Talk:Quantum nonlocality/Archive 1

Archive 1

Archive 2

Nonlocal without entanglement

Is this question still left open: can nonlocality happen to non-entangled systems, too?Mastertek (talk) 14:58, 23 October 2011 (UTC)

Not in the standard non-relativistic Quantum mechanical framework, no. A non-entangled or "separable" quantum state is one which may be prepared locally at each party's site, allowing for some shared randomness between the parties (i.e. they all know the outcome of a complicated coin-flip). Since each local state is itself just like a kind of hidden variable, almost by definition this can never lead to non-local measurement outcomes. --Sabri Al-Safi (talk) 10:27, 21 October 2011 (UTC)

Thanks. So that means instantaneous communication/quantum correction between non-entangled particles is nonexistent?Mastertek (talk) 14:58, 23 October 2011 (UTC)

That's correct. The kinds of correlations between non-entangled particles are no stronger than the kinds of correlations you can simulate just with classical shared randomness. --Sabri Al-Safi (talk) 00:54, 15 November 2011 (UTC)

Re: Grandfather

The statement: "...it would be possible to arrange for your grandfather to be killed before you are born,..." seems strange - my grandfather was dead before I was born, but not before my father was.Dave Catlin 06:14, 30 January 2007 (UTC)

I'm no physicist, but I have to agree that the grandfather statement sounds so scrambled that it casts doubt over this whole wiki. Maybe the editors have let the statement stand in that form for such a long time for that very purpose! :-) But, really, shouldn't it read as follows?: "...it would be possible for you to kill your own paternal grandfather before he sired your father..." or something like that? The Tetrast (talk) 16:13, 17 December 2007 (UTC)

Comment on regularization and nonlocal actions

"Incidentally, the regularization techniques used to deal with ultraviolet divergences all use nonlocal actions."

It is not true that *all* regularization techniques rely on nonlocal actions. To take some examples, dimensional regularization and Pauli-Villars regularization. In fact the latter is an example of a higher-derivative theory, but that does not make it nonlocal. Of course, there are some regularization techniques which rely on nonlocalization ... Amar 15:31, Jan 26, 2005 (UTC)

Explanations of nonlocality seem to be saying that two object that have had some of their properties syncronized, after having undergone an acceleration that doen't affect said properties, remain with properties syncronized. This is then claimed to be a violation of classical physics. user:hackwrenchRobert Claypool

The existence of nonlocal effects can be causally understood as follows:

Of all the experimental findings of matter and radiation on the smallest scale the question can be asked: given the action of all the forces alone and just as their properties have been measured and described, how can radiation or matter in any form be and remain organised out of its fundamental cmponent parts as photons and the subatomic particles of matter?

To which a reply could be: A cause needs to act universally in addition to the forces just so as to maintain or conserve this natural organisation despite the action of the forces. Such a cause could not be described as acting by pushing or pulling objects and so would have no strength that could measured to reduce or cease wth increasing distance. And so effects could occur between entangled pairs of photons or between subatomic components that need not vary in any way at at any distance.

And so that the effects of quantum entanglement are measured because a cause acts so as the conserve a correlation between forms of behaviour (such as 'spin up' in relation to 'spin down') because a cause acts so as to conserve this relationship despite the effects of experimental measurement. Andrew Daw 17.57 July 18th 2005

I would like to add that the second example of a nonlocal Lagrangian is not a good one. The term F^2 can be rewritten as A \partial^2 A, hence the Lagrangian F ( 1 + m^2/\partial^2 ) F is actually the Proca Lagrangian, which is local.

Metaphysics

Is it just me, or is the Metaphysics section pretty much nonsense? It should at least have a source cited. rodii 23:20, 14 December 2005 (UTC)

You are correct, it is nonsense. I am unable to turn it into anything useful, so I deleted it in my last edit. Dave Kielpinski 23:04, 17 December 2005 (UTC)

First paragraph and Metaphysics

I removed the misleading statement "Nonlocality in quantum mechanics, refers to the property of entangled quantum states in which both the entangled states "collapse" simultaneously upon measurement of one of their entangled components, regardless of the spatial separation of the two states." Quantum states do nothing of the sort. I added a new introductory statement, rewriting the rest of the introduction for style reasons. Dave Kielpinski 23:05, 17 December 2005 (UTC)

May I suggest that someone write an opening sentence that is more comprehensible to non-physicists. I take the point that the current first sentence is a good rigorous definition. But have pity on those of us who are generally literate in science but for whom "multiparty measurement correlations" and "hidden variable theories" do not ring all the bells. Start with a definition that may lack total rigor but which gets to the heart of the matter in comprehensible language. Then hit us with the proper physical definition, before we get all wrapped up in what Bob and Alice are doing. Ajrocke (talk) 13:57, 18 April 2011 (UTC)

You're right, of course. The difficulty is that a looser definition risks being flat-out false according to one or more of the various different interpretations of quantum theory. I'll give it a go anyway. --Sabri Al-Safi (talk) 09:33, 26 April 2011 (UTC)

I've given it the above mentioned go; looks a little better now. --Sabri Al-Safi (talk) 16:42, 26 April 2011 (UTC)

distant

when you say Separated, do you mean in space or can it also be in time? —The preceding unsigned comment was added by Dara.bayat (talk • contribs) 15:47, 7 February 2007 (UTC).

Theology

some theologians have already theorized that God exists in non-locality —The preceding unsigned comment was added by 165.173.126.109 (talk) 20:18, 21 March 2007 (UTC).

Some people think that ~Einstein deliberately wrote GR to conceal non-locality for exactly the opposite reason - that it was a conclusive proof of the non existence of god. Hyperspace theories are a long-term hobby of mine and one thing is obvious, in mapping linear hyperspace you eventually reach a point of infinite speeds and absolute zero energy (zero = zero). Einsteins version leaves a large vacant area where god can hide like some malevolent beast. Don't put that in the article because its definitely OR. Lucien86 (talk) 07:05, 23 August 2008 (UTC)

It--72.74.112.203 (talk) 15:00, 9 May 2009 (UTC) doesn't belong here, but either is it no true.

Proposed rewrite of article

Copied over from the Wikiproject talk page since it might be more relevant here:

There has been a lot of work done over the past few years on super-quantum non-locality, which comes out of work by Sandu Popescu and Daniel Rohrlich showing that the CHSH inequality can be violated to an absolute value of four, in breach of Cirel'son's bound for quantum correlations. This article discusses Nonlocality briefly, but it isn't entirely comprehensive. I was wondering if editors would object if I rewrote this article with a comprehensive review of non-local boxes, maximal violations of Bell's inequalities, etc. I think it's such a large topic that it merits its own article rather than being spread out over many. The topics I would like to cover would be (ion no particular order):

- brief overview of EPR, Bell's inequalities, and Cirelson's work (mostly linking to the existing articles on the subjects, so only one or two paragraphs)
- superquantum violations of the CHSH inequality, and how they are theoretically achievable
- the various classifications of non-signalling boxes (including local boxes) and what conditions a non-signalling box has to fulfil in terms of joint probability distributions
- links to applications of quantum non-locality (as appropriate) and a brief discussion of usefulness in computation (as per a paper by Linden, Popescu, Short and Winter)

I don't envisage removing any of the general overview of non-locality already in this article, but would combine it into appropriately titled sections, such as a general definition, and some philosophical aspects.

Since this was my undergrad specialism, I would like to be able to write it, but not without checking with you guys what you would like done first! Please get back to me --Fritzpoll 14:02, 6 May 2007 (UTC)

Removed lines

This line was removed:

"Simple example of a non-local object is a wave. Because all objects in Universe possess wave-particle duality, they are non-local too."

It is complete nonsense.

Kevin aylward 11:48, 15 May 2007 (UTC)

Why is it complete nonsense? Please elaborate. —Preceding unsigned comment added by 99.238.20.210 (talk) 00:41, 16 November 2007 (UTC)

It's not nonsense, it's just difficult to understand. Even a single-particle wavefunction spread over a volume of space has the property that detecting a particle at any location instantly prevents the detection of the particle at any other place. BruceThomson (talk) 12:19, 24 January 2008 (UTC)

My thought expirment

I have a thought expirment and i have gotten many answers but i want to know what random people on the internet that i've never meet think. in this expirment there is a space ship that is headed to alpha centuri at near light speed (lets say .99c). it will obviously take it about 4 years to get their. during this time, N.A.S.A is watching the ship through a telescope but as the years pass, the light they are seeing has more and more delay than it used to. after the first year or so the light they are seing is way behind the actual ship at the moment. before takeoff, a rope had been tied to the ship. this rope was 4 light years long. after the four yaer journey, the telescope still sees the ship moving outward but the rope has become taught and indicates that the ship has traveled 4 light years. did the rope violate locality and produce ftl communication? the rope never moved faster than light and therefore the information in the rope obeyed the universal speed limit. some say that for the rope to become taught force would have to travel down it at a sub light speed. force had been traveling down the rope at a sub light speed for 4 years. in fact, if the rope had been rolled around a spindel 1 meter in diameter and kept taught the entire trip, and a counter was counting how many times the spindel made a revolution, than at any point N.A.S.A could calculate the traveled distance by multiplying the number of revolutions by pi. Some say that the rope gave no new information because knowing the ships speed is enough to know where it was at a given time. that cannot be true because there would be no way to know that the ship had not had an accident on the way there. the rope proves where the ship is. once the rope is fully extended, it can be moved back and forth at a sublight speed to transmit more information faster than light.

Retrieved from "http://en.wikipedia.org/wiki/Talk:Nonlocality/Comments"

Nice thought experiment! Unfortunately, after about three years the rope that continues to spin out is just moving by momentum, and if the ship has an incident after that you will never know... Shucks, but it was fun even so. BruceThomson (talk) 12:12, 24 January 2008 (UTC)

You are assuming the rope is not stretchable, which is another way to say that the rope is solid in the direction of the rope. One consequence of special relativity is that solids do not exist because you end up in contradictions like this one. The example we got at the bachelor in physics was that of a long car crashing into a shorter garage at close to light speed. With high enough speed the back of the car will enter the garage before even knowing the front has crashed. So to answer your experiment more directly, the rope actually does not prove where the ship is.

Performing proposed rewrite

Performing the rewrite I promised so long ago - should be done in an hour. -Fritzpoll (talk) 21:25, 25 February 2008 (UTC)

Major rewrite incomplete; still need more material on the applications of non-locality. A bit of copyediting probably also wouldn't go amiss, and the littering of the existing references where relevant in the material. If you're not sure which reference goes with a particular statment, just {{fact}} it and I will sort it out tomorrow. I know some of the material on optics has been lost in this transition, but I suspect it may either be an application of NL, or otherwise migth only be worth mentioning in passing. Please make any comments you like - I'd love some feedback, even though I'm already aware of some imperfections! Best wishes - Fritzpoll (talk) 23:25, 25 February 2008 (UTC)

Good job! Much better than before. Boris Tsirelson (talk) 07:32, 26 February 2008 (UTC)

Comments on the rewrite

I have a few problems with the rewrite, not that it isn't much better than what was there before.

First, there are at least two concepts which are generally called "locality". One is the principle that spacelike separated systems can't communicate, which does hold in relativistic QM. The other is the locality assumption that goes into Bell-type derivations, which is violated by QM. Only the second one seems to be discussed in the article. I think that an article with the title of "nonlocality" needs to discuss both.

Second, the description of the EPR paradox is incorrect. What that section is actually describing is Bertlmann's socks. Such behavior is not nonlocal; if it were then classical special relativity would be nonlocal. This section needs to be rewritten.

Third, and related to the second point, I'm not too happy with the definition in the lead paragraph, even considered as a definition of the second kind of locality only. It's not clear exactly what it means to "treat systems as independent", and in fact the current EPR section illustrates the subtlety of that point, since Alice and Bob's systems can be treated independently in the experiment currently described there, contrary to what the section says. I think "classically independent" would be better, but it still seems too vague a definition to follow by a point-blank claim that QM violates it. (There is a locality principle that QM violates point blank, I'm just having a frustratingly hard time seeing how to state it.) -- BenRG (talk) 20:00, 26 February 2008 (UTC)

I'm also unhappy with the lead, and I encourage everyone to edit the article back up to scratch. What I was hoping to do was inspire a bit of re-writing by demolishing what was there before and clearly setting out nonlocality. As regards the EPR paradox section, It is some time since I properly read through the paper - I may have got the description here from a different, incorrect text. If it can be corrected in the meantime, please, be my guest. I assume it is the description rather than the historical aspects that are under question? Best wishes - Fritzpoll (talk) 20:24, 26 February 2008 (UTC)

I have some further thoughts on your comments:

• As regards your comment on locality as the maintenance of the no-signalling theorem: I agree. But this article is about nonlocality, and I do not recall seeing nonlocality used as meaning "not violating locality", which I fear would put an apparent contradiction into the article

• I have looked around, and indeed, the original EPR paper does not state the problem in this manner. The manner in which it is stated is akin to Bohm's simplification of the problem from the 1960s, and is the manner in which it is staed in Nielsen and Chuang's book (cited in the article) and also in Rae's Quantum Mechanics (IoP). Would you suggest quoting the Bohm paper, or is it just that the referencing does not make clear that it is not lifted from the EPR paper?

• I have tried to change the vagueries of the lead sentence, by defining it as the maintenance of correlations within systems regardless of spatial separation. Any better?  :)

Regards - Fritzpoll (talk) 20:54, 26 February 2008 (UTC)

The article was tagged inuse and I didn't want to step on your toes, but I could certainly make some edits of my own (but not at the moment, I'm going to bed). I'm sorry about being so unclear in my last post. The issue with the EPR section is that the EPR argument really needs noncommuting measurements to work. Any experiment where all the measurements commute is actually classical, and that's the case here. Spin-up and spin-down measurements on the same particle commute (they're the same measurement), and measurements on different particles commute (that's a QM expression of the other locality principle).

The locality principle can't be understood as a lack of correlation, because correlation is always maintained regardless of separation even classically. For example, if you flip a coin and write the result on one piece of paper and the opposite of the result on another, and send them off in different directions, the contents of the pieces of paper will be perfectly anticorrelated (over many repeats of the experiment) no matter how far they separate. I could describe that situation by the classical probabilistic superposition ${\frac {1}{2}}[\uparrow ]_{A}[\downarrow ]_{B}+{\frac {1}{2}}[\downarrow ]_{A}[\uparrow ]_{B}$, where the up and down arrows stand for heads and tails. Exactly how that differs from the quantum superposition ${\frac {1}{\sqrt {2}}}\left|\uparrow \right\rangle _{A}\left|\downarrow \right\rangle _{B}+{\frac {1}{\sqrt {2}}}\left|\downarrow \right\rangle _{A}\left|\uparrow \right\rangle _{B}$ is hard to articulate. It's nothing as simple as one being correlated and the other not. I can say exactly what the difference is within QM using density matrices, but the point of the locality condition is to apply to theories that aren't QM (and as many of them as possible).

I don't understand your first bullet point. -- BenRG (talk) 22:50, 26 February 2008 (UTC)

Ah, now I understand the other point. What we want is an experiment where Alice and Bob measure perform measurements on spin-half particles along different axes, which is probably the rest of Bohm's interpretation that I failed to include I believe, but I will leave these edits to you. As to my first bullet point, your comment previously was that the article should include the concept that space-like separated systems can't communicate, even in QM. This is simply the no-signalling theorem, and it may be worthy of an sub'heading within the article, but I don't think it requires a redefinition of the lead, since what the lead says does not contradict this. - Fritzpoll (talk) 00:08, 27 February 2008 (UTC)

I think we're disagreeing about whether the term "nonlocality" is used to refer to the possibility of ordinary causal influence across spacelike intervals. I still think it is. Here are some examples from the arXiv: [1][2][3]. -- BenRG (talk) 14:03, 27 February 2008 (UTC)

Those references clearly indicate my error in this matter. Unfortunately, my work in this area was largely confined to nonlocal boxes based on local plans and superquantum correlations, so, perhaps strangely, my understanding about the basics is not as deep (applies equally to the EPR paradox, which I will cover in a reply further down). The lead sentence to me covers your concerns, because it states that:

A physical system is said to exhibit nonlocality if operations carried out on one system influence the properties of some other, spatially separated system.

This seems to me quite general, and does not necessarily pin the definition down to quantum correlations. The "influence" can be causal within this definition. What perhaps needs adjusting is the material following this sentence (which in any case needs to be expanded per WP:LEAD) to ensure that a reader is not confused into thinking that this is purely a quantum effect. Our disagreement may have been a misunderstanding, as the way you worded it here seems to me perfectly reasonable! Oh, the vagueries of text! :) - Fritzpoll (talk) 18:16, 27 February 2008 (UTC)

EPR and Bell should not be confised.

EPR does not prove nonlocality (and is not intended to).

Till the end of his life Einstein emphasized that, in spite of the great success of quantum mechanics, return to the classical paradigm is still not excluded (that is, nature is hopefully local, still!..).

This is why I do not like the phrase "Such behaviour is non-local..." within the "EPR" section.

The result of Bell (two years after Einstein's death) is that under some (reasonable) assumptions about nature, IF quantum predictions are correct (at least, approximately) THEN nature is non-local.

The result of EPR is that under some (reasonable) assumptions about nature, if quantum predictions are correct then the coordinate and the momentum of a particle coexist (at least in some situations), in contrast to "the quantal wisdom".

And indeed, EPR treated coordinates and momenta rather than spins.

Boris Tsirelson (talk) 11:38, 27 February 2008 (UTC)

I agree that EPR and Bell shouldn't be confused, but I do think they were talking about exactly the same thing. EPR were not trying to show that quantum mechanics predicts that position and momentum sometimes coexist as elements of reality. They were trying to show that quantum mechanics is self-contradictory, by showing that it predicts that they do coexist in certain cases (the conclusion of their argument involving separated systems) and also predicts that they never coexist in any case (as follows directly from the uncertainty principle). Once an internal contradiction has been shown, every prediction of quantum mechanics becomes suspect. You can't just cherrypick one conclusion (that they do coexist) and ignore the other. EPR note that quantum mechanics can escape the contradiction if the reality of properties of the second system depends on the measurement done on the first system, but conclude that "no reasonable definition of reality could be expected to permit this". By "this" they're referring to exactly (the negation of) the locality condition that Bell later assumed when deriving his inequality. -- BenRG (talk) 14:03, 27 February 2008 (UTC)

I think the EPR section is suffering from confusing the actual EPR paper contents with interpretations of it involving spin. I attempted to deal with this by sourcing the formulation of the problem, but perhaps the text should be amended to emphasise this, since it is confusing?

I think it's reasonable to state that the behaviour described is nonlocal, since EPR were trying to show that quantum mechanics allowed the principle of locality (or perhaps, better put, the principle of local realism) to be violated. Perhaps I have misunderstood your point here, Boris, in which case, can you clarify? I think this article could benefit from someone other than me making the next few edits. As regards the EPR section, I doubt there is a conflict of interest if you want to edit it, since your paper is only cited further down (is the spelling incorrect??) - Fritzpoll (talk) 18:16, 27 February 2008 (UTC)

ToBenRG: You write "They were trying to show that quantum mechanics is self-contradictory". But they write that one of the two possibilities is that QM is incomplete. Probably you treat QM as containing the claim to be complete. In principle you can; however, I think, the opposite view is also legitimate. This is rather a matter of terminology, is not it? If you agree that it is, then we should decide which terminology prevails (and if they coexist, we should write so in the paper).Boris Tsirelson (talk) 18:35, 27 February 2008 (UTC)

Let me say it non-carefully, but boldly. It seems to me that roughly the message of EPR is "save locality!", while the message of Bell is "kill locality!" But probably it is just my "original research". Or not?

I mean: "save locality by enlarging the incomplete QM", and "kill locality by experiment that violates Bell inequality". Boris Tsirelson (talk) 18:40, 27 February 2008 (UTC)

To Fritzpoll: I am reluctant, for several reasons. First, I am very opinionable; in math this does not manifest itself (if only a little), but in NONLOCALITY it surely would. Second, I prefer exact formulations to understandable formulations, which (again) is not bad in math papers, but would harm in encyclopedia.

Indeed, the difference between spins and the original EPR should finally be settled; however, this difference is of no principal importance, and we may safely postpone it to the end of the work. (After all, it is easy to do.) What is really important, I think, is to clarify the principal points. Boris Tsirelson (talk) 18:58, 27 February 2008 (UTC)

The spelling of my name is historically correct. On the other hand, now many authors cite it as follows: "B.S. Cirel'son (Tsirelson)". The spelling of the journal was incorrect; I have corrected it (as well as "Pironio"). —Preceding unsigned comment added by Tsirel (talk • contribs) 20:36, 27 February 2008 (UTC)

I just have read EPR again. First, the simple point: they consider the state of two (spinless) particles such that (1) the sum of their momenta vanishes, and (2) the difference between their coordinates is equal to a given number. That is, the system as whole has momentum zero and completely indetermined position (of the center-of-mass), while the internal degree of freedom (of relative motion) has a given position and completely indetermined momentum.

Second, does QM claim its own completeness? It seems to me that the authors have some doubts here. On one hand, they write on page 778: "In quantum mechanics it is usually assumed that the wave function does contain a complete description..." On the other hand, the title is, "Can ... be considered complete?" rather than "...consistent?" (and the same wording appears here and there in the paper).

Third, the last phrase "We believe, however, that such a theory is possible" says just what I mean by "save locality by enlarging the incomplete QM".Boris Tsirelson (talk) 20:21, 27 February 2008 (UTC)

Well, I am trying to be bold! Let us see what will come out of it... Boris Tsirelson (talk) 16:58, 1 March 2008 (UTC)

Onwards from the rewrite...

Started a new section to make delineation clear! I like the content of the new lead - I think it makes it very clear, and is considerably better than what was there previously. I think I will attempt to tackle the applications section in due course, once I have collected some thoughts and ideas, and the all important references! I think then the task list will be:

• Complete applications section
• Clarify the EPR section
• General proofread/copyedit of the article

Once we have done that, I would like to submit it to peer review to garner some further insight, and maybe even inspire some other editors to participate in the clean up of the article! Does anyone have any further comments, or additions to the above task list. Fritzpoll (talk) 18:19, 1 March 2008 (UTC)

I am glad that you like it. Now about the EPR section. Maybe it is because I am an unexperienced wikipedian, but I do not understand, why should we work on a matter already described in Wikipedia. Why not just give a link, adding several words (but not formulas and details)? What do you think? 79.176.228.38 (talk) 18:58, 1 March 2008 (UTC)

Oops, I forgot to login (from a different computer). But it is me, indeed. Boris Tsirelson (talk) 19:02, 1 March 2008 (UTC)

I certainly think a simple description of the problem might suffice, with a {{mainarticle}} tag in place, provided the text suitably linked the issue to non-locality. I'm also going to clean up some of the text in this article, and hopefully (finallY!) get around to the applications section, since I think this will broaden interest in the topic. Just so difficult to write! - Fritzpoll (talk) 13:34, 12 March 2008 (UTC)

The more difficult to write, the more pleasure to see it written... Hope my contribution "(non)locality and (in)determinism" will be incorporated somehow (maybe in a different form). As you probably see, it intends (a) to explain the definition in more detail than fit in the lead, (b) to explain without quantum theory everything that can be so explained, (c) to go straight to the no-signaling setup. I feel that "no-signaling" (Popescu-Rohrlich etc) is your main interest here. And indeed, everything else is already explained in Wikipedia, more or less.Boris Tsirelson (talk) 11:38, 13 March 2008 (UTC)

I like the section, mainly because it reminds me of how I learnt quantum non-locality. It may need some fiddling so that it can be referenced. No-signalling is mostly what I did my undergraduate work on, so it is what I best understand (I think!). Once I've dumped in the applications, and tweaked your section slightly, I'll pop this into Wikipedia's peer review process to make sure it is tidy enough. We might even try to have it recognised as a good article. - Fritzpoll (talk) 11:47, 13 March 2008 (UTC)

To User:Enormousdude: (1) A usual wave on a see, treated in the framework of classical physics (classical hydrodynamics), is a local object. That is, no kind of action-on-a-distance (nor "passion-on-a-distance") is involved. If you mean rather, that the wave is extended in space, not a point, then of course it is nonlocal in this sense; but in this sense everything is evidently nonlocal for a trivial reason. Let us save the word "nonlocal" for a deeper meaning. (2) It seems, you mean that in quantum mechanics a particle has a wave function extended in space, THEREFORE nonlocal. The same objection... Do you agree? Boris Tsirelson (talk) 07:28, 29 March 2008 (UTC)

To Fritzpoll: In the article on p-value (for example), there is a section "Frequent misunderstandings", containing many items of the form "The p-value is not..." Maybe something like that could help also in "Nonlocality"? Boris Tsirelson (talk) 09:13, 29 March 2008 (UTC)

Or not "misunderstandings" but rather "different ideas of locality". Two important examples are Aharonov-Bohm effect and Nonlocal Lagrangian. Boris Tsirelson (talk) 09:08, 30 March 2008 (UTC)

To Orangedolphin: "the absence of non-local hidden variables has yet to be shown"?? The Bohm interpretation is just that: non-local hidden variables. In fact, this was the starting point for Bell. Boris Tsirelson (talk) 21:29, 11 August 2008 (UTC)

You state that the "Physical reality of entanglement has been demonstrated experimentally" but the reference you cite is that of Aspect, Dalibard and Roger (1982) which shows only the violation of Bell's inequalities. Bell's inequalities refer to a theory of local hidden variables but entanglement (which is to say nonlocality) would mean an absence of non-local hidden variables which has yet to be shown (by definition). Orangedolphin (talk) 00:04, 12 August 2008 (UTC)

Sorry, I do not understand you. First, why do you think that "entanglement (which is to say nonlocality) would mean an absence of non-local hidden variables"? NONlocality means ABSENCE OF LOCALITY, not absence of nonlocality! And absence of LOCALITY means here absence of LOCAL hidden variables. Right? Second. Once again, why do you think that "absence of non-local hidden variables has yet to be shown"? It cannot be shown, since a theory based on nonlocal hidden variables exists, see Bohm interpretation. Right? I wrote this argument to you yesterday, but it seems you did not note it, why? Boris Tsirelson (talk) 07:15, 12 August 2008 (UTC)

The absence of local hidden variables (which has been demonstrated experimentally) does not imply a nonlocal universe. We can still have non-local hidden variables which would give a local universe. When I said "the absence of non-local hidden variables has yet to be shown" I meant experimentally (the existence of a theory does not change this) sorry for not being clear in this respect. Orangedolphin (talk) 09:03, 12 August 2008 (UTC)

Please explain, first, what do you mean by a local universe (I really do not guess), and second, which situation could you treat as an experimental evidence of absence of nonlocal hidden variables. Boris Tsirelson (talk) 11:01, 12 August 2008 (UTC)

By "local universe" I mean causal, deterministic, NOT entangled. By "nonlocal universe" I mean NOT causal, NOT deterministic, entangled. We can never have evidence of the absence of non-local hidden variables since by definition they are hidden. Orangedolphin (talk) 11:23, 12 August 2008 (UTC)

Thus, in your opinion, action at a distance does not break locality! Is it your original way to use the word "locality", or did you find it somewhere? Boris Tsirelson (talk) 13:45, 12 August 2008 (UTC)

In my opinion "action at a distance" is the same as entanglement and a "nonlocal universe" so it does "break locality". I'm pretty sure I'm using the accepted definition of locality but if you can tell me where I'm going wrong I would be interested to know. Orangedolphin (talk) 15:07, 12 August 2008 (UTC)

"Action at a distance" is of course far not the same as entanglement (which is explained just in the Nonlocality article!), but well, let as leave it for now; I understand that your phrase "causal, deterministic, NOT entangled" should be read "causal, deterministic, without action at a distance". Nice! This is exactly what Bell and EPR mean; this means exactly "local hidden variables". This is why I do not understand your phrase "The absence of local hidden variables does not imply a nonlocal universe". Boris Tsirelson (talk) 19:49, 12 August 2008 (UTC)

Sorry to intrude on your page, but do we know there aren't nonlocal hidden variables? Orangedolphin (talk) 03:42, 30 September 2009 (UTC)

First, it is not "my page", of course. Second, nonlocal hidden variables can be introduced, of course; this is just the point of Bohmian mechanics. However, what for to introduce them? If we do not care about locality then what is the fuss at all? Boris Tsirelson (talk) 07:32, 30 September 2009 (UTC)

Now about your words "please could you cite a source that conflates violation of Bell's inequalities with proof of nonlocality, entanglement" in the "Citation needed|reason=". I am astonished. This was just the point of the groundbreaking paper of Bell. And of course this is the central point of all subsequent popular presentations of Bell inequalities; for example, see the nice papers by Mermin. Boris Tsirelson (talk) 07:45, 30 September 2009 (UTC)

The rewrite "sucks"

Rewrites ought to make articles more clear. This one's made it more difficult to understand for non-physicists / physics majors. Could someone who really understands this please write a summary for the Simple English Wikipedia? I think that would help. --75.31.188.205 (talk) 15:14, 2 December 2008 (UTC)

Be careful with links

To User:Kesaloma: when inserting a link please check it. The link to Action was irrelevant; and note that the meaning of "action" in the phrase "action at a distance" is far from that in the Action (physics) article. Also, Field theory should be disambiguated to Field theory (physics). Boris Tsirelson (talk) 12:58, 18 January 2009 (UTC)

Should this page be merged with Entanglement?

Perhaps this page should be merged with Entanglement? Orangedolphin (talk) 04:20, 30 September 2009 (UTC)

No, I think it's better as a standalone topic. I might return to complete its updating though, as I became sidetracked by a number of things, not least my ongoing PhD studies Fritzpoll (talk) 09:37, 30 September 2009 (UTC)

It would be nice, to complete its updating. Here is an interesting recent source: Non-locality and Communication Complexity, by Harry Buhrman, Richard Cleve, Serge Massar and Ronald de Wolf. Boris Tsirelson (talk) 10:24, 30 September 2009 (UTC)

Yeah, I got dragged of to sort out a non-physics article, and have never quite circled back here. On another note, a merge to quantum entanglement would mean avoiding discussion of the non-quantum nonlocality, such as the general meaning of the word and the superquantum form discussed by Popescu et al. Fritzpoll (talk) 12:34, 30 September 2009 (UTC)

Another source, not recent, but quite useful for non-experts (used in Bell's theorem): David Mermin, “Is the Moon There When Nobody Looks? Reality and the Quantum Theory,” Physics Today 38, 38-47, April, 1985.

Still another: Spooky Actions at a Distance? Boris Tsirelson (talk) 21:31, 30 September 2009 (UTC)

I have recently majorly cleaned up the article, making a lot of the descriptions more intuitive, making the notation consistent and linking the "Generalising Nonlocality" section to the "EPR Paradox" section. A lot of the explanations were confused or in the wrong order, and there were quite a few typos. I've also added some comments about recent investigations into nonlocality to do with information causality and communication complexity. I agree that the page should not be merged with the Entanglement page, and have in fact added a section on "Nonlocality and Entanglement" which briefly compares and contrasts nonlocality and entanglement, since these concepts are often confused with each other. — Preceding unsigned comment added by Sabri Al-Safi (talk • contribs) 17:03, 16 December 2010 (UTC)

Where is non-locality on Relativity?

A section on relativity might be helpful. As I understand it non-locality is a critical concept for General Relativity. -
General Relativity is defined non-locally for the general universe and extends over areas and scales where the speed of light shrinks to almost zero. Without non-locality GR cannot forbid FTL travel, worse there would be nothing holding the universe together on that huge macro scale and the whole thing would be dimensionally unstable. The solution was to define time as 'synchronous', or 'instantaneous', or in other words that time moves at infinite speed. We can calculate a very rough finite lower value for this speed- about 10^43 m/s - ie crossing the universe in about one unit of Plank time.
The non-locality in General Relativity conflicts with special relativity and the two theories are somewhat incompatible. (this is on my future list of things to investigate - but there are plenty of relativity specialists about, say on the GR page) Hope that helps. Lucien86 (talk) 18:00, 8 February 2010 (UTC)

Looking at the General relativity article I do not see such statements as "the non-locality in General Relativity conflicts with special relativity and the two theories are somewhat incompatible". Boris Tsirelson (talk) 18:55, 8 February 2010 (UTC)

As I said this is something that I need to read up on more sometime. BTW I'm not sure the Wikipedia article is the best source , I just checked it (and a few related) and it doesn't seem to even mention general synchronicity or non-locality at all. I am trying to clarify things a little on PF: GR and will hunt through my physics books. Lucien86 (talk) 18:38, 9 February 2010 (UTC)

Have done that research a while ago (its a big part of my own research anyway) but decided that it was to poorly defined and involved to much OR to be included in a Wikipedia page. The answer was that no-one really knows for sure, some will argue strongly that there can be no non-local component, while others argue the opposite, most don't really know. (its a pretty abstract question)

'Special relativity uses non-accelerating frames which can be extended non-locally without problem (but it is not a realistic model anyway).' 'General relativity is a far more complex proposition and itself doesn't directly allow non-locality, however there is still a way around this. Also without something more General relativity creates a pretty strange universe on large scales that exists in the past and future but not in the present. For other stars or even the sun to exist at the same time as us really some kind of FTL or simultaneity is needed - including simultaneity of time. The shape of the link is ongoing speculative research..' - Lucien86 (talk) 09:25, 4 August 2010 (UTC)

Article not for lay reader

I agree with a comment above that the article, as it stands now, requires quite a bit of physics (and math) background to understand. Maybe a rewrite to make it more layman-friendly? I know I could do it, but if someone else has more time than me, please go ahead :) Capricornis (talk) 18:26, 3 December 2010 (UTC)

The article is virtually useless for the lay reader. Writing it was just a waste of energy if it isn't for an exam. This is a constant wiki problem; it is the curse of the nonprofessional. Anyway, it doesn't have much practical use. NaySay (talk) 00:24, 11 June 2021 (UTC)

On removing content from user:Antichristos

I've taken out some content that was added a little while ago to this and several other articles by User:Antichristos (see Wikipedia talk:WikiProject Physics#Speed of gravity, which was a mixture of nonsense, improper synthesis, and confusing irrelevancy. I was a little reluctant to take out the Bohm quote, but I feel that without a better discussion of what he was talking about (which really belongs on other pages) it did more to confuse than to explain the subject. Rafaelgr (talk) 09:12, 20 January 2011 (UTC)

I agree. --Boris Tsirelson (talk) 06:06, 21 January 2011 (UTC)

I see that the content has been restored by an IP, possibly Antichristos. Will undo. Rafaelgr (talk) 11:13, 21 January 2011 (UTC)

And again.Rafaelgr (talk) 11:43, 21 January 2011 (UTC)

And again.Rafaelgr (talk) 11:49, 21 January 2011 (UTC)

"Local Realism & C.D." vs "Locality"

I occasionally edit this page in an attempt to make it more consistent, coherent and clear (this helps me somewhat; my PhD is in this area). I only just noticed that the Example section contains several references to "local realism and counterfactual definiteness". There is some debate over the exact assumptions of Bell's Theorem and many, many physicists use the term "local realism" in introductions to their papers (without really thinking about it, in my opinion). However, after some thought and research it seems apparent that the only real assumption here is Locality. "Local Realism" is confusing because the "realism" part could be referring to various possible definitions of philosophical realism, none of which seem revelent or at all questionable in the context. "Counterfactual Definiteness" is, I think, irrelevent:

"the ability to assume the existence of objects, and properties of objects, even when they have not been measured"

... Bell's Theorem certainly talks about statistical outcomes of measurements. But technically, I don't think anything of this sort need be assumed. I propose that throughout the article we talk about locality and not local realism, counterfactual definiteness or local hidden variables, except where it is relevent or makes for clearer exposition. Sources:

- Bell's "Speakable and Unspeakable...": I believe the paper "La Nouvelle Cuisine" has many pertinent comments along these lines.

- Travis Norsen "Against Realism" http://arxiv.org/abs/quant-ph/0607057: cites a barrage of arguments against using words like "realism" and "counterfactual definiteness". Very convincingly argued.

- Tim Maudlin "What Bell proved..." http://ajp.aapt.org/resource/1/ajpias/v78/i1/p121_s1?view=fulltext Sabri Al-Safi (talk) 13:44, 21 March 2011 (UTC)

I do not agree. Quantum theory is local in some sense, and this fact is worth to be reflected in the terminology. If we just say that Nature is not local, then, how do we explain the no-signaling? See also the external link to Citizendium; that article treats the relations between counterfactual definiteness, locality and a third assumption (or rather, group of assumptions). --Boris Tsirelson (talk) 14:25, 21 March 2011 (UTC)

Certainly quantum theory is not maximally nonlocal, nor does it permit superluminal signalling. However, Bell showed that no local theory can predict exactly the same results as quantum theory. There is no mention of counterfactual definiteness in his paper, and he makes an explicit point that predetermination of results (which I believe is what people mean by CFD in this context) is not a presupposition but may be derived in ANY local theory satisfying EPR correlations - that is what this article should state, instead of "the affirmative answer follows from the assumptions of local realism and counterfactual definiteness". Quantum theory still violates locality without any mention of CFD. I think the Citizendium article is not enlightening on this aspect. Sabri Al-Safi (talk) 14:56, 21 March 2011 (UTC)

"predetermination of results is not a presupposition but may be derived in ANY local theory" --- as for me, this is evidently wrong. By a local theory Bell means a local deterministic theory. Could you please show us the idea, how to derive predetermination of results if it is not assumed from the start? --Boris Tsirelson (talk) 15:57, 21 March 2011 (UTC)

You took my statement a little out of context: predetermination may be derived in any local theory which satisfies EPR correlations. Perhaps I worded it badly, in which case I apologise. Bell argued as follows: suppose a theory satisfies EPR correlations with respect to the singlet state. That is to say, if Alice and Bob measure spin along the same direction, they always acheive different results. Suppose also that the theory is local, in the sense that Alice's choice of measurement axis does not influence the result obtained by Bob. Then Alice can predict with certainty the outcome of any measurement by Bob in an arbitrary direction a, by measuring along a herself; however, what she predicts would be true whether or not she actually measures in that direction (even if she measures in an orthogonal direction!). Thus the outcome of Bob's measurement must in fact be predetermined according to the theory. To quote Bell himself in "Bertlmann's Socks and the Nature of Reality":

“It is important to note that to the limited degree to which determinism plays a role in the EPR argument, it is not assumed but inferred. What is held sacred is the principle of ‘local causality’ or ‘no action at a distance…’ It is remarkably difficult to get this point across, that determinism is not a presupposition of the analysis.”

These arguments, amongst others, are outlined in the sources I referenced. I should think that even if I am wrong, Bell is at least just as wrong. Sabri Al-Safi (talk) 17:06, 21 March 2011 (UTC)

"Suppose also that the theory is local, in the sense that Alice's choice of measurement axis does not influence the result obtained by Bob." --- This is the problem! If you assume determinism, or at least CFD, then this phrase makes sense. It means: "Imagine a different Alice's choice in the experiment made yesterday; would it lead to the same result of Bob?" It is implicitly assumed that "the same (not different) Alice's choice in the experiment made yesterday surely would lead to the same result of Bob!" Maybe this is a self-evident truth for you. But anyway, THIS is called CFD. True, it is assumed by EPR. Also locality is. So what? Quantum theory tells us that we cannot keep both. Does it tell us which one to abandon? Not just so; but it hints: CFD is the first candidacy to be abandoned. --Boris Tsirelson (talk) 18:35, 21 March 2011 (UTC)

I don't think we're implicitly assuming what you suggest. I am merely talking in terms of the outcome probabilities predicted by a theory for measurements performed on a system. Is CFD assumed simply by talking about such probabilities? Surely not, since any meaningful physical theory must make predictions on actual measurement outcomes, probabilistic or otherwise. Locality is then a statement about which such probability distributions are admissible - I don't have to imagine Alice performing the experiment at a different time, or anything like that. Sabri Al-Safi (talk) 11:22, 22 March 2011 (UTC)

Hey editors, is someone else interested in this discussion? I am getting tired (and not convinced). --Boris Tsirelson (talk) 15:03, 22 March 2011 (UTC)

I'm interested. I think Sabri is right, Boris, in that what you're saying seems to come down to the argument that a statement of the form "X influences Y" can only make sense under the assumption of counterfactuals. It is the case that quantum theory is local in 'some' important senses, it is false that it is local in the strongest possible sense.

Incidentally, I believe that at least some of the motivation for the articles Sabri cited was that some of the authors (I have in mind Tim Maudlin) knew that CFD is contradicted (restricted? limited?) by the Kochen-Specker theorem irrespective of locality.Rafaelgr (talk) 10:14, 23 March 2011 (UTC)

Very interesting. The naive statement of locality (an object is influenced only by its surroundings) doesn't appear to require further assumptions to make sense. However, if we require CFD whenever we try to impose locality on physical theories, it calls into question the usefulness of CFD as a separate assumption in this context.

I should say that I started out from an overly strong position here, but I still absolutely believe wikipedia misrepresents the current state of affairs with regards to the assumptions of Bell's Theorem. The fact is, that this is a serious, ongoing debate amongst philosophers and physicists, yet several articles blithely state things like "Bell's Theorem actually proves that every type of quantum theory must necessarily violate either locality or CFD." At the very least it should be indicated somewhere on these articles that academia is currently split on the subject. Sabri Al-Safi (talk) 15:13, 23 March 2011 (UTC)

(Unindent)

"It is the case that quantum theory is local in 'some' important senses, it is false that it is local in the strongest possible sense." --- Yes, here I agree, up to a terminological reservation: the idea you call "the strongest possible sense of locality" is also called "local realism", "CFD plus locality plus no-conspiracy" etc.

"I think Sabri is right, Boris, in that what you're saying seems to come down to the argument that a statement of the form "X influences Y" can only make sense under the assumption of counterfactuals." --- Yes, THIS is what I say. And here is what Sabri says: "The naive statement of locality (an object is influenced only by its surroundings) doesn't appear to require further assumptions to make sense." And you, Rafaelgr, say that Sabri is right. I got puzzled. Can you, Rafaelgr, define the notion "X influences Y" in the absence of CFD? When I try to do so, I only come to no-signaling; but no-signaling is not violated. --Boris Tsirelson (talk) 16:44, 23 March 2011 (UTC)

Let me get this right, Boris. Are you explicitly saying that locality is meaningless without the assumption of CFD? What about "local realism"? (whatever realism means as opposed to CFD). In this case, do you still subscribe to wikipedia's view that quantum theory must violate either CFD or locality/local realism? Such a statement is surely nonsense if locality requires CFD. Perhaps you will say that locality requires either CFD or realism. In this case I might as well give up entirely and get a job =D Sabri Al-Safi (talk) 10:50, 24 March 2011 (UTC)

(Toward consensus, wow...) Frankly, I do not know what is "realism" itself. But I observe that the so-called "local realism" is effectively the same as "CFD and locality".

Yes, (once again,) in the absence of CFD I see only one idea of locality: the no-signaling.

(And no-conspiracy is always assumed, explicitly or implicitly; otherwise everything is a nonsense anyway.)

Thus, if we retain CFD then QM negates locality. But if we sacrifice CFD then QM respects locality in the form of no-signaling.

Subjectively/personally I feel inclined to do just so: sacrifice CFD and, after licking the wound (sigh), be more-or-less happy with no-signaling. --Boris Tsirelson (talk) 14:07, 24 March 2011 (UTC)

Ah, I see: you dislike the phrase "Thorough logical analysis reveals that the affirmative answer follows from the assumptions of local realism and counterfactual definiteness." Yes, this is incorrect, I agree. --Boris Tsirelson (talk) 14:13, 24 March 2011 (UTC)

In the article Principle of locality I see: "...must violate either the principle of locality or the form of philosophical realism known as counterfactual definiteness"; probably this is better. --Boris Tsirelson (talk) 14:17, 24 March 2011 (UTC)

Amusingly, right afterwards it states "[clarification needed]"!. I feel like I have a better understanding of your position on this, Boris, however I don't think that Wikipedia is about what you or I take things to mean. It's about the current academic picture, and I believe there is still a large debate going on between academics on this issue. The interested Wikipedia reader is bewildered by the number of different accounts of Bell's theorem, and there is simply nothing to say "Here is where Bell assumes CFD; here is where Bell assumes locality; this is why Bell refers to this as a hidden variables...". Frankly, it seems like different accounts of the same topic have been written by different people with differing opinions, on the same Wikipedia. This isn't ideal (although inevitable at first) - at the least we should have consistency. At the next least, we should have an explicit clarification somewhere.--Sabri Al-Safi (talk) 10:43, 1 April 2011 (UTC)

Well, I only want our readers to be informed that CFD is relevant here (at least, in the opinion of a non-marginal part of these debating academics). --Boris Tsirelson (talk) 14:56, 1 April 2011 (UTC)

I think this is fair enough. Hopefully I might have more success with this in the Bell's Theorem article! --Sabri Al-Safi (talk) 18:57, 5 April 2011 (UTC)

Suggest Disambiguation page

It seems like a good idea to have the title "Nonlocality" as a disambiguation page, and to use the section "Different Notions of nonlocality..." as the main content of the disambiguation page, with appropriate links. This current page I suggest renaming "Quantum Nonlocality" in analogue to the page "Quantum Entanglement". This way we do not need to worry about deferring to other forms of nonlocality, as the reader will not be interested in e.g. non-local Lagrangians.

My reasoning is this: although there is a section dealing with ambiguity of the word "Nonlocality", the rest of the entire article is specifically about super-quantum nonlocality, i.e. inadequacy of local hidden variables or research in generalised super-quantum correlations. The overview and the section "Generalising Nonlocality" both talk about it in the "quantum" way - e.g. with LHVs and entanglement - even though these sections lie outside of the "Nonlocality in Quantum Mehanics" section.

If there is general agreement on this then I am happy to do it, though I have not done something like this before and would appreciate being pointed in the right direction. If there is not agreement then okay, however it does seem odd to admit that "nonlocality" has a general meaning, then only talk about it in the context of non-relativistic QM. --Sabri Al-Safi (talk) 12:11, 7 April 2011 (UTC)

Good call. I'd question if there would be any content left about non-quantum nonlocality, but that can't be helped. Currently the quantum nonlocality page is a redirect. You need the direct link to edit it. Tercer (talk) 17:08, 7 April 2011 (UTC)

I should also mention that the citation given on the line "One needs to distinguish between..." is a broken link. I'm pretty stumped as to what it could even refer to.--Sabri Al-Safi (talk) 17:15, 7 April 2011 (UTC)

Alright, since there don't seem to be any immediate glaring objections, I'll give this a go in the next few days, as and when I have free time. I'm aiming for it to look and represent something similar to the Entanglement page, in relation to Quantum Entanglement --Sabri Al-Safi (talk) 10:57, 12 April 2011 (UTC)

Okay, I've made what I think is a decent attempt at the conversion. Let me know if any details are unfinished --Sabri Al-Safi (talk) 12:05, 12 April 2011 (UTC)

You forgot to move the talk page; I took care of it. Tercer (talk) 14:06, 12 April 2011 (UTC)

Link added to "Wheeler-Feynman absorber theory" article

I'm doubt that Wheeler Feynman absorber theory is directly relevant to this article, but I'll raise a discussion since I don't know much about it. From a brief scan of W-F it appears there are one or two issues involving nonlocality, but with a meaning distinct from the quantum nonlocality described here. There is certainly nothing explicit about multipartite measurement correlations, local realism etc... Perhaps the article on "nonlocal Lagrangians" is more appropriate? — Preceding unsigned comment added by Sabri Al-Safi (talk • contribs) 09:28, 9 August 2011 (UTC)

Easy to understand intro

I rewrote the introduction so it is easier to understand. I used the writing technique of "progressive precision" or hierarchy of detail. Here is what I wrote:

---

Generally, quantum nonlocality is the phenomenon by which microscopic objects seem to interact instantaneously (or nearly instantaneously) at a distance without any apparent intervening force. The phenomenon violates the common notion that an object may be directly influenced only by its immediate surroundings (the principle of locality).

This would seem to allow for faster-than-light communication. However, because the distant connections are actually statistical probabilities involving correlated measurements (see examples below), as of 2012 faster-than-light communication has not been observed and therefore quantum nonlocality is still compatible with special relativity's speed limit.

As of 2012, quantum nonlocality has only been observed with quantum entanglement, which occurs when particles such as photons, electrons, and some larger objects, interact physically and then become separated. The interaction is such that each resulting member of a pair has the same quantum mechanical description (state), which is expressed as probabilities of factors such as position, momentum, spin, polarization, etc. Even though entanglement is compatible with relativity, it prompts more fundamental discussions concerning quantum theory. For example, it has been proposed that quantum mechanics cannot be more non-local without violating the Heisenberg uncertainty principle. A more general nonlocality beyond quantum entanglement, yet retaining compatibility with relativity, is an active field of theoretical investigation and has yet to be observed.

The experimental evidence of quantum nonlocality has resulted in the general rejection of a previous theory known as local hidden variable theory in which distant events were assumed to have no instantaneous (or at least faster-than-light) effect on local ones. However, whether quantum entanglement counts as action-at-a-distance hinges on the nature of the wave function and decoherence, issues over which as of 2012 there is still considerable debate among scientists and philosophers. Non-standard interpretations of quantum mechanics have varied in their response to quantum nonlocality. These include the Bohm interpretation, counterfactual definiteness, the consciousness causes collapse theory, and the EPR paradox thought experiment.

Quantum nonlocality is of much interest because of potential practical applications, such as quantum computing and faster-than-light communication.

---

Sparkie82 (t•c) 08:37, 21 December 2012 (UTC)

"The interaction is such that each resulting member of a pair has the same quantum mechanical description (state), which is expressed as probabilities of factors such as position,[6] momentum, spin, polarization, etc." — No, the marginal state (usually, mixed) of a member of the pair is of little interest; the state of the pair (usually, pure) is of interest; maybe this is difficult to understand, but this is one of the crucial points. The probabilities of a single spin (or whatever) are of little interest; probabilities of combinations of two spins are crucial. Boris Tsirelson (talk) 19:03, 21 December 2012 (UTC)

Yeah, that's wrong. How about, "The interaction is such that the indefinite quantum mechanical description (state) of one member of the entangled pair is correlated (or oppositely correlated) to the state of the other member of the pair. When one entangled member is measured and definitely known, the other then becomes entirely predictable (i.e., it instantly changes from being indefinite to definite)." — That's a bit longer, but more accurate and clearer, I think. Sparkie82 (t•c) 07:57, 26 December 2012 (UTC)

Yes, this would be much better. I'd say "indefinite quantum mechanical description (mixed state)", since "state" is the technical term for the "description", and "mixed" – for the "indefinite". "correlated (or oppositely correlated)" — I'd say just "correlated", since it does not mean "positively correlated". "the other then becomes entirely predictable" – well, one of its properties (for spin, one of its projections) becomes entirely predictable, others do not. Also, this happens in the simplest case, while in the general case the other changes its state, not necessarily to the pure state, maybe just to a "less mixed" state. Also, "changes its state" is not quite correct; a more correct is "its conditional state differs from the unconditional one". Of course, it is impossible to say such things without sacrificing either precision or accessibility. Boris Tsirelson (talk) 08:50, 26 December 2012 (UTC)

"The interaction is such that the quantum state of one member of the entangled pair is correlated to the state of the other member of the pair. When a property of one entangled member is measured and definitely known, the property of the other member then becomes entirely predictable (i.e., it instantly changes from being indefinite to definite)." -- removing some specificity to avoid issues of precision/accessibility -- even shorter/clearer (because this is only the lead). Added link to "Wave function collapse". (I'm going to go ahead and edit this version into the article since what is out there currently is incorrect.) Sparkie82 (t•c) 05:13, 28 December 2012 (UTC)

Following WP:BRD, I reverted a bold edit back to the version we have worked out above. The next step is to discuss the proposed changes here (if the editor wants to pursue the changes). Sparkie82 (t•c) 12:48, 18 February 2013 (UTC)

I was afraid that this might happen. Look, I'm not willing to argue to death about the lead, which I have done many times on other articles. I'm tired, and I have work to do. If for some reason you dislike my version so badly that you think it's better to discard it rather than improve on it, I'll just ask you please to remove some insanity that is in the current lead:

" Generally, quantum nonlocality is the phenomenon by which microscopic objects seem to interact instantaneously (or nearly instantaneously) at a distance without any apparent intervening force. The phenomenon violates the common notion that an object may be directly influenced only by its immediate surroundings (the principle of locality).

This would seem to allow for faster-than-light communication."

This is just wrong. Nonlocality is about the correlations that we observe in microscopic objects. They do not "seem" to interact instantaneously, nor there is any evidence that instant interactions is the mechanism through which nonlocal correlations come to existence. And this certainly does not seem to allow for faster-than-light communication, this sentence is just plain nonsense. Also

" A more general nonlocality beyond quantum entanglement, yet retaining compatibility with relativity, is an active field of theoretical investigation and has yet to be observed."

This sentence makes no sense, and if it were to be corrected to make sense, it would be a lie. First that quantum entanglement cannot be compared to nonlocality like this, they are completely different concepts. If one would write "A more general nonlocality beyond the nonlocality that can be produced from quantum entanglement" it would make sense, but than it's wrong, as nobody thinks that we'll ever observe such nonlocality. The research in this area is directed to why quantum theory cannot produce such superstrong nonlocality. Also wrong:

"The experimental evidence of quantum nonlocality has resulted in the general rejection of a previous theory known as local hidden variable theory in which distant events were assumed to have no instantaneous (or at least faster-than-light) effect on local ones"

Local hidden variable theories is not "a" theory, but a class of theories. And they assume no instantaneous action-at-a-distance and counterfactual definiteness, which is the crucial point why nonlocality does not imply action-at-a-distance. And finally:

"Quantum nonlocality is of much interest because of potential practical applications, such as quantum computing and faster-than-light communication."

This is also a lie. There are absolutely no physicists researching nonlocality to make faster-than-light communication. I am lost for words trying to explain how insane this would be. Also, the importance to nonlocality to quantum computation is tangential. People that are looking for practical applications study it mostly to make quantum key distribution, but also certification of randomness and some other things.

I am a researcher in nonlocality. Seeing the wikipedia article about my speciality in such a sorry state makes me wanna cry. We had a reasonable lead last year. What happened to it? Mateus Araújo (talk) 01:00, 14 March 2013 (UTC)

So it was actually you who put all this incorrect information here in the first place. Sorry for not checking the names. As per WP:BRD, then, I'm gonna revert your 21 December edit, since it has garnered only negative comments from me and Boris Tsirelson. Mateus Araújo (talk) 01:13, 14 March 2013 (UTC)

Does not allow for faster-than-light communication

I find this flat out statement misleading, as there is recent research establishing that quantum entanglement effects are faster than the speed of light (or possibly even instantaneous) or at least bounded i.e. the entangled particles know when to change at least 10000 times faster than the speed of light, and that no slower than light signals are occurring.

If the argument being proposed is that even though the particles change instantaneously upon measurement but cannot be ascertained FTL - I find misleading, as the actual change does take place FTL - and most reasonable people look at the problem of communication as the actual change between two distant objects, not how fast the change is actually ascertained (it would be like saying the speed of electricity isn't actually what it is because the brain cannot deduce information passed to it as fast as the electricity - which would mislead most reasonable people.)

Two recent articles on non-locality that show FTL transference of information:

-Francis, Matthew. Quantum entanglement shows that reality can't be local, Ars Technica, 30 October 2012 http://www.nature.com/news/quantum-teleportation-achieved-over-record-distances-1.11163

-Juan, Yin Bounding the speed of `spooky action at a distance' http://arxiv.org/pdf/1303.0614v1.pdf

I would like other opinions on this - and I am wondering if the statement above might be qualified, at least so it doesn't mislead the layman to believe there is no part of quantum non-locality that breaches the universal FTL speed bound assumption - which is in fact the most remarkable part of quantum non-locality that ought to be highlighted, not disguised. Jamenta (talk) 20:06, 27 March 2013 (UTC)

Surely controversial, but I am inclined to treat all that as illusions that emerge when one (implicitly) assumes counterfactual definiteness (while the nature does not). Boris Tsirelson (talk) 06:22, 28 March 2013 (UTC)

Certainly when the article was written, the point was not that the effect of measurement on, say, one half of an entangled photon pair would not instantaneously have an impact on the other half. The problem was that, if we examine spin states as an example, Bob must make a measurement of the correct type to transfer the information Alice is trying to provide to him. Since he can't know what measurement to make non-locally, the 'information' that Alice is trying to convey cannot be obtained. 193.117.31.2 (talk) 14:45, 25 September 2013 (UTC)

Alice and Bob

The Alice and Bob example doesn't specifically describe the interconnection (if any) between the apparatus in each separate laboratory. In fact, it doesn't describe the hypothetical switches-and-lights apparatus very well at all. A better explanation - or even a diagram - might be helpful for the naiive reader. However, since the discussion gets a bit arcane fairly rapidly, I don't feel comfortable tackling it myself. Thoughts?jxm (talk) 15:48, 23 August 2014 (UTC)

Proposed section: GHZ Game (a three party quantum game)

Suppose Alice, Bob, and Carol are three quantum information researchers each imprisoned in a different isolated lab and they are allowed to communicate only with a Referee. Everyday the Referee sends each one of them an email containing a single letter, either X or Y, with the condition that only an even number of them will receive Y; i.e., either all receive X (XXX), or one of them receives X and the other two receive Y (XYY, YXY, YYX). By the end of the day the researchers are required to respond to the email with either +1 or -1 and the Referee will extend their research funding if and only if the product of their replies is +1 in the case of XXX or -1 otherwise. As it is usual in these games, the participants can devise a strategy before the beginning of the game but once it begins no communication is permitted.

In the first glance it is easy to come up with a strategy that guarantees winning in 3/4 of the games, but is there a strategy that guarantees winning all the time?

Let $A_{X}$ denote Alice's response if she received an X and $A_{Y}$ denote her response if she received a Y and so on for $B_{X}$, $B_{Y}$, etc. Then, we can formulate the requirements for winning the game:

$A_{X}B_{X}C_{X}=+1$

$A_{X}B_{Y}C_{Y}=-1$

$A_{Y}B_{X}C_{Y}=-1$

$A_{Y}B_{Y}C_{X}=-1$

But here we have a contradiction because the product of the left sides is a perfect square and the outputs are all real:

$A_{X}^{2}A_{Y}^{2}B_{X}^{2}B_{Y}^{2}C_{X}^{2}C_{Y}^{2}=-1$

There could be other classic strategies with probabilistic outputs, however, the proof of impossibility of those is very similar to this case.

Now that the researchers are confident that classical strategies are not helpful in saving their funding, the trio decide to put their quantum information research to use. They begin by creating three entangled particles in the following state:

$|\gamma \rangle ={1 \over {\sqrt {2}}}(|000\rangle +|111\rangle )$

This entangled state is known as the GHZ-state for Greenberger, Horne, and Zeilinger, who studied it first in 1989 .

After taking one part of the state each, they go to their labs and measure their qubits according to the letter in the Referee's email: if the email contains X, the receiver measures their share in the basis of the eigenstates of the Pauli X operator:

$|+\rangle ={1 \over {\sqrt {2}}}(|0\rangle +|1\rangle )$

and

$|-\rangle ={1 \over {\sqrt {2}}}(|0\rangle -|1\rangle )$

and outputs +1 if the outcome of the measurement is $|+\rangle$ or -1 for $|-\rangle$. Similarly if the email contains Y, the receiver measures their share in the basis of the eigenstates of the Pauli Y operator:

$|+i\rangle ={1 \over {\sqrt {2}}}(|0\rangle +i|1\rangle )$

and

$|-i\rangle ={1 \over {\sqrt {2}}}(|0\rangle -i|1\rangle )$

and outputs +1 if the outcome of the measurement is $|+i\rangle$ or -1 for $|-i\rangle$.

Using this system, the probability of an odd number of researchers observing $|-\rangle$ if the Referee sent X to everyone (i.e., they all measured in $|+\rangle$ and $|-\rangle$ basis) will be zero; for instance:

$|(\langle -|\otimes \langle +|\otimes \langle +|)|\gamma \rangle |=0$,

so in the case of XXX, always an even number of measurements result in -1 and the final product will be +1.

For the other three cases the probability of an even number of researchers observing $|-\rangle$ or $|-i\rangle$ will be zero; for instance:

$|(\langle -i|\otimes \langle +i|\otimes \langle -|)|\gamma \rangle |=0$,

so always an odd number of measurements result in -1 and the final product will be -1.

Thus, using this quantum strategy, the researchers can guarantee perpetual funding.

The study of these quantum games and strategies has lead to interesting results in cryptography and other information theoretic fields. In this case, it is important to note that the labs were still isolated and no information was transferred. This seemingly paradoxical result is in fact due to the non-local nature of the correlations. In the quantum information literature these games are sometimes referred to as violations of Bell's inequalities because these quantum correlations cannot be explained by a local hidden variables theory. — Preceding unsigned comment added by Mahrud (talk • contribs) 16:19, 15 December 2014 (UTC)

Tsirelson's bound surely has and analogue for this game and that presents a problem. YohanN7 (talk) 16:55, 15 December 2014 (UTC)

No, it does not! The quantum bound for this case is trivial: equals 1. This is the point of GHZ. Boris Tsirelson (talk) 19:23, 15 December 2014 (UTC)

In that case inclusion somewhere seems like a good idea. I noticed that we don't have an article on the CHSH game. An article Quantum games could have entries for both this and the the CHSH game. Just a thought. YohanN7 (talk) 22:55, 15 December 2014 (UTC)

Incorrect wording in first example?

"Alice observes one of two outcomes, a0 and a1, and Bob also observes one of two outcomes, b0 and b1. There are 2^4 = 16 possible combinations of these 4 events:" - shouldn't this say, for example, something like 'Alice observes one of four outcomes, a0, a1, a0 & a1, !(a0 & a1)' And likewise for Bob? If they only observe one of two outcomes each then there's only 2^2 combinations possible.

No, this is not the text from the article. Look again:

Alice pushes one of two buttons, A0 and A1, and Bob also pushes one of two buttons, B0 and B1. Alice observes one of two outcomes, a0 and a1, and Bob also observes one of two outcomes, b0 and b1. There are 2⁴ = 16 possible combinations of these 4 events:

$\textstyle (AX,BY,ax,by)$

where each of X,Y,x,y is 0 or 1.

That is, for instance, one possibility is: Alice pushes the button A1 and observes the outcome a0, while Bob pushes B1 and observes b1. Here X=1, Y=1, x=0, y=0. This is one out of the 16 possibilities. There is no such thing as a0 & a1. Boris Tsirelson (talk) 06:40, 17 October 2015 (UTC)

Nonlocality without entanglement

It seems we need to discuss a reasonably substantive rewrite of parts of the article in order to make it better. As some discussion of this matter has already occurred on the talk pages of myself, Boris Tsirelson, and Tal Mor, I think it is best that we move to this central hub and as such, I will summarise what I consider to be the salient points.

1. The opening sentence is not sufficiently general to accommodate the subtler form of Nonlocality found in the paper "Quantum Nonlocality without Entanglement", the concept of which we would ideally wish to communicate in the article.
2. Whatever this opening sentence is changed to should probably still reflect that notions of nonlocality that are characterised by quantumly accessible probability distributions for experiments on separated systems that cannot be explained using local hidden variable models are the most common, and therefore that the reader can expect that often, the phrase "nonlocality" will refer to such notions.

It seems to me additionally that the flow of the article could do with substansive reworking. The sections and subsections don't really seem to me to work as they are. I would tentatively suggest that the article be changed into the following form:

1. An introduction, as discussed above
2. History, which can mention the EPR argument folllowed by mention of Bell's theorem. Perhaps this section is left without much mathematics
3. Notions of nonlocality: This section can have different subsections. The first could be "probabilistic nonlocality", which contains a proof of Bell's theorem of Probabilistic type. A Bell scenario would work well here, but there are proofs of this type by Mermin that might be more accessible, since they are not explicitly inequality-driven. The second could be "possibilistic nonlocality" which details the proof by Hardy which currently has its own subsection, as well as that of GHZ which is a natural candidate for making an appearance in this article. Thirdly, I suggest we include the concept of nonlocality without entanglement, which can then be presented as a different concept with, I think, less possibility of confusion.

Porphyro (talk) 15:28, 2 November 2016 (UTC)

I looked first at Porphyro's Talk page, and wrote a new suggestion there, BEFORE seeing the discussion in Tsirel Talk page and then the Talk page here. So let me, for now, just copy to here the idea I had, which might lead to an easy and optimal solution - to start by changing the TITLE of the current article. Here is the full suggestion: Note please that it might be very difficult and time consuming to reach an agreed definition of "quantum nonlocality". A better approach might be to change the title, so that it fits the current definition, e.g. "quantum nonlocality of a single quantum state". Then it is natural to put back my change from last week, but only into the section on nonlocality and entanglement (a section that MAYBE should become an article on its own). Best regards. Tal Mor (talk) 17:21, 4 November 2016 (UTC)

There are at least two types of quantum nonlocalities that do not follow the definition as given in the current article: nonlocality of a set of quantum states, and nonlocality as in the Aharonov-Bohm effect. Maybe there are more types. How can we contain all those types into a single definition? So my current suggestion is to change the title of the article, and let it be linked to other types of quantum nonlocality as well. Looking now at the suggestion by Porphyro (above), I think that both what he refers to as probabilistic nonlocality and possibilistic nonlocality, are special cases of what I called quantum nonlocality of a single quantum state. Tal Mor (talk) 18:26, 4 November 2016 (UTC)

I cannot resist to try a general definition of "quantum nonlocality", even though it could be "original research" not usable on Wikipedia; maybe it is not; and even if it is, maybe it can be somehow useful in this discussion.

Classical intuition suggests that every physical device can (in principle) be replaced with a very special device (such as Turing machine), if this device is needed only for a computation, and resources are not limited; see Hypercomputation#Criticism.

Similarly, classical intuition suggests that every physical channel can (in principle) be replaced with a very special channel (such as a telegraph), if this channel is needed only for communication, and resources are not limited.

The latter (unlike the former) contradicts to a number of predictions of quantum mechanics and physical experiments that confirm these predictions. In particular, Bell inequalities hold for classical channels (from the source to the detectors) but is violated for quantum channels. Such situations are called "quantum nonlocality".

Boris Tsirelson (talk) 20:58, 4 November 2016 (UTC)

A slogan, if you like: "No" to quantum hypercomputation, "yes" to quantum communication. Boris Tsirelson (talk) 21:25, 4 November 2016 (UTC)

I admit that I had not really thought about the Aharanov-Bohm effect when considering what forms of nonlocality we should consider in this article, but I don't like the idea of renaming this one- the inability of local hidden variable models to give rise to quantumly-accessible probability distributions is by far the most common usage of "quantum nonlocality" and I think the article should reflect that. For clarity, the two forms of nonlocality I mention above are both of this "single state" form, but I felt that it would be nicer to split them off into separate subsections. They perhaps represent different "degrees" of one type of nonlocality. I like Boris's idea of having a very general definition at the top, but cannot immediately see how this generalises to the other forms of nonlocality that Tal mentions.Porphyro (talk) 11:06, 7 November 2016 (UTC)

Currently, the article is wrong in my eyes, as it is too specific. Of course, I agree with Porphyro about "...by far the most..." and I agree with Boris that a it will be great to have a definition that contains all possible types of quantum nonlocality. I again agree with Porphyro that it is hard to see how Boris' suggestion generalizes quantum nonlocality to include the possible types we discuss here. What about a much more minimal modification? I can see another option (added to the one I suggested earlier, or changing the title): Change:

In theoretical physics, quantum nonlocality is the phenomenon by which measurements made at a microscopic level contradict a collection... To: In theoretical physics, quantum nonlocality most commonly refers to the phenomenon by which measurements made at a microscopic level contradict a collection... This then opens the door to a later sentence providing more options of quantum nonlocality.Tal Mor (talk) 16:25, 7 November 2016 (UTC)

I see: not just "is" but "most commonly refers to". Why not? OK with me. Boris Tsirelson (talk) 16:44, 7 November 2016 (UTC)

To Porphyro: Oops... you are right, the Aharonov–Bohm effect is beyond my formulation. It is rather about two local observers able to communicate and still less capable that one global observer; and in this case quantum communication does not help. Is there at all a single notion of nonlocality that embraces all these cases? Or is nonlocality a vague idea? Boris Tsirelson (talk) 16:47, 7 November 2016 (UTC)

I think unfortunately it is very difficult to capture all these notions with a single definition, and although I would prefer to find one than go with the "most commonly..", that solution is acceptable to me as well. Perhaps the closest we can get is something along these lines: "A system is said to exhibit nonlocality if operational predictions about the system rule out the possibility that it is amenable to a complete description made up of descriptions of its subsystems. Rather, the system must have some holistic properties that cannot be said to belong to any of its local subsystems". I think this idea captures what Tal calls nonlocality of a single state, and Aharanov-Bohm as you describe it, since the Holisitic properties are (almost by definition) not available to local observers but only to a global observer; I am not sure about nonlocality of a set of states. Porphyro (talk) 17:04, 7 November 2016 (UTC)

In addition to my above comment, I would still like to overhaul the flow of the article no matter which of these options we choose, but I'll refrain from doing so until after there's more of a consensus about how to move forward. Porphyro (talk) 17:07, 7 November 2016 (UTC)

Be careful not to go too advanced for a Wikipedia article. In particular, the lead is allowed to be less than exact. Just a thought. YohanN7 (talk) 17:09, 7 November 2016 (UTC)

Another attempt. On a semi-vague level, Bell theorem excludes local classical variables (hidden or not); "nonlocality without entanglement" excludes local classical unhidden variables; Aharonov–Bohm effect excludes local unhidden variables (classical or not). Really so? Does it mean that, most generally, nonlocality is something that excludes some class of local variables? Is it basically the same as "must have some holistic properties that cannot be said to belong to any of its local subsystems"? A bit more exact but still less than exact? Boris Tsirelson (talk) 17:41, 7 November 2016 (UTC)

I think that Wikipedia does not encourage "new research" in its articles. And I believe that finding one definition of quantum nonlocality that captures all its amazing aspect is actually a new research - maybe in (springer's) Foundation of Physics or in an even more philosophical journal. For the current discussion - I suggest we go with my simple correction ("is" replaced by "most commonly refers to"). In addition we can try to have a separate article about quantum nonlocality and entanglement (currently it is a Section, and this Section appears in two articles, and it is not identical in those two articles - the other article of course is Entanglement). I think we should leave the question of a single definition OPEN, as it is indeed open. I do like Boris' suggestion of hidden and unhidden... but this seems like doing research. We will have hard time citing papers trying to join all those into one definition (maybe we should write one). Tal Mor (talk) 18:57, 7 November 2016 (UTC)

I agree that we are flirting dangerously with "original research" at this point. I would not object to trying to write a paper if we felt we had something interesting to say, though.. Porphyro (talk) 22:05, 7 November 2016 (UTC)

In addition to the sentence --- In theoretical physics, quantum nonlocality most commonly refers to the phenomenon by which measurements made at a microscopic level contradict a collection... --- I also think we should explicitly follow the way the abstract of quantum nonlocality without entanglement (PRA, vol 59, page 1070, year 1999) clarifies why the topic of the paper is quantum nonlocality; We can add that, More generally, quantum nonlocality refers to situations in which observers allowed any sequence of local operations and classical communication fail to perform a task that observers allowed also quantum communication can do. Note however that such a definition may not capture the nonlocality of the Aharonov Bohm effect.Tal Mor (talk) 11:13, 9 November 2016 (UTC)

Good! (I guess, you mean that both phrases are to be inserted.) Boris Tsirelson (talk) 17:05, 9 November 2016 (UTC)

However, the matter is more subtle than it may seem. Classical communication between observers is unhidden. Hidden variables theory is rather about classical communication on the hidden level of nature, between hidden entities. Boris Tsirelson (talk) 05:57, 10 November 2016 (UTC)

The same ambiguity appears in "holistic properties that cannot be said to belong to any of its local subsystems" (not "to belong" but "to emerge from", I'd say); is it about observable properties, or hidden properties? Boris Tsirelson (talk) 06:02, 10 November 2016 (UTC)

...And the quantum electrodynamics (being a gauge QFT) is about quantum communication between electromagnetic potentials and other non-gauge-invariant "hidden entities". Boris Tsirelson (talk) 06:13, 10 November 2016 (UTC)

If I understand properly what you now say Boris, the term "More generally, quantum nonlocality refers..." that I used might not be optimal. Maybe "In addition, quantum nonlocality refers..." is better? Tal Mor (talk) 20:36, 15 November 2016 (UTC)

Neither wrong nor illuminating... At the very least, the "...without entanglement" article could be mentioned. Boris Tsirelson (talk) 05:37, 16 November 2016 (UTC)

To prevent further delay, I did minimal changes, and I would like to have your confirmation of course, prior to moving the page (or applying the changes) to the article itself. Please see my sandbox's sub-page on quantum nonlocality. — Preceding unsigned comment added by Tal Mor (talk • contribs) 13:06, 16 November 2016 (UTC)

I see: "...most commonly refers to the phenomenon..." in the lead, and "...and there exist non-entangled (separable) states that do produce some type of non-local behaviour. A well-known example of the first case is..." in Sect. "Nonlocality vs entanglement". Well, this is an improvement. Boris Tsirelson (talk) 16:43, 16 November 2016 (UTC)

I'm certainly more happy with Tal's proposed edits than I am with the article as it stands. Porphyro (talk) 14:10, 17 November 2016 (UTC)

Great! So I'll wait another day or two, and then I'll do the change exactly as suggested in my sandbox. In the meanwhile, we can also discuss what Refs to add (I cannot and should not be the one giving a citation to the original nonlocality without entanglement paper, of course). And I would also like to discuss, not here, but in Boris' Talk page, the slogan he suggested about yes/no to quantum hypercomputation/hypercommunication. Last but not least: I still do not believe it will be good to have here a definition of nonlocality, but it can be an interesting research. I would like to talk to Lev Vaidman about what wording would enable including the Aharonov-Bohm effect in the same (sufficiently-general) definition of nonlocality. Tal Mor (talk) 06:32, 18 November 2016 (UTC)

Is this also included in one of the three we discused, or is it a fourth form of nonlocality: interaction-free measurements (Elitzur-Vaidman, 1993)? Tal Mor (talk) 06:46, 18 November 2016 (UTC)

Elitzur–Vaidman bomb tester, you mean. A good question... I'll try to think. Boris Tsirelson (talk) 08:38, 18 November 2016 (UTC)

Is this "bomb tester" nonlocal? Assume existence of local observable classical states. Imagine that when a photon passes through the half-silvered mirror, it chooses one of the two paths, but something (call it "spy") emerges and travels the other path. Spies are not detected by photon detectors. So what? Which general assumption is needed in order to get a contradiction? Yes, some classical observable detects spies (since classical states are assumed to be observable). Quantum theory does not provide such observables. Should we assume that every classical observable has a counterpart in the quantum theory? Should we also assume that the well-known quantum description of that experiment (in terms of single-particle states) is true? Boris Tsirelson (talk) 09:53, 18 November 2016 (UTC)

I seem to recall a paper by Pawel Blasiak in which he presents a classical (local) model which could apply to the bomb tester but I'm not sure- it's along the same lines as your suggestion Boris in which there is a null element travelling across the other branch. I'll try and chase it up.Porphyro (talk) 11:14, 18 November 2016 (UTC)

I did the agreed change to the first parag. Now will do the change to the relevant section on entanglement and nonlocality. Tal Mor (talk) 07:18, 23 November 2016 (UTC)

I now did the agreed change (with 2-3 extra and minor changes) to the relevant Section. Boris and Porphyro: a. Please verify. b. Please be aware that on purpose I did not provide a Ref., since this is a self-ref and hence not so legitimate. Of course, it will be great if someone add the relevant ref. Tal Mor (talk) 07:28, 23 November 2016 (UTC)

If you wish to add, the Ref (a primary source; it is surely nice to also find secondary sources, and I will try that as well) is: "Quantum nonlocality without entanglement", by (note the alphabetic order, so please never cite it as Bennett-et-al, but use abbreviations or other methods), Charles H. Bennett, David P. DiVincenzo, Christopher A. Fuchs, Tal Mor, Eric Rains, Peter W. Shor, John A. Smolin, and William K. Wootters. Phys. Rev. A 59, 1070 – Published 1 February 1999. Tal Mor (talk) 07:31, 23 November 2016 (UTC)

Nice; I did. Boris Tsirelson (talk) 10:18, 23 November 2016 (UTC)

Thanks Boris. I think we're at a better point than we were before. Next week I will do some re-ordering to the page and seek feedback on it. Porphyro (talk) 15:35, 24 November 2016 (UTC)

Thanks a lot for all the help. Tal Mor (talk) 11:01, 30 November 2016 (UTC)

Modifying "importance" from medium to high

After browsing through Physics Project articles of various importance, it it quite clear in my eyes, that this change (to high) is justified.Tal Mor (talk) 14:01, 23 November 2016 (UTC)

I think this is more than reasonable Porphyro (talk) 18:56, 7 December 2016 (UTC)

Substansive Rewrite

I've had the time today to make a start on a substansive rewrite of parts of this article. Because this is quite a large change, I've mirrored the article in my userspace to make changes User:Porphyro/QML, and I'm hoping to get some consensus before I merge the changes into the main article. Let me summarise the changes I have made so far.

• I have made a few minor changes to the wording of the introduction
• I have removed the section "Examples" which had a communication scenario followed by a PR-box. I don't think this is helpful right at the top of an article on quantum nonlocality and the PR-box is given a treatment lower down the page.
• I have substantially reworked the section on EPR: Einstein said to Schroedinger that he "didn't give a sausage" (Einstein archives EA 22-47) which operators Bob's system was left in an eigenstate of during a nonlocality experiment; he is much more concerned with ontological incompleteness of quantum theory. As written the section was one long example of Bertlmann's socks and I didn't think that it was helpful.

In particular, I think that we could have a section on "nonlocality without entanglement" at the bottom of the history section, and perhaps wrap the "entanglement vs nonlocality" section into the main article somewhere so it doesn't float there by itself. The article can then finish with the treatment of superquantum nonlocality. Porphyro (talk) 12:33, 12 April 2017 (UTC)

Yes, I like such improvement. Some remarks (really, to the current version as well as to the proposed version):

• "In 1993, Lucien Hardy demonstrated a logical proof of quantum nonlocality that, like the GHZ proof..." — but the GHZ proof is not mentioned before (nor after).
• "Non-signaling adversaries have recently been considered in quantum cryptography.[27][citation needed]" — is the citation still needed?
• "In short, entanglement of a two-party state is necessary but not sufficient for that state to be nonlocal" — really? even for mixed states?
• "The polarisation can be either ↑ or ←" — rather, either vertical or horizontal; why these arrows? it is polarization, not spin.

Boris Tsirelson (talk) 13:54, 12 April 2017 (UTC)

Some comments-

• I think the Hardy paradox section needs substantial reworking. The GHZ issue is a very fair one. It's possible we could drop this section.
• This part is presumably talking about bound entanglement- do you mean "even for pure states"?
• I've seen that symbology used elsewhere but tend to agree with you that it's not so helpful for polarisations states- what would your preference be? Pipe/hyphen? Please also feel free to make any changes in the userspace

Porphyro (talk) 14:19, 12 April 2017 (UTC)

• Bound entanglement? Where is it described on Wikipedia?
• Pipe/hyphen, yes.

Also I observe that the transition from EPR to EPRB (Bohm's version of the gedankenexperiment) is implicit in your version. Is this intended?

In addition, in a blatant conflict of interests, I note that, on one hand, "Popescu-Rohrlich box" is the standard terminology, but on the other hand, the article by Popescu & Rohrlich of 1994 cites two articles of 1985 with more or less the same message, one being Khalfin & Tsirelson. Boris Tsirelson (talk) 14:42, 12 April 2017 (UTC)

Yes, I've adopted the notation more associated with EPRB but the argument in the article is Einstein's from his clarifying letter to Schrödinger in 1935. I'll make this a little more explicit. I tend to agree with you that if we're not doing a full treatment of bound entanglement, a small reference is unlikely to be helpful to anyone. Thank you for alerting me to the Khalin and Tsirelson paper. I will give it a read and then keep all these points in mind when I make my next round of changes, probably later this week. Porphyro (talk) 14:53, 12 April 2017 (UTC)

I have now enacted these changes. Porphyro (talk) 09:33, 20 April 2017 (UTC)

I see. But again, about the EPR state. I did not see the letter to Schrodinger. Does Einstein use spins (rather than coordinates and momenta) in this letter? Boris Tsirelson (talk) 12:19, 21 April 2017 (UTC)

No, the change in the letter is the shift in focus between different measurements Alice can make leading to Bob's state being steered into eigenstates of incompatible operators to the observation that he can be steered into different sets of states and so a putative real, local, state of his system must be compatible with two or more quantum states, leading to a conclusion that quantum mechanics isn't a complete description of nature. The article currently includes the phrase "with notation inspired by Bohr's take on EPR", but I should perhaps cite this. One source for this would be Jevtic and Rudolph, but this is arguably a conflict of interest on my part; Rudolph is my supervisor. Porphyro (talk) 12:23, 21 April 2017 (UTC)

I wonder, what is this (Bohr's?) notation. Really, EPR considers eigenstates of operators with continuous spectrum, which is mathematically incorrect. Well, sure, we can use a reasonable approximation; no need to assume the total momentum exactly zero, etc. But, in "History" section, do you really want to go that far from the historical facts? Or do I misunderstand? Boris Tsirelson (talk) 12:31, 21 April 2017 (UTC)

Ah! I meant to write Bohm rather than Bohr. I've corrected it. I personally think it's not too revisionist to present Einstein's concerns using some more modern notation, if only to keep things accessible- but I'm happy to take on board others opinions. Porphyro (talk) 12:36, 21 April 2017 (UTC)

Ah, Bohm! I start to understand. Looking at Jevtic and Rudolf, I observe "we attempt a little revisionist history". Well, if it helps, let it appear here; but please, do not try to convince the reader that this counterfactual history is THE history! We all know that the classical past is unique and not mutable...   :-)   Boris Tsirelson (talk) 12:43, 21 April 2017 (UTC)

I will try to make it clear that while the argument is Einstein's and is historical, the notation is not. Porphyro (talk) 12:45, 21 April 2017 (UTC)

But no, this is not about notation. I recall one my old publication, surely unnoticed and almost surely inavailable: 1996 B.S. Tsirelson, "Fine structure of EPR state and universal quantum correlation", Annals of the Israel Physical Society 12, 83-87. But I do not try to use it here. I only note that there I prove that the EPR state (as is!) is not at all isomorphic to the singlet state. And in particular, there I prove that the EPR state "contains" Hardy's one (1992, Phys Lett A 167 17-23), beyound singlet; and moreover, the EPR state contains EVERYTHING possible. Boris Tsirelson (talk) 12:55, 21 April 2017 (UTC)

Ah, I see what you're saying. I'll make it clear that I am changing both the notation and the state. I'll also take another look at the original document to try and make sure my understanding of what it is saying is correct. Porphyro (talk) 13:00, 21 April 2017 (UTC)

Possibilistic Nonlocality

@Porphyro: The Hardy 1993 article is cited; but there I do not see the specific state used here. Did you choose these coefficients yourself? Also, relations between these two (single-particle) bases are not specified. Boris Tsirelson (talk) 12:15, 21 April 2017 (UTC)

Interesting, those are the states I think most would associate with the paradox. I'll find a source and also make the other change you suggested. Thanks for your diligence here! Porphyro (talk) 12:19, 21 April 2017 (UTC)

I've found it a little harder than expected to find a source for those exact states- it seems to be a bit folkloric. However I've found a more general treatment, the states used here can be seen to be a special form of. Do you think this is sufficient? Porphyro (talk) 12:57, 21 April 2017 (UTC)

As for me, a folkloric version is OK (though, others may object), provided that I can check the details myself. And to this end I need complete specification of the two bases used. Boris Tsirelson (talk) 13:04, 21 April 2017 (UTC)

I added a specification of the states used, I'll add the reference for now and if there are objections I'm happy to consider other options. Porphyro (talk) 13:05, 21 April 2017 (UTC)

But I observe your GoodArticle nomination. I am afraid that the two are not compatible (the nomination and the folk lore). Boris Tsirelson (talk) 13:08, 21 April 2017 (UTC)

I appreciate that. I think this source should cover our bases- but if not, it would be nice to get some third party feedback on where improvement needs to be made, and also to revise the quality assessment for the article. Perhaps it's not ready for GA, but I would not say it is currently as bad as C-class. Porphyro (talk) 13:10, 21 April 2017 (UTC)

No, I fail to check the details. The equality

${\tfrac {1}{\sqrt {3}}}(|00\rangle +\left|01\right\rangle +\left|10\right\rangle )={\frac {1}{\sqrt {3}}}{\big (}{\sqrt {2}}\left|+0\right\rangle +{\tfrac {1}{\sqrt {2}}}\left(\left|0+\right\rangle -\left|0-\right\rangle \right){\big )}={\frac {1}{\sqrt {3}}}{\big (}{\sqrt {2}}|0+\rangle +{\tfrac {1}{\sqrt {2}}}(|1+\rangle +|1-\rangle ){\big )}$

is OK; so what? The first form shows that they never see $\left|11\right\rangle ;$ OK. But the third form shows that they never see $\left|0-\right\rangle$ (rather than $\left|-0\right\rangle$), and the second form does not show anything useful. Please look again. Boris Tsirelson (talk) 13:42, 22 April 2017 (UTC)

Boris, once again I thank you for the effort you're putting in, and again I apologise that I was sloppy in my initial change. I've corrected the second form to ${\frac {1}{\sqrt {3}}}\left({\sqrt {2}}\left|+0\right\rangle +{\frac {1}{\sqrt {2}}}\left(\left|+1\right\rangle +\left|-1\right\rangle \right)\right)$. The symmetry should now be clear. Porphyro (talk) 11:58, 23 April 2017 (UTC)

Now the first form shows that they never see $\left|11\right\rangle ,$ the second form shows that they never see $\left|-0\right\rangle ,$ and the third form shows that they never see $\left|0-\right\rangle ,$ which you did not claim; rather you claim they do see $\left|--\right\rangle .$ A mismatch. Boris Tsirelson (talk) 05:41, 24 April 2017 (UTC)

Yes and no. I claim that they never see $\left|11\right\rangle ,$$\left|0-\right\rangle ,$ or $\left|-0\right\rangle$, and that they do see $\left|--\right\rangle$. These four components are what's necessary for the paradox. As you point out, the article doesn't contain an explicit calculation that the state has overlap with $\left|--\right\rangle .$ I'm obviously open to putting one in! Porphyro (talk) 09:27, 24 April 2017 (UTC)

OK. Boris Tsirelson (talk) 15:15, 24 April 2017 (UTC)

GA Review

This review is transcluded from Talk:Quantum nonlocality/GA1. The edit link for this section can be used to add comments to the review.

Reviewer: Doctorg (talk · contribs) 22:36, 1 September 2017 (UTC)

@Porphyro:I am now starting this review. Thank you for the time you have put into this article and your work towards expanding Wikipedia’s quality content. I will add my comments into each of the following sections. Doctor^G  (talk) 22:36, 1 September 2017 (UTC)

GA review (see here for what the criteria are, and here for what they are not)

1. It is reasonably well written.

   a (prose, spelling, and grammar): b (MoS for lead, layout, word choice, fiction, and lists):

   This is a well written article that discusses the topic with grammatical and technical accuracy throughout. The lead section needs to more clearly summarize the article content and, topics introduced in the lead, should be discussed in the article (faster than light communication and special relativity are two examples). Doctor^G  (talk) 22:48, 1 September 2017 (UTC)

1. It is factually accurate and verifiable.

   a (reference section): b (citations to reliable sources): c (OR): d (copyvio and plagiarism):

   Every formula needs to be cited. Doctor^G  (talk) 23:24, 1 September 2017 (UTC)

1. It is broad in its coverage.

   a (major aspects): b (focused):

2. It follows the neutral point of view policy.

   Fair representation without bias:

   The article includes thoughts from multiple well known scientists which provides for the neutrality on the subject. Doctor^G  (talk) 22:51, 1 September 2017 (UTC)

1. It is stable.

   No edit wars, etc.:

   Article is stable, no edit wars detected. Doctor^G  (talk) 23:02, 1 September 2017 (UTC)

1. It is illustrated by images and other media, where possible and appropriate.

   a (images are tagged and non-free content have fair use rationales): b (appropriate use with suitable captions):

   Math formulas are used throughout to illustrate the written content. Doctor^G  (talk) 23:11, 1 September 2017 (UTC)

1. Overall:

   Pass/Fail:

   Immediate failure for copyright infringement and plagiarism. Much of the content is copied/pasted from other locations on the internet. Doctor^G  (talk) 23:37, 1 September 2017 (UTC)

Thank you for your work reviewing this article; I'll keep your comments in mind for future revision. I'm surprised by the final conclusion, though, as I'm not aware that any of the material is copied or pasted from elsewhere. Could you point out the relevant sections for me?

Porphyro (talk) 13:57, 2 September 2017 (UTC)

Copy a fe different sentences into Google and you will see verbatim content throughout the internet. I did this with a handful of sentences and had the same result each time.

could you be more specific? The only examples I (and the GA plagiarism detector) can find are sites that source this exact article and quote it wholesale. Porphyro (talk) 01:00, 3 September 2017 (UTC)

For starters, the opening sentence of the lead section returns 700 + results in a Google search. While it is certainly possible that some sites have linked to this article, or copied from it, it seems unlikely that it is in the hundreds. As I dug deeper, another line from the lead section also seems to have been published in a Journal of Physics conference proceedings in 2012. I am not saying you plagiarized, but the same content is in a lot of different places. Published articles are typically peer reviewed and it would probably be caught if it was copied from Wikipedia. I hope this all makes sense. Doctor^G  (talk) 02:22, 3 September 2017 (UTC)

It is obviously something that happens on occasion that some plagiarised material is added at some point by an editor and remains mostly unchanged throughout the editing process, and it is possible that part of the first sentence falls into this category. However I can't find any source on google that it could have said to be copied from. If you could tell me which sentence specifically you found in the JoP proceedings I will definitely take a look at it. the thing that surprised me, though, is that you believe that a lot of the article consists of plagiarised material; I strongly believe this is not the case. The reason for my concern is that the rest of your review gives me particular points that I can act on to improve the article but I can't remove plagiarism that isn't really present. Porphyro (talk) 03:17, 3 September 2017 (UTC)

I agree, someone may copy/paste content into an article from time to time. You asked for a specific reference so I used the opening sentence as my example. I can't give you a line by line breakout of everything that may be plagiarized, that is better handled by those (including yourself) who are actively working to improve the article. Do what I did, search for the first sentence on Google and you will find what I did. Your profile says you are a PhD student, you should have access to the publication I found. I suggest you do the research, rewrite anything that is plagiarized, and resubmit it for GA status. It's a great article, but there is a lot of content there from other sources and that makes it highly suspect. BTW, I have a doctorate and I teach grad students, I can usually spot plagiarism from a mile away. I'm not saying you did it, but it is definitely in there. Doctor^G  (talk) 00:42, 4 September 2017 (UTC)

I'm not asking for a line by line breakdown of plagiarised material, but if you found a specific example I don't think that it's unreasonable to ask you to specify what it is; please therefore specify the sentence that you have found elsewhere and I can investigate when that was added and check other material that was added at the time. The duplicate detector finds no plagiarised material on google and so i am sure you can see that it is frustrating for me that you suggest there is a large amount of plagiarised material but have not given an example. The first line of the article was recently debated and rewritten by myself, User:Tsirelson and others as you can see on the talk page and is demonstrably not copied verbatim from elsewhere. As you suggest, it is excessively onerous to google every single sentence of an article to look for similar sentences and I don't see why you expect me to do this to stave off accusations of plagiarism. I will in any case revise parts of the article to respond to your other (reasonable) criticisms. Porphyro (talk) 03:52, 4 September 2017 (UTC)

I think I already did that, but in case it was unclear, the very first paragraph in the lead section is on many pages throughout the Internet as well as in a published conference article from 2012. Doctor^G  (talk) 13:22, 4 September 2017 (UTC)

thank you for the clarification! Unfortunately I really cannot find the article you're referring to- I'm sorry if this is unhelpful. If you could give me the name it would be much appreciated. In any case it seems that the best course of action here will be the following, and I hope you'll agree. Firstly, to make the referencing changes you suggested and generally clean up the article following your points. Secondly, to reach out to editors experienced in dealing with plagiarised content on Wikipedia to get an expert pair of eyes on the content who will hopefully be able to give constructive advice. I ought to be able to do this in a couple of weeks time. Porphyro (talk) 18:58, 4 September 2017 (UTC)

Response to third opinion request:

Well, I had written a few paragraphs and they somehow disappeared on me when I pressed "Publish changes", so here we go a second time. The dispute is old but I don't see why a third opinion wouldn't be appropriate here. I don't see any copyright issues in the lead paragraphs here. Any text I was able to find online (either from Earwig's copyvio detector or spot-checking via Google) was copied from or mirrors Wikipedia's text, not the other way around. It might be helpful to have the reference information for the Journal of Physics article mentioned above, just to identify it and then check. But I'm not seeing any text lifted or plagiarized from copyrighted sources in the lead. Thanks, /wiae /tlk 17:22, 3 July 2018 (UTC)

Requested move 6 July 2018

The following is a closed discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review. No further edits should be made to this section.

The result of the move request was: no consensus to move to the proposed title at this time. Although there was some support for the request, it did not reach the level of consensus in favor of the change. In addition, the proposer also appears to wish to withdraw the request; see comments below relating to the possibility of opening a new discussion. Dekimasuよ! 21:22, 12 July 2018 (UTC)

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Quantum nonlocality → Local realism – If we reject local realism (as a true description of reality)—and obviously we should—then I think it's highly misleading to call the opposite of local realism by the term "quantum nonlocality". Logically, the opposite of "both local and real" is:

Either non-local ... or non-real ... or both non-local and non-real.

The title "quantum nonlocality" is not neutral on this; it strongly insinuates a preference for "non-local" over the alternatives. My proposed alternative title "local realism" can discuss all the same topics as currently in the article, while remaining neutral on what local realism should be replaced with. Well, it doesn't have to remain neutral, it can discuss the topic explicitly. But that's still better than specifically emphasizing non-locality in the article title.

The problem is not just picking sides, but picking the wrong side. If I had to describe mainstream quantum mechanics, I would say it violates realism long before I would say it violates locality. Why?

• Nonlocal has a technical meaning in physics (observable influences going faster than light), and according to this definition, quantum field theory is in fact a local theory. More concretely: In quantum field theory, if you want to calculate how the probability amplitude for a quantum field at point x is changing, all you need to know is what are the various fields at x and their derivatives. Faraway points (outside the light cone) are not included in the formulas either explicitly or implicitly.
• Nonlocal realism—a theory with no quantum superpositions, but with real physical influences traveling faster than light in a way that miraculously does not allow communication—is not how standard quantum mechanics works. Even if it's not definitively ruled out experimentally (I'm not sure), it's certainly not widely believed.

Again, with the title "Local Realism", the whole article can stay just about the same, and can talk about how local realism is not true, but it won't be repeatedly misleadingly insinuating that the primary alternative to local realism is non-locality as opposed to non-realism. --Steve (talk) 17:34, 6 July 2018 (UTC)

Oppose. I think this article should be merged into Principle of locality. I think the principles "locality" and "realism" need to be discussed together in one article. We already have a Principle of locality article where they are discussed along with the EPR paradox and Bell inequality, that is the place for this content. There is no need for either a Quantum nonlocality or a Local realism article; these will simply fragment this discussion. I also question whether mainstream sources support two different types of locality as Steve claims above; I don't have time right now to go through the article. Whether they do or not, for comprehensibility these issues need to be presented in a single article. --Chetvorno^TALK 19:07, 6 July 2018 (UTC)

• If there's a consensus for merging into Principle of locality, I'm happy to support that as well. Maybe it's even better, as long as there's someone willing to do the merging work. --Steve (talk) 21:30, 6 July 2018 (UTC)

• Oppose per nom and it seems fine as a stand-alone page and not a merge candidate. The nom begins with "Quantum nonlocality → Local realism – If we reject local realism—and obviously we should...". Then why would Wikipedia name the page with a title it rejects? The lead sentence itself includes "quantum nonlocality most commonly refers to the phenomenon by which measurements made at a microscopic level contradict a collection of notions known as local realism", which, again, seems counter to changing the name of the page to "local realism". Quantum nonlocality seems fine as a name per my, admittedly, limited understanding of the concept (and I look forward to reading the comments of those who don't have a "limited understanding" of the topic), and I'll keep watch on this discussion. Question, what is the common name of the topic if not "quantum nonlocality"? Randy Kryn (talk) 09:48, 7 July 2018 (UTC)

I added a parenthetical to the top, I meant "if we reject local realism as a true description of reality", I didn't mean "reject it as a wikipedia article title" in that particular sentence. In mainstream quantum mechanics, e.g. popular intro textbooks, I believe that the most common way to talk about the fact that QM violates local realism is just to say "QM violates local realism" without using a more specific term at all. One example is Nielsen & Chuang (quoted below). How would you feel about the alternative title "Quantum violation of local realism"? It's less punchy than "quantum nonlocality", but I'd take a less punchy title over an actively misleading title, when both are sufficiently common in the field. --Steve (talk) 14:44, 8 July 2018 (UTC)

• Oppose Local realism is defined in a way that is relatively theory-independent. The kind of nonlocality described by quantum mechanics is a specific type of violation that should not be the focus of an article called "local realism". The content of this article, about different flavours or manifestations of nonlocality within quantum theory, is currently very distinct from the article at Principle of locality and should not be merged. Agreed also that having an article called Local Realism about how local realism does not seem to hold in our universe is strange, at least. I can't see a good reason to merge the articles; nonlocality is the term used by most in the physics community to describe violations of local realism, though I do admit the philosophical point that the nom mentions. Part of the issue is that locality is not always used in a consistent way, which is illustrated by the QFT example. Porphyro (talk) 12:38, 7 July 2018 (UTC)

As a note, above the OP writes "Nonlocal has a technical meaning in physics (observable influences going faster than light)"; for the avoidance of any ambiguity, this property is referred to in the quantum foundations/quantum information community as *signalling*, which *locality* usually meaning local realism. Porphyro (talk) 09:18, 8 July 2018 (UTC)

I admit that some people use "quantum nonlocality" to mean "the opposite of local realism", but it is (1) actively misleading [as I argue at top] and (2) far from universal. (1) means we should avoid using it if we have any choice at all in the matter, and (2) means we do have a choice! For an example of (2), I just pulled out a popular quantum information textbook (Nielsen & Chuang) and there's a nice discussion of local realism (p116): "...The world is not locally realistic. Most physicists take the point of view that it is the assumption of realism which needs to be dropped from our worldview in quantum mechanics, although others have argued that the assumption of locality should be dropped instead." The book discusses local realism at length but the terms "nonlocality" or "quantum nonlocality" are not used anywhere in the book in a way that means "the opposite of local realism". This is the first book I checked, I'm not cherry-picking.

How would you feel about the alternative title "Quantum violation of local realism"? It's less punchy than "quantum nonlocality", but I'd take a less punchy title over an actively misleading title, when both are sufficiently common in the field. --Steve (talk) 14:44, 8 July 2018 (UTC)

So I think that Nielsen and Chuang are taking something of a "classical" definition of how to split "Local Realism" into Locality + Realism. It is not a trivial result, but dropping determinism is not enough to escape Bell's theorem. I think Bell must have known this (he proved that 3-dimensional quantum mechanics is contextual, thereby disproving the classical notion of realism, before he proved his celebrated theorem on nonlocality), but it's not particularly clear in his proof or the discussion. A more modern take on local realism is to define realism in the sense philosophers use it: that there is an actual state of reality, independent of our observations. This state does not have to assign values to measurable quantities in a deterministic way. Then, local realism says that the actual states of two separated subsystems can be thought of as being local to the systems: there is no holistic element of the state, and the local states have local dynamics. Dropping the assumption of determinism, given this definition of Kinematics (or equivalently, showing this is the same notion of local realism), is justified by an extension of Fine's theorem (for example by Abramsky and Brandenburger), and this is not a trivial mathematical result. In this view, I do not think that most people would want to give up on "realism" to recover "locality". However, it seems like there are many people here who use the more classical definitions, so in the interest of having a title that does not seem to anyone as prejudicial, I would support a title along the lines of "Quantum violation of local realism". For what it's worth, I picked up the first text I had to hand (Isham's Lectures on Quantum Theory), and his take-home message from Bell's theorem is "we are obliged to either stick to a pragmatic approach or strict instrumentalist interpretation, or else to accept the existence of a strange nonlocality that seems hard to reconcile with our normal concepts of spatial separation between independent entitites". Porphyro (talk) 09:48, 9 July 2018 (UTC)

Support per above. Also it would bring less fringe fans. MaoGo (talk) 15:13, 8 July 2018 (UTC)

• Support. See this ngram. "Quantum nonlocality" suggest that that the paradox can be resolved by assuming faster-than-light or higher dimensional communication between the entangled particles. But this is only one possibility, and not the most popular one among those who study this issue. Locality is discussed only in the context of the nonlocality issue, so merging these articles into a single locality article is a good suggestion as well. Locality (quantum)? Nine Zulu queens (talk) 00:10, 12 July 2018 (UTC)

  But that's like an n-gram using apples and oranges, the topics are not the same. Randy Kryn (talk) 00:17, 12 July 2018 (UTC)

Closing as the proposer, seems like "Quantum violation of local realism" has a better chance, and I think I've come to like it more myself too. I'll put up a separate proposal on that later. Thanks everyone for constructive dialog. --Steve (talk) 00:23, 12 July 2018 (UTC)

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The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page or in a move review. No further edits should be made to this section.

GA Review

This review is transcluded from Talk:Quantum nonlocality/GA2. The edit link for this section can be used to add comments to the review.

Reviewer: RockMagnetist (talk · contribs) 01:18, 8 December 2018 (UTC)

I am planning to review this article. RockMagnetist(talk) 01:18, 8 December 2018 (UTC)

General comments

I would like to begin by thanking the editors for their efforts on this article. There is a lot of good material, and contrary to the previous review, I don't see any evidence of plagiarism. But I do have some serious concerns.

Writing: context

My first concern comes under the heading of Criterion 1. An important element of good writing is that it provides context for the reader. This article repeatedly fails to do that. Consider the first paragraph of the lead, which mentions local realism, entanglement, local hidden variable theory and Bell's theorem (not to mention quantum and classical physics) without explaining any of them. One might forgive that if the first section in the body explains them, but instead we get a history section that leads off with a mention of a thought experiment and the Copenhagen interpretation, and then starts using ket notation! Any lay leader would be completely lost by this point.

Is it reasonable to expect this article to be written for a lay reader? Definitely! (At least in the first section or two.) This is a subject that is regularly mentioned in popular media, for example a Forbes article, another Forbes article, and a series of articles in Science News.

At a minimum, the first section of the introduction should say a little about the difference between quantum and classical mechanics, describe in lay terms what bothered Einstein about quantum, and explain what local realism and hidden variable theories are (and maybe how action at a distance fits in). And for the most part the bra-ket notation should be dropped; you can use terms like "up" and "down" or "color" like some of the sources below. And it wouldn't hurt to mention Einstein's term "spooky action at a distance" - it would make this article more approachable.

Some sources that might help with making this article more accessible are "The emperor's new mind" by Roger Penrose; "Is the moon there when nobody looks? Reality and the quantum theory" by N. David Mermin; "How the Hippies Saved Physics: Science, Counterculture, and the Quantum Revival" by David Kaiser; "The Quantum Handshake: Entanglement, Nonlocality and Transactions" by John G. Cramer; and the Stanford Encyclopedia articles "The Einstein-Podolsky-Rosen Argument in Quantum Theory" and "Action at a Distance in Quantum Mechanics". Those are just ideas - I won't require you to use them all. RockMagnetist(talk) 01:52, 8 December 2018 (UTC)

Experiments

Under Criterion 3, breadth of coverage, a glaring omission is the experiments. They are only discussed in the most cursory fashion. RockMagnetist(talk) 01:52, 8 December 2018 (UTC)

Illustrations

It would be nice to have an illustration or two. How about File:Bell-test-photon-analyer.png? RockMagnetist(talk) 01:52, 8 December 2018 (UTC)

Major rewrite

This article needs a major rewrite.

To begin, the current entry makes reference to “nonlocality without entanglement”. The latter refers to the existence of separable measurements which cannot be implemented via local operations and classical communication. This has nothing to do with the way in which most people currently understand the term “quantum non-locality”, it’s just a clever title for a paper. It is confusing and misleading. It must go.

More worrying is the discussion on supra-quantum nonlocality. There has been a lot of research in that topic, mainly between the years 2005 and 2014. During that time, there were many proposals of device-independent principles to constrain the set of physical correlations. Interestingly, three of them recover Tsirelson’s bound. There is also evidence that no set of “reasonable” device-independent principles will suffice to re-derive the set of quantum correlations.

In addition, there have been a few breakthroughs on the mathematics of the quantum set. For starters, there exist two different definitions of the quantum set of correlations: one where the operator algebras of Alice and Bob are required to commute; and another one, where they are required to act on different factors of a tensor product of Hilbert spaces. Tsirelson's problem consists in deciding whether the closures of these two sets coincide. One can characterize the "commuting" set of quantum correlations via hierarchies of semidefinite programs. This has allowed many authors to derive provable upper bounds on the maximum quantum violation of a number of interesting Bell inequalities. More recently, Slofstra showed that the “tensor set” of quantum correlations is not closed.

None of this appears in the current Wikipedia entry. Regarding physical reconstructions of the quantum set, one just finds several references to “uncertainty principles”. Namely, assuming that a number of local observables are subject to specific quantum constraints, one shows that some constraints on Bell inequality violations follow.

Such uncertainty-based principles cannot be falsified in a Bell scenario. In a Bell experiment, the local parties can choose to measure their systems with, say, either setting 1 or setting 2: no one promises them on top that settings 1, 2 correspond to observables satisfying a given uncertainty relation over all states of the theory; or that their statistics depend on the average value of a third setting. Uncertainty-based principles are hence not device-independent. They could perhaps be included in a Wikipedia entry on quantum foundations, along with, e.g., reconstructions of quantum theory based on generalized probabilistic theories. But they have no place in a quantum nonlocality entry. They must go too.

The section “nonlocality and uncertainty relations” is particularly embarrassing. It shows how to derive Tsirelson’s bound from the principle that covariance matrices must admit a positive semidefinite completion. This is mathematically equivalent to the principle of macroscopic locality [4], that is not cited (the article credits instead the much later work of Carmi and Cohen). Even though I am one of the authors of the macroscopic locality paper, I find it excessive to include the derivation of Tsirelson’s bound by macroscopic locality in a general Wikipedia entry on quantum nonlocality. This section is too specialized and must go too.

Finally, I also miss a section with all one can do with quantum nonlocality. Namely, witnessing the Hilbert space dimension, conducting quantum key distribution, etc.

Unless someone opposes, I will introduce major changes soon. Miguel Navascues (talk) 09:32, 2 December 2019 (UTC)

Wow, rewritten! Highly professional. Now reflects the recent progress. Thanks to Miguel Navascues. Boris Tsirelson (talk) 05:33, 6 December 2019 (UTC)