Spooky Action at a Distance Read online

  It also bugged him that his physics professors, like mine, had never once mentioned this phenomenon. When he probed them about it, they blew him off. Once, Maudlin recalls, he raised his hand in class and asked whether quantum theory might not give way to a deeper theory in which the seeming contradictions would make perfect sense. The professor dismissed the idea and went back to scribbling Greek letters on the blackboard. “He didn’t offer any explanation at all of why not,” Maudlin says. “So he shut down the question without answering it.”

  * * *

  To appreciate the mind block that Maudlin and I ran into, you have to go back to the famous debates between Einstein and another of the founders of quantum mechanics, the Danish physicist Niels Bohr, in the 1920s and ’30s. Einstein worried that nonlocality would contradict his theory of relativity and argued that it had to be a kind of illusion, reflecting our ignorance of some essential aspect of nature. Bohr argued … well, nobody is quite sure what Bohr argued. His reasoning gave “tangled” a whole new meaning, and his missives have been interpreted as either championing or contesting nonlocality. To the extent that anyone does understand what he said, he was asserting that it didn’t matter what weirdness lay behind the scenes, as long as the theory could predict what experiments saw.

  As anyone who has watched an American presidential debate knows, judgments about “win” or “lose” often have little to do with what the debaters actually say. Most physicists just wanted the Bohr-Einstein debate to be over, so they could get on with applying quantum mechanics to practical problems. Because Bohr promised closure, they rallied around him and wrote off Einstein as a has-been. One later wrote that Einstein’s “fame would be undiminished, if not enhanced, had he gone fishing instead.”

  Over the subsequent decades, physicists used quantum theory to do all sorts of useful calculations. They figured out transistors, lasers, and other mainstays of the modern world. So the collective decision to set aside questions about the theory’s deeper meaning seemed justified. Whenever those conceptual questions did come up, physicists deemed them “philosophical,” which wasn’t intended as a compliment, but as a way to deny that the questions were even worth asking. The English physicist Paul Dirac wrote, “It is only the philosopher, wanting to have a satisfying description of nature, who is bothered.”

  Because Maudlin was, in fact, bothered, he decided to go to graduate school in philosophy rather than physics. “I want to get to the bottom of everything,” Maudlin says. “That’s what you do as a philosopher.” Philosophy is distinguished not just by its interests, but by its methods: philosophers are trained in logic as opposed to mathematics or experiments. Maudlin gained a reputation among philosophers as Dr. Takedown, able to spot the flaw in almost any argument. Throughout his graduate studies and early years as a professor, Maudlin says, nonlocality sizzled in the back of his mind. But no one else he knew seemed interested, and in some ways philosophers were as much in thrall to the principle of locality as physicists were. Other things kept getting in the way of Maudlin’s thinking more about it—until the fall of 1990, when John Stewart Bell died.

  Bell had done more than anyone else to reopen the case of Einstein versus Bohr. He began to doubt Bohr’s victory while a university student in the early 1950s, but realized that airing his misgivings wouldn’t do his career any favors. By the mid-’60s, having made a name for himself by studying particles and designing particle accelerators, including the precursors of the Large Hadron Collider, Bell felt secure enough to revisit his youthful concerns. He showed that nonlocality was no longer just a matter for debate; you could play with it in the lab. Like Einstein, Bell struggled to convince his colleagues. His first paper on the subject was not cited by a single other paper for four years and not mentioned in textbooks until 1985. Even when Bell’s work did get attention, it was apt to be misinterpreted. One of his obituaries was titled “The Man Who Proved Einstein Was Wrong,” which totally missed the man’s point that nonlocality transcended the old debate. Einstein may have been wrong to think that nonlocality would prove to be merely apparent, but Bohr was wrong to have ignored it altogether.

  Like Einstein, Bell fretted that nonlocality defied the theory of relativity. Physicists can’t give up quantum theory; it passes all experimental tests. For relativity to be wrong is equally unthinkable. In a lecture in 1984, Bell concluded, “We have an apparent incompatibility, at the deepest level, between the two fundamental pillars of contemporary theory.” Even those who were otherwise sympathetic to him saw no such incompatibility. In creating relativity theory, Einstein thought about how we gather information. Signals such as light or sound must pass from objects in the world to our senses. If those signals travel instantaneously, they can conflict. Paradoxes ensue. Things both happen and don’t happen. The machinery of the universe seizes up. Yet quantum magic coins don’t pose any such risk. They are inherently incapable of signaling. They land on either heads or tails—you can’t dictate which. You have no way to control them in order to transmit a message or, indeed, do anything at all. So you could never use them to bring about a paradoxical situation. Danger averted.

  In other words, if entanglement is magic, it is not like a magic wand that you can wave to make things happen. Rather, the magic happens spontaneously, and you notice it only if you’re looking carefully. This is a very attenuated form of magic that won’t win you any wizarding cups. Most everybody reassured themselves that quantum mechanics and the theory of relativity live in “peaceful coexistence.”

  In Bell’s memory, several Rutgers University philosophers organized a symposium on quantum physics and asked Maudlin to speak. Picking up where he’d left off as a student, he proceeded to take down the lore that had grown up around Einstein’s and Bell’s findings. The conventional vision of theoretical harmony struck Maudlin as a trifle too harmonious. “Just pointing out that you can’t send signals never seemed enough for me to show that there was no fundamental conflict with relativity,” he says. Even if a pair of entangled particles can’t convey a signal, quantum theory still says that what happens to one instantly affects the other. The theory therefore requires that the universe have a kind of master clock, ensuring that what is 7:30 p.m. to one particle is 7:30 p.m. to another. And relativity theory denies any such thing. The reason they call it relativity theory is that the passage of time is relative. Two events that are simultaneous for one person can be sequential for another.

  Maudlin’s book grew out of his talk, and its publication coincided with a surge of interest in entanglement. Experimentalists, realizing that the phenomenon wasn’t as useless as they’d thought, were beginning to exploit it for cryptography and computers. For instance, Artur Ekert, a physicist at the University of Oxford and currently the director of the Centre for Quantum Technologies, proved in 1991 that entangled particles can create a communications channel so secure that not even the sneakiest government surveillance program could listen in. Once physicists were clued in to the importance of entanglement, they began to see it almost everywhere they looked. It occurs even in living organisms. In photosynthesis, entanglement accounts for the unexpectedly high efficiency with which molecules transfer light energy into chemical energy, thereby helping to enable life on our planet.

  By the turn of the millennium, Einstein’s paper that got it all started had become one of the most widely cited articles in the history of physics. Meanwhile, the old walls between physicists and philosophers were tumbling down. Zeilinger, the pioneering experimentalist, often disagrees with Maudlin, but exchanges ideas with him in a way that would have been unthinkable twenty years ago. “This connection between philosophy and physics is crucial to making real progress,” Zeilinger tells me.

  Quantum nonlocality is clearly not just a dinner act in Vegas, but an essential aspect of the world, and physicists and philosophers still don’t know what is behind the magic. Could the clues they seek lie in other domains of science? What can they learn from the other types of nonlocality that are out there in the world?

  The Skywatcher and the Ice Climber

  For most of the twentieth century, the peculiar synchronicity of entangled particles was the only type of nonlocality that rated any mention. But physicists gradually realized that other phenomena are suspiciously spooky, too. Those who study black holes think that matter in these cosmic vacuum cleaners may jump from one place to another without crossing the intervening distance, a type of nonlocality that is arguably even more baffling than the situation Einstein worried about.

  Black holes have long been physicists’ top choice for weirdest things in the universe. Ramesh Narayan has seen them in action. Like Galvez, Narayan says he came to his scientific passion late and almost by accident. As a boy, he had zero interest in astronomy. He’s one of the few astrophysicists I’ve met who doesn’t recall being obsessed with black holes as a child. Crystals were his thing. But in his first job, at the prestigious Raman Research Institute in Bangalore, in southern India, he found himself mingling with people exploring the mysteries of the universe, and soon he was hooked. He became an expert on cosmic flows of gas. The governing principle of these flows is simple: what goes down must come up. Whenever gas crashes into the surface of a star, it acts to warm up the star; the star, in turn, emits the energy back into space, typically as infrared radiation or visible light. “All the energy falling in has to come out,” explains Narayan, who is now a Harvard professor. But in the early 1990s astronomers noticed a peculiar exception to this rule at the center of our galaxy.

  The center of the galaxy is easy to see for yourself. The next time you go outside to gaze at the night sky, find the constellation of Sagittarius. From my hometown, it is easiest to see in the summer and early fall, hanging in the sky near the south horizon. It is supposed to look like an archer, but most astronomers think of it as a giant teapot. The spout points to the very center of the Milky Way. To our eyes, the center is just a fuzzy patch of sky, but in the 1940s telescopes began to reveal a cauldron of swirling gas there. At the very center, gas converges on a tiny region known as Sagittarius A*. This region is puzzlingly dim; less than 1 percent of the energy carried by the inflowing gas comes back out. “In front of our eyes, you see the energy going into the center and vanishing—poof,” Narayan says.

  That is the definition of a black hole. Its gravity is so powerful that what goes down never comes up. Artist’s conceptions sometimes depict a black hole as a giant funnel in space, but from the outside it looks more like a planet—a big, suspiciously dark planet. Material can and usually does orbit around it. But if you tried to touch what you thought was its surface, your hand would just pass through; the object is empty space. The supposed surface, or “event horizon,” is really just a hypothetical point of no return, where infalling gas or other matter could reverse course only by exceeding the speed of light. For Sagittarius A*, the event horizon is a sphere about 25 million kilometers across. Matter crossing it just keeps on going, like a car entering a one-way dead-end street, hurtling toward some uncertain and presumably unhappy fate. “It’s the one unique feature of a black hole,” Narayan says. “The black hole has no surface, and that makes all the difference. The gas and all the energy it’s carrying is just swallowed.”

  Well, what happens to the stuff, then? That’s the puzzle. Unfortunately, the two main theories that physicists have—gravity theory and quantum theory—reach diametrically opposite conclusions about the fate of swallowed material. Simply put, gravity theory says that falling into a black hole is irreversible, while quantum theory says nothing is irreversible. The former says stuff can’t get out; it’s assimilated into the black hole permanently. The latter says stuff must get out and partake in the life of the cosmos once again. What gives? This contradiction is a red warning light that some seemingly essential principle of modern physics must be wrong.

  Narayan’s observations can’t settle the issue. Resolving the contradictions of black holes will take a unified theory of physics, one that fuses quantum theory and gravity theory into a quantum theory of gravity. And many of those who have been seeking such a theory think the suspect principle will prove to be locality. If material can travel faster than light or vault from inside to outside without crossing the intervening space, it can slip the hole’s surly bonds.

  A leading proponent of this idea is Steve Giddings. He is a professor at the University of California, Santa Barbara, although, with his cargo shorts, fleece jacket, and untucked plaid shirts, you might mistake him for a mountain trekking guide. Which isn’t too far from the truth: he has appeared in both Scientific American and Climbing magazines. Giddings is accomplished at rock and ice climbing, downhill and cross-country skiing, mountaineering, and kayaking. He sees his scientific and outdoors passions as complementary. “I feel that these are two sides of intimately relating to nature,” he says. On camping trips as a boy, he read physics books; in college, he got a National Science Foundation grant to do research on gravitation, while on weekends he skied the backcountry. The summer after graduating, Giddings built his own kayak and paddled the Colorado River through the Grand Canyon. Then he hitchhiked to Denali National Park, the first of several trips there. He remembers watching a caribou and its calf cross his path, oddly untroubled by his presence. “Then I looked back and I saw why they weren’t fazed by me,” he says. “They were escaping from a big grizzly bear. At that point, the bear decided to follow me.” Recalling a park ranger’s briefing, Giddings stood his ground and yelled at the bear until it shuffled off in search of easier pickings.

  And then he moved to New Jersey. New Jersey has many charms, but mountains and canyons are not among them. Not that Giddings had a life anymore. Days, nights, weekdays, weekends revolved around cramming for exams. Princeton’s graduate physics program seemed intent on capsizing his kayak. “There wasn’t a lot of encouragement,” he says. “It was an atmosphere where the students felt very intimidated.” Giddings considered running, but stood his ground, and just as he passed his exams, in 1984, the field of theoretical physics lit up with excitement. Researchers around the world dropped everything else they were doing and took up string theory, a proposed unified theory of nature.

  String theory gets its name from the idea that subatomic particles are tiny rubber bands or guitar strings. What we perceive as different types of particles are really just those strings vibrating in different ways, making the world a symphony of unimaginable complexity. The theory had languished in obscurity since the late 1960s, and a tipping point came when its few proponents managed to persuade the majority of its internal consistency. “That was the thing to do—I got swept up in the waves,” Giddings recalls. The theory’s leading figure, Edward Witten, asked him to solve a crucial formula, and after months of struggle, trying one mathematical technique after another, he nailed it. Meanwhile, he met some fellow kayakers and discovered that the Garden State wasn’t completely undeserving of the name. “I started to realize, maybe this could work,” he says.

  Resolving the contradictions of black holes was one of the main reasons to seek a unified theory, and in 1990 Giddings decided to retrace the steps that led to a paradox, laid out by the celebrated University of Cambridge theorist Stephen Hawking in the mid-’70s. Hawking’s starting point was that decay is the rule of nature. Almost everything in this world eventually dies. Black holes are not exempt—nor could they be, if they are to form in the first place. Ruin is creation in reverse. “If you can make a black hole out of random junk, then you can have a black hole decay to random junk,” Giddings says.

  By Hawking’s analysis, decay doesn’t mean that the black hole’s innards ooze out. How could they? To escape the event horizon, the innards would have to ooze faster than light. Instead, the hole rots from the outside in. The horizon throws the electric field, magnetic field, and other force fields off balance, causing them to shed particles like so many rust flakes. A black hole equal in mass to our Sun releases about one particle per second, which is far too feeble for astrophysicists such as Narayan to detect with their instruments, but enough over trillions of years to reduce the hole to a jumbled, formless spray of particles. The structure the infalling matter had, the information it embodied, every last trace of its identity—all is lost. In other words, the black hole is not just irreversible in the sense that stuff can’t get back out. That might not be so troubling, since, from a godlike perspective, you could peer into the hole and reconstruct how objects came to be there. But the hole is also irreversible in that it obliterates matter with such thoroughness that not even a god could recover the original.

  As Hawking himself pointed out, his calculations were tough to do. He could track how the black hole affects the outgoing particles, but not how the outgoing particles affect the black hole—and this reciprocal effect could conceivably open a back door between exterior and interior, allowing infalling matter to reemerge. If so, falling into a black hole would be reversible after all, and the paradox would evaporate. So Giddings and several colleagues did a new analysis, based on string theory, to search for escape hatches and hiding spots Hawking’s calculations might have missed. They found none. Hawking had been right. “In these simple models, you really vindicate Hawking’s original picture,” Giddings says.

  So, there’s no easy way out of the paradox (let alone the hole). Some assumption that goes into the argument must be wrong, and there are only really two assumptions: reversibility and locality. At first, Hawking faulted the former. He suggested that quantum theory is wrong and falling into the hole is irreversible. Yet quantum mechanics seems to be an all-or-nothing package: if it breaks down anywhere, it breaks down everywhere. If it misfires in the way Hawking supposed, we should see parallel failings in ordinary settings, and we don’t. Hawking eventually came to agree that black holes must be reversible. By default, locality must be wrong. “I’m continuing to beat my head on the question of how the information gets out—it just seems to have to be nonlocal,” Giddings says.