One of the more unsettling discoveries in the past half century is that the universe is not locally real. “Real,” meaning that objects have definite properties independent of observation—an apple can be red even when no one is looking; “local” means objects can only be influenced by their surroundings, and that any influence cannot travel faster than light. Investigations at the frontiers of quantum physics have found that these things cannot both be true. Instead, the evidence shows objects are not influenced solely by their surroundings and they may also lack definite properties prior to measurement. As Albert Einstein famously bemoaned to a friend, “Do you really believe the moon is not there when you are not looking at it?”
This is, of course, deeply contrary to our everyday experiences. To paraphrase Douglas Adams, the demise of local realism has made a lot of people very angry and been widely regarded as a bad move.
Blame for this achievement has now been laid squarely on the shoulders of three physicists: John Clauser, Alain Aspect and Anton Zeilinger. They equally split the 2022 Nobel Prize in Physics “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science.” (“Bell inequalities” refers to the pioneering work of the Northern Irish physicist John Stewart Bell, who laid the foundations for this year’s Physics Nobel in the early 1960s.) Colleagues agreed that the trio had it coming, deserving this reckoning for overthrowing reality as we know it. “It is fantastic news. It was long overdue,” says Sandu Popescu, a quantum physicist at the University of Bristol. “Without any doubt, the prize is well-deserved.”
“The experiments beginning with the earliest one of Clauser and continuing along, show that this stuff isn’t just philosophical, it’s real—and like other real things, potentially useful,” says Charles Bennett, an eminent quantum researcher at IBM.
“Each year I thought, ‘oh, maybe this is the year,’” says David Kaiser, a physicist and historian at the Massachusetts Institute of Technology. “This year, it really was. It was very emotional—and very thrilling.”
Quantum foundations’ journey from fringe to favor was a long one. From about 1940 until as late as 1990, the topic was often treated as philosophy at best and crackpottery at worst. Many scientific journals refused to publish papers in quantum foundations, and academic positions indulging such investigations were nearly impossible to come by. In 1985, Popescu’s advisor warned him against a Ph.D. in the subject. “He said ‘look, if you do that, you will have fun for five years, and then you will be jobless,’” Popescu says.
Today, quantum information science is among the most vibrant and impactful subfields in all of physics. It links Einstein’s general theory of relativity with quantum mechanics via the still-mysterious behavior of black holes. It dictates the design and function of quantum sensors, which are increasingly being used to study everything from earthquakes to dark matter. And it clarifies the often-confusing nature of quantum entanglement, a phenomenon that is pivotal to modern materials science and that lies at the heart of quantum computing.
“What even makes a quantum computer ‘quantum’?” Nicole Yunger Halpern, a National Institute of Standards and Technology physicist, asks rhetorically. “One of the most popular answers is entanglement, and the main reason why we understand entanglement is the grand work participated in by Bell and these Nobel Prize–winners. Without that understanding of entanglement, we probably wouldn’t be able to realize quantum computers.”
For Whom the Bell Tolls
The trouble with quantum mechanics was never that it made the wrong predictions—in fact, the theory described the microscopic world splendidly well right from the start when physicists devised it in the opening decades of the 20th century.
What Einstein, Boris Podolsky and Nathan Rosen took issue with, laid out in their iconic 1935 paper, was the theory’s uncomfortable implications for reality. Their analysis, known by their initials EPR, centered on a thought experiment meant to illustrate the absurdity of quantum mechanics; to show how under certain conditions the theory can break—or at least deliver nonsensical results that conflict with everything else we know about reality. A simplified and modernized version of EPR goes something like this: Pairs of particles are sent off in different directions from a common source, targeted for two observers, Alice and Bob, each stationed at opposite ends of the solar system. Quantum mechanics dictates that it is impossible to know the spin, a quantum property of individual particles prior to measurement. When Alice measures one of her particles, she finds its spin to be either up or down. Her results are random, and yet, when she measures up, she instantly knows Bob’s corresponding particle must be down. At first glance, this is not so odd; perhaps the particles are like a pair of socks—if Alice gets the right sock, Bob must have the left.
But under quantum mechanics, particles are not like socks, and only when measured do they settle on a spin of up or down. This is EPR’s key conundrum: If Alice’s particles lack a spin until measurement, how then when they whiz past Neptune do they know what Bob’s particles will do as they fly out of the solar system in the other direction? Each time Alice measures, she effectively quizzes her particle on what Bob will get if he flips a coin: up, or down? The odds of correctly predicting this even 200 times in a row are one in 1060—a number greater than all the atoms in the solar system. Yet despite the billions of kilometers that separate the particle pairs, quantum mechanics says Alice’s particles can keep correctly predicting, as though they were telepathically connected to Bob’s particles.
Although intended to reveal the imperfections of quantum mechanics, when real-world versions of the EPR thought experiment are conducted the results instead reinforce the theory’s most mind-boggling tenets. Under quantum mechanics, nature is not locally real—particles lack properties such as spin up or spin down prior to measurement, and seemingly talk to one another no matter the distance.
Physicists skeptical of quantum mechanics proposed that there were “hidden variables,” factors that existed in some imperceptible level of reality beneath the subatomic realm that contained information about a particle’s future state. They hoped in hidden-variable theories, nature could recover the local realism denied to it by quantum mechanics.
“One would have thought that the arguments of Einstein, Podolsky and Rosen would produce a revolution at that moment, and everybody would have started working on hidden variables,” Popescu says.
Einstein’s “attack” on quantum mechanics, however, did not catch on among physicists, who by and large accepted quantum mechanics as is. This was often less a thoughtful embrace of nonlocal reality, and more a desire to not think too hard while doing physics—a head-in-the-sand sentiment later summarized by the physicist David Mermin as a demand to “shut up and calculate.”
The lack of interest was driven in part because John von Neumann, a highly regarded scientist, had in 1932 published a mathematical proof ruling out hidden-variable theories. (Von Neumann’s proof, it must be said, was refuted just three years later by a young female mathematician, Grete Hermann, but at the time no one seemed to notice.)
Quantum mechanics’ problem of nonlocal realism would languish in a complacent stupor for another three decades until being decisively shattered by Bell. From the start of his career, Bell was bothered by the quantum orthodoxy and sympathetic toward hidden variable theories. Inspiration struck him in 1952, when he learned of a viable nonlocal hidden-variable interpretation of quantum mechanics devised by fellow physicist David Bohm—something von Neumann had claimed was impossible. Bell mulled the ideas over for years, as a side project to his main job working as a particle physicist at CERN.
In 1964, Bell rediscovered the same flaws in von Neumann’s argument that Hermann had. And then, in a triumph of rigorous thinking, Bell concocted a theorem that dragged the question of hidden variables from its metaphysical quagmire onto the concrete ground of experiment.
Normally, hidden-variable theories and quantum mechanics predict indistinguishable experimental outcomes. What Bell realized is that under precise circumstances, an empirical discrepancy between the two can emerge. In the eponymous Bell test (an evolution of the EPR thought experiment), Alice and Bob receive the same paired particles, but now they each have two different detector settings—A and a, B and b. These detector settings allow Alice and Bob to ask the particles different questions; an additional trick to throw off their apparent telepathy. In local hidden-variable theories, where their state is preordained and nothing links them, particles cannot outsmart this extra step, and they cannot always achieve the perfect correlation where Alice measures spin down when Bob measures spin up (and vice versa). But in quantum mechanics, particles remain connected and far more correlated than they could ever be in local hidden-variable theories. They are, in a word, entangled.
Measuring the correlation multiple times for many particle pairs, therefore, could prove which theory was correct. If the correlation remained below a limit derived from Bell’s theorem, this would suggest hidden variables were real; if it exceeded Bell’s limit, then the mind-boggling tenets of quantum mechanics would reign supreme. And yet, in spite of its potential to help determine the very nature of reality, after being published in a relatively obscure journal Bell’s theorem languished unnoticed for years.
The Bell Tolls for Thee
In 1967, John Clauser, then a graduate student at Columbia University, accidentally stumbled across a library copy of Bell’s paper and became enthralled by the possibility of proving hidden-variable theories correct. Clauser wrote to Bell two years later, asking if anyone had actually performed the test. Clauser’s letter was among the first feedback Bell had received.
With Bell’s encouragement, five years later Clauser and his graduate student Stuart Freedman performed the first Bell test. Clauser had secured permission from his supervisors, but little in the way of funds, so he became, as he said in a later interview, adept at “dumpster diving” to secure equipment—some of which he and Freedman then duct-taped together. In Clauser’s setup—a kayak-sized apparatus requiring careful tuning by hand—pairs of photons were sent in opposite directions toward detectors that could measure their state, or polarization.
Unfortunately for Clauser and his infatuation with hidden variables, once he and Freedman completed their analysis, they could not help but conclude that they had found strong evidence against them. Still, the result was hardly conclusive, because of various “loopholes” in the experiment that conceivably could allow the influence of hidden variables to slip through undetected. The most concerning of these was the locality loophole: if either the photon source or the detectors could have somehow shared information (a plausible feat within the confines of a kayak-sized object), the resulting measured correlations could still emerge from hidden variables. As Kaiser puts it pithily, if Alice tweets at Bob which detector setting she’s in, that interference makes ruling out hidden variables impossible.
Closing the locality loophole is easier said than done. The detector setting must be quickly changed while photons are on the fly—“quickly” meaning in a matter of mere nanoseconds. In 1976, a young French expert in optics, Alain Aspect, proposed a way for doing this ultra-speedy switch. His group’s experimental results, published in 1982, only bolstered Clauser’s results: local hidden variables looked extremely unlikely. “Perhaps Nature is not so queer as quantum mechanics,” Bell wrote in response to Aspect’s initial results. “But the experimental situation is not very encouraging from this point of view.”
Other loopholes, however, still remained—and, alas, Bell died in 1990 without witnessing their closure. Even Aspect’s experiment had not fully ruled out local effects because it took place over too small a distance. Similarly, as Clauser and others had realized, if Alice and Bob were not ensured to detect an unbiased representative sample of particles, they could reach the wrong conclusions.
No one pounced to close these loopholes with more gusto than Anton Zeilinger, an ambitious, gregarious Austrian physicist. In 1998, he and his team improved on Aspect’s earlier work by conducting a Bell test over a then-unprecedented distance of nearly half a kilometer. The era of divining reality’s nonlocality from kayak-sized experiments had drawn to a close. Finally, in 2013, Zeilinger’s group took the next logical step, tackling multiple loopholes at the same time.
“Before quantum mechanics, I actually was interested in engineering. I like building things with my hands,” says Marissa Giustina, a quantum researcher at Google who worked with Zeilinger. “In retrospect, a loophole-free Bell experiment is a giant systems-engineering project.” One requirement for creating an experiment closing multiple loopholes was finding a perfectly straight, unoccupied 60-meter tunnel with access to fiber optic cables. As it turned out, the dungeon of Vienna’s Hofburg palace was an almost ideal setting—aside from being caked with a century’s worth of dust. Their results, published in 2015, coincided with similar tests from two other groups that also found quantum mechanics as flawless as ever.
Bell’s Test Reaches the Stars
One great final loophole remained to be closed, or at least narrowed. Any prior physical connection between components, no matter how distant in the past, has the possibility of interfering with the validity of a Bell test’s results. If Alice shakes Bob’s hand prior to departing on a spaceship, they share a past. It is seemingly implausible that a local hidden-variable theory would exploit these loopholes, but still possible.
In 2017, a team including Kaiser and Zeilinger performed a cosmic Bell test. Using telescopes in the Canary Islands, the team sourced its random decisions for detector settings from stars sufficiently far apart in the sky that light from one would not reach the other for hundreds of years, ensuring a centuries-spanning gap in their shared cosmic past. Yet even then, quantum mechanics again proved triumphant.
One of the principal difficulties in explaining the importance of Bell tests to the public—as well as to skeptical physicists—is the perception that the veracity of quantum mechanics was a foregone conclusion. After all, researchers have measured many key aspects of quantum mechanics to a precision of greater than 10 parts in a billion. “I actually didn’t want to work on it. I thought, like, ‘Come on; this is old physics. We all know what’s going to happen,’” Giustina says. But the accuracy of quantum mechanics could not rule out the possibility of local hidden variables; only Bell tests could do that.
“What drew each of these Nobel recipients to the topic, and what drew John Bell himself, to the topic was indeed [the question], ‘Can the world work that way?’” Kaiser says. “And how do we really know with confidence?” What Bell tests allow physicists to do is remove the bias of anthropocentric aesthetic judgments from the equation; purging from their work the parts of human cognition that recoil at the possibility of eerily inexplicable entanglement, or that scoff at hidden-variable theories as just more debates over how many angels may dance on the head of a pin. The award honors Clauser, Aspect and Zeilinger, but it is testament to all the researchers who were unsatisfied with superficial explanations about quantum mechanics, and who asked their questions even when doing so was unpopular.
“Bell tests,” Giustina concludes, “are a very useful way of looking at reality.”