From Clockwork to Crapshoot a history of physics From Clockwork to Crapshoot a history of physics Roger G. Newton The Belknap Press of Harvard University Press Cambridge, Massachusetts • London, England • 2007 Copyright © 2007 by the President and Fellows of Harvard College All rights reserved Printed in the United States of America Library of Congress Cataloging-in-Publication Data Newton, Roger G. From clockwork to crapshoot : a history of physics / Roger G. Newton p. cm. Includes bibliographical references. ISBN-13: 978–0–674–02337–6 (alk. paper) ISBN-10: 0–674–02337–4 (alk. paper) 1. Physics—History. I. Title. QC7.N398 2007 530.09—dc22 2007043583 To Ruth, Julie, Rachel, Paul, Lily, Eden, Isabella, Daniel, and Benjamin Preface This book is a survey of the history of physics, together with the as- sociated astronomy, mathematics, and chemistry, from the begin- nings of science to the present. I pay particular attention to the change from a deterministic view of nature to one dominated by probabilities, from viewing the universe as running like clockwork to seeing it as a crapshoot. Written for the general scientifically inter- ested reader rather than for professional scientists, the book presents, whenever needed, brief explanations of the scientific issues involved, biographical thumbnail sketches of the protagonists, and descrip- tions of the changing instruments that enabled scientists to discover ever new facts begging to be understood and to test their theories. As does any history of science, it runs the risk of overemphasizing the role of major innovators while ignoring what Thomas Kuhn called “normal science.”To recognize a new experimental or observa- tional fact as a discovery demanding an explanation by a new theory takes a community of knowledgeable and active participants, most of whom remain anonymous. The book is not a detailed history that judges the contributions of every one of the individuals involved in this enterprise, important as some of them may have been, nor does it trace the origin of every new concept to its ultimate source. More modest in scope at the historical micro-level, its focus is on the gen- eral development of ideas. viii Preface I have greatly benefited from conversations with my colleagues, especially Don Lichtenberg and Edward Grant, and from the re- sources of the Indiana University library, particularly from the help of Robert Noel, head of the Swain-Hall library. I also thank Holis Johnson, in our astronomy department, for his advice, and my wife, Ruth, for invaluable editorial assistance. Contents Prologue 1 1 Beginnings 4 2 The Greek Miracle 11 3 Science in the Middle Ages 41 4 The First Revolution 67 5 Newton’s Legacy 100 6 New Physics 121 7 Relativity 154 8 Statistical Physics 171 9 Probability 189 10 The Quantum Revolution 210 11 Fields, Nuclei, and Stars 248 12 The Properties of Matter 279 13 The Constituents of the Universe 290 Epilogue 308 Notes 313 Sources and Further Reading 316 Index 322 From Clockwork to Crapshoot a history of physics Prologue For well over six thousand years people have been assembling facts about their world, recording them when possible, and handing them down to their offspring. As this activity evolved into what is now called science, it became more than just a collection of disconnected facts. Important as these remained, they began to serve primarily to anchor a larger conception of the workings and structure of nature, a conception that forms one of the grandest achievements of the hu- man spirit. In the course of this long historical development, as more facts were discovered, concepts changed, sometimes radically. But certain large-scale themes persisted, informing the questions scien- tists and scientifically inclined philosophers asked and the kinds of answers they expected. One theme became clearly recognizable early on, and gradually separated the branch now called physics from the other parts of sci- ence: it was the ability to predict future events with some confidence of success. Whether dealing with the reliable functioning of instru- ments of agriculture, engineering, and war or with a description of the course of the awesome heavenly bodies, the goal of understand- ing was predictability and its causes. The Greek Leucippos articu- lated this purpose in the fifth century bce by declaring “Nothing 2 From Clockwork to Crapshoot happens without reason, everything has a cause and is the result of necessity.” Chinese Taoist writings expressed similar sentiments in the third century bce: “All phenomena have their causes. If one does not know these causes, although one may be right [about the facts], it is as if one knew nothing.”By the time of Aristotle, the explicit goal of physics (or “physiology,” as it was then called) was to understand causes and to use this understanding for prediction. Aristotle himself set down laws governing the movements of the heavenly bodies and rules determining the motions of objects on earth. Surviving the vicissitudes of history, including the fall of the Greek and Roman civilizations and the rise of Christianity and Islam, the effort to understand nature by discovering fundamental laws culmi- nated in the great scientific revolution led by Galileo Galilei and Isaac Newton. Aided by Newton’s new mathematics, determinism found its apex of expression at the end of the eighteenth century in Laplace, who foresaw a science to which “nothing would be uncertain and the future, as the past, would be present to its eyes.” However, since we could not possibly know enough to make such all-encompassing pre- dictions, we would have to make do with probabilities, and he pro- vided the needed mathematical tools. Before long, the pervading de- terminism was further eroded: the second half of the nineteenth century witnessed the direct introduction of the alien concept of probability into physics, and probability dominated basic science throughout the twentieth century. At the most fundamental level, chance took the place of necessity. But another equally important change accompanied the shift from causality to probability, as we shall see. With reliable prediction of future events at the submicroscopic level no longer within reach, the focus of explanatory theories moved away from their previous Aris- totelian purpose of postulating laws of motion to a rather more Pla- tonic goal of explaining structure. In a certain sense, How? questions were replaced by Why? Since the question “How does an electron move?” could not be answered, it was replaced by “Why does an elec- Prologue 3 tron exist?” and “Why does it have the mass it has?” Physics began to explain the detailed properties of matter, both in bulk and in the form of its constituents. The change in the explanatory aims of physics during the last half century goes even further. Physicists no longer consider it sufficient for physical understanding to point to a force with certain character- istics—or in more modern terminology, an interaction—as Newton had done for the solar system with the force of gravity. They believe that a unified and grand “final theory” will follow logically from a general abstract principle of symmetry, which they accept as an ax- iom, or else will simply arise because it is the only mathematically possible way for nature to be. If Galileo thought that mathematics was the language of nature, some physicists now go even further, be- lieving that the laws of nature should be mathematical theorems. The course of this development will be outlined in this book. one Beginnings When and where what we now call science started, nobody knows. But we do know that it slowly grew from humans’ awe at the heavens and from the arts of healing, hunting, construction, and war. Seeds of it appeared during the Stone Age, in the invention of such hunt- ing implements as the bow and arrow. While the early hunters surely did not try to understand the flight of the arrow, the arrow’s effec- tiveness depended on the predictability of its flight, which was taken for granted. Similarly, agriculture owed its development to experi- mentation and reliance on previously observed outcomes. Very early hints of what eventually grew into medical science appeared in Mes- opotamia—roughly the region now making up Iraq and Syria—and in Egypt, where circumcision was practiced as early as 4000 bce. Signs of its use have been found in bodies exhumed from prehistoric graves, and the operation is clearly depicted on the walls of a tomb of the sixth dynasty in Egypt, which ruled c. 2625–2475 (Fig. 1). Imhotep, the earliest physician known by name—an astronomer and an architect as well, later venerated as a god—lived about 3000 bce. Good archeological evidence also indicates that trepanation—cut- ting out disks from the skull—was performed on living people in prehistoric times. And some of them survived the procedure—we Beginnings 5 know this because living bone tends to heal itself, and new growth has been unambiguously identified on some of these skulls. Why and how this delicate operation was done is unknown. Interest in the heavens served less practical purposes and may therefore perhaps be regarded as the proper precursor of “pure sci- ence,” but that interest seems always to have been closely associated with religion. Individual stars were identified either with specific gods or with the homes of deities. Capricious as the gods were in general, the regular movements of their celestial images offered a re- assuring sign of order, while unusual events such as eclipses of the moon or the sun were frightening disruptions of that order. Anyone able to predict these unsettling phenomena was regarded as possess- ing extraordinary powers. Though it could be found in other places as well, an intense interest in the movements of the heavenly bodies is definitely known to have existed quite early in Egypt, as the devel- [To view this image, refer to the print version of this title.] Figure 1 A representation of circumcision with a stone knife, beginning of the Sixth Dynasty in Egypt.