: the state of being dependent for existence on or determined in nature, value, or quality by relation to something else
a: a theory which is based on the two postulates (1) that the speed of light in a vacuum is constant and independent of the source or observer and (2) that the mathematical forms of the laws of physics are invariant in all inertial systems and which leads to the assertion of the equivalence of mass and energy and of change in mass, dimension, and time with increased velocity —called also special relativity, special theory of relativity
b: an extension of the theory to include gravitation and related acceleration phenomena —called also general relativity, general theory of relativity
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 alsoequivalence principle, space-time.