The arguments thus far presented show that the space and time determinations derived from electromagnetics and optics are in all probability also those holding for the entire physical universe. Such is indeed Einstein’s attitude when he unhesitatingly applies the relativistic transformations to all manner of phenomena. But it is precisely because as a result of this application he has anticipated phenomena at variance with those deduced from the classical transformations, and it is precisely because his anticipations have been verified by highly accurate experiments, that his assumption of the universal validity of the relativistic transformations must be considered justified. On no account, therefore, may we regard time and simultaneity, when defined by optical signals, as constituting a special kind of optical or electromagnetic time and simultaneity, at variance with equally justified determinations issuing from mechanical processes or sound transmissions. In short, if we wish to co-ordinate natural phenomena with simplicity, we must thus conceive the various time-flows and simultaneities defined by Einstein, to represent the times imposed upon us, not by the caprice of a mathematician, not by optical transmissions alone, but by the entire physical world. The times of relativity are therefore those which will govern all physical processes, not merely optical ones; and for this reason we must regard them as representing the real time-flows of the universe. An understanding of this point is essential. For instance, in the general theory we shall see that Einstein’s law of gravitation necessitates a slowing down of time in the proximity of large masses of matter. If we accept the general significance of this fact as affecting all physical phenomena, it will entail the slowing down in the rate of vibration of an atom located in a gravitational field. This is the celebrated Einstein shift-effect, which has since been verified. Before it was detected, however, and while it was still considered doubtful, some thinkers had argued that it might perfectly well prove to be non-existent on the ground that the atom might not act like a perfect clock and might fail to adjust itself to the local rate of time-flow. Einstein, however, refused to accept this means of escape and stated that the entire theory would have to be rejected if the effect were not verified, for the times of relativity should be of universal significance or else worthless.