This report on the BBC site suggests that optical photons have been prepared that travel slower than c in vacuum. How is this possible?
I believe that Paul's (http://www.mathpages.com/home/kmath210/kmath210.htm) source answers it brilliantly. "The main bunch of riders - may be moving at a constant speed. But within the bunch an individual rider may moving more slowly, dropping back for a rest or a drink." Coupled with Mr. Witthoft's response "What you're missing, Dirk, is that there is no "single photon" while in the wavelike behaviour in free space. The mask is absolutely forcing the wave from a single emitted photon to become a multispectral packet, and the phase vs group velocity rules apply. This article looks suspiciously like a lot of hullaballoo over nothing." you have your anwser.