A tachyon / (http://en.wikipedia.org/wiki/Help:IPA_for_English)ˈtæki.ɒn/ (http://en.wikipedia.org/wiki/Help:IPA_for_English) or tachyonic particle is a hypothetical particle (http://en.wikipedia.org/wiki/Particle) that always moves faster than light (http://en.wikipedia.org/wiki/Faster-than-light). The word comes from the Greek (http://en.wikipedia.org/wiki/Greek_language): ταχύς (http://en.wiktionary.org/wiki/%CF%84%CE%B1%CF%87%CF%8D%CF%82) or tachys, meaning "swift, quick, fast, rapid", and was coined by Gerald Feinberg (http://en.wikipedia.org/wiki/Gerald_Feinberg).[1] Most physicists think that faster-than-light particles cannot exist because they are not consistent with the known laws of physics.[2][3] If such particles did exist, they could be used to build a tachyonic antitelephone (http://en.wikipedia.org/wiki/Tachyonic_antitelephone) and send signals faster than light, which (according to special relativity (http://en.wikipedia.org/wiki/Special_relativity)) would lead to violations of causality (http://en.wikipedia.org/wiki/Causality_%28physics%29).[3] Potentially consistent theories that allow faster-than-light particles include those that break Lorentz invariance (http://en.wikipedia.org/wiki/Lorentz_invariance), the symmetry underlying special relativity, so that the speed of light is not a barrier.
http://en.wikipedia.org/wiki/Tachyon (http://en.wikipedia.org/wiki/Tachyon)