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What does it feel like to be both alive and dead?<\/p>\n
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\u2014the 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 Schr\u00f6dinger\u2019s 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\u2014a so-called superposition between life and death. But does the cat experience being in superposition?<\/p>\n
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. \u201cThe \u2018Wigner\u2019s friend\u2019 thought experiment shows that things can become very weird if the observer is also observed,\u201d says Nora Tischler, a quantum physicist at Griffith University in Australia.<\/p>\n
Now Tischler and her colleagues have carried out a version of the Wigner\u2019s friend test. By combining the classic thought experiment with another quantum head-scratcher called entanglement\u2014a phenomenon that links particles across vast distances\u2014they 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<\/em> on August 17, has implications for the role that consciousness might play in quantum physics\u2014and even whether quantum theory must be replaced.<\/p>\n The new work is an \u201cimportant step forward in the field of experimental metaphysics,\u201d says quantum physicist Aephraim Steinberg of the University of Toronto, who was not involved in the study. \u201cIt\u2019s the beginning of what I expect will be a huge program of research.\u201d<\/p>\n 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\u2014sometimes called the Copenhagen interpretation\u2014says that until a system\u2019s 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\u2014and Wigner would never see her or him in superposition.<\/p>\n This view has since fallen out of favor. \u201cPeople in the foundations of quantum mechanics rapidly dismiss Wigner\u2019s view as spooky and ill-defined because it makes observers special,\u201d 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.<\/p>\n 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\u2019s test. The most exotic is the \u201cmany worlds\u201d view, which says that whenever you make a quantum measurement, reality fractures, creating parallel universes to accommodate every possible outcome. Thus, Wigner\u2019s friend would split into two copies and, \u201cwith good enough supertechnology,\u201d 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.<\/p>\n The alternative \u201cBohmian\u201d 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 \u201ccubism,\u201d is actually short for quantum Bayesianism, a reference to 18th-century mathematician Thomas Bayes\u2019s 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. \u201cMeasurement outcomes must be regarded as personal to the agent who makes the measurement,\u201d says Ruediger Schack of Royal Holloway, University of London, who is one of QBism\u2019s founders.\u00a0According to QBism\u2019s tenets, quantum theory cannot tell you anything about the underlying state of reality, nor can Wigner use it to speculate on his friend\u2019s experiences.<\/p>\n Another intriguing interpretation, called retrocausality, allows events in the future to influence the past. \u201cIn a retrocausal account, Wigner\u2019s friend absolutely does experience something,\u201d says Ken Wharton, a physicist at San Jose State University, who is an advocate for this time-twisting view. But that \u201csomething\u201d the friend experiences at the point of measurement can depend upon Wigner\u2019s choice of how to observe that person later.<\/p>\n The trouble is that each interpretation is equally good\u2014or bad\u2014at predicting the outcome of quantum tests, so choosing between them comes down to taste. \u201cNo one knows what the solution is,\u201d Steinberg says. \u201cWe don\u2019t even know if the list of potential solutions we have is exhaustive.\u201d<\/p>\n 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\u2014explaining 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. <\/strong>Collapse theories, however, posit that a limit will one day be reached, regardless of how carefully experiments are prepared. \u201cIf you try and manipulate a classical observer\u2014a human, say\u2014and treat it as a quantum system, it would immediately collapse,\u201d says Angelo Bassi, a quantum physicist and proponent of collapse theories at the University of Trieste in Italy.<\/p>\n Tischler and her colleagues believed that analyzing and performing a Wigner\u2019s 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\u2019s 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 Schr\u00f6dinger\u2019s paradoxical cat can be both alive and dead before it is observed.<\/p>\n Such pairs of photons can be prepared together\u2014entangled\u2014so that their polarizations are always found to be in the opposite direction when observed. That may not seem strange\u2014unless 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\u2019s photon instantly syncs to vibrating vertically. Because the two photons appear to communicate faster than the speed of light\u2014something prohibited by his theories of relativity\u2014this phenomenon deeply troubled Albert Einstein, who dubbed it \u201cspooky action at a distance.\u201d<\/p>\n These concerns remained theoretical until the 1960s, when physicist John Bell devised a way to test if reality is truly spooky\u2014or if there could be a more mundane explanation behind the correlations between entangled partners. Bell imagined a commonsense theory that was local\u2014that 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\u2019s 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.<\/p>\n Such \u201cBell tests\u201d have since been carried out, with a series of watertight versions performed in 2015, and they have confirmed reality\u2019s spookiness. \u201cQuantum foundations is a field that was really started experimentally by Bell\u2019s [theorem]\u2014now over 50 years old. And we\u2019ve spent a lot of time reimplementing those experiments and discussing what they mean,\u201d Steinberg says. \u201cIt\u2019s very rare that people are able to come up with a new test that moves beyond Bell.\u201d<\/p>\n The Brisbane team\u2019s aim was to derive and test a new theorem that would do just that, providing even stricter constraints\u2014\u201clocal friendliness\u201d bounds\u2014on the nature of reality. Like Bell\u2019s theory, the researchers\u2019 imaginary one is local. They also explicitly ban \u201csuperdeterminism\u201d\u2014that 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\u2014it is not relative to anyone or anything.<\/p>\n Testing local friendliness requires a cunning setup involving two \u201csuperobservers,\u201d Alice and Bob (who play the role of Wigner), watching their friends Charlie and Debbie. Alice and Bob each have their own interferometer\u2014an apparatus used to manipulate beams of photons. Before being measured, the photons\u2019 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\u2019s interferometer, and its partner is sent to Bob\u2019s. Charlie and Debbie are not actually human friends in this test. Rather, they are beam displacers at the front of each interferometer. When Alice\u2019s 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\u2019s friend Charlie \u201cmeasuring\u201d the polarization. (Debbie similarly resides in Bob\u2019s interferometer.)<\/p>\n Alice then has to make a choice: She can measure the photon\u2019s 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\u2014the equivalent of keeping the lab door closed. Alice can then directly measure her photon\u2019s 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.<\/p>\n Tischler and her colleagues carried out 90,000 runs of the experiment. As expected, the correlations violated Bell\u2019s original bounds\u2014and 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\u2019s 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. \u201cIf you violate them, you learn more about reality,\u201d she adds. Namely, if your theory says that \u201cfriends\u201d 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\u2014and, to some physicists, distinctly distasteful\u2014implications.<\/p>\n \u201cThe paper is an important philosophical study,\u201d 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. \u201cI am thrilled to see an experiment behind philosophical studies,\u201d Reilly says. Steinberg calls the experiment \u201cextremely elegant\u201d and praises the team for tackling the mystery of the observer\u2019s role in measurement head-on.<\/p>\n Although it is no surprise that quantum mechanics forces us to give up a commonsense assumption\u2014physicists knew that from Bell\u2014\u201cthe advance here is that we are a narrowing in on which of those assumptions it is,\u201d 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.<\/p>\n 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.<\/p>\n Vaidman, who was also not involved in the new work, is less enthused by it, however, and criticizes the identification of Wigner\u2019s friend with a photon. The methods used in the paper \u201care ridiculous; the friend has to be macroscopic,\u201d he says. Philosopher of physics Tim Maudlin of New York University, who was not part of the study, agrees. \u201cNobody thinks a photon is an observer, unless you are a panpsychic,\u201d he says. Because no physicist questions whether a photon can be put into superposition, Maudlin feels the experiment lacks bite. \u201cIt rules something out\u2014just something that nobody ever proposed,\u201d he says.<\/p>\n Tischler accepts the criticism. \u201cWe don\u2019t want to overclaim what we have done,\u201d she says. The key for future experiments will be scaling up the size of the \u201cfriend,\u201d 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\u2019s consciousness cannot be put into superposition. In turn, that conclusion would suggest that Wigner was right that consciousness causes collapse. \u201cI don\u2019t think I will live to see an experiment like this,\u201d Wiseman says. \u201cBut that would be revolutionary.\u201d<\/p>\n 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. \u201cIt\u2019s not inconceivable to me that quantum computers will be the paradigm shift to get to us into AGI,\u201d she says. \u201cUltimately, we need a theory of everything in order to build an AGI on a quantum computer, period, full stop.\u201d <\/strong><\/p>\n 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 \u201cinteresting and provocative,\u201d he says. \u201cIt\u2019s becoming conceivable that larger- and larger-scale computational devices could, in fact, be measured in a quantum way.\u201d <\/strong><\/p>\n 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\u2019s 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.<\/p>\n \u201cThe new paper is another confirmation that we have a problem with current quantum theory,\u201d says Renner, who was not involved in the work. He argues that none of today\u2019s 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. \u201cIt still worries me, though: If everything is just personal to me, how can I say anything relevant to you?\u201d 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.<\/p>\n Still, those who strongly believe their favorite interpretation is right see little value in Tischler\u2019s study. \u201cIf you think quantum mechanics is unhealthy, and it needs replacing, then this is useful because it tells you new constraints,\u201d Vaidman says. \u201cBut I don\u2019t agree that this is the case\u2014many worlds explains everything.\u201d<\/p>\n For now, physicists will have to continue to agree to disagree about which interpretation is best or if an entirely new theory is needed. \u201cThat\u2019s where we left off in the early 20th century\u2014we\u2019re genuinely confused about this,\u201d Reilly says. \u201cBut these studies are exactly the right thing to do to think through it.\u201d<\/p>\n [Disclaimer: The author writes frequently for the Foundational Questions Institute, which sponsors research in physics and cosmology, and partially funded the Brisbane team\u2019s study.]<\/em><\/p>\n<\/div>\nA Matter of Taste<\/h2>\n
A Way to Watch Wigner\u2019s Friend<\/h2>\n
Reconsidering Reality<\/h2>\n