Spooky Action at a Distance Read online

  “I always thought, and still do, that the discovery and proof of the nonlocality is the single most astonishing discovery of twentieth-century physics,” says Tim Maudlin, a professor at New York University and one of the world’s leading philosophers of physics. In a paper in the late 1990s, he summed up the implications: “The world is not just a set of separately existing localized objects, externally related only by space and time. Something deeper, and more mysterious, knits together the fabric of the world. We have only just come to the moment in the development of physics that we can begin to contemplate what that might be.”

  At the same time, precisely because so much is at stake, other scientists tell me nonlocality can’t be real—that one or another of the nonlocal phenomena will turn out to be a misinterpretation, and that it is a mistake to lump them together. Physicists have had enormous success with spatial reasoning and won’t give it up lightly. One skeptic, Bill Unruh, who is a physics professor at the University of British Columbia, feels much as Einstein did: “If I have to know everything about the universe to know anything, if we take nonlocality seriously, if what happens here depends on what the stars are doing, it makes physics virtually impossible. What makes physics possible is that the world is partitionable. If we really do have to look to the stars to see our future, then I don’t see how we can do physics anymore.”

  Apart from its inherent fascination, nonlocality is an ideal case study for scientific disputes. The disagreements between people such as Maudlin and Unruh are intellectually pure. No economic interests make you suspect ulterior motives. No lobbyists from Exxon-Mobil roam the halls. The adversaries have no overt personal animosity; many are friends. The mathematics is fairly simple; the experimental findings, undisputed. And still the debates drag on for generations. Today’s scholars rehearse arguments that go back to Einstein and those he sparred with in the 1920s and ’30s. Why is that? And what are the rest of us to do when the experts can’t agree?

  Consider the highest-profile scientific debate of recent times: climate change. Most climate scientists think human activity is warming the planet, some holdouts still disagree—and to someone reading a newspaper or surfing the web, the arguments can be baffling. Most people don’t have time to become experts in general circulation models or measurements of longwave radiation. But one thing we will see is that a debate can be resolved in a practical sense regardless of whether the experts go on arguing. In the case of climate change, the public already knows what it needs to. There’s a good chance of climate disaster and it’s only prudent to manage the risk, just as you don’t need a Ph.D. in combustion theory to know you should buy fire insurance for your home. Likewise, in the case of nonlocality, even the most die-hard skeptic now accepts that something very weird is going on, something that forces us to go beyond our deepest-held notions of space and time, something that we need to grasp if we are to know how the universe was born and how the natural world fits together in perfect unity.

  The social stories are not just a sideline to the science. They are directly pertinent, because in a fluid area of research, where ideas jostle and nothing is entirely clear, the conventional ways that people outside science assume it operates—through the application of fact, logic, equations, experiments—aren’t enough to bring closure. Scientists have to reach into gut feelings, metaphorical connections, and judgment calls about the adequacy of their basic principles. In deciding to explore nonlocality, I set off down what looked like a leisurely nature walk, but soon found myself entangled in an exotic rain forest, filled with glistening leaves, labyrinthine byways, and tempting handholds swarming with fire ants. Some physicists thrill to the rebelliousness of questioning one of the oldest and deepest concepts in science. Others shudder at the madness. If locality fails, does it mean our universe is ultimately incomprehensible, as Einstein feared, or can physicists find some other way for it to make sense?

  1

  The Many Varieties of Nonlocality

  Enrique Galvez’s lab at Colgate University is about the size of a two-car garage and, like most people’s garages, jammed with stuff. Along the walls are workbenches loaded with toolboxes, electronic gear in various stages of disrepair, and, on the left side as you enter, the most frequently used piece of equipment: the coffee pot. In the middle of the room are a pair of optical benches: industrial-strength steel platforms, each the size of a dining-room table, covered with a pegboard-like grid of holes for attaching mirrors, prisms, lenses, and filters. “It’s like playing with Erector sets all over again,” says Galvez, a mellow Peruvian who looks remarkably like Al Franken.

  If anyone has taken it on himself to show the world what quantum entanglement looks like, it’s Galvez. Entanglement is the best known of several types of nonlocality that modern physicists have observed, and the one that spooked Einstein. The word “entanglement” has connotations similar to a romantic entanglement: a special and potentially troublesome relationship. Two particles that are entangled with each other are not literally intertwined, like balls of yarn; rather, they have a peculiar bond that transcends space. You can see this effect by creating, deflecting, and measuring beams of light—not ordinary flashlight beams, but beams of entangled photons. The earliest versions of the experiment, done in the 1970s at Berkeley and Harvard, involved mad-scientist contraptions of broiling-hot ovens, stacks of glass panes, and clattering teletypewriters. Galvez has taken advantage of Blu-ray lasers and optical fibers to miniaturize the setup, so that it now fits on a classroom desk.

  Most experimental physicists I’ve met are tinkerers at heart, as fascinated by cool stuff as by the mysteries of the universe. An experimentalist at the Centre for Quantum Technologies in Singapore told me that, in his lab, incoming students have to pass a test. There’s not a single physics question on it. Instead, they have to tell the story of how they took apart some household appliance and managed to get it back together, hopefully before their family found out. Apparently, clothes washers are a popular choice. Galvez, for his part, says his childhood passion was chemistry—of the blowing-up variety. Growing up in a middle-class neighborhood in Lima, he and some friends once tried to make gunpowder. All they got was a smoke bomb, which is perhaps just as well. “It was much more fun than something exploding,” Galvez recalls. “It probably wasn’t very healthy.”

  Galvez says he found his calling as a nonlocality crusader almost by accident. In common with the majority of physicists, he didn’t give much thought to the phenomenon until the late 1990s, when a colleague stopped by his office with some dramatic news: the Austrian physicist Anton Zeilinger and his lab mates had used entanglement to teleport particles from one place to another. Teleport?! No fan of Star Trek could fail to be impressed. Although Zeilinger’s team had beamed only single photons rather than an entire starship landing party, the coolness factor rivaled that of smoke bombs. And the procedure was straightforward. Suppose you want to teleport a photon from the left side of your lab to the right. First, you prime the teleporters by creating a pair of entangled photons and positioning one on each side of the lab. Then, you take the photon you want to beam and let it interact with the left particle. Because the entangled particles have a special bond between them, the interaction is immediately felt on the right, allowing the photon to be reconstituted there. (Some quibble whether the procedure should really be called teleportation; they consider it closer in spirit to identity theft. The experimentalists strip the left particle of its properties and thrust those properties onto the right particle. But a particle is nothing more than the sum of its properties, so these two characterizations amount to the same thing.)

  Galvez and his colleague already had all the gear, and before long, they were beaming particles across their lab, too. “We were trying to figure out teleportation just for the fun of it,” Galvez says. Another colleague suggested they design an entanglement experiment that even a physics-for-poets class could do. It doesn’t do teleportation, but achieves the first and most important step in the
process—namely, creating and distributing the entangled photons. As simple as the apparatus looks now, the team sweated over it for two years. Galvez began to run summer workshops for ALPhA, a physics-education group, to show teachers how to do the experiment, and he posted his instruction manuals online so that do-it-yourselfers can entangle particles in their basements. The former president of ALPhA, David Van Baak, exclaims: “We’re past the stage where entanglement is a research-university-only affair. It’s getting out to the masses.”

  On the day I visit Galvez’s lab, one of his optical benches is given over to the entanglement experiment, the aim of which is not only to demonstrate entanglement, but also to explore what might be causing it. I recognize the setup as basically a high-tech Rube Goldberg coin flipper in which photons assume the role of coins. They are either “heads” or “tails” depending on whether they pass through a filter or not. The system is tuned so they have a 50-50 chance of getting through, like flipping a fair coin. The basic plan is to create a pair of these coins, flip both at the same time, see which sides they land on, create another pair, flip them, and so on. Repeat thousands of times and add up the statistics. It seems like a lot of effort for a predictable result, until you remember that we’re talking about quantum coins. Clearly, thinking of particles as coins is a metaphor, but as long as you don’t take it too literally, it’s completely kosher. Physicists themselves understand phenomena in terms of metaphor.

  To set the apparatus into motion, Galvez fires an ultraviolet laser through a series of optical elements that ensure proper alignment of the light. The beam strikes a small crystal of barium borate, a material discovered by Chinese scientists in the early 1980s, which splits the ultraviolet beam into two red beams. The splitting occurs particle by particle: if you could zoom in and view the beam as a stream of photons, you would see some of the ultraviolet photons hit the crystal and divide their energy into identical twin red photons. Voilà, coins. Located just upstream of the crystal is an optical element known as a waveplate, which Galvez uses to control the output of the crystal. Depending on how he sets the waveplate, the red photons are either entangled or not.

  1.1. Setup of quantum entanglement experiment. (Illustration by Jen Christensen)

  Once the red beams diverge, they cease to interact. Galvez aims each beam at a polarizing filter, much like the ones that photographers screw onto the front of their lenses to cut down on glare. The filter lets photons through or blocks them depending on their orientation—their polarization. Galvez can turn a dial on the side of the filter to control which photons make it. For this experiment, he sets both filters to the same setting, one that admits half the photons at random, thereby simulating coin flips.

  Photons that make it through the filters are sent to detectors that convert them to electrical pulses. These detectors are the you-break-’em-you-bought-’em part of the system. Being sensitive enough to pick up individual photons, they run $4,000 apiece and are easily damaged by bright light. Even with the room lights off, the detectors pulse wildly, because the minutest sliver of light will set them off. Watching them gives me a new appreciation for how bright a supposedly dark room can be. We have to make sure our phones and laptops are fully off; a single glowing LED might spoil the experiment. “A while back we had to put black tape over anything that lit in the lab,” Galvez says. “You would be surprised how many of those lights there are.” He drapes a black velvet cloth over the devices and draws a thick curtain around the entire bench.

  Finally, the detectors are wired into a meter with three digital readouts, located safely outside the curtain. Two show the number of photons that make it through the left and right polarizing filters. When Galvez switches on the laser, those numbers flash by like milliseconds on a stopwatch. The third readout shows the “coincidences”—when both photons in a pair make it through their respective filters. In terms of the coin metaphor, a coincidence means that both coins have landed on heads. These coincidences are Galvez’s window into quantum nonlocality.

  Having given me the tour, Galvez is ready to take some data. To verify that everything is working properly, he first simulates flipping ordinary coins by setting the waveplate to produce unentangled photons. The meter reads about twenty-five coincidences per second. For comparison, you’d get one hundred coincidences per second if every single photon in every single pair made it through the filters. So, the coincidence rate is about a quarter of its maximum possible value. This is just what you’d expect from the laws of chance. If you take two coins and flip them, each will come up heads about half the time, so both will be heads about a quarter of the time.

  Now Galvez adjusts the waveplate to generate entangled photons. The coincidence rate jumps to about fifty per second. A change from twenty-five to fifty on a digital readout in a basement lab might not seem like much. But that’s physics for you. It takes effort to peer beneath the surface of the world around us, and the clues are subtle, but they are no less dramatic for that. All those years of waiting and preparing for this moment have paid off, because when I see that fifty, I realize what I am seeing, and I shiver. The photons are behaving like a pair of magic coins. Galvez flips thousands of such pairs, and both always land on the same side: either both heads or both tails. That kind of thing doesn’t happen by pure dumb luck.

  1.2. Sample results of coins experiment. If you flip a pair of ordinary coins, they’ll land on the same side half the time, on average. But if you flip a pair of suitably prepared quantum entangled “coins,” they will always land on the same side.

  If a friend of mine did this trick at a party—flip pairs of coins so that both came up heads twice as often as they rightfully should—I’d assume it was a practical joke. My friend might have gone to a magic shop and bought double-sided coins, which look the same on both sides, making the outcome of a flip preordained. Could an equivalent stunt explain the pattern I was seeing in Galvez’s lab? To test for such trickery, Galvez uses a tactic proposed by the Irish particle physicist John Stewart Bell in the 1960s. He turns one of the filters by an angle of 90 degrees, which, like flipping a coin with your left hand rather than your right, doesn’t alter the probability of a particle getting through; if the outcome really is predetermined, nothing should change. But this seemingly innocuous change does have an effect on the photons. The coincidence meter drops nearly to zero—meaning that if one photon gets through, the other never does. In other words, the magic coins have switched from always landing on the same side to always landing on opposite sides. A practical jokester would need some extra sleight of hand to pull off this trick. By making further refinements, Galvez rules out any conceivable chicanery.

  I go over and look at the optical bench again. Those filters are separated by the width of my hand. Experiments by Zeilinger and others have stretched the distance to one hundred miles, and researchers at the Centre for Quantum Technologies are working on a space-based version that will go even farther. For a tiny particle, that might as well be the other side of the universe. The photons manage to coordinate their behavior across that gap. They are not in contact, and no known force links them, yet they act as one. When Galvez dials the polarizer filter on the left side of his lab bench and a photon passes through, the photon will be polarized in the same direction as the filter. Its entangled partner follows in lockstep: it acquires the same polarization and will respond accordingly to its own filter. So, what happens on the left affects the photon on the right, even when there’s no time for any kind of influence to cross the gap. Indeed, such an influence would need to travel from left to right instantly—that is, infinitely fast, which is plainly faster than light, in apparent defiance of the theory of relativity. This is one of the many mysteries posed by nonlocality. Physicists have commented that it is as close to real magic as they’ve ever seen. “Students love it,” Galvez says. “The good students say, ‘I want to figure this out.’”

  Shut Up and Calculate

  Is nonlocality just a carnival freak show—f
un to ooh and aah over, but having no broader implications—or does it belong on the center stage of physics? For most of the twentieth century, physicists treated it as a freak show, and as a student I adopted this attitude, too. It wasn’t until years later, when I delved into Tim Maudlin’s book Quantum Nonlocality and Relativity, that I appreciated the depth of the mystery.

  Sitting in his George Nakashima–furnished living room, Maudlin tells me he’ll never forget the moment he learned about quantum nonlocality. One day in the fall of 1979, while a physics major at Yale University, he opened up the latest issue of Scientific American magazine. The cover story was about dung beetles, but he flipped past it and landed on an article on the early entanglement experiments. For particles to act as if by magic stunned Maudlin. “I remember the day when I read that article,” he says. “My roommates remember that day. I walked around and around my room. The world wasn’t what I thought it was. It bugged the hell out of me.”