tachyon

Definition: A hypothetical particle that has a speed in excess of the speed of light, c. According to electromagnetic theory, a charged particle travelling through a medium at a speed in excess of the speed of light in that medium emits Cerenkov radiation. A charged tachyon would emit Cerenkov radiation even in a vacuum. No such particle has yet been detected. According to the special theory of relativity, it is impossoble to accelerate a particle up to the speed of light because its energy would have to become infinite. The theory, however, does not forbid the existence of particles with speed greater than the speed of light in a vacuum. In such cases, the energy of the particle would be imaginary, according to the equations of special relativity. Tachyons are a staple particle in science fiction, often associated with time travel (or time disturbances) and faster than light travel.