Einstein used an unstated assumption in SRT theory that seems like it cannot be correct. In his clock synchronization of sending a signal from A to a mirror at B with the timed return of the signal, etc., he assumes that the reflection at the mirror is instantaneous. I am sure it is PDQ, but can it be *instant*, with absolutely no lag between the arrival and departure of a photon? If that photon carries momentum, then an instant turnaround would deliver an infinite impulse to the electrons in the mirror. It probably doesn’t make any difference in the theory, but I am just curious. Is his synchronization scheme actually used in practice? If someone could do a Mossbauer type experiment with the photons / gammas reflected between emission and reception, would there be a Doppler type redshift of the spectral line due to recoil of the electrons in the reflector?