One of the most unsettling things learned in the last 50 years is that the universe is not real where you are. “Real” means that things have clear properties even when no one is looking at them. For example, an apple can be red even when no one is looking at it. “Local” means that things can only be affected by their surroundings and that effects can’t move faster than light. Researchers in quantum physics have found that these two ideas can’t both be true. Instead, the evidence shows that things are affected by more than just their surroundings and that they may not have clear properties before they are measured. “Do you really think the moon doesn’t exist when you aren’t looking at it?” Albert Einstein asked a friend this famous question.
This goes against everything we know and see every day. To paraphrase Douglas Adams, the end of local realism has made a lot of people very angry and is generally seen as a bad thing.
The accomplishment has now been attributed to three physicists: John Clauser, Alain Aspect, and Anton Zeilinger. They were awarded the Nobel Prize in Physics in 2022 in equal parts “for experiments with entangled photons, establishing the violation of Bell inequalities, and pioneering quantum information science.” (“Bell inequalities” alludes to the early 1960s pioneering work of Northern Irish physicist John Stewart Bell, who established the groundwork for this year’s Physics Nobel.) Colleagues felt that the trio deserved this punishment for upending reality as we know it. “This is wonderful news. “It had been a long time coming,” Sandu Popescu, a quantum physicist at the University of Bristol, says. “There is no doubt that the award 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.”
It took a long time for quantum foundations to move from the fringe to the mainstream. From about 1940 until as late as 1990, people often thought the topic was either philosophy or nonsense. Many scientific journals wouldn’t publish papers about quantum foundations, and it was hard to find academic jobs that let you do research in this area. In 1985, Popescu’s advisor told him that he shouldn’t get a Ph.D. in that field.
“He said ‘look, if you do that, you will have fun for five years, and then you will be jobless,’” Popescu says.
Quantum information science is one of the most exciting and important parts of physics right now. It links Einstein’s theory of general relativity to quantum mechanics by looking at how black holes behave, which is still a mystery. It tells how quantum sensors, which are used to study everything from earthquakes to dark matter, are made and how they work. Quantum entanglement is a very important part of modern materials science and is at the heart of quantum computing. This book explains how it works and why it can be hard to understand.
“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.”
WHOM DOES THE BELL RING?
Quantum mechanics has never been wrong because it made wrong predictions. In fact, when it was first made by physicists in the early 20th century, the theory described the microscopic world very well from the start.
In their famous 1935 paper, Einstein, Boris Podolsky, and Nathan Rosen wrote about what they didn’t like about the theory: the uncomfortable things it said about the real world. Their analysis, which is known by the initials EPR, was based on a thought experiment that was meant to show how crazy quantum mechanics is, to show how the theory can break or at least give results that don’t make sense and go against everything else we know about reality. A version of EPR that is easier to understand and more up-to-date goes something like this: Particles are sent in pairs in different directions from a single source. These particles are aimed at two observers, Alice and Bob, who are at opposite ends of the solar system. Quantum mechanics says that you can’t know a particle’s spin, which is a quantum property, before you measure it. Alice finds that when she measures one of her particles, its spin is either up or down. Even though her results are random, she knows right away that Bob’s particle must be down when she measures up. At first glance, this doesn’t seem so strange. Maybe the particles are like a pair of socks: if Alice gets the right sock, Bob must get the left.
But according to quantum mechanics, particles are not like socks, and they don’t know whether they are spinning up or down until they are measured. This is the biggest problem with EPR: If Alice’s particles don’t have a spin until they are measured, how do they know what Bob’s particles will do as they leave the solar system in the opposite direction? Every time Alice takes a measurement, she is, in effect, asking her particle what Bob will get if he flips a coin: up or down? Even if you got this right 200 times in a row, the odds are 1 in 1060, which is more than all the atoms in the solar system. Quantum mechanics says that even though the pairs of particles are separated by billions of kilometers, Alice’s particles can keep making correct predictions, as if they were telepathically linked to Bob’s particles.
Even though the EPR thought experiment was meant to show how quantum mechanics isn’t perfect, when it’s done in the real world, the results support the most strange parts of the theory. Quantum mechanics says that nature is not real locally. Before being measured, particles don’t have properties like spinning up or down, and they seem to talk to each other no matter how far apart they are.
Some physicists who didn’t believe in quantum mechanics thought that there were “hidden variables” that existed in a level of reality below the subatomic level and held information about how a particle would behave in the future. In hidden-variable theories, they hoped that nature could get back the local realism that quantum mechanics took away.
“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.
Physicists, on the other hand, didn’t agree with Einstein’s “attack” on quantum mechanics. They mostly accepted quantum mechanics as it was. This was often less a thoughtful acceptance of nonlocal reality than a desire to not think too hard while doing physics. The physicist David Mermin later called this “head-in-the-sand” thinking a demand to “shut up and calculate.”
Part of the reason why people weren’t interested was that in 1932, a well-known scientist named John von Neumann published a mathematical proof that showed hidden-variable theories couldn’t be true. Von Neumann’s proof was shown to be wrong by a young woman mathematician named Grete Hermann just three years later, but no one seemed to notice at the time.
The problem of nonlocal realism in quantum mechanics would lie dormant for another 30 years until John Bell broke it in a decisive way. From the beginning of his career, Bell was bothered by the orthodoxy of quantum physics and interested in theories with hidden variables. In 1952, he found out that fellow physicist David Bohm had come up with a workable nonlocal hidden-variable interpretation of quantum mechanics, which von Neumann had said was impossible. This gave him an idea. Bell thought about the ideas for years, when he wasn’t busy with his main job as a particle physicist at CERN.
In 1964, Bell found the same flaws that Hermann had found in von Neumann’s argument. Then, in a great example of careful thinking, Bell came up with a theorem that brought the question of hidden variables out of the muddy waters of metaphysics and onto the solid ground of experiment.
Theories with hidden variables and quantum mechanics usually predict the same experimental results. What Bell realized is that there can be a real difference between the two in certain situations. In the famous Bell test, which is an updated version of the EPR thought experiment, Alice and Bob get the same pair of particles, but now their detectors are set to A and a and B and b, respectively. With these settings, Alice and Bob can ask the particles different questions, which is another way to make it look like they can talk to each other. In local hidden-variable theories, where their states are fixed and nothing connects them, particles can’t get around this extra step, and they can’t always get the perfect correlation where Alice measures spin down when Bob measures spin up (and vice versa). But in quantum mechanics, particles stay linked and have a lot more things in common than they ever could in local hidden-variable theories. In a word, they are mixed up.
By measuring the correlation between many different pairs of particles, you could show which theory was right. If the correlation stayed below a limit based on Bell’s theorem, it would mean that hidden variables are real. If it went above Bell’s limit, however, the strange rules of quantum mechanics would rule. Even though Bell’s theorem could help figure out what reality is, it was published in a relatively unknown journal and didn’t get much attention for years.
THE BELL CHIMES FOR THEE
In 1967, when John Clauser was a graduate student at Columbia University, he found a copy of Bell’s paper by accident in the library. He was fascinated by the idea of proving hidden-variable theories right. Clauser wrote to Bell two years later and asked if the test had ever been done. Clauser’s letter was one of the first things Bell heard about what he had done.
Five years later, Clauser and his graduate student Stuart Freedman did the first Bell test. Bell had encouraged them to do so. Clauser had permission from his bosses but not much money, so he became good at “dumpster diving” to get equipment, which he and Freedman then put together with duct tape, as he said in an interview years later. In Clauser’s setup, which was about the size of a kayak and had to be tuned by hand, pairs of photons were sent in opposite directions toward detectors that could measure their state, or polarization.
Unfortunately for Clauser and his love of hidden variables, he and Freedman could not help but come to the conclusion that they had found strong evidence against them once they had finished their analysis. Still, the result was not very conclusive because the experiment had a number of “loopholes” that could have let the influence of hidden variables slip through without being noticed. The one that worried them the most was the locality loophole: if either the photon source or the detectors could have shared information (which is possible in an object the size of a kayak), the correlations that were measured could still come from hidden variables. Kaiser puts it simply: if Alice tweets to Bob which setting she’s using on the detector, that interference makes it impossible to rule out hidden variables.
It is easier to say how to close the locality loophole than to do it. While photons are moving, the setting on the detector must be changed quickly. By “quickly,” we mean in a matter of nanoseconds. In 1976, a young French optics expert named Alain Aspect came up with a way to make this switch happen very quickly. Clauser’s results were supported by the experimental results of his group, which were published in 1982. It seemed very unlikely that there were local hidden variables.
In response to Aspect’s first results, Bell wrote, “Perhaps Nature is not as strange as quantum mechanics.” “But from this point of view, the experimental situation is not very hopeful.”
But there were still other holes, and Bell didn’t get to see them closed because he died in 1990. Even Aspect’s experiment could not completely rule out local effects because it was too short. Clauser and others had also figured out that Alice and Bob could come to the wrong conclusions if they didn’t make sure to look for an unbiased, representative sample of particles.
Anton Zeilinger, an ambitious and friendly Austrian physicist, jumped at the chance to close these holes faster than anyone else. In 1998, he and his team improved on Aspect’s earlier work by doing a Bell test over a distance of nearly half a kilometer. This was the first time this had ever been done. Experiments in the size of a kayak were no longer a good way to figure out that reality doesn’t have a single location. In 2013, Zeilinger’s group finally did the next logical thing, which was to close more than one loophole 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 thing that was needed to make an experiment that closed multiple holes was a 60-meter tunnel that was perfectly straight and had access to fiber optic cables. As it turned out, the dungeon of Vienna’s Hofburg palace was almost the perfect place, except that it was covered in dust from a century ago. Their results, which were published in 2015, were the same as those of two other groups that did similar tests and found that quantum mechanics is still as good as ever.
BELL’S TEST HITS THE HEIGHTS
One big hole still needed to be closed, or at least made smaller. Any physical connection between components that happened in the past, no matter how long ago, could change the results of a Bell test. If Alice shakes Bob’s hand before getting on a spaceship, that means they’ve known each other in the past. It seems unlikely that a local hidden-variable theory would take advantage of these holes, but it is still possible.
In 2017, Kaiser and Zeilinger were part of a team that did a cosmic Bell test. Using telescopes in the Canary Islands, the team chose random settings for the detectors based on the positions of stars in the sky that were so far apart that light from one star wouldn’t reach the other for hundreds of years. This made sure that the stars’ pasts were not the same for hundreds of years. Even so, quantum mechanics came out on top again.
One of the main problems with trying to explain the importance of Bell tests to the general public and to skeptic physicists is that most people think that quantum mechanics is true. After all, scientists have measured many important parts of quantum mechanics with an accuracy of 10 parts in a billion or more.
“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 couldn’t 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.”