The Order of Time Read online

  RIVERHEAD BOOKS

  An imprint of Penguin Random House LLC

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  New York, New York 10014

  Published by Riverhead Books 2018

  Copyright © 2017 by Adelphi Edizioni SPA, Milano

  Translation copyright © 2018 by Simon Carnell and Erica Segre

  First published in Italy by Adelphi Edizioni SPA under the title L’ordine del tempo 2017

  English translation published simultaneously in Great Britain by Allen Lane, an imprint of Penguin Random House UK and in the United States of America by Riverhead Books, an imprint of Penguin Random House LLC

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  Library of Congress Cataloging-in-Publication Data

  Names: Rovelli, Carlo, author. | Segre, Erica, translator. | Carnell, Simon, translator.

  Title: The order of time / Carlo Rovelli ; translated by Erica Segre and Simon Carnell.

  Other titles: Ordine del tempo. English

  Description: First American edition. | New York : Riverhead Books, 2018. | Originally published in Italian: L’ordine del tempo (Milan: Adelphi Edizioni, 2017). | Includes bibliographical references and index.

  Identifiers: LCCN 2017060293 | ISBN 9780735216105 (hardcover) | ISBN 9780735216129 (ebook)

  Subjects: LCSH: Space and time. | Time. | Presentism (Philosophy). | Cosmology.

  Classification: LCC QC173.59.S65 R6813 2018 | DDC 530.11—dc23

  LC record available at https://lccn.loc.gov/2017060293

  p. cm.

  Version_1

  FOR ERNESTO, BILO, AND EDOARDO

  CONTENTS

  Title Page

  Copyright

  Dedication

  Author's Note

  Perhaps Time Is the Greatest Mystery

  Part I THE CRUMBLING OF TIME

  1. Loss of Unity

  2. Loss of Direction

  3. The End of the Present

  4. Loss of Independence

  5. Quanta of Time

  Part II THE WORLD WITHOUT TIME

  6. The World Is Made of Events, Not Things

  7. The Inadequacy of Grammar

  8. Dynamics as Relation

  Part III THE SOURCES OF TIME

  9. Time Is Ignorance

  10. Perspective

  11. What Emerges from a Particularity

  12. The Scent of the Madeleine

  13. The Source of Time

  The Sister of Sleep

  Image Credits

  Notes

  Index

  About the Author

  The verses that open each chapter, unless otherwise indicated, are from versions of Horace’s Odes translated by Giulio Galetto and published in a charming small volume entitled In questo breve cerchio (Verona: Edizioni del Paniere, 1980); English translations by Erica Segre and Simon Carnell.

  PERHAPS TIME IS THE GREATEST MYSTERY

  Even the words that we are speaking now

  thieving time

  has stolen away,

  and nothing can return. (I, 11)

  I stop and do nothing. Nothing happens. I am thinking about nothing. I listen to the passing of time.

  This is time, familiar and intimate. We are taken by it. The rush of seconds, hours, years that hurls us toward life then drags us toward nothingness. . . . We inhabit time as fish live in water. Our being is being in time. Its solemn music nurtures us, opens the world to us, troubles us, frightens and lulls us. The universe unfolds into the future, dragged by time, and exists according to the order of time.

  In Hindu mythology, the river of the cosmos is portrayed with the sacred image of Shiva dancing: his dance supports the coursing of the universe; it is itself the flowing of time. What could be more universal and obvious than this flowing?

  And yet things are somewhat more complicated than this. Reality is often very different from what it seems. The Earth appears to be flat but is in fact spherical. The sun seems to revolve in the sky when it is really we who are spinning. Neither is the structure of time what it seems to be: it is different from this uniform, universal flowing. I discovered this, to my utter astonishment, in the physics books I read as a university student: time works quite differently from the way it seems to.

  In those same books I also discovered that we still don’t know how time actually works. The nature of time is perhaps the greatest remaining mystery. Curious threads connect it to those other great open mysteries: the nature of mind, the origin of the universe, the fate of black holes, the very functioning of life on Earth. Something essential continues to draw us back to the nature of time.

  Wonder is the source of our desire for knowledge,1 and the discovery that time is not what we thought it was opens up a thousand questions. The nature of time has been at the center of my life’s work in theoretical physics. In the following pages, I give an account of what we have understood about time and the paths that are being followed in our search to understand it better, as well as an account of what we have yet to understand and what it seems to me that we are just beginning to glimpse.

  Why do we remember the past and not the future? Do we exist in time, or does time exist in us? What does it really mean to say that time “passes”? What ties time to our nature as persons, to our subjectivity?

  What am I listening to when I listen to the passing of time?

  This book is divided into three unequal parts. In the first, I summarize what modern physics has understood about time. It is like holding a snowflake in your hands: gradually, as you study it, it melts between your fingers and vanishes. We conventionally think of time as something simple and fundamental that flows uniformly, independently from everything else, from the past to the future, measured by clocks and watches. In the course of time, the events of the universe succeed each other in an orderly way: pasts, presents, futures. The past is fixed, the future open. . . . And yet all of this has turned out to be false.

  One after another, the characteristic features of time have proved to be approximations, mistakes determined by our perspective, just like the flatness of the Earth or the revolving of the sun. The growth of our knowledge has led to a slow disintegration of our notion of time. What we call “time” is a complex collection of structures,2 of layers. Under increasing scrutiny, in ever greater depth, time has lost layers one after another, piece by piece. The first part of this book gives an account of this crumbling of time.

  The second part describes what we have been left with: an empty, windswept landscape almost devoid of all trace of temporality. A strange, alien world that is nevertheless still the one to which we belong. It is like arriving in the high mountains, where there is nothing but snow, rocks, and sky. Or like it must have been for Armstrong and Aldrin when venturing onto the motionless sand of the moon. A world stripped to its essence, glittering with an arid and troubling beauty. The physics on which I work—quantum gravity—is an attempt to understand and lend coherent meaning to this extreme and beautiful landscape. To the world without time.

  The third part of the book is the most difficult, but also the most vital and the one that most closely involves us. In a world without time, there must still be something that gives rise to the time that we are accustomed to, with its order, with its past that is different from the future, with its smooth flowing. Somehow, our time must emerge around us, at least for us and at our scale.3

  This is the return journey, back toward the time lost in the first part of the book when pursuing the elementary grammar of the world. As in a crime novel, we are now going in search of a guilty party: the culprit who has created time. One by one, we discover the constituent parts of the time that is familiar to us—not, now, as elementary structures of reality, but rather as useful approximations for the clumsy and bungling mortal creatures we are: aspects of our perspective, and aspects, too, perhaps, that are decisive in determining what we are. Because the mystery of time is ultimately, perhaps, more about ourselves than about the cosmos. Perhaps, as in the first and greatest of all detective novels, Sophocles’ Oedipus Rex, the culprit turns out to be the detective.

  Here, the book becomes a fiery magma of ideas, sometimes illuminating, sometimes confusing. If you decide to follow me, I will take you to where I believe our knowledge of time has reached: up to the brink of that vast nocturnal and star-studded ocean of all that we still don’t know.

  PART 1

  THE CRUMBLING OF TIME

  1 LOSS OF UNITY

  Dances of love intertwine

  such graceful girls

  lit by the moon

  on these clear nights. (I, 4)

  THE SLOWING DOWN OF TIME

  Let’s begin with a simple fact: time passes faster in the mountains than it does at sea level.

  The difference is small but can be measured with precision timepieces that can be bought today on the internet for a few thousand dollars. With practice, anyone can witness the slowing down of time. With the timepieces of specialized laboratories, this slowing down of time can b
e detected between levels just a few centimeters apart: a clock placed on the floor runs a little more slowly than one on a table.

  It is not just the clocks that slow down: lower down, all processes are slower. Two friends separate, with one of them living in the plains and the other going to live in the mountains. They meet up again years later: the one who has stayed down has lived less, aged less, the mechanism of his cuckoo clock has oscillated fewer times. He has had less time to do things, his plants have grown less, his thoughts have had less time to unfold. . . . Lower down, there is simply less time than at altitude.

  Is this surprising? Perhaps it is. But this is how the world works. Time passes more slowly in some places, more rapidly in others.

  The surprising thing, perhaps, is that someone understood this slowing down of time a century before we had clocks precise enough to measure it. His name, of course, was Albert Einstein.

  The ability to understand something before it’s observed is at the heart of scientific thinking. In antiquity, Anaximander understood that the sky continues beneath our feet long before ships had circumnavigated the Earth. At the beginning of the modern era, Copernicus understood that the Earth turns long before astronauts had seen it do so from the moon. In a similar way, Einstein understood that time does not pass uniformly everywhere before the development of clocks accurate enough to measure the different speeds at which it passes.

  In the course of making such strides, we learn that the things that seemed self-evident to us were really no more than prejudices. It seemed obvious that the sky was above us and not below; otherwise, the Earth would fall down. It seemed self-evident that the Earth did not move; otherwise, it would cause everything to crash. That time passed at the same speed everywhere seemed equally obvious to us. . . . Children grow up and discover that the world is not as it seemed from within the four walls of their homes. Humankind as a whole does the same.

  Einstein asked himself a question that has perhaps puzzled many of us when studying the force of gravity: how can the sun and the Earth “attract” each other without touching and without utilizing anything between them?

  He looked for a plausible explanation and found one by imagining that the sun and the Earth do not attract each other directly but that each of the two gradually acts on that which is between them. And since what lies between them is only space and time, he imagined that the sun and the Earth each modified the space and time that surrounded them, just as a body immersed in water displaces the water around it. This modification of the structure of time influences in turn the movement of bodies, causing them to “fall” toward each other.4

  What does it mean, this “modification of the structure of time”? It means precisely the slowing down of time described above: a mass slows down time around itself. The Earth is a large mass and slows down time in its vicinity. It does so more in the plains and less in the mountains, because the plains are closer to it. This is why the friend who stays at sea level ages more slowly.

  If things fall, it is due to this slowing down of time. Where time passes uniformly, in interplanetary space, things do not fall. They float, without falling. Here on the surface of our planet, on the other hand, the movement of things inclines naturally toward where time passes more slowly, as when we run down the beach into the sea and the resistance of the water on our legs makes us fall headfirst into the waves. Things fall downward because, down there, time is slowed by the Earth.5

  Hence, even though we cannot easily observe it, the slowing down of time nevertheless has crucial effects: things fall because of it, and it allows us to keep our feet firmly on the ground. If our feet adhere to the pavement, it is because our whole body inclines naturally to where time runs more slowly—and time passes more slowly for your feet than it does for your head.

  Does this seem strange? It is like when, watching the sun going down gloriously at sunset, disappearing slowly behind distant clouds, we suddenly remember that it’s not the sun that’s moving but the Earth that’s spinning, and we see with the unhinged eye of the mind our entire planet—and ourselves with it—rotating backward, away from the sun. We are seeing with “mad” eyes, like those of Paul McCartney’s Fool on the Hill: the crazed vision that sometimes sees further than our bleary, customary eyesight.

  TEN THOUSAND DANCING SHIVAS

  I have an enduring passion for Anaximander, the Greek philosopher who lived twenty-six centuries ago and understood that the Earth floats in space, supported by nothing.6 We know of Anaximander’s thought from other writers. Only one small original fragment of his writings has survived—just one:

  Things are transformed one into another according to necessity,

  and render justice to one another

  according to the order of time.

  “According to the order of time” (κατὰ τὴν τοῦ χρόνου τάξιν). From one of the crucial, initial moments of natural science there remains nothing but these obscure, arcanely resonant words, this appeal to the “order of time.”

  Astronomy and physics have since developed by following this seminal lead given by Anaximander: by understanding how phenomena occur according to the order of time. In antiquity, astronomy described the movements of stars in time. The equations of physics describe how things change in time. From the equations of Newton, which establish the foundations of mechanics, to those of Maxwell for electromagnetic phenomena; from Schrödinger’s equation describing how quantum phenomena evolve, to those of quantum field theory for the dynamics of subatomic particles: the whole of our physics, and science in general, is about how things develop “according to the order of time.”

  It has long been the convention to indicate this time in equations with the letter t (the word for “time” begins with t in Italian, French, and Spanish, but not in German, Arabic, Russian, or Mandarin). What does this t stand for? It stands for the number measured by a clock. The equations tell us how things change as the time measured by a clock passes.

  But if different clocks mark different times, as we have seen above, what does t indicate? When the two friends meet up again after one has lived in the mountains and the other at sea level, the watches on their wrists will show different times. Which of the two is t? In a physics laboratory, a clock on a table and another on the ground run at different speeds. Which of the two tells the time? How do we describe the difference between them? Should we say that the clock on the ground has slowed relative to the real time recorded on the table? Or that the clock on the table runs faster than the real time measured on the ground?

  The question is meaningless. We might just as well ask what is most real—the value of sterling in dollars or the value of dollars in sterling. There is no “truer” value; they are two currencies that have value relative to each other. There is no “truer” time; there are two times and they change relative to each other. Neither is truer than the other.

  But there are not just two times. Times are legion: a different one for every point in space. There is not one single time; there is a vast multitude of them.

  The time indicated by a particular clock measuring a particular phenomenon is called “proper time” in physics. Every clock has its proper time. Every phenomenon that occurs has its proper time, its own rhythm.

  Einstein has given us the equations that describe how proper times develop relative to each other. He has shown us how to calculate the difference between two times.7

  The single quantity “time” melts into a spiderweb of times. We do not describe how the world evolves in time: we describe how things evolve in local time, and how local times evolve relative to each other. The world is not like a platoon advancing at the pace of a single commander. It’s a network of events affecting each other.

  This is how time is depicted in Einstein’s general theory of relativity. His equations do not have a single “time”; they have innumerable times. Between two events, just as between the two clocks that are separated and then brought together again, the duration is not a single one.8 Physics does not describe how things evolve “in time” but how things evolve in their own times, and how “times” evolve relative to each other.*