tachyon

A tachyon is a hypothetical particle that would always travel faster than light. The concept was popularized in discussions of special relativity, with Gerald Feinberg coining the term in the 1960s. In conventional particles, the rest mass is real; for tachyons the rest-mass squared would be negative, written as m0^2 = -μ^2 with μ > 0. This leads to an energy–momentum relation E^2 = p^2 c^2 - μ^2 c^4. In such a framework, the velocity v = pc^2 / E would exceed the speed of light, and tachyons would not possess a rest frame.

The notion raises significant causal questions. Because superluminal signaling can, in some frames, imply events that precede their causes, tachyons are associated with potential paradoxes such as the tachyonic antitelephone. In the absence of empirical evidence, they remain speculative, and many physicists treat them with caution. Some theoretical constructions accommodate tachyonic solutions but do not require real, propagating superluminal particles.

In quantum field theory, the term “tachyonic mass” is often used to describe instability in a field rather than a true faster-than-light particle. In string theory, tachyons can arise in unstable configurations and may indicate a process—tachyon condensation—that leads to a more stable vacuum rather than a physical superluminal particle.

Overall, tachyons are a theoretical possibility with provocative implications for causality and relativity, but no experimental observation supports their existence. They continue to be discussed mainly as a tool for exploring the foundations and limits of physical theories.