Relativity, Theory of, a description of space and time as determined by physical measurements. The origins of the theory date back to the principles of relative motion formulated by scientists in the 17th century. In its present form, however, the theory is largely the work of Albert Einstein (1879-1955). According to Einstein's theory, space and time are relative concepts, and measurements of space and time depend on the state of motion of the observer.

Einstein's theory consists of two parts: (1) the special, or restricted, theory, which concerns measurements made by observers moving at constant velocity with respect to each other; and (2) the general theory, which expands the special theory to include measurements by observers whose relative velocity is changing. The general theory applies the principles of relativity to gravitation. The special theory of relativity was published in 1905; the general theory, in 1916.

Einstein's theory of relativity has been of great importance in modern physics. For example, the special theory showed scientists that it is possible to unleash the energy contained in the nucleus of the atom. The theory has influenced all branches of physics dealing with electromagnetic radiation and high-speed particles. It has had a profound effect on astronomy and the related science of cosmology, which attempts to explain the origin and structure of the universe.

The body of scientific principles developed before Einstein's time is referred to as classical physics. When applied to everyday situations, these principles are still valid. The theory of relativity differs significantly from classical physics only when dealing with objects moving at extremely high speed, with objects having very strong gravitational fields, or with the universe on a broad scale.