Quantum theory is often viewed as a strange and mysterious model where objects behave in illogical ways. While it’s true that quantum objects behave in ways that are counterintuitive, we actually understand the behavior quite well. In fact, many of these strange behaviors are used in modern astronomy. Take, for example, the quantization of magnetic moments.

Most atoms have a small magnetic field. This magnetic field can be approximated as a small magnet, just as the Earth’s magnetic field is sometimes treated as a magnetic. The strength of that imaginary magnet is given by the magnetic moment of the atom. With this in mind, the orientation of an atom’s magnetic field can be represented by the orientation of the magnet.

The basic setup of a Stern-Gerlach experiment. Credit: Wikipedia

Suppose, then, that we were to toss atoms through an inhomogeneous magnetic field. Individually the atoms have no particular orientation, so we would expect that the orientation of their magnetic moments are entirely random. As a result, some of the atoms would be more strongly attracted toward the north direction of the magnetic field, while others would be more attracted to the south, and everywhere in between. If the atoms really did act like tiny magnets, we would expect to see the beam of atoms spread out evenly by the magnetic field. In fact, what we see is that the atoms either move toward the north or south, and that’s it. Instead of spreading out evenly, the atoms lock into specific orientations. This experiment is named the Stern-Gerlach experiment, after the physicists who first performed it in 1922, and it demonstrates one of the basic aspects of quantum theory. When you try to measure the state of a quantum system, the results you get are often snapped to discrete results. It would be like measuring the height of a random collection of people, and finding they are all exactly either 5 ft or 6 ft tall.

The Zeeman effect.

As strange as this is, we actually use a similar effect to measure the strength of magnetic fields in the Sun. Since electrons also have magnetic moments, strong magnetic fields can cause their energy levels in an atom to shift. As a result, the emission lines an atom gives off can be shifted by magnetic fields. Emission lines can even be split slightly, which is known as the Zeeman effect. We see this effect near sunspots, which is how we know that sunspots are cooled by magnetic dampening.

That’s part of the real power of astrophysics. Once we understand a phenomena, however strange, we can use it has a tool to study the stars.

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Measuring the magnetic fields of our galaxy poses an interesting challenge.  The galactic magnetic field doesn’t emit or absorb light, and of course we can’t directly measure it at various places like we can for Earth’s magnetic field. The galactic magnetic field does, however, interact strongly with things such as ionized gas and electrically charged dust, so we can indirectly measure the field by the way it affects these things.One way that the field has been measured is by looking at the radio bursts of pulsars.  When pulsars emit a burst of radio waves, they are polarized.  That is, the oscillation of the radio waves has a particular orientation, similar to the way you can shake a jump rope up and down, or side to side.  When a radio wave passes through a region of ionized gas, it causes the gas to vibrate slightly.  But if there is a magnetic field in the region of the gas, the gas can move easily along the magnetic field, but not in other directions.  As a result, the polarization of the radio waves rotates, and by measuring this rotation we can determine the strength of the magnetic field between us and the pulsar.  The advantage of this method is that it is relatively easy to do, but the big disadvantage is that we can only measure the magnetic field along the direction of a pulsar.

Another way is to look at the light emitted by ionized gas as it moves through the magnetic fields. The charged particles of the plasma spiral along the magnetic field causing them emit radio waves through what is known as synchrotron radiation. These are typically in the form of radio waves, and are also polarized. With this method we can determine the magnetic field wherever there is ionized gas (which is pretty much everywhere), but it is a much fainter signal than that of pulsars.  This emission of polarized radio waves by ionized gas also poses a challenge for detecting evidence of inflation in the early universe. You might recall an earlier post talking about how such radio loops due to galactic magnetic fields look very similar to the B-mode polarization observed by BICEP2. Discerning the difference between polarization due to cosmic inflation, and polarization due to magnetic fields is very difficult.

Now new data from the Planck satellite should make that a bit easier. The Planck team has recently released a map of the galactic magnetic field obtained from emitted polarized light.

In addition to ionized gas, there is also a significant amount of dust throughout the galaxy. This dust is typically cold and doesn’t emit much light, but does emit a small amount in radio and microwaves.  Since most dust particles are not spherical, most of the emitted light is polarized along the length of the grain.  If the grains were simply floating randomly, then all that polarization would wash out and we would just see a faint glow from the dust without any orientation. But in the presents of a magnetic field, the grains will tend to align with the magnetic field. As a result, the light from the dust is polarized along the direction of the magnetic field. By measuring this orientation we can determine the direction of the magnetic field. The stronger the magnetic field, the more strongly the dust will align with the field, so the strength of the polarization also tells us the strength of the magnetic field.

Planck can measure the light emitted by these dust grains very precisely, and the result can be seen in the image. It is the most precise map of our galaxy’s magnetic field ever obtained.  This is a significant achievement in itself, but this map will also allow astronomers to take into account the effect of radio loops on B-mode polarization. With this map we can better filter out galactic effects when looking at the cosmic microwave background. So later if Planck observes B-mode polarization similar to BICEP2, we can be sure that it really is due to inflation.

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There’s a new paper in Nature announcing the observation of magnetic monopoles.  Just to be clear, this is not the discovery of some new kind of particle.  Rather, these are simulated monopoles within a Bose-Einstein condensate.  These effective monopoles are useful because they can be used to study the ways in which magnetic monopoles interact.  While it is mainly a study of condensed matter physics, magnetic monopoles are extremely interesting to astrophysicists because they could solve one of the great mysteries of the universe.

Small threads suspended in oil align with the electric field of a charge. Credit: NASA.

Many types of particles, such as protons and electrons, have electric charge.  Electrons and protons are electric monopoles, though we don’t usually refer to them that way.  This means they have a single charge, with protons being a positive monopole, and electrons being a negative one.  In our everyday experience, electrons and protons (and neutrons) tend to be bound together into atoms.  From at human-scale distances the positive and negative charges average out, so we don’t really notice them, but it is fairly easy to separate some of the positive and negative charges.  If you’ve ever gotten a static shock, it’s because you’ve separated a few too many positive and negative charges, and the shock is them coming together again.

Iron filings alined with a magnet’s field.
Credit: Newton Henry Black, Harvey N. Davis .

Magnets behave very differently.  A magnet has a north and south pole, similar to charges, but you can’t separate the poles.  If you break a magnet in half, then instead of getting a north magnet and a south magnet, you get two separate magnets, each with a north and south pole.  The reason for this is that the atoms or molecules that form a magnet each act as a small magnet, and are aligned to make a stronger one.  So it is impossible to separate the north and south poles in such a magnet.

The mystery comes from the fact that electricity and magnetism are connected as a fundamental force called electromagnetism.  One consequence of this is that moving charges can create magnetic forces and moving magnets can create electric forces, which lead to electromagnetic waves (light).  The equations that describe electromagnetism are symmetrical when it comes to treating electricity and magnetism.  But electric charge breaks this symmetry, because there are electric monopoles (charges), but there are no magnetic monopoles.

This might seem like much of a mystery, since it may just be the way the universe works.  Just because we’d like a theory to be symmetrical doesn’t make it so.  But if there were true magnetic monopoles, it would solve another mystery, known as charge quantization.  The charge of any particle can be given as an integer number of electron charges, either positive, negative, or zero.  This means that charge is quantized into charges of +2, -1, 0, etc.  You never see anything with a fractional charge.  (Some of you may ask “what about quarks?”, but these are always bound such that the total charge is an integer.)  Quantized charge is a basic aspect of physics, but we aren’t entirely sure why charge is quantized.

If magnetic monopoles exist, then this would solve the charge quantization mystery.  If you had a number of electric and magnetic charges, the electromagnetic field they produce would have a an angular momentum (rotation) that depends on value of the electric and magnetic charges.  In quantum mechanics angular momentum is quantized, which means the charges would also be quantized.  What’s particularly interesting is that even a single magnetic monopole would quantize all the electric charges in the universe.

There’s just one problem:  they’ve never been observed.  Several projects have tried to detect them, but none have been successful.  It’s possible that magnetic monopoles are too light to be observed by current detectors, but it’s also possible that they don’t exist.  The theoretical framework that proposes monopoles is compelling, but that’s not enough to support their existence.

But this new work in condensed matter may give clues on how we could detect magnetic monopoles (if they exist).

Paper:  Ray MW, Ruokokoski E, Kandel S, Möttönen M, Hall DS. Observation of Dirac monopoles in a synthetic magnetic field. Nature. 2014;505(7485):657-660.

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Path of a charge in a magnetic field.

One of the more interesting aspects of magnetic fields is that they don’t really affect the speed of a charged particle. Instead the magnetic field causes a charge to spiral along the magnetic field. For this reason magnetic fields tend to trap charges along their field lines. The charges generally stay trapped unless they collide with each other (not likely in interstellar space) or interact with something else, such as our atmosphere.

It is this effect that causes the aurora we see in the polar regions of our planet. The solar wind given off by our sun consists of charged particles, and those particles get trapped by the sun’s magnetic field. As a result they spiral outward until they interact with Earth’s magnetic field. Most of the solar wind gets repelled by our magnetic field, but some of the charged particles get trapped by Earth’s field. Those particles then spiral along our field and are funneled to either the north or south magnetic pole. When these charges hit our upper atmosphere they can ionize oxygen and nitrogen atoms. Oxygen excitations tend to be greenish, while Nitrogen tends to be reddish, though you can get some other colors as well.

Of course describing all this mathematically takes a bit of computational vector calculus. You can see a basic plot of the motion of a charge above.

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