Talk:Bell's theorem

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Recent reverts[edit]

I have, like, no time or energy to put into this article now, but I did want to say that I think the text removed here is a bad addition. Squeezing every possible qualification into the paragraph that is supposed to be the most broadly comprehensible is a bad idea. The intro is already overlong; if anything, it needs to be condensed (and if we are to add any more to it, it needs to be condensed significantly first). Moreover, per house style, the intro is supposed to provide a capsule summary of the main article that follows, and putting emphasis upon a point that the main article doesn't is giving that point undue weight. (The article itself doesn't even say "noncontextual" until it gets into Gleason's theorem.) No doubt the page needs improvements, but I don't think this is one of them. XOR'easter (talk) 20:56, 27 February 2024 (UTC)[reply]

Without mentioning noncontextuality, the conclusion drawn is literally incorrect. In order to describe a theorem, one must state all of the hypotheses of the theorem. As written the article is misleading. Physicalisms (talk) 18:58, 7 March 2024 (UTC)[reply]
This isn't an issue of mathematics, it's one of language and technical writing. No one disagrees as to what the theorem is or what it fundamentally requires, but we have to explain it in the format of an encyclopedia article. It's fine to have an incomplete description after the end of the first paragraph.
(This reminds me tangentially of the saga where Wikipedians came to heartache and an ArbCom decision over the ordering and treatment of explanations in the Monty Hall problem article.) Remsense诉 19:37, 7 March 2024 (UTC)[reply]
The term "noncontextual" is confusing, and does not add anything. Roger (talk) 20:11, 7 March 2024 (UTC)[reply]

Mermin-Peres magic squares.[edit]

The Mermin-Peres magic square analysis seems missing here. As a simplified "game" it might make some of this more accessible.

Mermin and Peres both worked on simplified Bell models as summarized in:

  • Aravind, Padmanabhan K. "Quantum mysteries revisited again." American Journal of Physics 72.10 (2004): 1303-1307.

These models take the form of a "game" described in the common sense of a game but treated as in game theory. The wikipedia treatment of the Mermin-Peres magic square is buried inside Quantum pseudo-telepathy. Johnjbarton (talk) 17:59, 27 March 2024 (UTC)[reply]

I disagree. The rules of the game are more complicated, the proof of the local bound is more complicated, and the quantum strategy is much more complicated. The only good thing about the magic square is that the quantum probability of victory is 1. Which is rather nice but conceptually irrelevant for Bell's theorem, and in any case already covered by the GHZ game here. Tercer (talk) 19:55, 27 March 2024 (UTC)[reply]

The Copenhagen Interpretation[edit]

The section of the current article called "The Copenhagen Interpretation" seems to be WP:SYNTH to me.

For example the first part of the section covers Bohr's reply to EPR. (I'm not sure why that is even in here.) This is "Bohr's interpretation". The paragraph starts with refs to papers that are not about Bells theorem at all. Johnjbarton (talk) 15:27, 1 May 2024 (UTC)[reply]

I check some more refs then deleted the first paragraph, off topic and not useful elsewhere. Johnjbarton (talk) 15:38, 1 May 2024 (UTC)[reply]
Saying something about Bohr's reply to EPR makes sense here, in principle, as Bell '64 built on top of EPR. I don't see how the removed material qualifies as WP:SYNTH, because it didn't advance a new conclusion (e.g., "Based on what Bohr wrote in 1935, he would have responded thusly to Bell in 1964..."). Maybe it was unsuitable for other reasons (e.g., too long), but I don't think WP:SYNTH was a problem there. And the opening of the deleted paragraph performed the important role of saying what the "Copenhagen interpretation" is, or rather, what the Copenhagen interpretations are. I don't know if just providing a wiki-link is enough, but I've added one. XOR'easter (talk) 17:07, 1 May 2024 (UTC)[reply]
What I object to is the idea that Bohr's view on EPR becomes "Copenhagen" view on Bell. There is no "Copenhagen" view on Bell because there is no "Copenhagen". Half the paragraph was devoted to shoring up a definition of "Copenhagen" because it is not a thing.
There is a "standard" "conventional" or "orthodox" interpretation which is also not crisply defined but all we need are refs about Bell theorem from standard text books, not a big intro.
If we do that, which I think is wikipedia, then what happens to Bohr on EPR? Without the implicit claim that Bohr speaks on behalf of "Copenhagen" when he speaks about EPR, we need a ref to include Bohr on EPR in an article on Bell. Otherwise it is off topic.
We could make Bohr/EPR/Bell a topic as there are references that do exactly that, eg:
  • Dickson, Michael. "Bohr on bell: A proposed reading of bohr and its implications for bell’s theorem." Non-locality and Modality. Dordrecht: Springer Netherlands, 2002. 19-35.
But Bohr/ERP can be discussed directly in the History without the "Copenhagen" baggage. Johnjbarton (talk) 17:36, 1 May 2024 (UTC)[reply]
I don't think a paragraph that prominently included the text There is no definitive historical statement of what is the Copenhagen interpretation. In particular, there were fundamental disagreements between the views of Bohr and Heisenberg. was "shoring up a definition" for the Copenhagen interpretation. Rather the opposite: it went out of its way to say that there is no unique definition of a single Copenhagen interpretation, only a collection of features that more-or-less distinguish this collection of separate ideas from other collections. Cutting that makes the subsection slightly worse, I think.
Bohr's reply to EPR makes sense as background to explain what "rejecting counterfactual definiteness" means. I think that qualifies it for being on-topic, but reading the subsection in its shorter form, I think we might be OK without it.
The question of how Bohr might have reacted to Bell's theorem is a matter of speculation (Dickson uses lots of language like "Bohr would presumably argue") and would of course have to be labeled as such. XOR'easter (talk) 18:07, 1 May 2024 (UTC)[reply]
Ok, my main objection here is to emphasize "Copenhagen" and exclude "orthodox" as a result. I get the sense that throughout wikipedia "Copenhagen" is a code-word for "old, incorrect". The term is not often used in textbooks or even in QM articles. Johnjbarton (talk) 18:16, 1 May 2024 (UTC)[reply]
And when the textbooks do use it, they just copy the same lazy explanation that the previous textbooks copied from the textbooks before that, ignoring everything that historians of science have been saying since the 1970s. XOR'easter (talk) 18:20, 1 May 2024 (UTC)[reply]

Alternative organization for the Interpretations section[edit]

Since I know that the many indistinguishable interpretations of QM have an equal number of ardent followers I'll suggest this reorg before editing.

I think the more effective categories for interpretations is "Hidden variables" vs "No variables". This grouping would make the implications of Bell's theorem for interpretations clearer. See for examples

  • Werner, Reinhard F. "What Maudlin replied to." arXiv preprint arXiv:1411.2120 (2014).
  • Żukowski, Marek. "Bell’s theorem tells us not what quantum mechanics is, but what quantum mechanics is not." Quantum [Un] Speakables II: Half a Century of Bell's Theorem (2017): 175-185.

Any "classical" interpretation must be non-local because of Bell; any non-classical interpretation is unaffected. Johnjbarton (talk) 17:41, 1 May 2024 (UTC)[reply]

I'm not sure what this reorganization would achieve. Surely organizing a section called "Interpretations" by the name of the interpretation makes sense enough? XOR'easter (talk) 18:09, 1 May 2024 (UTC)[reply]
Yes, but I don't think that is what we have. Under "Copenhagen" we have most of the modern interpretations. We have "Non-local hidden variables". And we have MWI and Superdeterminism. Johnjbarton (talk) 18:22, 1 May 2024 (UTC)[reply]
The "Copenhagen" section has one line about modern interpretations that are close to the Copenhagen collection, which sounds reasonable. Really, the odd one out is "Superdeterminism", which isn't really an interpretation but a feature that (a small number of people) have advocated for an interpretation to include. How about this: Move the line about the transactional interpretation into the next subsection, change "Superdeterminism" to "Proposals about retrocausality and superdeterminism", and change "Non-local hidden variables" to "Bohmian mechanics"? There have been other proposals to bring retrocausality into the story somehow, e.g., by Wharton and Argaman. XOR'easter (talk) 18:42, 1 May 2024 (UTC)[reply]
Yes, good. But then where do these things go?
  • Fuchs, Christopher A., and Asher Peres. "Quantum theory needs no ‘interpretation’." Physics today 53.3 (2000): 70-71. "...the violation of Bell’s inequality by quantum theory is not a proof of its nonlocality. Quantum theory is essentially local. Bell’s discovery was that any realistic theory that could mimic quantum mechanics would necessarily be nonlocal."
  • Bub, Jeffrey. "Why the quantum?." Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 35.2 (2004): 241-266. "The separability and locality conditions, formulated as constraints on probabilities, are equivalent to the assumption that correlations can be reduced to a common cause, and Bell’s derivation of an inequality (violated by certain quantum correlations) from these conditions is an elegant demonstration of a surprising implication of Einstein’s insight: the impossibility of embedding the quantum correlations in a common cause theory."
These seem like interpretation, but to classify them as "Copenhagen" seems inconsistent with any textbook or literature outside of the narrow historical discussions on "Copenhagen". Johnjbarton (talk) 22:11, 1 May 2024 (UTC)[reply]
Bub, along with Pitowsky, Brukner, and Zeilinger can be sorted into the post- or neo-Copenhagen informational interpretations. I'm not sure it's a good idea to cite the Fuchs–Peres paper; that was a slog of a compromise in order to produce something with which neither author disagreed (there's a whole chapter in Fuchs' Coming of Age with Quantum Information about the writing process!), and Fuchs later decided that a more radical position was necessary, so it's not really an essay that anyone stood by. Citing Peres' textbook or any of his solo-author papers would be a better way to represent his position. XOR'easter (talk) 02:15, 2 May 2024 (UTC)[reply]
In Interpretations of quantum mechanics there is a section "Quantum information theories" and it is backed by the (famous?) poll as a category separate from Copenhagen. I suggest a section with a name including "quantum information".
Peres' book has quite a lot about Bell, but that makes it difficult to boil down to a sentence.
This article definitely needs some content about Zeilinger's work. Johnjbarton (talk) 02:53, 2 May 2024 (UTC)[reply]
The write-up of that poll describes "information-based interpretations" as among the "intellectual offsprings" of Copenhagen. Maybe we should collect them together in a subsection called, e.g., "Copenhagen and neo-Copenhagen"? I don't have strong feelings here (though I do tend to find short subsections kind of choppy, as a matter of personal taste). XOR'easter (talk) 03:11, 2 May 2024 (UTC)[reply]
If we go strictly by the ref, we would have "information-based interpretations" and include the other (among the offspring) Copenhagen-like interpretations under their own names. To me, that matches the modern scene. While we can't find "Copenhagen" to ask how Bell works, refs on information-based interpretations will claim eg complementarity.
Also I think coining "neo-Copenhagen" as "information-based interpretations" would conflict with
  • De Muynck, Willem M. "Towards a neo-Copenhagen interpretation of quantum mechanics." Foundations of Physics 34 (2004): 717-770.
Johnjbarton (talk) 03:28, 2 May 2024 (UTC)[reply]
Using neo-Copenhagen to refer to that general territory is not my coinage; see, e.g., [1][2][3][4]. But I don't think the terminology of classification is very standardized at all here. Cf. Cabello [5]. Perhaps a subsection heading like "Copenhagen-type and information-based" would be viable.
We obviously don't have a response from Bohr to Bell's theorem. We do, however, have authors like Peierls and Omnès, who tried to present what they saw as Copenhagen, and who had things to say about Bell's work. XOR'easter (talk) 12:07, 2 May 2024 (UTC)[reply]
Wow thanks for the refs!
I agree that we cannot plan to "get it right", just try to make some improvement. Cabello's 2x2 organization is worth considering for our interpretations article. In the context of this page his participatory realism (Type II) category supports a "Copenhagen-type and information-based" section. Johnjbarton (talk) 15:22, 2 May 2024 (UTC)[reply]
Just to wrap up, I think we agree:
  • section name changes to "Copenhagen-type and information-based"
  • Add a few sentences about information based, eg Zeilinger.
Johnjbarton (talk) 15:49, 2 May 2024 (UTC)[reply]

violation of a Bell inequality cannot be interpreted as a proof of non-locality.[edit]

The article cites:

  • Brown, Harvey R.; Timpson, Christopher G. (2016). "Bell on Bell's Theorem: The Changing Face of Nonlocality". In Bell, Mary; Gao, Shan (eds.). Quantum Nonlocality and Reality: 50 years of Bell's theorem. Cambridge University Press. pp. 91–123. arXiv:1501.03521. doi:10.1017/CBO9781316219393.008. ISBN 9781316219393. S2CID 118686956.

when it says:

  • "Therefore, the violation of a Bell inequality cannot be interpreted as a proof of non-locality."

The ref is a cited but unpublished arxiv article. But it does not support the current content and the current content does not make sense anyway. The question we should address is "how does Bell theorem impact MWI?"

I added a quote to the ref to support a change to the sentence. Johnjbarton (talk) 16:00, 2 May 2024 (UTC)[reply]

Well I am trying to make sense of this section but it is tough sledding. The first paragraph ends with:
  • "Therefore, a violation of a Bell inequality can be interpreted as a demonstration that measurements have multiple outcomes."
citing
The paper does not claim that Bell tests demonstrate multiple outcomes as implied. Obviously that would be news. In fact the section of the cited article is called "Irrelevance of Bell’s theorem". Johnjbarton (talk) 17:21, 2 May 2024 (UTC)[reply]
I changed both paragraphs to match what I read in the refs. In both cases I removed concluding sentences that implied consequences beyond what the refs say. Johnjbarton (talk) 17:54, 2 May 2024 (UTC)[reply]
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