Speed of light in vacuum
| Value | 299,792,458 m/s |
| Status | Exact by definition — no uncertainty |
| Source | SI Brochure, 9th edition (2019); defined by the 17th CGPM (1983) |
| Categories | Universal & Atomicphysicsrelativity |
| meter per second | 299,792,460 m/s |
| kilometer per hour | 1,079,252,800 km/h |
| foot per second | 983,571,060 ft/s |
| mile per hour | 670,616,630 mph |
| knot | 582,749,920 kn |
| foot per minute | 59,014,263,000 ft/min |
| centimeter per second | 29,979,246,000 cm/s |
| meter per hour | 1,079,252,800,000 m/h |
| meter per day | 25,902,068,000,000 m/d |
| foot per day | 84,980,539,000,000 ft/d |
Learning zone
Light in vacuum travels at the same speed for every observer, whatever their motion — the experimental fact Michelson and Morley stumbled into in 1887 and Einstein promoted to a postulate in 1905. Because c is a universal invariant rather than a property of any material, the 17th CGPM stopped measuring it in 1983: the metre was redefined as the distance light travels in 1/299 792 458 of a second, and c became exact by fiat. Every laser interferometer and every GPS satellite has been calibrating length against time ever since.
The trap is the word "vacuum". In glass, water or optical fibre light propagates at c/n — about 2.0 × 10⁸ m/s in a typical fibre, which is why a transatlantic packet cannot beat about 30 ms one way. And c is not merely a speed limit for light: it is the conversion factor between space and time in the metric, so it appears in E = mc², in the Lorentz factor, and in the Schwarzschild radius of things that emit no light at all.