CHAPTER XII
ON THE NATURE OF THINGS
Theseus. The best in this kind are but shadows; and the worst are no worse, if imagination amend them.
The constructive results of the theory of relativity are based on two principles which have been enunciated—the restricted principle of relativity, and the principle of equivalence. These may be summed up in the statement that uniform motion and fields of force are purely relative. In their more formal enunciations they are experimental generalisations, which can be admitted or denied; if admitted, all the observational results obtained by us can be deduced mathematically without any reference to the views of space, time, or force, described in this book. In many respects this is the most attractive aspect of Einstein's work; it deduces a great number of remarkable phenomena solely from two general principles, aided by a mathematical calculus of great power; and it leaves aside as irrelevant all questions of mechanism. But this mode of development of the theory cannot be described in a non-technical book.
To avoid mathematical analysis we have had to resort to geometrical illustrations, which run parallel with the mathematical development and enable its processes to be understood to some extent. The question arises, are these merely illustrations of the mathematical argument, or illustrations of the actual processes of nature. No doubt the safest course is to avoid the thorny questions raised by the latter suggestion, and to say that it is quite sufficient that the illustrations should correctly replace the mathematical argument. But I think that this would give a misleading view of what the theory of relativity has accomplished in science.
The physicist, so long as he thinks as a physicist, has a definite belief in a real world outside him. For instance, he believes that atoms and molecules really exist; they are not mere inventions
