Tachyons
A tachyon or a 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, 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. No experimental evidence for the existence of such particles has been found.
E. C. G. Sudarshan, V.K Deshpande, and Baidyanath Misra were the first to propose the existence of particles faster than light and named them "meta-particles". After that the possibility of particles moving faster than light was also proposed by Robert Ehrlich and Arnold Sommerfeld, independently of each other. In the 1967 paper that coined the term, Gerald Feinberg proposed that tachyonic particles could be quanta of a quantum field with imaginary mass. However, it was soon realized that excitations of such imaginary mass fields do not under any circumstances propagate faster than light, and instead, the imaginary mass gives rise to an instability known as tachyon condensation. 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: tachy, meaning rapid. 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.
