Portions of this entry contributed by Leonardo Motta

Portions of this entry contributed by Waldyr A. Rodrigues, Jr.

Tachyons are a putative class of particles which able to travel faster than the speed of light. Tachyons were first proposed by physicist Arnold Sommerfeld, and named by Gerald Feinberg. The word tachyon derives from the Greek (tachus), meaning "speedy." Tachyons have the strange properties that, when they lose energy, they gain speed. Consequently, when tachyons gain energy, they slow down. The slowest speed possible for tachyons is the speed of light.

Tachyons appear to violate causality (the so-called causality problem), since they could be sent to the past under the assumption that the principle of special relativity is a true law of nature, thus generating a real unavoidable time paradox (Maiorino and Rodrigues 1999). Therefore, it seems unavoidable that if tachyons exist, the principle of special relativity must be false, and there exists a unique time order for all observers in the universe independent of their state of motion.

Tachyons can be assigned properties of normal matter such as spin, as well as an antiparticle (the antitachyon). And amazingly, modern presentations of tachyon theory actually allow tachyons to actually have real mass (Recami 1996).

It has been proposed that tachyons could be produced from high-energy particle collisions, and tachyon searches have been undertaken in cosmic rays. Cosmic rays hit the Earth's atmosphere with high energy (some of them with speed almost 99.99% of the speed of light) making several collisions with the molecules in the atmosphere. The particles made by this collision interact with the air, creating even more particles in a phenomenon known as a cosmic ray shower. In 1973, using a large collection of particle detectors, Philip Crough and Roger Clay identified a putative superluminal particle in an air shower, although this result has never been reproduced.

Speed of Light, Superbradyon, Superluminal