WHAT IS REAL?
The Unfinished Quest for the Meaning of Quantum Physics
Are atoms real? Of course they are. Everybody believes in atoms, even people who don’t believe in evolution or climate change. If we didn’t have atoms, how could we have atomic bombs? But you can’t see an atom directly. And even though atoms were first conceived and named by ancient Greeks, it was not until the last century that they achieved the status of actual physical entities — real as apples, real as the moon.
The first proof of atoms came from 26-year-old Albert Einstein in 1905, the same year he proposed his theory of special relativity. Before that, the atom served as an increasingly useful hypothetical construct. At the same time, Einstein defined a new entity: a particle of light, the “light quantum,” now called the photon. Until then, everyone considered light to be a kind of wave. It didn’t bother Einstein that no one could observe this new thing. “It is the theory which decides what we can observe,” he said.
Which brings us to quantum theory. The physics of atoms and their ever-smaller constituents and cousins is, as Adam Becker reminds us more than once in his new book, “What Is Real?,” “the most successful theory in all of science.” Its predictions are stunningly accurate, and its power to grasp the unseen ultramicroscopic world has brought us modern marvels. But there is a problem: Quantum theory is, in a profound way, weird. It defies our common-sense intuition about what things are and what they can do.
“Figuring out what quantum physics is saying about the world has been hard,” Becker says, and this understatement motivates his book, a thorough, illuminating exploration of the most consequential controversy raging in modern science.
The debate over the nature of reality has been growing in intensity for more than a half-century; it generates conferences and symposiums and enough argumentation to fill entire journals. Before he died, Richard Feynman, who understood quantum theory as well as anyone, said, “I still get nervous with it…I cannot define the real problem, therefore I suspect there’s no real problem, but I’m not sure there’s no real problem.” The problem is not with using the theory — making calculations, applying it to engineering tasks — but in understanding what it means. What does it tell us about the world?
From one point of view, quantum physics is just a set of formalisms, a useful tool kit. Want to make better lasers or transistors or television sets? The Schrödinger equation is your friend. The trouble starts only when you step back and ask whether the entities implied by the equation can really exist. Then you encounter problems that can be described in several familiar ways:
Wave-particle duality. Everything there is — all matter and energy, all known forces — behaves sometimes like waves, smooth and continuous, and sometimes like particles, rat-a-tat-tat. Electricity flows through wires, like a fluid, or flies through a vacuum as a volley of individual electrons. Can it be both things at once?
The uncertainty principle. Werner Heisenberg famously discovered that when you measure the position (let’s say) of an electron as precisely as you can, you find yourself more and more in the dark about its momentum. And vice versa. You can pin down one or the other but not both.
The measurement problem. Most of quantum mechanics deals with probabilities rather than certainties. A particle has a probability of appearing in a certain place. An unstable atom has a probability of decaying at a certain instant. But when a physicist goes into the laboratory and performs an experiment, there is a definite outcome. The act of measurement — observation, by someone or something — becomes an inextricable part of the theory.
The strange implication is that the reality of the quantum world remains amorphous or indefinite until scientists start measuring. Schrödinger’s cat, as you may have heard, is in a terrifying limbo, neither alive nor dead, until someone opens the box to look. Indeed, Heisenberg said that quantum particles “are not as real; they form a world of potentialities or possibilities rather than one of things or facts.”
This is disturbing to philosophers as well as physicists. It led Einstein to say in 1952, “The theory reminds me a little of the system of delusions of an exceedingly intelligent paranoiac.”
So quantum physics — quite unlike any other realm of science — has acquired its own metaphysics, a shadow discipline tagging along like the tail of a comet. You can think of it as an “ideological superstructure” (Heisenberg’s phrase). This field is called quantum foundations, which is inadvertently ironic, because the point is that precisely where you would expect foundations you instead find quicksand.
Competing approaches to quantum foundations are called “interpretations,” and nowadays there are many. The first and still possibly foremost of these is the so-called Copenhagen interpretation. “Copenhagen” is shorthand for Niels Bohr, whose famous institute there served as unofficial world headquarters for quantum theory beginning in the 1920s. In a way, the Copenhagen is an anti-interpretation. “It is wrong to think that the task of physics is to find out how nature is,” Bohr said. “Physics concerns what we can say about nature.”
Nothing is definite in Bohr’s quantum world until someone observes it. Physics can help us order experience but should not be expected to provide a complete picture of reality. The popular four-word summary of the Copenhagen interpretation is: “Shut up and calculate!”
For much of the 20th century, when quantum physicists were making giant leaps in solid-state and high-energy physics, few of them bothered much about foundations. But the philosophical difficulties were always there, troubling those who cared to worry about them.
Becker sides with the worriers. He 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. It is, he suggests, a “strange assemblage of claims.”
An American physicist, David Bohm, devised a radical alternative at midcentury, visualizing “pilot waves” that guide every particle, an attempt to eliminate the wave-particle duality. For a long time, he was mainly lambasted or ignored, but variants of the Bohmian interpretation have supporters today. Other interpretations rely on “hidden variables” to account for quantities presumed to exist behind the curtain. Perhaps the most popular lately — certainly the most talked about — is the “many-worlds interpretation”: Every quantum event is a fork in the road, and one way to escape the difficulties is to imagine, mathematically speaking, that each fork creates a new universe.
So in this view, Schrödinger’s cat is alive and well in one universe while in another she goes to her doom. And we, too, should imagine countless versions of ourselves. Everything that can happen does happen, in one universe or another. “The universe is constantly splitting into a stupendous number of branches,” said the theorist Bryce DeWitt, “every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies of itself.”
This is ridiculous, of course. “A heavy load of metaphysical baggage,” John Wheeler called it. How could we ever prove or disprove such a theory? But if you think the many-worlds idea is easily dismissed, plenty of physicists will beg to differ. They will tell you that it could explain, for example, why quantum computers (which admittedly don’t yet quite exist) could be so powerful: They would delegate the work to their alter egos in other universes.
Is any of this real? At the risk of spoiling its suspense, I will tell you that this book does not propose a definite answer to its title question. You weren’t counting on one, were you? The story is far from finished.
When scientists search for meaning in quantum physics, they may be straying into a no-man’s-land between philosophy and religion. But they can’t help themselves. They’re only human. “If you were to watch me by day, you would see me sitting at my desk solving Schrödinger’s equation…exactly like my colleagues,” says Sir Anthony Leggett, a Nobel Prize winner and pioneer in superfluidity. “But occasionally at night, when the full moon is bright, I do what in the physics community is the intellectual equivalent of turning into a werewolf: I question whether quantum mechanics is the complete and ultimate truth about the physical universe.”