In physics, the speed of light (usually denoted c) is a physical constant, the speed at which electromagnetic radiation, such as light, travels in free space (i.e., perfect vacuum). Its value is 299,792,458 metres per second (see the table on the right for conversions). In the theory of special relativity, c is an important constant connecting space and time in the unified structure of spacetime, defines the conversion between mass and energy (E = mc2),[1] and is an upper bound on the speed at which matter and information can travel.[2][3] This constant is significant in the understanding and study of astronomy, space travel, and other fields.
In any inertial frame of reference, independently of the relative velocity of the emitter and the observer, c is the speed of all massless particles and associated fields, including all electromagnetic radiation in free space,[4] and it is believed to be the speed of gravity and of gravitational waves.[5][6]
For much of human history the nature of light, including whether it was instantaneous or simply travelled very quickly, was unknown. By the 11th century many scientists had suggested that light had a finite speed but it was not until the 17th century that Ole Rømer demonstrated this by observing small differences in the apparent orbital period of Jupiter's moon Io. Using these observations, Christiaan Huygens estimated the speed of light to be about 220,000 km per second. Since then, scientists have devised more sophisticated techniques to improve the precision of measurement. By 1975, the speed of light was known to be 299,792,458 metres per second with a relative uncertainty of 4 parts per billion, mostly due to the uncertainty in the length of the metre. 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 a fixed, exact value by definition.[7][8]
The actual 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, the refractive index of glass is typically around 1.5, meaning that light in glass travel at c/1.5 ≈ 200,000 km/s; the refractive index of air is about 1.0003, so the speed of light in air is very close to that in vacuum.
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Important consequence of this model is, it enables to estimate the resulting deceleration, which is equal to product of Hubble constant and speed of light , ...
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