What Is Real?

Introduction

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.

Figure I.1. Schrödinger’s cat. When the metal gives off radiation, the Geiger counter will register it and drop the hammer, releasing the cyanide and killing the cat.

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.