tachyonic

Tachyonic is an adjective used in physics to describe phenomena associated with tachyons, hypothetical particles that would move faster than light and would possess imaginary rest mass. In special relativity, a particle with rest mass m0 has the dispersion relation E^2 = p^2 c^2 + m0^2 c^4. If m0^2 is negative (a tachyonic mass term), the relation implies superluminal behavior in a formal sense. Tachyons have never been observed, and their existence would raise fundamental questions about causality and the structure of spacetime.

In quantum field theory, a tachyonic mass term refers to a negative value of m^2 in the Lagrangian for a field. Such a term signals an instability of the chosen vacuum, causing the field to develop a nonzero vacuum expectation value and leading to spontaneous symmetry breaking. A classic example is the tachyonic mass term in the Higgs mechanism, where the potential drives the field away from the origin and into a broken-symmetry vacuum, resulting in a massive spectrum for physical excitations after symmetry breaking.

In string theory and related areas, tachyons are often interpreted not as actual faster-than-light particles but as indicators of instability in a given background. Tachyon condensation can describe transitions to more stable configurations, and the appearance of tachyons in a spectrum typically signals the need to reformulate the vacuum rather than to posit superluminal signals. These ideas have been explored in various contexts, including bosonic string theory and brane dynamics.

Experiments have not detected tachyons, and the theoretical implications—especially regarding causality and Lorentz invariance—make their physical realization uncertain. The term tachyonic is widely used, however, to describe negative-m^2 modes and related instabilities across high-energy, cosmological, and condensed-matter contexts.