Let us take, for simplicity, two hydrogen atoms, of which one emits energy which the other absorbs. But for the theory of quanta, and such phenomena as the photo-electric effect, a supposition of this sort would be impossible. If the energy radiated from a hydrogen atom in the form of light really has the shape of a spherical wave, it is impossible that the whole of it should be absorbed by one other atom, any more than the whole of the light radiated from the sun can fall on the earth. But if the light emitted by a single atom travels in a straight line (approximately), like a material particle, then it may happen to hit one atom and be absorbed whole, just as Jonah might have been swallowed by another whale. We shall have to suppose, in this case, that the spherical distribution of light round a radiating body is a statistical phenomenon, like bullets fired from a fort in all directions. This suggests the hypothesis which we have already considered in Chapter XIII., according to which nothing at all happens between the emission of light by one body and its absorption by another. In that case, empty space collapses just as the electron did, and only the surface of the electron remains. This, however, seems hardly a tenable view. The intervening space might be described as non-existent from a metrical point of view, since the interval between the emission and the absorption of a light-ray is zero; but from an ordinal point of view this is not the case, since, if and are two points on a light-ray, we can distinguish the case in which the ray goes from to from that in which it goes from to . This difference can be stated in metrical terms. For example: Let us take as our time co-ordinate the proper time of no matter what body; whatever body we choose, will be earlier than , or else, whatever body we choose, will be earlier than . Again: Suppose that at and there are mirrors, which reflect part of the[Pg 330] ray in such a way that an observer sees both reflected rays. Then either every such observer will see the reflection from before that from , or else every such observer will see the reflection from before that from . We can free this from dependence on an observer by the following method of statement: Let be a point on the ray reflected from , and a point on the ray reflected from , so chosen that the interval between and is time-like. Then, however and may be chosen, either is always before , or is always before . This is stated in the language of the special theory, but it is still valid, mutatis mutandis, in the general theory. Thus when we say that the interval between two points on a light-ray is zero we are not denying that there is an important sense in which one is earlier than the other, and in which one can be regarded as cause and the other as effect. This suggests that the zero interval is not quite so significant as it might seem to be, and I cannot therefore