Speed of Light

SYMBOL: c
STATUS: Exact — defines the metre (1983)
VALUE: 299792458 m·s⁻¹

The exact speed at which light and all massless particles travel through vacuum, fixed by definition of the metre.

Fixed at exactly 299,792,458 m/s in 1983, when the metre was redefined as the distance light travels in 1/299,792,458 seconds.
Einstein's special relativity rests on its invariance: every inertial observer measures the same value.
Michelson and Morley's 1887 interferometer found no variation with Earth's motion, dooming the luminiferous aether.