Speed of light

n. permanent physical speed of the electromagnetic rays in space, unit of distance per time within which light moves (approximately 299,792.5 kilometers per second / 186, 200, 000 miles per second)

"Lightspeed" redirects here. For other uses, see Lightspeed (disambiguation).For the song "Speed of Light" see Episode (album)The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning "swiftness". It is the speed of all electromagnetic radiation, including visible light, in a vacuum. More generally, it is the speed of anything having zero rest mass.
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Noun
1. the speed at which light travels in a vacuum; the constancy and universality of the speed of light is recognized by defining it to be exactly 299,792,458 meters per second
(synonym) light speed, c
(hypernym) speed, velocity

The speed of an electromagnetic wave in free space, precisely 299,792,458 m/s. Note 1: The preceding figure is precise because by international agreement the meter is now defined in terms of the speed of light. Note 2: The speed of an electromagnetic wave, e.g. , light, is equal to the product of the wavelength and the frequency. Note 3: In any physical medium, the speed of light is lower than in free space. Since the frequency is not changed, the wavelength is also decreased. [After FAA]

The speed of photons of electromagnetic radiation in a vacuum. The value is equal to 3 x 108 m/s. According to the theory of special relativity, nothing can travel faster than this. See universality of speed of light.

One of the cornerstones of special relativity, it states that the speed of light is the same whether the observer measuring it is stationary or in constant motion. This was first proved experimentally by Michelson and Morley from 1881 onwards, in their failure to detect the ether. The ether was the hypothetical medium which pervaded the whole of space and through which electromagnetic radiation was thought to have propagated. In an attempt to measure the difference in light's velocity through this ether, it was shown that all experiments return the same value for the speed of light, regardless of the relative velocity of the experiment which researchers expected to be superimposed upon that of light. At a stroke, this disproved that the ether existed and paved the way for Einstein to develop the theory of special relativity.