speedoflight

The speed of light, denoted c, is the speed at which electromagnetic radiation propagates through vacuum. It is a fundamental constant of nature and the upper bound on the speed of information and matter. Since 1983, c is exact: 299,792,458 meters per second, and the meter is defined as the distance light travels in vacuum in 1/299,792,458 of a second.

In physics, c arises from Maxwell's equations and is the invariant speed in special relativity. It acts as the universal speed limit, and many relativistic effects (time dilation, length contraction) depend on ratios to c. Massless particles travel at c; particles with mass can approach it but cannot exceed it.

In media other than vacuum, light travels more slowly, with speed v = c/n, where n is the refractive index. The phase velocity, group velocity, and dispersion can differ, leading to phenomena such as refraction and optical delay. The value of c in vacuum is independent of source or observer.

Historical notes: early measurements of light’s speed were refined over the 17th to 19th centuries, culminating in high-precision modern techniques using laser interferometry and timing. CODATA provides the standard value, used to define the meter and to calibrate instruments. The constant c underpins much of physics, including the structure of spacetime in relativity and the synchronization of clocks and standards worldwide.