# tachyon

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## tachyon

(tăk`ēŏn'), hypothetical elementary particle**elementary particles,**

the most basic physical constituents of the universe.

**Basic Constituents of Matter**

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.

**.....**Click the link for more information. that travels only at speeds exceeding that of light. According to the theory of relativity

**relativity,**

physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference.

**.....**Click the link for more information. , 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

**photon**

, the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum. The photon has no charge and no mass. About the beginning of the 20th cent.

**.....**Click the link for more information. , 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

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

*m*

^{2}is a property of the particle, independent of its momentum and energy. Three cases may be considered:

*m*

^{2}may be positive, zero, or negative. The case

*m*

^{2}> 0 applies for atoms, nuclei, and the macroscopic objects of everyday experience. The positive root

*m*is called the rest-mass. If

*m*

^{2}= 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

*m*

^{2}< 0.

In general, the particle speed is given by Eq. (2). If *m*^{2} < 0,

(2)

Eq. (1) implies *E*>

*c*

**p**and Eq. (2) gives

*v*<

*c*. If

*m*

^{2}= 0, then

*E*=

*c*

**p**and

*v*=

*c*. In case

*m*

^{2}< 0, one finds

*E*<

*c*

**p**and

*v*>

*c*. Tachyons exist only at faster-than-light speeds.

*See*Elementary particle, Relativity

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

## tachyon

[′tak·ē‚än] (particle physics)

A hypothetical particle that travels faster than light, consistent with the theory of relativity.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.