Relativity

Article By:

Wald, Robert M. Department of Physics, University of Chicago, Chicago, Illinois.

Adler, Ronald J. Lockheed Palo Alto Research Laboratories, Palo Alto, California, and Physics Department, San Francisco State University, San Francisco, California.

Last reviewed:January 2022

DOI:https://doi.org/10.1036/1097-8542.580100

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• Relativity, published January 2018:Download PDF Get Adobe Acrobat Reader
• Relativity, published 2014:Download PDF Get Adobe Acrobat Reader

A general theory of physics, primarily conceived by German-born U.S. theoretical physicist Albert Einstein, which involves a profound analysis of time and space, leading to a generalization of physical laws with far-reaching implications in important branches of physics and in cosmology. Historically, Einstein (Fig. 1) developed his theory of relativity in two stages. His initial formulation in 1905 (now known as the special, or restricted, theory of relativity) does not treat gravitation; and one of the two principles on which it is based, the principle of relativity, with the other being the principle of the constancy of the speed of light, stipulates the form invariance of physical laws only for inertial reference systems. Both restrictions were removed by Einstein in his general theory of relativity developed in 1915, which exploits a deep-seated equivalence between inertial and gravitational effects, and leads to a successful “relativistic” generalization of Isaac Newton's theory of gravitation. In practice, the special theory of relativity can be combined with quantum mechanics as relativistic quantum mechanics, describing elementary particles and their interactions via the three forces of nature (with the exception of gravity), conveyed by the standard model of particle physics. General relativity, which is incompatible with quantum mechanics, is the description of gravity fundamental to modern physics (Fig. 2). See also: Classical mechanics; Elementary particle; Fundamental interactions; Gravity; Light; Newton's laws of motion; Physics; Relativistic mechanics; Relativistic quantum theory; Standard model

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