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 we shall see in what way this result becomes modified in the general theory of relativity.
is the energy taken up, as judged from a co-ordinate system moving with the body.
[14]As judged from a co-ordinate system moving with the body.