The Enigma Of The Universe : Generalization by Einstein

As we have seen, in the special theory of relativity, only those systems are considered which are in uniform rectilinear motion. Such systems are also known as 'inertial' systems, i.e. systems in which the law of inertia holds good so that a body not subjected to a force remains at rest or in uniform motion. Now, according to the special theory of relativity, in all inertial systemsthe physical laws have the same form. Einstein extended this principle of equivalence to systems moving in any way, even with accelerated velocity and, in particular, to a special case of accelerated motion involved in the phenomenon of gravitation.^[1] Thus, the general theory of relativity states that 'the laws of nature are same for all the systems whatsoever their motion may be.'