Relativity, the group of theories formed by Albert Einstein to explain concepts about space, time, matter, energy, and gravity. Special relativity begins with the behavior of light and draws the conclusion that the speed of light is a limit that can be approached but not reached by any material object (i.e. any object with mass). The speed of light as represented in the famous equation E= mc² is that of light in a vacuum, and the speed of light differs when it is travelling through other mediums. When particles are somehow caused to travel faster than light through a clear medium, like water, the resulting blue-ish glow emitted is called Cherenkov radiation.

Travelling faster than the speed of light, according to Einstein, is impossible. For new research that suggests faster than light speed travel might be possible, read on here and here.