A principle that postulates the equivalence of the description of the universe, in terms of physical laws, by various observers, or for various frames of reference.

A theory formulated by Einstein that describes the relations between measurements of physical phenomena by two different observers who are in relative motion at constant velocity (the special theory of relativity) or that describes how a gravitational field can be replaced by a curvature of space-time (the general theory of relativity).
 


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