A tachyon (pronounced /ˈtæki.ɒn/; Greek: ταχύς, takhus, "swift" + English: -on "elementary particle") is a hypothetical subatomic particle that moves faster than light. In the language of special relativity, a tachyon is a particle with space-like four-momentum and imaginary proper time. A tachyon is constrained to the space-like portion of the energy-momentum graph. Therefore, it cannot slow down to subluminal speeds.
The first description of tachyons is attributed to German physicist Arnold Sommerfeld. However, it was George Sudarshan, Olexa-Myron Bilaniuk, Vijay Deshpande, and Gerald Feinberg (who originally coined the term in the 1960s) who advanced a theoretical framework for their study.
If tachyons were conventional, localizable particles that could be used to send signals faster than light, this would lead to violations of causality in special relativity. But in the framework of quantum field theory, tachyons are understood as signifying an instability of the system and treated using tachyon condensation, rather than as real faster-than-light particles, and such instabilities are described by tachyonic fields. Tachyonic fields have appeared theoretically in a variety of contexts, such as the bosonic string theory. According to the contemporary and widely accepted understanding of the concept of a particle, tachyon particles are too unstable to be treated as existent. By that theory, faster than light information transmission and causality violation with tachyons are impossible.
Conventional massive particles which travel slower than the speed of light are sometimes termed "bradyons" or "tardyons" in contrast, although these terms are only used in the context of discussions about tachyons.
Despite the theoretical arguments against the existence of tachyon particles, experimental searches have been conducted to test the assumption against their existence; however, no experimental evidence for the existence of tachyon particles has been found.
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