A hypothetical particle that has a speed in excess of the speed of light. According to electromagnetic theory, a charged particle travelling through a medium at a speed in excess of the speed of light in that medium emits Cherenkov radiation. A charged tachyon would emit Cherenkov radiation even in a vacuum. No such particle has yet been detected. According to the special theory of relativity, it is impossible to accelerate a particle up to the speed of light because its energy E, given by E+mc2/√(1 − v2/c2), would have to become infinite. The theory, however, does not forbid the existence of particles with v > c (where c is the speed of light). In such cases the expression in the brackets becomes negative and the energy would be imaginary.