This Twist on Schrödinger’s Cat Paradox Has Major Implications for Quantum Theory
A laboratory demonstration of the classic “Wigner’s friend” thought experiment could overturn cherished assumptions about reality
- By Zeeya Merali on August 17, 2020
Credit: Getty Images
What does it feel like to be both alive and dead?
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.
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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.
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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.”
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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.
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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.”
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