theory of relativity

relativity el`a*tiv"i*ty (-t?v"?-t?), n. 1. The state of being relative; as, the relativity of a subject. --Coleridge. [1913 Webster]

2. One of two theories (also called {theory of relativity}) proposed by Albert Einstein, the {special theory of relativity}, or the {general theory of relativity}. The {special theory of relativity} or {special relativity} is based on the proposition that the speed of light is a constant no matter how observed, and is independent of the motion of the observer. From this follows several principles, such as the increase of mass with velocity (which has been confirmed: see {relativistic mass equation}) and the impossibility of acceleration to a speed greater than that of light; the equivalence of mass and energy, expressed by the famous equation E = mc^{2}; and time dilation, which is the apparent slowing of a clock in a system, as observed by an observer in a system moving relative to the clock. The {general theory of relativity} is based on the proposition that there is no physical difference between gravitational force and the force produced by acceleration. From this follow several results, of which the bending of light rays in a gravitational field and the equivalence of the inertial and gravitational masses have been verified. The possible existence of {black holes} (believed by many astronomers to have been adequately proven) is another consequence of the theory. [PJC]