A tachyon () or tachyonic particle is a hypothetical particle that always travels faster than light. Most physicists believe that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. If such particles did exist, and could send signals faster than light, then according to the theory of relativity they would violate causality, leading to logical paradoxes of the ‘killing your own grandfather’ type. Tachyons would also exhibit the unusual property of increasing in speed as their energy decreases, and would require infinite energy to slow down to the speed of light. No experimental evidence for the existence of such particles has been found.
In the 1967 paper that coined the term, Gerald Feinberg proposed that tachyonic particles could be made from excitations of a quantum field with imaginary mass. However, it was soon realized that Feinberg's model did not in fact allow for superluminal (faster-than-light) speeds. Nevertheless, in modern physics the term tachyon often refers to imaginary mass fields rather than to faster-than-light particles. Such fields have come to play a significant role in modern physics.
The term comes from the Greek: ταχύ, tachy, meaning swift. The complementary particle types are called luxons (which always move at the speed of light) and bradyons (which always move slower than light); both of these particle types are known to exist.