient Greek city of Magnesia and so they were named magnets. They are attracted to each other as well as bits of iron. Not surprisingly, when the mysterious properties of lodestones were discovered, they were soon surrounded by superstition and were thought to possess magical powers. By around 1200, the Chinese realized that lodestone could be very useful - as a magnetic compass. Within about 100 years, compasses were being used around the world for navigation at sea. This discovery was a major breakthrough - it paved the way for the golden age of worldwide exploration. At the time, ship navigators believed that their compasses lined up with the North Star, Polaris, or perhaps some large magnetic island at the North Pole. What they couldn't figure out was why their compass needles shifted direction as they sailed across the globe and why the north on their compasses did not always line up exactly with Polaris. These discrepancies eventually lead Renaissance scientist William Gilbert to conclude in 1600 that Earth itself must be a giant magnet, and that this is actually what the compass needle responds to. But what mechanism made it point north remained a nagging question, and until Christian Oersted's experiments in 1819, the inner workings of magnets remained a complete mystery.

Oersted found out quite by accident one day that an electric current made a compass needle sitting nearby twitch. An electric current produced a magnetic force! This was to mark the beginning of our current understanding of magnetism.

We now know that magnetism is a widespread phenomenon evident throughout the universe. It is as much a part of subatomic particles as it is of giant stars, and magnetism is never uncoupled from electricity. In fact, magnetism is part of electromagnetism, one of the four fundamental forces in physics, which also include gravity, the strong force and the weak force. Like the other fundamental forces, magnetism arises from interactions that take place at the atomic level. We don't really know how gravity works yet, so this could possibly be an exception. That magnetism is such a basic part of the workings of the universe might seem odd to those of us who know magnetism only from playing with magnets. It doesn't feel like an all-encompassing phenomenon of nature. But, as I will show you, magnets are actually a rarity in the world of magnetism - and ferromagnetism, which magnets display, is almost an accident of nature, or at least an accident of atomic structuring, as we will see.

What is Magnetism? A First Look

Let's start with atoms. Every single atom of matter is composed of electrically charged particles. Negatively charged electrons orbit around a nucleus composed of neutrally charged neutrons and positively charged protons. In electrically neutral atoms, the number of protons equals the number of electrons. Some atoms are more electrically conductive than others. Let's take copper as an example. Copper atoms contain especially motile unpaired valence electrons. Valence electrons tend to fill the outermost orbitals in atoms. These are the electrons that participate in chemical bonds because they can be shared between atoms. In the case of copper, these electrons are more strongly bound to the special lattice arrangement that metal atoms like to form than they are to any individual atom. When no current is applied to the lattice, these electrons tend to spin in random directions, resulting in a zero net current. But when current is applied something interesting happens.

If we pull copper into a thin wire and connect its ends across the two terminals of a typical DC (direct current) battery, electric charge within the valence electrons will drift toward the positive terminal. Like charges repel each other and opposite charges attract each other, analogous the north and south magnetic poles of a magnet. The negatively charged electrons are attracted to the positively charged terminal. The motile valence electrons are charge carriers, and moving charge is exactly what an electric current is. What caught Oersted by surprise here is that just such a current created a magnetic force, as if a magnetic field suddenly appeared near the wire. In fact, it did.

Oersted's discovery produced a great deal of excitement in the scientific community, resulting in a deluge of experimentation. One of the scientists in this community, Michael Faraday, studied the magnetism around wires carrying DC current and established the concept of the magnetic field. You can visualize this field yourself simply by scattering some iron filings on a piece of paper over top of an ordinary magnet. The filings will arrange themselves along the magnetic field lines of the magnet, and you will see something like this pattern:

The Magnetosphere

These lines are also often called magnetic flux lines. In outer space, free electrons and ions tend to attach themselves to these field lines, sometimes even becoming trapped. These lines emanate from a planet or moon that happens to have internally moving electric currents. These planets and moons are in essence giant magnets in space, and just as William Gilbert suggested, Earth is indeed a giant magnet. Powerful magnetic fields surround these planets and moons and interact with charged particles streaming from the Sun to create what we call a magnetosphere. Just as William Gilbert suggested, Earth is indeed a giant magnet. Earth's magnetosphere creates a gigantic protective bubble against a constant and deadly stream of fast moving charged particles emanating from the Sun. Without our planet's magnetic field, neither we nor any creature would be able to survive for long. This is how the magnetosphere is structured:

The red lines represent magnetic field lines. Charged particles are accelerated along these lines and, where they converge near the poles, they collide with with charged particles high up in the atmosphere. These collisions create the glow of the Northern and Southern Lights. If you would like to know more about how this mechanism works, please see my article called, "The Northern Lights."

James Maxwell, a contemporary of Faraday, took Faraday's concept of a magnetic field and put it on firm mathematical ground with his famous set of equations, the Maxwell equations. Knowing that electricity and magnetism are intertwined, he renamed this field the electromagnetic field. These equations describe how electric charges and currents act as sources for electric and magnetic fields. A changing electric field generates a changing magnetic field and vice versa. They represent a major breakthrough in physics, underlying many fields of study such as classical electrodynamics, optics, and electric circuits, and they are the foundation for all modern electrical and communications technologies. Maxwell's equations also provided theoretical footing for Einstein's breakthrough theory of special relativity to follow. Physicists are now developing electromagnetism even further as they incorporate it into gauge theory, quantum electrodynamics, electroweak theory and ultimately, the standard model of particle physics. We will explore the theory behind Maxwell's equations and how relativity relates to electromagnetism in later articles in the magnetism series.

For now, let's take a closer look at the mysterious magnet of the ancient Greeks, the lodestone. We now know that where there is magnetism there must be electricity, so we have to ask - how did these rocks get magnetized? There were no appreciable electric currents nearby in the city of Magnesia were there?

Lodestone

Lodestone is actually very unique. It is composed of a mineral called magnetite, which is relatively common in igneous, metamorphic and sedimentary rock. It is a significant component of black sands such as those around the coast of California and those off western New Zealand. There are also huge deposits of magnetite in banded iron formations around the world. This is a visually stunning example of magnetite crystals obtained from a mine in New York:

Copyright: Rob Lavinsky, IRocks.com

In this sample of quartz beach sand from India, magnetite, along with other heavy minerals, forms dark bands:

All lodestones are made of magnetite but not all magnetite is lodestone. All magnetite, and all lodestone, is composed of iron and oxygen, called iron oxide, but only lodestone is a natural permanent magnet. There are several different kinds of iron oxides, all made of iron and oxygen but with different atomic makeups and molecular structures. We will explore some of these differences in the next article in this series, on the mechanism of magnetism.

Magnetite, like iron and steel, is attracted to a magnetic field, but it does not stay magnetized itself. Only a small portion of magnetite is permanently magnetized as lodestone. For magnetite to turn into lodestone, it must have a particular crystalline structure and it must be mixed with another ferric oxide mineral called maghemite. No one is quite sure how lodestone achieves this special structure but one theory is that it is magnetized by strong magnetic fields surrounding lightning bolts. The fact that lodestone tends to be found scattered on the surface of Earth rather than buried deep supports this theory. You can read more about lodestone and modern testing that supports this theory here.

Of all the thousands of different minerals on Earth, lodestone is one of only two natural magnets. The other natural magnet is pyrrholite. It is an iron sulfide mineral that is only weakly magnetic. Lodestone itself is extremely rare as well. What if no one had discovered lodestone? Would we know as much about magnetism as we do?

Magnetism is a fairly simple behavior with complex variations. Find out what we know about the mechanism of magnetism in the next article in this series, "The Mechanisms of Magnetism."