# Light and magnetic fields

#### Andy

Didnt think that this should go in the theoretical physics section, but if it should feel free to move it.

We all know that a large gravitational field can bend light, but my question is would it be possible for a large enough magnetic field to bend light? I know that this doesnt happen around the Sun or at least i think it doesnt, but if it was more concentrated, if its possible for a magnetic field to be of a higher concentration, could it bend light?

I think thats long winded for what i want to ask, but you should get the idea.

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A

No.
Gravitational fields bend space and time, so the
light rays passing through the bent area, bend with
it.

Magnetic and electrostatic fields, are supposed to be
completely different from gravitational fields, since
they don't bend space+time.

These instead operate by sending out virtual photons,
which ultimately lead to a force being exerted.

#### HallsofIvy

Homework Helper
It is also true that magnetic and electric fields add linearly without affecting one another. Since light is a wave in a magnetic-electric field, magnetic fields cannot affect light.

#### mmwave

Originally posted by HallsofIvy
It is also true that magnetic and electric fields add linearly without affecting one another. Since light is a wave in a magnetic-electric field, magnetic fields cannot affect light.
This is certainly true in a vacuum or a linear medium. In non-linear media two light waves can interact to produce new frequencies as in a Fabry-Perot cavity modulator.

P

#### pmb

Originally posted by Andy
Didnt think that this should go in the theoretical physics section, but if it should feel free to move it.

We all know that a large gravitational field can bend light, but my question is would it be possible for a large enough magnetic field to bend light? I know that this doesnt happen around the Sun or at least i think it doesnt, but if it was more concentrated, if its possible for a magnetic field to be of a higher concentration, could it bend light?

I think thats long winded for what i want to ask, but you should get the idea.
Since the field has energy and since energy has mass and since mass is the source of gravity then it follows that a magnetic field will general a gravitational field and such a field can delfect light.

Even light can generate a gravitational field and deflect light! :-)

Pete

#### McQueen

Originally posted by HallsofIvy
Since light is a wave in a magnetic-electric field, magnetic fields cannot affect light.
This is a wrong statement , magnetic fields do affect light , they change the spectral output of light .

#### chroot

Staff Emeritus
Gold Member
Originally posted by McQueen
This is a wrong statement , magnetic fields do affect light , they change the spectral output of light .
Well, light doesn't have any quality called 'spectral output' that I know of!

I think you're talking about the hyperfine splitting, and how magnetic fields can affect the light produced by atoms immersed in the field -- and this is correct.

However, once emitted by the atom, the photons themselves are not affected by the magnetic field in any way.

- Warren

#### Creator

Originally posted by HallsofIvy
It is also true that magnetic and electric fields add linearly without affecting one another. Since light is a wave in a magnetic-electric field, magnetic fields cannot affect light.
Well, as mmwave pointed out, this is only true in a vacuum. Magnetic field effects upon light are observed in solids and liquids, generally referred to as magneto-optic effects.

Generally a magnetic field aligned to the propogation axis of light causes left and right circularly polarized light to travel at different speeds in the substance, (i.e., the index of refraction for left and right circular polarization is changed).
The most common example is Faraday rotation, where, due to the above effect, the plane of polarization of linearly polarized light is rotated as it passes thru the material, the amount of rotation being proportional to the magnetic field and the path length through the material.

Many ingenious & useful devices have been developed as a result of these effects.

Creator

P.S. Magneto-optic effects also take place in plasmas, where, for ex., Faraday rotation is observed in polarized radio waves through the earth's ionosphere.

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#### Creator

Originally posted by mmwave
This is certainly true in a vacuum or a linear medium. In non-linear media two light waves can interact to produce new frequencies as in a Fabry-Perot cavity modulator.
Thanks mmwave.
Furthermore, (and somewhat contradicting my previous post), according to quantum electrodynamics two light waves DO interact even in the vacuum, violating the (classical) superposition principle.

This is a very small interaction but represents a non-linear departure from the classical predictions. It's a result of the virtual polarization of the vacuum which causes a non-vanishing photon scattering cross section. In QED photon-photon scattering is predicted, but very small.

ALSO, let it be known that photon interaction with the nuclear coulomb field is also shown to occur and has been measured exactly as predicted by QED; it is usually referred to as Delbruck scattering.

Creator

#### Creator

Re: Re: Light and magnetic fields

Originally posted by pmb
Since the field has energy and since energy has mass and since mass is the source of gravity then it follows that a magnetic field will general a gravitational field and such a field can delfect light.

Even light can generate a gravitational field and deflect light! :-)
pmb,
It may interest you to know (and I found quite interesting) that, using Quantum Field Theory, Tolman, Ehrenfest, & Podolsky found that two light beams propogating in the same direction do not interact gravitationally; BUT two beams going in opposite directions DO interact! Two different derivations are given in A. Zee's book entitled, Quantum Field Theory ....

However, I'm not sure that proof has ever been given of gravitating magnetic fields.

Creator

#### jeff

Originally posted by Creator
...using Quantum Field Theory, Tolman, Ehrenfest, & Podolsky found that two light beams propogating in the same direction do not interact gravitationally; BUT two beams going in opposite directions DO interact!
That's true only for low energy graviton exchange. If we include higher energy exchanges, we do see the expected gravitational interaction of photons, whatever their relative momentum.

Originally posted by Creator
However, I'm not sure that proof has ever been given of gravitating magnetic fields.
Magnetic and electric fields are different aspects of the electromagnetic field, which is light.

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#### Creator

Originally posted by jeff
That's true only for low energy graviton exchange. ...
Yes, it was shown to be so in the 'weak field limit'. However I still find it interesting. I'm not quite sure what 'high energy exchange' would represent physically for two photon beams.

Magnetic and electric fields are different aspects of the electromagnetic field, which is light.
Yes, Jeff, I know, but that is still not considered explicit proof that magnetic fields gravitate.

Creator

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#### jeff

Originally posted by Creator
...not quite sure what 'high energy exchange' would represent physically for two photon beams.
Then maybe you should find out.

Originally posted by Creator
...that [Magnetic and electric fields are different aspects of the electromagnetic field, which is light] is still not considered explicit proof that magnetic fields gravitate.
If you accept that magnetic fields are just photons, then how can you doubt that light gravitates? After all, light has energy, and all energy gravitates.

#### jeff

Re: Re: Light and magnetic fields

Originally posted by pmb
Since the field has energy and since energy has mass and since mass is the source of gravity then it follows that a magnetic field will general a gravitational field and such a field can delfect light.
Why do you insist on inserting this extra step in your reasoning by saying "energy has mass and mass generates gravity" when mass-energy equivalence allows you to dispense with this and just say that energy gravitates?

I looked at your site and you make a big deal out of the issue of whether or not the use of the term "relativistic mass" is legitimate. But really it's just a convention, some people like to use it - I'm one of them - and others think it's better not to, and I think they're being silly.

P

#### pmb

Re: Re: Re: Light and magnetic fields

Originally posted by jeff
Why do you insist on inserting this extra step in your reasoning by saying "energy has mass and mass generates gravity" when mass-energy equivalence allows you to dispense with this and just say that energy gravitates?

I looked at your site and you make a big deal out of the issue of whether or not the use of the term "relativistic mass" is legitimate. But really it's just a convention, some people like to use it - I'm one of them - and others think it's better not to, and I think they're being silly.
So you have your opinions and I have mine. I thought you agreed to disagree? What is it that you want?

I don't make a big deal of "relativistic mass." It's simply something I'm studying. I use that web site to record what I'm doing. The site is has a purpose which is not what you may think it is. I solidify concepts in my mind by putting them in writing. And I put them on the internet because it's easier to communicate with people I know in e-mail this way. I don't have to hope that they can read MS - Word etc. And since I discuss this a huge amount on the internet then it's a great way to discuss this since it's easier to see equations this way.

Regarding why I refer to mass instead of energy - I've explained that to you already. Was I unclear?

Pmb

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