Tachyon (from Greek tachýs, "fast"): a hypothetical elementary particle with a potentially infinite speed.

►Special relativity theory rules out the acceleration of particles of matter up to the ►speed of light or beyond. However, the equations of relativity theory admit of different and alternative solutions, one of which allows for particles that constantly move at a speed faster than that of the speed of light and can never actually be decelerated to the speed of light. This solution was first published in 1967 by the physicist Gerald Feinberg, who also coined the word "tachyons" as a name for these hypothetical particles.

Of course, establishing that tachyons are theoretically possible is one thing; proving that they actually exist is quite another. If they do exist, though, they must have certain properties that could be detected in experiments. They are always located outside of the ►event horizon's cone of light. Unlike regular matter, upon losing energy they actually become faster; when the energy deprivation is total, they move at infinite speed. Moreover, only neutral particles could reach this state of total energy loss + infinite speed. The reason is that charged particles moving faster than light would emit a certain radiation, the so-called Cherenkov radiation. (This can be shown even for non-tachyonic particles if they move through a medium in which light is slower than in a vacuum — such as water — at nearly the speed of light.) Charged tachyons, however, would emit Cherenkov radiation even in a vacuum and thus would be continuously losing energy without ever reaching the point of completely lacking it. For similar reasons, ►quarks cannot be tachyons, either.

By process of elimination, that leaves us with neutrinos as the only known elementary particles whose existence in the form of tachyons cannot at present be ruled out. One way to settle the issue would be by determining the mass of neutrinos. Neutrino mass is one of the ► ten puzzles of science.