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Not sure about in QM, but IIRC in classical oscillator theory, photons move at c (i.e. vacuum speed of light) between atoms. They're not really absorbed and re-emitted by the atoms, though. What happens is that the electric field of the photon drives charges into oscillation, and those oscillating charges radiate a field which is slightly out-of-phase with the photon. The superposition of the photon and the radiated field is slightly retarded w.r.t. the original field, and so the photon is 'delayed' a bit at each atom. On a large enough scale, this looks like the photon is slower. -- DrBob |
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