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  Quantum Strangeness

  Quantum Strangeness

  Wrestling with Bell’s Theorem and the Ultimate Nature of Reality

  George Greenstein

  The MIT Press

  Cambridge, Massachusetts

  London, England

  © 2019 Massachusetts Institute of Technology

  All rights reserved. No part of this book may be reproduced in any form by any electronic or mechanical means (including photocopying, recording, or information storage and retrieval) without permission in writing from the publisher.

  This book was set in Stone Serif by Westchester Publishing Ser vices. Printed and bound in the United States of America.

  Library of Congress Cataloging­in­Publication Data

  Names: Greenstein, George, 1940­ author.

  Title: Quantum strangeness : wrestling with Bell’s Theorem and the ultimate nature of reality / George S. Greenstein ; foreword by David Kaiser.

  Description: Cambridge, MA : The MIT Press, [2019] | Includes bibliographical references and index.

  Identifiers: LCCN 2018043232 | ISBN 9780262039932 (hardcover : alk. paper) Subjects: LCSH: Bell, J. S. | Quantum theory. | Physics­­Philosophy.

  Classification: LCC QC174.12 .G7325 2019 | DDC 530.12­­dc23 LC record

  available at https://lccn.loc.gov/2018043232

  10 9 8 7 6 5 4 3 2 1

  To Guy Blaylock and Arthur Zajonc

  Close friends, valued colleagues, and fellow Wrestlers

  “I think I can safely say that nobody understands quantum mechanics.”

  Richard Feynman, The Character of Physical Law

  Contents

  Foreword by David Kaiser xi

  Acknowledgments xvii

  1. The Great Predictor 1

  Background to Bell

  2. Silence 11

  3. Half a Theory? 17

  4. The Solvay Battles 21

  5. Spin 29

  6. An Impoverished Language? 33

  7. The EPR Paradox 37

  8. Hidden Variables 41

  Bell’s Theorem

  9. A Hidden Variable Theory 45

  10. Bell’s Theorem 55

  11. Stigma 63

  Experimental Metaphysics

  12. Experimental … 71

  13. … Metaphysics 93

  14. Nonlocality 97

  15. Quantum machines 101

  16. A New Universe 113

  x Contents

  Appendix 1: The GHZ Theorem 121

  Appendix 2: Further Reading 125

  Notes 129

  Index 135

  Foreword

  Physics has more than its share of mind­ bending ideas: the slowing clocks and shrinking meter sticks of relativity; enormous coagulations of matter, like black holes, that can rupture space­ time itself. Yet the strangest ideas of all are clustered in quantum theory, physicists’ remarkably successful

  description of matter and energy at atomic scales. Here we find descriptions of objects that seem to act as if they were in two places at once; of particles that can tunnel through walls; of Erwin Schrödinger’s twice­ fated cat, trapped in a zombie­ like state of being both alive and dead. For all that, Schrödinger himself declared one idea in particular, quantum entanglement, to be “the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.”1

  Schrödinger had done so much to contribute to quantum theory; his

  “wave function,” obeying an equation he first published in 1926, remains

  central to physicists’ efforts to describe quantum systems quantitatively.

  Almost a decade later, in 1935, Schrödinger coined the term “entanglement,” though by then his enthusiasm for quantum theory had begun to wane. That same year his friend Albert Einstein teamed up with two younger colleagues, Boris Podolsky and Nathan Rosen, to issue his own, latest challenge to quantum theory. In their famous “EPR” paper (named for the authors’ initials), they described a system involving a pair of entangled particles emitted from a central source. Physicists could perform various measurements on one particle, and thereby learn something about the second particle, far off in the distance. Indeed, the EPR authors concluded, physicists should be able to glean more information about the far­ away particle than could be accounted for within quantum theory.2

  Each particle, it seemed self­ evident to Einstein and his coauthors, should possess definite properties on its own, independent of what physicists

  xii Foreword

  happened to choose to measure. If physicists elected to measure the first

  particle’s position at a given moment, for example, they would learn about the position of the second particle, which had headed off at the same speed but in the opposite direction from the first particle. Or the physicists might choose to measure the momentum of the first particle, and thereby learn

  about the second particle’s momentum. But surely the second particle had

  definite values for these and any other properties the physicists might have chosen to investigate, regardless of what choices the physicists had made.

  After all, Einstein’s own theory of relativity made clear that no signal or influence could travel faster than the speed of light— so nothing the physicists might have chosen to do to the first particle should have been able to affect the second particle, which had traveled so far away. If relativity really set an absolute speed limit on how quickly A could influence B, then the second particle would need to carry all its own information with it, as it traveled through space; there would be no time to receive an update on

  what values for various properties it should have, based on the outcomes of measurements on the first particle. Therefore, the EPR authors concluded

  with a flourish, there existed “elements of reality”— real, definite properties of that second particle— about which quantum theory offered no description. Quantum mechanics, they argued, was incomplete.3

  Within weeks a response came from Niels Bohr, the Danish physicist

  who had helped to craft quantum theory and who served as a kind of

  spokesperson for the emerging work. Bohr’s response to EPR was rapid,

  but abstruse; to this day, it remains difficult to parse Bohr’s argument. Central to his response, however, was a denial that objects in the subatomic realm really must carry complete sets of properties on their own. Rather,

  Bohr insisted, a particle might have no definite value of, say, momentum, until subjected to a particular measurement— as if a person had no definite weight until stepping on a bathroom scale. (Years later, Einstein famously asked a colleague if quantum physicists really believed that the moon was

  only there when someone chose to look.) Most important to physicists at

  the time, it seems, was that Bohr had responded at all. More recent scholarship has indicated that Einstein and Bohr were largely talking past each other— a series of miscommunications exacerbated by the rise of fascism in Europe, which had driven Einstein to emigrate to the United States, thereby ending the late­ night, face­ to­ face discussions that the two had enjoyed

  Foreword xiii

  during happier times. Each physicist died, decades later, having failed to convince the other when it came to quantum entanglement.4

  The debate seemed to linger, with no clear resolution, for years. A young

  physicist from Northern Ireland, John S. Bell, shared many of the reservations about quantum theory that Einstein had expressed. Indeed, Bell had nursed a private concern about the topic since his student days, growing

  frustrated when students and teachers seemed to parrot Bohr’s responses.

  Bell was quickly counseled t
o keep such “philosophical” concerns to himself— by the 1950s, quantum mechanics had moved to the very center of physicists’ expanding efforts to understand everything from nuclear reactions to superconductivity to the properties of little devices like transistors.

  In every single case, the equations of quantum theory provided a remarkable match to experiments. So why, Bell’s teachers pressed, should they continue to fret over the abstract questions that had distracted old­ timers like Einstein and Bohr?5

  Bell dove into mainstream topics for his research in high­ energy physics, even as his thoughts kept returning to nagging questions about quantum

  entanglement. Finally, during a sabbatical in the United States in 1964, Bell brought many of his ideas to fruition. He tweaked the famous EPR thought

  experiment, focusing on specific combinations of measurements that

  physicists could perform on each of the two entangled particles. In just a few lines of algebra, Bell demonstrated that Einstein’s pair of assumptions— that particles carry definite properties on their own, prior to and independent of measurement, and that no influence can travel faster than light— necessarily led to a contradiction with quantum theory. Bell identified a quantitative upper limit for how often the outcomes of certain combinations of measurements on the two particles could ever line up, if they behaved in accordance with Einstein’s assumptions. If, instead, the particles were governed by quantum theory, then the measurements on each particle should be

  more strongly correlated, surpassing the upper bound that Bell had derived.

  If quantum theory were true, in other words, then performing a measurement over here really would seem to affect the behavior of some other tiny bit of matter, observed arbitrarily far away.6

  On paper, Bell showed, the contrast was as clear as day: an Einstein­ like world limited to one side of the bound; a quantum­ mechanical world clearly surpassing that limit. The central question that had absorbed Einstein and

  xiv Foreword

  Bohr for decades could be posed in a laboratory, not just debated in a smoke­

  filled room. Bell published his paper, and then … nothing. Years went by

  before he heard so much as a peep of interest from the physics community.

  In time, Bell’s elegant paper happened to catch the eye of a few unconventional physicists, who recognized the magnitude of Bell’s achievement. If they followed Bell’s reasoning, and really conducted experiments of the sort he had described, they might be able to learn something deep about how

  the world works. Pioneering physicists like Abner Shimony, John Clauser,

  Michael Horne, Stuart Freedman, Alain Aspect, and a handful of others began to realize that by testing Bell’s inequality in a laboratory, they could subject abstract, metaphysical mysteries to experimental investigation. Hence the

  term used in George Greenstein’s lovely book: experimental metaphysics.7

  For nearly half a century, physicists have subjected Bell’s inequality to

  experimental test. Every single published result has been consistent with the predictions of quantum theory, showing correlations among measurements

  on pairs of entangled particles in excess of Bell’s bound. Yet from the start, Bell, Shimony, Clauser, Horne, Aspect, and others have recognized that each of these tests has been subject to one or more “loopholes”: little conceptual escape hatches by which an Einstein­ like interpretation could still account for the experimental results.

  Perhaps the particles, or other elements of the apparatus, had shared information (at or below the speed of light) during a particular series of measurements, thereby arranging for the measurements to line up the way they did.

  Or perhaps the detectors that had been used to measure the particles were

  inefficient, and failed to register any definite outcome some fraction of the time. Then it would be possible, at least in principle, for the strong correlations that showed up in those measurements that were successfully recorded to be but a rare statistical hiccup, some fluke that would have been washed out had all of the particles actually been measured. Or perhaps the striking correlations arose from some common cause, deep in the experiment’s past, which had somehow nudged the selection of measurements to be performed and

  tipped off the particles in advance. After all, as Schrödinger himself acknowledged back in 1935, one should hardly be surprised when a student aces an examination, if she had received a copy of the questions ahead of time.8

  Experimental groups around the world have tackled these loopholes,

  usually one at a time, since the early 1980s.9 Only as recently as 2015

  have various groups succeeded in measuring significant violations of Bell’s

  Foreword xv

  inequality while closing not one but two of the stubborn loopholes.10 More recently, my own colleagues and I have conducted experiments to address

  that strange, third loophole— the one regarding the seemingly random

  selection of measurements to perform on the entangled particles— by turning the universe itself into a pair of random­ number generators. Yet again we have found— as have our colleagues around the world— the strong

  correlations that Bell had first identified.11

  Why go through all the trouble? Aren’t the dozens of previous experiments sufficient for us to declare the question settled? More than just stubbornness is at stake. In fact, quantum entanglement now lies at the center of a booming field dubbed “quantum information science,” which promises major new devices. Quantum encryption and quantum computers—

  each of which has progressed well into the beta­ testing stage— will function as promised only if entanglement is real. If, for some reason, entanglement were merely an artifact, and the world really were governed by Einstein’s

  assumptions, then quantum encryption simply would not be secure, and

  quantum computers would fail to deliver the anticipated exponential speedup compared to ordinary machines. These days, practical technologies built around quantum entanglement constitute a multibillion­ dollar industry—

  contributing a new set of imperatives to keep testing Bell’s inequality, in addition to the deep, metaphysical questions.12

  Despite all the theoretical progress over the past half century, and the

  recent experimental advances in addressing various loopholes, we are still left with vexing questions. How could the world really work that way? How could two little specks of matter act in concert, even after they had moved arbitrarily far apart? In recent years, physicists have gone to extremes trying to articulate, in everyday scenarios, what entanglement implies about the

  world. There have been elaborate fables about cops chasing quantum robbers; quantum soufflés that do or do not rise; tales of twins who order drinks in bars an ocean apart; and so on.13

  Amid all these discussions, George Greenstein’s book is a special delight.

  With patience and clarity, Greenstein guides readers along this extraordinary conceptual journey. We are neither hushed nor rushed; never reprimanded,

  as John Bell himself had been as a student, to simply take this or that result on faith. Rather, Greenstein shares his own earnest struggles to come to

  grips with the ideas, to sit with them, to try to puzzle through what they might imply about the workings of nature at its most fundamental. And

  xvi Foreword

  he does all this with simple illustrations and virtually no mathematics. His book is an invitation and a primer for those new to the topic, and a timely reminder to fellow specialists, who have long since grown comfortable with the mathematical formalism of quantum theory, that this central element

  of physicists’ toolkit retains a beguiling strangeness at its core.

  David Kaiser

  Germeshausen Professor of the History of Science

  and Professor of Physics

  Massachusetts Institute of Technology

  October 2
018

  Acknowledgments

  This book is the product of many minds.

  Guy Blaylock, Danny Greenerger, David Harrison, David Kaiser, Preston

  Stahley, and Arthur Zajonc read the manuscript and gave me the immeasurable benefit of their advice. Andrew Fraknoi and John Harte read an earlier article and similarly gave me much help. And over the years I have benefited greatly from— and enjoyed— illuminating conversations with

  innumerable colleagues and friends: some of them scientists working in the field, others nonscientists who nevertheless had insights that gently prodded me in productive directions.

  Renate Bertleman, John Clauser, and Jain Wei Pan kindly contributed

  their own personal photographs of people and equipment.

  To all my deepest thanks.

  1 The Great Predictor

  It was many years ago that I first encountered the Great Predictor.

  I was thrilled to meet him. I’d been looking forward to the encounter for

  years. The Great Predictor was famous— world famous. He was legendary

  for the number of his predictions, and for their amazing accuracy. Many

  people had relied on those predictions, and always with profit. And he could tell you the most amazing stuff— this, that, and the other thing.

  What intrigued me the most, however, was how bizarre were some of the

  predictions. “Tomorrow you will be in two places at once” was one. “On

  Wednesday an event will occur for which there is no cause” was another.

  How could such things be? I was captivated by the strangeness of these

  prognostications. Could such weird things really come to pass? That’s why

  I had been so anxious to meet the Predictor. For years I had looked forward to finally getting to know him.

  At long last I was getting my wish. I was twenty years old, and I was

  thrilled. I was sitting in a classroom, in college, on the first day of a course called Introduction to Quantum Mechanics.

  Quantum mechanics is one of the glories of our age. The theory lies at

  the very heart of modern society. I once saw an estimate that a full third of our economy involves products based on it. Much of science involves it