superluminality

Superluminality is the term used for phenomena in which an observed or effective velocity exceeds the speed of light in vacuum, c. It is a label applied to a variety of situations where something appears to travel faster than light, though not all such cases involve a real violation of relativity.

In physics, special relativity forbids the transmission of information or energy faster than c in vacuum. Many apparent superluminal effects arise from measurements in media, wave phenomena, or projection effects that create the illusion of faster-than-light motion. For waves, the phase velocity can exceed c without carrying information, and even the group or signal velocity remains limited by causality in most contexts. In some media with unusual dispersion, group velocities can temporarily exceed c, but the leading edge that carries information does not.

Notable contexts include Cherenkov radiation, which occurs when a charged particle moves faster than light in a medium (though still below c in vacuum); and apparent superluminal motion observed in astrophysical jets, where projection effects in relativistic motion make features seem to travel faster than light. In cosmology, the expansion of space can cause distant objects to recede faster than light without violating relativity.

Theoretical discussions also address tachyons, hypothetical particles that would always move faster than c, though there is no experimental evidence for them. Some quantum phenomena and interpretations of tunneling times have been described as superluminal, but no information-carrying signal has been demonstrated to propagate superluminally.

See also tachyon, Cherenkov radiation, faster-than-light.