The author of Mind-Body Problems explains that, while quantum mechanics has been confirmed by countless experiments as well as by computer chips, it “defies common sense.” Specifically, it creates doubt about what “the facts” are.
Instead of the fabled cat in a box, imagine that a friend of Wigner is inside a laboratory monitoring a radioactive specimen. When the specimen decays, a detector flashes.
Now imagine that Wigner is outside the lab. If Wigner’s friend sees the detector flash, he knows that the specimen has decayed. But to Wigner, standing outside the lab, the specimen, his friend and the entire lab hover in a blur of possible states. Wigner and his friend seem to occupy two distinct realities.
In 2020, physicists performed a version of Wigner’s thought experiment and concluded that his intuitions were correct. In a story for Science headlined “Quantum paradox points to shaky foundations of reality,” physics reporter George Musser says the experiment calls objectivity into question. “It could mean there is no such thing as an absolute fact,” Musser writes, “one that is as true for me as it is for you.”John Horgan, “Does Quantum Mechanics Reveal That Life Is But a Dream?” at Scientific American (February 2, 2022)
When physicists tried a version of Wigner’s thought experiment in 2020, they found that his intuitions checked out. A fact that they found unsettling.
Now, researchers in Australia and Taiwan offer perhaps the sharpest demonstration that Wigner’s paradox is real. In a study published this week in Nature Physics, they transform the thought experiment into a mathematical theorem that confirms the irreconcilable contradiction at the heart of the scenario. The team also tests the theorem with an experiment, using photons as proxies for the humans. Whereas Wigner believed resolving the paradox requires quantum mechanics to break down for large systems such as human observers, some of the new study’s authors believe something just as fundamental is on thin ice: objectivity. It could mean there is no such thing as an absolute fact, one that is as true for me as it is for you.
“It’s a bit disconcerting,” says co-author Nora Tischler of Griffith University. “A measurement outcome is what science is based on. If somehow that’s not absolute, it’s hard to imagine.”George Musser, “Quantum paradox points to shaky foundations of reality” at Science (August 17, 2020) The paper discussed requires a subscription.
The entangled photons showed an “irreconcilable mismatch between the friends and the Wigners.” Musser thinks that one of four basic assumptions in physics has to give:
Few physicists believe superdeterminism could be to blame. Some see locality as the weak point, but its failure would be stark: One observer’s actions would affect another’s results even across great distances—a stronger kind of nonlocality than the type quantum theorists often consider. So some are questioning the tenet that observers can pool their measurements empirically. “It could be that there are facts for one observer, and facts for another; they need not mesh,” says study co-author and Griffith physicist Howard Wiseman. It is a radical relativism, still jarring to many. “From a classical perspective, what everyone sees is considered objective, independent of what anyone else sees,” says Olimpia Lombardi, a philosopher of physics at the University of Buenos Aires.George Musser, “Quantum paradox points to shaky foundations of reality” at Science (August 17, 2020) The paper discussed requires a subscription.
Horgan links the paradox to a newer approach to quantum mechanics:
A newish interpretation of quantum mechanics called QBism (pronounced “Cubism,” like the art movement) makes subjective experience the bedrock of knowledge and reality itself. David Mermin, a prominent theorist, says QBism can dispel the “confusion at the foundations of quantum mechanics.” You just have to accept that all knowledge begins with “individual personal experience.”John Horgan, “Does Quantum Mechanics Reveal That Life Is But a Dream?” at Scientific American (February 2, 2022)
QBists are often charged with solipsism: a belief that the world exists only in the mind of a single agent. This is wrong. Although I cannot enter your mind to experience your own private perceptions, you can affect my perceptions through language. When I converse with you or read your books and articles in Nature, I plausibly conclude that you are a perceiving being rather like myself, and infer features of your experience. This is how we can arrive at a common understanding of our external worlds, in spite of the privacy of our individual experiences.N. David Mermin, “Physics: QBism puts the scientist back into science” at Nature (March 26, 2014)
Essentially, QBism makes the observer a part of the scene observed. That doesn’t mean anything goes. It just means that everyone’s knowledge is gained from a perspective. In that case, there isn’t abstract knowledge of a situation; there is only knowledge as perceived by an observer.
One thing QBism certainly does is eliminate the possibility that the mind is just an illusion, as some philosophers and neuroscientists have proposed. In that case, there would be no knowledge at all. Qbism makes the observer real by making the observer part of the experiment.
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