Revolutions in Twentieth-Century Physics

Transcription

1 Revolutions in Twentieth-Century Physics The conceptual changes wrought by modern physics are radical, important, and fascinating, yet they are only vaguely understood by people working outside the field. Exploring the four pillars of modern physics relativity, quantum mechanics, elementary particles, and cosmology this clear and lively account, that will interest anyone who has wondered what Einstein, Schrödinger, Feynman, Hubble, and others were really talking about. The book discusses quarks and leptons, antiparticles and Feynman diagrams, curved spacetime, the Big Bang and the expanding Universe. Suitable for undergraduate students on nonscience majors as well as science majors, it uses problems and worked examples to help readers develop an understanding of what recent advances in physics actually mean. david j. griffiths is an Emeritus Professor of Physics at Reed College. He is the author of three highly regarded physics textbooks: Introduction to Electrodynamics (fourth edition, Pearson, 2012), Introduction to Quantum Mechanics (second edition, Pearson, 2004), and Introduction to Elementary Particles (second edition, Wiley-VCH, 2008).

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3 Revolutions in Twentieth-Century Physics DAVID J. GRIFFITHS Reed College

4 University Printing House, Cambridg eicb2i8bs,iu nited Kingdom Cambridge University Press is part of the University of Cambridge. It furthers the University s mission by disseminating knowledge in the pursuit of education, learning and research at the highest international levels of excellence. Information on this title: / C D. Griffiths 2013 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2013 Reprinted 2015 Printed in the United States of America by Sheridan Books, Inc. A catalogue record for this publication is available from the British Library Library of Congress Cataloguing in Publication data Griffiths, David J. (David Jeffery), 1942 Revolutions in twentieth-century physics / David Griffiths, Reed College. pages cm Includes index. ISBN (pbk.) 1. Quantum theory Popular works. 2. General relativity (Physics) Popular works. I. Title. QC G dc ISBN Paperback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

5 Contents Preface page ix 1 Introduction: classical foundations Preliminaries Units Scientific notation Significant digits Mechanics Kinematics Dynamics Forces Newton s law of universal gravitation Planetary motion Coulomb s law Conservation laws Momentum Work and power Energy Conservation of energy Gravitational potential energy Waves Velocity, wavelength, and frequency Interference Standing waves 36 2 Special relativity Einstein s postulates The principle of relativity 40 v

6 vi Contents The universal speed of light Implications The relativity of simultaneity Time dilation Lorentz contraction Paradoxes The paradox of Lorentz contraction The paradox of time dilation The barn and ladder paradox The twin paradox Relativistic mechanics Mass and momentum Energy Massless particles The structure of spacetime 64 3 Quantum mechanics Photons Planck s formula The photoelectric effect The Compton effect de Broglie s hypothesis The Bohr model Allowed energies Quantum mechanics Wave particle duality The wave function Born s statistical interpretation Indeterminacy Uncertainty Tunneling What s so funny about quantum mechanics? Three schools of thought The EPR paradox Bell s theorem Nonlocality Schrödinger s cat 95 4 Elementary particles The early period ( ) Electrons, protons, and neutrons 99

7 Contents vii Atoms Nuclei The middle ages ( ) Neutrinos ( ) Mesons ( ) Strange particles ( ) The modern era ( ) The Eightfold Way (1961) The quark model (1964) The November revolution (1974) The Standard Model (1978) Interactions Electrodynamics Chromodynamics Weak interactions Conservation laws Unification Cosmology Expansion of the Universe Stars and galaxies The cosmological redshift Hubble s law The Big Bang The Cosmic Microwave Background Blackbody radiation Penzias and Wilson The origins of matter Light elements Heavy elements Stars and galaxies Remaining mysteries Dark matter The shape of the Universe The future Dark energy 163 Index 166

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9 Preface The twentieth century saw four astonishing revolutions in physics: relativity, quantum mechanics, elementary particles, and cosmology. Each one radically changed our understanding of the Universe. There were also, of course, extraordinary breakthroughs in technology (electronics, lasers, computers) that had a much larger influence on our daily lives, but did not carry the same conceptual impact. This is a book about those four revolutions. It is intended for anyone with a serious interest in the great ideas that have shaped modern physics: advanced high school students or freshman physics majors who would like a taste of what lies ahead; undergraduates who do not intend to major in the sciences but are curious to know about some of the most profound intellectual achievements of our time; general readers who have heard about quarks and quanta, Albert Einstein and the Big Bang, and would like to know what all the fuss is about. I should tell you up-front what this book is not. It is not another breathless account of the fantastic speculations that seem to dominate much of contemporary theoretical physics things you may have read about, or seen on NOVA. Apart from a few footnotes and an occasional parenthetical remark, there is nothing here about superstrings or extra dimensions or multiple universes. We re dealing with well-established, robustly confirmed facts. In a way, modern physics has been a victim of its own success. The revolutions described in this book account so perfectly for everything that is known about our world, that anyone hoping to come up with the next great idea is forced to rely more on imagination than on observation. There is nothing wrong with this inspired conjecture occupies an honored place at the very pinnacle of scientific discovery. Pauli predicted the neutrino, Dirac the antiparticle, Yukawa the meson, and Gell-Mann the omega-minus and the quark, using nothing but pencil and paper, well before these particles were detected in the laboratory. ix

10 x Preface But I do think it is important to distinguish between knowledge and speculation. Some authors leave the impression that you could buy a six-pack of black holes and a bag of Higgs bosons at any convenience store. No doubt many of the current conjectures will turn out to be true, but that s not what this book is about. It s about the rock solid foundations on which any future developments must inevitably rest. They are already astounding enough to make a great story. This book is not for casual bed-time reading, and it is not for everyone. You need to know some mathematics arithmetic for sure, and a little algebra in places. There is no honest way to explain this material without it. But if you are intimidated by the sight of an equation, please don t give up too quickly. Physics tends to seem extremely difficult when you first encounter it, but quite simple once you understand it. Understanding comes with familiarity and practice. There are quite a few problems sprinkled through the text, and I urge you to work them all. Students often tell me, I understand the concepts; I just can t do the problems. They are fooling themselves. The only sure test of whether you understand the concepts is precisely your ability to work the problems. But it does take time and practice. There are no short cuts. Modern physics is built on a venerable foundation going back to Galileo and Newton, so I begin with a survey of the essential ideas, laws, and terminology we inherit from the classical era. I don t pretend this is anywhere near complete I will concentrate on those pieces of the subject that are essential to the story that follows. We need to know about mass and velocity, force and energy, momentum and wavelength, gravity and electric charge. These things are the focus of Chapter 1. The subsequent chapters treat relativity, quantum mechanics, elementary particles, and cosmology, in that order. In principle the sequence of these topics is interchangeable, but I think this roughly chronological ordering makes the best sense. However, if you are only interested in one or two of them, you should be able to read the chapters independently. I thank the students in my Master of Arts in Liberal Studies (MALS) course, for whom much of this was written, and especially Trina Marmarelli, who read the first draft with great care and fixed many awkward passages. This book draws on material from my three advanced undergraduate textbooks: Introduction to Electrodynamics, 3rd ed., (Pearson, Printed and electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey); Introduction to Quantum Mechanics, 2nd ed., (Pearson, Printed and electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey); Introduction to Elementary Particles, 2nd ed., (Wiley-VCH, Copyright Wiley-VCH Verlag GmbH & Co. KgaA). Material from these textbooks is reproduced with the kind permission of Pearson and Wiley-VCH.