Speed of light

In physics, the speed of light in vacuum, commonly denoted by c, is seen as one of the fundamental constants of nature. The main postulate of special relativity asserts that the velocity of light is independent of the motion of the light source; the speed of light is the same in any inertial frame (coordinate system moving with constant velocity), irrespective whether the light is emitted by a body in uniform motion or by a body at rest.

Already Galileo Galilei suspected that light has a finite velocity and claimed that he tried in vain to measure it. About forty years later, in 1675, the Danish atronomer Rømer estimated that it takes about 11 minutes (660 seconds) for light to travel from the Sun to the Earth. He could make this estimate by observing eclipses of the first satellite of the planet Jupiter. A few years earlier Cassini had deduced from observations of Mars that the distance from Sun to Earth was about 139⋅10⁶ km, so that the speed of light was estimated to be 2.1⋅10⁸ m/s, which is about 30% lower than the modern value. Later Rømer's value was refined, by similar astronomical observations, to 499 seconds. In 1849 Fizeau determined by Earth-bound experiments that c is 3.15⋅10⁸ m/s. Modern work brought this value down to just under 3⋅10⁸ m/s.

In 1975 the 15th CGPM (Conférence Générale des Poids et Mesures, General Conference on Weights and Measures), noticing that the uncertainty in the determination of the meter was about as large as the uncertainty in the measurements of the speed of light, recommended the definition

c ≡ 299 792 458 m/s (exact).

The great precision in the determination of c was due to the development of microwave and laser optics. A few years later (at the 17th CGPM in 1983) the metre was redefined as the length of the path travelled by light in vacuum during a time interval of 1/c of a second. This definition of the meter was possible, because earlier, in 1968, the second was defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.