Light travels at 299,792,458 meters per second, as was definitively stated in 1983 and has believed to have been a constant since that time. But that may not be the case.
Now, according to the Christian Science Monitor:
A pair of studies suggest that this universal constant might not be so constant after all. In the first study, Marcel Urban from the University of Paris-Sud and his team found that the speed of light in a vacuum varies ever so slightly.
This happens because what we think of as nothing isn't really nothing. Even if you were to create a perfect vacuum, at the quantum level it would still be populated with pairs of tiny "virtual" particles that flash in and out of existence and whose energy values fluctuate. As a consequence of these fluctuations, the speed of a photon passing through a vacuum varies, about 50 quintillionths of a second per square meter.
That may not sound like much, but it's enough to point the way toward a new underlying physics.