# Speed of light

Jump to: navigation, search

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)[1] 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. A few years later (at the 17th CGPM in 1983)[2] 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.[3] Reference was made to commonly employed methods and corrections to insure measurements referred to vacuum. The numerical value for c0 was selected to correspond well with the measured speed of light using wavelength measurements in order to cause the least dislocation in switching international standards of length.

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.[4] On the other hand, in Si units, because of the use of a defined value c0:[5]

“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

## Reference

1. Bureau International des Poids et Mesures (Brochure on SI units, 8th ed.; pdf page 65, paper page 157) From the website of the Bureau International des Poids et Mesures
2. Resolution 1, 17th Meeting of the General Conference on Weights and Measures, 1983.
3. See, for example, the official definition and the BIPM brochure on the SI units: The International System of Units §2.1.1 Unit of length (metre), page 112, 8th edition of 2006.
4. Markus Reiher, Alexander Wolf (2009). Relativistic quantum chemistry: the fundamental theory of molecular science. Wiley-VCH, p. 7. ISBN 3527312927.
5. Rindler, W (2006). Relativity: Special, General, and Cosmological, 2nd ed.. Oxford University Press, p. 39. ISBN 0198567316.
 Some content on this page may previously have appeared on Citizendium.