At the Root of Things The Subatomic World K20506_FM.indd 1 7/10/14 2:38 AM This page intentionally left blank At the Root of Things The Subatomic World Palash B. Pal Saha Institute of Nuclear Physics Calcutta, India Translated from Bengali by Sushan Konar K20506_FM.indd 3 7/10/14 2:38 AM CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2015 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Version Date: 20140623 International Standard Book Number-13: 978-1-4665-9130-1 (eBook - PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. 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Das This page intentionally left blank Contents List of Figures xi List of Tables xiii Preface xv Translator’s Foreword xix Mathematical Symbols xxi 1 Conservation laws 1 1.1 Preamble......................... 1 1.2 Conservationofmomentum . 3 1.3 Conservationofangularmomentum . 8 1.4 Conservationofelectriccharge . 16 1.5 Thebeginningofchemistry . 20 1.6 Conservationofenergy. 23 1.7 Thedanger........................ 26 1.8 Thesynthesis ...................... 30 1.9 Relativity......................... 33 1.10 Noether’stheorem . 37 2 Waves and particles 43 2.1 FromDemocritustoDalton . 43 2.2 Newton,Huygens. 45 2.3 Young,Fresnel...................... 48 vii viii Contents 2.4 Maxwell ......................... 51 2.5 Hertz,Lenard ...................... 54 2.6 Planck .......................... 56 2.7 Einstein.......................... 59 2.8 Bose,Bohr ........................ 62 2.9 deBroglie ........................ 63 2.10 Jekyll,Hyde ....................... 65 3 Insider’s story of the atomland 67 3.1 Dalton .......................... 67 3.2 Mendeleev........................ 70 3.3 Thomson......................... 73 3.4 Rutherford........................ 74 3.5 Mosley .......................... 78 3.6 Bohr............................ 83 3.7 Heisenberg........................ 87 3.8 Schr¨odinger ....................... 94 3.9 Pauli ...........................101 3.10 Inshort..........................104 4 Smaller than the atom 105 4.1 Preamble.........................105 4.2 Dirac ...........................106 4.3 Pauli ...........................112 4.4 Yukawa..........................115 4.5 Rabi’squestion . .119 4.6 Lawrence’slegacy. .125 4.7 Gell-Mann,Zweig. .127 4.8 Feynman.........................132 4.9 Richter,Ting,Lederman . .134 4.10 Anelementaryconsideration . .138 5 Destination unification 140 5.1 Behindtheapparent . .140 5.2 Beyondtheapparent . .143 Contents ix 5.3 Giveandtake ......................145 5.4 Thesymmetries . .154 5.5 Oldwineinanewbottle . .160 5.6 The“colors” .......................162 5.7 Thepictorialdescription . .166 5.8 SubtleistheLord . .170 5.9 Theunifyinghand . .174 5.10 Untiedknots,unexploredhorizons . 180 Suggested reading 185 Index 189 This page intentionally left blank List of Figures 1.1 Kepler’s law ....................... 10 1.2 Torque required to rotate a disc. ............ 11 1.3 Spin of a cricket ball. .................. 12 1.4 Force on a particle moving in an elliptical orbit. 13 1.5 Momentum of a particle in an elliptical orbit. 14 1.6 Franklin’s experiment with charged objects. 16 1.7 Force between two current-carrying wires. 26 1.8 Rotational symmetry. .................. 40 2.1 Arrangement for producing Newton’s rings. 49 2.2 Newton’s rings ...................... 50 2.3 Hertz’s instrument .................... 55 3.1 Plum-pudding model of an atom. ........... 74 3.2 Rutherford scattering according to the plum- pudding model. ..................... 75 3.3 Rutherford scattering, as it would have been, for an atom modeled after a mango. ............. 76 3.4 The Bohr model of an atom. .............. 77 3.5 Hypothetical energy states. .............. 84 3.6 Emission of a photon by an electron in an atom. 86 3.7 Waves in a string with two ends rigidly fixed. 96 3.8 Specifying the position of the point P. ......... 97 3.9 Electronic energy levels in a hydrogen atom. 99 xi xii List of Figures 5.1 Two current-carrying wires. ...............146 5.2 Feynman diagram depicting interaction between two electrons. ......................148 5.3 More complicated Feynman diagrams. ........149 5.4 Two streams of electrons falling on a screen. 156 5.5 Binding of quarks in a hadron by gluon exchange. 164 5.6 Gluon exchange mediates strong interactions. 165 5.7 Decay of the muon. ...................167 5.8 Decay of the neutron. ..................168 5.9 Orientation of little magnets in a piece of iron. 171 List of Tables 3.1 The periodic table .................... 69 3.2 Ranges of the atomic quantum numbers. 100 4.1 The leptons and their lepton numbers. ........125 4.2 The quarks, their charges, and baryon numbers. 137 5.1 Superposition of two beams of electrons . 157 5.2 Effect of an extra half rotation on one beam . 159 xiii This page intentionally left blank Preface This book is a translation of my Bengali book Ki diye somosto-kichu gorha. This preface, however, is not a trans- lation of the preface of the Bengali book. It has been written specifically for this English edition. The book is about the structure of matter, that is to say, on the fundamental building blocks that constitute every- thing in the universe. The story has been divided into five chapters. In Chapter 1, I introduce a summary of classical (or pre-quantum) physics in order to set the stage for later developments in quantum physics, which are essential for the study of elementary particles. I present this summary from a somewhat unusual point of view, viz., keeping a fo- cus on various conservation laws. In Chapter 2, I discuss the emergence of quantum theory from studies on radia- tion of heat and photoelectric effect, developments that lead to the concept of the duality between waves and particles. Chapter 3 discusses how quantum theory helped us under- stand the structure of atoms. Chapter 4 starts with the dis- covery of particles that were not constituents of atoms, for example, the positron and the muon. Then it goes on to re- veal the world of dozens and dozens of particles that were discovered experimentally in the middle of the twentieth century, and then describes how the concept of quarks re- xv xvi Preface duced this maze into something simple and tractable, and ends with the quarks and leptons, which are considered the fundamental particles today. Of course all physical phe- nomena involve interactions between these particles, and in Chapter 5 I introduce the fundamental interactions, de- scribe the basic nature of quantum theories of these interac- tions, and end with a discussion of how these interactions might be unified. I believe that no prior knowledge of physics is neces- sary for understanding the subject matter of this book. A very rudimentary acquaintance with mathematics is neces- sary for understanding the few mathematical formulas that appear in the book. To be precise, one only needs to know a few pieces of mathematical notation, which have all been summarized on page xxi. However, the main flow of ar- gument can be followed even without paying attention to most of these equations. The Bengali original of the book was first published in 1987. The present translation mostly follows the second edi- tion, published in 1997. I used the word ‘mostly’, because a literal translation was not attempted for many reasons. First, the material of the original book, although quite up- to-date at the time it was published, became outdated in places owing to new discoveries that have occurred since then. So I had to have discussion sessions with the trans- lator, to delete a few lines or add a new paragraph here and there to bring the book up to date. Such updating has taken place in only the last few sections of Chapters 4 and 5. Second, the translation gave me a chance to re-evaluate the original, and I was dissatisfied with some lines of the orig- inal, sometimes because the translator pointed them out to me and sometimes otherwise. I felt that they should be Preface xvii modified, and so we did just that. Such lines are strewn all over the book, but fortunately there are not many of them. There is a third reason. A few years ago I was asked to contribute an article to the science magazine Resonance. Rather than attempting something completely from scratch, I penned an article based on the fifth chapter of my book Ki diye..