Saturday, October 29, 2011

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 (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 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,

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."

Friday, October 28, 2011

Magnetism Explained: 2 The Mechanism of Magnetism

What is a Magnet?

A magnet is simply any material that produces a magnetic field around itself. Lodestones produce magnetic fields and so do the magnets below. They attract each other as well as various iron-containing steel items:

Copyright: wikipedia user:Omegatron

The Magnetic Dipole (And the Mystery of the Monopole)

Every magnet is a magnetic dipole. This means that all magnets have two opposing poles - a north pole and a south pole. Curiously, no magnetic monopole has ever been found. All magnets, no matter how small, must have two poles. Two opposite charges separated in space also create a dipole, an electric dipole in this case, with a positive end and a negative end. But electricity also has a monopole - a point charge, which can be either entirely positive or negative. No one is sure why electricity would have this basic unit but not magnetism, especially considering that both forces are part of one force, electromagnetism, as mentioned in the previous article, "An Introduction to Magnetism."

Magnetic Fields Don't Always Come From Magnets

Every magnet produces a magnetic field but magnetic fields are not all  produced by magnets. As we learned in the previous article, a moving electric charge or a changing electric field can also produce a magnetic field.  If you want to dissect magnetism down to its most fundamental units, you will discover that atoms, electrons and even atomic nuclei all possess magnetism. They, like all charged subatomic particles, have spin, and these tiny moving electric charges create tiny magnetic fields, and a force comes from this, which is called the magnetic moment. The magnetic moment is defined as the force that a magnet can exert on electric currents as well as the torque that a magnetic field can exert on a current. This torque is the force that tends to line up the magnetic moment with the magnetic field. Both magnetic moment and magnetic field are vector quantities and they are directly proportional to each other.

Charged subatomic particles are tiny magnetic dipoles. Magnetism in charged particles can arise in two ways: from the intrinsic (built-in) spin of a charged particle as well as from that particle's motion through space. An electron has a built-in spin of 1/2. All electrons have this spin - it is part of what makes an electron an electron. As a result, every electron has an intrinsic magnetic moment. It also moves around a nucleus, which creates an additional moving charge and an additional magnetic field. Every magnetic field exerts a force. It is the force that changing electric fields or moving charges exert on each other. The magnetic force of a magnet is technically the net sum of these forces. Because magnetic force is a vector force, it has strength and direction. Opposite-directed magnetic forces produced inside a magnet cancel each other out. That is why we say "net force."

The Magnetic Moment of Electrons: Why Isn't Wood Magnetic?

Generally, we talk about the spins of electrons when we talk about magnetism - these contribute by far the most magnetic force within materials. Let's explore why this is so.

Inside a block of wood, a material we don't think of as magnetic at all, magnetism exists. All of the wood's atoms, and in fact all atoms, have magnetic moments, and these moments are the net sum of various subatomic magnetic moments. However, this material displays zero magnetism. Why?

First of all, let's take a deeper look into the how the magnetic moments of electrons add up inside atoms. For each atom, individual electron spins are added to get a total spin. However, some rules must be followed when this is done. As I mentioned above, each electron acts like a tiny magnet, creating its own magnetic field. Only when these dipoles are aligned parallel to each other, can they be added together to create a larger magnetic field. And here's the hitch: electrons come in up/down pairs. Any filled valence shell of electrons has a zero net magnetic moment because all the oppositely spinning electrons cancel each other out. Some (electrically neutral!) atoms come with partially filled valence electron orbitals, as shown in this periodic table:

The number of unpaired electrons per atom is labeled in colour.

In these atoms, you get (usually) one or (occasionally) more unpaired electrons. And in these atoms, as a result, you get a net non-zero atomic magnetic moment. Because of Hund's rules, the first few electrons in a shell all tend to have the same spin (they don't get paired up right away) and atoms with this arrangement tend to have the largest atomic magnetic dipole moment. Notice where iron stands in the table above. It has four unpaired electrons, so iron atoms can exhibit a relatively large magnetic torque force. It is no surprise then that most magnets have a large iron component in them. These unpaired electron dipoles (we tend to just call these "spins" for simplicity) tend to line up with external magnetic fields, while paired electrons aren't so easily influenced. When a number of these atomic spins line up within a material, we say it is magnetized. And we can measure that with instruments.

Fine, you say, but how does this explain why wood isn't magnetic?

We've now covered the atomic contribution of electrons. What about the nucleus? The intrinsic spins of protons and neutrons inside the nucleus can also contribute to the magnetic moment of an atom, but they cancel each other out when both are in equal numbers. Most atoms, unless they are radioactive, exist in this balanced, or ground, state, and thus have zero net magnetic moment. Some elements, however, come in different (and sometimes stable) isotopes, however. Some of these isotopes can contain uneven numbers of neutrons and protons inside their nuclei. Therefore, one isotope of an element may have a nuclear magnetic moment while another isotope does not. Carbon-12 has 6 protons and 6 neutrons so it has a zero net nuclear magnetic moment. However, carbon-14 (yes it is radioactive - it's used in carbon dating), with 6 protons and 8 neutrons, does have one. You might find this idea intuitively weird - how can the number of electrically neutral particles in a nucleus have any effect on its overall magnetism? This question has a fairly complicated answer. It is because the magnetic moment of any particle is closely related to angular momentum. According to the nuclear shell model, the magnetic moment of a particle is a product of its orbital angular momentum and its spin. The angular momentum of a particle is a vector measurement of its inertia and its angular velocity. In subatomic particles such as neutrons and protons, this value is built-in and permanent; it's not related to their movement through space, like it is in larger systems. This value differs slightly between neutrons and protons, so it affects their arrangement within the nucleus - the nucleus isn't just a smeared positive charge in the center of an atom nor is it a single point of positive charge right in the center. The nucleus is a more complicated system that displays variance of charge across its radius. This leads to magnetic anisotropy in atomic nuclei with uneven numbers of protons and neutrons. This means that these isotopes have a directionally dependent magnetic moment. There is a lot of research going on right now to test various theories about how this magnetic moment arises in nuclei. Most nuclei with uneven numbers of protons and neutrons tend to be unstable, and they eventually decay into stable nuclei. For this reason most of the atoms in the materials we see everyday do not contribute any nuclear magnetic moment. Even when it is present, it is very small force that is difficult to detect and measure.

So, when we talk about the magnetism of atoms, we have the nuclear magnetic moment and the net magnetic moment of electrons to contend with. We can fairly confidently ignore the nuclear component for the reasons explained above. This still doesn't explain why wood isn't magnetic, does it?

Finally, an Answer

All these magnetic moments add up to give a total magnetic moment for an atom, but as mentioned, we usually concern ourselves just with the net magnetic moment contributed by the spins of the atom's unpaired valence electrons. We are now ready to look back at our wood sample. The reason this material shows no magnetism is not that it is made up of atoms that don't have strong magnetic moments. Wood consists of about 50% carbon, 40% oxygen and 6% hydrogen, and traces of other elements. All three of these atoms have unpaired valence electrons. The catch is that they tend to be snugly bound up in covalent bonds. The unpaired electrons in wood tend to form stable pairs across atoms, so they now act as paired electrons. The molecular structures of various wood tissues also don't allow much electron motility. There aren't any unpaired motile electrons available for magnetism in this material, even though it consists of atoms that have unpaired electrons. Therefore, there are no electrons motile enough to line up with an external magnetic field and wood, as a result, exhibits no magnetism. Magnets, in contrast, have unpaired electrons that are freely available to line up their spins with each other and we can measure a net magnetic force very easily.

Magnetism is really all about the arrangements atoms find themselves in. These atomic arrangements determine how easily the spins of unpaired valence electrons can line up with each other. Atoms bond with each other into molecules to make matter. A number of atoms have measurable magnetic moments. But that does not mean that materials composed of these atoms display magnetism. The oxygen atoms in wood, for example, each have two unpaired electrons but they do not contribute to a magnetic moment in wood because the oxygen is chemically bound into various complex molecules, like cellulose. This is the case with almost all of the various kinds of molecules that make up our planet. Only very rarely do materials happen to have atomic arrangements that display magnetism. That is why we have so few natural magnets on Earth and it is why almost all magnetite is not lodestone.

Magnetism in Molecules Under Extreme Conditions

As the case with wood illustrates, atoms within most molecules do not contribute magnetism to materials. However, some molecules do display magnetic properties. Oxygen molecules (O2) display strong magnetism in response to an external magnetic field because they have unpaired spins that remain once two atoms bond with each other. Here, a trickle of transparent liquid molecular oxygen is deflected and attracted toward a magnet:

At very cold temperatures, molecular oxygen forms a highly magnetic solid. Solid oxygen is the only elementary molecular magnet known. Oxygen, nitrogen and hydrogen also become metallic at very low temperatures or under very high pressures. Hydrogen atoms, each with one unbound electron, metalize into a solid crystal lattice in which the unbound electrons act like conduction electrons in a metal. This structure also gives solid hydrogen, like solid oxygen, magnetic properties, and perhaps solid hydrogen, like solid oxygen is a magnet as well. Jupiter, Saturn, Uranus and Neptune all have mysteriously powerful magnetic fields. Most physicists link this magnetism to the flow of electric currents inside the planets, within metallic hydrogen in its liquid phase. This flow of current is in effect a dynamo, much like the one inside Earth but composed of different elements. It is this dynamo that generates the planet's magnetic field. Only Jupiter and Saturn are large enough to compress hydrogen into a metallic solid, however.  It is the solid phase of these elements that displays strong magnetism so it remains unknown whether hydrogen in this solid state also contributes to the magnetic fields of these two planets, which are exceptionally powerful. It is technically extremely challenging to make solid oxygen (freezing point is -219°C) and even more challenging to make solid hydrogen (freezing point is -259°C) but research is underway, testing the physical properties of these fascinating materials.

Magnetars Are The Most Powerful Magnets in The Universe

Magnetars are special neutron stars with magnetic fields hundreds of millions times stronger than any man-made magnet. Magnetic field strength is measured in units called Gauss. Magnetars have magnetic fields of around 1015 Gauss. To illustrate how strong that is, if we tried to make a magnet stronger than 4.5 x 105 Gauss, the physical stress of such a field would blow the magnet apart. Magnetars are held together only by unbelievably powerful gravitational forces. No one is entirely sure what mechanism is responsible for the magnetic field of a magnetar but physicists, using the latest in computer modelling, have an idea. They know that when a magnetar forms (after a very massive star blows up in a supernova), it consists of a boiling dense fluid, mostly made up of free neutrons. Free electrons affect this fluid's buoyancy and drive powerful convective currents. Free electrons and free protons set up powerful electric currents in the fluid. Young magnetars spin extremely fast and this spinning motion combined with convective motion drags magnetic field lines through the star. This process builds up the star's magnetic field, thus creating a dynamo effect, similar to the dynamos inside Earth and other planets. This dynamo acts to magnify the magnetic field. If you want to know more about magnetars, please see my article, "Stellar Objects Part 4: Magnetars."

New research suggests that the strongest theoretical magnetic field that could exist is between 1049 and 1053 Gauss. Any field stronger than this would break down the vacuum of space and spontaneously decay.

To sum up then, making a magnet is a bit like making a cake. Not only do you need the right ingredients (atoms with unpaired electrons), you also need to combine them in the right way to create a structure (atomic arrangement) that will display macroscopic magnetism. In the next article, "Ferromagnetism," we will explore the most common mechanism through which materials display magnetism, and what makes magnets magnets.

Thursday, October 27, 2011

Magnetism Explained: 3 Ferromagnetism

Ferromagnetism is the basic mechanism by which certain materials form permanent magnets or are attracted to magnets. This is the strongest type of magnetism and it's the only kind that can be felt, such as the pull of a fridge magnet toward your fridge. Ferromagnetism is the everyday magnetism we are all familiar with.

Ferromagnetic materials can be divided into "soft" materials like iron, which can be magnetized but do not tend to stay magnetized, and "hard" materials like lodestone, which are permanent magnets; they tend to stay magnetized. We make a permanent magnet from a ferromagnetic material such as ferric oxide by melting it and subjecting it to a very powerful magnetic field while it cools. This process aligns its atoms into a precise microcrystalline structure. In the case of lodestone, most physicists think that lightning does this job. Permanent magnets have a quality called high coercivity. This means that they have a high threshold against demagnetization. However, even permanent magnets can be demagnetized and I will explain how in a bit.

As explained in the previous article, ferromagnetism is a property of not just of which kinds of atoms (iron, iron oxide or another metal, for example) are in a material, but of its crystalline structure and atomic organization as well. This means that one ferric oxide (Fe2O3 for example) might be less magnetic than another one (FeOFe2O3 for example), depending on how the atoms are arranged with one another.

This is the crystalline structure of hematite (Fe2O3), which is weakly ferromagnetic:

Copyright: materialscientist(talk)

In contrast, this is the crystalline structure of magnetite (FeOFe2O3), which is strongly ferromagnetic (it includes lodestone):

Copyright: materialscientist(talk)

There are also ferromagnetic metal alloys whose constituents themselves are not ferromagnetic. These are called Heusler alloys. An example is an alloy of copper, magnesium and tin. None of these metals in pure form display measurable magnetism but together they do. In an alloy, these metals exist as ions in different oxidation states. This allows them to form into a crystalline structure that provides an easy way for electrons to be exchanged between different metal ions. In other words, this makes it easier for the electrons to line up their spins. Below is a diagram of the general lattice structure of Heusler alloys, where X, Y and Z represent the three constituent metals.

Notice how orderly the lattice is arranged. Metallic lattices like this one create environments in which the electron spins can easily line up. That is why the atoms in most magnets happen to be arranged in these metallic lattices.

The mechanism responsible for making this metallic lattice magnetic is a quantum mechanical effect. Electrons are fermions and that means that they have a 1/2 spin. Because of this spin, no two electrons can occupy the same space (bosons with whole integer spins can). However, they can come close and reduce their overall potential energy in the process. In these alloys, and in fact in all ferromagnetic materials, a mechanism called the exchange interaction allows the wave functions of neighbouring electrons from two atoms to line up and create an overall wave function of lower potential energy. As a result, the spins of electrons in ferromagnetic materials tend to want to be lined up parallel to each other. Electrons also have intrinsic magnetic moments but this contribution to overall magnetism is negligible, as I mentioned earlier, in comparison to this essentially electric interaction.

In addition to Heusler alloys, we have alloys like stainless steel, which display an opposite effect. Stainless steel is composed almost entirely of ferromagnetic metals and yet it, itself, is generally non-magnetic. Like steel, stainless steel is made of carbon and iron. However, stainless steel contains other components as well such as chromium and nickel, depending on the kind of stainless steel. When these metals are added, they affect the atomic lattice structure of the steel. In 300 series stainless steel, both chromium and nickel are added and these steels are not magnetic. But 400 series stainless steels, which contain chromium but not nickel, tend to be magnetic. The lattice structure tends to be preserved in this combination of elements. That is why your stainless steel sink is probably not magnetic but some of your stainless steel knives might be.

Most ferromagnetic materials are crystalline, allowing them to achieve the structure and exchange interaction I have outlined, but some are not. These metal alloys are made ferromagnetic when they are very rapidly cooled, so no crystals can form. However, the same exchange interaction does occur between their electrons. Some of these amorphous alloys have advantages over crystalline structures, such as resistance to changes in temperature and to changing electric fields. A lot of research is going on right now to find out how magnetism arises in these materials and how they can be used.

Ferromagnetism Example: Rare-earth Magnets and Computer Hard Drives

A relatively new kind of very strong magnet can be made from rare earths rather than metals, and here, the exchange mechanism is similar but it can occur across as many as 7 unpaired electrons with aligned spins. The electrons are strongly localized so they retain their magnetic moments very well. The crystalline structures of rare earth magnets also have very high magnetic anisotropy. That means that they have a very stable atomic alignment making them exceptionally easy to magnetize in one direction but they resist magnetization in other directions. The most common rare earth magnet is made of an alloy of neodymium, iron and boron. These magnets can be twice as powerful as ordinary ferrite (iron) magnets but they are very brittle and vulnerable to corrosion, so they are usually plated with nickel. Here is one such nickel-plated magnet from the bracket of a computer's hard drive:

These hard drive magnets are powerful, making them excellent for magnetic data storage. The magnetic flux patterns on them can last many years and they can be easily erased and rewritten. As I write this article on my computer, data is "written" on my hard drive disk. A pattern of tiny electrical impulses representing these words passes through a coil in the recording head. This produces a related pattern of magnetic fields. These fields, very close to the disk, alter the magnetic orientations of bit regions on the disk itself. The magnetized bits on my hard drive disk now represent my data.

Magnetic Domains

In the previous article I said that when a number of spins line up in a material it is considered magnetized. Regions in a ferromagnetic material in which all the spins are lined up in one direction display uniform magnetization, and are called a magnetic domains. A magnetic domain simply means that the individual magnetic moments of the electrons are aligned parallel with each other, all pointing in the same direction. It is what makes a ferromagnetic material magnetic. Differently aligned domains can exist within one sample. This means that a magnet, for example, might not be homogenously magnetic, and its net magnetic strength, as a result, will be lower. However, just as the magnetic moments of individual electrons tend to line up parallel to an external magnetic field, so do domains. Like individual electrons, domains tend to favour this arrangement in order to maintain a slightly lower potential energy state. So, even when a magnetic field is removed, the electrons in a ferromagnetic material tend to maintain their orientation, even when the material is not a permanent magnet. The natural kinetic or vibratory movement of atoms, however, tends to destroy this alignment over time. How long electrons manage to maintain their orientation determines how permanent, or hard, a magnet is. Now, can you guess how heat might affect the arrangement of magnetic domains within a material? You can check your answer in a moment.

This is how the domains look inside a saturated magnet displaying strong ferromagnetism:

This is a ferromagnetic sample containing various aligned and unaligned domains:

This sample will exhibit weaker magnetism than the saturated magnet diagramed above. Remember that magnetism is a vector force, so a component of the lower right domain will contribute, along with the lower right and upper left domains, to the sample's overall magnetic strength.

Earth is One Big Ferromagnet

Iron is ferromagnetic, and when it is incorporated through natural processes into iron oxides, and other minerals, it can be used to record ancient changes in Earth's magnetic field. Scientists can take samples of these minerals and measure their residual magnetization, the magnetization left behind in ferromagnetic materials after an external magnetic field is removed. This data tells them that Earth's magnetic field not only fluctuates in intensity, it completely reverses at random intervals, averaging once every several hundred thousand years. These geomagnetic field reversals happen when the flow of molten metals within Earth's outer core is disrupted. An impact with another large body like an asteroid could cause such a disruption but some scientists think that internal triggers, such as the subduction of large plates, may be possible triggers as well. There is no universally agreed upon model for why these reversals occur. Geomagnetic field data also allows these scientists (called paleomagnetists) to calculate past motions of continents and ocean floors, providing proof to the theory of continental drift.

Curie Temperature

A magnet that has all of its domains aligned is called a saturated magnet. It displays maximum possible magnetic strength. Several processes can desaturate a magnet, causing some or all of its domains to become randomly aligned. One way is to heat it. If you heat a magnet above what is called Curie temperature, the atoms attain so much kinetic energy, they are vibrating so violently, that they can no longer maintain their special structure and the electron spin alignment gives way. The magnetic domains become disrupted and the magnet loses its magnetism. Dropping or banging a magnet against something will do the same job. Non-permanent ferromagnetic materials lose their domain alignment over time regardless of how they are treated. How fast they lose their alignment determines how soft they are. Each ferromagnetic material has its own Curie temperature, above which its magnetism is eliminated. The Curie temperature for iron, for example, is 770°C. An iron oxide magnet heated to this temperature is no longer a magnet. The Cure temperature for rare earths is well below room temperature. This means that in pure form at room temperature, rare earths like neodymium are not magnetic at all. Only when they are alloyed with other metals do they become powerful and permanent magnets, such as the hard drive magnet described above, at room temperature. The Curie temperature of neodymium alloy magnets is quite high, between 310°C and 400°C. Yet, if you put a hard drive magnet in a self-cleaning oven and turned on its cleaning cycle (it heats to 500°C), you would destroy its magnetism.


Another way to demagnetize a magnet is to subject it to a changing electric field. Here, the magnet is retracted through an electric coil with alternating (AC) current. The magnet's magnetic domains become randomized and it is demagnetized. This process is called degaussing. If you hear a brief but loud thunk when you turn on your TV, it is likely degaussing itself. Most computer monitors also degauss automatically. In this case, you might hear a "dwoing" sound when you turn it on. This process is believed to improve picture quality. Degaussing is also used to demagnetize naval submarines to make them harder to detect by other submarines and naval ships. Over time, these vessels acquire a potentially detectable magnetic signature due to interaction with Earth's magnetic field, so they are routinely demagnetized within a changing electric field. This can be done by draping heavy gauge copper cables over the vessel and pulsing very high electric current through them as shown here:

In addition to ferromagnetism, there are other kinds of magnetic order within materials. These will be explored in the next article, "Other Kinds of Magnetic Ordering."

Wednesday, October 26, 2011

Magnetism Explained: 4 Other Kinds of Magnetic Ordering

Ferromagnetism is the most commonly understood kind of magnetism. However, there are several other kinds of magnetic ordering as well. These other kinds of magnetism are generally much less powerful than ferromagnetism and sensitive instruments are required to measure their strength. In this article we will explore these mechanisms, as well as some useful applications of magnetism.


Every material is diamagnetic. This process creates a magnetic field within a material that is opposite to an externally applied one, causing a repulsive effect. This happens even in ferromagnetic materials, which are, overall, attracted to each other. Here, an external magnetic field alters the orbital velocity of electrons in atoms, altering their intrinsic magnetic dipole moment. This effect is usually very weak and sensitive instruments are required to measure lines of magnetic flux curving away from the material. It is easily overcome by the attractive magnetic force acting between ferromagnetic materials.

Magnetic flux, once again, is simply a measurement of the quantity of magnetism. These are the lines you see when you hold a magnet near iron filings. In superconductors, a powerful quantum effect, called the Meissner effect, can emerge where these lines of flux are completely blocked, except for a very thin surface layer. This powerful diamagnetism is the result of a completely different mechanism than the one described above. An external magnetic field sets up electric currents near the superconductor's surface that cancel out the applied magnetic field. This powerful repulsive effect can overcome gravity, resulting in magnetic levitation. Below, strongly diamagnetic pyrolytic carbon (the grey square with a hole in it) levitates above four permanent magnets.

This kind of diamagnetism is used to support maglev trains, with the great advantage of frictionless transport, shown here.

This maglev train runs in northern Germany.


In contrast to diamagnetism's repulsive response to magnetism, paramagnetic materials are attracted. It is a weak phenomenon, again requiring sensitive measuring instruments, and, like diamagnetism, the response is transient. It dissipates once the external magnetic field is removed. When it is removed, the kinetic motion of atoms randomizes the spin orientations, much the same as it does with non-permanent ferromagnetic materials. Whereas ferromagnetism relies on the spin alignment of two or more unpaired electrons, which then act as a single quantum unit, paramagnetism relies on the parallel alignment of lone electrons that do not pair up. These lone valence electrons are what make metals such as magnesium and lithium paramagnetic, for example. These atoms have intrinsic magnetic dipoles thanks to the spin of their unpaired electrons but only when a magnetic field is applied do the dipoles line up in one direction and the material displays magnetic attraction. The process of lining up in this case, creates torque on the magnetic dipoles, and they are actually in a higher potential energy state than when they are not in a magnetic field. In nonpermanent ferromagnetic materials, on the other hand, spin alignment reduces overall potential energy. It is a preferred state that is overcome by the kinetic energy of the atoms.

A paramagnetic sample placed within a magnetic field:

The same sample right after the magnetic field is removed:

Once the magnetic field is removed, the atoms quickly lose their alignment, a bit like a spring bouncing back once it is no longer compressed.


In antiferromagnetic materials, the magnetic moments of atoms align in a pattern with neighbouring spins on adjacent sublattices pointing in opposite directions (see the diagram below). As a result, these materials have no net magnetic moment. Antiferromagnetic materials are relatively uncommon - most are transition metal compounds, usually oxides, such as nickel oxide, for example, but some metals such as chromium and alloys such as iron manganese are also antiferromagnetic. This kind of magnetic ordering tends to be a low temperature arrangement, above which the kinetic energy of the atoms becomes large enough to destroy the atomic ordering. Like ferromagnetic materials, every antiferromagnetic material has its own temperature, in this case called its Néel temperature. Antiferromagnetic solids exhibit special behavior in an external magnetic field, depending on the temperature. At temperatures below Néel temperature, an antiferromagnetic material displays no magnetism. Its atoms are arranged as in the diagram below. As it is gradually heated, the atoms gradually break free of the orderly arrangement and align with the external magnetic field. The material begins to exhibit a weak magnetism that reaches its maximum strength at Néel temperature. As the material continues to be heated, its magnetism progressively weakens as the kinetic energy of the atoms overcomes their alignment with the field, until eventually it displays weak paramagnetism.

These materials are unique in that they have more than one optimal ground state, which represents a minimum potential energy. In one dimension (a single line of atoms), you get only one choice of spin ground state - up/down/up/down, but in two dimensions, like this diagram, you have several possible ground states.

In more complex 3-dimensional lattices, the number of possible ground states increases exponentially. This is called geometric frustration, where a system cannot find a single ground state of lowest potential energy. Frustration is a fascinating phenomenon displayed by some crystals where, even at absolute zero, molecular motion still occurs because atoms cannot settle on a single ground state, a startling example of a system with non-zero entropy at absolute zero temperature.

Hematite is an Interesting Magnetic Specimen

Remember the example of hematite as a ferromagnetic material, in the previous article on ferromagnetism? Hematite is a mineral ranging in colour from black to steel to brown to red. This is an example from Michigan. The hematite is the very shiny black deposits in this reddish ore. The yellow dots are simply reflections from the light source that was used:

Copyright: DanielCD

Hematite is very strong but brittle. It can be made into jewelry or other objects such as this bear carving:

Hematite, a ferric oxide, with the formula Fe2O3, can also display antiferromagnetism. This is how it works:

Iron and oxygen ions crystalize into a fairly closely packed hexagonal structure where the magnetic moments of the Fe3+ ions are ferromagnetically coupled within what are called the c-planes of the framework. These same ions are also antiferromagnetically coupled between the c-planes. All 3-dimensional crystals have three lattice planes, a, b and c. The c-plane is one of three 2-dimensional planes within a crystal. Above 10°C, the spin moments in the c-plane are slightly canted or tilted. As a vector result of these two kinds of contributing magnetism, hematite is weakly magnetized in the c-plane. It is a weak permanent magnet at room temperature, which you can attest to if you happen to own a hematite bracelet. Some people believe this material has curative properties. However, below 10°C, the direction of the antiferromagnetism changes and becomes exactly parallel to the c-plane axis. Now, hematite shows no spin canting and becomes a perfect antiferromagnet. It now has zero net magnetic moment and it displays no magnetism. This temperature-dependent transition in spin is called the Morin transition. If you heat the hematite once again, this time above its Néel temperature (675°C), like other antiferromagnetic materials, it becomes paramagnetic. It shows a very weak and transient attraction to a magnetic field.


In ferrimagnetic materials, the magnetic moments are lined up on different sublattices in opposite directions, somewhat like, but different from, antiferromagnetic materials (compare the two diagrams). The main difference here is that opposing moments have unequal strengths so a net magnetism results, as shown in the diagram below using two different sized arrows. Each arrow represents a magnetic moment.

For ferrimagnetism to happen, the sublattices must consist of two different materials or two different oxidation states of one element, such as Fe2+ and Fe3+ ions, which differ in the strength of their magnetic moments. Ferrites and magnetic garnets exhibit this kind of magnetism. Magnetite, which consists of both Fe2+ and Fe3+ ions, was classified as a ferromagnet before Néel's discovery of ferrimagnetism and antiferromagnetism in 1948. Now, it is more correctly classified as a ferrimagnet. Like ferromagnets, ferrimagnets retain their magnetism in the absence of a magnetic field, at least for a while, depending on the magnet's hardness.

Molecule-based Magnets

These magnets display ferromagnetism much like typical metal and metallic oxide magnets do and they can be hard or soft, depending on the strength of their coercive field, just like metal-based magnets. However, in metal magnets, magnetism arises from the alignment of the spins of unpaired electrons that are lined up in a crystalline structure to create domains. Molecular magnets instead rely on their molecular structure to align electrons. Most are purely organic molecules, which tend to come with horribly long names, such as p-nitrophenyl nitronyl nitroxide. Instead of being made at high temperatures within a strong magnetic field, molecular magnets are made in low-temperature liquid solutions. Here they can be chemically tailored to fine-tune their magnetic properties. These magnets need to be very cold (usually around -200°C) in order to display magnetism. At room temperature, the kinetic energy of the molecules tends to destroy their molecular alignment. This is a problem similar to that with superconductors. So far, most are simply laboratory curiosities with no practical applications. However, a group of Canadian researchers recently discovered a new molecular magnet, which is an organic metal hybrid made up of two parts nickel ions to one part organic ligand. This new class of molecular magnets can work at room temperature. No one is sure yet why they work but this discovery is likely creating a buzz in labs around the world. Molecular magnets might be easier to make than traditional ones and easier to mold into shapes, which is difficult with metallic magnets because they lose their magnetism when heated. These magnets might be useful some day to make thin magnetic films or in new kinds of computer memory storage, for example.

Electromagnets, Electric Motors and Electric Generators

In the 1800's, Andre-Marie Ampere discovered that two wires carrying parallel electric currents in the same direction attract each other, and two wires containing currents running in opposite directions repel each other. This phenomenon seemed reminiscent of when you put two opposite magnetic poles close to each other and they attract each other, and vice versa. This force is called the magnetic force. Now, if you form these wires into loops and create circular currents, you can get an attractive or repulsive force between the loops, depending on whether the currents in them are in the same, or in opposite, directions. If you then create a coil of many loops, a solenoid in other words, and run current (in the same direction) through it, you create an electromagnet. The more loops you add, the stronger your electromagnet becomes. The resulting magnetic field creates a magnetic force that stops as soon as the applied current is shut off.

Using just a few inexpensive and readily available items you can make your own electromagnet and use it to explore how magnetism and electricity work together. This excellent and easy to follow 10-minute video shows you how, and it is appropriate for anyone around age 10 to 110:

Electromagnets are very useful and widely used. They are essential components in the electric motors inside your household appliances, and in generators, loudspeakers, hard disk drives, MRI machines and the list goes on.

In electric motors, a current-carrying wire is bent into a loop. When the two sides of that loop are placed perpendicular to a magnetic field (the loop is placed between the north and south poles of two magnets), the loop will experience a torque and start to rotate. This relationship is described by the motor principle. This 9-minute video (its animation is a bit dated but it's good) explains the motor principle very well and how electric motors work:

The rotation of the loop is used to do work, such as driving a turbine or a rotor in a washing machine, for example. This 8-minute video (with great graphics and animation) explains in greater detail how a typical electric motor is set up, because the host builds one for you in cyberspace:

You can also use mechanical energy to make electricity. Generators work on the same principle as electric motors, but backward, as illustrated in this excellent Scienceman digital lesson, which shows you how a generator creates AC current:

In the next and final article, we will explore how magnetism and electricity are unified as one fundamental force, electromagnetism.

Tuesday, October 25, 2011

Magnetism Explained: 5 Electromagnetism

Magnetism + Electricity = Electromagnetism

Magnetism and electricity cannot be separated from one another. Whenever you have a moving charge, you have magnetism and whenever you have a changing magnetic field, you have electricity. For example, let's move a copper (electrically conductive) wire through the magnetic field of a stationary magnet. You will get eddy currents of electricity in the wire because of the magnetic force acting on the electrons inside it. Now, let's hold the wire stationary and move a magnet across it. Here, exactly the same eddy currents will be produced but classical electromagnetic theory says they are the result of an applied electric force, rather than a magnetic force! Albert Einstein called this the "moving magnet and conductor problem" in his 1905 paper on special relativity. The current in the wire experiences a magnetic force in the frame of reference of the magnet and an electric force in the frame of reference of the wire itself. This consistent and predictable alteration from one measurement to another is called a Lorentz transformation. It simply means that varying measurements of magnetic and electric fields can be converted into each other's frame of reference. This transformation holds up equally well for space and time under the theory of special relativity, which is explored in my article, "Time." In our case, the same phenomenon is either electric or magnetic in nature depending on your frame of reference. Electricity and magnetism are different aspects of the same force, the electromagnetic force. Maxwell's equations predict this transformation and it is through them that the unified concept of electromagnetism was created.

Electromagnetism = Light

The electromagnetic force is behind almost all the phenomena we encounter in daily life, with the exception of gravity. It holds electrons and protons together inside atoms (as virtual photons) and it holds atoms together within matter. It governs all the processes involved in chemistry, and that is why it should come as no surprise that the ways in which some atoms arrange themselves and interact electrically with each other has so much to do with distinguishing one kind of magnetism from another, as we have seen in the previous two articles, "Ferromagnetism" and "Other Kinds of Magnetic Ordering."

Michael Faraday and James Maxwell gave us our first understanding of how magnetism and electricity work together as one unified force. Maxwell used his electric and magnetic equations to derive the waveform of electromagnetism. His wave equation predicted the speed of light, leading him to conclude that light itself is an electromagnetic wave. A light wave, according to his formulae, consists of a spatially shifting electric field causing changes in a magnetic field. Each field causes the other one to shift as they move forward in the same direction, and this is how electromagnetic (EM) fields propagate. Photons are the force carriers of propagating EM fields. This animation shows how it works, with the electric field in the vertical plane, Z, and the magnetic field in the horizontal plane, X.

Copyright: Lookang

Λ (lambda) represents one wavelength, E represents the electric oscillation and B represents the magnetic oscillation. This particular EM wave is polarized - each of the red and blue fields is perfectly aligned along one plane. All EM radiation, including visible light, propagates as a transverse oscillating wave of electric and magnetic fields. The energy of this propagation travels at the speed of light, as photons. All EM radiation behaves like this. The only part that changes, making red light different from blue light, for example, is the wavelength. When wavelength changes, frequency changes as well, because the two are inversely proportional to each other. Blue visible light  has more energy than red light because its wavelength, about 400 nm, is shorter than that for red light, which is about 700 nm. Shorter wavelength EM waves  have more energy than longer waves. You can think of an EM wave as a skipping rope. If you try to make your skipping rope have more waves in it, you need to apply more energy with your hand. This 5-minute NASA video introduces you to EM waves, and shows you how EM radiation fits into our modern lives:

How Is An EM Wave Created?

We know that when electrons move, they create a magnetic field. When electrons move back and forth, or oscillate, their electric and magnetic fields change together and this creates an EM wave. An EM wave consists of one or more photons, which are spontaneously emitted from atoms with sufficiently high kinetic energy. Here is an example of how it works: An atom absorbs energy when a photon collides with it. If the photon has enough energy, the atom it strikes will become excited. The atom's potential energy increases, as a result, and one of its electrons moves from ground state to an excited state. The atom, like any physical system, wants to maintain the lowest possible energy state, so the extra energy that the electron absorbed is released as a photon (with exactly the same energy). The atom emits the photon and as it does so, the excited electron moves back down to its ground state. The atom returns to its original energy state. For interest's sake, if the incoming photon has extremely high energy, say a gamma ray for example, the electron will move up to state of maximum potential energy and then leave the atom altogether. The atom loses this electron and becomes ionized. If you are interested in examples of phenomena that rely on photon absorption and emission, take a look at my articles called "Radiation: What is Happening in Japan?" and "The Northern Lights." Every kind of atom, every element, has its own ionization energy, above which it becomes ionized.

This brief 2 - minute animation will help you visualize what is going on when an atom emits a photon:

A photon is absorbed when it hits opaque matter. This collision must have enough energy to cause an electron within an atom of the opaque matter to oscillate (this is a transfer of kinetic energy) with enough energy to emit a new photon. Otherwise, the matter is simply warmed up. Its electrons, and therefore its atoms, have increased kinetic energy in general but they are not energetic enough to be in an excited state. If you heat matter, electrons in this matter will eventually begin to emit low energy photons such as infrared photons. You can sense this kind of EM radiation, called thermal radiation, when you stand near a warm stove, for example. Getting back to our matter, if you continue to heat it, the electrons will emit higher and higher-energy photons.

If you want to know more about the nature of light, please see my article called, "The Nature of Light."

Electromagnetic Waves Within Magnetic and Electric Fields

All EM waves can, and do, interfere with each other all the time. This is why we have to deal with radio interference and dropped cell phone calls. However, because EM waves are electric/magnetic in nature, you might be wondering how EM waves act when they travel through magnetic and electric fields, which they, of course, do all the time. Light and other kinds of EM radiation from the Sun, for example, must go through Earth's magnetic field, before it can strike Earth's surface. It is important to keep in mind that electric and magnetic fields can and do bend the paths of charged particles. This is how the cathode rays in old-fashioned TV's work, for example. EM radiation is indeed an electromagnetic phenomenon but, and here is the bullet point, the force carriers themselves, photons, are not charged particles. For this reason they do not interact with magnetic and electric fields, with one very small "but."

There are a few very small and rare interactions that do occur but these kinds of effects not usually apparent to us in our daily lives - if light were bent by EM fields, we would never get clear TV reception nor would we get reliable radio reception. Both TV and radio transmitters create powerful EM fields around them that would bend any EM radiation coming from them (here, I'm using old-fashioned TV's again not digital ones).  But occasional effects do happen and the physics involved can get complicated. One explanation for how it can happen looks at EM radiation at the quantum level. A phenomenon called Delbruck scattering can occur where powerful EM fields can break down a photon into an electron and a positron, both of which carry a charge. When this happens, the path of an EM wave can be altered - usually a type of scattering occurs when the positron and electron, once formed, rapidly annihilate each other and two new lower-energy photons created in the annihilation carry on, in altered directions. This may be part of the answer, but it is not the whole story, as very powerful EM Fields are required. What happens in the far tamer EM fields around Earth?

EM waves do not tend to be affected by fields that are not rapidly changing. Earth's magnetic field is relatively constant and almost all EM radiation passes right through it unaffected. If EM waves pass through static magnetic or electric fields (fields that don't change over time) in a linear medium, they are not affected at all. The vacuum of space is a static linear field. Static simply means non-changing but what does linear mean? In nonlinear fields, interactions can occur, and those effects can be unpredictable. To understand what a nonlinear field is we must acquaint ourselves with systems theory. According to this theory, a field is linear when it satisfies the superposition principle. This means that the net response of a linear system to two or more different stimuli is simply the sum of those responses, which would have been caused by each stimulus alone. If two or more stimuli are applied to a nonlinear system, its net response is not the sum of the stimuli. Other interactions occur between, for example, the stimuli themselves, and this can change the net response, often with unpredictable results (this underlies chaos in chaos theory. In reality, almost all systems are nonlinear in nature, and engineers have to deal with them all the time. For example, the wing of this plane creates a tip vortex, made visible by using coloured smoke, which results in powerful turbulence. Fluid turbulence is an excellent example of a chaotic nonlinear system. This particular phenomenon happens to develop through a strange attractor. This is why planes must maintain a set distance away from each other when they land - the turbulence of landing planes is unpredictable so pilots cannot adjust for it.

Climate and weather are other common examples of complex nonlinear systems. They have a lot of different variables and those variables interact with each other in chaotic and unpredictable ways.

Faraday Effect

In the case of EM waves, an example of an interaction with a nonlinear field is the Faraday effect. Here, light interacts with a magnetic field in a medium. The Faraday effect causes the plane of polarization of light to rotate. This diagram explains how it works. B is the magnetic field and E represents the polarized electric oscillation of an EM wave.

Copyright: User:DrBob

You can see a large Faraday effect rotation in magnetic garnets, which are transparent to some of the light going through them. Consider a beam of polarized light as a superposition of equal amounts of right and left circularly polarized beams. The magnetic field (inside the garnet) causes each component to experience a slightly different refractive index and that causes the rotation, as one circularly polarized beam slows down and emerges very slightly out of phase with the other component.

This effect happens to radio waves passing through Earth's ionosphere. The ionosphere contains charged ions (most of the effect comes from free electrons), which create a magnetic field. This field, along with Earth's intrinsic magnetic field, rotates the polarization of radio waves. The amount of rotation varies with electron density in the ionosphere and this, in turn, is significantly affected by sunspot activity on the Sun. And, as we just learned, the effect itself is unpredictable. This effect diminishes greatly at higher frequencies, so radar and satellite communications (between 1 and 15 GHz) are not generally affected. (You may have heard of radar jamming, radar noise and clutter, etc. - these are all the effects  of interference). However, interference in VHF televisions (30 - 300 MHz) caused by the Faraday effect can be a serious problem in Japan. HF ham radio operators can also experience signal fading due to ionic interference when they transmit across the poles where the magnetic field is intense. This is an unpredictable Faraday effect that is further complicated by other phenomena such as absorption, multipathing, etc.

Kerr Effect

Along with the Faraday effect, the Kerr effect can affect EM waves. While the Faraday effect tends to cause problems mostly within radio frequencies, the Kerr effect is generally described in terms of visible light. Like the Faraday effect, the Kerr effect is a response to a nonlinear field, in this case an electric field, which can change the refractive index of a material. Here, the effect is not quite as random in nature as the Faraday effect because the change in refractive index is directly proportional to the square of the electric field. All materials show a Kerr effect, but some liquids, such as nitrobenzene, display it particularly strongly. When an electric field is applied to this liquid, its (anisotropic) molecules easily align with the field, changing opaque molecules into transparent ones. This allows someone to control the amount of light passing through a transparent cell filled with this fluid, simply by applying a transient electric field to it. It's a very rapid and reversible effect, limited only by how fast the electric field can be changed. You may have heard of special high-speed cameras capable of taking still images with exposure times as short as 10 billionths of a second. These cameras, called rapatronic cameras, make use of a Kerr cell shutter to film such events as nuclear tests. Click here to see some famous and eerie rapatronic images of atmospheric nuclear tests done in the 1950's in Nevada. The Kerr effect can also come from the electric field produced by the EM radiation itself. Here, light, through the Kerr effect, can change the refractive index of the media through which it is passing. It is responsible for nonlinear optical effects such as self-focusing and modulational instability. This kind of Kerr effect only becomes significant with very intense beams of EM radiation such as lasers.

If you are unnerved by these seemingly mutually exclusive behaviours of EM radiation - it's a wave with electric and magnetic components, no - it's a charge-less particle, the photon, you are in good company. If you want to explore how EM radiation is both, take a look at my article called, "The Nature of Light," where I attempt to explain how both descriptions of light are true.

Geomagnetic Storms

You may have heard about how magnetic storms can potentially cause extensive damage to communications and navigation systems, satellites, the electric grid, and even pipelines. Is this not an example of magnetism effecting EM radiation? Not exactly - this is a great example of magnetic fields interacting with electric fields and the damage-causing mechanism involved is electromagnetic induction, Michael Faraday's famous discovery, and I will explain why.

As you learned in the introductory article in this series, Earth is surrounded by a gigantic magnetosphere. Magnetic storms are caused by periods of unusually intense solar wind. This usually happens when the Sun experiences a coronal mass ejection in which a slew of charged particles is released into space.  This creates a shock wave of solar wind that strikes Earth's magnetosphere and compresses it, while transferring energy from the solar wind magnetic field to Earth's magnetic field. Both interactions increase the movement of charged particles through our magnetosphere. These particles cause especially dramatic aurorae (Northern and Southern Lights), which in themselves are fairly benign. The damage is caused when these solar charged particles increase the electric current in the magnetosphere and ionosphere and push the boundary of the magnetosphere toward Earth's surface. At this point you get communications disruptions as charged particles interact with radio waves, etc. Even more damage happens when Earth's magnetic field lines get interrupted. This can happen with particularly violent magnetic storms. They separate and then snap back together, throwing a tremendous amount of radiation back toward Earth's surface. This is when you get extensive power outages and damage to hardware as electrical components are overloaded through the process of electromagnetic induction. The influx of charged particles induces changing magnetic fields. A changing magnetic field creates a current. Such induced currents are added to the current already within current-carrying wires and can overload them and blow up transformers. These induced currents can also corrode oil and water pipelines, creating a long-term problem. The current created by these charged particles creates a magnetic force that snaps the magnetosphere boundary back out away from Earth, like a rubber band. The largest magnetic storm ever recorded happened in 1859, when communications was restricted to telegraphs. Telegraph operators received shocks as induced electrical currents traveled up the telegraph wires. Paper caught fire. And many telegraph systems continued to receive and send signals even after the operators disconnected the batteries. Such a storm today would cause immeasurable damage.


Magnetism is not a mysterious property of specially treated iron. It is simply the net result of the motion of electrons inside atoms. When the spins of two nearby electrons line up parallel to each other, they create a tiny electrical current and a tiny magnetic field is created. Billions of atoms within molecular arrangements that allow some electron spins to line up with each other create magnetic domains and when enough of these domains line up parallel with each other in a material, you have a magnet. Different kinds of magnetism are simply the net magnetic results of different kinds of atomic arrangements within materials.

Subatomic particles display magnetism as well. Here, it is called the magnetic moment. It is built into electrons and atomic nuclei and it adds up to create atomic and molecular magnetic moments. Magnetism at this level can affect the chemistry of atoms as they combine into molecules but it is not powerful enough to affect the magnetism of macroscopic objects, which is based on the much more powerful magnetic effects of electron spins that are paired across atoms.

When an atom's electron attains sufficient kinetic energy, it vibrates fast enough to create a tiny changing electric charge, a current in essence, and this current spontaneously creates a changing magnetic field. This is the basic mechanism underlying all EM radiation, such as light, gamma rays, and radio waves. The modulation of electric and magnetic fields contains energy, and that energy travels at light speed, along a transverse EM wave, as photons.

Sunday, October 16, 2011


Does time exist? If it does, can it move backward as well as forward? Is time eternal?

These are deceptively simple questions that are almost impossible to answer. I hope to give you an idea of where science is at right now as physicists grapple with time.

We experience time moving forward, set with a regular ticking away of passing moments (physicists call this a metric). But is this just our perception of time? Neuroscientists know that time cannot be directly perceived. It must be reconstructed in the brain, and it is variable - duration is subjective, depending on who is experiencing it. Drugs such as LSD, psilocybin and mescaline, hypnosis, and several clinical disorders can affect time perception. Some physicists now believe that time does not even exist.

In previous articles I have mentioned space-time and particles that move backward through time. You may have found these ideas hard to digest. They don't make sense based on our reality. We see change all around us - objects moving, plants growing and decaying, heavenly bodies traversing the sky. Without time, wouldn't everything be still? Not necessarily. Scientists now view time as an emergent property of reality. This means that, just as our perception of a solid table emerges from a swarm of spinning zipping particles, our perception of the one-way flow of time emerges from the basic ingredients of the universe. Many believe time is our perception of entropy, and I will flesh out this idea later.

Throughout my articles I have been harping on the need for a unified theory of everything. The Holy Grail in physics is the successful merging of two fundamental theories - relativity and quantum mechanics. Some physicists now believe that in order to achieve this, we must reconceptualize time. This is not an easy task.

Let's begin with how our understanding of time has evolved.

Before Newton, it was thought that time could not be measured and in fact it could not occur without a change occurring somewhere in the universe. Time was considered the measure of intervals between changes in the universe.

Newtonian Time

In the late 17th century, Isaac Newton viewed time as consistent and irreversible. His laws of motion depend on it. This is a quote about time from his masterpiece, "Principia:"

"Absolute, true and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration. Relative, apparent and common time is any sensible and external measure (precise or imprecise) of duration by means of motion; such a measure - for example, an hour, a day, a month, a year - is commonly used instead of true time."

His concept of time held up particularly well when working out the motions of planets and moons. But it was not to last. Accurate measurements of the speed of light threw this concept a curveball.

In the late 1800's, physicists came up against a real problem in physics. If nothing, not even light, can travel faster than light speed, what happens when a light source is shone from the front of a traveling object? Doesn't that light surpass light speed? Albert Einstein, thinking over the relationships between velocity, time and distance, realized that time must be motion-dependent.

Einsteinian Time

Many of us still think of time in terms of Newtonian time. Almost all daily phenomena make sense within this framework. However, for those who are studying objects at very high speeds or within very intense gravitational fields, the Newtonian framework must be abandoned in favour of Einstein's special relativity. Few of us have really thought through the implications of Einsteinian time, but if we want to get a feel for how our universe works, we must get a basic handle on special relativity.

Idea # 1: Time is "Stretchy"

Let's warm up our brains with two videos. The first one describes classic relativity and the second describes and compares special relativity to it. Both are brief, about 8-9 minutes, and they are both easy to follow. (These are two of seven videos on relativity from ScienceTV. If you would like to be a relativity expert watch all of them - they're all fun!)

Classical Relativity

Special Relativity

To understand special relativity, we must first accept that time cannot be separated from the dimensions of space. In other words, time depends on what is happening within these other three dimensions. In order to describe any event, that is, any physical situation or occurrence, we need four measurements - three (usually) for space (up/down, forward/backward, left/right) and one (usually) for time. The "usually"' means that according to some contemporary theories, such as string theory, more than four dimensions may exist, here we are focusing strictly on time however.

Time runs slower, it stretches out, when one object moves faster relative to another object. Clocks on board the space shuttles as they orbited Earth ran slower than our Earth-bound clocks relative to us. The word relative is important. For the shuttle crews the clocks kept time perfectly - they did not experience any slowing of time. This concept, called time dilation, is famously expressed in the movie, "Contact." Recall when Palmer tells Arroway that if she ends up traveling anywhere near light speed to the nearest star, Vega, and back in the space travel device, she will be only four years older but over fifty years will have passed on Earth, everyone she knows and loves will be dead.

Now let's explore the nuts and bolts of special relativity. First of all, when an object travels at close to light speed relative to you, it not only experiences time dilation relative to you, it experiences something called Lorentz contraction. It you click this link you can see the graphs for them. You will notice that each effect does not become significant until you get very close to light-speed. The MIT Game Lab just came out with a free downloadable open-source game called "A Slower Speed of Light." As a player, you can experience your surroundings change as you approach relativistic speeds.

Time dilation and Lorentz contraction show us how time is inextricably wound up in the other three dimensions of space. As I think about this for a bit, it raises some very disconcerting questions, such as how has time been affected by the evolution of the universe? When the universe was very young, many physicists believe that the dimensions of space were tightly curled up and these dimensions themselves unfolded faster than light speed (cosmic inflation). How would time behave under these extreme circumstances? To an outside observer, what would our universe look like as it was born? I leave this is an example of the immense implications of Einstein's theory of special relativity. We know it's true but it is very hard to wrap our heads around it. We live our lives in Newtonian time so it is very hard to reconcile these concepts with reality.

Let's focus now on time dilation. The speed of light works very well to help illustrate how time slows down for an observer. If you click this animation, it will show you very nicely how light within a moving apparatus must travel a longer distance relative to a stationary observer than it does when the observer is moving right along at the same velocity. The light speed in each case is the same.* It all depends on the observer relative to the event. That's the crux of relativity. It shows us what time dilation does to atomic clocks, biological clocks, and in fact all processes and phenomena. It means that two events can occur at the same time in one reference frame and at different times if viewed from another one. Time is not an impartial score keeper outside of our universe - it is a function of our universe, perhaps even an artifact. We have just tasted the unsettling malleability of time.

This short 1.3-minute video illustrates how time is stretched relative to an observer:

*That light speed remains constant is a simple but extremely important point. If you think about it, physicists are saying that if we hold light speed as a constant in the universe we must let go of universal time as a constant. One or the other must go, according to relativity, and mountains of experimental evidence back up the consistency of light speed.

Physicists are using special relativity to great advantage in particle accelerators. Most of the interesting particles they want to observe and study last just infinitesimally small fractions of a second, once they are created in high velocity collisions between particles. Take the muon for example. Muons disintegrate in just 2 millionths of a second before they explode into electrons and neutrinos, not much time for study! But if they are created in an accelerator where they are traveling at about 99.5% the speed of light, they can get them to last ten times longer (from the physicists perspective! The muon itself, if it could feel, would not feel it lived any longer). Thanks to relativity, the muon would also be much more massive and its shape, if one could see it, would appear flattened.

To bring home the point of stretchy time even further, it is not just velocity that dilates time. Gravity does the same thing. Gravitation slows time down. Even an atomic clock placed just a few feet higher than another identical one in the same room runs just a tiny bit faster. The gravitational field in which it finds itself is just a tiny bit less intense so to a person observing the two clocks side by side, that one runs just a bit faster. This effect is significant enough that global positioning satellites have to have their clocks adjusted because their clocks run a bit faster than those on Earth's surface (more so than their relative velocity slows them down).

This means that there is a local proper time for every infinitesimal region of space, where the gravitational field in that space affects light and matter equally. It means that no two synchronized clocks, unless they were somehow perfectly superimposed on each other, can remain synchronized. This is a consequence of Einstein's law of general relativity.

Einstein managed to erode time as a stand-alone phenomenon. Its meaning is severely limited unless it is incorporated into space-time.

Now that we have expanded our understanding of time by weaving it into the three dimensions of space, let's examine its directional nature.

Idea # 2: Time is a One-way Arrow. Or is It?


All the three spatial dimensions have two arrows, two directions, such as east/west or up/down. But time seems to have just one arrow - forward. Why?

In 1927, British astronomer Arthur Eddington coined the phrase, "arrow of time." He viewed time as the direction of the progressive increase of randomness in the universe, that it is a property of entropy, in other words. What is entropy? The idea of entropy was introduced by Ludwig Boltzmann in the 1870's. You can think of entropy in this sense as a measure of microscopic randomness. According to the second law of thermodynamics, entropy increases with time. Although entropy cannot measure time, according to this theory it can be used to empirically distinguish between future and past, in a closed system. In an open system, some objects, such the human body, for example, decrease in entropy, at the expense of the environment, which then increases entropy, with the overall system gaining entropy over time. The interesting part about the second law of thermodynamics is that it is, by nature, a statistical law. No macroscopic system has ever been observed to violate the second law of thermodynamics.

Maxwell's demon is an excellent thought experiment that tests our understanding of entropy and the second law. This is how physicist James Maxwell set it up in 1867:

A container is divided into two parts by an insulated wall. It has a door that allows only fast-moving energetic or "hot" molecules of gas to flow through to only one chamber in the container. That chamber gradually heats up while the other chamber cools down. This violates the second law of thermodynamics because no work is expended to decrease the entropy of a closed system. A level of order has been imposed on it.

Fellow physicists cleverly pointed out that the entropy of the door itself was neglected in the experiment. It must acquire information about the molecules and that, by itself, would require energy. The expenditure of energy by the door would increase the overall entropy of the system. Real-life Maxwell demons are now widely used, such as single-atom traps and molecular mechanisms used in nanotechnology. None of them violate the second law of thermodynamics.

If you feel a bit fuzzy about Maxwell's Demon, try out this 13-minute online lecture:

You might be able to come up with many examples that demonstrate time's irreversible one-way arrow which also rely on entropy. For example, ice cubes melt in a cup of hot coffee; they never reassemble themselves out of it.

Time in the Mirror?

The one-way time arrow required by entropy seems to break down when we examine the universe at the quantum level. You might recall me talking about CPT symmetry in my antimatter article. CPT stands for charge, parity and time symmetries. All physical laws are perfectly symmetrical with respect to these three values. Before the 1950's, physicists were confident that no system violates charge or parity symmetry. All phenomena should be perfectly reversible in these two regards, as long as the second law of thermodynamics isn't violated. This means that any particle can be interchanged with its antiparticle (c-parity) and left and right can be swapped (p-symmetry) without violating any laws of physics. All charged particles with spin 1/2 (these include the electrons and quarks that make up matter) have antimatter counterparts of opposite charge and opposite parity. They are like mirrors of each other except that the reflection in a mirror is 2-dimensional - a true p-transformation is a reversal in all three spatial axes. This should be expected of nature.

Quantum Time

However, in the 1950's and 1960's, physicists discovered that both charge and parity symmetry are violated during special kinds of nuclear decay, a phenomenon of the weak fundamental force. They are described in detail here. Symmetry is restored if time is permitted to run both backwards and forwards (t has a minus and a plus value) in the formulae used to describe the decay process. If time runs backwards on a macroscopic scale, cause and effect are lost and the universe becomes a jumbled series of unrelated events. The second law of thermodynamics becomes meaningless. However, what if time can run in both directions at the quantum level? Recall that thermodynamics is a statistical theory. As long as events add up to its predictions, the theory remains intact. An individual event, however, such as a particle moving backward through time, may not violate the law. Physicist Richard Feynman followed the argument that by inverting time, one can invert the charge and parity of a particle so, by extension, antiparticles travel backward through time. All of the fundamental laws of quantum physics allow for time reversibility: a time-reversed process is allowed where parity is switched and particles are switched with antiparticles. The problem is how to reconcile this with the irreversibility of entropy.

Some physicists believe that the answer comes at least in part from reexamining the second law of thermodynamics. What we see as the irreversibility of entropy might be the result of the extremely low entropy conditions of the early universe very shortly after the Big Bang. Energy was so uniform at this point that it has been a theoretical challenge to figure out how stars and galaxies later formed. Entropy itself has been refined over the decades as well. We now consider entropy to be the measure of the total number of different individual microscopic states of an object that, macroscopically, are indistinguishable. An omelet has higher entropy than an egg because there are more ways to rearrange its atoms while keeping it an omelet than there are for an egg. Entropy tends to increase over time because there are more ways to be high entropy than there are for low entropy. The gist of this reasoning is that, if increasing entropy is the result of how this particular universe unfolded, then time may likewise be reversible. Perhaps another universe could have unfolded instead, one with decreasing entropy and in which time runs backward. The question of why entropy was so low in the beginning of our universe remains however. And, as you can see, this remains a partial answer.

Idea #3: Time as Artifact

One of the central goals in modern physics is to reconcile quantum mechanics with relativity. A theory of quantum gravity would go a long way toward bridging the gap and, as a result, this field of research is being hotly pursued. Physicists John Wheeler and Bryce DeWitt attempted to formulate a quantum gravity theory as early as the 1960's. They rewrote Einstein's equations for gravity in the same form as the equations for electromagnetism, with the idea being that a quantum theory of electromagnetism could then be applied to gravity as well. The equation they came up with had a weird result - it had no time variable in it. Some physicists regarded this result as the further denigration of time as an independent phenomenon, that time may in fact be nothing more than an artifact of changing physical phenomena such as the relationships between objects in motion. Time in this sense is simply a way to relate physical systems to one another, much like money is used to measure the transactions of objects of value. This has spurred many physicists to come up with ways to test the validity of time itself, a quest in progress.

Life As a Photon?

If you read my article on the nature of light, you might have felt frustrated at Richard Feynman's apparent acceptance of the notion of photons sensing where they will ultimately go. How can any particle try out infinite paths and require no time at all to do it? There is something fishy about QED and yet there is already a mountain of direct experimental evidence that it works. I can't give you a satisfying answer here but I can suggest that particles such as photons do not experience the universe like we do.

Let's explore this notion further. First I recommend a really cool 1992 essay written by astronomer, John Reed, called, "The life of a Photon." It describes the lifespan, from emission to absorption, of a single photon that happens to reach your retina from a distant star in the Andromeda galaxy while you are staring up at the night sky. The author does not insert special relativity into his description so let's consider that. The photon is traveling at exactly light speed. From our perspective it took two million years to make the trip. From its perspective it took an instant, right? Now consider a photon from the universe's origin, a part of the microwave background, striking a TV antenna. For it, the entire universe just flashed instantly into existence. To an object approaching the speed of light, the universe has a width approaching zero (this is called Lorentz contraction, a consequence of special relativity) and everything in the universe from its beginning to the present exists within one flat 2-dimensional plane, with the latest version of things directly ahead and the most distant past visible around the circumference. You can get an approximation of light speed but this description relies on Lorentz transformation equations and when you plug the speed of light into them you get an undefined answer.

At present we don't really have any way of knowing just what "life" from the perspective of a photon, where time stops, is like. And we have no way to ask photons what they are up to as they go about their spooky business. What these examples point out, however, is that our understanding of time is incomplete. We don't have the theoretical framework yet in which to test the veracity of time.

You might be tempted to throw out time altogether, and some physicists are considering doing just that. Let's consider this viewpoint as we take a look at physicist Sascha Vongehr's article in which he argues that light not only does not exist, it cannot exist. He makes an excellent and fascinating argument that, although about photons, builds upon John Reed's exploration and helps us expand our reference frame to ask what time really means. His argument that light cannot exist is based on the logic that, for a photon, time is squeezed down to zero, and if it does not exist in time how can it exist at all? An analogous argument is made based on Lorentz contraction. Distance, to a photon, is likewise squeezed down to zero, so the photon does not travel anywhere. By these arguments, photons don't really exist. It's an excellent thought experiment to ponder over. Let's use this logic to further explore "life" as a photon. To a photon, the universe has no time and no width. Each photon experiences the universe as a photo-like snapshot, regardless of how far it travels before it is absorbed. Each photon's (disk shaped) snap shot would be unique, reflecting its point of absorption (end or death from our perspective) in the exact middle and its point of origin smeared thinly around the circumference. Two photons with an intersecting time line (briefly co-existing, according to our frame of reference) would have a section of overlapping view in their snap shot. Consider how daunting a task it might be for an intelligent light-speed dwelling being to figure out the dimensions of time and the third spatial dimension. Photons would have to compare their snap shots and wonder why some have overlapping parts and others do not. The center of each photon's snap shot would reflect a unique point in their 2-dimensional world, and perhaps a comparison between the sizes of individual snap shots would hint at the hidden dimension of time. This is the same kind of quandary we find ourselves in when we speculate that we are three-dimensional projections within a four-dimensional (four spatial dimensions + time) universe. This is the basis of the holographic universe theory. I think this logic does a better job of placing time as a dimension on par with spatial dimensions, asking us to further explore the multidimensionality of the universe, than it does trying to prove that time does not exist.

So, as a result, I think it is premature to throw out time yet. Many experts find the notion of timelessness completely unpalatable. For one thing, the absence of time forces us to rethink quantum mechanics. Consider Schrodinger's famous cat for example. Its life-or-death fate depends on which quantum state a particle collapses into once a measurement takes place. If time does not exist the fate of the cat can never be resolved. It could be dead with respect to itself, alive from my point of view and dead once again according to you. To extrapolate from this, the quantum state of every particle in the universe would be both collapsed and uncollapsed simultaneously, depending only on the reference point of who is observing it.

Time is, perhaps surprisingly, treated much like Newtonian time in quantum mechanics. In the equations, it acts like a constant, and the equations work very well, with the caveat that "t" can be negative or positive as I mentioned earlier. This is extremely pesky to those trying to reconcile this with Einsteinian time, where time (and space) treads upon slippery ground. No, it goes even further - it is the slippery ground.

As another possible partial answer, there may be a way to view time that respects both its quantum reversibility and entropy's one-way arrow. In 2008, Physicist Claus Kiefer suggested that while the universe may be timeless, if it were broken into pieces, some of the pieces could serve as clocks for the others. We perceive time because we are one of those pieces. This idea is reminiscent of time as a consequence of relationships between physical systems. And, it humbly brings us back to a point before all this Newtonian/Einsteinian time confusion began.

I think this idea, or any idea about time, needs a fully fleshed out theory of quantum gravity before we can really begin to understand the nature of time within the universe as a whole. And here we have a conundrum: quantum gravity might need a better understanding of time in in order to be worked out. As an intriguing side note, some physicists are working on a theory of quantum gravity based on the holographic principle (and string theory). According to it, gravity is so much weaker than the other three fundamental forces because it originates in a fourth spatial dimension. We experience it merely as a shadow. This holographic idea seems to be gaining traction - for example it can be used to solve a puzzle called the black hole information paradox, but it is not without controversy, as this article attests to one of Stephen Hawking's famous heated arguments over it.

Time is an extreme puzzle to physicists right now. All possibilities are up in the air, and rather than fret about it I urge you to let various ideas sink in over time (pardon the pun), to mull things over for yourself, and to appreciate what an exciting and rare opportunity this is. We are alive when things are about to get very interesting. All it is going to take is another Einstein-like mind to finish what was started 100 years ago.