## Speed of light |

speed of light takes about 8 minutes 17 seconds to travel the average distance from the surface of thesunlight to thesun .earth exact values metres per second 299792458 perplanck length planck time

(i.e., )planck units 1 approximate values (to three significant digits) kilometres per hour 1080000000 miles per second 186000 miles per hour ^{[1]}671000000 per dayastronomical units 173 ^{[note 1]} per yearparsecs 0.307 ^{[note 2]}approximate light signal travel times **distance****time**one foot 1.0 ns one metre 3.3 ns from to earthgeostationary orbit 119 ms the length of earth's equator 134 ms from to earthmoon 1.3 s from to earth (1sun )au 8.3 min one light year 1.0 year one parsec 3.26 years from to sun (1.3 pc)nearest star 4.2 years from the nearest galaxy (the ) to earthcanis major dwarf galaxy 25000 years across the milky way 100000 years from the to earthandromeda galaxy 2.5 million years from earth to the edge of the observable universe 46.5 billion years special relativity principle of relativity theory of relativity doubly special relativity de sitter invariant special relativity general relativity

alternative formulations

of special relativitythe

**speed of light**in , commonly denotedvacuum **c**, is a universal important in many areas ofphysical constant . its exact value is defined as 299792458 metres per second (approximately 300000 km/s, or 186000 mi/sphysics ^{[note 3]}). it is exact because by international agreement a is defined as the length of the path travelled by light in vacuum during a time interval of metre ^{1}⁄_{299792458} .second ^{[note 4]}^{[3]}according to ,special relativity *c*is the upper limit for the speed at which conventional matter and can travel. though this speed is most commonly associated withinformation , it is also the speed at which alllight andmassless particles perturbations travel in vacuum, includingfield andelectromagnetic radiation . such particles and waves travel atgravitational waves *c*regardless of the motion of the source or the of the observer. particles with nonzeroinertial reference frame can approach c, but can never actually reach it. in therest mass ,special and general theories of relativity *c*interrelates , and also appears in the famous equation ofspace and time mass–energy equivalence *e*=*mc*^{2}.^{[4]}the speed at which light propagates through

, such as glass or air, is less thantransparent materials *c*; similarly, the speed of in wire cables is slower thanelectromagnetic waves *c*. the ratio between*c*and the speed*v*at which light travels in a material is called therefractive index *n*of the material (*n*=*c*/*v*). for example, for the refractive index of glass is typically around 1.5, meaning that light in glass travels atvisible light *c*/ 1.5 ≈ 200000 km/s (124000 mi/s); the for visible light is about 1.0003, so the speed of light in air is about 299700 km/s (186220 mi/s), which is about 90 km/s (56 mi/s) slower thanrefractive index of air *c*.for many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. in communicating with distant

, it can take minutes to hours for a message to get from earth to the spacecraft, or vice versa. the light seen from stars left them many years ago, allowing the study of the history of the universe by looking at distant objects. the finite speed of light also limits the data transfer between the cpu and memory chips inspace probes . the speed of light can be used withcomputers measurements to measure large distances to high precision.time of flight firstole rømer that light travels at a finite speed (as opposed to instantaneously) by studying the apparent motion ofdemonstrated in 1676 's moonjupiter . in 1865,io proposed that light was an electromagnetic wave, and therefore travelled at the speedjames clerk maxwell *c*appearing in his theory of electromagnetism.^{[5]}in 1905, postulated that the speed of lightalbert einstein *c*with respect to any inertial frame 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 in doing so showed that the parameter*c*had relevance outside of the context of light and electromagnetism.after centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299792458 m/s (983571056 ft/s; 186282.397 mi/s) with a

of 4 parts per billion. in 1983, themeasurement uncertainty was redefined in themetre (si) as the distance travelled by light in vacuum in 1/299792458 of ainternational system of units .second - numerical value, notation, and units
- fundamental role in physics
- faster-than-light observations and experiments
- propagation of light
- practical effects of finiteness
- measurement
- history
- see also
- notes
- references
- further reading
- external links

Exact values | |
---|---|

299792458 | |

(i.e., | 1 |

Approximate values (to three significant digits) | |

1080000000 | |

186000 | |

^{[1]} | 671000000 |

173^{[Note 1]} | |

0.307^{[Note 2]} | |

Approximate light signal travel times | |

Distance | Time |

one | 1.0 |

one | 3.3 ns |

from | 119 |

the length of Earth's | 134 ms |

from | 1.3 |

from | 8.3 |

one | 1.0 year |

one | 3.26 years |

from | 4.2 years |

from the nearest galaxy (the | 25000 years |

across the | 100000 years |

from the | 2.5 million years |

from Earth to the edge of the | 46.5 billion years |

The **speed of light** in **c**, is a universal ^{[Note 3]}). It is exact because by international agreement a ^{1}⁄_{299792458} ^{[Note 4]}^{[3]} According to *c* is the upper limit for the speed at which conventional matter and *c* regardless of the motion of the source or the *c* interrelates *E* = *mc*^{2}.^{[4]}

The speed at which light propagates through *c*; similarly, the speed of *c*. The ratio between *c* and the speed *v* at which light travels in a material is called the *n* of the material (*n* = *c* / *v*). For example, for *c* / 1.5 ≈ 200000 km/s (124000 mi/s); the *c*.

For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. In communicating with distant

*c* appearing in his theory of electromagnetism.^{[5]} In 1905, *c* with respect to any inertial frame 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 in doing so showed that the parameter *c* had relevance outside of the context of light and electromagnetism.

After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299792458 m/s (983571056 ft/s; 186282.397 mi/s) with a