Concept in physics that measurements change when considered by observers in various states of motion. In classical physics, it was assumed that all observers anywhere in the universe would obtain identical measurements of space and time intervals. According to relativity theory, this is not so; all measurements depend on the relative motions of the observer and the observed. There are two distinct theories of relativity, both proposed by Albert Einstein. The special theory of relativity (1905) developed from Einstein's acceptance that the speed of light is the same in all reference frames, irrespective of their relative motion. It deals with non-accelerating reference frames, and is concerned primarily with electric and magnetic phenomena and their propagation in space and time. The general theory (1916) was developed primarily to deal with gravitation and involves accelerating reference frames. Both theories are major milestones in the history of modern physics. See also equivalence principle, space-time.

This entry comes from Encyclopædia Britannica Concise.
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