This book was published posthumously in 1947. Sir James Jeans had seen and read the first proofs, but he died before the final touches could be added. Since publication a number of expert reviewers and readers have voluntarily supplied the publishers with notes of misprints in dates and names, and of places where the latest findings on matters dealt with had been overlooked. In the ordinary course Sir James Jeans would himself have revised his book in the light of these indications. As this was no longer possible the publishers have taken expert advice, and after collating the comments of several readers have put the preparation of the second edition into the hands of Mr P. J. Grant of the Cavendish Laboratory. They are grateful to all those who have helped in this way.

The three centuries we have just had under discussion formed a sort of intellectual ‘golden age’ in which science made more progress than in three millennia of Babylon and Egypt. But as this period approached its end, a change set in, and by the middle of the fourth century b.c., Greek culture had definitely begun to decline, and Greek science with it. A few years later, the decline was accelerated by the invasion and military conquest of the country by Alexander the Great. Yet events which seemed to be disastrous to science at the time may perhaps have been a piece of good fortune in disguise.

For Alexander now decided to celebrate his victories and consolidate his empire by building a new capital which was to be the most magnificent city in the world. He chose a site on the flat lands where the Nile ran into the sea, and called the still unborn city Alexandria, after himself.

He died in 323 b.c., his grandiose scheme still incomplete, and his kingdom was divided among all who could lay hands on a piece of it. Egypt fell to the lot of one of his generals, Ptolemy, who chose the still unfinished Alexandria as his capital and, more ambitious even than Alexander, aspired to make it the world's capital not only for government and commerce but for culture and intellect as well.

In the present chapter we examine the first three centuries of Greek scientific progress; our period begins with the earliest impact of oriental scientific ideas on Ionian Greece, and ends with the conquest of Greece by Alexander the Great (332 b.c.), the death of Aristotle (322 b.c.), a general decline of science and art in Greece, and the foundation of the City of Alexandria and of its university (323 b.c.), which was to be the intellectual centre of the world for many generations to come. In brief, we study Greek science in the period of Greece's intellectual greatness.

This science was almost entirely mathematical. The Greeks had nothing of our elaborate equipment of laboratories and observatories. Indeed, their equipment was limited to their own brains, but these were of the very best; just as Aeschylus and Sophocles exhibit mental powers comparable with those of Shakespeare, so Archimedes and Aristarchus exhibit powers comparable with those of Newton. Thus they could attack their various problems only by reflection and contemplation, aided at most by a minimum of observation, and when physics and astronomy creep in, it is in the form of philosophical speculation rather than of true science as we understand it to-day.

It will be convenient to discuss the early Greek mathematics, physics and astronomy separately, and in this order.

We now approach an era which, if not so strikingly brilliant as its great predecessor, was at least one of solid and steady progress. It produced no second Newton, but provided an abundance of first-class investigators. The studious and talented amateur could still accomplish scientific work of the highest value, for a single mind could carry a good working knowledge of a substantial part of science; the days of immense stacks of literature and teams of experts, each understanding only one corner of a subject of research, had not yet arrived. They were, however, on the way, for there was already a tendency for the various sciences to unite into one, to lose their identity as detached units, and become merged into one single field of knowledge which was too vast for anyone to comprehend the whole, or even a large fraction of it. We note the appearance of such words as thermodynamics, astrophysics and electrochemistry.

MECHANICS

Mechanics looms large in the story of eighteenth-century progress. Galileo and Newton had opened the road, but a lot remained to be done in extending their gains and filling up lacunae. Newton's laws of motion were applicable only to particles, i.e. to pieces of matter which were small enough to be treated as points, and so could have definite positions, velocities and accelerations unambiguously assigned to them.

There are a vast number of detailed and comprehensive histories, both of general science and of special departments of science. Most of these are admirable for the scientific reader, but the layman sometimes cannot see the wood for the trees. I have felt no ambition (nor competence) to add to their number, but have thought I might usefully try to describe the main lines of advance of physical science, including astronomy and mathematics but excluding all points and side-issues, in language non-technical enough to be understood by readers who have no scientific attainments or knowledge.

I hope that such a book may prove of interest to the general educated reader, perhaps also to those who are beginning the study of physics, and possibly to students of other subjects who wish to know something of how physical science has grown, what it has done, and what it can do.

The two centuries from 1687 to 1887 may appropriately be described as the mechanical age of physics. Science seemed to have found that we lived in a mechanical world, a world of particles which moved as the forces from other particles made them move, a world in which the future is completely determined by the past. In 1687 Newton's Principia had interpreted the astronomical universe very successfully in this way. Before 1887 Maxwell had interpreted radiation in an essentially similar way, teaching that it consisted of disturbances travelling through an ether under the direction of mechanical laws. Finally, in 1887 Hertz produced radiation of Maxwellian type from electric sources in the laboratory, and demonstrated its similarity to ordinary light. This seemed to fit a final keystone into the structure which had been built up in the preceding two centuries.

Most physicists now thought of this structure as standing foursquare, complete and unshakable. It was hard to imagine the physicists of the future finding any more exciting occupation than dotting the i's and crossing the t's of the mechanical explanation of the universe, and carrying the measurement of physical quantities to further decimal places.

Little did anyone imagine how completely different the actual course of events would be. Yet the year 1887, which had provided a keystone to the structure, also saw the structure begin visibly to totter; it was the year of the famous Michelson-Morley experiment, which first showed that there was something wrong with the foundations.

Here and there, in the history of human thought and action, we find periods to which the epithet ‘great’ may properly be applied—in Greece the fourth century before Christ; in England the Elizabethan age; in the domain of science the seventeenth century, the ‘century of genius’, to which we now come.

It would be very undiscerning to suppose that such a period of greatness could arrive as a mere accident, a specially brilliant galaxy of exceptional minds just happening to be born at one particular epoch. Mental ability is believed to be transmitted in accordance with the laws of heredity, in which case the laws of probability will see to it that no abrupt jump occurs from one generation to the next. Thus a period of greatness must be attributed to environment rather than to accident; if an age shows one particular form of greatness, external conditions must have encouraged that form. For instance, the sixteenth century was an age of great explorers because conditions then specially favoured exploration; the pioneering voyages of Columbus, Vasco da Gama, Cabot, Magellan and others had drawn attention to the wealth of new territory awaiting discovery, while men had learned to build ships which could defy the worst fury of the ocean.

Science usually advances by a succession of small steps, through a fog in which even the most keen-sighted explorer can seldom see more than a few paces ahead. Occasionally the fog lifts, an eminence is gained, and a wider stretch of territory can be surveyed—sometimes with startling results. A whole science may then seem to undergo a kaleidoscopic rearrangement, fragments of knowledge being found to fit together in a hitherto unsuspected manner. Sometimes the shock of readjustment may spread to other sciences; sometimes it may divert the whole current of human thought.

Events of this last kind are rare, but instances come readily to mind. We are likely to think first of the results of replacing the geocentric astronomy of mediaeval times by the Copernican system—man saw that his home was not the majestic fixed centre of the universe round which all else had to revolve, but one of many fragments of matter which were themselves revolving round a very ordinary one of the myriads of stars in the sky. Or we may think of the implications of the Darwinian biology—man saw that his body had not been specially designed for himself, the lord of creation, but was an adaptation and development of the bodies of animals which had preceded him on earth, and were in fact his own ancestry; all terrestrial creatures, even the meanest, proved to be his bloodrelations, and if he had dominion over them it was only because he happened to have been born into the clever branch of the big family.

We have now concluded our summary of the findings of modern physics, and may turn to consider how these findings affect the practical problems of philosophy and of everyday life. But let us first recapitulate the conclusions we have reached in our scientific discussion.

Recapitulation

Because we are human beings and not mere animals, we try to discover as much as we can about the world in which our lives are cast. We have seen that there is only one method of gaining such knowledge—the method of science, which consists in a direct questioning of nature by observation and experiment.

The first thing we learn from such questioning is that the world is rational; its happenings are not determined by caprice but by law. There exists what we have called a ‘pattern of events’, and the primary aim of physical science is the discovery of this pattern. This, as we have seen, will be capable of description only in mathematical terms.

The new quantum theory explained in the preceding chapter has provided a mathematical description of the pattern of events which is believed to be complete and perfect. For it enables us—in principle at least—to predict every possible phenomenon of physics, and not one of its predictions has so far proved to be wrong. In a sense, then, we might say that theoretical physics has achieved the main purpose of its being, and that nothing remains but to work out the details.

The new physics just described was still based largely on Newtonian ideas. Indeed, in its theoretical aspects, it might not unfairly be described as a final attempt to explain the world in materialistic terms—as particles being pushed and pulled about in space and time. Nevertheless, the new physics had found it necessary to abolish most of the forces of pushing and pulling, replacing the gradual changes of motion of the particles under these forces by sudden and unpredictable jumps. These appeared to involve violations of the law of causality, both in the disintegration of radioactive atoms and also in the internal changes of ordinary atoms. We seemed to see Fate defying this law as she picked out certain atoms for disintegration or collapse and, by her apparently capricious acts, sent the universe along one path or another according to her whim.

On such lines the new physics had explained many phenomena which had hitherto seemed inexplicable, but it had by no means met with complete success. For instance, while it gave a perfect interpretation of the simplest spectrum of all, namely that of the hydrogen atom, it failed with more complex spectra. This was not necessarily a fatal objection; a few emendations and possibly a few new ad hoc assumptions might have effected a complete reconciliation, although this seems improbable.

The aim of the present book is very simply stated; it is to discuss—and to some extent to explore—that borderland territory between physics and philosophy which used to seem so dull, but suddenly became so interesting and important through recent developments of theoretical physics.

The new interest extends far beyond the technical problems of physics and philosophy to questions which touch human life very closely, such as materialism and free-will. Thus I hope the book may interest many who are neither physicists nor philosophers by profession, and to this end I have made the discussion as simple as possible, avoiding technicalities when I could, and, when I could not, explaining them. I have also tried to arrange the book so that a reading of the first two chapters and the last shall give an intelligible view of the main argument and conclusions of the whole; many readers may prefer to read these three chapters first.

I need hardly add that my acquaintance with philosophy is simply that of an intruder, and nothing could be further from my intentions than to pose as an authority on questions of pure philosophy. If I had to choose a sub-title for my book, it might well be ‘The reflections of a physicist on some of the problems of philosophy”.

Pre-Newtonian Mechanics

The earliest attempts to discover the pattern of events were limited, naturally enough, to the visible movements of objects either on what we have called the man-sized scale or on the far grander scale of astronomy—these were the only movements which could be studied without instrumental aid.

The movements of the astronomical bodies were treated only in their geometrical aspect. The ‘fixed stars’ hardly came under discussion at all, since they appeared to have no motion beyond their diurnal rotation round the pole. This was of course a consequence of their great distance from the earth, but it was explained by supposing them to be immovably attached to a sphere which rotated round the earth as centre.

There remained the sun, moon, and planets. A whole succession of astronomers—from Aristarchus through Ptolemy to Copernicus and Kepler—had investigated the paths in which these bodies moved, but had shown very little concern as to why they moved in these particular paths rather than in others. Aristotle's pronouncement that a circular motion was natural to all bodies, because the circle was the perfect geometrical figure, seems to have stifled curiosity fairly thoroughly for nearly two thousand years; it was uncritically accepted by Copernicus, and even at one time by Galileo.

It was different with terrestrial bodies; there had been many attempts to explain their movements in what we should now describe as dynamical terms.

Preliminary Survey

With the coming of the twentieth century, there came into being a new physics which was especially concerned with phenomena on the atomic and sub-atomic scale. It brought with it a new way of interpreting the phenomena of inanimate nature, which was destined in time to sweep away all the difficulties besetting the old classical mechanics. A preliminary glance over the vast territory of this new physics reveals three outstanding landmarks.

First we notice an investigation which Prof. Planck of Berlin published in 1899. His aim was so to amend the classical mechanics that it should fit the observed facts of radiation, and show why the energy of bodies was not wholly transformed into radiation. We have already seen that this was likely to involve giving up either continuity or causality or the representation of phenomena as changes taking place in space and time. Actually his investigation seemed to show that continuity had to be given up, suggesting that in the last resort changes in the universe do not consist of continuous motions in space and time, but are in some way discontinuous.

The classical mechanics had envisaged a world constructed of matter and radiation, the matter consisting of atoms and the radiation of waves. Planck's theory called for an atomicity of radiation similar to that which was so well established for matter.

THE SOURCES OF KNOWLEDGE

We have already noticed how knowledge is gained by establishing relations between an inner process of understanding in our private minds and the facts of that public outer world which is common to us all. As Plato pointed out, the use of a common language is based on the supposition that such relations can be established by all of us.

In the period we have been considering, science claimed only one source of knowledge of the facts and objects of the outer world, namely the impressions they make on the mind through the medium of the senses. Yet the untrustworthiness of the senses had been one of the commonplaces of philosophy from Greek times on, and if the same facts and objects of the outer world made different impressions on different minds, where did science stand? If we trusted to individual sense-impressions, we could never get beyond the position described by Protagoras (c. 481—411 B.C.): ‘What seems to me is so to me, what seems to you is so to you’; each individual would become his own final arbiter of truth, and there could be no body of objective knowledge. Six centuries before Christ, in the earliest days of Greek philosophy, Thales of Miletus had urged the importance of gaining a substratum of facts, independent of the judgment of individuals, on which a body of objective knowledge could be built.