Tachyon (redirected from Tachions)

tachyon (tăk`ēŏn'), hypothetical elementary particle that travels only at speeds exceeding that of light. According to the theory of relativity, the speed of light is the limiting velocity for all ordinary material particles. Particles having nonzero rest mass can approach, but not reach, the speed of light, since their mass would become infinite at that speed. On the other hand, particles with zero rest mass, such as the photon, must always travel at the speed of light; they cannot be brought to rest or even slowed down. Theorists have argued that since nothing in principle prohibits the existence of a third class of particles that travel only at speeds exceeding that of light, such particles, called tachyons [Gr. tachys,=swift] may exist although no evidence for them has been found. In the terminology of the theory, the particles that travel only at the speed of light are called luxons, and those that travel at lesser speeds are called tardyons. Like the original theory of relativity, the theory of tachyons has several aspects that appear to contradict common sense but that are fully self-consistent. For example, a tachyon must have an imaginary (in the mathematical sense) rest mass, or proper mass, and it must travel faster rather than slow down when it loses energy.

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tachyon

Hypothetical subatomic particle whose velocity is always greater than that of light. Its existence appears consistent with the theory of relativity. Just as an ordinary particle such as an electron can exist only at speeds less than that of light, a tachyon can exist only at speeds greater than that of light. At such speeds, its mass would be real and positive. On losing energy, a tachyon accelerates; the faster it travels, the less energy it has. The existence of tachyons has not been established experimentally.

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Tachyon

A hypothetical faster-than-light particle consistent with the special theory of relativity. According to this theory, a free particle has an energy E and a momentum p which form a Lorentz four-vector. The length of this vector is a scalar, having the same value in all inertial reference frames. One writes

(1) 

Eq. (1), where c is the speed of light and the parameter m² is a property of the particle, independent of its momentum and energy. Three cases may be considered: m² may be positive, zero, or negative. The case m² > 0 applies for atoms, nuclei, and the macroscopic objects of everyday experience. The positive root m is called the rest-mass. If m² = 0, the particle is called massless. A few of these are known: the electron neutrino, the muon neutrino, the photon, and the graviton. The name tachyons (after a Greek word for swift) has been given to particles with m² < 0.

In general, the particle speed is given by Eq. (2). If m² < 0,

(2) 

Eq. (1) implies E > c p and Eq. (2) gives v < c. If m² = 0, then E = c p and v = c. In case m² < 0, one finds E < c p and v > c. Tachyons exist only at faster-than-light speeds. See Elementary particle, Relativity