PENGUIN BOOKS

REALITY IS NOT WHAT IT SEEMS

‘The world’s most inspirational physics teacher’ Daily Telegraph

‘The new book by Carlo Rovelli is an utter joy’ Adam Rutherford, author of A Brief History of Everyone Who Ever Lived

‘Be prepared for your intellectual foundations to be vaporised … Carlo Rovelli will melt your synapses with this exploration of physical reality and what the universe is formed of at the very deepest level … Quantum gravity is so new that there aren’t many popular books about it. You couldn’t be in better hands than Rovelli, a world expert’ Tara Shears, The Times Higher Education

‘A hugely engaging book, which leaves the head spinning … From evaporating black holes to quantum fields, Rovelli is a charming, thought-provoking tour guide’ Manjit Kumar, Prospect

‘Theoretical physicist Carlo Rovelli plumbs this cosmic mystery [gravity] with the historical awareness and flair one might expect from the author of Seven Brief Lessons on Physics’ Andrew Robinson, Daily Telegraph, Books of the Year

‘Covering much of the same ground as his Seven Brief Lessons on Physics, this explains with style and charm how general relativity may yet be integrated with quantum theory’ Daily Telegraph, Books of the Year

‘Carlo Rovelli’s Reality is Not What it Seems did not disappoint. Bearing in mind the giddy expectations his surprise 2015 smash hit Seven Brief Lessons on Physics gave rise to, that’s quite a feat’ The Big Issue, Best Books of 2016

‘A global superstar … Professor Rovelli is making the grammar of the universe accessible to a new generation’ Channel 4 News

‘A top-flight theoretical physicist with a rock star reputation’ BBC Radio 4 World at One

‘It’s not a scientific treatise. It’s a paean to the wonder of the natural world … I scraped a C in my Physics O-level and haven’t been near a physics textbook since. If I can understand – and even enjoy – Rovelli’s book, then anyone can … beautiful’ William Cook, Spectator

‘Rovelli writes with elegance, clarity and charm … A joy to read, as well as being an intellectual feast’ Michael Brooks, New Statesman

‘Rovelli offers a soaring vision’ Tom Whipple, The Times

‘A comprehensive guide to the bewitching adventure of physics’ Lewis Jones, Daily Telegraph

‘Carlo Rovelli is the man behind the gigantic triumph of scientific popular writing, Seven Brief Lessons on Physics … His new book sets out to explore and explain some of the more difficult notions of theoretical physics. Space and time, the nature of reality, black holes, quantum theory – there doesn’t seem to be much that Rovelli won’t attempt to explain to us non–scientists, and Reality Is Not What It Seems has more of those answers to offer’ Cultured Vultures

ABOUT THE AUTHOR

Carlo Rovelli is a theoretical physicist who has made significant contributions to the physics of space and time. He has worked in Italy and the US, and is currently directing the quantum gravity research group of the Centre de physique théorique in Marseille, France. His Seven Brief Lessons on Physics is an international bestseller translated into forty-one languages.

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First pubished in Italian under the title La realtà non è come ci appare by Raffaello Cortina Editore SpA 2014

This translation first published by Allen Lane 2016

Published in Penguin Books 2017

Copyright © Raffaello Cortina Editore SpA, 2014

Translation copyright © Simon Carnell and Erica Segre, 2016

The moral right of the author and translators has been asserted

Cover credit: Coralie Bickford-Smith

ISBN: 978-0-241-25797-5

Contents

Author’s Note

Preface: Walking along the Shore

PART ONE
Roots

1 Grains

2 The Classics

PART TWO
The Beginning of the Revolution

3 Albert

4 Quanta

PART THREE
Quantum Space and Relational Time

5 Spacetime is Quantum

6 Quanta of Space

7 Time Does Not Exist

PART FOUR
Beyond Space and Time

8 Beyond the Big Bang

9 Empirical Confirmations?

10 Quantum Black Holes

11 The End of Infinity

12 Information

13 Mystery

Notes

Annotated Bibliography

Follow Penguin

Author’s Note

During my entire research life, friends and curious people have asked me to explain what was going on in quantum gravity research. How was it possible to study new ways of thinking about space and time? Over and over again I have been asked to write a popular account of this research. While books on cosmology or string theory abound, a book describing the research on the quantum nature of space and time, and on loop quantum gravity in particular, did not yet exist. I have long hesitated, because I wanted to concentrate on research. Some years ago, after completing my technical book on the subject, I felt that the collective work of many scientists had moved the topic to a stage mature enough for a popular book. The landscape we are exploring is enchanting: why keep it hidden?

But I still delayed the project, because I could not ‘see’ the book in my head. How to explain a world without space and time? One night in 2012, during a long solitary drive from Italy to France, I realized that the only way to explain in a comprehensible manner the ongoing modifications of the notions of space and time was to tell the story from the beginning: starting from Democritus, all the way through to the quanta of space. After all, this is how I understand the story. I began to design the entire book in my mind while driving, and got increasingly excited, until I heard a police car’s sirens telling me to pull over: I was driving far above the speed limit. The Italian policemen asked me politely if I was crazy to drive at that speed. I explained that I had just found the idea I’d been seeking for so long; the policeman let me go without a ticket, and wished me good luck with the book. This is the book.

This book was written and first published in Italian at the beginning of 2014. Shortly afterwards, I wrote a few articles on fundamental physics for an Italian newspaper. A prestigious Italian publisher, Adelphi, asked me for an extended version of these articles, to appear as a small booklet. This is the origin of the short book Seven Brief Lessons on Physics, which to my immense surprise has become an international bestseller and has opened a beautiful channel of communication between me and so many wonderful readers all over the world. The Seven Lessons were thus written after this book, and to some extent they are a synthesis of some of the topics you find here. If you have read Seven Brief Lessons on Physics and want to know more, to journey deeper into the strange world that book sketched, here you can find more.

While the account of established physics I give here is presented from the peculiar perspective in which I understand it, it is largely uncontroversial. However, the part of this book that describes current research in quantum gravity is my own personal understanding of the state of the art. This is the region at the boundary between what we have understood and what we do not yet understand, and is still far from achieving consensus. Some of my physicist colleagues will agree with what I write here; others won’t. This is true for all presentations of ongoing research at the frontiers of knowledge, but I prefer to state it upfront and clearly. This is not a book about certainties: it is a book about the adventure of moving towards the unknown.

As a whole, this is a travel book describing one of the most spectacular journeys that humanity has taken: a journey out of our limited and parochial views of reality, towards an increasingly vast understanding of the structure of things. A magical journey out of our common-sense view of things, far from complete.

Marseille, 4 May 2016

Preface: Walking along the Shore

We are obsessed with ourselves. We study our history, our psychology, our philosophy, our gods. Much of our knowledge revolves around man himself, as if we were the most important thing in the universe. I think I like physics because it opens a window through which we can see further. It gives me the sense of fresh air entering the house.

What we see out there through the window is constantly surprising us. We have learned a great deal about the universe. In the course of the centuries we have come to realize just how very many wrong ideas we had. We thought that the Earth was flat, and that it was the still centre of our world. That the universe was small, and unchanging. We believed that man was a breed apart, without kinship to the other animals. We have learned of the existence of quarks, black holes, particles of light, waves of space, and of the extraordinary molecular structures in every cell of our bodies. The human race is like a growing child who discovers with amazement that the world consists not just of his bedroom and playground, but that it is vast, and that there are a thousand things to discover, and innumerable ideas quite different from those with which he began. The universe is multiform and boundless, and we continue to stumble upon new aspects of it. The more we learn about the world, the more we are amazed by its variety, beauty and simplicity.

But the more we discover, the more we understand that what we don’t yet know is greater than what we know. The more powerful our telescopes, the stranger and more unexpected are the heavens we see. The closer we look at the minute detail of matter, the more we discover of its profound structure. Today we see almost to the Big Bang, the great explosion from which, 14 billion years ago, all the galaxies were born – but we have already begun to glimpse something beyond the Big Bang. We have learned that space is curved, but already foresee that this same space is woven from vibrating quantum grains.

Our knowledge of the elementary grammar of the world continues to grow. If we try to put together what we have learned about the physical world in the course of the twentieth century, the clues point towards something profoundly different from what we were taught at school. An elementary structure of the world is emerging, generated by a swarm of quantum events, where time and space do not exist. Quantum fields draw space, time, matter and light, exchanging information between one event and another. Reality is a network of granular events; the dynamic which connects them is probabilistic; between one event and another, space, time, matter and energy melt in a cloud of probability.

This strange new world is slowly emerging today from the study of the main open question posed in fundamental physics: quantum gravity. It’s the problem of coherently synthesizing what we have learned about the world with the two major discoveries of twentieth-century physics: general relativity and quantum theory. To quantum gravity, and the strange world that this research is unfolding, this book is dedicated.

The book is a live coverage of the ongoing research: what we are learning, what we already know, and what we think we are beginning to understand, about the elementary nature of things. It starts from the distant origin of some key ideas that we use today to order our understanding of the world and describes the two great discoveries of the twentieth century – Einstein’s general relativity and quantum mechanics – trying to put into focus the core of their physical content. It tells of the picture of the world which is emerging today from research in quantum gravity, taking into account the latest indications given by nature, such as the confirmation of the cosmological standard model obtained from the Planck satellite and the failure at CERN to observe the super-symmetric particles that many expected. And it discusses the consequences of these ideas: the granular structure of space; the disappearance of time at small scale; the physics of the Big Bang; the origin of black-hole heat – up to the role of information in the foundation of physics.

In a famous myth related by Plato in the seventh book of The Republic, some men are chained at the bottom of a dark cave and see only shadows cast upon a wall by a fire behind them. They think that this is reality. One of them frees himself, leaves the cave and discovers the light of the Sun, and the wider world. At first the light, to which his eyes are unaccustomed, stuns and confuses him. But eventually he can see, and returns excitedly to his companions to tell them what he has seen. They find it hard to believe.

We are all in the depths of a cave, chained by our ignorance, by our prejudices, and our weak senses reveal to us only shadows. If we try to see further, we are confused: we are unaccustomed. But we try. This is science. Scientific thinking explores and redraws the world, gradually offering us better and better images of it, teaching us to think in ever more effective ways. Science is a continual exploration of ways of thinking. Its strength is its visionary capacity to demolish preconceived ideas, to reveal new regions of reality, and to construct novel and more effective images of the world. This adventure rests upon the entirety of past knowledge, but at its heart is change. The world is boundless and iridescent; we want to go and see it. We are immersed in its mystery and in its beauty, and over the horizon there is unexplored territory. The incompleteness and the uncertainty of our knowledge, our precariousness, suspended over the abyss of the immensity of what we don’t know, does not render life meaningless: it makes it interesting and precious.

I have written this book to give an account of what for me is the wonder of this adventure. I’ve written with a particular reader in mind: someone who knows little or nothing about today’s physics but is curious to find out what we know, but also what we don’t yet understand, about the elementary weave of the world – and where we are searching. And I have written it to try to communicate the breathtaking beauty of the panorama of reality which can be seen from this perspective.

I’ve also written it for my colleagues, fellow travellers dispersed throughout the world, as well as for the young women and men with a passion for science, eager to set out on this journey for the first time. I’ve sought to outline the general landscape of the structure of the physical world, as seen by the double lights of relativity and of quantum physics, and to show how they can be combined. This is not only a book of divulgation; it’s also one which articulates a point of view, in a field of research where the abstraction of the technical language may sometimes obscure the wide-angle vision. Science is made up of experiments, hypotheses, equations, calculations and long discussions; but these are only tools, like the instruments of musicians. In the end, what matters in music is the music itself, and what matters in science is the understanding of the world which science provides. To understand the significance of the discovery that the Earth turns around the Sun, it is not necessary to follow Copernicus’s complicated calculations; to understand the importance of the discovery that all living beings on our planet have the same ancestors, it is not necessary to follow the complex arguments of Darwin’s books. Science is about reading the world from a gradually widening point of view.

This book gives an account of the current state of the search for our new image of the world, as I understand it today. It is the reply I would give to a colleague and friend asking me, ‘So, what do you think is the true nature of things?’, as we walk along the shore, on a long midsummer’s evening.

Part One

ROOTS

This book begins in Miletus, twenty-six centuries ago. Why begin a book about quantum gravity with events, people and ideas so ancient? I hope the reader eager to get on to quanta of space will not hold this against me. For it is easier to understand ideas by starting with the roots from which they have grown, and an important number of the ideas which turned out to be effective for understanding the world originated over two thousand years ago. If we briefly retrace their birth, they become clearer, and the later steps turn out to be simpler and natural.

But there’s more. Certain problems first posed in antiquity continue to be crucial to our understanding of the world. Some of the most recent ideas about the structure of space utilize concepts and issues introduced then. In speaking of these distant ideas, I put on to the table questions which are going to be central to quantum gravity. This makes it also possible, when treating of quantum gravity, to distinguish between the ideas which go back to the very origin of scientific thought, even if we are unfamiliar with them, and those which are radically new. The connection between problems posed by the scientists of antiquity, and solutions found by Einstein and quantum gravity, is, as we shall see, surprisingly close.