A tachyon ( /ˈtæki.ɒn/) or tachyonic particle is a hypothetical particle that always moves faster than light. The word comes from the Greek: ταχύς or tachys, meaning "swift, quick, fast, rapid", and was coined by Gerald Feinberg in a 1967 paper.[1] Feinberg proposed that tachyonic particles could be quanta of a quantum field with negative squared mass. However, it was soon realized that excitations of such imaginary mass fields do not in fact propagate faster than light[2], but instead represent an instability known as tachyon condensation. Nevertheless, they are still commonly known as "tachyons"[3], and have come to play an important role in modern physics.[4]

Most physicists think that faster-than-light particles cannot exist because they are not consistent with the known laws of physics.[4][5] If such particles did exist, they could be used to build a tachyonic antitelephone and send signals faster than light, which (according to special relativity) would lead to violations of causality.[5] Potentially consistent theories that allow faster-than-light particles include those that break Lorentz invariance, the symmetry underlying special relativity, so that the speed of light is not a barrier.

Despite theoretical arguments against the existence of faster-than-light particles, experiments have been conducted to search for them. No compelling evidence for their existence had been found.[6]

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