Does light have magnetism?

November 27, 2010 5:40 PM

Does light have magnetism?
posted by Optamystic to Science & Nature (21 answers total)

No.
posted by axiom at 5:41 PM on November 27, 2010

It depends on how you're using the word "magnetism".

Light is an oscillation (or excitation) of the electromagnetic field. So, yes, light does involve the magnetic field.

Perhaps if you expand on what your question is I can answer more accurately.
posted by Salvor Hardin at 5:55 PM on November 27, 2010

If, as axiom took it, your question is whether or not light is affected by magnetic fields (perhaps you're imagining light being bent by a strong magnet or something), then the answer is no. Light has no magnetic charge or moment, and therefore does not interact with external magnetic fields.
posted by Salvor Hardin at 6:13 PM on November 27, 2010 [2 favorites]

Light IS magnetism. Photons (light particles) are manifestations of the electromagnetic field. If your question is whether light is attracted by a magnetic, the answer is no.
posted by auto-correct at 6:34 PM on November 27, 2010

This is kind of a weird question. Every magnetic field is made of photons.
posted by empath at 6:47 PM on November 27, 2010

The superposition principle, as applied to electromagnetism, means that two electrical fields created by two electrical charges add up just like any other two numbers. Two plus three still equals five. Of course, we have to fancy this up by giving these fields a direction as well as a size and call the result a vector, so sometimes you have to worry about fields being in different directions, but let us ignore that for now.

A photon, being a self-recreating electromagnetic disturbance (a changing electrical field creates a magnetic field; a changing magnetic field creates an electric field. Start it up just right and you get something that moves forward as it constantly recreates itself) obeys the superposition principle like any other EM field and its path would be unaffected by traveling through any ordinary electromagnetic field. Imagine two rocks thrown into a pond, a few feet apart — the ripples pass through one another. That is the superposition principle in action, and in that same way, a photon (ripple) travels through a magnetic field (water), and photons can pass through one another.

The caveat: according to my old Ohanian E&M text, there are notes to the effect that, theoretically, this would not hold at very high flux densities, where you would see non-linear effects and the photons would interact with one another. And by "very high," we're talking lots and lots of zeroes, where you're entering the realm of "conditions probably not achievable by humanity within the next few centuries, if ever."

So, a laser beam won't be deflected by anything but passing through a strong gravitational field or traveling from one medium to another.
posted by adipocere at 6:50 PM on November 27, 2010 [2 favorites]

The interaction of light with a magnetic field (where the polarization vector is rotated) is called the Faraday effect.
posted by achmorrison at 7:23 PM on November 27, 2010

Light has no magnetic charge or moment, and therefore does not interact with external magnetic fields.

The classical Faraday effect is a good example of why this statement is false. However, one might argue that the Faraday effect is really an interaction of light with polarized matter, which is true enough. But, a magnetic field in perfect vacuum can also induce a Faraday effect due to vacuum birefringence. It's a small effect, but has definitely been observed.
posted by fatllama at 7:42 PM on November 27, 2010 [1 favorite]

Huh?

Consider an EM wave with some frequency f. If f is around 100,000 GHz, then the wave is near visible light frequencies. Presumably we have no argument that photons can be observed for this sort of wave (clicks on a photomultipler tube), yes? Imagine dialing down f... 6 orders of magnitude lower and we have microwaves... still photons, though their wavelengths are approaching centimeters. A few more orders of magnitude down and we have radio waves. Still photons, but with meter sized wavelengths.

What happens as we go lower and lower? How about 60 Hz power line frequencies? How about 1 Hz? What happens as we approach dc or "0 Hz". The EM energy is still mediated by photons of ever longer wavelengths. Since you can only localize an observed photon to approximately one wavelength, these low-frequency photos become harder and harder to observe. (You'd need a photomultipler tube sized in kilometers and somehow shielded to everything else but the powerline frequency photons to observe clicking). Also not helping is the fact that we need more and more low frequency photons to account for a certain energy because of Planck's law, E = h f. In other words, the discrete nature of low-frequency photons is harder to detect because for a given total energy in the wave there are vastly more low-frequency photons.

Quiz: How many photons make up a truly static magnetic field? An infinite number. What is the energy of each? Zero. Enter renormalization and field theory.
posted by fatllama at 7:56 PM on November 27, 2010 [4 favorites]

Fatllama...you can reflect light which is photons...can you reflect electromagnetic waves?

Particle-Wave Duality. All light is made up of EM waves and all EM waves are also photons.
posted by delmoi at 8:08 PM on November 27, 2010

(And yes most EM waves can be reflected if you have some material that reflects them. Different materials reflect different frequencies)
posted by delmoi at 8:08 PM on November 27, 2010

Fatllama...you can reflect light which is photons...can you reflect electromagnetic waves?

Well, radar dishes are made of metal for just that reason... Metal reflects microwaves, radio waves, and light up to the ultraviolet (with some exceptions, colors) pretty well due to the free charges (electrons) in the metal that easily respond to sufficiently low frequencies.

One needs a macroscopic object much larger than the wavelength of light to really reflect the wave and not just diffract it. When the wavelengths get long (frequencies get small), that can be difficult. And, such an object has to share the polarizable characteristic of metal.

The ionosphere does pretty well for wavelengths longer than 10 m or so. This is because the ionosphere is, in a certain sense, just like a metal: it is a plasma with lots of free electrons to be perturbed by, to absorb, and to reflect electromagnetism. In fact, the ionosphere is an important reflector used by radio operators to get signals over the horizon of the earth.
posted by fatllama at 8:13 PM on November 27, 2010

Technically, light isn't so much reflected as it is absorbed and re-emitted. Light only goes in one direction--straight.
posted by empath at 9:09 PM on November 27, 2010

empath: "Technically, light isn't so much reflected as it is absorbed and re-emitted. Light only goes in one direction--straight."

Ah... that sounds much better to me than some of the above. Not that I know anything.
posted by InsertNiftyNameHere at 11:40 PM on November 27, 2010

fatllama: "Huh?

Consider an EM wave with some frequency f. If f is around 100,000 GHz, then the wave is near visible light frequencies. Presumably we have no argument that photons can be observed for this sort of wave (clicks on a photomultipler tube), yes? Imagine dialing down f... 6 orders of magnitude lower and we have microwaves... still photons, though their wavelengths are approaching centimeters. A few more orders of magnitude down and we have radio waves. Still photons, but with meter sized wavelengths.

What happens as we go lower and lower? How about 60 Hz power line frequencies? How about 1 Hz? "

So when I beat a drum at 1 beat per second, my ears are "hearing" photons? (serious question). What part of the drum is electrical or magnetic? (again, no offense intended)
posted by InsertNiftyNameHere at 11:45 PM on November 27, 2010

A few clarifications:

Sound is not an electromagnetic wave. Sound is a wave in air, whereas light is a wave in the electromagnetic field.

"Photon" and "Electromagnetic waves" are two different ways to describe the same phenomenon (particle-wave duality as said above), of which visible light is one example (others are microwaves, x-rays, etc).

And joining the chorus that it's awfully hard to answer this question without a definition of what the author meant by 'magnetism'.
posted by Lady Li at 12:32 AM on November 28, 2010

Sound is also carried by a medium: it's a pressure wave in the air around us. Light is carried by no medium whatsoever: there's no matter or field that's doing the waving.

One answer to the original question is that magnetic fields are communicated by photons, which are the particles which propagate the electro-magnetic field. The light that our eyes see is also composed of photons, but the electro-magnetic influence they propagate carries no net magnetic flux, so they're not "magnetic" in that sense.

On the particle level, every photon is part of a self-sustaining electro-magnetic wave, so all light is magnetic on the quantum scale.
posted by pharm at 1:07 AM on November 28, 2010

And you apparently need field strengths of magnetars before magnetic fields will do anything interesting to photons.
posted by oonh at 6:50 AM on November 28, 2010

So when I beat a drum at 1 beat per second, my ears are "hearing" photons? (serious question). What part of the drum is electrical or magnetic? (again, no offense intended)

Lady Li and pharm have it. Ever been to a ballpark and seen a batter make a huge hit before hearing the crack of the bat a fraction of a second later? Or hear thunder 10 seconds after seeing the lightning flash? Sound and light are both vibrations, but of very different things. Light could propagate in a vacuum, but sound cannot since there would be no gas molecules to transmit the pressure wave.

The presence of atmosphere has almost no effect on (visible) light; on Earth, air makes the observed speed of light about 0.03% slower than in vacuum. Glass, a material with much higher density than air, slows down light by about 30% (this leads to refraction, very useful in lenses).

You might ask: if light can freely propagate in vacuum (i.e. "nothing is waving"), why would it slow at all when going through matter? Well, empath is correct above about reflection being really absorption and reemission; transmission of light through matter is also a process of absorption and reemission. All atoms are to some degree polarizable by light because atoms are made of positive and negative charges that can be pulled slightly apart by passing electro-magnetic waves.
posted by fatllama at 8:49 AM on November 28, 2010

So when I beat a drum at 1 beat per second...

Oh, and to address one possible confusion... When you beat a drum one time per second, you are not really exciting much sound at all at 1 Hz. The drum head vibrates at the many frequencies corresponding to its normal modes. These frequencies are set by the drum geometry, the head tension and material density, environmental factors to a limited extent, and where on the drum one beats. I don't know for sure, but a typical snare drum probably makes sound uniformly spread between 10's of Hz through several 1000 Hz; middle A on a piano is 440 Hz for reference.

You might feel a true sonic excitation at 1Hz with your body, but not using your ears since they aren't sensitive to sound below about 20 Hz.
posted by fatllama at 9:32 AM on November 28, 2010

Oh, heavens. There is no need to discuss photons or quantum physics in this thread. Classical physics is just fine. First, a brief review of electricity and magnetism.

Electric and magnetic fields are well described by Maxwell's equations. These equations require some degree of comfort with calculus to appreciate and understand fully, but when it comes down to it, they essentially provide rules for how 1) electric and magnetic fields are generated, 2) the particular ways in which they can curve, and 3) how time-varying electric and magnetic fields are related to one another. Points 1 and 3 are relevant here.

Electric fields are generated simply by the existence of charge. If the charge is stationary, the field is stationary. If the charge is moving, the field will be constantly changing. Such fields impart forces on any other charge.

Magnetic fields are generated by the presence of currents, or moving charges. If the charge is moving in a steady way, the field is stationary. (For example, a stationary loop of wire with a constant current of 1A going around it). If the current is changing, the field will be changing. A simple example of this can be described by two conductors--just some metal spheres, maybe--with charge moving back and forth between them. These fields impart forces on other moving charges. For example, the electrons flying from the back of your old CRT monitor or TV are re-directed by magnetic fields generated by little loops of current-carrying wire so that they land in just the right spot.

A key idea encoded in Maxwell's equations is that changing electric fields produce magnetic fields, and changing magnetic fields produce electric fields in a very specific way. So, take our example of the spheres with charge moving back and forth. What you find is that away from the spheres, you get oscillating electric and magnetic fields that travel away from the source (the spheres) as waves. What's more, they always travel away at a very specific speed known as the speed of light, and that the electric and magnetic components are always perpendicular to one another, and both are perpendicular to the direction of motion. (This is called a "transverse wave"). If you put a charge in the way of such a wave, the fields will tend to cause the charge to move around.

This is exactly how you see! Little bits of charge in your incandescent lamp are rapidly moving back and forth, and thus they generate waves of intertwined electric and magnetic fields. These fields travel toward your eyes. When they hit cells in your retina, they cause little bits of charge to move back and forth. Your brain interprets the little wiggles and you see a lamp. So in this sense, yes, "light has magnetism," although that's not how it would normally be phrased.
posted by dsword at 9:56 AM on November 28, 2010