1591: "Bell's Theorem"

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ps.02
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Re: 1591: "Bell's Theorem"

Postby ps.02 » Wed Oct 21, 2015 3:59 pm UTC

orthogon wrote:There's also something meta or self-referential about the way I created the phrase itself by a process of tirare ex rectum.

Quite. A connection I completely missed. Carry on, and of course, illegitimi non carborundum.

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TomPace101
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Re: 1591: "Bell's Theorem"

Postby TomPace101 » Thu Oct 22, 2015 1:09 am UTC

doogly wrote:...
Nature is, as far as we can tell, entirely local. It is possible that quantum gravity will futz with this, but that'd be it.

Maybe doogly and I are using the word "locality" differently, or maybe I misunderstood the post, or maybe I have misunderstood the whole EPR issue itself, but something is amiss here somewhere.

Here's what Griffiths himself had to say in his textbook, in discussing the reaction after Aspect et al.'s experimental investigation of Bell's theorem. (Emphasis in the original).
The real shock was in the demonstration that nature itself is fundamentally nonlocal. Nonlocality, in the form of the instananeous collapse of the wave function (and for that matter also in the symmetrization requirement for identical particles) had always been a feature of the orthodox interpretation, but before Aspect's experiment it was possible to hope that quantum nonlocality was somehow a nonphysical artifact of the formalism, with no detectable consequences. That hope can no longer be sustained, and we are obliged to reexamine our objection to instantaneous action-at-a-distance.

If you want to read more, this is in Section 12.2, for the 2nd edition at least.

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doogly
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Re: 1591: "Bell's Theorem"

Postby doogly » Thu Oct 22, 2015 1:36 am UTC

Yeah, Griffiths is not that great.
Instantaneous collapse is a lie for children. How could anything happen "instantly," when a notion of "now" or "this instant" makes no sense at spacelike separated points? What's going on is not an objective collapse,* and is better thought of in terms of decoherence.

Perhaps it would be better to say that quantum states are a-local. They live in Hilbert space, not configuration space. I think many people can manage to make it through a semester of quantum mechanics and sort of nod along when they are told that Hilbert space is a thing, but their internal model of a quantum state is still some sort of fuzzy probabilistic cloud existing in (t,x,y,z). They can be forgiven this misunderstanding; Schrodinger persisted in this, and he was pretty good. But, a misunderstanding it remains.

It looks like Griffiths isn't helping. I'd definitely prefer something like a-local. I coooould just chalk it up to a question of taste and language, but I think his footnote here is actually misleading due to his talk of things being "instantaneous."

To be specific about what I mean for nonlocality, it is [A(x),B(y)] != 0 for (x,y) spacelike separated.

*some people do believe in this, I have been reminded in another thread. I think it is silly but it's probably more "unorthodox" than "heresy."
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Re: 1591: "Bell's Theorem"

Postby Kit. » Thu Oct 22, 2015 8:42 am UTC

So, for those who "believe" in Copenhagen interpretation, it can be said that, although Alice's and Bob's results are not independent, we have no way to tell which one of them causes the collapse, which one is acting upon another. Thus, no non-local action and no non-local information exchange.

Do I understand you correctly?

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TomPace101
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Re: 1591: "Bell's Theorem"

Postby TomPace101 » Thu Oct 22, 2015 10:43 am UTC

So in digging around some more, I have seen that apparently the subject of locality is actually a controversial one, even among physicists. For example, the wikipedia article https://en.wikipedia.org/wiki/Principle_of_locality includes a section on the Copenhagen interpretation which lays out both doogly's position and the one espoused by Griffiths. So there's disagreement there even without hidden variables and so forth.

Also, this recent blog post (http://backreaction.blogspot.com/2015/10/book-review-spooky-action-at-distance.html) reviewing a book on the subject explains that
Locality and non-locality are topics as confusing as controversial, both in- and outside the community, and George’s book is a great introduction to an intriguing development in contemporary physics. It’s a courageous book. I can only imagine how much headache writing it must have been, after I once organized a workshop on nonlocality and realized that no two people could agree on what they even meant with the word.

and a bit further down
In his book, George lays out how the attitude of scientists towards nonlocality has gone from acceptance to rejection and makes a case that now the pendulum is swinging back to acceptance again. I think he is right that this is the current trend (thus the workshop).

Interesting stuff. I have learned something from this exchange, including not to assume that such basic questions are actually "settled".

There's also a new experiment that is circulating in the news now that claims to have closed the final loopholes in Aspect et al's work. I wonder if the timing of the comic is not coincidental.

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Re: 1591: "Bell's Theorem"

Postby doogly » Thu Oct 22, 2015 11:39 am UTC

Kit. wrote:So, for those who "believe" in Copenhagen interpretation, it can be said that, although Alice's and Bob's results are not independent, we have no way to tell which one of them causes the collapse, which one is acting upon another. Thus, no non-local action and no non-local information exchange.

Do I understand you correctly?

"Copenhagen" is an ambiguous label for an interpretation. I would think of "mature, modern Copenhagen" being the decoherence emphasising view. There is no "the collapse," that notion is useful when you are separating out your test system and environment clearly. Then you can talk about collapse. But you don't want to build your interpretations around this sort of arbitrary distinction, no matter how useful for clear and quick calculation. So it's a decoherence / consistent histories sort of view. But some people (probably MWI or Bohm enthusiasts, who would like "Copenhagen" people to seem ridiculous, but also some actually ridiculous people) take the collapse ontologically seriously.

Regarding Alice and Bob here, the results are indeed independent, but they are tightly and non-classically correlated. Bob's result doesn't *depend* on Alice's result. He can say putatively, "Well, if she does her job today, she'll get the other spin." But he can't even be sure she showed up in the lab at all that day. Maybe she just fucked off to the beach. I mean, why not?

As for Bee's post on the pendulum going back to non-locality, I think she's talking more specifically about quantum gravity. This is one reason why it's good to use a specific criterion like [A(x),B(y)] != 0 for locality; this way you can be very clear when a theory violates it. If there is a definition of locality under which standard qm can be considered non local, it is wacky.
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Re: 1591: "Bell's Theorem"

Postby Kit. » Thu Oct 22, 2015 12:25 pm UTC

doogly wrote:Regarding Alice and Bob here, the results are indeed independent, but they are tightly and non-classically correlated. Bob's result doesn't *depend* on Alice's result. He can say putatively, "Well, if she does her job today, she'll get the other spin." But he can't even be sure she showed up in the lab at all that day. Maybe she just fucked off to the beach. I mean, why not?

I'm not sure I would like to use the word "independent" like this. For me, using your "electron in a box" example, in a single-electron system, a success in observing the electron in the box and a space-like separated failure in observing the electron outside the box are definitely not "independent".


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