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Does Light have an Magnetic Field in a Vacuum?

  1. May 14, 2008 #1
    Light, aka electromagnetic radiation, is said to have electric and magnetic fields as it propagates thru space. When I hold a 1 Tesla rare earth magnet next to a beam of red laser light, nothing happens. Why?
     
    Last edited: May 14, 2008
  2. jcsd
  3. May 14, 2008 #2

    Doc Al

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    Staff: Mentor

    What do you expect to happen?
     
  4. May 14, 2008 #3
    I kinda expected the light to deflect to some extent.
     
  5. May 14, 2008 #4

    Doc Al

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    Staff: Mentor

    Why?
     
  6. May 14, 2008 #5
    It seems that the magnetic field of the magnet should interact with the magnetic field of the light thus causing some change.
     
  7. May 14, 2008 #6
    Yes, but it would be negligible you couldn't notice it with your eyes.
     
  8. May 14, 2008 #7
    Can you suggest what conditions might be needed to make the deflection detectable by some form of instrumentation?
     
  9. May 15, 2008 #8
    Maxwell's equations are linear and there is therefore no interaction between magnetic fields. The magnetic field of the dipole magnet satisfies the Maxwell equations. The electromagnetic waves from your laser is another solution. Add them up and the sum of the two solutions is another solution.

    So, this suggests that there shouldn't be a deflection. This is not 100% true. The most important effect that does lead to a deflection is the fact that the magnet induces changes in the air that the laser light moves through. Even though the full problem is linear, if you pretend that the medium isn't there, then it is effectivel nonlinear.

    But even in a perfect vacuum there are QED effects due to virtual electron positron pairs. The magnet effectively changes the QED vacuum and then the laserlight that moves through it will be deflected. When light enters a region with a magnetic field, then it behaves as if there is an index of fraction. This effect is largest if the light enters the region orthogonal to the magnetic field. The two polarizations states of the light with the magnetic component parallel or orthogonal to the external field have different indices of refraction. That causes the so-called vacuum birefringence effect.


    See here for details
     
  10. May 16, 2008 #9
    Well, said
     
  11. May 16, 2008 #10

    Hi Count!

    Hmmm.
    If memory serves me right, achiral (non-polarized) laser light - when shone on various materials produces an Inverse Faraday Effect that can be interpreted various ways, but it still represents a significant change in the EM nature of the light that emerges and hits the phase sensitive optical detector. Myron Evans and Jean-Pierre Vigier are both busy developing a new quantum theory that is mixed into Evans's "Generally Covariant Unified Field Theory" as a result of discovering this phenomenon a few years ago.

    The much older, Optical Faraday Effect produces a similar phenomenon when various wavelengths of noncoherent light are passed thru achiral materials that are envelop by a horseshoe type permanent magnet.

    Both effects seem to involve magnetic field changes almost exclusive of the electric field of light.

    We can debate the change as being a change of spin or orbital angular momentum when the transverse EM fields of light interact with solid matter, but we are still ignoring the longitudinal nature of the magnetic field in light which is almost never addressed and is considered too weak to be relevant.

    Keeping the above in mind, if light should not interact with a rare earth magnet, then why do both incoherent and coherent forms of light interact with the magnetic nature of materials in either the presence or the absence of a permanent magnet?

    On top of that we can add - the spin polarized nature of electrons that result from laser light being shone on GaAs and many other achiral materials. How did non-polarize light produce spin polarized electrons?

    Any insight will be greatly appreciated. Thanks!
     
  12. May 17, 2008 #11

    pam

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    Michael Faraday showed in 1845 that a magnetic field rotated the plane of polarized light
    (the Faraday affect mentioned above).
    Michael was not a member of PF, so his result was not posted to this thread.
     
  13. May 17, 2008 #12
    Magnetic fields do indeed affect light. It is called the Zeeman effect. It is a very small effect.
     
  14. May 18, 2008 #13
    Isn't the Zeeman effect indirect in its effect on light? That is, its magnetic field must first affect an atom to alter the resultant spectra. This seems to deny such an action in vacuo.
     
  15. May 18, 2008 #14
    Yes, after reading a little more I see Zeeman effect is indirect - it only affects the emitting atom; not the light itself.
     
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