# Speed of light

• exact values sunlight takes about 8 minutes 17 seconds to travel the average distance from the surface of the sun to the earth. 299792458 1 1080000000 186000 671000000 173[note 1] 0.307[note 2] time 1.0 ns 3.3 ns 119 ms 134 ms 1.3 s 8.3 min 1.0 year 3.26 years 4.2 years 25000 years 100000 years 2.5 million years 46.5 billion years

the speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. its exact value is defined as 299792458 metres per second (approximately 300000 km/s, or 186000 mi/s[note 3]). 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 ​1299792458 second.[note 4][3] according to special relativity, c is the upper limit for the speed at which conventional matter and information can travel. though this speed is most commonly associated with light, it is also the speed at which all massless particles and field perturbations travel in vacuum, including electromagnetic radiation and gravitational waves. 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 approach c, but can never actually reach it. in the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence e = mc2.[4]

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 299700 km/s (186220 mi/s), which is about 90 km/s (56 mi/s) slower than 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 space probes, 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 in computers. the speed of light can be used with time of flight measurements to measure large distances to high precision.

ole rømer first demonstrated in 1676 that light travels at a finite speed (as opposed to instantaneously) by studying the apparent motion of jupiter's moon io. in 1865, james clerk maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed c appearing in his theory of electromagnetism.[5] in 1905, albert einstein postulated that the speed of light 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 measurement uncertainty of 4 parts per billion. in 1983, the metre was redefined in the international system of units (si) as the distance travelled by light in vacuum in 1/299792458 of a 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