- A tachyon or tachyonic particle is a hypothetical particle that always moves 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 taychonic antitelephone and send signals faster than light, which (according to special relativity) would lead to violations of causality.
In special relativity, a faster-than-light particle would have space like you’re momentum in contrast to ordinary particles that have time like four momentum. Although in some theories the mass of tachyons is regarded as imaginary, in some modern formulations the mass is considered real,the formulas for the momentum and energy being redefined to this end. Moreover, since tachyons are constrained to the space like portion of the energy–momentum graph, it could not slow down to subliminal speeds.
The reinterpretation principle asserts that a tachyon sent back in time can always be reinterpreted as a tachyon traveling forward in time, because observers cannot distinguish between the emission and absorption of tachyons. The attempt to detect a tachyon from the future (and violate causality) would actually create the same tachyon and send it forward in time (which is causal).
However, this principle is not widely accepted as resolving the paradoxes. Instead, what would be required to avoid paradoxes is that unlike any known particle, tachyons do not interact in any way and can never be detected or observed, because otherwise a tachyon beam could be modulated and used to create an anti-telephone or a “logically pernicious self-inhibitor”.All forms of energy are believed to interact at least gravitationally, and many authors state that superluminal propagation in Lorentz invariant theories always leads to causal paradoxes.
Fields with non canonical kinetic terms
By modifying the kinetic energy of the field, it is possible to produce Lorentz invariant field theories with excitations that propagate superluminally.However, such theories, in general, do not have a well-defined Cauchy problem.(for reasons related to the issues of causality discussed above), and are probably inconsistent quantum mechanically.
Tachyons have appeared in many works of fiction. They have been used as a standby mechanism upon which many science fiction authors rely to establish faster than light communication, with or without reference to causality issues. The word tachyon has become widely recognized to such an extent that it can impart a science-fictional connotation even if the subject in question has no particular relation to superluminal travel (a form of technobabble, akin to positrinic brain).