Speed

The speed of light in free space (a vacuum) is an important physical constant usually denoted by the letter c. It is the speed of all electromagnetic radiation, including visible light, in free space. More generally, it is the speed of anything having zero rest mass. more...

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In metric units, the speed of light in vacuum is exactly 299,792,458 metres per second (1,079,252,849 km/h). The fundamental SI unit of length, the metre, has been defined since October 21, 1983, as the distance light travels in a vacuum in 1/299,792,458 of a second; any increase in the measurement precision of the speed of light would refine the definition of the metre, but not alter the numerical value of c. The approximate value of 3×10⁸ m/s is commonly used in rough estimates. In imperial units, the speed of light is about 670,616,630.6 miles per hour or 983,571,056 feet per second, which is about 186,282.397 miles per second, or roughly one foot per nanosecond. See also the later section of this article at \"Speed of light set by definition\".

The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in a vacuum. The ratio of the speed of light in the vacuum to the observed phase velocity is called the refractive index of the medium. See dispersion (optics). In general relativity c remains an important constant of spacetime, however the concepts of 'distance', 'time', and therefore 'speed' are not always unambiguously defined due to the curvature of spacetime caused by gravitation. When measured locally, light in a vacuum always passes an observer at c.

Overview

The speed of light in vacuum is now viewed as a fundamental physical constant. This postulate, together with the principle of relativity that all inertial frames are equivalent, forms the basis of Einstein's theory of special relativity. According to the currently prevailing definition, adopted in 1983, the speed of light is exactly 299,792,458 metres per second (approximately 3×10⁸ metres per second, or about 30 centimetres (1 foot) per nanosecond). See metre.

Experimental evidence has shown that the speed of light is independent of the motion of the source. It has also been confirmed experimentally that the two-way speed of light (for example from a source, to a mirror, and back again) is constant. It is not, however, possible to measure the one-way speed of light (for example from a source to a distant detector) without some convention as to how clocks at the source and receiver should be synchronized. Einstein (who was aware of this fact) postulated that the speed of light should be taken as constant in all cases, one-way and two-way.

It is worth noting that it is the constant speed c, rather than light itself, that is fundamental to special relativity; thus if light is somehow manipulated to travel at less than c, this manipulation will not directly affect the theory of special relativity.

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