## ::Speed of light

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{{#invoke:Infobox|infobox}} The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its value is exactly {{safesubst:#invoke:val|main}} (≈{{safesubst:#invoke:val|main}}), as the length of the metre is defined from this constant and the international standard for time.<ref name="penrose">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> According to special relativity, c 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 c regardless of the motion of the source or the inertial reference frame of the observer. In the theory of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc2.<ref name=LeClerq>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of radio waves in wire cables is slower than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ {{safesubst:#invoke:val|main}}; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about {{safesubst:#invoke:val|main}} (about {{safesubst:#invoke:val|main}} slower than c).

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 c appearing in his theory of electromagnetism.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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,<ref name="stachel">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> and explored the consequences of that postulate by deriving the special theory of relativity and showing that the parameter c 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 {{safesubst:#invoke:val|main}} 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/{{safesubst:#invoke:val|main}} of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.<ref name=BIPM_SI_units>{{#invoke:citation/CS1|citation |CitationClass=citation }}</ref>

Speed of light sections
Intro  Numerical value, notation, and units  Fundamental role in physics  Faster-than-light observations and experiments  Propagation of light  Practical effects of finiteness  Measurement  History  See also  Notes  References  Further reading  External links

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