Speed Of Light

Overview

The value is precisely

c = 2.997 924 58 × 10⁸ metres per second,

Constant in all Reference Frames

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 particular, a meter is defined as exactly c/299792458 * 1 second. 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: tachyon

History

Galileo Galilei as far as we know was the first person to suspect that light might have a finite speed and attempt to measure it-but people before Galileo probably thought of lights (i.e. stars, suns) as constants anyway. He wrote about his unsuccessful attempt using lanterns flashed from hill to hill outside Florence. The speed of light was first measured in 1676, some decades after Galileo's attempt, by Rømer, who was studying the motions of Jupiter's moonss. A plaque at the Observatory of Paris, where the Danish astronomer happened to be working, commemorates what was, in effect, the first measurement of a universal quantity made on this planet. Rømer published his result, which had an error of 10-25%, in Journal des Scavans.