A tachyon () or tachyonic particle is a hypothetical particle that always travels faster than light. 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 send signals faster than light. According to the theory of relativity this would violate causality, leading to logical paradoxes such as the grandfather paradox. Tachyons would 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 verifiable 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) particles or signals and that tachyonic fields merely give rise to instabilities, not causality violations. Nevertheless, in modern physics the term often refers to imaginary mass fields rather than to faster-than-light particles. Such fields play a significant role in modern physics. The term comes from the , tachy, meaning . 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. History The term was coined by Gerald Feinberg in a 1967 paper titled "Possibility of faster-than-light particles". He had been inspired by the science-fiction story "Beep" by James Blish. Feinberg studied the kinematics of such particles according to special relativity. In his paper, he also introduced fields with imaginary mass (now also referred to as tachyons) in an attempt to understand the microphysical origin such particles might have. The first hypothesis regarding faster-than-light particles is sometimes attributed to German physicist Arnold Sommerfeld, who, in 1904, named them "meta-particles". Bilanuik, Deshpande and Sudarshan discussed this more recently in their 1962 paper on the topic and in 1969. The possibility of existence of faster-than-light particles was also proposed by Lev Yakovlevich Shtrum in 1923. In September 2011, it was reported, in a major release by CERN, that a tau neutrino had traveled faster than the speed of light; however, later updates from CERN on the OPERA project indicate that the faster-than-light readings were due to a faulty element of the