Wow. I wouldn't have thought it possible, but matter traveling in a Gamma Ray Burst has now been clocked at very near the speed of light - in fact only different from the speed of light by three parts in a million.
Gamma-ray bursts (GRBs) are powerful explosions occurring in distant galaxies, that often signal the death of stars. They are so bright that, for a brief moment, they almost rival the whole Universe in luminosity. They last, however, for only a very short time, from less than a second to a few minutes. Astronomers have long known that, in order to emit such incredible power in so little time, the exploding material must be moving at a speed comparable with that of light, namely 300 000 km per second. By studying the temporal evolution of the burst luminosity, it has now been possible for the first time to precisely measure this velocity.
GRB's are the most energetic events since the Big Bang, but because the burst itself only lasts seconds, the key to analysis is "catching them in the act" and watching as they evolve over time. This has been accomplished by linking satellite detection of GRB events to special ground based telescopes that can quickly point to the correct spot in the sky.