It takes time for a photon to move in your reference frame, but time within the photon's own reference frame is not advancing at all during that. Within the pho

It takes time for a photon to move in your reference frame, but time within the photon's own reference frame is not advancing at all during that. Within the photon's reference frame, the photon exists instantaneously, simultaneously, at its emitter and absorber. Its whole existence "brings together" the spacetime it was emitted from and the spacetime it is absorbed into, at a single 4D pinch-point. It's like the whole universe is squished flat into two hyperplanes of "everything behind the photon at time of emission" and "everything ahead of the photon at time of absorption", and those two hyperplanes have no distance between them. > Doesn't the atmosphere and passing through my glasses slow it down a wee bit? When a photon is travelling through anything other than vacuum, it's not "slowed down." It's repeatedly being absorbed and re-emitted. (Or rather, it's being absorbed, and new photons that happen to be mostly equivalent are being emitted.) The refractive index of a material is effectively a measurement of the likelihood of absorption, times the average per-particle time-delay between absorption and re-emission.