Now a planet revolving on its axis in space is nothing but a huge top; and if, for instance, we stand at the non-rotating point of a planet, say the North Pole (so as to be in a Galilean frame), we should be perfectly justified in measuring our unit durations by the successive rotations of our planet with respect to the fixed stars. Consider, then, two planets rotating at exactly the same speed when held side by side, and let us call one of these planets the earth. Regardless of whether these two planets be set flying apart or held together, the observer on either planet will be justified in measuring his unit time-stretches in terms of his own planetary rotations. Classical science assumed that since time was one and universal, both planets would always beat out exactly the same unit time through their rotations, whether they were in relative motion or at rest, so it mattered little which particular planet was selected. Under the circumstances, from motives of convenience, the earth was chosen as standard. The novel point of Einstein’s theory consists in proving that this attitude is untenable. Assuming the various planets to rotate with the same speed when their axes are at relative rest, each different observer must henceforth abide by the rotations of his own planet and not by those of the other planets when relative motion is present. Only thus can the special principle of relativity be maintained. In particular, if we assume one rotating planet to be transferred to a distant star, then returned near the rotating earth as in the example of the two twins, it would be found that, owing to the solidarity between the optical clocks and the rotations of the planets, the wandering planet would have revolved less often during its trip than would have been the case with the earth. So we see that the slowing down we have mentioned is absolutely general, and must indeed be so if the special principle of relativity is to endure. Furthermore, we see that the great difference between classical science and relativity consists, not in having substituted an optical clock for the earth’s rotation, since both these determinations of time would be identical, but in having proved that the earth’s rotation or the earth’s optical clock is valid only as a measure of time for an earth observer or for one at rest with respect to him. [Pg 234]