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speed of light
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relativistic velocity combination
invariance of the speed of light length contraction and time dilation Figure 6: Fizeau's method for measuring the velocity of light. Figure 7: Michelson's Mount Wilson experiment, 1927.
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speed of light

physics
Also known as: c, electromagnetic constant, velocity of light
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light
physical constant
velocity
E = mc2

speed of light, speed at which light waves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per second.

relativistic velocity combination
relativistic velocity combinationIf you race at a beam of light, why doesn't the light approach you faster than the speed of light? If you run away, why doesn't the light approach you slower than the speed of light? This video is an episode in Brian Greene's Daily Equation series.
See all videos for this article

The speed of light is considered a fundamental constant of nature. Its significance is far broader than its role in describing a property of electromagnetic waves. It serves as the single limiting velocity in the universe, being an upper bound to the propagation speed of signals and to the speeds of all material particles. In the famous relativity equation, E = mc2, the speed of light (c) serves as a constant of proportionality, linking the formerly disparate concepts of mass (m) and energy (E).

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Adam Augustyn.
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special relativity
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relativity of simultaneity
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Understand the proof of Albert Einstein's special relativity equation
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special relativity

physics
Also known as: special theory
Written and fact-checked by
The Editors of Encyclopaedia Britannica
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.
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Last Updated: • Article History

special relativity, part of the wide-ranging physical theory of relativity formed by the German-born physicist Albert Einstein. It was conceived by Einstein in 1905. Along with quantum mechanics, relativity is central to modern physics.

relativity of simultaneity
relativity of simultaneityBrian Greene derives the time discrepancy between two people who are moving relative to one another but who disagree on what happens at the same time. This video is an episode in his Daily Equation series.
See all videos for this article

Special relativity is limited to objects that are moving with respect to inertial frames of reference—i.e., in a state of uniform motion with respect to one another such that one cannot, by purely mechanical experiments, distinguish one from the other. Beginning with the behaviour of light (and all other electromagnetic radiation), the theory of special relativity draws conclusions that are contrary to everyday experience but fully confirmed by experiments that examine subatomic particles at high speeds or measure small changes between clocks traveling at different speeds. Special relativity revealed that the speed of light is a limit that can be approached but not reached by any material object. It is the origin of the most famous equation in science, E = mc2, which expresses the fact that mass and energy are the same physical entity and can be changed into each other. (For a more-detailed treatment of special relativity, see relativity: Special relativity.)

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Barbara A. Schreiber.
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