CHAPTER V
GENERALISED RELATIVITY
Weight and inertia—Ambiguity of the Newtonian law—Equivalence of gravitation and accelerated movement—Jules Verne’s projectile and the principle of inertia—Why rays of light are subject to gravitation—How light from the stars is weighed—An eclipse as a source of light.
We are now on the threshold of the great mystery of gravitation.
In the [preceding chapter] we saw how Einstein brought under one magnificent law both the slow movements of massive objects and the far more rapid movements of light. They had hitherto been separate and anarchic provinces of the universe. We now know that the same laws govern mechanics and optics. If for a time it appeared otherwise, it was because at velocities which approach that of light the lengths and masses of objects experience in the eyes of the observer an alteration which is imperceptible at familiar speeds. It is in its power of synthesis that Einstein’s mechanics is so splendid. Thanks to it, we perceive more unity, more harmony, more beauty, than formerly in this astounding universe, in which our thoughts and our anxieties are so ephemeral.
The theory of Relativity, however, has up to the present not touched a phenomenon that is fundamental, essential, ubiquitous in our cosmos. I mean gravitation, the mysterious property of bodies which rules the tiny atom no less than the most gigantic star, and directs their paths in majestic curves.
The universal attraction which, as far as earth is concerned, we call weight was a kind of steep-cliffed island in the sea of phenomena, something unrelated to the rest of natural philosophy.
The Einsteinian mechanism, as we have described it up to now, passed by this island, taking no notice of it. For that reason it was, in this form, known as “the theory of Special Relativity.” In order to convert it into a perfect instrument of synthesis, the phenomenon of gravitation had to be introduced. It is thus that Einstein crowned his work, and his system assumed the form which is well called “the theory of General Relativity.”
Einstein has drawn gravitation from its “splendid isolation,” and has annexed it, docile and vanquished, to the triumphal chariot of his mechanics. He has, moreover, given Newton’s famous law a more correct form, and experiment, the supreme judge, has declared this the only just form.
How he did this, by what subtle and powerful chain of reasoning, by what calculations based upon facts, I will now endeavour to tell; and I will again do my best to avoid the network of barbed wire of mathematical terminology.