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.
A bradyon, also known as a tardyon or ittyon, is a particle that travels slower than light. 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 special relativity, since bradyons 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).
All massive particles that have been experimentally found are bradyons. There are two major ways to define a bradyon:
1. A bradyon is any particle with real rest mass.
2. From statement no.1, we can use the Lorentz factor to create an alternative definition. A bradyon is any particle moving slower than light.
Assuming that tachyons exist, 'bradyon' is not a synonym for 'massive particle' (i.e. a particle having a non-zero mass). Any bradyon has a real rest mass, whereas any luxon has zero (both real and imaginary. In fact, the complex argument of 0 is not defined) rest mass. Tachyons have imaginary masses.
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.
Tachyons are predicted by bosonic string theory and also the NS (which is the open bosonic sector) and NS-NS (which is the closed bosonic sector) sectors of RNS Superstring theory before GSO projection. However, due to the Sen conjecture—also known as tachyon condensation—this is not possible. This resulted in the necessity for the GSO projection.
- Bilaniuk, O.-M.P.; Sudarshan, E.C.G. (1969). "Particles beyond the Light Barrier". Physics Today 22 (5): 43–51. Bibcode:1969PhT....22e..43B. doi:10.1063/1.3035574.
- Folman, R.; Recami, E. (1995). "On the Phenomenology of Tachyon Radiation". arXiv:hep-th/9508166 [hep-th].
- The dictionary definition of tardyon at Wiktionary
- The dictionary definition of tachyon at Wiktionary
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