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Vol. XVI, No. 2
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principle states : “It is impossible to distinguish between the motion generated by inertial forces (such as centrifugal force, force producing uniform acceleration) and that by gravitational force "'5
This is as shown above, due to the fact that the effects of a gravi. tational field are precisely the same as those due to uniform acceleration of the material frame of reference relative to which the phenomena are observed, this acceleration being equal and opposite to the acceleration which the gravitational field would give to a particle in the frame.6
It also follows that one cannot speak of the absolute acceleration of a reference frame, only a relative one, just as it followed from the special theory of relativity that one cannot speak of the absolute velo. city of a reference frame, only a relative one. Again, it is clear that inertial mass and gravitational mass are equal. For all bodies which are free of any forces will move with uniform velocity relative to an inertial reference frame, no matter what their inertial masses are, and they should, therefore, have the same acceleration relative to an accelerated reference frame. In other words all bodies irrespective of their masses would move with the same acceleration in a homogeneous gravitational field.
Curvature of Space
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 sattelite in terms of ther 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 moying 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 ta
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