That question irked and inspired Hungarian-American physicist Eugene Wigner in the 1960s. He was frustrated by the paradoxes arising from the vagaries of quantum mechanics—the theory governing the microscopic realm that suggests, among many other counterintuitive things, that until a quantum system is observed, it does not necessarily have definite properties. Take his fellow physicist Erwin Schrödinger’s famous thought experiment in which a cat is trapped in a box with poison that will be released if a radioactive atom decays. Radioactivity is a quantum process, so before the box is opened, the story goes, the atom has both decayed and not decayed, leaving the unfortunate cat in limbo—a so-called superposition between life and death. But does the cat experience being in superposition?
Wigner sharpened the paradox by imagining a (human) friend of his shut in a lab, measuring a quantum system. He argued it was absurd to say his friend exists in a superposition of having seen and not seen a decay unless and until Wigner opens the lab door. “The ‘Wigner’s friend’ thought experiment shows that things can become very weird if the observer is also observed,” says Nora Tischler, a quantum physicist at Griffith University in Brisbane, Australia.
Now Tischler and her colleagues have carried out a version of the Wigner’s friend test. By combining the classic thought experiment with another quantum head-scratcher called entanglement—a phenomenon that links particles across vast distances—they have also derived a new theorem, which they claim puts the strongest constraints yet on the fundamental nature of reality. Their study, which appeared in Nature Physics on August 17, has implications for the role that consciousness might play in quantum physics—and even whether quantum theory must be replaced.
The new work is an “important step forward in the field of experimental metaphysics,” says quantum physicist Aephraim Steinberg of the University of Toronto, who was not involved in the study. “It’s the beginning of what I expect will be a huge program of research.”
A MATTER OF TASTE
Until quantum physics came along in the 1920s, physicists expected their theories to be deterministic, generating predictions for the outcome of experiments with certainty. But quantum theory appears to be inherently probabilistic. The textbook version—sometimes called the Copenhagen interpretation—says that until a system’s properties are measured, they can encompass myriad values. This superposition only collapses into a single state when the system is observed, and physicists can never precisely predict what that state will be. Wigner held the then popular view that consciousness somehow triggers a superposition to collapse. Thus, his hypothetical friend would discern a definite outcome when she or he made a measurement—and Wigner would never see her or him in superposition.
This view has since fallen out of favor. “People in the foundations of quantum mechanics rapidly dismiss Wigner’s view as spooky and ill-defined because it makes observers special,” says David Chalmers, a philosopher and cognitive scientist at New York University. Today most physicists concur that inanimate objects can knock quantum systems out of superposition through a process known as decoherence. Certainly, researchers attempting to manipulate complex quantum superpositions in the lab can find their hard work destroyed by speedy air particles colliding with their systems. So they carry out their tests at ultracold temperatures and try to isolate their apparatuses from vibrations.
Several competing quantum interpretations have sprung up over the decades that employ less mystical mechanisms, such as decoherence, to explain how superpositions break down without invoking consciousness. Other interpretations hold the even more radical position that there is no collapse at all. Each has its own weird and wonderful take on Wigner’s test. The most exotic is the “many worlds” view, which says that whenever you make a quantum measurement, reality fractures, creating parallel universes to accommodate every possible outcome. Thus, Wigner’s friend would split into two copies and, “with good enough supertechnology,” he could indeed measure that person to be in superposition from outside the lab, says quantum physicist and many-worlds fan Lev Vaidman of Tel Aviv University.
The alternative “Bohmian” theory (named for physicist David Bohm) says that at the fundamental level, quantum systems do have definite properties; we just do not know enough about those systems to precisely predict their behavior. In that case, the friend has a single experience, but Wigner may still measure that individual to be in a superposition because of his own ignorance. In contrast, a relative newcomer on the block called the QBism interpretation embraces the probabilistic element of quantum theory wholeheartedly (QBism, pronounced “cubism,” is actually short for quantum Bayesianism, a reference to 18th-century mathematician Thomas Bayes’s work on probability.) QBists argue that a person can only use quantum mechanics to calculate how to calibrate his or her beliefs about what he or she will measure in an experiment. “Measurement outcomes must be regarded as personal to the agent who makes the measurement,” says Ruediger Schack of Royal Holloway, University of London, who is one of QBism’s founders. According to QBism’s tenets, quantum theory cannot tell you anything about the underlying state of reality, nor can Wigner use it to speculate on his friend’s experiences.
Another intriguing interpretation, called retrocausality, allows events in the future to influence the past. “In a retrocausal account, Wigner’s friend absolutely does experience something,” says Ken Wharton, a physicist at San Jose State University, who is an advocate for this time-twisting view. But that “something” the friend experiences at the point of measurement can depend upon Wigner’s choice of how to observe that person later.
The trouble is that each interpretation is equally good—or bad—at predicting the outcome of quantum tests, so choosing between them comes down to taste. “No one knows what the solution is,” Steinberg says. “We don’t even know if the list of potential solutions we have is exhaustive.”
Other models, called collapse theories, do make testable predictions. These models tack on a mechanism that forces a quantum system to collapse when it gets too big—explaining why cats, people and other macroscopic objects cannot be in superposition. Experiments are underway to hunt for signatures of such collapses, but as yet they have not found anything. Quantum physicists are also placing ever larger objects into superposition: last year a team in Vienna reported doing so with a 2,000-atom molecule. Most quantum interpretations say there is no reason why these efforts to supersize superpositions should not continue upward forever, presuming researchers can devise the right experiments in pristine lab conditions so that decoherence can be avoided.Collapse theories, however, posit that a limit will one day be reached, regardless of how carefully experiments are prepared. “If you try and manipulate a classical observer—a human, say—and treat it as a quantum system, it would immediately collapse,” says Angelo Bassi, a quantum physicist and proponent of collapse theories at the University of Trieste in Italy.
A WAY TO WATCH WIGNER’S FRIEND
Tischler and her colleagues believed that analyzing and performing a Wigner’s friend experiment could shed light on the limits of quantum theory. They were inspired by a new wave of theoretical and experimental papers that have investigated the role of the observer in quantum theory by bringing entanglement into Wigner’s classic setup. Say you take two particles of light, or photons, that are polarized so that they can vibrate horizontally or vertically. The photons can also be placed in a superposition of vibrating both horizontally and vertically at the same time, just as Schrödinger’s paradoxical cat can be both alive and dead before it is observed.
Such pairs of photons can be prepared together—entangled—so that their polarizations are always found to be in the opposite direction when observed. That may not seem strange—unless you remember that these properties are not fixed until they are measured. Even if one photon is given to a physicist called Alice in Australia, while the other is transported to her colleague Bob in a lab in Vienna, entanglement ensures that as soon as Alice observes her photon and, for instance, finds its polarization to be horizontal, the polarization of Bob’s photon instantly syncs to vibrating vertically. Because the two photons appear to communicate faster than the speed of light—something prohibited by his theories of relativity—this phenomenon deeply troubled Albert Einstein, who dubbed it “spooky action at a distance.”
These concerns remained theoretical until the 1960s, when physicist John Bell devised a way to test if reality is truly spooky—or if there could be a more mundane explanation behind the correlations between entangled partners. Bell imagined a commonsense theory that was local—that is, one in which influences could not travel between particles instantly. It was also deterministic rather than inherently probabilistic, so experimental results could, in principle, be predicted with certainty, if only physicists understood more about the system’s hidden properties. And it was realistic, which, to a quantum theorist, means that systems would have these definite properties even if nobody looked at them. Then Bell calculated the maximum level of correlations between a series of entangled particles that such a local, deterministic and realistic theory could support. If that threshold was violated in an experiment, then one of the assumptions behind the theory must be false.
Such “Bell tests” have since been carried out, with a series of watertight versions performed in 2015, and they have confirmed reality’s spookiness. “Quantum foundations is a field that was really started experimentally by Bell’s [theorem]—now over 50 years old. And we’ve spent a lot of time reimplementing those experiments and discussing what they mean,” Steinberg says. “It’s very rare that people are able to come up with a new test that moves beyond Bell.”
The Brisbane team’s aim was to derive and test a new theorem that would do just that, providing even stricter constraints—“local friendliness” bounds—on the nature of reality. Like Bell’s theory, the researchers’ imaginary one is local. They also explicitly ban “superdeterminism”—that is, they insist that experimenters are free to choose what to measure without being influenced by events in the future or the distant past. (Bell implicitly assumed that experimenters can make free choices, too.) Finally, the team prescribes that when an observer makes a measurement, the outcome is a real, single event in the world—it is not relative to anyone or anything.
Testing local friendliness requires a cunning setup involving two “superobservers,” Alice and Bob (who play the role of Wigner), watching their friends Charlie and Debbie. Alice and Bob each have their own interferometer—an apparatus used to manipulate beams of photons. Before being measured, the photons’ polarizations are in a superposition of being both horizontal and vertical. Pairs of entangled photons are prepared such that if the polarization of one is measured to be horizontal, the polarization of its partner should immediately flip to be vertical. One photon from each entangled pair is sent into Alice’s interferometer, and its partner is sent to Bob’s. Charlie and Debbie are not actually human friends in this test. Rather, they are beam displacers at the front of each interferometer. When Alice’s photon hits the displacer, its polarization is effectively measured, and it swerves either left or right, depending on the direction of the polarization it snaps into. This action plays the role of Alice’s friend Charlie “measuring” the polarization. (Debbie similarly resides in Bob’s interferometer.)
Alice then has to make a choice: She can measure the photon’s new deviated path immediately, which would be the equivalent of opening the lab door and asking Charlie what he saw. Or she can allow the photon to continue on its journey, passing through a second beam displacer that recombines the left and right paths—the equivalent of keeping the lab door closed. Alice can then directly measure her photon’s polarization as it exits the interferometer. Throughout the experiment, Alice and Bob independently choose which measurement choices to make and then compare notes to calculate the correlations seen across a series of entangled pairs.
Tischler and her colleagues carried out 90,000 runs of the experiment. As expected, the correlations violated Bell’s original bounds—and crucially, they also violated the new local-friendliness threshold. The team could also modify the setup to tune down the degree of entanglement between the photons by sending one of the pair on a detour before it entered its interferometer, gently perturbing the perfect harmony between the partners. When the researchers ran the experiment with this slightly lower level of entanglement, they found a point where the correlations still violated Bell’s bound but not local friendliness. This result proved that the two sets of bounds are not equivalent and that the new local-friendliness constraints are stronger, Tischler says. “If you violate them, you learn more about reality,” she adds. Namely, if your theory says that “friends” can be treated as quantum systems, then you must either give up locality, accept that measurements do not have a single result that observers must agree on or allow superdeterminism. Each of these options has profound—and, to some physicists, distinctly distasteful—implications.
“The paper is an important philosophical study,” says Michele Reilly, co-founder of Turing, a quantum-computing company based in New York City, who was not involved in the work. She notes that physicists studying quantum foundations have often struggled to come up with a feasible test to back up their big ideas. “I am thrilled to see an experiment behind philosophical studies,” Reilly says. Steinberg calls the experiment “extremely elegant” and praises the team for tackling the mystery of the observer’s role in measurement head-on.
Although it is no surprise that quantum mechanics forces us to give up a commonsense assumption—physicists knew that from Bell—“the advance here is that we are a narrowing in on which of those assumptions it is,” says Wharton, who was also not part of the study. Still, he notes, proponents of most quantum interpretations will not lose any sleep. Fans of retrocausality, such as himself, have already made peace with superdeterminism: in their view, it is not shocking that future measurements affect past results. Meanwhile QBists and many-worlds adherents long ago threw out the requirement that quantum mechanics prescribes a single outcome that every observer must agree on.
And both Bohmian mechanics and spontaneous collapse models already happily ditched locality in response to Bell. Furthermore, collapse models say that a real macroscopic friend cannot be manipulated as a quantum system in the first place.
Vaidman, who was also not involved in the new work, is less enthused by it, however, and criticizes the identification of Wigner’s friend with a photon. The methods used in the paper “are ridiculous; the friend has to be macroscopic,” he says. Philosopher of physics Tim Maudlin of New York University, who was not part of the study, agrees. “Nobody thinks a photon is an observer, unless you are a panpsychic,” he says. Because no physicist questions whether a photon can be put into superposition, Maudlin feels the experiment lacks bite. “It rules something out—just something that nobody ever proposed,” he says.
Tischler accepts the criticism. “We don’t want to overclaim what we have done,” she says. The key for future experiments will be scaling up the size of the “friend,” adds team member Howard Wiseman, a physicist at Griffith University. The most dramatic result, he says, would involve using an artificial intelligence, embodied on a quantum computer, as the friend. Some philosophers have mused that such a machine could have humanlike experiences, a position known as the strong AI hypothesis, Wiseman notes, though nobody yet knows whether that idea will turn out to be true. But if the hypothesis holds, this quantum-based artificial general intelligence (AGI) would be microscopic. So from the point of view of spontaneous collapse models, it would not trigger collapse because of its size. If such a test was run, and the local-friendliness bound was not violated, that result would imply that an AGI’s consciousness cannot be put into superposition. In turn, that conclusion would suggest that Wigner was right that consciousness causes collapse. “I don’t think I will live to see an experiment like this,” Wiseman says. “But that would be revolutionary.”
Reilly, however, warns that physicists hoping that future AGI will help them home in on the fundamental description of reality are putting the cart before the horse. “It’s not inconceivable to me that quantum computers will be the paradigm shift to get to us into AGI,” she says. “Ultimately, we need a theory of everything in order to build an AGI on a quantum computer, period, full stop.”
That requirement may rule out more grandiose plans. But the team also suggests more modest intermediate tests involving machine-learning systems as friends, which appeals to Steinberg. That approach is “interesting and provocative,” he says. “It’s becoming conceivable that larger- and larger-scale computational devices could, in fact, be measured in a quantum way.”
Renato Renner, a quantum physicist at the Swiss Federal Institute of Technology Zurich (ETH Zurich), makes an even stronger claim: regardless of whether future experiments can be carried out, he says, the new theorem tells us that quantum mechanics needs to be replaced. In 2018 Renner and his colleague Daniela Frauchiger, then at ETH Zurich, published a thought experiment based on Wigner’s friend and used it to derive a new paradox. Their setup differs from that of the Brisbane team but also involves four observers whose measurements can become entangled. Renner and Frauchiger calculated that if the observers apply quantum laws to one another, they can end up inferring different results in the same experiment.
“The new paper is another confirmation that we have a problem with current quantum theory,” says Renner, who was not involved in the work. He argues that none of today’s quantum interpretations can worm their way out of the so-called Frauchiger-Renner paradox without proponents admitting they do not care whether quantum theory gives consistent results. QBists offer the most palatable means of escape, because from the outset, they say that quantum theory cannot be used to infer what other observers will measure, Renner says. “It still worries me, though: If everything is just personal to me, how can I say anything relevant to you?” he adds. Renner is now working on a new theory that provides a set of mathematical rules that would allow one observer to work out what another should see in a quantum experiment.
Still, those who strongly believe their favorite interpretation is right see little value in Tischler’s study. “If you think quantum mechanics is unhealthy, and it needs replacing, then this is useful because it tells you new constraints,” Vaidman says. “But I don’t agree that this is the case—many worlds explains everything.”
For now, physicists will have to continue to agree to disagree about which interpretation is best or if an entirely new theory is needed. “That’s where we left off in the early 20th century—we’re genuinely confused about this,” Reilly says. “But these studies are exactly the right thing to do to think through it.”
Disclaimer: The author frequently writes for the Foundational Questions Institute, which sponsors research in physics and cosmology and partially funded the Brisbane team’s study.