Posted: 20.09.2014

The above principle of equivalence formed the basis of Einstein's new theory of gravitation. Using higher mathematics (involving tensors), he derived his famous field-equations, which embodies the law of gravitation. He refused to believe that any such thing as a 'gravitational force of attraction', which was believed by Newton to exist between masses, actually existed. He explained the motion of a body such as a falling apple or an orbiting satellite in terms of altogether a new concept of "Curvature of Space-time" in which it is embedded. Thus, Einstein, discarding the Newtonian "action-at a-distance" concept, reverted to field-point of view.

To understand his view, we take a simple example. Consider a stone attached to the end of string and whirled round, the stone moving along a circular path. We know that a tension is developed in the string called the centripetal force. To an observer on the stone there is a repulsion called centrifugal force equal and opposite to the centripetal force, while to the one who is whirling the stone there is no such centrifugal force. Einstein saw that the apparent repulsive force is due to the curvilinear motion of the stone and he extended the idea of such curved motions to a sort of curved space in order to explain gravitational force.^{[1]} Again, since the centrifugal forces had to be considered as due to physical properties of empty space, Einstein turned to the hypothesis that the gravitational forces also are due to the properties of empty-space. Also, it is known that the forces of gravity are produced by masses. If, therefore, gravitation is connected with properties of space, these properties of space must be caused or influenced by the masses.^{[2]} Thus Einstein explained the phenomenon of gravitation by stating that the presence of matter gives an appreciable curvature to the surrounding space (or more precisely, distorts the curvature of the Minkowski's four-dimensional space-time continuum) and the matter falls down, as it were along the slop of this curvature according to some definite law.

According to Einstein, gravitation is an inalienable property of all masses; where there is a mass, a gravitational field is generated around it in the same way as a magnetic field is created around a magnet. The field itself is nothing but a change in the space-time continuum. In other words, the geometrical properties of space-time continuum are determined by the physical matter existing in it and not intrinsic properties of space and time themselves. These properties in turn determine the motions of that matter.