Vacuum speed of light
|
|
| Exact values | |
|---|---|
| metres per second | 299792458 |
|
Planck length per Planck time (i.e., Planck units) |
1 |
| Approximate values (to three significant digits) | |
| kilometres per hour | 1080 million (1.08×109) |
| miles per second | 186000 |
| miles per hour | 671 million (6.71×108) |
| astronomical units per day | 173 |
| parsecs per year | 0.307 |
| Approximate light signal travel times | |
| Distance | Time |
| one foot | 1.0 ns |
| one metre | 3.3 ns |
| from geostationary orbit to Earth | 119 ms |
| the length of Earth's equator | 134 ms |
| from Moon to Earth | 1.3 s |
| from Sun to Earth (1 AU) | 8.3 min |
| one light year | 1.0 year |
| one parsec | 3.26 years |
| from nearest star to Sun (1.3 pc) | 4.2 years |
| from the nearest galaxy (the Canis Major Dwarf Galaxy) to Earth | 25000 years |
| across the Milky Way | 100000 years |
| from the Andromeda Galaxy to Earth | 2.5 million years |
| from Earth to the edge of the observable universe | 46.5 billion years |
| <1638 | Galileo, covered lanterns | inconclusive |
| <1667 | Accademia del Cimento, covered lanterns | inconclusive |
| 1675 | Rømer and Huygens, moons of Jupiter | 220000 |
| 1729 | James Bradley, aberration of light | 301000 |
| 1849 | Hippolyte Fizeau, toothed wheel | 315000 |
| 1862 | Léon Foucault, rotating mirror | 298000±500 |
| 1907 | Rosa and Dorsey, EM constants | 299710±30 |
| 1926 | Albert A. Michelson, rotating mirror | 299796±4 |
| 1950 | Essen and Gordon-Smith, cavity resonator | 299792.5±3.0 |
| 1958 | K.D. Froome, radio interferometry | 299792.50±0.10 |
| 1972 | Evenson et al., laser interferometry | 299792.4562±0.0011 |
| 1983 | 17th CGPM, definition of the metre | 299792.458 (exact) |
The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299792458 metres per second (approximately 3.00×108 m/s, approximately 186,282 mi/s); it is exact because the length of the metre is defined from this constant and the international standard for time. According to special relativity, c is the maximum speed at which all matter and hence information in the universe can travel. It is the speed at which all massless particles and changes of the associated fields (including light, a type of electromagnetic radiation, and gravitational waves) travel in vacuum. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the theory of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc2.
The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of radio waves in wire cables is slower 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, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200,000 kilometres (120,000 mi) /s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 299,700 kilometres (186,200 mi) /s (about 90 kilometres (56 mi) /s slower than c).
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