BBC has a small article on how Prof Daniele Faccio of Heriot-Watt University and Prof Miles Padgett of Glasgow slowed a photon to less than c in "free space", which I pressume to mean a vacuum. Done by "changing the photon's shape" via some medium. When the photon returned to "free space" it continued it's retarded velocity. Is this pucky? Or is there some known reason that a photons "shape" can effect what it's maximum velocity is? I pressume that the retarded velocity is still the maximum for the particular photon since they're massless. article Edit: reading about light through mediums maybe this means the photon maintains it's slowed group velocity after leaving the medium.