supraluminal

Supraluminal is an adjective used in physics to denote motion, communication, or effects that exceed the speed of light in vacuum, c, or that appear to do so. In strict physical terms, relativity forbids the transfer of information or matter faster than c. Supraluminal descriptions typically refer to apparent speeds arising from geometry and relativistic effects rather than actual superluminal signaling.

Apparent supraluminal motion is well documented in astronomy, where jets from active galactic nuclei or quasars can display transverse speeds greater than c when projected on the sky. The effect results from highly relativistic speeds and small angles between jet direction and the observer's line of sight; the observed transverse motion can exceed c, even though the intrinsic velocity remains less than c.

In other contexts, phase velocity can exceed c in some media, but this does not convey information faster than light; group velocity and signal velocity remain limited. Also recession due to expanding space can create superluminal apparent recession without violating relativity.

Etymology: supraluminal is formed from supra- meaning above and luminal referring to light. The term is closely related to superluminal, though usage varies; both describe speeds beyond c in common parlance but not as a violation of relativity.

Notes: The distinction between apparent and actual supraluminal phenomena is important in physics.