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.

The universality of speed of light in vacuum, and its propagation being independent of complications like dichroism, anisotropy, dispersion and nonlinearity meant that all observers readily could measure lengths using the transit time of light. In 1975 the 15th CGPM (Conférence Générale des Poids et Mesures, General Conference on Weights and Measures) recommended a defined speed of light in the SI system of units:

c ≡ 299 792 458 m/s (exactly), where 'm' = metre, 's' = second,

which is, of course, the same thing as stating the metre is traversed with the transit time of 1/299 792 458 s. The numerical value for c in principle could be chosen to be any numerical value whatsoever, because a change in value simply corresponds to a different choice for the length of the metre, which is an arbitrary unit selected by international convention. A few years later (at the 17th CGPM in 1983) this suggestion was adopted, and the metre was redefined as the length of the path traveled by light in vacuum during a time interval of 1/c of a second, and the notation c₀ suggested for the defined value of the speed of light in vacuum. The value for c₀ was selected to correspond well with the measured speed of light using the then standard metre defined in terms of a wavelength of krypton-86 radiation. That choice was made to limit any dislocation in switching to the new international standard of length. Reference was made to commonly employed methods and corrections to insure measured transit times in real media could be adjusted to refer to vacuum.

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. Greater precision in time determinations became possible with the development of microwave and laser optics, allowing extension of time measurements to higher frequency transitions in the optical regime. Comparison of lengths by comparing their transit times is now far more accurate than methods based upon counting wavelengths.

In systems of units like Atomic units where lengths are not measured by times of transit, but are independent units (for example, the Bohr radius), the speed of light is not a matter of definition but of measurement. In such units, the improvement in time measurements leads to a more accurate determination of the speed of light. On the other hand, in Si units, because of the use of a defined value c₀:

“Note that consequently the speed of light is and remains precisely 299 792 458 meters per second; improvements in experimental accuracy will modify the meter relative to atomic wavelengths, but not the value of the speed of light!”

-W Rindler, Relativity: Special, General, and Cosmological, p. 39