By Adam Becker
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An Editor's Choice, New York Times Book Review
Longlisted for PEN/E.O. Wilson Prize for Literary Science Writing
Longlisted for Goodreads Choice Award
The soundest fact may fail or prevail in the style of its telling.
—Ursula K. Le Guin
The objects in our everyday lives have an annoying inability to appear in two places at once. Leave your keys in your jacket, and they won’t also be on the hook by the front door. This isn’t surprising—these objects have no uncharted abilities or virtues. They’re profoundly ordinary. Yet these mundane things are composed of a galaxy of the unfamiliar. Your house keys are a temporary alliance of a trillion trillion atoms, each forged in a dying star eons ago, each falling to Earth in its earliest days. They have bathed in the light of a violent young sun. They have witnessed the entire history of life on our planet. Atoms are epic.
Like most epic heroes, atoms have some problems that ordinary humans don’t. We are creatures of habit, monotonously persisting in just one location at a time. But atoms are prone to whimsy. A single atom, wandering down a path in a laboratory, encounters a fork where it can go left or right. Rather than choosing one way forward, as you or I would have to do, the atom suffers a crisis of indecision over where to be and where not to be. Ultimately, our nanometer Hamlet chooses both. The atom doesn’t split, it doesn’t take one path and then the other—it travels down both paths, simultaneously, thumbing its nose at the laws of logic. The rules that apply to you and me and Danish princes don’t apply to atoms. They live in a different world, governed by a different physics: the submicroscopic world of the quantum.
Quantum physics—the physics of atoms and other ultratiny objects, like molecules and subatomic particles—is the most successful theory in all of science. It predicts a stunning variety of phenomena to an extraordinary degree of accuracy, and its impact goes well beyond the world of the very small and into our everyday lives. The discovery of quantum physics in the early twentieth century led directly to the silicon transistors buried in your phone and the LEDs in its screen, the nuclear hearts of the most distant space probes and the lasers in the supermarket checkout scanner. Quantum physics explains why the Sun shines and how your eyes can see. It explains the entire discipline of chemistry, periodic table and all. It even explains how things stay solid, like the chair you’re sitting in or your own bones and skin. All of this comes down to very tiny objects behaving in very odd ways.
But there’s something troubling here. Quantum physics doesn’t seem to apply to humans, or to anything at human scale. Our world is a world of people and keys and other ordinary things that can travel down only one path at a time. Yet all the mundane things in the world around us are made of atoms—including you, me, and Danish princes. And those atoms certainly are governed by quantum physics. So how can the physics of atoms differ so wildly from the physics of our world made of atoms? Why is quantum physics only the physics of the ultratiny?
The problem isn’t that quantum physics is weird. The world is a wild and wooly place, with plenty of room for weirdness. But we definitely don’t see all the strange effects of quantum physics in our daily lives. Why not? Maybe quantum physics really is only the physics of tiny things, and it doesn’t apply to large objects—perhaps there’s a boundary somewhere, a border beyond which quantum physics doesn’t work. In that case, where is the boundary, and how does it work? And if there is no such boundary—if quantum physics really applies to us just as much as it applies to atoms and subatomic particles—then why does quantum physics so flagrantly contradict our experience of the world? Why aren’t our keys ever in two places at once?
Eighty years ago, one of the founders of quantum physics, Erwin Schrödinger, was deeply troubled by these problems. To explain his concerns to his colleagues, he devised a now-famous thought experiment: Schrödinger’s cat (Figure I.1). Schrödinger imagined putting a cat in a box along with a sealed glass vial of cyanide, with a small hammer hanging over the vial. The hammer, in turn, would be connected to a Geiger counter, which detects radioactivity, and that counter would be pointed at a tiny lump of slightly radioactive metal. This Rube Goldberg contraption would be set off the moment the metal emitted any radiation; once that happens, the Geiger counter would register the radiation, which would release the hammer, smashing the vial and killing the cat. (Schrödinger had no intention of actually conducting this experiment, to the SPCA’s relief.) Schrödinger proposed leaving the cat in the box for a certain period of time, then opening the box to find the cat’s fate.
The radiation emitted by the lump of metal is composed of subatomic particles, breaking away from the atoms in the metal and flying off at high speeds. Like all sufficiently tiny things, those particles obey the laws of quantum physics. But, instead of reading Shakespeare, the subatomic particles in the metal have been listening to the Clash—at any particular moment, they don’t know whether they should stay or they should go. So they do both: during the time the box is closed, the indecisive lump of radioactive metal will and won’t emit radiation.
Thanks to these punk-rock particles, the Geiger counter will and won’t register radiation, which means the hammer will and won’t smash the vial of cyanide—so the cat will be both dead and alive. And this, Schrödinger pointed out, is a serious problem. Maybe an atom can travel down two paths at once, but a cat certainly can’t be both dead and alive. When we open the box, the cat will be either dead or alive, and it stands to reason that the cat must have been one or the other the moment before we opened the box.
Yet many of Schrödinger’s contemporaries piled on, denying exactly that point. Some claimed that the cat was in a state of dead-and-alive until the moment the box was opened, when the cat was somehow forced into “aliveness” or “deadness” through the action of looking inside the box. Others believed that talking about what was going on inside the box before it was opened was meaningless, because the interior of the unopened box was unobservable by definition, and only observable, measurable things have meaning. To them, worrying about unobservable things was pointless, like asking whether a tree that falls in the forest makes a sound when nobody’s around to hear it.
Schrödinger’s concerns about his cat weren’t allayed by these arguments. He thought that his colleagues had missed the point: quantum physics lacked an important component, a story about how it lined up with the things in the world. How does a phenomenal number of atoms, governed by quantum physics, give rise to the world we see around us? What is real, at the most fundamental level, and how does it work? Yet Schrödinger’s opponents carried the day, and his concerns about what was actually happening in the quantum world were dismissed. The rest of physics simply moved on.
Schrödinger was in a minority, but he wasn’t alone. Albert Einstein also wanted to understand what was really happening in the quantum world. He debated Niels Bohr, the great Danish physicist, over the nature of quantum physics and reality. The Einstein-Bohr debates have entered into the lore of physics itself, and the usual conclusion is that Bohr won, that Einstein’s and Schrödinger’s concerns were shown to be baseless, that there is no problem with reality in quantum physics because there is no need to think about reality in the first place.
Yet quantum physics is certainly telling us something about what is real, out in the world. Otherwise, why would it work at all? It would be very difficult to account for its wild success if it had no connection to anything real in the world. Even if the theory is simply a model, surely it’s modeling something and doing a reasonably good job of it. There must be some thing that ensures the predictions of quantum physics come to pass, with phenomenally high precision.
But figuring out what quantum physics is saying about the world has been hard. This is, in part, due to the sheer weirdness of the theory. Whatever is in the world of the quantum, it is nothing familiar at all. The seemingly contradictory nature of quantum objects—atoms that are here and there at the same time, radiation that has both been emitted and remains latent in its source—isn’t the only alien aspect of the theory. There are also instantaneous long-distance connections between objects: subtle, useless for direct communication, but surprisingly useful for computation and encryption. And there does not appear to be any limit to the size of object that is subject to quantum physics. Ingenious devices built by experimental physicists coax larger and larger objects to display strange quantum phenomena almost monthly—deepening the gravity of the problem that no such quantum phenomena are seen in our everyday lives.
These phenomena aren’t the only challenge to deciphering the message of quantum physics. They’re not even the largest challenge. Despite the fact that every physicist agrees that quantum physics works, a bitter debate has raged over its meaning for the past ninety years, since the theory was first developed. And one position in that debate—held by the majority of physicists and purportedly by Bohr—has continually denied the very terms of the debate itself. These physicists claim that it is somehow inappropriate or unscientific to ask what is going on in the quantum realm, despite the phenomenal success of the theory. To them, the theory needs no interpretation, because the things that the theory describes aren’t truly real. Indeed, the strangeness of quantum phenomena has led some prominent physicists to state flatly that there is no alternative, that quantum physics proves that small objects simply do not exist in the same objectively real way as the objects in our everyday lives do. Therefore, they claim, it is impossible to talk about reality in quantum physics. There is not, nor could there be, any story of the world that goes along with the theory.
The popularity of this attitude to quantum physics is surprising. Physics is about the world around us. It aims to understand the fundamental constituents of the universe and how they behave. Many physicists are driven to enter the field out of a desire to understand the most basic properties of nature, to see how the puzzle fits together. Yet, when it comes to quantum physics, the majority of physicists are perfectly willing to abandon this quest and instead merely “shut up and calculate,” in the words of physicist David Mermin.
More surprising still is that this majority view has, time and again, been shown not to work. Despite the popular view among physicists, Einstein clearly got the better of Bohr in their debates and convincingly showed there were deep problems that needed answering at the heart of quantum physics. Simply dismissing questions about reality as “unscientific,” as some of Schrödinger’s opponents did, is an untenable position based on outdated philosophy. And some dissenters from the majority have developed alternative approaches to quantum physics that clearly explain what is going on in the world without sacrificing any of the theory’s accuracy.
The existence of these viable alternatives puts the lie to the idea that we are forced to give up on reality in quantum physics. Yet most physicists still subscribe to some form of this idea. It’s still what’s taught in classrooms, and it’s still the picture that’s usually painted for the public. Even when the alternatives are mentioned, they are mentioned as just that—alternatives to the default, despite the fact that the default is entirely unworkable. Thus, nearly a century after quantum theory was first developed—after it has thoroughly altered the world and the lives of every single human in it, both for better and worse—we still don’t know what it’s telling us about the nature of reality. This thoroughly strange story is the subject of this book.
This is an astonishing state of affairs, and hardly anyone outside of physics knows about it. But why should anyone else care? After all, quantum physics certainly works. For that matter, why should physicists care? Their mathematics makes accurate predictions; isn’t that enough?
But science is about more than mathematics and predictions—it’s about building a picture of the way nature works. And that picture, that story about the world, informs both the day-to-day practice of science and the future development of scientific theories, not to mention the wider world of human activity outside of science. For any given set of equations, there’s an infinite number of stories we could tell about what those equations mean. Picking a good story, and then searching for holes in that story, is how science progresses. The stories told by the best scientific theories determine the experiments that scientists choose to perform and influence the way that the outcomes of those experiments are interpreted. As Einstein pointed out, “The theory decides what we can observe.”
The history of science bears this out over and over again. Galileo didn’t invent the telescope—but he was the first to think of pointing a good one at Jupiter, because he believed that Jupiter was a planet, like Earth, that went around the Sun. After that, telescopes were used regularly to look at everything from comets to nebulae to star clusters. But nobody bothered to use a telescope to find out whether the Sun’s gravity bent starlight during a solar eclipse—not until Einstein’s theory of general relativity predicted just such an effect, over three centuries after Galileo’s discovery. The practice of science itself depends on the total content of our best scientific theories—not just the math but the story of the world that goes along with the math. That story is a crucial part of the science, and of going beyond the existing science to find the next theory.
That story also matters beyond the confines of science. The stories that science tells about the world filter out into the wider culture, changing the way that we look at the world around us and our place in it. The discovery that the Earth was not at the center of the universe, Darwin’s theory of evolution, the Big Bang and an expanding universe nearly 14 billion years old, containing hundreds of billions of galaxies, each containing hundreds of billions of stars—these ideas have radically altered humanity’s conception of itself.
Quantum physics works, but ignoring what it tells us about reality means papering over a hole in our understanding of the world—and ignoring a larger story about science as a human process. Specifically, it ignores a story about failure: a failure to think across disciplines, a failure to insulate scientific pursuits from the corrupting influence of big money and military contracts, and a failure to live up to the ideals of the scientific method. And this failure matters to every thinking inhabitant of our world, a world whose every corner has been reshaped by science. This is a story of science as a human endeavor—not just a story about how nature works but also about how people work.
A Tranquilizing Philosophy
The people of Tlön are taught that the act of counting modifies the amount counted, turning indefinites into definites. The fact that several persons counting the same quantity come to the same result is for the psychologists of Tlön an example of the association of ideas or of memorization.
—Jorge Luis Borges, “Tlön, Uqbar, Orbus Tertius”
This epistemology-soaked orgy ought to come to an end.
—Albert Einstein, letter to Erwin Schrödinger, 1935
The Measure of All Things
Two great theories shook the world and shattered the earth in the first quarter of the twentieth century, scattering the remains of the physics that had come before and forever altering our understanding of reality. One of these theories, relativity, was developed in true science-fiction fashion, by a lone genius working in splendid isolation, who had left the academy only to return triumphant with profound truth in his hand—this was, of course, Albert Einstein.
The other theory, quantum physics, had a more difficult birth. It was a collaborative effort involving dozens of physicists working over the course of nearly thirty years. Einstein was among them, but he was not their leader; the closest thing this disorganized and unruly band of revolutionaries had was Niels Bohr, the great Danish physicist. Bohr’s Institute for Theoretical Physics in Copenhagen was the mecca of quantum physics in its infancy, with nearly every big name in the field for fifty years studying there at one point or another. The physicists who worked there made profound discoveries across nearly every field of science: they developed the first genuine theory of quantum physics, found the underlying logic of the periodic table of the elements, and used the power of radioactivity to reveal the basic workings of living cells. And it was Bohr, along with a group of his most talented students and colleagues—Werner Heisenberg, Wolfgang Pauli, Max Born, Pascual Jordan, and others—who developed and championed the “Copenhagen interpretation,” which rapidly became the standard interpretation of the mathematics of quantum physics. What does quantum physics tell us about the world? According to the Copenhagen interpretation, this question has a very simple answer: quantum physics tells us nothing whatsoever about the world.
Rather than telling us a story about the quantum world that atoms and subatomic particles inhabit, the Copenhagen interpretation states that quantum physics is merely a tool for calculating the probabilities of various outcomes of experiments. According to Bohr, there isn’t a story about the quantum world because “there is no quantum world. There is only an abstract quantum physical description.” That description doesn’t allow us to do more than predict probabilities for quantum events, because quantum objects don’t exist in the same way as the everyday world around us. As Heisenberg put it, “The idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them, is impossible.” But the results of our experiments are very real, because we create them in the process of measuring them. Jordan said when measuring the position of a subatomic particle such as an electron, “the electron is forced to a decision. We compel it to assume a definite position; previously, it was, in general, neither here nor there.… We ourselves produce the results of measurement.”
Statements like these sounded ludicrous to Albert Einstein. “The theory reminds me a little of the system of delusions of an exceedingly intelligent paranoiac,” he said in a letter to a friend. Despite his crucial role in the development of quantum physics, Einstein couldn’t stand the Copenhagen interpretation. He called it a “tranquilizing philosophy—or religion” that provides a “soft pillow to the true believer… [but it] has so damned little effect on me.” Einstein demanded an interpretation of quantum physics that told a coherent story about the world, one that allowed answers to questions even when no measurement was taking place. He was exasperated with the Copenhagen interpretation’s refusal to answer such questions, calling it an “epistemology-soaked orgy.”
Yet Einstein’s pleas for a more complete theory went unheard, in part because of John von Neumann’s proof that no such theory was possible. Von Neumann was arguably the greatest mathematical genius alive. He had taught himself calculus by the age of eight, published his first paper on advanced mathematics at nineteen, and earned a PhD when he was twenty-two. He played a crucial role in building the atomic bomb, and he was one of the founding fathers of computer science. He was also fluent in seven languages. His colleagues at Princeton said, only half-joking, that von Neumann could prove anything—and anything he proved was correct.
Von Neumann published his proof as part of his textbook on quantum physics in 1932. There’s no evidence that Einstein was even aware of this proof, but many other physicists were—and for them, merely the idea of a proof from the mighty von Neumann was enough to settle the debate. The philosopher Paul Feyerabend experienced this firsthand after attending a public talk given by Bohr: “At the end of the lecture [Bohr] left, and the discussion proceeded without him. Some speakers attacked his qualitative arguments—there seemed to be lots of loopholes. The Bohrians did not clarify the arguments; they mentioned the alleged proof by von Neumann and that settled the matter… like magic, the mere name of ‘von Neumann’ and the mere word ‘proof’ silenced the objectors.”
At least one person did notice a problem with von Neumann’s proof shortly after it was published. Grete Hermann, a German mathematician and philosopher, published a paper in 1935 criticizing von Neumann’s proof. Hermann pointed out that von Neumann failed to justify a crucial step, and thus the whole proof was flawed. But nobody listened to her, partly because she was an outsider to the physics community—and partly because she was a woman.
Despite the flaw in von Neumann’s proof, the Copenhagen interpretation remained totally dominant. Einstein was painted as an old man out of touch with the rest of the world, and questioning the Copenhagen interpretation became tantamount to questioning the massive success of quantum physics itself. And so quantum physics continued for the next twenty years, piling success upon success, without any further questions about the hole at its heart.
Why does quantum physics need an interpretation? Why doesn’t it simply tell us what the world is like? Why was there any dispute between Einstein and Bohr at all? Einstein and Bohr certainly agreed that quantum physics worked. If they both believed the theory, how could they disagree about what the theory said?
Quantum physics needs an interpretation because it’s not immediately clear what the theory is saying about the world. The mathematics of quantum physics is unfamiliar and abstruse, and the connection between that mathematics and the world we live in is hard to see. This is in stark contrast with the theory quantum physics replaced, the physics of Isaac Newton. Newton’s physics describes a familiar and simple world with three dimensions, filled with solid objects that move in straight lines until something knocks them off their paths. The math of Newtonian physics specifies the location of an object using a set of three numbers, one for each dimension, known as a vector. If I’m on a ladder, two meters off the ground, and that ladder is three meters in front of you, then I could describe my position as (zero, three, two). The zero says that I’m not off to one side or the other, the three says I’m three meters in front of you, and the two says I’m two meters above you. It’s fairly straightforward—nobody runs around deeply worried about how to interpret Newtonian physics.
But quantum physics is significantly stranger than Newtonian physics, and its math is stranger too. If you want to know where an electron is, you need more than three numbers—you need an infinity of them. Quantum physics uses infinite collections of numbers called wave functions to describe the world. These numbers are assigned to different locations: a number for every point in space. If you had an app on your phone that measured a single electron’s wave function, the screen would just display a single number, the number assigned to the spot where your phone is. Where you’re sitting right now, the Wave-Function-O-Meter™ might display the number 5. Half a block down the street, it’d display 0.02. That’s what a wave function is, at its simplest: a set of numbers, fixed at different places.
Everything has a wave function in quantum physics: this book, the chair you’re sitting in, even you. So do the atoms in the air around you, and the electrons and other particles inside those atoms. An object’s wave function determines its behavior, and the behavior of an object’s wave function is determined in turn by the Schrödinger equation, the central equation of quantum physics, discovered in 1925 by the Austrian physicist Erwin Schrödinger. The Schrödinger equation ensures that wave functions always change smoothly—the number that a wave function assigns to a particular location never hops instantly from 5 to 500. Instead, the numbers flow perfectly predictably: 5.1, 5.2, 5.3, and so on. A wave function’s numbers can go up and down again, like a wave—hence the name—but they’ll always undulate smoothly like waves too, never jerking around too crazily.
Wave functions aren’t too complicated, but it’s a little weird that quantum physics needs them. Newton could give you the location of any object using just three numbers. Apparently, quantum physics needs an infinity of numbers, scattered across the universe, just to describe the location of a single electron. But maybe electrons are weird—maybe they don’t behave the way that rocks or chairs or people do. Maybe they’re smeared out, and the wave function describes how much of the electron is in a particular place.
But, as it turns out, that can’t be right. Nobody’s ever seen half of an electron, or anything less than a whole electron in one well-defined place. The wave function doesn’t tell you how much of the electron is in one place—it tells you the probability
- "In What Is Real? Adam Becker tells a fascinating if complex story of quantum dissidents...An excellent, accessible account."—Wall Street Journal
- "A thorough, illuminating exploration of the most consequential controversy raging in modern science...[Becker] leads us through an impressive account of the rise of competing interpretations, grounding them in the human stories, which are naturally messy and full of contingencies. He makes a convincing case that it's wrong to imagine the Copenhagen interpretation as a single official or even coherent statement."—New York Times Book Review
- "Becker's book is one of the first attempts we have at telling this story in a way that acknowledges how it actually turned out--acknowledges, that is, who won these debates about the Copenhagen interpretation, who lost them, who pretended otherwise, and how they got away with it.... He has clearly done extensive and meticulous historical research."—David Z. Albert, NewYork Review of Books
- "Splendid.... With deeply detailed research, accompanied by charming anecdotes about the scientists...[Becker] hopes to convince us that the Cophenhagen interpretation has had too great an influence on physics for historically contingent reasons."—Washington Post
- "Becker...make[s] a case for the importance of philosophy. That's a key call, with influential scientists such as Neil deGrasse Tyson dismissing the discipline as a waste of time. What Is Real? is an argument for keeping an open mind."—Nature
- "A riveting storyteller, Becker brings to life physicists who have too long remained in the shadow of Bohr and Einstein.... What Is Real? offers an engaging and accessible overview of the debates surrounding the interpretation of quantum mechanics."—Science
- "Impressive...[Becker's] strength is the excavation of stories that show how deeply quantum physics was in thrall to the personalities of its developers. The cast is colourful and expansive, and provides engaging drama...The subtext running through this hugely enjoyable book is that, if we still have a long way to go before we understand reality, we may only have our own prejudices to blame."—New Scientist
- "A joy to read...For anyone who has been intrigued by other popular accounts of the quantum world but came away feeling somewhat cheated by the Copenhagen sleight-of-hand."—Physics World
- "Remarkable...What Is Real? is a superb contribution both to popular understanding of quantum theory and to ongoing debates among experts...It deserves wide attention and careful study."—Physics Today
- "Spellbinding....This very book could prove to be a watershed moment for the physics community if it faces up to its own past and its present....If you have any interest in the implications of quantum theory, or in the suppression of scientific curiosity, What is Real? is required reading. There is no more reliable, careful, and readable account of the whole history of quantum theory in all its scandalous detail."—Boston Review
- "Becker has done a great service in putting this fascinating story together into a single easily-digestible volume that is gripping, authoritative, and true....I sincerely hope it gains an extremely wide readership and manages to have a powerful influence."—Quantum Times
- "A page turner...Becker writes very well...To any one with more than a passing interest in QM, how it came to be the way it is, and how it might be otherwise, this book will be irresistible."—MAA Reviews
- "What Is Real? cuts through the confusion, providing a vivid account of this often arcane field, its history, and its numerous controversies."—Gizmodo
- "[Becker] unveils a story of the competing schools of thought within the weirdest parts of weird physics."—Popular Mechanics
- "Becker...takes readers on a deep dive into the battle for the heart of quantum physics."—Unbound Worlds
- "Admirably explains the intricacies of quantum physics...[Becker] brings to the foreground the hegemonic nature of a dominant scientific theory in crushing dissent."—Scroll India
- "Tremendously appealing...[Becker] smoothly, easily dramatizes the great debates and the outsized personalities of quantum physics and fits it all into an enthusiastic, readable narrative."—Open Letters Monthly
- "[A] fresh debut.... Vivid biographical portraits enliven even dense theoretical explanations with wit and bite.... With his crisp voice, Becker lucidly relates the complicated history of quantum foundations."—Publishers Weekly (starred review)
- "[Becker's] research is as detailed and meticulous...The narrative is enlivened by the personalities of key scientists and physicists....A fascinating revision of pivotal decisions around quantum physics discoveries."—Library Journal
- "A useful introduction to the history of quantum theory for scientifically inclined readers."—Kirkus Reviews
- "Quantum mechanics is an extraordinarily successful theory, but for decades physicists have been in stubborn denial about what the theory actually says about the nature of reality. Adam Becker's wonderful book recounts the colorful history of the reality debates, and makes the case that Einstein was right all along."—Sean Carroll, author of The BigPicture
- "In this immensely well-researched book, Adam Becker explores the little-known history of principled critiques of the quantum physics establishment, tracing a fascinating skein from the Einstein-Bohr debates all the way to the proliferation of alternatives in the 1970s and beyond. Thorough in its detailed treatment of the key players and their contributions, What Is Real? offers an essential guide to the great quantum controversies that are still raging strong."—Paul Halpern, author of TheQuantum Labyrinth
- "Quantum mechanics is astoundingly successful, but its proper meaning and interpretation remains an open question that vexes physicists to this day. Adam Becker brings this topic to life by explaining the science, and the ongoing human struggle to make sense of the quantum world. His book reads like a novel."—Art Friedman, co-author of Quantum Mechanics and Special Relativity and Classical Field Theory
- On Sale
- Sep 3, 2019
- Page Count
- 384 pages
- Basic Books