|MadSci Network: Astronomy|
Relativity does not allow objects to move faster than light. The theory of relativity proposes the speed of light in vacuum is ultimate speed. In a medium, light is slowed down due to the refractive index. So speeds higher than the speed of light in a medium are allowed. In fact there are very nice experiments with very slow light pulses in certain media, see e.g. this article for a very nice reference, or search the MadSci archives for "slowing of speed of light". Light can be slowed down (the so called group velocity of a light pulse, which is the velocity a bunch of light travels at, but not a single light ray) is slowed down to 38 miles an hour in this experiment.
Relative velocities higher than the speed of light in vacuum are not allowed. The best one can do, according to relativity, is to create a situation where it looks like something is travelling at such a high speed, but actually is not! All one can do is playing tricks. These tricks may be very effective though. For some reference see e.g. the article Faster- than-light from the Wikipedia archive. These tricks basically use that the distortion of space-time also distorts the very idea of distance. It seems that someone is travelling with a speed faster than light but actually he is taking some kind of a shortcut, such as a wormhole. But they are compatible with the speed of light as ultimate speed and with relativity.
Try the links in the MadSci Library for more information on Astronomy.