Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Are photon energies constant for any observers?

  1. May 26, 2015 #1
    Title should be: Are photon energies constant along null geodesics? [Mentor's note: Title corrected]

    As I understand it in general relativity the paths of light rays, given by null geodesics, are scale invariant.

    Is that correct?

    Now Maxwell's equations in flat Minkowski spacetime are also scale invariant.

    General curved spacetime is locally flat in the vicinity of any point along a trajectory in spacetime.

    Do these facts imply that the energy/momentum of photons is actually constant along the trajectory of the light beam?

    Observers themselves and their measuring equipment do have fixed length scales so that their energies are not scale invariant. The energies of the emitting and absorbing atoms change but the energy of the photon itself is constant. This would be an explanation of the gravitational redshift effect without the assumption of changing photon energy.

    To summarise: the scale of a photon is set purely by the emitting atom - after emission neither Maxwell's laws nor general relativity change the photon's scale.
    Last edited: May 26, 2015
  2. jcsd
  3. May 26, 2015 #2


    User Avatar

    Staff: Mentor

    However observers modiffract different speeds relative to one another will measure different energies for the photon as they measure different frequencies. This is just the Doppler effect at work.
  4. May 26, 2015 #3
    What happens if the emitter and the absorber are both locally in free-fall frames?

    Would there be no Doppler effect in this case - just a difference in energy due to the different clock rates at emitter and absorber?
  5. May 26, 2015 #4


    User Avatar

    Staff: Mentor

    There is still Doppler if there is relative motion.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook