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Bell proved that there is no local hidden variable model that is consistent with quantum theory. He said the same thing as Von Neumann (quantum mechanics can't
by pjs_ 10y ago
Bell proved that there is no local hidden variable model that is consistent with quantum theory. He said the same thing as Von Neumann (quantum mechanics can't be understood merely in terms of hidden variables), only more robustly. No new physics was discovered.
- effie 10y agoNo, that's not the whole story. Bell pointed out that Bohm's theory, already known but ignored, is a counterexample to von Neumann's and others' claims on impossibility of hidden variables. He analyzed how that is possible and found out: 1) von Neumann made some unreasonable assumptions in his analysis; 2) the fixation on hidden variables is missing the point; the real surprise is that locality, not theory with hidden variables, is inconsistent with quantum theory. That's quite a discovery. http://www.ijqf.org/wps/wp-content/uploads/2014/11/Bricmont-Bell-paper.pdf http://www.ijqf.org/wps/wp-content/uploads/2014/11/Bricmont-... https://arxiv.org/pdf/1408.1826.pdf https://arxiv.org/pdf/1408.1826.pdf
- loup-vaillant 10y agoWait a minute, my understanding of QM rejects both non-locality and hidden variables. Put it that way however, you kinda have to believe in macroscopic decoherence, that is, Everet's multiverse: there are 2 universes, one in which the cat is dead, and one in which the cat is alive, and observing one universe doesn't rule out the existence of the other. (Similarly, sending a photon (or settlers!) outside of the observable universe doesn't end its existence.) Non-locality sounds way, way weirder to me than macroscopic decoherence. Stuff like quantum collapse is really an additional, un-falsifiable claim: why the universe would zero-out precisely the stuff that the math says we can't observe?
- ajkjk 10y agoEr, are you familiar with the Bohmian mechanics [1] that was mentioned above? It has, basically, 'non-local hidden variables', and that works fine. [1] (https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory)
- loup-vaillant 10y agoHmm, nope, sorry. I'll look that up.
- omginternets 10y agoThis sounds really interesting but my physics are abysmal. Could someone kindly explain this thread in conceptually simple terms?
- Retric 10y ago> conceptually simple terms? Physics does not seem to operate on conceptually simple terms. Three options, it is simple when approached from some way we have yet to think of. There is some inherent mistake we are making with a huge range of current experiments. Or, as seems most likely the universe operates very differently on different scales making it conceptually difficult to deal with.
- ajkjk 10y agoIt's pretty hard to say anything meaningful about quantum mechanics and above without being precise with the math (case in point, there are so many frustrating examples of people who don't have the math down making statements that aren't consistent with it). Here's a summary, cause talking about this stuff is fun, with the caveat that it won't Quantum mechanics in its usual interpretations involves some randomness, where a measurement can give multiple results with different probabilities. The interpretations tell us that this is truly random; that the values are not known until you measure them, and then afterwards they are known and true everywhere. The obvious objection is "hey, maybe the values were there already, but until we measured them we just didn't know what they were". A theory with this property is called a 'hidden variable' theory. A 'local' hidden variable is one attached to a particle, such as its velocity. If you measured two entangled particles at the same time that are some difference apart, QM shows us that the results of one can be influenced by the results of the other - for example, if you measure one particle as having a positive spin on the Z-axis, the other would have to have a negative spin, in a certain experiment. Excluding superluminal communication (one particle did not send a message to the other), we might guess: well, one had spin up and the other had spin down to begin with, and we were just measuring them to find out which one was which. Bell's Theorem [1] proves (in an experimentally verifiable way) that this is not the case. There is no way that local hidden variables can reproduce the results of quantum mechanics. This is one of the most amazing discoveries of the 20th century, in my opinion. Nonlocal hidden variables still work, which is what Bohmian mechanics is. You're allowed to say "there is a variable accessible to every measurement that determines what the result of a measurement is. In Bohmian mechanics - which is mathematically equivalent to regular QM, just more complex - there is a 'pilot wave' that is computed from the whole configuration of the universe, and then is used to determine what the spin of a particule is. Basically you get to pick between nondeterminism (randomness) and a global function that influences everything that's much more complicated. The theory is appealing to many because it avoids non-determinism. It's unappealing because it's strictly more complicated than the interpretations that don't have this extra object, but predicts nothing beyond them. By Occam's razor it's not as good as the simpler interpretations. It is appealing to people who really don't want to accept the possibility of randomness in the universe, which I have no problem with. Not that it's not worth time investigating it, because it's interesting. (Some people think it's not worth splitting hairs over interpretations over QM, because they don't provide falsifiable predictions and so this stuff isn't science but philosophy. I disagree with this. Finding that one explanation is simpler than another is finding something, and constitutes, in my opinion, valid evidence for that explanation in a scientific sense.) [1] https://en.wikipedia.org/wiki/Bell's_theorem https://en.wikipedia.org/wiki/Bell's_theorem
- SomeStupidPoint 10y agoRegular QM has non-locality. Entanglement is non-local, at least any way that I've heard it to explain things like delayed choice erasers and the behavior of entangled anyons in confined electron gases. (Anyons (particularly fused anyons) themselves seem to have nonlocal quantum vales assocaited with them.)
- AgentME 10y agoThe many-worlds interpretation doesn't need non-locality.
- Jweb_Guru 10y agoMany-worlds still has nonlocal entanglement, IIRC, and all the weird phenomena therein.
- DavidPlumpton 10y agoIt does not require FTL signalling for the entanglement to work.
- deleted 10y ago[deleted]
- Jweb_Guru 10y agoCare to make rigorous how other interpretations require FTL signaling? For that matter, can you actually rigorously explain what it means for worlds to "branch", or why Born probabilities should be interpreted as though they are probabilities in MWI? Saying "all the possibilities actually happen!" doesn't really explain the correlations we actually observe in any interesting way.
- effie 10y agoYou make some good points. Regarding the Born probabilities, the extrapolation of Schroedinger equation to the whole universe seems to make the concept of probability superfluous and the Born rules lose their sense.