tachyons

Tachyons are hypothetical particles that would travel faster than light in a vacuum. The term was introduced by physicist Gerald Feinberg in 1967 to describe particles with imaginary rest mass, a concept arising from extending the relativistic energy-momentum relation to include negative mass-squared. The name comes from the Greek tachys, meaning swift.

In special relativity, the energy-momentum relation is E^2 = p^2 c^2 + m^2 c^4. If m^2 < 0, one can obtain solutions corresponding to speeds exceeding c. In such formulations, tachyons would inherently move faster than light, and their presence can lead to counterintuitive and problematic features, including potential causality violations in certain reference frames.

Despite extensive discussion, there is no experimental or observational evidence for tachyons. They remain theoretical curiosities and are often treated as speculative extrapolations of known physics. In quantum field theory and string theory, tachyons frequently appear not as real faster-than-light particles but as indicators of instability. A negative mass-squared term, or tachyonic mass, signals that a given vacuum or background is unstable and may decay to a more stable configuration through a process known as condensation.

Examples of contexts in which tachyonic ideas appear include the bosonic string spectrum, where tachyons indicate an unstable theory, and models involving tachyon condensates in cosmology or field theory. Overall, tachyons play a role as a theoretical tool for exploring causality, stability, and the boundaries of relativistic physics, rather than as established physical particles.