Back Ligsnelheid Afrikaans Lichtgeschwindigkeit ALS የብርሃን ፍጥነት Amharic Velocidat d'a luz AN سرعة الضوء Arabic سرعة النور ARZ পোহৰৰ বেগ Assamese Velocidá de la lluz AST İşıq sürəti Azerbaijani Яҡтылыҡ тиҙлеге Bashkir
 | |
| Exact value | |
|---|---|
| metres per second | 299792458 |
| Approximate values (to three significant digits) | |
| kilometres per hour | 1080000000 |
| miles per second | 186000 |
| miles per hour[1] | 671000000 |
| astronomical units per day | 173[Note 1] |
| parsecs per year | 0.307[Note 2] |
| 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 the nearest star to Sun (1.3 pc) | 4.2 years |
| from the nearest galaxy to Earth | 70000 years |
| across the Milky Way | 87400 years |
| from the Andromeda Galaxy to Earth | 2.5 million years |
| Special relativity |
|---|
 |
The speed of light in vacuum, commonly denoted c, is a universal physical constant exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second. The speed of light is the same for all observers, no matter their relative velocity. It is the upper limit for the speed at which information, matter, or energy can travel through space.[2][3][4]
All forms of electromagnetic radiation, including visible light, travel at the speed of light. For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and sensitive measurements, their finite speed has noticeable effects. Much starlight viewed on Earth is from the distant past, allowing humans to study the history of the universe by viewing distant objects. When communicating with distant space probes, it can take hours for signals to travel. In computing, the speed of light fixes the ultimate minimum communication delay. The speed of light can be used in time of flight measurements to measure large distances to extremely high precision.
Ole Rømer first demonstrated that light does not travel instantaneously by studying the apparent motion of Jupiter's moon Io. In an 1865 paper, James Clerk Maxwell proposed that light was an electromagnetic wave and, therefore, travelled at speed c.[5] Albert Einstein postulated that the speed of light c with respect to any inertial frame of reference is a constant and is independent of the motion of the light source.[6] He explored the consequences of that postulate by deriving the theory of relativity and, so showed that the parameter c had relevance outside of the context of light and electromagnetism.
Massless particles and field perturbations, such as gravitational waves, also travel at speed c in vacuum. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Particles with nonzero rest mass can be accelerated to approach c but can never reach it, regardless of the frame of reference in which their speed is measured. In the theory of relativity, c interrelates space and time and appears in the famous mass–energy equivalence, E = mc2.[7]
In some cases, objects or waves may appear to travel faster than light. The expansion of the universe is understood to exceed the speed of light beyond a certain boundary. The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of electromagnetic 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 ≈ 200000 km/s (124000 mi/s); the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s (56 mi/s) slower than c.
- ^ Larson, Ron; Hostetler, Robert P. (2007). Elementary and Intermediate Algebra: A Combined Course, Student Support Edition (4th illustrated ed.). Cengage Learning. p. 197. ISBN 978-0-618-75354-3.
- ^ Moses Fayngold (2008). Special Relativity and How it Works (illustrated ed.). John Wiley & Sons. p. 497. ISBN 978-3-527-40607-4. Extract of page 497.
- ^ Albert Shadowitz (1988). Special Relativity (revised ed.). Courier Corporation. p. 79. ISBN 978-0-486-65743-1. Extract of page 79.
- ^ Peres, Asher; Terno, Daniel R. (6 January 2004). "Quantum information and relativity theory". Reviews of Modern Physics. 76 (1): 93–123. arXiv:quant-ph/0212023. Bibcode:2004RvMP...76...93P. doi:10.1103/RevModPhys.76.93. ISSN 0034-6861. S2CID 7481797.
- ^ Gibbs, Philip (1997). "How is the speed of light measured?". The Physics and Relativity FAQ. Archived from the original on 21 August 2015.
- ^ Stachel, J. J. (2002). Einstein from "B" to "Z" – Volume 9 of Einstein studies. Springer. p. 226. ISBN 978-0-8176-4143-6.
- ^ See, for example:
- Feigenbaum, Mitchell J.; Mermin, N. David (January 1988). "E = mc2". American Journal of Physics. 56 (1): 18–21. Bibcode:1988AmJPh..56...18F. doi:10.1119/1.15422. ISSN 0002-9505.
- Uzan, J-P; Leclercq, B (2008). The Natural Laws of the Universe: Understanding Fundamental Constants. Springer. pp. 43–44. ISBN 978-0-387-73454-5.
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