Principle of equivalence

In the physics of relativity, the equivalence principle is applied to several related concepts dealing with gravitation and the uniformity of physical measurements in different frames of reference. They are related to the Copernican idea that the laws of physics should be the same everywhere in the universe, to the equivalence of gravitational and inertial mass, and also to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.
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Noun
1. (physics) the principle that an observer has no way of distinguishing whether his laboratory is in a uniform gravitational field or is in an accelerated frame of reference
(hypernym) principle, rule
(classification) physics, physical science, natural philosophy

A fundamental principle of general relativity. It states that an observer, in a closed system, would be unable to tell whether the force acting upon him was gravity or whether the entire system, in which he was placed, was being accelerated. In other words: the effect of gravity can be counteracted by a correctly chosen and applied (equal but opposite) acceleration. By thinking of gravity as an accelerated frame of reference, general relativity can predict its effects.