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Un nuovo tentativo di aggiustare la meccanica quantistica

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Un accessibile long-form del New York Times (link alternativo) sulla ricerca di Angelo Bassi alle fondamenta della meccanica quantistica, e quindi sulla natura della fisica e sul conoscibile.

Angelo Bassi è un fisico teorico all’Università di Trieste.

Still, a veritable zoo of conjectures for what quantum mechanics might really imply about the world has been floated by physicists and philosophers over the years, including some that postulate parallel universes and others a special status for the human mind. And the theory’s completeness is still questioned by a handful of skeptics, including the Nobel laureate Steven Weinberg, whose own textbook on the subject expresses his hope that a better theory will emerge and reveal the story quantum mechanics refuses to tell.

Schrödinger coined a term — “entanglement” — for the way quantum mechanics itself may account for its reluctance: the wave functions of any two interacting objects, including observer and observed, get wove into one. This puts a researcher probing the subatomic world into a position similar to that of one water droplet trying to deduce the dimensions of another by touching it: Since the end result is one big droplet, the observing droplet can work out the volume of the observed (by subtracting its own initial volume from that of the big droplet) but can’t glean its original shape. Entanglement could be responsible for keeping objective reality behind a veil.

Nonetheless, Schrödinger also came up with his famous cat paradox to argue that quantum mechanics can’t be the whole story.

He imagined a cat locked in a box with a vial of poison and a radioactive substance that, his equation predicts, has a 50 percent chance of emitting a particle that breaks the vial and kills the cat in the time before a researcher is scheduled to look inside. Now, before that observation, quantum mechanics represents the particle with a wave function that encapsulates its two potential destinies (emitted or not) and that suggests that the particle has realized neither. At the same time, entanglement interweaves that wave function with those of the vial and cat, uniting their fates. This leads to a patently absurd description of the situation in the box before it’s opened: The particle is neither emitted nor not; the vial is neither broken nor not; and the cat is neither dead nor alive. Clearly, concluded Schrödinger, something is missing in this picture.

But what’s missing, says the orthodoxy, is an understanding of what physics is really about. “Physics,” Bohr wrote, “is to be regarded not so much as the study of something a priori given, but as the development of methods for ordering and surveying human experience.”

If all we ask of physics is that it describe human experience, then the paradox goes away. Quantum mechanics predicts, correctly, that the researcher, upon opening the box, is as likely to find the happy outcome as the alternative. And that’s it. To ask the cat’s condition before that is, from the orthodoxy’s perspective, as inappropriate as asking which way is north from the North Pole.

Of all the weird things about quantum mechanics, this limitation on the knowable is the weirdest, and the most profound. It suggests that scientists’ most accurate model of the world can’t describe whatever goings-on underlie our observations — or even be specific about what “observations” actually are, and what their effects are. Do they affect the cat? Or do they only happen in the observer’s mind? And although most physicists today have given up hope of answering such questions, Schrödinger, like Einstein, never did. He called its lack of description a “much overrated provisional aspect” of the theory he helped invent, one that resulted, he believed, from an all-too-human desire of his fellow physicists to believe they had found in quantum mechanics a lasting truth.

Weinberg, who wrote a book titled “Dreams of a Final Theory,” mused by phone with me about the possibility that quantum mechanics really is the truth, such that the ultimate theory that physicists dream of would only address human experience and say nothing about nature beyond that. “That would, to me, be horrible,” he said. “In fact, I might almost conclude that if that’s what it is, the hell with it.”

Still, largely because quantum mechanics has passed so many extraordinarily precise tests, collapse models are generally dismissed when considered at all, and few physicists think Bassi will succeed. Even Weinberg, on the phone, characterized his quest as “interesting” but “to some extent whistling in the dark.”

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