Quantum Physics: the measurement problem (General)

by David Turell , Monday, May 22, 2023, 23:56 (340 days ago) @ David Turell

Still no solution for it:

https://www.scientificamerican.com/article/quantum-theorys-measurement-problem-may-be-a...

"In textbook quantum theory, before collapse, the system is said to be in a superposition of two states.

"This collapse-inducing process is the murky source of the measurement problem: it’s an irreversible, one-time-only affair—and no one even knows what defines the process or boundaries of measurement. What amounts to a “measurement” or, for that matter, an “observer”? Do either of these things have physical constraints, such as minimal or maximal sizes?

"Quantum Theory's 'Measurement Problem' May Be a Poison Pill for Objective Reality
A core mystery of quantum physics hints that objective reality is illusory—or that the quantum world is even weirder than expected.

"At the heart of this bizarreness is what’s called the measurement problem...the measurement causes the system’s multiple possible states to randomly “collapse” into one definite state. But this accounting doesn’t define what constitutes a measurement—hence, the measurement problem.

"Attempts to avoid the measurement problem—for example, by envisaging a reality in which quantum states don’t collapse at all—have led physicists into strange terrain where measurement outcomes can be subjective.

" In a recent preprint, the trio proved a theorem that shows why certain theories—such as quantum mechanics—have a measurement problem in the first place and how one might develop alternative theories to sidestep it, thus preserving the “absoluteness” of any observed event.

"But their work also shows that preserving such absoluteness comes at a cost many physicists would deem prohibitive. “It’s a demonstration that there is no pain-free solution to this problem,” Ormrod says. “If we ever can recover absoluteness, then we’re going to have to give up on some physical principle that we really care about.”

"Holding on to absoluteness of observed events, it turns out, could mean that the quantum world is even weirder than we know it to be.

"Gaining a sense of what exactly Ormrod, Venkatesh and Barrett have achieved requires a crash course in the basic arcana of quantum foundations.

"In textbook quantum theory, before collapse, the system is said to be in a superposition of two states, and this quantum state is described by a mathematical construct called a wave function, which evolves in time and space. This evolution is both deterministic and reversible: given an initial wave function, one can predict what it’ll be at some future time, and one can in principle run the evolution backward to recover the prior state.

"This collapse-inducing process is the murky source of the measurement problem: it’s an irreversible, one-time-only affair—and no one even knows what defines the process or boundaries of measurement. What amounts to a “measurement” or, for that matter, an “observer”? Do either of these things have physical constraints, such as minimal or maximal sizes? And must they, too, be subject to various slippery quantum effects, or can they be somehow considered immune from such complications? None of these questions have easy, agreed-upon answers
"Given the example system, one model that preserves the absoluteness of the observed event—meaning that it’s either heads or tails for all observers—is the Ghirardi-Rimini-Weber theory (GRW). In GRW, quantum systems can exist in a superposition of states until they reach some as-yet-underdetermined size, at which point the superposition spontaneously and randomly collapses, independent of an observer. Whatever the outcome—heads or tails in our example—it shall hold for all observers.

:But GRW, which belongs to a broader class of “spontaneous collapse” theories, seemingly runs afoul of a long-cherished physical principle: the preservation of information....By postulating a random collapse, GRW theory destroys the possibility of knowing what led up to the collapsed state—which, by most accounts, means information about the system prior to its transformation becomes irrecoverably lost. “[GRW] would be a model that gives up information preservation, thereby preserving absoluteness of events,” Venkatesh says.

***

"So if a theory is Bell nonlocal, it implicitly acknowledges the free will of the experimenters. “What I suspect is that they are sneaking in a free choice assumption,” Wiseman says.

"This is not to say that the proof is weaker. Rather it would have been stronger if it had not required an assumption of free will. As it happens, free will remains a requirement. Given that, the most profound import of this theorem could be that the universe is nonlocal in an entirely new way. If so, such nonlocality would equal or rival Bell nonlocality, an understanding of which has paved the way for quantum communications and quantum cryptography.

"In the end, only experiments will point the way toward the correct theory, and quantum physicists can only prepare themselves for any eventuality."

Comment: the universe is non-local. Any theory must include that concept. This dense quantum theorizing may not compute with all readers but it the basis of our reality.