A tachyon is a hypothetical particle that travels faster than the speed of light (and therefore, also, theorectically, travels back in time).
The existence of tachyons (Greek, tachys, for "swift) is allowed by equations of Einstein's special theory of relativity. However, although searches have been carried out for tachyons, the results have so far proved negative.
The existence of tachyons were first proposed by the physicist Arnold Sommerfeld, and the particles were named in the 1967 by Gerald Feinberg.
Particles that travel slower than light are called tardyons (or bradyons in more modern usage).
Particles, such as photons, that travel exactly at the speed of light are called luxons.
