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## Measurements on an entangled state

The diagram of Bob and Alice shows their axes rotated by 45 degrees. The text does explain why this is necessary. — Preceding unsigned comment added by 62.56.70.12 (talk) 08:42, 21 March 2012 (UTC)

I agree, I also think the diagram is confusing. Suggest rotating the Bob circle 45 degrees anti-clockwise and removing the caption '45 (degrees)'. Then it makes sense. Count ludwig (talk) 13:34, 7 April 2013 (UTC)
I also agree with Count ludwig.Septate (talk) 11:35, 6 July 2014 (UTC)

The section under "the crux of the matter" is wrong. By only measuring the x or z axis you cannot distinguish between a classical system with hidden variables and a quantum system. One has to measure at 45 degrees also. The diagram is right, the description is wrong. --Jules — Preceding unsigned comment added by 83.82.131.247 (talk) 13:32, 23 October 2015 (UTC)

I agree that the text is confusing / misleading, but I disagree that you need to "measure 45 degrees also" to distinguish between classical and quantum. I think you are referring to an experiment measuring polarization angles of entangled photons in the 45 degree planes X=0 and X=Z. This example is about measuring spins of an entangled electron and positron about orthogonal axes X=0 and Z=0. (But I may be wrong)

"You might imagine that, when Bob measures the x-spin of his positron, he would get an answer with absolute certainty, since prior to this he hasn't disturbed his particle at all. Bob's positron has a 50% probability of producing +x and a 50% probability of −x—so the outcome is not certain. Bob's positron "knows" that Alice's electron has been measured, and its z-spin detected, and hence B's z-spin has been calculated, but the x-spin of Bob's positron remains uncertain."

1) I've fixed the above, and hopefully made the meaning of the text clearer. I removed a bit of passive voice and highlighted where the reader's assumptions might follow ("It's as if"), and where they might be contradicted ("But it turns out that").
2) But I don't understand the point about "certainty" vs "50% probability" of a measurement (and nor does the person who put an HTML comment at that point).
2a) What is the difference in certainty between Bob making his measurement before Alice makes hers, or after, or what if she doesn't make a measurement at all?
2b) Is it because he measures his positron's "x"-spin twice and gets a different result the second time? Except he doesn't, because "he hasn't disturbed his particle at all."
3) But I think he *has* measured it already, and this is the "crux of the matter". In a classical system it's the same the second time ("certain"), but in a quantum system it can be different ("50% probability").
4) Actually, I think the text in the whole section "Measurements on an entangled state" is practically unintelligible - a reader would have to know what entanglement is and how it works already before they can make any sense of the text, and even then it is still confusing.
5) So I propose a rewrite. — Preceding unsigned comment added by Count ludwig (talkcontribs) 18:43, 12 January 2017 (UTC)

## Section under "the crux of the matter" is wrong

The section under "the crux of the matter" is wrong. By only measuring the x or z axis you cannot distinguish between a classical system with hidden variables and a quantum system. One has to measure at 45 degrees also. The description is wrong.

Sorry, I meant to put this somewhere else, but I do not know how to delete this.

## The EPR paradox and gravity

We now know that the speed of gravity, usually is close to the speed of light in the void (classical void) but in the real world occur disturbances because complex arrangements of matter, interlink particles in ways that cause "cohesion distortions". Gravity is not a fundamental force, the "graviton" is simply the superluminal "briefion" that connects briefly any particle, not necessarily only parts of the same compound particle. The graviton/briefion is nothing other than any virtual boson (can also be individual particle) of the other three fundamental forces• the gravitational field is a compound statistical mechanism of the electroweak gauge field and the strong/chromodynamic gauge field while the two interact indirectly (not as a first step of a feynogram [Feynman diagram] but as a further step, thus intermediate steps are required, and that constitutes gravity so weak, for it's only a secondary statistical effect of the interaction of the electroweak gauge field with the strong/chromodynamic gauge field inside the Higgs connection field) through the Higgs connection field. In huge accumulations of matter, statistically few entanglements occur, the actual entanglements are not enough to justify gravity, but we have to calculate the infinite virtual particles created using as a measure of time the ultimate Planck sequence. That infinity becomes renormalized due to the curvature of spacetime that doesn't allow infinite perfect alignments when relativistically observed at Planck sized microholograms (ultimate quantization of spacetime). Thus a non infinite amount of entanglements occurs. These entanglements transfer instantaneously quantum information, but the electroweak gauge field and the strong/chromodynamic gauge field continue to transfer information at luminal (thus not instantaneous) speed. All particle arrangements should constantly lose energy toward nothingness, because almost all virtual particles dissapear without being materialized (objectified). Gravity is the mechanism that maintans the overall energy (although no gravitational system is closed, and all lose energy towards the Universe; even the Universe isn't a closed system inside the Megaverse, and becomes so diffused with the passage of time so it reaches the upper limit of quantum decohesion, thus then the virtual particles are compelled to become actual [materialized/objectified] to fill the gap, this event is a Big Bang without singularity, and it occurs when entanglements are no longer possible) through the Higgs connection field, and brings matter closer to the center of gravity, in order energy is maintained.

— Preceding Steven Weinberg comment added by Steven Weinberg (talk) 23:29, 29 April 2016 (UTC)

## Implications for Quantum Mechanics

The claim written in that section: "The EPR paradox has deepened our understanding of quantum mechanics by exposing the fundamentally non-classical characteristics of the measurement process. Before the publication of the EPR paper, a measurement was often visualized as a physical disturbance inflicted directly upon the measured system. For instance, when measuring the position of an electron, one imagines shining a light on it, thus disturbing the electron and producing the quantum mechanical uncertainties in its position. Such explanations, which are still encountered in popular expositions of quantum mechanics, are debunked by the EPR paradox, which shows that a "measurement" can be performed on a particle without disturbing it directly, by performing a measurement on a distant entangled particle. "

is basically nonsense as many college level texbooks still teach that and I would say that the measurement problem has not really been solved. Moreover it contraddicts Heisenberg uncertainty principle. — Preceding unsigned comment added by 155.69.199.255 (talk) 10:49, 10 January 2017 (UTC)

Do you doubt that a "measurement" can be performed on a particle without disturbing it directly? Boris Tsirelson (talk) 11:30, 10 January 2017 (UTC)
About Heisenberg uncertainty principle. Without EPR one could hope that q and p (the coordinate and the momentum of a given particle at a given moment) cannot be known both, but still, can exist both (hidden variables). According to EPR, there is no such hope (assuming locality, of course); if q and p exist both, then they can be known both. True, this way we can know their past values, before the measurement, not their current values; but it would violate the uncertainty principle, still. Boris Tsirelson (talk) 12:37, 10 January 2017 (UTC)
The HUP has really little to do with measurements. The uncertainty lies in the states whether we measure them or not. But, I'd give you right that in order to make a measurement on a system, then we have mess with it. Period. Then one can argue. Is there really something like a system of two distant entangled particles comprised in such a way that when one measures, one messes only with one of the particles? I don't know. The statement in the article is strong indeed. YohanN7 (talk) 13:03, 10 January 2017 (UTC)
To explain where my ignorance comes from; one is in the beginning taught to think about particles in a system to not have individuality (this is part of disabusing people from thinking classically about QM). Now one is asked to again think of particles in a system to have some sort of individuality. YohanN7 (talk) 13:46, 10 January 2017 (UTC)
Thinking more about it, the statement in the article could be weakened (or strengthened if you will) to claim that we can find out facts about a particle without doing measurements on it. This would be cleaner and leave measurements and their effect out of the discussion. They tend to blur. YohanN7 (talk) 14:05, 10 January 2017 (UTC)
No individuality? Just holism? Then, why locality, at all? Boris Tsirelson (talk) 16:23, 10 January 2017 (UTC)
Like I said – I don't know. I just accept QM and mathematical facts like Bell's theorem. Then I try hard not to think hard about various "paradoxes" and what they mean. Bell's theorem b t w, I tend to think about as an expression of conservation laws (typically angular momentum) of nature (the form forced by QM). Goofy? YohanN7 (talk) 09:31, 11 January 2017 (UTC)
Oops, no! I think about it as something purely informational. Generally not related at all to any conservation law. See also [1]. Like Aaronson: User:Tsirel#Quantum mechanics is not a physical theory. Boris Tsirelson (talk) 18:57, 11 January 2017 (UTC)
I fully agree with User:Tsirel#Quantum mechanics is not a physical theory. (But I wonder what editor Chjoaygame would have to say about that.) QM is a mathematical framework that can be applied to yield physical theories. YohanN7 (talk) 13:16, 13 January 2017 (UTC)
To be more specific. Conservation laws emerge from symmetries, and are violated when space-time is far from flat. But Bell inequalities are still the same (as well as their quantum counterparts). Relevant devices may differ, but the maximum over all possible devices is still 0.75 (or 0.853...). Boris Tsirelson (talk) 19:13, 11 January 2017 (UTC)
That (broken conservation laws) is definitely new to me. Need to digest this. YohanN7 (talk) 13:21, 13 January 2017 (UTC)
Except for electric charge conservation, though. Boris Tsirelson (talk) 16:02, 13 January 2017 (UTC)

## usefulness

For those with phd in theoretical physics - perhaps, for those who peruse wikipedia this is garbage, not explained at all for layperson Juror1 (talk) 14:27, 16 June 2017 (UTC)

## Landau's contribution

Landau's contributions do matter (and are cited more than once in my articles). However, "any model whether local or non-local will obey Bell's inequality"?? Landau did not (and could not) write anything like that. Probably, the anonymous editor means Landau's Proposition 2: "In a classical theory with joint distributions |R|<=2." However, in the absence of locality the observable R is irrelevant; conditional probabilities are relevant. Moreover, Bell's work on this matter started with the observation that a nonlocal classical theory can reproduce quantum predictions; namely, the De Broglie–Bohm theory does. Boris Tsirelson (talk) 17:47, 22 August 2017 (UTC)

Interesting Ray Streater claims that both you and Landau proved such a thing. Would love to hear you hear your thoughts on Streater :) 197.234.164.85 (talk) —Preceding undated comment added 19:38, 22 August 2017 (UTC)
With every respect to Ray Streater, "such a thing" is wrong (see above), and therefore all its proofs (if any) must be erroneous, and their authors must be guilty.   :-)   (I happened to be guilty, shame on me, but not in this case.) Boris Tsirelson (talk) 20:00, 22 August 2017 (UTC)
Can you elaborate on your statement that R is irrelevant without locality? I have re-read Landau's paper as well as Streater's argument. Streater's view is that locality is not being used in Landau's proof only the assumption that R is a combination of observables represented by random variables on a joint probability space. If by R being irrelevant without locality, you mean that you are supposing a non-local mechanism that prevents the observables being represented as random variables on a joint probability space, well our assumption is ruling out that possibility. By the assumption any non-local mechanism present would have to be one that does not affect our ability to use the joint probability space. But in that case the non-local mechanism does not block the derivation of |R|<=2. So assuming locality in addition to the ability to use a joint probability space is redundant. A similar argument is made by Hess et al. in this paper https://www.researchgate.net/publication/308130326_Counterfactual_Definiteness_and_Bell%27s_Inequality where they note that locality is a redundant assumption if one assumes counterfactual definiteness (in the manner they define it). 197.234.164.85 (talk) —Preceding undated comment added 13:32, 23 August 2017 (UTC)
Sure.
Bell scenario is not about the expectation of the "Bell observable", that is, an observable of the form ${\displaystyle \sum _{i,j}c_{i,j}A_{i}B_{j}.}$ Rather, it is about a linear combination ${\displaystyle \sum _{i,j}c_{i,j}E(AB|i,j)}$ of conditional expectations of the product ${\displaystyle AB}$ under different ${\displaystyle i,j.}$ One may treat the settings ${\displaystyle i,j}$ as non-random parameters, which leads to ${\displaystyle \sum _{i,j}c_{i,j}E(A_{i,j}B_{i,j}),}$ since in the absence of locality each parameter may influence each spin. Alternatively, one may treat ${\displaystyle i,j}$ as random variables (and indeed, nowadays they are randomized, intentionally and carefully). In both cases one may assume (in addition) the usual ("classical") probability theory. In both (equivalent) cases the classical upper bound for CHSH is 4, not 2.
Locality says that each spin measurement is influenced by one setting (not both); and then, indeed, one may use the expectation of the "Bell observable" as an equivalent formulation.
It is vital for Bell scenario to be formulated in phenomenological ("experiment-related") terms, that is, in terms of two spatially separated devices, each with its input and output (setting and outcome). Not in terms of an algebraic expression in the framework of a given formalism (classical or quantum). Without locality the "spatially separated" means nothing, and Bell inequality fails evidently.
Boris Tsirelson (talk) 18:11, 23 August 2017 (UTC)
The same applies to counterfactual definiteness (in the manner I define it). Boris Tsirelson (talk) 18:20, 23 August 2017 (UTC)