I have seen in a description of QED that it predicts that light can move faster or slower than the speed of light, but on average it moves at the speed of light. I wanted to know if htis was true and if it is, is the reason behind it due to the fact that the phase velocity (since we are dealing with only a photon) of light can be slower or faster than c, but the group velocity (that of a wave packet, or a group of photons) has to be c. If my description is correct then it seems that classical electrodynamics is explaining how a photon can more faster or slower than the speed of light. Is there any connection between how this classical description fits a quantum description. (This is however assuming that all I have said is correct so far. There could be errors.) Thanks in advance to whoever answers any of these questions.