- 2 Physics the dependence of various physical phenomena on relative motion of the observer and the observed objects, especially regarding the nature and behavior of light, space, time, and gravity.
The concept of relativity was set out in Einstein’s special theory of relativity, published in 1905. This states that all motion is relative and that the velocity of light in a vacuum has a constant value that nothing can exceed. Among its consequences are the following: the mass of a body increases and its length (in the direction of motion) shortens as its speed increases; the time interval between two events occurring in a moving body appears greater to a stationary observer; and mass and energy are equivalent and interconvertible. Einstein’s general theory of relativity, published in 1915, extended the theory to accelerated motion and gravitation, which was treated as a curvature of the space-time continuum. It predicted that light rays would be deflected and shifted in wavelength when passing through a substantial gravitational field, effects that have been experimentally confirmed