"In the past few years, scientists have found ways to make light go both faster and slower than its usual speed limit, but now researchers at the University of Rochester published a paper on May 12 in Science on how they've gone one step further: pushing light into reverse. As if to defy common sense, the backward-moving pulse of light travels faster than light.
Confused? You're not alone. ................................... ." read the full article at
http://www.spaceflightnow.com/news/n0605/14light/
So it seems that 'C' is not an absolute.
Doug