Technically, photons always travel at c. Slow moving light is caused by the medium continually absorbing, delaying, then re-emitting the photon, or by other st

Technically, photons always travel at c. Slow moving light is caused by the medium continually absorbing, delaying, then re-emitting the photon, or by other stuff which I won't pretend to understand [1]. Either way, rest assured that electromagnetic propagation always travels at c WRT any inertial frame. [1] http://en.wikipedia.org/wiki/Slow_light http://en.wikipedia.org/wiki/Slow_light