Tachyon

A tachyon is a hypothetical particle that always travels faster than light, consistent only with the new laws of physics so that causality is not violated. Tachyons exhibit the exotic property of increasing in speed as their energy decreases, and would require infinite energy to slow down to the speed of light.

If all matter is divided into three general classes:


 * Tardyons can move at any velocity less than c = speed of light (normal matter). These are also called bradyons and are massive particles.
 * Luxons can travel only at c (photons and neutrinos). These are massless particles.
 * Tachyons can only be superluminal, greater than c. They have an imaginary or complex mass. They speed up as they lose energy. At zero energy they have infinite velocity and are present everywhere in the universe at the same time (transcendental tachyon).

An electron and a positron, both tardyons, undergo "annihilation" when brought together with the release of gamma-ray photons, which are luxons. A neutron, a tardyon, can decay into an antineutrino, which is a luxon. A gamma-ray (luxon) striking an atomic nucleus gives rise to an electron/positron pair (both tardyons). Conversion between tardyons and luxons are common, and conversion from tardyons to tachyons and vice versa are moderated by luxons.

The speed of a tachyon increases as its energy decreases. Therefore, just as bradyons are forbidden to break the light-speed barrier, so too are tachyons forbidden from slowing down to below c, because infinite energy is required to reach the barrier from either above or below.

A tachyonic field is a quantum field with an imaginary mass. The Higgs field also has imaginary mass but has been known to exist since the discovery of the Higgs boson.

Tachyons are used to send signals faster than light. A tachyonic antitelephone is a hypothetical device that could be used to send signals into one's own past. In 1907 Albert Einstein presented a thought experiment of how faster-than-light signals can lead to a paradox of causality, which was described as a means "to telegraph into the past".

They are also used to build tachyon spaceships.