The World According to Physics

Book Preface

This book is an ode to physics. I first fell in love with physics when I was a teenager. Admittedly, this was partly b ecause I realised I was good at it. The subject seemed to be a fun mix of puzzle- solving and common sense, and I enjoyed playing with the equations, manipulating the algebraic symbols, and plugging in numbers so that they revealed the secrets of nature. But I also realised that if I wanted satisfying answers to the many deep questions about the nature of the universe and the meaning of existence bubbling up in my teenage mind, then physics was the subject I had to study. I wanted to know: What are we made of? Where do we come from? Does the universe have a beginning, or an end? Is it finite in extent, or does it stretch out to infinity? What was this thing called quantum mechanics that my father had mentioned to me? What is the nature of time? My quest to find answers to these questions has led to a life spent studying physics. I have some answers to my questions now; others I am still searching for. Some people turn to religion or some other ideology or belief system to find answers to life’s mysteries. But for me, there is no substitute for the careful hypothesising, testing, and deducing of facts about the world that are the hallmark of the scientific method. The understanding we have gained through science— and physics in particular—of how the world is made up and how it works is, in my view, not just one of many equally valid ways of reaching the ‘truth’ about reality. It is the only reliable way we have. No doubt many people never fell in love with physics, as I did. Perhaps they were turned off from studying science because they de cided, or perhaps w ere told by others, that it is a hard— or a geeky—subject. And to be sure, getting to grips with the subtleties of quantum mechanics can bring on a headache. But the wonders of our universe can and should be appreciated by every one, and gaining a basic understanding doesn’t take a lifetime of study. In this book, I want to describe why physics is so wonderful, why it is such a fundamental science, and why it is so crucial to our understanding of the world. The grand scope and sweep of physics today are breathtaking. That we now know what (almost) everything we see in the world is made of and how it holds together; that we can trace back the evolution of the entire universe to fractions of a second after the birth of space and time themselves; that through our knowledge of the physical laws of nature we have developed, and continue to develop, technologies that have transformed our lives— this is all pretty staggering. I still find myself thinking, as I write this: How can anyone not love physics? This book is intended to serve as an introduction to some of the most profound and fundamental ideas in physics. But the topics I cover are not ones you will likely have encountered at school. For some readers, the book may be a first invitation into physics— one that will entice you to learn more about it, maybe even pursue it as a lifelong journey of study and discovery, as I have. To others, who may have gotten off on the wrong foot with physics early on, it may serve as a gentle reintroduction. For many, it may provoke wonder at just how far humanity has come in its quest to understand. To convey a working knowledge of what physics tells us about the nature of our world, I have selected an array of the most important concepts in modern physics and attempted to show how they link together. We’ll survey the vast range of this conceptual landscape, from the physics of the largest cosmic scales to that of the smallest quantum level; from physicists’ quest to unify the laws of nature to their search for the simplest possible physical principles governing life; from the speculative frontiers of theoretical research to the physics that underpins our everyday experiences and technologies. I will also offer readers some new perspectives: ideas that we physicists have learnt to accept, but which we haven’t done a very good job of conveying to t hose outside our innermost circles of experts. For example, down at the subatomic scale, separated particles communicate with each other instantaneously despite being far apart, in a way that violates common sense. This property, called nonlocality, may force us ultimately to revise our entire understanding of the structure of space itself. But, sadly, many non-physicists— and indeed some physicists— misunderstand or misinterpret what this really means. A criticism levelled (usually by theoretical physicists) at many popul ar science books covering fundamental concepts in physics is that they don’t always help the lay reader grasp what these concepts actually mean. In my view, this is because the physicists who truly understand the concepts, and who write the research papers and come up with new theories, are not necessarily the best at explaining their own ideas to non- physicists. But, in turn, t hose who may have more experience and success with communicating their work to the public may not understand certain concepts deeply enough themselves to go beyond simple analogies. Even if one understands the physics and can successfully (I hope) communicate with non-physicists, it is not a small challenge to explain terms like gauge invariance, duality, eternal inflation, the holographic principle, conformal field theories, anti-de Sitter spaces or vacuum energy in a way that conveys real insight into the physics involved, without involving complex mathematics. I have done my best, but t here may well be some readers who feel I could have done better. And, of course, this will be true. Nevertheless, if you wish to delve more deeply into any par tic u lar topics which I only touch on briefly here, then t here are many books that do this brilliantly. I list at the end of the book some of those I believe you would find the most accessible and enlightening. Many of the books on this list describe the journey of scientific pro gress— how physics has developed over the millennia since the ancient Greeks, how discoveries were made, and how theories and hypotheses were proposed and discarded. These books often focus on the revolutions that have overturned previously held views about the universe and describe the leading players in these historical accounts. In this short book, however, I won’t look back on how far we’ve come; nor will I say too much about how far we have yet to go (since I don’t know, and also because I suspect it is still a long way), although I will focus in chapter8 on what we know we don’t know.

I have no particular theory to plug, either. For example, when it comes to reconciling quantum mechanics with general relativity (the holy grail of modern theoretical physics), I do not subscribe to either of the two main camps working towards this goal: I am neither a string theory advocate nor a loop quantum gravity fan,1 since neither theory falls within my part icu l ar specialism; and when it comes to interpreting the meaning of quantum mechanics, I am neither a ‘Copenhagenist’ nor a ‘many worlds’ enthusiast.2 But, this won’t stop me from being somewhat polemical about these issues now and then.

CONTENTS
Preface vii
1 The Awe of Understanding 1 2 Scale 24 3 Space and Time 53 4 Energy and Matter 82 5 The Quantum World 108 6 Thermodynamics and the Arrow of Time 139 7 Unification 166 8 The Future of Physics 192 9 The Usefulness of Physics 237 10 Thinking like a Physicist 259
Acknowl edgments 283 Further Reading 289 Index 299