Page:L. Silberstein - The Theory of Relativity.djvu/18

 instance of motion of a particular body or also of almost any observable motion of bodies in general, leading to a high degree of completeness, exactness and simplicity, while other frameworks (moving in an arbitrary manner relatively to those) give of the same phenomena a most complicated, intricate and confused picture.

Suppose that somebody, ignorant of the work of Copernicus, Galileo and Newton, but otherwise gifted with the highest experimental abilities and mathematical skill (a quite imaginary supposition, being hardly consistent with the first one), chooses the interior of an old-fashioned coach, driven along a fairly rough road, as his laboratory and tries to investigate the laws of motion of bodies enclosed together with him in the coach—say, of a pendulum or of a spinning top—and selects that vehicle as his system of reference. Then his tangible bodies and his conceptual 'material points,' starting from rest or any given velocity, would describe the most wonderful paths, in incessant shocks and jerky motions; the axis of his 'free gyroscope' would oscillate in a most complicated way,—never disclosing to him the constancy of the vector known to us as the 'angular momentum,' i.e. the rotatory analogue of Newton's first law of motion. Nor would the uniform translational motion have for him any peculiarly simple or generally noteworthy properties at all. His mechanical experience being, in a word, full of surprises, he would soon give up his task of stating any laws of motion whatever with reference to the coach. Getting out of it on to firm ground, he will readily find out that the earth is a much better system of reference. With this framework, smoothness and simplicity will take the place of hopeless irregularity. Undoubtedly, this property must have been remarked in a very early stage of man's history, and the above example will appear to the least trained student of mechanics of our present times trivial and simply ridiculous. 'Of course,' he would say, 'the motions of material bodies relatively to that coach are so very complicated, for that vehicle is itself moving in a highly complicated way.' He would hardly consider it worth while to add 'relatively to the earth.' The coach being such a small, insignificant thing in comparison with the terrestrial globe, it would seem extravagant to our interlocutor, if somebody insisted rather on saying that it is the earth which moves in such a complicated way relatively