Definition
The speed of light in vacuum, denoted c, is the universal constant c = 299,792,458 m/s exactly.
It is the speed at which all massless particles — photons, gluons (confined), gravitons (predicted) — propagate through empty space. By the principles of special relativity, it is also the upper limit on the speed at which energy, matter, or information can travel.
Why it matters
How it works
Ole Rømer made the first quantitative measurement in 1676 by timing eclipses of Jupiter's moon Io as Earth moved toward and away from Jupiter — Io's eclipses came earlier when Earth was closer, indicating a finite travel time for light. James Bradley refined the value via stellar aberration in 1728. Albert Michelson and others tightened the measurement through the 19th and 20th centuries.
In 1865 Maxwell derived from his electromagnetic equations a wave speed 1/√(μ₀ε₀) — numerically equal to the measured speed of light — and concluded that light is an electromagnetic wave. Maxwell's equations contain c as a fundamental constant without specifying a preferred rest frame.
Einstein's 1905 special relativity took this seriously: light moves at c in every inertial frame, period. This single postulate forces time dilation, length contraction, the relativity of simultaneity, and a cosmic speed limit on matter. Accelerating a massive object to c requires infinite energy, by the relativistic energy formula E = γmc².
Since 1983 the metre has been defined as the distance light travels in 1/299,792,458 of a second. The speed of light is therefore no longer measured experimentally — it is fixed by convention, and any improvement in our ability to time light's travel sharpens the metre instead. The current SI system extends this approach: c, ℏ, e, k_B, N_A, K_cd, and the cesium hyperfine transition are all defined exactly, and units of length, mass, and time follow from them.