The value is precisely
We are accustomed to the additive rule of velocities: if two cars approach each other, each travelling at a speed of 50 miles per hour, we expect that each car will perceive the other as approaching at a combined speed of miles per hour (to a very high degree of accuracy).
At velocities approaching or at the speed of light, however, it becomes clear from experimental results that this additive rule no longer applies. Two spaceships approaching each other, each travelling at 90% the speed of light relative to some third observer between them, do not perceive each other as approaching at 90 + 90 = 180% the speed of light; instead they each perceive the other as approaching at slightly less than 99.5% the speed of light.
This last result is given by the Einstein velocity addition formula:
Contrary to our usual intuitions, regardless of the speed at which one observer is moving relative to another observer, both will measure the speed of an incoming light beam as the same constant value, the speed of light.
Albert Einstein developed the theory of relativity by applying the (somewhat bizarre) consequences of the above to classical mechanics. Experimental confirmations of the theory of relativity directly and indirectly confirm that the velocity of light has a constant magnitude, independent of the motion of the observer.
Since the speed of light in vacuum is constant, it is convenient to measure both time and distance in terms of . Both the SI unit of length and SI unit of time have been defined in terms of wavelengths and cycles of light. In 1983 the metre was redefined in term of c. In particular, one meter is defined as 299792458^{-1}c s. This relies on the constancy of the velocity of light for all observers. Distances in physical experiment or astronomy are commonly measured in light seconds, light minutes, or light years.
Exceeding the group velocity of light in this manner is comparable to exceeding the speed of sound by arranging people in a distantly spaced line of people, and asking them all to shout "I'm here!", one after another with short intervals, each one timing it by looking at their own wristwatch so they don't have to wait until they hear the last person shouting.
The speed of light may also appear to be exceeded in some phenomena involving evanescent waves. Again, it is not possible that information is transmitted faster than .
See also: faster-than-light, tachyon
In 2003, Mikhail Lukin, with scientists at Harvard University and the Lebedev Institute in Moscow, succeeded in completely halting light by directing it into a mass of hot rubidium gas, the atoms of which, in Lukin's words, "[behaved] like tiny mirrors" (Dumé, 2003), due to an interference pattern in two "control" beams. (Dumé, 2003)
It is a bizarre coincidence that the average speed of the earth in its orbit is very close to one ten-thousandth of this, actually within less than a percent. This gives a hint as to how Rømer measured light's speed. He was recording eclipses of Jupiter's moon Io: every day or two Io would go into Jupiter's shadow and later emerge from it. Rømer could see Io blink off and then later blink on, if Jupiter happened to be visible. Io's orbit seemed to be a kind of distant clock, but one which Rømer discovered ran fast while Earth was approaching Jupiter and slow while it was receding from the giant planet. Roemer measured the cumulative effect: by how much it eventually got ahead and then eventually fell behind. He explained the measured variation by positing a finite velocity for light.