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The speed of light in vacuum, commonly denoted c , is a universal physical constant important in many areas of physics. Its exact value is defined as 299 792 458  metres per second (approximately 300 000  km/s, or 186 000  mi/s).Note It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of ​ 1⁄ 299 792 458 second.Note According to special relativity, c is the upper limit for the speed at which conventional matter, energy or any information can travel through coordinate space. Though this speed is most commonly associated with light, it is also the speed at which all massless particles and field perturbations travel in vacuum, including electromagnetic radiation (of which light is a small range in the frequency spectrum) and gravitational waves. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Particles with nonze...

The speed of light in vacuum is usually denoted by a lowercase c , for "constant" or the Latin celeritas (meaning "swiftness, celerity"). In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used c for a different constant that was later shown to equal √ 2 times the speed of light in vacuum. Historically, the symbol V was used as an alternative symbol for the speed of light, introduced by James Clerk Maxwell in 1865. In 1894, Paul Drude redefined c with its modern meaning. Einstein used V in his original German-language papers on special relativity in 1905, but in 1907 he switched to c , which by then had become the standard symbol for the speed of light. Sometimes c is used for the speed of waves in any material medium, and c 0 for the speed of light in vacuum. This subscripted notation, which is endorsed in official SI literature, has the same form as other related constants: namely, μ 0 for the vacuum permeability or magnetic constant, ε 0 for the v...

The speed at which light waves propagate in vacuum is independent both of the motion of the wave source and of the inertial frame of reference of the observer.Note This invariance of the speed of light was postulated by Einstein in 1905, after being motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous aether; it has since been consistently confirmed by many experiments. It is only possible to verify experimentally that the two-way speed of light (for example, from a source to a mirror and back again) is frame-independent, because it is impossible 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 at the detector should be synchronized. However, by adopting Einstein synchronization for the clocks, the one-way speed of light becomes equal to the two-way speed of light by definition. The special theory of relativity explores the consequences of t...

There are situations in which it may seem that matter, energy, or information travels at speeds greater than  c , but they do not. For example, as is discussed in the propagation of light in a medium section below, many wave velocities can exceed  c . For example, the phase velocity of X-rays through most glasses can routinely exceed c , but phase velocity does not determine the velocity at which waves convey information. If a laser beam is swept quickly across a distant object, the spot of light can move faster than  c , although the initial movement of the spot is delayed because of the time it takes light to get to the distant object at the speed  c . However, the only physical entities that are moving are the laser and its emitted light, which travels at the speed  c from the laser to the various positions of the spot. Similarly, a shadow projected onto a distant object can be made to move faster than  c , after a delay in time. In neither case does any matter, energy, or informat...

In classical physics, light is described as a type of electromagnetic wave. The classical behaviour of the electromagnetic field is described by Maxwell's equations, which predict that the speed  c with which electromagnetic waves (such as light) propagate in vacuum is related to the distributed capacitance and inductance of vacuum, otherwise respectively known as the electric constant ε 0 and the magnetic constant μ 0 , by the equation c = 1 ε 0 μ 0   . {\displaystyle c={\frac {1}{\sqrt {\varepsilon _{0}\mu _{0}}}}\ .} In modern quantum physics, the electromagnetic field is described by the theory of quantum electrodynamics (QED). In this theory, light is described by the fundamental excitations (or quanta) of the electromagnetic field, called photons. In QED, photons are massless particles and thus, according to special relativity, they travel at the speed of light in vacuum. Extensions of QED in which the photon has a mass have been considered. In...

The speed of light is of relevance to communications: the one-way and round-trip delay time are greater than zero. This applies from small to astronomical scales. On the other hand, some techniques depend on the finite speed of light, for example in distance measurements. Small scales In supercomputers, the speed of light imposes a limit on how quickly data can be sent between processors. If a processor operates at 1   gigahertz, a signal can travel only a maximum of about 30 centimetres (1 ft) in a single cycle. Processors must therefore be placed close to each other to minimize communication latencies; this can cause difficulty with cooling. If clock frequencies continue to increase, the speed of light will eventually become a limiting factor for the internal design of single chips. Large distances on Earth Given that the equatorial circumference of the Earth is about 40 075  km and that c is about 300 000  km/s , the theoretical shortest time for a piece of information to travel ...

There are different ways to determine the value of c . One way is to measure the actual speed at which light waves propagate, which can be done in various astronomical and earth-based setups. However, it is also possible to determine c from other physical laws where it appears, for example, by determining the values of the electromagnetic constants ε 0 and μ 0 and using their relation to c . Historically, the most accurate results have been obtained by separately determining the frequency and wavelength of a light beam, with their product equalling c . In 1983 the metre was defined as "the length of the path travelled by light in vacuum during a time interval of ​ 1⁄ 299 792 458 of a second", fixing the value of the speed of light at 299 792 458  m/s by definition, as described below. Consequently, accurate measurements of the speed of light yield an accurate realization of the metre rather than an accurate value of c . Astronomical measurements Outer space is a convenie...

History of measurements of  c (in km/s) <1638 Galileo, covered lanterns inconclusive: 1252 Note <1667 Accademia del Cimento, covered lanterns inconclusive: 1253 1675 Rømer and Huygens, moons of Jupiter 220 000 ‒27% error 1729 James Bradley, aberration of light 301 000 +0.40% error 1849 Hippolyte Fizeau, toothed wheel 315 000 +5.1% error 1862 Léon Foucault, rotating mirror 298 000 ± 500 ‒0.60% error 1907 Rosa and Dorsey, EM constants 299 710 ± 30 ‒280 ppm error 1926 Albert A. Michelson, rotating mirror 299 796 ± 4 +12 ppm error 1950 Essen and Gordon-Smith , cavity resonator 299 792 .5 ± 3.0 +0.14 ppm error 1958 K.D. Froome, radio interferometry 299 792 .50 ± 0.10 +0.14 ppm error 1972 Evenson  et al. , laser interferometry 299 792 .4562 ± 0.0011 ‒0.006 ppm error 1983 17th CGPM, definition of the metre 299 792 .458  (exact) exact, as defined Until the early modern period, it was not known whether light travelled instantan...