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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)
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)
"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.
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)
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)
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).
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).
It-- 72.74.112.203 ( talk) 15:00, 9 May 2009 (UTC) doesn't belong here, but either is it no true.
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)
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)
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 the rewrite I promised so long ago - should be done in an hour. - Fritzpoll ( talk) 21:25, 25 February 2008 (UTC)
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)
Regards - Fritzpoll ( talk) 20:54, 26 February 2008 (UTC)
A physical system is said to exhibit nonlocality if operations carried out on one system influence the properties of some other, spatially separated system.
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:
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)
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)
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)
Perhaps this page should be merged with Entanglement? Orangedolphin ( talk) 04:20, 30 September 2009 (UTC)
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)
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)
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 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)
(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)
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)
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)
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)
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. [1] However, because the distant connections are actually statistical probabilities involving correlated measurements (see examples below), as of 2012 [update] faster-than-light communication has not been observed and therefore quantum nonlocality is still compatible with special relativity's speed limit.
As of 2012 [update], quantum nonlocality has only been observed with quantum entanglement, which occurs when particles such as photons, electrons, and some larger objects, [2] [3] [4] [5] 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, [6] 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 [7] 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 [update] there is still considerable debate among scientists and philosophers. [8] Non-standard interpretations of quantum mechanics have varied in their response to quantum nonlocality. These include the Bohm interpretation [9], 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)
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 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)
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)
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 denote Alice's response if she received an X and denote her response if she received a Y and so on for , , etc. Then, we can formulate the requirements for winning the game:
But here we have a contradiction because the product of the left sides is a perfect square and the outputs are all real:
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:
This entangled state is known as the GHZ-state for Greenberger, Horne, and Zeilinger, who studied it first in 1989 [10].
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:
and outputs +1 if the outcome of the measurement is or -1 for . Similarly if the email contains Y, the receiver measures their share in the basis of the eigenstates of the Pauli Y operator:
and outputs +1 if the outcome of the measurement is or -1 for .
Using this system, the probability of an odd number of researchers observing if the Referee sent X to everyone (i.e., they all measured in and basis) will be zero; for instance:
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 or will be zero; for instance:
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
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"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.
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.
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:
Porphyro ( talk) 15:28, 2 November 2016 (UTC)
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)
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)
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'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.
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)
@ 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)
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 . The symmetry should now be clear. Porphyro ( talk) 11:58, 23 April 2017 (UTC)
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Reviewer: Doctorg ( talk · contribs) 22:36, 1 September 2017 (UTC)
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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.
DoctorG
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22:36, 1 September 2017 (UTC)
Porphyro ( talk) 13:57, 2 September 2017 (UTC)
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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) |
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)
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:
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?
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)
Support per above. Also it would bring less fringe fans. MaoGo ( talk) 15:13, 8 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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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)
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.
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)
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)
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)
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)
![]() | This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
Archive 1 | Archive 2 |
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)
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)
"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.
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)
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)
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).
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).
It-- 72.74.112.203 ( talk) 15:00, 9 May 2009 (UTC) doesn't belong here, but either is it no true.
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)
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)
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 the rewrite I promised so long ago - should be done in an hour. - Fritzpoll ( talk) 21:25, 25 February 2008 (UTC)
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)
Regards - Fritzpoll ( talk) 20:54, 26 February 2008 (UTC)
A physical system is said to exhibit nonlocality if operations carried out on one system influence the properties of some other, spatially separated system.
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:
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)
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)
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)
Perhaps this page should be merged with Entanglement? Orangedolphin ( talk) 04:20, 30 September 2009 (UTC)
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)
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)
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 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)
(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)
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)
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)
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)
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. [1] However, because the distant connections are actually statistical probabilities involving correlated measurements (see examples below), as of 2012 [update] faster-than-light communication has not been observed and therefore quantum nonlocality is still compatible with special relativity's speed limit.
As of 2012 [update], quantum nonlocality has only been observed with quantum entanglement, which occurs when particles such as photons, electrons, and some larger objects, [2] [3] [4] [5] 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, [6] 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 [7] 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 [update] there is still considerable debate among scientists and philosophers. [8] Non-standard interpretations of quantum mechanics have varied in their response to quantum nonlocality. These include the Bohm interpretation [9], 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)
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 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)
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)
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 denote Alice's response if she received an X and denote her response if she received a Y and so on for , , etc. Then, we can formulate the requirements for winning the game:
But here we have a contradiction because the product of the left sides is a perfect square and the outputs are all real:
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:
This entangled state is known as the GHZ-state for Greenberger, Horne, and Zeilinger, who studied it first in 1989 [10].
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:
and outputs +1 if the outcome of the measurement is or -1 for . Similarly if the email contains Y, the receiver measures their share in the basis of the eigenstates of the Pauli Y operator:
and outputs +1 if the outcome of the measurement is or -1 for .
Using this system, the probability of an odd number of researchers observing if the Referee sent X to everyone (i.e., they all measured in and basis) will be zero; for instance:
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 or will be zero; for instance:
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
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"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.
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.
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:
Porphyro ( talk) 15:28, 2 November 2016 (UTC)
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)
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)
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'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.
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)
@ 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)
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 . The symmetry should now be clear. Porphyro ( talk) 11:58, 23 April 2017 (UTC)
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Reviewer: Doctorg ( talk · contribs) 22:36, 1 September 2017 (UTC)
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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.
DoctorG
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Porphyro ( talk) 13:57, 2 September 2017 (UTC)
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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) |
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)
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:
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?
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)
Support per above. Also it would bring less fringe fans. MaoGo ( talk) 15:13, 8 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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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)
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.
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)
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)
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)
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)