In addition to the existence of space-time and its invariant intervals, the really novel views which the theory forces upon us consist in the relativity of such primary concepts as length, duration, simultaneity, mass, force, etc., to which temperature should also be added. We also obtain new views on causal connections, and a better understanding of the principle of action and of the principles of conservation. Matter becomes fused with energy and disappears as a substance, and conservation is not rigorous. Furthermore, matter possibly creates space-time, and all motion may be relative. As for mass, it may be of an entirely relational nature, just like weight.
If we confine ourselves to the better-established philosophical aspects of the theory, we may say that it proves that nature is ruled by mathematical laws, at least to a first approximation; but this fact was already known to scientists many years ago, following Newton’s discoveries in celestial mechanics. So far as the unity of nature is concerned (a belief which was indissolubly connected with the very existence of science), the theory merely confirms the scientific attitude by revealing a degree of unity of sublime simplicity and beauty. When it comes to deciding whether nature must be viewed in an idealistic or a realistic sense, relativity affords little new information; and, as before, those who wish to adopt a realistic attitude towards the objective world of science are at liberty to doso, though, as we have mentioned, there are a number of reasons which render this philosophy most unsatisfactory. At any rate, the sole difference between relativity and classical science, in their bearing on realism and idealism, consists in the fact that the objective world of science is now one of space-time, no longer one of separate space and time.
The theory is undoubtedly opposed to the view that we can speculate to any advantage on things which are beyond the control of experiment and observation; so the theory is distinctly anti-metaphysical in this respect. But this again brings no additional information to scientists; for the anti-metaphysical attitude was already that of science prior to Einstein’s discoveries, and was expressed in Planck’s statement: “What can be measured is what exists.”
For similar reasons the theory brings home to us with increased force a truth already recognised by science, namely, that space, time and simultaneity are concepts which can be studied only by empirical methods, by the use of rods, clocks and physical processes, and, more generally, by investigating the simplest co-ordinations of physical laws. In particular, the problems of determining whether a simultaneity of external events has any meaning, and, if so, whether it is a relative or an absolute concept, or, again, whether the universe is finite or infinite, are questions which the theoretical physicist alone is in a position to discuss—and then only after he has been informed of the results of ultra-precise observation.A priorispeculations are worthless.
Relativity has also brought about the fusion of two realms of knowledge which had hitherto been developed independently of each other; we refer to geometry and physics. This fusion is illustrated by the fundamental rôle that is played by the geometrical-quantities in the laws of mechanics, in that of gravitation, and in many other physical phenomena. As Weyl tells us, this synthesis may be indicated by the schemeAlthough not much more can be said at the present time, one may realise that even this fragmentary accretion to our knowledge of nature is of an importance which it would be difficult to overestimate.