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FQXI ARTICLE

September 28, 2021

Why Did Nature Choose Quantum Theory?

Schrödinger’s cat may help reveal why some physical theories are better at describing reality than others.

FQXi Awardees: Jonathan Barrett, Stefano Pironio

November 21, 2011

JONATHAN BARRETT

Royal Holloway, University of London

Today, Barrett is particularly fascinated by quantum theory, which describes the physics of the atomic realm. He and his colleague Stefano Pironio, at the Free University of Brussels, Belgium, are pondering what makes quantum theory, rather than an alternative physical model, the best bet for navigating

Barrett’s first encounters with the quantum theory weren’t too memorable, however. "It seemed like a perfectly good bit of technical physics," he recalls, "but not the beautiful and poorly understood thing I know it is now." Quantum mechanics has a reputation for being bizarre; its conventional interpretation tells us, for instance, that reality is indeterministic at its core. Physicists cannot calculate the precise outcomes of quantum experiments before they have been performed; they can only work out the probabilities of getting a certain result. But being a probabilistic theory is not enough to define what is special about quantum mechanics. In fact, in recent years, physicists have come to realize that there is a whole zoo of alternative probabilistic theories sharing many of quantum theory’s other mysterious-sounding features—such as entanglement, interference, teleportation, and nonlocality. Yet these alternatives have been rejected by nature. Studying these alternatives has already told physicists much about what isn’t unique to quantum theory. But what

With the help of an FQXi grant of over $110,000, Barrett and Pironio are proposing a new tack on this subtle problem: "What we want to know is whether facts about time can explain why quantum theory has the structure it does," says Barrett.

Schrödinger’s Clock?

The duo’s collaboration started when Pironio was a Ph.D. student in Brussels and shared an office with Barrett, then a postdoc, who was examining the family of probabilistic theories to which quantum physics belongs. Pironio thought that the key feature distinguishing quantum theory from its siblings might be

Reality Check

Nature allows certain exotic effects predicted by quantum theory to be

demonstrated in the lab. But what makes nature prefer one theory to another?

Credit: ©Institute for Quantum Optics and Quantum Information, University of Vienna

Talk about time’s arrow also raises questions about cause and effect. In everyday life, we know intuitively that effects cannot occur before the events that caused them. But quantum theory is notoriously fuzzy about which events cause others. Take entanglement for instance, the quantum phenomenon in which two or more particles become inextricably intertwined in such a way that measuring the properties of one seemingly influences the properties of its partners, no matter how far apart they are separated. Physicists shy away from saying that the measurement on the first particle

Stefano Pironio

Free University of Brussels.

"This is a fruitful topic," says Guido Bacciagaluppi, a philosopher of physics at the University of Aberdeen, UK. "Causality in quantum mechanics and causality in Bayesian networks are now two highly developed areas."

John Cramer, a physicist at the University of Washington in Seattle, adds that examining why alternatives to quantum theory are not realized in nature could have a long-term payoff for those currently struggling to unite the theory with Einstein’s description of gravity. "Such an approach could conceivably provide insights into the structure that a theory of quantum gravity might have to have," he says.

Barrett’s ambitions are more modest, however. When asked what he hopes to have achieved in ten years from now, he chuckles. "If this were a job interview, I would go on about being an international leader in the field, establishing a broad base of research income." He pauses. "I think the honest answer is that I hope to have had an idea. A good one. Something totally different from anything I’m imagining right now."

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NUNO OLIVEIRA wrote on April 4, 2012

As regards my Computer Program, I could say that, in an Open environment, where Determinism is the only acting force, that alternative probabilistic theory - which might violate the 2nd law of thermodynamics (increase of entropy as defined by a regular sense of time compass - which is mentioned in the article is probably the exception that confirms the fact: that of the entire pertinence of quantum mechanics to explain reality. How? Determinism seems (scientifically speaking) to be far away from...

As regards my Computer Program, I could say that, in an Open environment, where Determinism is the only acting force, that alternative probabilistic theory - which might violate the 2nd law of thermodynamics (increase of entropy as defined by a regular sense of time compass - which is mentioned in the article is probably the exception that confirms the fact: that of the entire pertinence of quantum mechanics to explain reality. How? Determinism seems (scientifically speaking) to be far away from...

MICHAEL J. BURNS wrote on March 15, 2012

What I have found from mathematical and analytical study (thanks to Spinoza and Leibniz) is that the mathematical system which constitutes a physical theory must have a single premise as well as a lock in theorem for its subject matter to be prominently and reliably true.

Pure spacetime, Einstein-Davis and Kaluza-Klein theory, has a single premise - the reliability of the metric.It also has a lock in theorem, the Bianchi identities. So denizens of spacetime cannot directly experience a...

What I have found from mathematical and analytical study (thanks to Spinoza and Leibniz) is that the mathematical system which constitutes a physical theory must have a single premise as well as a lock in theorem for its subject matter to be prominently and reliably true.

Pure spacetime, Einstein-Davis and Kaluza-Klein theory, has a single premise - the reliability of the metric.It also has a lock in theorem, the Bianchi identities. So denizens of spacetime cannot directly experience a...

DOUGLAS LIPP wrote on February 16, 2012

Specific to De Broglie (never actually read his paper):

- follow me here please: here, "X" = wavelength because I don't know how to make the wavelength lambda symbol

IF: X=h/p (De Broglie equation) , where X = wavelength, h = Planck constant, p = momentum

THEN: X = h/mv : because p ( momentum ) = mass multiplied by velocity {m = mass, v = velocity )

and, (please confirm my math here )

equivalent equation : m v X = h

equivalent equation: m = h /...

Specific to De Broglie (never actually read his paper):

- follow me here please: here, "X" = wavelength because I don't know how to make the wavelength lambda symbol

IF: X=h/p (De Broglie equation) , where X = wavelength, h = Planck constant, p = momentum

THEN: X = h/mv : because p ( momentum ) = mass multiplied by velocity {m = mass, v = velocity )

and, (please confirm my math here )

equivalent equation : m v X = h

equivalent equation: m = h /...

read all article comments