In What Is Real? The Unfinished Quest for the Meaning of Quantum Physics, Adam Becker traces decades of experiments, alternative philosophies, and surprising drama in the quantum physics boys’ club to three intriguing possibilities: “Either nature is nonlocal in some way, or we live in branching multiple worlds despite appearances to the contrary”—or quantum physics is incomplete. Here, Becker lays out a fundamental problem with quantum physics, despite its success and usefulness.
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?
The seemingly contradictory nature of quantum objects—atoms that are capable of being here and there at the same time—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.
So what is going on down there in the realm of electrons and nuclei? And how does it give rise to the familiar world around us? Surprisingly, despite the fact that quantum physics has been around for nearly a century, there are no widely-accepted answers to these questions among physicists. The question of how to interpret quantum physics is hotly debated, and has been since the days of Albert Einstein, Niels Bohr, and the other early quantum pioneers of the 1920s. There are a wide variety of possible answers, all deeply strange, and all giving radically different pictures of the quantum realm: waves that communicate faster than light, a multitude of nearly-identical universes continually spinning off from each other, and dozens more options. And we don’t know which answer is the right one—which means that after all this time, we still don’t really know what quantum physics is telling us about the nature of reality.
Yet why do the answers to such questions matter at all? After all, quantum physics certainly works. Why should anyone outside of physics care about these conundrums at the heart of an otherwise-successful theory? 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. Science and culture form an undivided whole, now more than ever, in our world whose every corner has been reshaped by human activity. If the past is any guide at all, finding the answer to the puzzle of quantum theory will ultimately affect the daily lives of every human being, not just the professional lives of physicists.