I've read ZapperZ's thread on the speed of photons through a solid and am still confused. If you imagine an experiment where two emitters A and B are set up to emit one photon each simultaneously towards two receptors (also A and B)in a vacuum 558,000 miles away. The receptors will sense the arrival of the photons approximately 3 seconds after they leave the emitters and should “see” them simultaneously. Are the photons that arrive at the receptors the same photons that left the emitters? If you now put a piece of glass 186,000 miles thick between the B emitter and B receptor and repeat the experiment the B receptor will “see” the photon slightly later then the A receptor. If the answer to my first question was yes, is the photon that hits the B receptor in the second test the same photon that left the B emitter?