New pathways in science

NEW ,PATHWAYS. IN SCIENCE by

SIR ARTHUR EDDINGTON

MESSENGER LECTURES 1934

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CAMBRIDGE AT THE UNIVERSITY PRESS

1947

Printed in Great Britain at the University Press, Cambridge (Brooke Crutchley, University Printer) and published by the Cambridge University Press Cambridge, and Bentley House, London Agents for Canada and India: Macmillan Copyrighted in the United States of America by The Macmillan Company

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CONTENTS

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Preface Chapter I Science and Experience IT ill IV V VI

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Dramatis Personae The End of the World The Declinb· of Determinism ~determinacy and Q!!antum Theory ,

Probability VII The Constitution of.the Stars VIII Subat~)1nic Energy

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IX Cosmic Clouds and Nebulae X The Expanding Universe XI The Constant~' of Nature

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XII The Theory of Groups XIII Criticisms and Controversies XIV Epilogue .

Index Plate

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27 50 72 92

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135 160 184 206 229 255 278 309 327

PLATES Electrons and Positrons

Facing page 28 By permission of Prof. P. M. S. Blackett r84

Gaseous Nebula (Cygnus) 3 Dark Nepulosity-The I;Iorse's Heacl

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4 Spiral Nebula (Canes Venatici)

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PREFACE THIS volume contains the Messenger Lectures which I delivere,d at Cornell University in April and May 1934. Chapters II and VIII have been added; the remaining chapters correspond to the twelve lectures of the course. It was one of the conditions of the lectureship that the lectures should be published. Except for a small book on the Expanding Universe, my last spell of writing was about six years ago, when Stars and Atoms (1927), The Nature of the Physical World (1928) and Science and the Unseen World (1929) practically exhausted all that it was then in my mind to say. A scientific writer is placed in a difficulty by his earlier books; either his new book will appear as a rather disjointed addendum to them, or he must perfunctorily go over again a great deal of matter which he has no wish to rewrite. Being unwilling to adopt the second alternative, I determined to make what I could of whatever had come to my mind in the last six years. Accordingly I spoke at Cornell on a variety of topics, using as a nucleus the material contained in a number of addresses and lectures which I had had occasion to deliver since 1929, and adding other subjects to which I had been giving attention. The general plan was that each lecture should have a separate theme, except that Indeterminism was spread over two lectures. The choice of subjects has allowed a certain amount of continuity of treatment; but there has been no attempt to provide a systematic introduction to modern scientific thought. Perhaps the biggest gap is the absence of any account of the elementary ideas of the theory of relativity;

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r could not bring myself to go over again the ground covered in Chapters I, IT, III, VI, VII of The Nature ofthe Physical World altering the treatment and. illustrations merely for the sake of alteration. In the opening lecture I try to explain the philosophical outlook of modem science, as I understand it, and show how the scientific picture of the world described in physics 'is related to the "familiar story" in our minds. Chapter n is an interpolation containing a summary of our knowledge of atomic physics, etc., which some readers may find necessary for an understanding of subsequent chapters and others may find useful as a reminder. Then follow four lectures which have something in common; they are .concerned with the consequences of the statistical type of law, first introduced into physics in the subject of thermodynamics, which has in recent years completely driven out the older causal type of law from the foundations of physics. The last of these four lectures, on Probability, has besides its application to statistical law a more elementary interest. Then follows a complete change of subject, and the next four lectures are devoted to a~trophysics. Starting with the sun and familiar stars, we advance to greater distances till we reach the system of milliards of galaxies which constitutes the universe. This last subject has been treated more fully in my recent book The Expanding Universe; I here give a much shorter account. In this lecture (Chapter x) we meet the elusive "cosmical constant" which takes us back to the fundamental conceptions of physics again for the "llext two chapters. Chapter XI is, I realise, much too severe for this kind of book; I'can only plead that the subject which has occupied me for the last five years, almost to the exclusion of any oilier research, was bound to spill over into any course .

PREFACE

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of lectures I might give. The next lecture, on Theory of Groups. was something of an experiment; but it, more nearly than any other part of the book, touches the key-note of scientific philosophy. The chapter "Criticisms and Controversies" may by its title lead the reader to expect a comprehensive series of answers to the multitudinous points raised by critics and reviewers, and by many who have contributed valuable discussion of the views which I have advocated. I think: that a little reflection will show that this was impracticable with any reasonable allotment of space. If a criticism can be answered briefly and decisively it seems scarcely worth while to inform the world in general that so-and-so has raised it. If it is more arguable, a lengthy explanation and discussion. of it is usually necessary. For the most part I am content to think that if my contentions are of value they will :find their proper level without continual parental intervention to save them from determined opponents-and sometimes from over-enthusiastic friends. But I would express here my gratitude for many articles by philosophers and others courteously discussing my writings. Sometimes I have appreciated the justice of the criticism. and it has had its due influence in maturing my views. Often I would have liked to write a reply in the hope of advancing an understanding on both sides; but such a reply requires at least as much time and care as an independent article, and with rare exceptions I have had to let, the opportunity go by. In the concluding lecture I return again to the philosophical outlook of Chapter I, but this time I refer to that part of" the problem of experience" which the methods of physics do not profess to treat. Parts of this lecture are taken from an address which I gave in a broadcast symposium on Science and Religion.

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PREFACE

As usual, notwithstanding my efforts to simplify things, , I have to impose a rather heavy strain 011 the attention of the reader. Since the chapters are to a considerable extent independent, the difficulty tends to increase towards the ends of the chapters. There is hope of a respite when the next chapter begins. These lectures carry for me happy memories of the weeks which I spent in Cornell University. To the friends who welcomed me, and to the large audiences who encouraged me, I dedicate them gratefully. A. S. E. CAMBRIDGE

September I934

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CHAPTER I

SCIENCE AND EXPERIENCE Does the harmony which human intelligence thinks it discovers in Nature ~xist apart from such intelligence? Assuredly no. A reality completely mdependent of the spirit that conceives it, sees it or feels it, is an impossibility. A world so external as that, even if it existed, would be for ever inaccessible to us. What we call "objective reality" is, strictly speaking, that which is common to several thinking beings and might be common to ali; this common pare, we shall see, can only be the harmony expressed by mathematical laws. POINCARB, The Value of Science.

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As a conscious being I am involved in a story. The perceiving part of my mind tells me a story of a world around me. The story tells of familiar objects. \ It tells of colours, sounds, scents belonging to these objects; of boundless space in which they have their existence, and ofan ever-rolling stream of time \ bringing change and incident. \ It tells of other life than mine busy about its oV(l1 purposes. ' As a scientist I have become mistrustful of this story. In many instances it has become clear that things are not what they seem to be. According to the story teller I have now in front of me'a substantial desk; but I have learned frQm physics that the desk is not at all the continuous substance that it is supposed to be in the story. It is a host of tiny ~lectric charges darting hither and thither with inconceivable velocity. Instead ofbeing solid substance my desk is more like a swarm of gnats. So I have come to realise that I must not put overmuch confidence in the story teller who lives in my mind. On the other hand, it would not do to ignore him altogether, since his story generally h~,s some foundation of truth more ENPS

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NEW PATHWAYS IN SCIENCE

especi~y in those anecdotes that concern me intimately. For I am given a part in the story, and if I do not take my

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cue with the other actors it is the worse for me. For example, there suddenly enters into the story a motor car coming'__ rapidly towards the actor identified with mysel£ As a scientist I cavil at many of the particulars given by the story teller-the substantiality, the .colour, the rapidly increasing size of the object approaching-but I accept his suggestion that it is wisest to jump out of the way. There are ponderous treatises on my shelves which tell another story of the world around me. We call this the scientific story. One of our first tasks must be to try to understand the relation between the familiar story and the scientific story of what is happening around us. At one time there was no very profound difference between the two versions. The scientist accepted the familiar story in its main outline; only he corrected a few facts here and there, and elaborated a few details. But latterly the familiar story and the scientific story have diverged more and more widely -.until it has become hard to recognise that they have anything in common. N ot ~ontent with upsetting fundamentally our ideas of material substance, physics has played strange pranks with our conceptions of space and time. Even causality has undergone transformation. Physical science now deliberately aims at presenting a new version of the story of our experience from the very beginning, rejecting the familiar story as too erratic a foundation. But although we try to make a clean ~tart, rejecting instinctive or traditional interpretations of experience and accepting only the kind of knowledge which can be inferred by strictly scientific methods, we cannot cut ourselves loose altogether from the familiar story teller. We lay down'the principle that he is always to be mistrusted; but we 'cannot do without him in science. What I mean is this: we rig up some delicate physical experiment 'with galvanom~ters,

SCIENCE AND EXPERIENCE

3 , micrometers, etc., specially designed to eliminate the fallibility of human perceptions; but in the end we must trust to our perceptions to tell us the result of the experiment. Even if the apparatus, is self-recor~g we employ our senses to read the records. So, having set the experiment going; we turn to the familiar story teller and say "Now put that into your story". He has perhaps just been telling us that the moon is about the size of a dinner plate~ or something equally crude and unscientific; but at our interruption he breaks off to inform us that there is a spot of light coinciding with division No. 53 on the scale of our galvanometer. And this time we . believe him-more or less. At any rate we use this information as the basis of our scientific conclusions. If we are to begin actually at the beginning we must inquire why we trust the story teller's information about galvanometers in spite of his general untrustworthiness. For presumably his tertile invention is quite capable of" embroidering)1 even a galvanometer. I do not want to spend time over points which no scienti:' fically-:-minded person disputes; so I will assume that you agree that the only channel of communication between the story teller who lives in your mind and the external world which his story professes to describe is the nervous system in your body. In so far as your familiar conception or picture of what is goum on around you is founded on your sense of sight, it depends on impulses transmitted along the optic nerves which connect the retina with the brain. Similarly for your other sense organs. You do not, of course, perceive the impulses themselves; the story teller has worked them up into a vivid story. The inside of your head must be rather like a newspaper office. It is connected with the outside world by nerves which play the part of telegraph wires. Messages from the outside world arrive in code along these wires; the whole substratum of fact is contained in these code messages. ,Within the office they are made up into a pre1-2.

NEW PATHWAYS IN SCIENCE 4 sentable story, pardy by legitimate use of accumulated. experience but also with an admixture of journalistic imagination; and it is this free translation of the original messages that our consciousness becomes aware of. If we had a complete record of the impulses transmitted along the nerves we should have all the material which the story teller can have had as a foundation for his story-in so far as his story relates to the external world. And it is to this material that we must appeal if we wish to discover the truth behind the story. To appreciate the task of physical science let us then suppose that we are in possession of these datathe dots and dashes, or whatever the signals are, that arrive at the brain cells at the terminations of the nerves. All that physical science can assert about the external world must be inferable from these. If there is any part of our conception of the physical universe which cannot have come to us in the form ofnerve signals we must cut it out. As in a beleaguered city there spread circumstantial rumours of happenings in the world outside which cannot have been received from without, so in our minds there arise all sorts of conceptions of entities and phenomena in the ~xternal world .which cannot have been transmitted to us from outside. They do not conform to the type of message which the narrow threads of communication will bear. Weare continually making the mistake of the man who, on receiving a telegram, thinks that the handwriting is that of the sender. The nlessages as we become aware of them in consciousness are dressed up with conceptions of colour, spatiousness, substance. This dress is no part of the message as it was handed in by the ' external universe. It is assumed after the message arrives; for the transmitting mechanism is by its very nature incapable of conveying such forms of conception. . This limitation of transmitting mechanism is strikingly illustrated when we talk with a colour-blind person. We know fronl his amazing mistakes that there is a big difference

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5 between his perception of his surroundings and ours. But he is quite unable to convey to us how his perception differs. When he confuses red with green, does he see both colours as red or both as green or as some hue unknown to us? He has no means of telling us. The intrinsic nature of his perception is trapped in his mind. It cannot flow out along his n~rves; nor could it travel up