Tachyon
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.
[3] 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.
[9] The term tachyon was coined by Gerald Feinberg in a 1967 paper titled "Possibility of faster-than-light particles".
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.
[14][15][16] One curious effect is that, unlike ordinary particles, the speed of a tachyon increases as its energy decreases.
As noted by Albert Einstein, Richard C. Tolman, and others, special relativity implies that faster-than-light particles, if they existed, could be used to communicate backwards in time.
[19][20][21] In this framework, neutrinos experience Lorentz-violating oscillations and can travel faster than light at high energies.
[22] If tachyons can transmit information faster than light, then, according to relativity, they violate causality, leading to logical paradoxes of the "kill your own grandfather" type.
[23] 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.
In the paper that coined the term "tachyon", Gerald Feinberg studied Lorentz invariant quantum fields with imaginary mass.
[3] Because the group velocity for such a field is superluminal, naively it appears that its excitations propagate faster than light.
However, it was quickly understood that the superluminal group velocity does not correspond to the speed of propagation of any localized excitation (like a particle).
Instead, the negative mass represents an instability to tachyon condensation, and all excitations of the field propagate subluminally and are consistent with causality.
Perhaps the most famous is the Higgs boson of the Standard Model of particle physics, which has an imaginary mass in its uncondensed phase.
In general, the phenomenon of spontaneous symmetry breaking, which is closely related to tachyon condensation, plays an important role in many aspects of theoretical physics, including the Ginzburg–Landau and BCS theories of superconductivity.
[29][28] 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.
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 positronic brain).
