On the subject of discontinuity, there is disagreement between Schrödinger and other physicists. Most of them maintain that quantum changes--_i.e._ the changes that occur in an atom when it radiates or absorbs energy--must be discontinuous. Schrödinger thinks otherwise. This is a matter in debate among experts, as to which it would be rash to venture an opinion. Probably it will be decided one way or other before very long. The main point for the philosopher in the modern theory is the disappearance of matter as a “thing”. It has been replaced by emanations from a locality--the sort of influences that characterise haunted rooms in ghost stories. As we shall see in the next chapter, the theory of relativity leads to a similar destruction of the solidity of matter, by a different line of argument. All sorts of events happen in the physical world, but tables and chairs, the sun and moon, and even our daily bread, have become pale abstractions, mere laws exhibited in the successions of events which radiate from certain regions. CHAPTER X RELATIVITY We have seen that the world of the atom is a world of revolution rather than evolution: the electron which has been moving in one orbit hops quite suddenly into another, so that the motion is what is called “discontinuous”, that is to say, the electron is first in one place and then in another, without having passed over any intermediate places. This sounds like magic, and there may be some way of avoiding such a disconcerting hypothesis. At any rate, nothing of the sort seems to happen in the regions where there are no electrons and protons. In these regions, so far as we can discover, there is continuity, that is to say, everything goes by gradual transitions, not by jumps. The regions in which there are no electrons and protons may be called “æther” or “empty space” as you prefer: the difference is only verbal. The theory of relativity is especially concerned with what goes on in these regions, as opposed to what goes on where there are electrons and protons. Apart from the theory of relativity, what we know about these regions is that waves travel across them, and that these waves, when they are waves of light or electromagnetism (which are identical), behave in a certain fashion set forth by Maxwell in certain formulæ called “Maxwell’s equations”. When I say we “know” this, I am saying more than is strictly correct, because all we know is what happens when the waves reach our bodies. It is as if we could not see the sea, but could only see the people disembarking at Dover, and inferred the waves from the fact that the people looked green. It is obvious, in any case, that we can only know so much about the waves as is involved in their having such-and-such causes at one end and such-and-such effects at the other. What can be inferred in this way will be, at best, something wholly expressible in terms of mathematical structure. We