Relativity
. Relativity, theory, developed in the early 20th century, which originally attempted to account for certain anomalies in the concept of relative motion, but which in its ramifications has developed into one of the most important basic concepts in physical science (see Physics ). The theory of relativity, developed primarily by German American physicist Albert Einstein , is the basis for later demonstration by physicists of the essential unity of matter and energy, of space and time, and of the forces of gravity and acceleration (see Acceleration ; Energy ; Gravitation ). CLASSICAL PHYSICS Physical laws generally accepted by scientists before the development of the theory of relativity, now called classical laws, were based on the principles of mechanics enunciated late in the 17th century by the English mathematician and physicist Isaac Newton . Newtonian mechanics and relativistic mechanics differ in fundamental assumptions and mathematical development, but in most cases do not diffe