The physical world : an inspirational tour of fundamental physics

573 Pages·2017·26.24 MB·English

by  Nicholas Manton

THE PHYSICAL WORLD

The Physical World
An Inspirational Tour of Fundamental Physics
Nicholas Manton
ProfessorattheDepartmentofAppliedMathematicsandTheoreticalPhysics,
UniversityofCambridge
Nicholas Mee
TrinityCollege,Cambridge
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Preface
Writingthisbookhasbeenagreatpleasure.Ithasgivenustheopportunitytothinkdeeply
about all the major branches of physics and how they unite to form a coherent picture of
the physical world. The result is a panoramic survey of the subject that is both concise
and comprehensive. Mathematics is the natural language for describing so much of the
world around us, as many commentators such as Richard Feynman and Jim Al Khalili
have stressed, and is essential for a true understanding of physics. Our account is therefore
necessarilymathematical.Weprovideclearexplanationsofthemathematicalreasoningthat
underpins modern fundamental physics and have not shied away from presenting the key
equations and their solutions.
There has never been a greater appetite for scientific explanations of the universe. Our
aim is to offer an inspirational tour of fundamental physics that is accessible to anyone who
hastakenatleasthighschoolphysicsandmathematics.Forthosejustleavingschool,itcan
be read as a taster of much of the physics covered in a university course. This book should
also suit those who have taken a maths course for scientists or engineers at university and
desire a survey of fundamental physics up to the frontiers of current research. It might also
appeal to mathematicians and computer scientists who wish to know more about physics.
Explaining the physical world is the work of many generations. The best theories of
today are built on the great theories of the past, so it is scarcely possible to appreciate
modernphysicswithoutagoodunderstandingoftheestablishedphysicsofNewton,Maxwell,
Einstein and the many others who have contributed to our understanding of the universe.
For this reason, much of the book may look historical. This material is, however, presented
in a modern style that may be very different from its original formulation. A key feature of
our approach is a unifying idea that runs as a theme throughout The Physical World. This
is the variational principle, of which the most important example is the principle of least
action. Almost all successful theories of physics can be formulated using this idea and it
lies at the heart of modern physics. Our tour also takes us around the highlights of recent
years,includingthesurveysofthecosmicmicrowavebackgroundbyWMAPandPlanck,the
discoveryoftheHiggsbosonattheLargeHadronColliderandthediscoveryofgravitational
waves by LIGO.
We would like to record here our thanks to the many friends, colleagues and relations
whohaveencouragedustowritethisbook.WeespeciallythankJohnBarrowandJonathan
Evans for their encouragement and advice.
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Nick Manton thanks Anthony Charlesworth, Helena Aitta, Roger Heumann and Alan
Smith for discussions and their interest in this book. He is also grateful to the physics
societiesofDulwichCollege,hisoldschool,andCharterhouseforopportunitiestopresentthe
principle of least action to a sixth-form audience. He particularly thanks Alasdair Kennedy,
whowasuntil recentlyatDulwich. HealsothanksAnneli andBenfortheirencouragement,
and their patience with all the rounds of writing and editing.
Nick Mee would like to thank his parents for their enduring support. He is very grateful
toJohnEastwoodforhisassistanceatnumerousbooksigningevents,includingtheOctober
2013 Society for Popular Astronomy Convention in Cambridge which led to a chance
encounter with Nick Manton that resulted in the writing of this book. He would also like to
thankJonathanEvansforhishospitalityinGonvilleandCaiusCollegeduringthisevent.He
also thanks Mark Sheeky and Debra Nightingale for their help and inspiration. He extends
a warm thank you to all the members of his newsletter group and the readers of his blog
for their encouragement and enthusiasm. He especially thanks Angie for her patience and
fortitude throughout another long project.
WethankSonkeAdlungandAniaWronskiofOxfordUniversityPressfortheirpersonal
involvement with bringing this book to publication, and Suganiya Karumbayeeram and
colleagues for their role in the production of the book. We also thank Mhairi Gray for
compiling the index. We would like to express our gratitude to the many authors whose
work we consulted while writing the book. At the end of each chapter, we have included a
Further Reading section that lists some key books and papers.
Contents
0 Introduction 1
1 Fundamental Ideas 4
1.1 Variational Principles 4
1.1.1 Geometrical optics—reflection and refraction 6
1.1.2 The scope of variational principles 9
1.2 Euclidean Space and Time 10
1.3 Partial Derivatives 14
1.4 e, π and Gaussian Integrals 18
1.4.1 Radioactive decay 19
1.4.2 Waves and periodic functions 21
1.4.3 The Gaussian integral 21
1.4.4 The method of steepest descents 24
2 Motions of Bodies—Newton’s Laws 26
2.1 Introduction 26
2.2 Newton’s Laws of Motion 27
2.3 The Principle of Least Action 29
2.3.1 Motion in one dimension 31
2.3.2 A simple example and a simple method 32
2.3.3 Motion in a general potential and Newton’s second law 35
2.3.4 The calculus of variations 36
2.3.5 The unimportance of the endpoints 38
2.4 The Motion of Several Bodies and Newton’s Third Law 39
2.5 Motion of One Body in Three Dimensions 42
2.5.1 The harmonic oscillator 43
2.6 Central Forces 44
2.6.1 Circular orbits 46
2.7 The Attractive Inverse Square Law Force 48
2.8 G and the Mass of the Earth 52
2.9 Composite Bodies and Centre of Mass Motion 53
2.10 The Kepler 2-Body Problem 56
2.10.1 Binary stars 57
2.11 Lagrangian Points 58
2.12 Conservation of Energy 63
2.13 Friction and Dissipation 65
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3 Fields—Maxwell’s Equations 67
3.1 Fields 67
3.2 The Scalar Field Equation 69
3.3 Waves 72
3.4 Divergence and Curl 74
3.5 Electromagnetic Fields and Maxwell’s Equations 75
3.5.1 What Maxwell’s equations tell us 78
3.6 Electrostatic Fields 81
3.6.1 Charge and dipole moment 84
3.7 Electromagnetic Waves 86
3.8 Magnetostatics 90
3.9 Principle of Least Action for Electromagnetic Fields 92
3.10 The Lorentz Force 93
3.10.1 The Lorentz force from the principle of least action 97
3.11 Field Energy and Momentum 98
3.12 Dynamics of Particles and Fields 100
4 Special Relativity 103
4.1 Introduction 103
4.2 Lorentz Transformations 105
4.3 Relativistic Dynamics 110
4.3.1 Comparison of Newtonian and relativistic dynamics 114
4.3.2 E =mc2 116
4.4 More on 4-Vectors 117
4.5 The Relativistic Character of Maxwell’s Equations 118
4.6 Relativistic Principles of Least Action 122
5 Curved Space 126
5.1 Spherical Geometry 126
5.1.1 Geodesics 127
5.2 Non-Euclidean, Hyperbolic Geometry 128
5.3 Gaussian Curvature 129
5.4 Riemannian Geometry 132
5.4.1 Simple examples of metrics 134
5.5 Tensors 137
5.5.1 Covariant derivatives and Christoffel symbols 140
5.5.2 Christoffel symbols in plane polar coordinates 143
5.6 The Riemann Curvature Tensor 143
5.6.1 Riemann curvature in plane polar coordinates 145
5.6.2 Riemann curvature on a sphere 145
5.6.3 The 3-sphere 146
5.7 The Geodesic Equation 146
5.7.1 Geodesics in plane polar coordinates 149
5.7.2 The equation of geodesic deviation 149
5.8 Applications 152
Contents ix
6 General Relativity 158
6.1 The Equivalence Principle 158
6.2 The Newtonian Gravitational Field and Tidal Forces 159
6.3 Minkowski Space 162
6.4 Curved Spacetime Geometry 164
6.4.1 Weak gravitational fields 166
6.5 The Gravitational Field Equation 167
6.5.1 The energy–momentum tensor 168
6.5.2 The Einstein tensor and the Einstein equation 170
6.5.3 Determining the constant of proportionality 172
6.6 The Classic Tests of General Relativity 173
6.6.1 The perihelion advance of Mercury 173
6.6.2 The deflection of starlight 174
6.6.3 Clocks and gravitational redshift 174
6.7 The Schwarzschild Solution of the Einstein Equation 177
6.7.1 The Newtonian limit 179
6.8 Particle Motion in Schwarzschild Spacetime 180
6.9 Light Deflection in Schwarzschild Spacetime 183
6.10 The Interior Schwarzschild Solution 185
6.11 Black Holes 187
6.11.1 Eddington–Finkelstein coordinates 189
6.11.2 The Kerr metric 191
6.12 Gravitational Waves 195
6.12.1 The detection of gravitational waves 196
6.13 The Einstein–Hilbert Action 199
7 Quantum Mechanics 203
7.1 Introduction 203
7.2 Position and Momentum in Quantum Mechanics 205
7.3 The Schr¨odinger Equation 207
7.3.1 The free particle 210
7.3.2 The harmonic oscillator 212
7.4 Interpretation of Wavefunctions—Observables 214
7.4.1 Position probabilities 215
7.4.2 Other physical quantities—hermitian operators 216
7.4.3 Measurements of observables 218
7.5 Expectation Values 220
7.6 After a Measurement 222
7.7 Uncertainty Relations 222
7.8 Scattering and Tunnelling 225
7.9 Variational Principles in Quantum Mechanics 227
8 Quantum Mechanics in Three Dimensions 231
8.1 Introduction 231
8.2 Position and Momentum Operators 232
8.2.1 Particle in a box 234
8.3 Angular Momentum Operators 235