Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality
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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.
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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
INDE X
ABOUT THE AUTHOR
Prologue
THE STORY OF NATURE TAUNTING US
T he 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.”
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 (inc
luding 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.
1
THE CASE OF THE EXPERIMENT WITH TWO HOLES
Richard Feynman Explains the Central Mystery
There is nothing more surreal, nothing more abstract than reality.
—Giorgio Morandi
R ichard Feynman was still a year away from winning his Nobel Prize. And two decades away from publishing an endearing autobiographical book that introduced him to non-physicists as a straight-talking scientist interested in everything from cracking safes to playing drums. But in November 1964, to students at Cornell University in Ithaca, New York, he was already a star and they received him as such. Feynman came to deliver a series of lectures. Strains of “Far above Cayuga’s Waters” rang out from the Cornell Chimes. The provost introduced Feynman as an instructor and physicist par excellence, but also, of course, as an accomplished bongo drummer. Feynman strode onto the stage to the kind of applause reserved for performing artists, and opened his lecture with this observation: “ It’s odd, but in the infrequent occasions when I have been called upon in a formal place to play the bongo drums, the introducer never seems to find it necessary to mention that I also do theoretical physics.”
By his sixth lecture, Feynman dispensed with any preamble, even a token “Hello” to the clapping students, and jumped straight into how our intuition, which is suited to dealing with everyday things that we can see and hear and touch, fails when it comes to understanding nature at very small scales.
And often, he said, it’s experiments that challenge our intuitive view of the world. “ Then we see unexpected things,” said Feynman. “We see things that are very far from what we could have imagined. And so our imagination is stretched to the utmost—not, as in fiction, to imagine things which aren’t really there. But our imagination is stretched to the utmost just to comprehend those things which are there. And it’s this kind of a situation that I want to talk about.”
The lecture was about quantum mechanics, the physics of the very small things; in particular, it was about the nature of light and subatomic bits of matter such as electrons. In other words, it was about the nature of reality. Do light and electrons show wavelike behavior (like water does)? Or do they act like particles (like grains of sand do)? Turns out that saying yes or no would be both correct and incorrect. Any attempt to visualize the behavior of the microscopic, subatomic entities makes a mockery of our intuition.
“ They behave in their own inimitable way,” said Feynman. “Which, technically, could be called the ‘quantum-mechanical’ way. They behave in a way that is like nothing that you have ever seen before. Your experience with things that you have seen before is inadequate—is incomplete. The behavior of things on a very tiny scale is simply different. They do not behave just like particles. They do not behave just like waves.”
But at least light and electrons behave in “exactly the same” way, said Feynman. “ That is, they’re both screwy.”
Feynman cautioned the audience that the lecture was going to be difficult because it would challenge their widely held views about how nature works: “ But the difficulty, really, is psychological and exists in the perpetual torment that results from your saying to yourself ‘But how can it be like that?’ Which really is a reflection of an uncontrolled, but I say utterly vain, desire to see it in terms of some analogy with something familiar. I will not describe it in terms of an analogy with something familiar. I’ll simply describe it.”
And so, to make his point over the course of an hour of spellbinding oratory, Feynman focused on the “ one experiment which has been designed to contain all of the mystery of quantum mechanics, to put you up against the paradoxes and mysteries and peculiarities of nature.”
It was the double-slit experiment. It’s difficult to imagine a simpler experiment—or, as we’ll discover over the course of this book, one more confounding. We start with a source of light. Place in front of the source a sheet of opaque material with two narrow, closely spaced slits or openings. This creates two paths for the light to go through. On the other side of the opaque sheet is a screen. What would you expect to see on the screen?
The answer, at least in the context of the world we are familiar with, depends on what one thinks is the nature of light. In the late seventeenth century and all of the eighteenth century, Isaac Newton’s ideas dominated our view of light. He argued that light was made of tiny particles, or “corpuscles,” as he called them. Newton’s “corpuscular theory of light” was partly formulated to explain why light, unlike sound, cannot bend around corners. Light must be made of particles, Newton argued, since particles don’t curve or bend in the absence of external forces.
In his lecture, when Feynman analyzed the double-slit experiment, he first considered the case of a source firing particles at the two slits. To accentuate the particle nature of the source, he urged the audience to imagine that instead of subatomic particles (of which electrons and particles of light would be examples), we were to fire bullets from a gun—which “come in lumps.” To avoid too much violent imagery (what with bombs in the prologue, and a thought experiment with gunpowder to come), let’s imagine a source that spews particles of sand rather than bullets; we know that sand comes in lumps, though the lumps are much, much smaller than bullets.
First, let’s do the experiment with either the left slit or the right slit closed. Let’s take it that the source is firing grains of sand at high enough speeds that they have straight trajectories. When we do this, the grains of sand that get through the slits mostly hit the region of the screen directly behind the open slit, with the numbers tapering off on either side. The higher the height of the graph, the more the number of grains of sand reaching that location on the screen.
Now, what should we see if both slits are open? As expected, each grain of sand passes through one or the other opening and reaches the other side. The distribution of the grains of sand on the far screen is simply the sum of what goes through each slit. It’s a demonstration of the intuitive and sensible behavior of the non-quantum world of everyday experience, the classical world described so well by Newton’s laws of motion.
To be convinced that this is indeed what happens with particles of sand, let’s orient the device such that the sand is now falling down onto the barrier with two slits. Our intuition clearly tells us that two mounds should form beneath the two openings.
Turning the experiment back to its original position, let’s dispense with the sand and consider a source that’s emitting light, and assume that light’s made of Newtonian corpuscles. Informed by our experiment with sand particles, we’d expect to see two strips of light on the screen, one behind the right slit and one behind the left slit, each strip of light fading off to the sides, leading to a distribution of light that is simply the sum of the light you’d get passing through each slit.
Well, that’s not what happens. Light, it seems, does not behave as if it’s made of particles.
Even before Newton’s time, there were observations that challenged his theory of the particle nature of light. For example, light changes course when going from one medium to another—say, from air to glass and back into air (this phenomenon, called refraction, is what allows us to make optical lenses). Refraction can’t be easily explained if you think of light as particles traveling through air and glass, because it requires positing an external force to change the direction of light when it goes from air to glass and from glass to air. But refraction can be explained if light is thought of as a wave (the speed of the wave would be different in air than in glass, explaining the change in direction as light goes from one type of material to another). This is exactly what Dutch scientist Christiaan Huygens proposed in the 1600s. Huygens argued that light is a wave much like a sound wave, and sin
ce sound waves are essentially vibrations of the medium in which they are traveling, Huygens argued that light too is made of vibrations of a medium called ether that pervades the space around us.
This was a serious theory put forth by an enormously gifted scientist. Huygens was a physicist, astronomer, and mathematician. He made telescopes by grinding lenses himself, and discovered Saturn’s moon Titan (the first probe to land on Titan, in 2005, was named Huygens in his honor). He independently discovered the Orion nebula. In 1690, he published his Traité de la Lumière (Treatise on Light ), in which he expounded his wave theory of light.
Newton and Huygens were contemporaries, but Newton’s star shone brighter. After all, he had come up with the laws of motion and the universal law of gravitation, which explained everything from the arc of a ball thrown across a field to the movement of planets around the sun. Besides, Newton was a polymath of considerable renown (as a mathematician, he gave us calculus, and even ventured into chemistry, theology, and writing biblical commentaries, not to mention all his work in physics). It was no wonder that his corpuscular theory of light, despite its shortcomings, overshadowed Huygens’s ideas of light being wavelike. It’d take another polymath to show up Newton when it came to understanding light.
—
Thomas Young has been called “ The Last Man Who Knew Everything.” In 1793, barely twenty years of age, he explained how our eyes focus upon objects at different distances, based partly on his own dissection of an ox’s eyes. A year later, on the strength of that work, Young was made a Fellow of the Royal Society, and in 1796 he became “ doctor of physic, surgery, and midwifery.” When he was in his forties, Young helped Egyptologists decipher the Rosetta stone (which had inscriptions in three scripts: Greek, hieroglyphics, and something unknown). And in between becoming a medical doctor, getting steeped in Egyptology, and even studying Indo-European languages, Young delivered one of the most intriguing lectures in the history of physics. The venue was the Royal Society of London, and the date, November 24, 1803. Young stood in front of that august audience, this time as a physicist describing a simple and elegant homespun experiment, which, in his mind, had unambiguously established the true nature of light and proved Newton wrong.