Through two doors at once : the elegant experiment that captures the enigma of our quantum reality

227 Pages·2018·12.57 MB·English

by  Ananthaswamy

ALSO BY ANIL ANANTHASWAMY
The Edge of Physics
The Man Who Wasn’t There

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Copyright © 2018 by Anil Ananthaswamy
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Portions of chapters 5 and 6 appeared in New Scientist magazine. Bohmian trajectories in chapter 6
reproduced with permission from Chris Dewdney. The de Broglie-Bohm and the many-interacting worlds
trajectories in the epilogue reproduced with permission granted by Howard Wiseman on behalf of his
coauthors.
Illustrations credit: Roshan Shakeel
LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA
Names: Ananthaswamy, Anil, author.
Title: Through two doors at once : the elegant experiment that captures the enigma of our quantum reality /
Anil Ananthaswamy.
Description: New York, New York : Dutton, an imprint of Penguin Random House LLC, [2018] | Includes
bibliographical references and index.
Identifiers: LCCN 2018008272 | ISBN 9781101986097 (hardcover) | ISBN 9781101986110 (ebook) |
Subjects: LCSH: Quantum theory—Popular works. | Wave theory of light—Popular works. | Reality—
Popular works.
Classification: LCC QC174.123.A53 2018 | DDC 530.12—dc23
LC record available https://lccn.loc.gov/2018008272
While the author has made every effort to provide accurate telephone numbers, Internet addresses, and other
contact information at the time of publication, neither the publisher nor the author assumes any
responsibility for errors or for changes that occur after publication. Further, the publisher does not have any
control over and does not assume any responsibility for author or third-party websites or their content.
Version_1
To my parents
Allow me to express now, once and for all, my deep respect for the
work of the experimenter and for his fight to wring significant facts
from an inflexible Nature . . . [which] says so distinctly “No” and so
indistinctly “Yes” to our theories.
—Hermann Weyl, German mathematician, 1885– 1955
CONTENTS
ALSO BY ANIL ANANTHASWAMY
TITLE PAGE
COPYRIGHT
DEDICATION
EPIGRAPH
PROLOGUE
The Story of Nature Taunting Us
1. THE CASE OF THE EXPERIMENT WITH TWO HOLES
Richard Feynman Explains the Central Mystery
2. WHAT DOES IT MEAN “TO BE”?
The Road to Reality, from Copenhagen to Brussels
3. BETWEEN REALITY AND PERCEPTION
Doing the Double Slit, One Photon at a Time
4. FROM SACRED TEXTS
Revelations about Spooky Action at a Distance
5. TO ERASE OR NOT TO ERASE
Mountaintop Experiments Take Us to the Edge
6. BOHMIAN RHAPSODY
Obvious Ontology Evolving the Obvious Way
7. GRAVITY KILLS THE QUANTUM CAT?
The Case for Adding Spacetime into the Mix
8. HEALING AN UGLY SCAR
The Many Worlds Medicine
EPILOGUE
Ways of Looking at the Same Thing?
NOTES
ACKNOWLEDGMENTS
INDEX
ABOUT THE AUTHOR
Prologue
THE STORY OF NATURE TAUNTING US
The office is simply the most uncluttered of any physicist’s office I have ever
seen. There’s a chair alongside a small table, with nothing on it. No books,
no papers, no lamp, no computer, nothing. A sofa graces the office. Large
windows overlook a small lake, the trees around which are bare, except for a few
stragglers that are holding on to their fall foliage, defying the approaching winter
in this part of Ontario, Canada. Lucien Hardy puts his laptop on the table—
pointing out that he does most of his work in cafés and figures that all he needs
in his office is a café-like small table to set down his laptop.
There is the obligatory blackboard, taking up most of one wall of his office.
It doesn’t take long for Hardy to spring up and start chalking it up with diagrams
and equations—something that most of the quantum physicists I meet seem
inclined to do.
We start talking about some esoteric aspect of quantum physics, when he
stops and says, “I started off the wrong way.” To reset our discussion, he says,
“Imagine you have a factory and they make bombs.” He has my attention.
He writes two names on the blackboard: Elitzur and Vaidman. He is talking
about something called the Elitzur-Vaidman bomb puzzle. Named after two
Israeli physicists, the puzzle exemplifies the counterintuitive nature of the
quantum world in ways that non-physicists can appreciate. It confounds
physicists too in no small measure.
The problem goes something like this. There’s a factory that makes bombs
equipped with triggers. The triggers are so sensitive that a single particle, any
particle, even a particle of light, can set them off. There’s a big dilemma,
however. The factory’s assembly line is faulty. It’s churning out both good
bombs with triggers and bad bombs without triggers. Hardy writes them as
“good” and “bad” and quips about the quotation marks: “Obviously, you may
have a different moral perspective on it.”
have a different moral perspective on it.”
The task is to identify the good bombs. This means having to check whether
the bombs have triggers. But examining each bomb isn’t the correct strategy,
because in order to do so, you’d need to shine light on it, however faint, and that
would cause a good bomb to explode. The only ones left unexploded would be
the duds without triggers.
So, how does one solve this problem? If it helps, we are allowed one
concession: we can detonate some bombs, as long as we are left with some good,
undetonated bombs.
From our everyday experience of how the world works, this is an impossible
problem to solve. But the quantum world—the world of very small things like
molecules and atoms and electrons and protons and photons—behaves in bizarre
ways. The physics that governs the behavior of this microscopic world is called
quantum physics or quantum mechanics. And we can use quantum physics to
find good bombs without setting them off. Even with a simple setup, it’s
possible to salvage about half the good bombs. It involves using a modern
variation of a 200-year-old experiment.
Called the double-slit experiment, it was first done in the early 1800s to
challenge Isaac Newton’s ideas about the nature of light. The experiment took
center stage again in the early twentieth century, when two of the founders of
quantum physics, Albert Einstein and Niels Bohr, grappled with its revelations
about the nature of reality. In the 1960s, Richard Feynman extolled its virtues,
saying that the double-slit experiment contained all of the mysteries of the
quantum world. A simpler and more elegant experiment would be hard to find,
the workings of which a high school student can grasp, yet profound enough in
its implications to bewilder brains like Einstein’s and Bohr’s, a confusion that
continues to this day.
This is the story of quantum mechanics from the perspective of one classic
experiment and its subtle, sophisticated variations (including one that, as we’ll
see, solves the Elitzur-Vaidman bomb puzzle), whether these variations are
carried out as thought experiments by luminous minds or painstakingly
performed in the basement labs of physics departments. It’s the story of nature
taunting us: catch me if you can.