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The speed of light, usually denoted by c, is a physical constant important in many areas of physics. Light and all other forms of electromagnetic radiation always travel at this speed in empty space (vacuum), regardless of the motion of the source or the inertial frame of reference of the observer. Its value is exactly 299,792,458 metres per second^{[1]} (approximately 186,282 miles per second). In the theory of relativity, c interrelates space and time, and appears in the famous equation of mass–energy equivalence E = mc^{2}.^{[2]} It is the speed of all massless particles and associated fields in vacuum, and it is predicted by the current theory to be the speed of gravity and of gravitational waves and an upper bound on the speed at which energy, matter, and information can travel.
The speed at which light propagates through transparent materials, such as glass or air, is less 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 ≈ 200,000 km/s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is very close to c.
In most practical cases, light can be thought of as moving instantaneously, but for long distances and very sensitive measurements the finite speed of light has noticeable effects. In communicating with distant space probes, it can take minutes to hours for the message to get from Earth to the satellite and back. The light we see from stars left them many years ago, allowing us to study 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 chips and from chip to chip. Finally, 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 travelled at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. In 1905, Albert Einstein postulated that the speed of light in vacuum was independent of the source or inertial frame of reference, and explored the consequences of that postulate by deriving the theory of special 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 299,792,458 m/s with a relative 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}⁄_{299,792,458} 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.^{[3]}
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