Relativity, Principle of

one of the fundamental physical laws. According to the principle of relativity, any process will occur identically in an isolated material system that is in a state of rest and in an identical material system that is in a state of uniform linear motion. The state of motion or rest is defined here with respect to an arbitrarily chosen inertial frame of reference; these states are physically completely equivalent. Another way to formulate the principle of relativity is that the laws of physics have the same form in all inertial frames of reference. The principle of relativity and the postulate that the speed of light in a vacuum is independent of the motion of the light source were the basis for Einstein’s special theory of relativity.