Talk:Principle of locality

Local realism

The term "local realism" usually redirected to EPR paradox. A while back I changed it to here because it provides a more concise version of it. However, maybe the term should redirect to Local hidden-variable theory as it is a formal article on the topic. Any thoughts are welcome. ReyHahn (talk) 08:56, 12 October 2022 (UTC)

I took a stab at (some of) the items on your summary under the field theory topic. I'm replying here because my stab is an attempt to (compactly) introduce the issues of local realism. Please see User:Johnjbarton/sandbox/locality.

I would like to find a good reference for why a local hidden-variable theory is a form of local realism. The compact and on-point review by Lambare says that one meaning of "local realism" is "local hidden-variable theory", so the text I wrote about this is ok, but to me this referenced assertion is not adequate for readers. Johnjbarton (talk) 23:42, 5 October 2023 (UTC)

It is starting to take shape. I am working on the Nobel prize 2023 articles and went to look back what the scientific comittee wrote about the Nobel in Physics of 2022 [1], as you can see they use local realism and local hidden variables as synonyms. See also the intro of this article [2] when it describes Bell's theorem (caution:the experiment has been criticized for being misinterpreted).

I just had an idea, what if we revert the order? Call the section "Bell theorem" or something like that. In that way we start by saying that Bell considered a classical model (local hidden variables) and showed that if experiments were carried one can distinguish between that model and quantum mechanics. Then talk experiments. Then discuss a bit on the implications of these violations: "If the principle of locality is violated by quantum mechanics is a matter of interpretation of quantum mechanics. A term often used is local realism, the intersection between locality and another term 'realism' which...". I think leaving the well understood stuff first and then describing the terminology issues might be better.--ReyHahn (talk) 00:23, 6 October 2023 (UTC)--ReyHahn (talk)

Thanks. One of the papers that cite An experimental test of non-local realism is a review Brunner, Nicolas, et al. "Bell nonlocality." Reviews of modern physics 86.2 (2014): 419. Their intro has a similar argument but even clearer. It does involve some math which I generally like to avoid using. However in this case the key issues are factorization vs non-factorization so I think the symbolic "picture" given by a few equations may make the explanation clearer.

In addition, this Brunner review touches on the role of the formal definition of "locality", as distinct from the simple intuitive definition I used in the sandbox draft. Thus it gives us a way of addressing your "point of conflict 2, locality". It also helps me sort out something Lambare discusses in his intro: some Bell analysis uses a formal definition of locality that embodies (or is parameterized by) hidden variables, so for this group the combination "local realism" is built into their definition of locality. Lambare essentially concludes that identification is justified when discussing Bell results because other ways of defining "realism" fail even without Bell tests.

I'm ok with any kind of reordering but I want to keep focus on "principle of locality" and not get caught up in myriad other Bell issues. For example we may want to cut down the experiments section for this reason.

I understand you are working on the attasecond article, so I will work on my draft with these new inputs. Soon however I would like to evaluate if the draft is at least as good as the corresponding part of the current article and move the draft into place to continue work. Johnjbarton (talk) 16:27, 6 October 2023 (UTC)

Just be careful, saying that realism fails independently of locality is still debated. The more we can avoid it in this mess the better. Also being too precise on the definition of locality can produce some disagreements. The experiments are just given here to show that it has indeed been tested deeply. I think it is important to say so. I’m not working in the attosecond physics article (I am working in the articles of the laureates) but that article needs to be Wp:TNTed. ReyHahn (talk) 16:41, 6 October 2023 (UTC)

What does the EPR paper say?

Johnjbarton I saw your recent edit [3]. What is exactly in conflict here? I think that the EPR paper is important to introduce the topic but I agree that the wording is loose. Can we work it back in? ReyHahn (talk) 19:28, 4 October 2023 (UTC)

I'm not against describing the EPR paper as the spark for the quantum locality discussions. I don't think it is essential in this article (on "principle of locality"); I would agree that it would be essential in the quantum nonlocality article.

I think the article should be about the title topic and the EPR paper does not directly address that topic. Rather, the paper is about the completeness of the wavefunction as a description of reality, using the principle in its analysis. Thus it is an application of a (classical) locality principle.

Unfortunately the paper never says such a thing. It never even uses the word "local" or "locality", or "simultaneous" or "speed of light" or "entanglement". So the sentence I deleted was an unsourced fabrication. Later analysis might claim the the EPR paper, when interpreted in modern terms "meant" those things, but we need a reference for that.

I hope this answers your questions.

Now how to proceed? The topic is quite complex, although I fear that it is one of these topics where "everyone knows" the answer.

My plan was to find a ref that amounts to your "EPR paper is important to introduce the topic", then try to work on quantum nonlocality with the idea of eventually reworking the current article to avoid the history altogether, focusing just on the "principle of locality" for QM. WDYT? Johnjbarton (talk) 21:12, 4 October 2023 (UTC)

Hmmm I will have to look at it again, I did not remember that EPR did no use "locality" in its terminology, you may have a point here. What about EPR-Bohm? I advice intense caution, the sources are even to this day are murky. Philosophers have taken some ground in this matter. The lack of consensus on which words to use make this topic awful. I guess we can source it but I would advise to intentionally use ambiguous terms in the points of conflict, if not we will have to go into the math but it might get too technical.--ReyHahn (talk) 21:20, 4 October 2023 (UTC)

I found a ref with a clear intro, take a look. Johnjbarton (talk) 21:36, 4 October 2023 (UTC)

I think a lot of this issue is the combination "local realism". Since we famously have dozens of serious contenders for QM "reality", the combination is almost impossible to manage. See eg Federico Laudisa,

"How and when did locality become ‘local realism’? A historical and critical analysis (1963–1978)",

Studies in History and Philosophy of Science, Volume 97, 2023, Pages 44-57, ISSN 0039-3681, https://doi.org/10.1016/j.shpsa.2022.11.008. Johnjbarton (talk) 21:45, 4 October 2023 (UTC)

Lovely article from Reid and Drummond, first time looking at it. However I would avoid "quantum nonlocality" in the text that you added, also it sparked a debate between them? or with somebody else? Local realism basically means local hidden variable theory, from what I have read authors use "local" and "real" in different ways making it almost impossible to be clear on where they differ, however when local realism is used altogether it is clear what it is being violated here.--ReyHahn (talk) 21:49, 4 October 2023 (UTC)

From the review: "Owing to the subsequent work of Bell, we now know what EPR did not know: local realism, the “realistic philosophy of most working scientists” (Clauser and Shimony, 1978), is itself in question. Thus, an experimental realization of the EPR proposal provides a way to demonstrate a type of entanglement inextricably connected with quantum nonlocality." So maybe the link can stay if we work on the text somehow? Johnjbarton (talk) 22:01, 4 October 2023 (UTC)

The link to quantum nonlocality? Its already set as Template:Main article. My problem with using "quantum nonlocality" in an article about the principle of locality is that it leads to the conclusion that quantum implies violation of locality (some but not all agree on this), when in fact it implies violation of local realism (everyone agrees on this). So it leads to confusion when terms are not defined in advance, even if vaguely.--ReyHahn (talk) 22:06, 4 October 2023 (UTC)

I agree on "quantum nonlocality". Not sure about the replacement. I think it implies a lot of understanding by readers to connect the dots.

Clauser, John F., and Abner Shimony. "Bell's theorem. Experimental tests and implications." Reports on Progress in Physics 41.12 (1978): 1881.

Says:

"Consequently, it can now be asserted with reasonable confidence that either the thesis of realism or that of locality must be abandoned."

I think that is what you are saying. Local + realism fails; pick one. Johnjbarton (talk) 22:12, 4 October 2023 (UTC)

I used this "Hence, the discussion, for the greater part of the subsequent 30 years, was pursued perhaps more at physicists’ cocktail parties than in the mainstream of modern research." from the end of the EPR history in Clauser and Shimony to swap the hidden variables link with simply "the nature of reality". They then move on to Bell just like the article. Johnjbarton (talk) 22:27, 4 October 2023 (UTC)

Yes, this is what I am saying (note that John Bell personally disagrees with the idea that violating realism is enough, Bell argued that his theorem implies that quantum mechanics necessarily violates locality, so those that side with Bell have a different mantra). I agree with the current paragraph, thanks.--ReyHahn (talk) 22:53, 4 October 2023 (UTC)

"relativistic quantum field theory is manifestly local"

In their review "Quantum information and relativity theory" Asher Peres and Daniel R. Terno Rev. Mod. Phys. 76, 93 – Published 6 January 2004 have a section called "Quantum nonlocality?"

"Bell’s theorem (1964) asserts that it is impossible to mimic quantum theory by introducing a set of objective local “hidden” variables. It follows that any classical imitation of quantum mechanics is necessarily nonlocal. However Bell’s theorem does not imply the existence of any nonlocality in quantum theory itself."

(Notice that they nicely side step "realism" by using "classical").

This seems relevant to the article. (To be sure, it would be relevant to quantum nonlocality but I'm not sure I have the patience). Johnjbarton (talk) 02:54, 5 October 2023 (UTC)

My summary on the problem of discussing locality in quantum mechanics in depth:

• Most physicists (except for superdeterminists and loophole researchers) agree that the Bell tests show that quantum mechanics violates local hidden variables/local realism.
• Everybody agrees that entanglement cannot be used to send information faster than light.
• EPR authors supported local realism, Bohr supported that there were violations of realism and not locality, Bohm and Bell supported violations of locality and not realism.
• Point of conflict 1: realism is badly defined, EPR seem to connect it to counterfactual definiteness even if this term is rarely used. Bell attached realism to determinism. Bohr seems to associate it to the probabilistic or non probabilistic nature of quantum mechanics. Modern authors define more complicated terms.
• Point of conflict 2: locality is badly defined and it is sometimes mixed with causality or "spooky" action-at-a-distance. Some authors argue that all interactions in physics are local and associate it to the lack of action-at-a-distance. Some see the violations of Bell theorem as to be in conflict with special relativity, so argue that locality must remain. Bohmians/pilot waveist/Bell supporters, argue that locality is violated as "classical" probabilties (again vaguely defined here) cannot be written in as being independent of the two particles (you need nonlocal hidden variables). For even more conflict, relativistic quantum field theory "locality" is included as one of the Wightman axioms but it is unclear if it is the same locality as the one in local realism. Quantum nonlocality=violation of local hidden variables is a misnomer in this sense as it supports one vision of the argument, quantum nonlocality clearly does not mean violation of Wightman axiom.
• Point of conflict 3: not everybody likes the term local realism. Bell argued that locality is always violated and the it should be preferable to talk about "local causality" (I admit that I still do not get the difference between local causality and locality/causality in the Bell sense). But that discussion should be resolved in another article on Bell's beable theory.
• Wiki decisions to make : keep/remove the term "local realism" from this page. If removed it can be replaced with local hidden variables which seems to be a more clear model. However then people looking for local realism in Wikipedia would be directed (as it was a few years ago) to EPR paradox which is a bad thing to do as the authors barely use the term and the resolution is in another page (Bell theorem). Explain/avoid to explain what quantum nonlocality means, dig deep into the definitions in conflic above. Explain/avoid to explan the position of Einstein/Bohr/Bell on this.

Hope this helps.--ReyHahn (talk) 12:17, 5 October 2023 (UTC)

Your summary fairly represents the sources I have read. Per Wikipedia, I think this is the point of view that should be reported (except of course the editorializing aspects). I'm in favor of keeping eg "local realism" even though I personally think it is impossible to define. We need to report on the difficulties in definition, as annoying as that is. Johnjbarton (talk) 15:12, 5 October 2023 (UTC)

New outline?

I have new material for quantum locality issues, but it is hard to work into the current chronological outline. An alternative outline:

Principle of Locality

• History
  • current pre-quantum
  • EPR,
  • Bell,
  • Modern (one paragraph summaries, no tech)
• Models of locality
  • Action at a distance
  • Bell screening
  • Quantum wavefunction
    • Bell local hidden variables proof and consequences
  • Field theory
    • Maxwell
    • QFT
  • Local realism definitional issues.

The History section would contain most of the current article; action at a distance could be merged in there. This section would have no discussion of current issues beyond mentioning "ongoing research". A causal reader might drop off here.

The later section would be more technical and mention unsettled aspects of locality and issues of definitions. Johnjbarton (talk) 14:42, 9 October 2023 (UTC)

Works for me.--ReyHahn (talk) 22:03, 9 October 2023 (UTC)

New version of QM section in place.

I re-wrote and expanded the QM section. I tried to focus on "locality" and not on eg QM, realism, photons etc.

The new version does not mention "counterfactual definiteness", which is not a difficult concept but is a very challenging pair of words to encounter. I feel if it is included it needs a full (short) paragraph to avoid being confusing jargon.

The new version does not mention non-communications. Worth adding, but the old version had no reference.

The new version cut the experimental section. We could say more, but I'm not keen on a mini-history of loophole-reduction. That info is not about locality but about Bell experiments and details. To say more I think we could outline one modern experiment with some kilometer bits to emphasize the locality test.

The realism bits are smaller; I'm kinda thinking about whether we should have Realism (physics) instead. The QFT section needs work.

Before these kinds of additions I think the best thing is to edit the new text to ensure it does the basic job adequately. And I would like to bring more from action at a distance over for eventual merge.

Please review. Johnjbarton (talk) 18:48, 16 October 2023 (UTC)

On a first read it is a great job! You manage to avoid most of my concerns. I'll be checking on it and editing a thing or two in the next couple days. Some quick answers:

• It is ok that realism does not appear as such in the article, maybe it is not needed and thus conterfactual definiteness neither. As long as local realism is there I'm good with it. For the moment, I do not think realism deserves its own article is a mess and the article on counterfactual definiteness might be enough.
• I still think that experiments could have a place here. The point is not to explain them but to provide insight on how well tested this is. Quantum mechanics has been experimentally shown to violate local realism. Pleople reading the article might benefit from this point being clear. Many people referenced this article after the 2022 Nobel Prize in Physics.
• The same thing as the above point with non communication. This is an important point. Many people on the web, after hearing about entanglement, try to figure a way to use entanglement as a way to send information faster than light. This should be clear.
• I am favorable with a merge with action at a distance.

Thanks again.--ReyHahn (talk) 19:14, 16 October 2023 (UTC)

Comment 2: I see that the "Models of locality for quantum mechanics" subsection do not need any quantum mechanics, can they be discussed in a preliminary section (before quantum mechanics sections)? Maybe just called "models of locality".--ReyHahn (talk) 20:59, 16 October 2023 (UTC)

We could try that. This would give the article a bit less of a historical slant. Johnjbarton (talk) 23:21, 16 October 2023 (UTC)

Also pinging a user with whom I had had previous discussions on the subtlety of the terms used in related wikipages. @Tercer: you might want to give the new version of the article a read (if you are interested).--ReyHahn (talk) 22:11, 16 October 2023 (UTC)

Thanks for the ping. The new version is a definite improvement over the previous one. I did fix a couple of mistakes. I'm unhappy with the subsections "Action at a distance" and "No future-input dependence". They are making a tempest in a tea pot, and the illustrations are unhelpful. Moreover, they are using jargon that is only really used by Wharton and Argaman.

More generally, I'm a bit worried that this article is becoming rather redundant with Bell's theorem and Local hidden-variable theory. Tercer (talk) 09:21, 17 October 2023 (UTC)

There does seem to be a considerable amount of redundancy in this corner of the encyclopedia, but I'm not sure what to do about that. XOR'easter (talk) 22:48, 17 October 2023 (UTC)

Thank for both of your feedback. I realize there is a huge overlap, throw also in quantum nonlocality and EPR paradox (probably there are more). This calls for an global article but the effort would be huge. As for the diagrams and models of locality, I am neutral, I'm fine with keeping them for illustrative purpose but I am fine with removing them per concerns of their lack of notability.--ReyHahn (talk) 23:00, 17 October 2023 (UTC)

In addition to those, we also have Aspect's experiment, Bell test, CHSH inequality, counterfactual definiteness, and probably others. The one that sticks out to me the most is local hidden-variable theory, because the title doesn't match the content. It's supposed to be about local hidden-variable theories, but it's actually a worse version of the Bell's theorem article. It would make more sense to rip out most of the text and replace it with material on LHV models, like the ones in section 3 of Bell's 1964 paper. XOR'easter (talk) 20:58, 25 October 2023 (UTC)

My quick assessment for the articles discussed above:

• Local hidden-variable theory is missing material and the first 2/3 is off base.
• quantum nonlocality I don't have patience enough to wade through this one. Seems like it is at least 3 articles pasted together.
• CHSH inequality 50% too long.
• Counterfactual definiteness needs a little work but not bloated with Bell.
• Aspect's experiment too long, a couple of section should be deleted to start.
• Einstein–Podolsky–Rosen paradox just needs a little work.
• Bell test Kinda listy but I think it gives the character of the subject. Could be trimmed a bit.

I think an article about the principle of locality must discuss Bell; I think don't think this article overly long or detailed for that goal.

The only question I have in my mind about the current article is whether it is the correct place for the discussion of "local-realism". We could move that topic to Bell's theorem but that article covers a lot of topics unrelated to locality.

In my opinion we should declare victory here and move on to articles with more problems. Johnjbarton (talk) 21:45, 25 October 2023 (UTC)

The explanation for Local hidden-variable theory is simple: it was written by Caroline Thompson, a crackpot that was very active in the articles about Bell in the early Wikipedia. That's why it's mostly about loopholes, she was a firm believer that loopholes would allow one to evade Bell's theorem. She died before the loophole-free experiments were performed. Tercer (talk) 07:31, 26 October 2023 (UTC)

Thanks.

• Realism: good let's leave it as now. (The physical concept of "realism" is messy and philosophical, but the only danger is fixing on one definition; I think we could present multiple perspectives. But another time).
• I agree that the results of experiments need more emphasis. I will take a try.
• Non-communications is strictly speaking orthogonal to locality. It's really only an issue after you discover that QM gives correlations. I think it can be mentioned in connection with experimental-results-show-correlations additions.

Johnjbarton (talk) 23:32, 16 October 2023 (UTC)

Would this order work for you? See User:ReyHahn/locality.--ReyHahn (talk) 07:39, 17 October 2023 (UTC)

There is something wrong with the following passage: In the 1935 EPR paper, Albert Einstein, Boris Podolsky and Nathan Rosen imagined such an experiment. They observed that quantum mechanics predicts "two spatially separated particles which have both perfectly correlated positions and momenta." Some comparison in missing. In classical physics any two particles that interact are correlated due to conservation of energy, momentum, angular momentum and so on. The problem of the EPR paradox is not that the two are correlated, but that the correlation seem to go against quantum mechanics uncertainty or more specifically against the incompatibility of observables. One could think that if we have two entangled particles with "perfect correlations" if Alice measures first the position of her particle and then Bob (who is far away) measures the momentum of his particle then we have all the information of the system. Thus we know position and momentum of both particles (this is not possible e.g. Heisenberg uncertainty principle). This is what is wrong, and in order for quantum mechanics to solve this, some nonlocal effect has to update what Bob will measure (instantaneously) right after Alice's measurement.--ReyHahn (talk) 07:59, 17 October 2023 (UTC)

Maybe we can write They observed that quantum mechanics predicts "two spatially separated particles which have both perfectly correlated positions and momenta." This seemed to be contradiction with the indeterminacy of quantum states. (if not we need more words).--ReyHahn (talk) 08:04, 17 October 2023 (UTC)

Sorry, I hadn't seen that you were already arguing about the problematic part of the EPR section. I hope you're happy with what I wrote. Tercer (talk) 09:22, 17 October 2023 (UTC)

Thanks, yes I prefer your new version.--ReyHahn (talk) 09:37, 17 October 2023 (UTC)

@Johnjbarton: could you chip in on the comments we have made about the locality diagrams. I suggest to split them as in User:ReyHahn/locality. Tercer suggested to remove them entirely, concerned that it might no be a very conventional way to describe the issue.--ReyHahn (talk) 15:52, 21 October 2023 (UTC)

Bell's local causality can be described using math, words, or diagrams, to varying degrees and effect. For the Wikipedia reader, I think diagrams should be tried when possible. Words are especially ineffective here, judging by the number of papers which seem to consider "locality" definitions as much as physics. Bell used a diagram similar to the last on in the article; it seemed clear and effective to me.

For a reader familiar with the topic, the last diagram in the article is all that is needed. But I thought it would help to break the diagram down in stages (Bell has one intro stage diagram similar to "Alice and Bob in spacetime").

If this were an article about Bell, then the two intermediate diagrams, action at a distance and no-future, are not needed. But this an article on locality and these two broaden the section, directing the readers attention to "locality models", of which Bell's is one.

Your reordering looks fine to me. Johnjbarton (talk) 20:51, 21 October 2023 (UTC)

Glove analogy

On this section Imagine that rather than quantum systems, Alice ad Bob are studying gloves. When Alice sees a right handed glove, she expects Bob will find a left handed glove. This type of explanation for the experimentally-confirmed quantum correlations is not valid': the hidden handedness variable does not exist in quantum mechanics is misleading. As long as Alice and Bob are measuring the same observable (handedness of gloves, or let's say spin component in the z-axis), then the glove analogy is fine. I wonder if we can avoid this analogy altogether as it is the one bringing confusion to the subject. A better analogy would be Mermin's device, but that would be a hassle to introduce.--ReyHahn (talk) 10:26, 17 October 2023 (UTC)

I don't think it's misleading. The point is only to introduce the usual hidden-variable explanation that is valid in classical cases, and claim that it doesn't work in the quantum case. Now you're worrying about *proving* that it doesn't work in the quantum case, but I don't think we need yet another proof of Bell's theorem.

Your edit, on the other hand, I find misleading. You replaced "is not valid" with "is not always valid". This implies that it is sometimes valid, but this is false. It's not as if Nature uses hidden variables when you're not violating a Bell inequality, and switches them off as soon as you do a Bell test. No. The correlations of an entangled state are not mere ignorance of an underlying value, even when you're just measuring it in a fixed basis. Tercer (talk) 16:07, 18 October 2023 (UTC)

I understand what you mean (hidden variables do not pop into existence at certain angles) but I am not sure that it is what most people get out of the analogies. I decided to drop the analogy. Popular culture and introductory discussions are repeating the fallacy that the glove experiment is different than entanglement because the "state is not defined until measurement" or whatever. That's not true but the real difference is so subtle that the glove analogy cannot provide the right image. That's why I want to avoid associating entanglement to that analogy. It is clear that if we did the experiments with the same angle for both detectors, we cannot violate Bell's inequalities (and hidden variables could be at work, in principle). In that sense when the angles are the same, the glove box analogy is on the contrary not that bad, but in the opposite sense that we want to use it here. The glove example shows that these correlations are not helpful for communication (when Alice gets her glove out of the box, she knows the handedness of Bob's glove, instantaneously!). --ReyHahn (talk) 18:44, 18 October 2023 (UTC)

The difference is not subtle. The handedness of the glove is determined before the measurement. The spin of the particle is not. Tercer (talk) 07:45, 19 October 2023 (UTC)

Again that is right but my point is that we have to convince people that just think in terms of classical mechanics that there is something different between entanglement and classical experiments. We can tell them that the difference is that handedness is not determined before measurement. Let me argue along those lines (not my position): Ok, let us think that the handedness of the gloves is undefined through some very complex chaotic chemical reaction (add some quantumness if you like) that finishes when you open the box. So what? How is that different from entanglement experiments? Bell test (with same angle detectors) or chaotic gloves there is nothing unusual about it. There is nothing different in terms of physical predictions. How do we even know that quantumness is just not another chaotic system? Only if we consider more observables (more than just handedness of gloves) then we can notice that something is different, but then the whole glove analogy breaks down. That's why Mermin's device is a better analogy because the detectors can measure more than one thing. Note that EPR authors were clever too, they did not argue in terms of gloves, they had to introduce relations between momentum and position (incompatible observables). The glove analogy is distracting from the issue, we should really consider saying something more about the incompatibility of observables. Sidenote: All this conversation remind me that there is another bad article that we should revise at some point quantum indeterminacy.--ReyHahn (talk) 08:25, 19 October 2023 (UTC)

Convincing people that quantum mechanics is different is important, but that's a different point. Here the goal is simply to say: "in the classical case there's obviously no nonlocality, because there's a hidden variable explaining it. In the quantum case this explanation does not hold." Explaining what does hold in the quantum case is another subject entirely. Tercer (talk) 10:10, 21 October 2023 (UTC)

Can we address that with a simple phrase without falling for the glove analogy? I mean Bell and co. argue that (aside from superdeterministic loopholes) the problem here is on locality not on the undeterminacy of the variables previous to measurement (I think that you would agree that this is not about "realism" as defined in the article).--ReyHahn (talk) 16:20, 21 October 2023 (UTC)

Yes, Bell is arguing that there is nonlocality, that's why it is important for him to emphasize that the quantum correlations are not of the glove sort, that can be easily explained by local hidden variables.

I don't see how avoiding a concrete example would help any reader. I mean, we have just shown the general abstract case with the factorizability equation. The gloves show what this explanation via factorizability means. Tercer (talk) 17:58, 21 October 2023 (UTC)

I say that there are examples but the glove one is not a good one. The section read This type of explanation for the experimentally-confirmed quantum correlations is not valid: the hidden handedness variable does not exist in quantum mechanics. My problem with this is that it sort of implies that realism (pre-existence of some values before measurement) is why quantum is different from classical. That is not what Bell says. What he seems to imply is that the difference is not on the hidden values but on the locality condition.--ReyHahn (talk) 18:09, 21 October 2023 (UTC)

No, it doesn't at all imply that "realism" is why quantum is different than classical. I advise you to read Bell's paper (La Nouvelle Cuisine), but other than that I give up. Tercer (talk) 19:12, 21 October 2023 (UTC)

I apologize if this went far for too long, but I think you are misreading what I have said. Maybe I am misreading you too. For the third time, Bell has never implied that realism is why quantum is different from classical. The glove analogy as stated so carelessly is.--ReyHahn (talk) 19:20, 21 October 2023 (UTC)

Bell himself wrote the glove analogy. Tercer (talk) 20:45, 21 October 2023 (UTC)

He was more careful. Please let us try to focus on how to word it you insist in keeping it there. Bell's discussed this in a clear context related to his theorem, he did not just simply say that gloves are different from quantum entanglement, he argued that it was unhelpful. In the words of Bell The analogy of the gloves is not a good one. Common sense does not work here. --ReyHahn (talk) 21:04, 21 October 2023 (UTC)

Realism

@Tercer: Why is this [4] controversial? It is clearly the usual definition and below we explain that it is complicated.--ReyHahn (talk) 09:55, 19 October 2023 (UTC)

As I wrote in the edit summary, this "realism" boils down to simply determinism. The usual definition of realism is the existence of an observer-independent world. This "realism" was not even invented by Bell or CHSH, it was invented by d'Espagnat and was always controversial even in the quantum nonlocality community. Most authors (including me) avoid "realism" completely and adopt less misleading expressions.

The phrasing before was more careful about it, in your version it sounded like the definition of "realism" is straightforward. Tercer (talk) 10:08, 21 October 2023 (UTC)

If you want to add something about determinism that can be added but that does not change that it is a common definition. I am not equating it with anything and the text afterwards says it is not used by Bell.--ReyHahn (talk) 10:17, 21 October 2023 (UTC)

Numerous papers physics papers refer to "local realism" and two review articles discuss this specific combination in detail. The review articles call out the special meaning of the combination in contrast to individual meanings of the words. Our article here needs just enough of the "realism" issue to surface these two points (to be clear: 1) "local realism" is a compound synonym for Bell's "local causality" 2) it is not a synonym for "realism").

"Realism" is certainly controversial and any particular definition won't satisfy everyone. We could consider a separate article on "Realism (physics)" to (attempt to) report the many sides of that issue. Here the short quote from the review is presented as just one simplistic definition, not to explore realism but to make the point that "local realism" is now a technical term. Johnjbarton (talk) 15:32, 21 October 2023 (UTC)

I think we all agree on that. I was just trying to make the phrase "A review of papers using this phrase suggests that a common (classical) physics definition of realism is" into "One common (classical) physics definition of realism is" just because that definition is common enough. Do we agree that it is one common definition? Even if vague and not necessarily tied to Bell's theorem.--ReyHahn (talk) 15:49, 21 October 2023 (UTC)

Ok thanks, I was unsure of the context. The "A review..." bit was just to head off "that's not what realism really means" kinds of edits and to avoid the appearance of editorializing. Johnjbarton (talk) 15:56, 21 October 2023 (UTC)