As these oscillations start and end with the beginning and end of the trip, we must assume that the trip, which lasted twenty years according to the traveller, will have lasted forty years in our own estimation. In other words, any registering device would show when both optical clocks were again placed side by side, that our earth clock had performed oscillations during the trip, whereas the moving clock had performed just half that number, namely, . Obviously, there is no illusion about the matter; for, as we have said, the oscillations of the two clocks could perfectly well be registered graphically, so that we should be dealing with a definite enumeration of registrations which any third party could verify. It is quite easy to understand why these extraordinary results must take place. The cause is to be ascribed entirely to this invariant velocity credited to any light wave in all Galilean frames. Were it not for this postulate, the mere fact that the total length of the zigzag line was twice that of the up-and-down line would not connote any such difference in duration. Classical science would have assumed that since the path was twice as long, the velocity of the light wave along this path would have measured out twice as great; so that finally both durations would have been the same, in full accord with the classical belief in the uniqueness of time. In short, the comparative retardation of the moving clock follows as an inevitable necessity when once we agree to accept the validity of the postulate of the invariant velocity. On no account is it permissible to accept the postulate and then deny its mathematical consequences. If, therefore, the example of the trip to the star is too great a tax on our credulity, we must reject the postulate of invariant velocity and be faced with all the difficulties which surrounded electrodynamics prior to Einstein’s discoveries.