NEW PATHWAYS IN SCIENCE by SIR ARTHUR EDDINGTON M. A., D. Sc., LL. D., F. R. S. Plumian Professor of Astronomy and Experimental Philosophy in the University of Cambridge MESSENGER LECTURES 1934 CAMBRIDGE AT THE UNIVERSITY PRESS 1935 PRINTED IN GREAT BRITAIN CONTENTS Preface page vi Chapter I Science and Experience I II Dramatis Personae 27 Til The End of the World 50 IV The DeclinaofT terminism 72 V Indeterminacy and Quantum Theory 92 VI Probability no VII The Constitution of the Stars 135 VIII Subatomic Energy 160 IX Cosmic Clouds and Nebulae 184 X The Expanding Universe 206 XI The Constants of Nature 229 XII The Theory of Groups 255 XIII Criticisms and Controversies 278 XIV Epilogue 309 Index 327 PLATES Plate i Electrons and Positrons Facing page 28 By permission of Prof. P. M. S. Blackett 2 Gaseous Nebula Cygnus 184 3 Dark Nebulosity The Horses Head 202 4 Spiral Nebula Canes Venatici 206 PREFACE THIS volume contains the Messenger Lectures which I delivered at Cornell University in April and May 1934. Chapters n and vin 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 Vlll PREFACE I could not bring myself to go over again the ground covered in Chapters I, n, m, vi, vn of The Nature of the 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 modern 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 astrophysics. 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...