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Q & A: magnetic fields and light

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Most recent answer: 10/22/2007
Q:
I have a question about the effects of magnetic fields on light. Since magnetic force moves charged particles and photons are oppositely charged particles(quark antiquark) would a magnetic field that is strong enough bend or refract light. As you can probably guess im only a high school sophomore(im currently taking stats. and in the fall im taking trig.) so i wont understand big fancy calculas based equation. AP Calculas is next year. :)
- Chase (age 16)
Edon, OH
A:
Actually, fixed magnetic fields have no effect on light propagating through a vacuum and (even for rather large field strengths) negligible effect on light propagating through most materials. Quark-antiquark pairs form another category of particle (meson) altogether, not light. Light is not composed of charged particles.

The interesting cases where magnetic fields do affect light propagation are in materials exhibiting the Faraday effect. In these materials, a magnetic field can change the way the charged particles (mainly electrons) respond to the light electromagnetic field. As a result, the polarization of the light (the plane in which the electric field points) rotates as the light propagates through the material. The direction of rotation depends on which way the field points.

Mike W.

Actually, relativistic quantum mechanics tells us that photons are constantly splitting into pairs of oppositely-charged particles (usually e+e- pairs) which re-annihilate back into the orignal photons. This process violates energy and momentum conservation, but Heisenberg’s uncertainty principle tells us that’s okay as long as the time and distance scales are small (uncertainty in momentum*uncertainty in position > Planck’s constant, and
uncertainty in energy*uncertainty in time > Planck’s constant). The charged pair of particles is called "virtual".

Very high-energy photons which propagate through materials interact electromagnetically with the charged components of the materials (nuclei and electrons). The photons can split into e+e- pairs, and if an external photon (from a nucleus, say) knocks into the e+ or the e-, these particles can lead real existences. This process is called "photon conversion" into an e+e- pair. It is most often observed where electric fields are strong (near heavy nuclei), but presumably can be induced by static magnetic fields which change rapidly in space as well.

Another effect brought on by the virtual pair creation and annihilation is the screening of fields. The produced pairs are pushed and pulled by electrical and magnetic forces, where the e+ is pushed one way in an electric field, and the e- is pushed the other way. Taken together, all of the virtual pairs reduce the strength of an electric field around a point charge. The e+ and e- have spin and therefore some magnetism, too, and so the magnetic field of a spinning electron is also modified by the cloud of virtual e+e- pairs around it. This affects the reaction of a spinning electron to an external magnetic field, and is called the anomalous magnetic moment of the electron (called g-2 in the biz).

These effects are all very very tiny, though, but with care, they can be measured very precisely.

Tom

(published on 10/22/2007)

Follow-Up #1: Polarizability of the vacuum with a magnet

Q:
Is there any experiments related to interaction betwen a static magnetic field and e+ e- pairs resulted from constantly splitting of the photons?
As I understand, these experiments should be undertaken in a vacuum conditions. Can you, please, send me few links related to this subject?
- Toma Augustinov
Romania
A:
The effect exists in theory but it is extremely small, I mean really, really small.  Several experiments have been attempted but no real conclusive results have been obtained.  One such experiment can be found here:

LeeH

(published on 10/22/2007)

Follow-up on this answer.