Discovering the Natural Laws: The Experimental Basis of Physics PDF
Preview Discovering the Natural Laws: The Experimental Basis of Physics
Discovering the Natural Laws The Experimental Basis of Physics by Milton A. Rothman Illustrations by the Author DOVER PUBLICATIONS, INC., New York Chapter 2 of this book is an expanded version of an article that first appeared in The Physics Teacher, Copyright © 1970 American Associa tion of Physics Teachers. Copyright © 1972 by Doubleday & Company, Inc. New material copyright © 1989 by Milton A. Rothman. All rights reserved under Pan American and International Copyright Conventions. Published in Canada by General Publishing Company, Ltd., 30 Lesmill Road, Don Mills, Toronto, Ontario. Published in the United Kingdom by Constable and Company, Ltd., 10 Orange Street, London WC2H 7EG. This Dover edition, first published in 1989, is an unabridged, enlarged republication of the work first published by Doubleday & Company, Inc., Garden City, New York, 1972, in its “Science Study Series.” For the Dover edition, the author has written a new chapter (Chapter 10), added references to that chapter, and prepared a separate index to that chapter. Manufactured in the United States of America Dover Publications, Inc., 31 East 2nd Street, Mineola, N.Y. 11501 Library of Congress Cataloging-in-Publication Data Rothman, Milton A. Discovering the natural laws : the experimental basis of physics / by Milton A. Rothman ; illustrations by the author, p. cm. Reprint, with new ch. 10. Originally published: 1st ed. Garden City, N.Y. : Doubleday, 1972. Includes bibliographical references. ISBN 0-486-26178-6 1. Physical measurements. 2. Physics—Experiments. I. Title. QC39.R68 1989 530—dc20 89-17055 CIP ACKNOWLEDGMENTS Chapter Zy in a more condensed version, has previously ap peared in The Physics Teacher. I would like to thank Dr. Clifford Swartz, Editor of that journal, for permission to use that material. I would also like to give thanks to the hbraries of Trenton State College and Princeton University for their hospitality, to Miss Nancy Bemarkt for much manuscript typing, to Mr. Gerald Nicholls for many discussions, and above all to my family for their patience and forbearance, a necessary in gredient of every book. Contents Introduction ix 1 How Do We Know What We Know? i 2 Definitions, Experiments, and the Laws of Motion 13 3 The Four Forces of the Physicist 48 4 The Gravitational Force 62 5 The Conservation Laws: Historical Background 83 6 The Conservation Laws: Modem Experiments 101 7 The Principle of Relativity 131 8 Electromagnetism 160 9 What Is Forbidden? 183 10 Epilogue (1989) 201 Appendix I Wavelengths, Frequencies, and Energy 215 Appendix II Energy and Momentum Relationships for Moving Particles 218 Appendix III The Michelson-Morley and Kennedy- Thomdike Experiments 223 References 231 Index 237 Index to Chapter 10 242 Introduction Every book should have a justification. Why should I add another volume to the hundreds of works already devoted to the fundamental laws of physics? The reason became clear to me when—after a considerable amount of writing and teaching about the laws of nature—I suddenly realized that I really did not know how accurately the law of conservation of energy had been verified, and I did not know what were the latest and most accurate experiments testing the validity of this law. This lack of knowledge on my part led me to suspect that many other professional physicists were in a similar state of ignorance. If that is the case, pity the poor science teacher who tries to justify to his students a belief in the absolute truth of the great laws of nature. Either he must present the laws in an authoritarian fashion as being handed down from on high, or else he searches through the textbooks to find some historical experiments by Galileo or Newton or Joule. From these imperfect experiments he must extrapolate to a perfect belief in a perfect law. The confusion has been compounded by the long contro versy concerning the mental processes that take place during the act of inductive inference. That is, how does a scientist arrive at a law such as conservation of energy when the ex periments on which the law is based are of less than perfect accuracy and precision? The question is supposedly answered by discussions of how we reach general conclusions after making a sufficient number of specific observations. The im IX Discovering the Natural Laws plication made by many scientists and philosophers is that somehow we do have knowledge that energy is perfectly conserved. But if we look with a realistic and critical eye at what physicists actually do, we suddenly realize that as a matter of fact we do not have the knowledge that energy is perfectly conserved. We may think we do, but that is an other story entirely. And if, in fact, the law is perfectly true, we have no way of knowing it. In this matter of understanding the logic of validation, the psychologists have been ahead of the physicists. When a psychologist performs an experiment, he knows that his results are going to be imperfect, and he will couch his con clusions in a manner somewhat like this; "Conservation of energy is a hypothesis that has been verified at a o.ooi level of significance.” Which means that there is a o.i percent chance that the hypothesis is false. There have been times when some theoretical physicists tried to lead us into perfect belief by arguments based on the concept of symmetry. We know that each conservation law is associated with a certain symmetry of space and time. For example, momentum is conserved if the properties of space do not change as you move from one place to another. Angular momentum is conserved if the properties of space do not change as you rotate from one position to another. Irhe parity of a system is a property that has to do with the symmetry of the system on both sides of a center line. When a system obeys the law of conservation of parity, we mean that if this system is refiected in a mirror it is impossible to distinguish the mirror image from the “real” system. More technically, if the law of conservation of parity holds true in a given physical reaction, then the description of that reac tion is completely unchanged if the directions “right” and “left” are interchanged in that description. The type of argument that used to be stylish during the thirties claimed that since nature hked symmetry (as well as simplicity and elegance) it followed that these conserva tion laws had to be absolutely true. Such arguments met Introduction their downfall in 1957 when the Nobel Prize winners T. D. Lee and C. N. Yang cast a cold eye on the experimental evi dence and pointed out that in certain types of elementary- particle interactions involving the weak nuclear force, parity was not conserved. This meant that conservation of parity was not a universal law, and that it was possible to distin guish between left and right in an absolute way.^ Clearly, it is not possible for nature to like or dislike any thing, It is people who like symmetry, and we obtain our knowledge of symmetry in natiure from observations that show certain quantities (momentum, energy, etc.) to be conserved. We do not obtain knowledge of the conservation laws from an a priori belief in symmetry. It is very impor tant to keep this distinction in mind. Since experimental evidence is the foundation of omr knowledge of the laws of nature, it appears that there is a need for a compilation of the best evidence supporting these laws. This book is an attempt to describe a number of the experiments dealing with the most important of the laws. It is by no means a complete and exhaustive work; such a proj ect would be encyclopedic. Although the experimental data are the raw materials for our theories, such data cannot be understood without a cer tain amoimt of earKer theory. A complex feedback spiral in volving both theory and experiment is necessary in order to build up a structure of knowledge, approaching but never reaching completeness. For this reason I include some dis cussion of the philosophy behind the verification of scientific laws. In this area I acknowledge the strong influence of Karl R. Popper, whose ideas have greatly clarified for me the psychology of scientific discovery.^ In the final chapters of this book I have attempted to show how knowledge of the fundamental laws may be applied to a critical examination of certain prevalent beliefs. You might find it odd that I include topics such as ESP and astrology, but it is my opinion that there is no dividing line between science and everyday life—if the laws of nature work in the XI
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