A theory developed by Albert Einstein (1878-1955) and published in 1905, which describes how physical phenomena are observed between unaccelerated (inertial) reference frames in relative motion.
The Special Theory of Relativity was a direct consequence of the fact, proved experimentally, that the speed of light (c) in a vacuum is measured to be the same by all observers, regardless of their state of motion or the motion of the source of light. This had been verified by the famous Michelson-Morley experiment in 1887. It was also predicted theoretically (1873) by the equations describing electromagnetic radiation derived by James Clerk Maxwell. The other underlying principle of the theory is the so-called principle of relativity. This states that no physical experiment can be devised to detect a uniform state of motion. In other words, neither location in space and time, nor uniform motion, affects the description of physical reality. The concept of motion through some absolute framework of space and time was swept away by the ideas of relativity.
A number of important consequences flow from these principles, relating to the way time intervals, lengths and masses are measured between reference frames in relative motion. Time intervals appear extended (time dilation), lengths foreshortened (the Lorentz contraction) and masses increase in reference frames moving relative to the observer, though the effects become significant only when the relative speed is a significant fraction of that of light. Using b = relative velocity/c, and the subscript o to mean "in the reference frame of the observer", the transformations are: . The three dimensions of space and time are considered to describe four-dimensional spacetime with the characteristic that the interval, is the same for all reference frames.
Another result from the theory is the concept of rest mass and the equivalence of mass and energy as expressed in the relationship E = mc². This formula gives the amount of energy released when mass is annihilated.
The predictions of Special Relativity have been totally verified, particularly in the physics of atoms and elementary particles.

See also: General Relativity.