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.
It seemed for a time that the ether, an elastic medium thought to be present throughout space, would provide a method for the measurement of absolute motion, but certain experiments in the late 19th cent. gave results unexplained by or contradicting Newtonian physics. Notable among these were the attempts of A. A. Michelson and E. W. Morley (1887) to measure the velocity of the earth through the supposed ether as one might measure the speed of a ship through the sea. The null result of this measurement caused great confusion among physicists, who made various unsuccessful attempts to explain the result within the context of classical theory.