Look up bradyon, tardyon, or ittyon in Wiktionary, the free dictionary.

Look up tachyon in Wiktionary, the free dictionary.

The term massive particle refers to particles which have non-zero rest mass. They are divided in two major groups, bradyons and tachyons. The existence of massive superluminal particles is a controversial issue.

Bradyons [link]

A bradyon, also known as a tardyon or ittyon,^[1] is a particle that travels slower than light.^[2] The term "bradyon", from Greek: βραδύς (bradys, “slow”), was coined to contrast with "tachyon", from Greek: ταχύς (tachys, “swift, quick, fast, rapid”), which refers to hypothetical particles that travel faster than light.

As predicted by relativity, since baryons travel slower than light, they can also be defined as particles that have a non-zero rest mass. However, if tachyons exist, then not all massive particles are bradyons, since tachyons are also massive, but with an imaginary rest mass (see Tachyon).

Tachyons [link]

A tachyon is a hypothetical subatomic particle that always moves faster than light. In the language of special relativity, a tachyon would be a particle with space-like four-momentum and imaginary proper time. A tachyon would be constrained to the space-like portion of the energy-momentum graph. Therefore, it cannot slow down to subluminal speeds.

References [link]

1. ^ Bilaniuk, O.-M.P.; Sudarshan, E.C.G. (1969). "Particles beyond the Light Barrier". Physics Today 22 (5): 43–51. DOI:10.1063/1.3035574. 
2. ^ Folman, R.; Recami, E. (1995). "On the Phenomenology of Tachyon Radiation". arXiv:hep-th/9508166 [hep-th]. 

This particle physics-related article is a stub. You can help Wikipedia by expanding it. v t e