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.