The theory of relativity, or simply relativity, refers specifically to two theories: special relativity and general relativity.
The modern theory is an extension of the simpler Galilean or Newtonian concept of relativity, which holds that the laws of mechanics are the same in one system as in another system in uniform motion relative to it. Thus, it is impossible to detect the motion of a system by measurements made within the system, and such motion can be observed only in relation to other systems in uniform motion. The older concept of relativity assumes that space and time are correctly measured separately and regards them as absolute and independent realities. The system of relativity and mechanics of Galileo and Newton is perfectly self-consistent, but the addition of Maxwell's theory of electricity and magnetism to the system leads to fundamental theoretical difficulties related to the problem of absolute motion.
Einstein's initial prominence stemmed from his spectacular advances in theoretical physics, in particular his Special Theory of Relativity (1905) and his General Theory of Relativity (1916), in which he demonstrated the relationship between mass, energy, and gravitation.