A direct comparison of this relation with experiment is not possible at the present time, owing to the fact that the changes in energy to which we can subject a system are not large enough to make themselves perceptible as a change in the inertial mass of the system. is too small in comparison with the mass , which was present before the alteration of the energy. It is owing to this circumstance that classical mechanics was able to establish successfully the conservation of mass as a law of independent validity.
Let me add a final remark of a fundamental nature. The success of the Faraday-Maxwell interpretation of
electromagnetic action at a distance resulted in physicists
becoming convinced that there are no such things as instantaneous actions at a distance (not involving an intermediary medium) of the type of Newton's law of
gravitation. According to the theory of relativity, action at a distance with the velocity of light always takes the place of instantaneous action at a distance or of action at a distance with an infinite velocity of transmission. This is connected with the fact that the velocity plays a fundamental rôle in this theory. In [part:II]Part II we shall see in what way this result becomes modified in the general theory of relativity.