Image Description: John Stewart Bell (1928-1990), the Northern Irish physicist whose work sparked a quiet revolution in quantum physics. Credit: Peter Menzel/Science Source |
The fact that the universe is not locally real is one of the
more disquieting discoveries of the last half-century. "Real," which
indicates that objects have defined attributes independent of observation—for
example, an apple can be red even when no one is looking; "local,"
which means that objects can only be impacted by their surroundings, and that
any effect cannot travel faster than light. Quantum physics researchers have
discovered that these concepts cannot both be true. Instead, the evidence
suggests that objects are not only influenced by their surroundings, and that
they may lack distinct properties prior to measurement. "Do you honestly
believe the moon isn't there when you're not gazing at it?" Albert
Einstein famously asked a friend.
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.
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.”
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.”
WHOM DOES THE BELL RING?
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 1 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 IS RINGING 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 GOES TO 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.”
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