<P> In 1905 Einstein postulated from the outset that the speed of light in vacuum, measured by a non-accelerating observer, is independent of the motion of the source or observer . Using this and the principle of relativity as a basis he derived the special theory of relativity, in which the speed of light in vacuum c featured as a fundamental constant, also appearing in contexts unrelated to light . This made the concept of the stationary aether (to which Lorentz and Poincaré still adhered) useless and revolutionized the concepts of space and time . </P> <P> In the second half of the 20th century much progress was made in increasing the accuracy of measurements of the speed of light, first by cavity resonance techniques and later by laser interferometer techniques . These were aided by new, more precise, definitions of the metre and second . In 1950, Louis Essen determined the speed as 7008299792500000000 ♠ 299 792. 5 ± 1 km / s, using cavity resonance . This value was adopted by the 12th General Assembly of the Radio - Scientific Union in 1957 . In 1960, the metre was redefined in terms of the wavelength of a particular spectral line of krypton - 86, and, in 1967, the second was redefined in terms of the hyperfine transition frequency of the ground state of caesium - 133 . </P> <P> In 1972, using the laser interferometer method and the new definitions, a group at the US National Bureau of Standards in Boulder, Colorado determined the speed of light in vacuum to be c = 7008299792456200000 ♠ 299 792 456. 2 ± 1.1 m / s . This was 100 times less uncertain than the previously accepted value . The remaining uncertainty was mainly related to the definition of the metre . As similar experiments found comparable results for c, the 15th General Conference on Weights and Measures in 1975 recommended using the value 7008299792458000000 ♠ 299 792 458 m / s for the speed of light . </P> <P> In 1983 the 17th CGPM found that wavelengths from frequency measurements and a given value for the speed of light are more reproducible than the previous standard . They kept the 1967 definition of second, so the caesium hyperfine frequency would now determine both the second and the metre . To do this, they redefined the metre as: "The metre is the length of the path travelled by light in vacuum during a time interval of 1 / 7008299792458000000 ♠ 299 792 458 of a second ." As a result of this definition, the value of the speed of light in vacuum is exactly 7008299792458000000 ♠ 299 792 458 m / s and has become a defined constant in the SI system of units . Improved experimental techniques that prior to 1983 would have measured the speed of light, no longer affect the known value of the speed of light in SI units, but instead allow a more precise realization of the metre by more accurately measuring the wavelength of Krypton - 86 and other light sources . </P>

Who discovered the speed of light in a vacuum
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