Tachyon 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.

In 1985, Chodos proposed that neutrinos can have a tachyonic nature.

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.

Because a tachyon would always travel faster than light, it would not be possible to see it approaching. After a tachyon has passed nearby, an observer would be able to see two images of it, appearing and departing in opposite directions.

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.

In special relativity, a faster-than-light particle would have space-like four-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 spacelike portion of the energy–momentum graph, they could not slow down to subluminal (meaning slower-than-light) 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”.

In modern physics, all fundamental particles are regarded as excitations of quantum fields. There are several distinct ways in which tachyonic particles could be embedded into a field theory.

•             Fields with imaginary mass

•             Lorentz-violating theories

•             Fields with non-canonical kinetic term

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