| header9 = Approximate light signal travel times
| label10 = Distance
| data10 = Time
| label11 = one foot
| data11 = 1.0 ns
| label12 = one metre
| data12 = 3.3 ns
| label15 = from geostationary orbit to Earth
| data15 = 119 ms
| label16 = the length of Earth's equator
| data16 = 134 ms
| label17 = from Moon to Earth
| data17 = 1.3 s
| label18 = from Sun to Earth (1 AU)
| data18 = 8.3 min
| label20 = one light year
| data20 = 1.0 year
| label21 = one parsec
| data21 = 3.26 years
| label22 = from nearest star to Sun ()
| data22 = 4.2 years
| label23 = from the nearest galaxy (the Canis Major Dwarf Galaxy) to Earth
| data23 =
| label24 = across the Milky Way
| data24 =
| label25 = from the Andromeda Galaxy to Earth
| data25 = 2.5 million years
| label26 = from Earth to the edge of the observable universe
| data26 = 46.5 billion years
The speed of light in vacuum, commonly denoted , is a universal physical constant important in many areas of physics. Its precise value is (approximately ), since the length of the metre is defined from this constant and the international standard for time. According to special relativity, is the maximum speed at which all matter and information in the universe can travel. It is the speed at which all massless particles and changes of the associated fields (including electromagnetic radiation such as light and gravitational waves) travel in vacuum. Such particles and waves travel at regardless of the motion of the source or the inertial reference frame of the observer. In the theory of relativity, interrelates space and time, and also appears in the famous equation of mass–energy equivalence .
The speed at which light propagates through transparent materials, such as glass or air, is less than ; similarly, the speed of radio waves in wire cables is slower than . The ratio between and the speed at which light travels in a material is called the refractive index of the material (). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at ; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about (about slower than ).
For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. In communicating with distant space probes, it can take minutes to hours for a message to get from Earth to the spacecraft, or vice versa. The light seen from stars left them many years ago, allowing the study of the history of the universe by looking at distant objects. The finite speed of light also limits the theoretical maximum speed of computers, since information must be sent within the computer from chip to chip. The speed of light can be used with time of flight measurements to measure large distances to high precision.
Ole Rømer first demonstrated in 1676 that light travels at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed appearing in his theory of electromagnetism. In 1905, Albert Einstein postulated that the speed of light with respect to any inertial frame is independent of the motion of the light source, and explored the consequences of that postulate by deriving the special theory of relativity and showing that the parameter had relevance outside of the context of light and electromagnetism.
After centuries of increasingly precise measurements, in 1975 the speed of light was known to be with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/ of a second. As a result, the numerical value of in metres per second is now fixed exactly by the definition of the metre.
==Numerical value, notation, and units==
The speed of light in vacuum is usually denoted by a lowercase ''c'', for "constant" or the Latin ラテン語:''celeritas'' (meaning "swiftness, celerity"). Historically, the symbol ''V'' was used for as an alternative symbol for the speed of light, introduced by James Clerk Maxwell in 1865. In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used ''c'' for a different constant later shown to equal times the speed of light in vacuum. 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.〔
"The origins of the letter c being used for the speed of light can be traced back to a paper of 1856 by Weber and Kohlrausch () Weber apparently meant c to stand for 'constant' in his force law, but there is evidence that physicists such as Lorentz and Einstein were accustomed to a common convention that c could be used as a variable for velocity. This usage can be traced back to the classic Latin texts in which c stood for 'celeritas' meaning 'speed'."
Sometimes ''c'' is used for the speed of waves in ''any'' material medium, and ''c''0 for the speed of light in vacuum.〔See for example:
*〕 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 vacuum permittivity or electric constant, and ''Z''0 for the impedance of free space. This article uses ''c'' exclusively for the speed of light in vacuum.
Since 1983, the metre has been defined in the International System of Units (SI) as the distance light travels in vacuum in of a second. This definition fixes the speed of light in vacuum at exactly .〔
As a dimensional physical constant, the numerical value of ''c'' is different for different unit systems.
In branches of physics in which ''c'' appears often, such as in relativity, it is common to use systems of natural units of measurement or the geometrized unit system where .〔
〕 Using these units, ''c'' does not appear explicitly because multiplication or division by 1 does not affect the result.
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