1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why doesn't a magnet interfere with light?

  1. Sep 17, 2010 #1
    Since light consists in part of magnetic fields, I was wondering why the magnetic field of a magnet never has any effects on the light passing through it. Is it because the magnet's field is static while the light is moving?
     
  2. jcsd
  3. Sep 17, 2010 #2
    I could be wrong but I suspect a magnetic field would affect a beam of light if it were concentrated in a space similar in scale to the wavelength of the light.
     
  4. Sep 17, 2010 #3

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    I believe it is because photons are not charged particles and therefore do not react to the electromagnetic force. (Yet they are the force carrier for that force oddly enough)
    I believe the magnetic field is cancelled out by the electric field as it moves. This is slightly similar to an atom with equal numbers of protons and electrons. They balance each other out and therefore will not be attracted to or repulsed from a magnetic field as a whole.
     
    Last edited: Sep 17, 2010
  5. Sep 17, 2010 #4

    OmCheeto

    User Avatar
    Gold Member


    Magnets do interfere with light.

     
    Last edited by a moderator: May 4, 2017
  6. Sep 17, 2010 #5
    The classical theory of an electromagnetic wave goes something like this...

    A potential difference occurs between two points be it the two ends of a transmitting antenna or two energy states in an atom. As that potential voltage collapses it creates an expanding magnetic field. When the electrical potential is depleted the magnetic field is at it's maximum. With no more electrical potential to sustain it, it starts to collapse, in the process it induces a potential voltage, this voltage exists whether or not there are charged particles for it to affect. when the magnetic field has fully collapsed the potential voltage is at it's maximum and the cycle repeats. One consequence of this theory is that each magnetic field component will be oriented opposite from the previous magnetic field component. If an outside magnetic field is large compared to the wavelength of the light then it will distort each magnetic field component of the light just a little and it will affect every other cycle oppositely. If the magnetic field is concentrated enough to affect just one wave and to affect it significantly then I think that the effect would be noticeable.
     
  7. Sep 17, 2010 #6

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    Do they? I'd never heard that before. Where can i read more on this?
     
  8. Sep 17, 2010 #7
    Only in the presence of certain materials. Not in empty space!

    Photons do not interact with each other. Electromagnetism (light, radio, and all those waves) is regulated by the superposition principle, which would not hold if waves interacted with each other. They can only sum with each other.
     
  9. Sep 17, 2010 #8

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    What do you mean by "They can only sum with each other"?
     
  10. Sep 18, 2010 #9
    The theory of electromagnetism is about the electric and the magnetic field. It is about both, combining it. The Maxwell equations are describing both, a magnetic field is nothing but the field of moved charge. These equations are linear. Linearity means always that the superposition principle is applyiable. The equations are perfectly valid in the domain of special relativity, it was one of the reasons Einstein discovered the SRT. If you accept that light is nothing but an "electromagnetic effect" you will see that the superposition holds: There is no interaction. An interaction would mean an nonlinear effect which is given in masses having interaction but not in the issue you wrote about.

    Best regards,
    Jens
     
  11. Sep 18, 2010 #10

    Pythagorean

    User Avatar
    Gold Member

    superposition is a fundamental feature of linearity:

    F(x) + F(y) = F(x+y)

    if y and x are coupled to each other (i.e the waves interact) this needn't be true.
     
  12. Sep 18, 2010 #11

    OmCheeto

    User Avatar
    Gold Member

    Hmm.. I think I might have missed that part. Though it is possible that my wash machine of a brain took the Faraday Effect and the article I saw a few years ago and produced an erroneous conclusion.
    Ah ha! The following indicates that I was confused:

    Never mind.
     
    Last edited by a moderator: May 4, 2017
  13. Sep 18, 2010 #12
    Two electromagnetic waves (or photons, which are physically the same thing) don't interact with each other.

    All that happens if they get near is that they add up (summation). This is what fundamentally causes interference. But this isn't really an interaction. Both remain independent of each other. Once two photons have crossed, they forget they've met, as if the other never existed.

    That's why a magnet won't affect a light beam. A light beam ('s field) technically adds to nearby magnet's field, but they don't affect each other. A particle in that region of space will feel the effects of both fields however.
     
  14. Sep 18, 2010 #13
    like sound waves they (light waves) just pass through each other.
    the net 'field' at any time is just the (linear) sum of the individual fields.
    in other words, the fields just 'superpose' on top of each other
     
  15. Sep 18, 2010 #14
    The thing to keep in mind, is that the vector potential has a greater physical significance than the magnetic field. Consider Feynman, vol 2, section 15-5, "the vector potential and quantum mechanics". The idea here is that if a beam of light passes through a region of space where the vector potential is non-zero (even if the magnetic field is zero), that the light beam will interact with the vector potential, and this will have a measurable effect, in the sense that the vector potential will influence the quantum phase of the light beam.

    The statement "the magnetic field of a magnet never has any effects on the light passing through it" is false. One very clear way to see this is to realize that photons do interact in QED through virtual particle production, this accounts for example, for the anomalous magnetic moment of the electron.

    The physical significance of the vector potential over the magnetic field can be seen from a more fundamental perspective. In order to quantize the electromagnetic field, it is customary to work in the Lorentz gauge and to consider photons as the harmonic frequencies of the vector potential.
     
    Last edited: Sep 18, 2010
  16. Sep 18, 2010 #15
    Maybe I should have specified that I meant an empirically visible effect, such as image distortion, prism effect, etc. Are there any observable effects of the virtual particle production you're talking about?
     
  17. Sep 18, 2010 #16
  18. Sep 18, 2010 #17
    one way to see this would be to compute the cross section for photon photon scattering to some order in the fine structure constant. In other words, the effect is that two photons will have some QED scattering interaction. I'm not sure about macroscopic effects...
     
    Last edited: Sep 18, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook