A tachyon or tachyonic particle is a hypothetical particle that always moves faster than light. The word comes from theGreek: ταχύς or tachys, meaning “swift, quick, fast, rapid”, and was coined by Gerald Feinberg. Most physicists think that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. 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. 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.
In the 1967 paper that coined the term, 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, and instead represent an instability known as tachyon condensation. Nevertheless, negative squared mass fields are commonly referred to as “tachyons”, and in fact have come to play an important role in modern physics.
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 has been found.