A " tachyon " is a particle that can travel at speeds greater than the speed of light . The term comes from the Greek "tachy-", meaning "fast" .

They have been the subject of interest in theoretical physics because of their implications for the theory of relativity and quantum physics.

In physics , particles are classified based on their speed and energy . According to Einstein's special theory of relativity, tachyons would have imaginary energy and "imaginary" mass (using complex numbers), which defies conventional logic, since their speed would increase as they lose energy rather than decrease. This concept is interesting because it would, in principle, allow tachyons to travel backwards in time, which opens up a number of temporal paradoxes.

Although tachyons appear in some models of string theory, where their existence is associated with instabilities in space-time, their physical existence is still unproven. At present, their role is rather theoretical and speculative within theories seeking a unified description of quantum physics and gravity.