In preparing a new edition, I have taken advantage of many suggestions made by reviewers and readers, for all of which I offer my thanks. Thus I hope that my presentation of the subject has gained in smoothness and lucidity. The scientific discoveries of the past year—especially those of the uncharged neutron and the apparently short-lived positive electron—have necessitated some restatement in matters of detail, but none in the main argument of the book.

We must now leave the vastness of astronomical space, to pass to the other extreme of the scale of size and explore the innermost recesses of the ultra-microscopic atom. While the phenomena of astronomy may shew us the nature of space and time, it is here, if anywhere, that we may hope to discover the true nature of matter and of material objects, the contents of space and time.

The Structure of Matter

We have seen how the atomic concept of matter gradually gained scientific recognition, and finally appeared to be securely established when Maxwell and others shewed that a gas could be pictured as consisting of hard bullet-like atoms or molecules flying about indiscriminately at speeds comparable with those of ordinary rifle bullets. The impact of these bullets produced the pressure of the gas; the energy of their motion was the heat-energy of the gas, so that heating up the gas resulted in its bullets travelling faster; the viscosity of a gas was caused by the drag of one bullet on another on the rare occasions on which actual collisions occurred, and so on. These concepts made it possible to explain a great number of the observed properties of gases, both qualitatively and quantitatively, with great exactness. Yet a residue obstinately defied explanation, and it is only recently that an explanation of these has been obtained, in terms of new and very different concepts to which we shall shortly pass.

Twentieth-Century Physics

A century which has run less than a third of its course has already witnessed two great upheavals in physical science. These are associated with the words Relativity and Quanta, and have forced the physicist of to-day to view nature against a background of ideas which is very different from that of his nineteenth-century predecessor.

The latter thought of nature as an assemblage of objects located in space and continually changing with the passage of time. It was something entirely detached from, and external to, himself; something which he could study and explore from a distance as the astronomer studies the surface of the sun through his telescope, or the explorer the desert from his aeroplane. He thought of the apparatus of his laboratory as the astronomer thinks of his telescope, or the explorer of his field-glass; it shewed him things which were there whether he looked at them or not, which had been there before the first man appeared on earth, and would still be there after the last man had been frozen to extinction. Finally he accepted a “common-sense” view of nature, believing that there was no great difference between appearance and reality; the possibility that things were not as they seemed might provide an admirable subject for a debating society of philosophers, but was of as little practical concern to the scientist as to the farm-labourer.

Although he may not have realised it, this complex of beliefs constituted a philosophical creed in itself.

We have already pictured the new-born child trying to correlate the events and objects which affect its senses, thereby taking its first steps towards becoming a scientist. Gradually it makes the discoveries which we express by saying that the events can be arranged in time, and that the objects in which they appear to originate can be arranged in space. Thus space and time form a sort of framework for the sense impressions which the child receives from the external world. The child does not of course concern itself with metaphysical questions as to the fundamental nature of space and time, and neither shall we here; only the simplest properties of space and time, as perceived by us, are relevant at the present stage of our discussion.

Rudimentary Views of Space and Time

The child finds that the events of its day come in simple sequence, like beads on a string. The string is what we call time, and the order of events relative to one another can be fully described by the words “earlier” and “later”. Adjacent events need not be contiguous; just as there may be stretches of a string which are not occupied by beads, so the child may experience uneventful periods of time. Time passes through our minds like tape through a chronograph; any small fragment of it may or may not have events impressed on it.

We have seen that our whole knowledge of the external world of physics may be pictured as arising from the impact of photons of energy either on our sense organs or on our physical instruments. As these photons occur in such profusion and variety, it might have been hoped that they would give us an almost perfect knowledge of the outer world.

Yet, as a means of acquiring knowledge, photons suffer from one very serious limitation. They are indivisible; no experiment has ever revealed a fraction of a photon or given any reason for supposing that energy can be either emitted or absorbed in fractions of photons. Thus the only means which are at our disposal for the study of physical nature suffer from a certain coarse-grainedness.

This is of little consequence as regards direct study by our senses, since these are even more coarse-grained. Each sense has its perceptions limited by a certain “threshold of sensation”, and if the stimulus of a physical effect falls below this, the sense in question registers nothing at all. We cannot experience the sweetness of a single molecule of sugar, nor the smell of a single molecule of musk; neither can we hear a bell at more than a certain limit of distance, nor see a star which is below a certain limit of faintness. In general we cannot experience a single photon; thousands at least are necessary to attain the threshold of sensation.

After undergoing a succession of kaleidoscopic changes, theoretical physics appears to have attained a state of comparative quiescence, in which there is fairly general agreement about essentials. In the following pages I have tried to depict the present situation in broad outline and in the simplest possible terms. I have drawn my picture against a roughly sketched background of rudimentary philosophy–the philosophy of a scientist, not of a metaphysician–because I believe, in common with most scientific workers, that without a background of this kind we can neither see our new knowledge as a consistent whole, nor appreciate its significance to the full. Statements made without reference to such a background–as, for instance, that “an electron consists of waves of probability” or that “the principle of indeterminacy shews that nature is not deterministic”–can convey at best only a minute fraction of the truth.

I have tried to exhibit the new knowledge in such a way that every reader can form his own judgment as to its philosophical implications. There is room for much legitimate difference of opinion as to what precisely these are; yet few, I think, will be found to doubt that some reorientation of scientific thought is called for.

Action at a Distance

Primitive man saw nature as a collection of objects which acted on one another, if at all, by direct contact; he was familiar with the pressure of wind and water on his body, the fall of raindrops on his skin, the thrust by an enemy, but action at a distance was somewhat of a rarity in his scheme of things.

Early science hardly advanced on this view, picturing matter as consisting of hard objects, no two of which could occupy the same space because one invariably pushed the other out of the way by direct contact. The science of a later era, however, found many instances of action at a distance. A magnet attracts iron filings to itself from a distance, and is itself acted on by the yet more distant magnetic poles of the earth; two electrified bodies attract or repel one another across the intervening space according as they are charged with opposite or similar kinds of electricity; the sun attracts the planets, and the earth the falling apple. In none of these cases can anything tangible be found to transmit the attractions and repulsions. It is true that the space between the interacting objects will often be occupied by air, but this does not transmit the action; electrified bodies and magnets attract rather more forcibly in a perfect vacuum than in air, while an apple falls more freely and rapidly when there is no air-resistance to break its fall.

Thermodynamics

The province of atomic physics is to discuss the nature of particular events, and it has been very successful in shewing us how it is that certain kinds of events occur, while others do not. Yet this can give us but little information as to what is happening to the universe as a whole. Another branch of physics, known as thermodynamics, takes this problem in hand; it does not concern itself with individual events separately, but studies events in crowds, statistically. Its province is to discuss the general trend of events, with a view to predicting how the universe as a whole will change with the passage of time.

The science of thermodynamics had its origin in severely practical problems relating to the efficiency of engines, but it was soon extended to cover the operations of nature as a whole. All this happened in the days when nature was assumed, without question, to be mechanical and deterministic. In what follows, we shall not treat nature as mechanical, but for the moment we shall treat it as though it were strictly deterministic.

On a deterministic view of nature, the universe never has any choice; its final state is inherent in its present state, just as this present state was inherent in its state at its creation. It must inevitably move along a single road to a predestined end, like a train rolling along a single-track line, on which there are no junctions of any kind.