Talk:Einstein–Podolsky–Rosen paradox/Archive 2

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EPR paradox?

Some questions conserning the EPR paradox

1.Considering a particle with zero spin decaying in to two particles A and B with non zero spin. Is the behavior of the two new particles A and B and their " choice " of spin direction, due to conservation of spin in an interaction, caused by the collapsing of one two particle wavefunction AB at the time of measurement?

2.By this I mean that each of the two wavefunction states{Aup, Bdown},{Adown,Bup) are states of ONE wave(AB), not two seperate wavefunctions A and B and is therefore no more a paradox then the instantanious collapse of the wavefunction in general( the wavefunction occupies space).

3.Does Reactions with larger arrangements of states also chooses all the states at the same time, only the information we get out of measuring one particle does'nt tell us as much about the other particles, cause there are many wavefunction states that contains the measurement?

4.Can the measurement of one of the paricles be considered as cutting the tie between the two paticles and splitting the one twoparticle statefunction AB in to two singleparticle statefunctions A and B?

Any help in clarifying these things would be great!JanBanan 23:13, 29 January 2006 (UTC)

Retrieved from "http://en.wikipedia.org/wiki/Talk:EPR_paradox/archive_1"

Why doesnt anyone answer? =JanBanan 21:13, 10 February 2006 (UTC)=

WP is not really the place to pose these types of questions, although there is the "science reference desk", Wikipedia:Reference_desk/Science and if you post there, maybe someone will be able to answer. Alos at wikiprojet physcis, maybe. Personally, I didn't nderstand the questions. e.g. question 1: the partcles A nd B don't choose a spin, the spin remains indterminate, in an "entangled state". question 2, Yes, except there is no collapse. Note tht the choice of basis for a vector space is completely arbitary. If you understand linear algebra, then measuring spin is like picking a set of coordinates for a vector space: there are no coords until you pick coords. A vector space does not "cllapse" when you pick coords. linas 23:16, 10 February 2006 (UTC)

Some questions ...

This and most other QM pages are really good but some questions arose when I read this article which I feel went unanswered.

1. Why is Bob's electron's spin "tilted" 45 degrees in the illustration (EPR-paradox-illus.png)?

2. If Alice measure the z-axis and get +z then we know that Bob will get -z. But if Bob instead measure the x-axis and get +x then he can go over to Alice and tell her this. Wouldn't that go against the uncertainty principle since the spin of both axis are now known (alice: +z, -x; bob: -z, +x). Even though Alice can't measure the x-axis she will know the outcome because it is the opposite of Bob's electron. The way I understand the uncertainty principle is that it's not something you can "work around". Thanks in advance --Narcala 04:10, 3 February 2006 (UTC)

First sentence is totally incorrect.

This first sentence is so blatently incorrect it grates:

"... which demonstrates that the result of a measurement performed on one part of a quantum system can have an instantaneous effect on the result of a measurement performed on another part, regardless of the distance separating the two parts."

There is no demonstration of effect. No effect is even described. Rather a contradiction in a set of assumptions is presented. This contradiction implies one of the assumptions is false. To understand this we must make all assumptions explicit. The business about locality is not the key issue it was simply one of many ways to set up the experiment so that the two measurements can be made independently of each other. One need only assume that it is possible --at all-- to decouple two systems which are none the less correlated (entangled) by prior actions. Separating them spatially is simply one easy way to do this.

The thought experiment demonstrates the incompatability of the two main assumptions: Classical Deterministic Reality and Quantum Theory (and now empirical data!)

One can derive Bell's inequality by simply assuming that the probabilities of outcomes for any experiment derives from a probability distribution over a classical state space for the whole universe. Allow the two experiments to "talk to each other" if you like, interject however many hidden variables you like. The base assumption dictates that the probabilities form a measure over the presumed set of physical states. The additivity and positivity of the probability measure yields a pseudo-metric:

$d(A,B)=Pr(A\cap {\overline {B}})+Pr({\overline {A}}\cap B)$

which obeys the triangle inequality (Bell's inequality):

$d(A,B)+d(B,C)-d(A,C)\geq 0$

[work it out and you get: ] $d(A,B)+d(B,C)-d(A,C)=2Pr(A\cap {\overline {B}}\cap C)+2Pr({\overline {A}}\cap B\cap {\overline {C}})\geq 0$

In quantum theory the correlation probabilities are not a metric but rather the square of a metric (magnitude of the correlation amplitudes) and so in some cases violate the triangle inequality.

The conclusion then is that quantum theory invalidates the assumption that we may describe what happens in the lab using a model of classical deterministic reality. Reality isn't real! Note that if you do allow violation of locality you also enable time travel and hence you can no longer speak meaningfully of any experimental outcome as some future event may change the outcome. This is why --I presume-- Einstein used this as the basis for the thought experiment.

Regards, James Baugh

If it grates, please replace the sentence with something suitable then. --CSTAR 17:04, 16 February 2006 (UTC)

--- I agree, the first sentence is incorrect. The EPR paradox is not a paradox because of instantaneous effects or anything like it, it's a paradox because EPRs construction ostensibly allows them to predict more about a physical system than is allowed by quantum mechanics. There is no "spooky action at a distance", thus. The point about rephrasing is well taken, also by me; I suggest the entire opening paragraph be rewritten to summarize EPR's original argument. If nobody does this shortly, I think I'll try myself, and start by posting a suggestion here, on the discussion page. --Agger 10:36, 29 March 2006 (UTC)

I agree, however, what's in the page is the result of compromise between various competing interests, including the one of saying something intelligible in an intro graf. Please see the discussions above. Note that the main points of EPR: elements of reality and apparent incompleteness of quantum mechanics with regard to it (which I believe is the point of your statement "it's a paradox because EPRs construction ostensibly allows them to predict more about a physical system than is allowed by quantum mechanics.") is described, correctly I believe, elsewhere in this article. Nevertheless, please make improvements. --CSTAR 17:36, 29 March 2006 (UTC)

causality violation

About this part: It turns out that quantum mechanics violates the principle of locality without violating causality. Causality is preserved because there is no way for Alice to transmit messages (i.e. information) to Bob by manipulating her measurement axis. Whichever axis she uses, she has a 50% probability of obtaining "+" and 50% of obtaining "-", completely at random; according to quantum mechanics, it is fundamentally impossible for her to influence what result she gets. Furthermore, Bob is only able to perform his measurement once: there is a fundamental property of quantum mechanics, known as the "no cloning theorem", which makes it impossible for him to make a million copies of the electron he receives, perform a spin measurement on each, and look at the statistical distribution of the results. Therefore, in the one measurement he is allowed to make, there is a 50% probability of getting "+" and 50% of getting "-", regardless of whether or not his axis is aligned with Alice's.

Can't we influence the behavior of electron A by detecting electron B, and use that to transmit information?

Check the experiment setting at the end of this page: http://www.joot.com/dave/writings/articles/entanglement/spookiness.shtml

A scientist at detector A can change the rotation of the polarizing filter, and as a result the recieved pattern at detector B changes. So a watcher at detector B can recieve information regarding the filter, and supposedly this information travels instantly, faster than light.

To demonstrate that FTL is not possible I've expanded the paragraph at the end of the locality section to read:

In recent years, however, doubt has been cast on EPR's conclusion due to developments in understanding locality and especially quantum decoherence. The word locality has several different meanings in physics. For example, in quantum field theory "locality" means that quantum fields at different points of space do not interact with one another. However, quantum field theories that are "local" in this sense appear to violate the principle of locality as defined by EPR, but they nevertheless do not violate locality in a more general sense. Wavefunction collapse can be viewed as an epiphenomen of quantum decoherence, which in turn is nothing more than an effect of the underlying local time evolution of the wavefunction of a system and all of its environment. Since the underlying behaviour doesn't violate local causality it follows the neither does the additional effect of wavefunction collapse, whether real or apparent. Therefore, as outlined in the example above, the EPR experiment (nor any quantum experiment) does not demonstrate that FTL signalling is possible.

--Michael C. Price ^talk 20:04, 28 June 2006 (UTC)

I'm uneasy with this change. An important point which seems to be getting diluted with this addition is that quantum mechanics is, in some sense, nonlocal. Now we have to define exactly what notion of locality it is that quantum mechanics violates, but this can be done, and there is such a notion. I don't imagine that how you interpret wavefunction collapse has any impact on this rather fundamental property of all quantum theories, which is the source of the EPR paradox. -lethe ^{talk +} 20:56, 28 June 2006 (UTC)

You're correct that the "non-local" implication of QM is being diluted with my change. However this seems to be an inevitable consequence of the decoherence perspective. The key thing is that the EPR nonlocality was only ever an epiphenomen or an appearance; EPR (along with most physicists) always accepted that you couldn't actually use entanglements to break locality with FTL information transmission. Nonlocality was more a matter of interpretation, rather than anything real. As such we shouldn't be too surprised that it can vanish with a shift in perspective. --Michael C. Price ^talk 21:12, 28 June 2006 (UTC)

I had thought that the way to deal with this was always statistical, as in the no-communication theorem. MAybe it's the same thing, but it's not clear to me yet.--CSTAR 21:18, 28 June 2006 (UTC)

The notion of locality that would be violated by FLT is a different concept than the nonlocality exhibited by quantum mechanics. The fact that the state space of a composite system contains entangled states is sometimes called nonlocality or the failure of local realism. Decoherence and MWI do not remove entangled states from quantum mechanics. Decoherence and MWI do remove the appearance of violation of faster than light information, but since that was never violated anyway, even in Copenhagen, this is a bit of a strawman. -lethe ^{talk +} 02:36, 29 June 2006 (UTC)

Entanglement is often taken as implying nonlocality, which is why the terms are sometimes blurred. Looking at Bell's theorem, though, we see that what is really implied is

entanglement ==> (non-local realism or counterfactual definiteness)

Which of the consequents you opt for is interpretation-dependent. Most non-decoherent interpretations of QM opt for nonlocality, MWI opts for CFD. --Michael C. Price ^talk 07:17, 29 June 2006 (UTC)

I think I probably don't understand the subtleties well enough to argue further, so I'll defer to your judgement on this. -lethe ^{talk +} 15:40, 29 June 2006 (UTC)

Entangled cat ...

To avoid repeating the same text, I am putting a link to my comment about Schrödinger's cat: [[1]]. David R. Ingham 20:51, 23 August 2006 (UTC)

"yields" in the second paragraph

Shouldn't that be "seems to yield". At worst, the existence of a paradox is in question. Weinberg's view (and mine), in particular, does not appear to lead to a paradox. David R. Ingham 03:36, 24 August 2006 (UTC)

The problem with this article, and in general with popular treatments of quantum mechanics

These problems never arise until classical descriptions are introduced. Mathematically, QM is local and deterministic, so these are problems with the rules for making classical approximations and not with QM itself. This has been conspicuously missing from nearly all articles in Wikipedia that I have read. This must be a result of the failure of ordinary language to describe the quantum world. This failure appears to accumulate, within Wikipedia and elsewhere, as each author repeats and interprets what he has read in ordinary language from others, without referring to the actual mathematics. No words fully describe QM. QM is expressible only in terms of complex numbers.

To be honest, my picture is less widely accepted by the mathematically conversant than I first assumed. But an encyclopedia article must accommodate the views of Steven Weinberg:

Physics Today, April 2006, "Weinberg replies", p. 16,

... but the apparatus that we use to measure these variables—and we ourselves—are described by a wave function that evolves deterministically. So there is a missing element in quantum mechanics: a demonstration that the deterministic evolution of the wave function of the apparatus and observer leads to the usual probabilistic rules [Copenhagen interpretation].

And I add that the rules for making classical approximations that rule out those approximations that lead to the EPR paradox are also missing. David R. Ingham 05:26, 24 August 2006 (UTC)

Quotes

See also [[2]].

Note

Note the mention of CFD, which, perhaps, should be more prominent. What Bell's Theorem really proved was that every quantum theory must either violate locality or CFD. --Michael C. Price ^talk 03:54, 24 August 2006 (UTC)

paradox?

EPR is not a "paradox" , it has no logical contradictions ,it is a theorem.

Second paragraph

It says:

2) Quantum mechanics is incomplete in the sense that some element of physical reality corresponding to B cannot be accounted for by quantum mechanics (that is, some extra variable is needed to account for it.)

Penrose points out that the number of variables is exponential in the number of particles. That would seem to be plenty of variables. David R. Ingham 19:59, 26 August 2006 (UTC)

Plenty <> enough. --Michael C. Price ^talk 20:15, 26 August 2006 (UTC)

Designing a telescope: an example of the use of classical approximations

Most of the calculations of the performance of an (optical) telescope are done with geometric optics. This is because it is large compared to the wavelength. When shot noise is included in the performance evaluation, it is usually said that quantum effects are being included, but an equally good statement is that Newton's particle classical mechanics optics is used. So at this point the description is classical, in the sense that Isaac Newton would have understood it. There is no consideration of wave phase.

But a telescope, like an EPR experiment, is intended for a very specific purpose, in this case measuring angle. So one must add to this description an independent estimate of diffraction effects. We know that there is an aperture limit to resolution, which is also called the uncertainty principle. Greater precision can be found by methods like Physical Optics. The aperture limit can be combined with the ray calculation by doing a convolution of the spot shapes on the focal surface. The physical optics calculation takes the ray optics as input. One does not consider doing the whole calculation with wave optics or quantum mechanics (which are roughly synonymous in this case).

Following this example, it is reasonable to describe the two notebooks recording spins in an EPR experiment classically, with the added and independently calculated constraint that when brought together they must show the electron pair in a singlet state. On the other hand it is not unreasonable to follow the usual Copenhagen rules for making classical approximations in experiments and patch them up with non-locality.

What I find very surprising is that so many people still seem to think that these rules of thumb for doing experiments are somehow fundamental to QM. Even Roger Penrose is not clear about this point.

Let me add that, as Penrose points out, QM provides a huge number of variables to account for entanglement, in addition to those needed for the correspondence principle, so there is no danger of over-specifying the system by using parallel calculations. David R. Ingham 16:12, 28 August 2006 (UTC)

Fission: a second example

Back when I was last doing nuclear physics, the usual way to model fission was with a combination of the liquid drop model and the shell model. (Nuclear [model]s are needed for all but the lightest nuclei, because the number of variables in a straight forward calculation depends exponentially on the number of nuclei, in this case around 200, not to mention mesons.) A pure liquid drop model describes nuclear matter as a homogeneous classical liquid, with electric charge but no detailed structure. This is a complete classical description, similar to those that are used to describe instruments. To this is added small quantum corrections such as the result that, like atoms, nuclei with closed shells have lower energies than those with partly filled shells. Since this involves the far sides of the nuclei where the surfaces are not in contact, it appears to be non-local from the classical view. It differs from the "many world" understanding of EPR in that the shell model resembles an independent particle model and so it is not mainly the entanglement that is involved but the one-body nucleon wavefunctions. David R. Ingham 18:22, 9 September 2006 (UTC)

Is another example needed?

Roger Penrose gives several examples in different parts of The Road to Reality. It appears to me, at this point, that the example given here has an obvious "hidden variables" explanation, while one or more of Penrose's examples do not. There is at least one that, interpreted according to the Copenhagen wave function collapse rules for expressing experimental results purely classically, seems to clearly show non-locality. (Of course non-locality is clearly not in the mathematical formalism of QM.) In these cases, the probabilities for the same measurement done by Bob at point B are changed by what Alice measures at point A. I see no obvious "hidden variables" explanation. Penrose's description is something like to say that the fact that the particles are entangled and part of the same quantum state makes an action on one felt by the other, even though the separation is space-like. To me, this, like the Copenhagen rules, may be useful in certain common classes of cases but cannot be fundamental. These examples, to me, and I think Penrose feels to most field theorists, require dropping the assumption that the results of an experiment can always be described in purely classical terms.

On the other hand, these examples are more complicated than the one given and might be too technical and too detailed to be usefully included here. There is some discussion already about the hidden variables approach not always giving correct results and about whether or not it can be patched up to give correct results.

Another point is which best illustrated what E, P and R originally published. I don't know the answer to that question but have the impression from Penrose that it may have been closer to his examples than to that given here. David R. Ingham 16:12, 31 August 2006 (UTC)

And the relevance to the article is? --Michael C. Price ^talk 17:48, 31 August 2006 (UTC)

It is that, as it stands, the article encourages the hidden variable theory, while another example would not. Maybe if original paper by EP&R is explainable by hidden variables, then you are right and the other example should go in the hidden variables article, but if not then this page seems incomplete. David R. Ingham 18:40, 7 September 2006 (UTC) Looking briefly at the original paper, it does seem similar to the example given here already. The example that is not explainable by hidden variables is attributed to Lucien Hardy and the pair of particles are in a spin one state. David R. Ingham 19:13, 7 September 2006 (UTC)

This article has turned into a mess! A reasonable person would be unlikely to read the original paper and connect it to most of the material presented here. We should present the historical context and the actual arguments of the paper. The stuff on Bell's Theorem should not be included directly, except as to the extent it updates the conclusions of the paper. The purpose of this article should not be to provide the entire history of the subject of entanglement, as this is covered better under other WP articles. It is my intention to begin a major update of this article with the purpose of making this informative to the lay reader.-DrChinese 18:25, 20 September 2006 (UTC)

Extraordinarily badly written article - makes several statements, starts substantiating them, and suddenly deviates. A major major edit is required.Ntveem 11:07, 27 December 2006 (UTC)

For instance - the article says that a theory is complete if every Element of Physical Reality is accounted for. What does it mean by the convenient "accounted for"? Later, the article says "In QM, x and z spin cannot have definite values at the same time. If QM is complete according to the def of complete theory above, then x and z spin cannot be EPR's at the same time." Does that mean that QM (a theory) decides whether a thing is an EPR or not? Has this article been written for a technical audience or an audience of laymen? A bit of clarity of thought would be nice.Ntveem 11:27, 27 December 2006 (UTC)

"EPRB" in intro removed

I just removed "It is sometimes referred to as the EPRB paradox for David Bohm, who converted the original thought experiment into something closer to being experimentally testable." I hadn't heard this, and googling "eprb paradox" yields 324 hits, so i removed it. It would need a citation if re-inserted. Tempshill 03:02, 13 June 2007 (UTC)

Surely the B would be J.S. Bell? 1Z 11:33, 13 June 2007 (UTC)

What is measurement?

See my user page. --Pateblen 12:55, 1 August 2007 (UTC)

Trying to make it a little clearer

Hi all,

I would be grateful if anyone could tell me whether my recent changes to the 'Explanation of the paradox' section are an improvement, or whether they contain false statements. I've just tried to make things a little clearer, but I thought I'd better check it with some experienced people first. Mark J (talk) 20:19, 27 January 2009 (UTC)

Typo in the fifth paragraph

Hi.

It seems to me that there is a typo in the “EPR Paradox” article, in the first sentence of the fifth paragraph (which begins with “The EPR paradox is a paradox”).

The typo is an extra parenthesis after the word “locality”, which confuses the meaning of the sentence.

I propose an edit that removes this erroneous parenthesis:

CHANGE:

(referred to as locality), realism

TO:

(referred to as locality, realism

Proof that a change is needed:

There are seven parentheses in that sentence. Parentheses must occur in pairs. There should be an even number of them. The extra parenthesis is a left parenthesis. The logic of the sentence dictates that the parenthesis I mentioned above is the superfluous one.

Ronjoseph (talk) 22:21, 16 February 2009 (UTC)

Typo in the fifth paragraph reconsidered

I recounted the parentheses in the sentence described above and found that there are six, not seven parentheses. I now believe the typo is not an extra parenthesis, but rather a misplaced parenthesis. I believe the parenthesis after the word “locality” should be placed after the word “realism”.

Proposed edit:

CHANGE: (referred to as locality), realism (not to be confused with philosophical realism,

TO: (referred to as locality, realism (not to be confused with philosophical realism),

Ronjoseph (talk) 11:55, 20 February 2009 (UTC)

Typo in the fifth paragraph corrected

I corrected the misplaced parenthesis after the word “locality" which was described in the previous two comments.

I CHANGED: (referred to as locality), realism (not to be confused with philosophical realism,

TO: (referred to as locality, realism (not to be confused with philosophical realism),

Ronjoseph (talk) 21:34, 15 March 2009 (UTC)

Hello,

I took out the reference to Bohms simple explanation because the explanation is apparently left out - the next sentence is about Greene. —Preceding unsigned comment added by Jinxman1 (talk • contribs) 04:20, 6 April 2009 (UTC)

The key to the EPR paradox is this:

The x spin and y spin of a given particle are so called non-commuting variables (this is a known fact). Like position and momentum, all non-commuting variables are not simultaneously measurable, according to quantum mechanics. The x spin of one particle of the separated pair has to be the opposite of the x spin of the other particle (this is also a known fact). In an EPR experiment, Alice can measure her particle’s x spin as plus, thus determining Bob’s particle’s x spin as minus, since they must be opposite. Bob can, at the same time measure his particle’s y spin as plus, thus determining Alice’s y spin as minus. Thus both x spin and y spin of both particles have been simultaneously measured, refuting the QM claim regarding non-commuting variables. Pateblen 05:07, 1 August 2007 (UTC)

Ahah! This site explains it more clearly: http://plato.stanford.edu/entries/qt-entangle/. "Either correlation can be observed, but the subsequent measurement of momentum, say, after establishing a position correlation, will no longer yield any correlation in the momenta of the two particles. It is as if the position measurement disturbs the correlation between the momentum values."

I suggest changing the article to read:

Suppose Alice measures the z-spin and obtains +z, so that the quantum state collapses into state I. Alice then measures the x-spin and finds -x. Bob then measures the x-spin. According to quantum mechanics, when the system is in state I, Bob's x-spin measurement will have a 50% probability of producing +x and a 50% probability of -x. Because Alice measured the z-spin first, the x-spin entanglement was lost.

...or something along those lines. --Theropod-X 20:30, 24 June 2006 (UTC)

The takeaway from the above confusion should be that the article itself is confusing. It's certainly not the worst offender on Wikipedia, but it could definitely stand some editing with an eye towards grammar and style. 64.109.251.85 23:52, 27 March 2007 (UTC)

I, for one, still don't get it. I think the explanation hinges on the following:

So how does Bob's electron know, at the same time, which way to point if Alice decides (based on information unavailable to Bob) to measure x and also how to point if Alice measures z?

I don't follow the point being made here. Can we rephrase this without the rhetorical question? --Doradus 15:43, 12 June 2007 (UTC)

I agree, that explanation is confusing, I don't get the rhetorical question either (what does the electron need to know?..).

Here is my take on the paradox: QM says that one cannot have a well-defined spin simultaneously along both the x and z axes (thecorresponding operators do not commute). Suppose that an electron is known to be in a state with spin up along the z axis. Then QM predicts that a subsequent measurement of its spin along the x axis will have a 50 percent probability of being spin up and a 50 percent probability of being spin down along that axis. This is what it means to say that the spin cannot be simultaneously well-defined along both axes.

Now suppose that Alice and Bob perform measurements on their respective electrons along an x or z axis of their own choosing. For example, Bob might flip a coin to determine which axis he will measure along, whereas Alice might roll some dice to determine her axis. They repeat the experiment a number of times, each time randomly choosing their own axes and recording the results. Finally they get together and compare notes. QM predicts that when Alice measures along the x axis and Bob along the z, there will be a 50 percent probability that Alice measures spin up and 50 percent of the time Bob will measure spin down. Furthermore QM predicts that on those occasions when both measured along the same axis, not one time will both measure spin up. Rather if Alice measures spin-up along the z axis, then Bob will measure spin-down along that axis. Somehow Bob's electron is forced into a spin-down state along the z axis whenever Alice measures hers to be spin-up along the z axis. And whenever Aliced measures spin-down along the x axis, Bob's electron will instantly go into a well-defined spin up state along the x axis. How is this possible without some sort of spooky action at a distance, given that QM says that an electron cannot have a well-defined spin simulateneously along both the x and z axes? This is the paradox. EPR used this paradox to argue that either QM violates causality or is incomplete. Ty8inf (talk) 06:18, 24 March 2008 (UTC)

Sorry, still not with you. If the particles are created in pairs such that for the pair total x-spin=0 and total y-spin=0 then it follows naturally that x1=-x2; ie, that when Alice measures +x spin Bob must measure -x spin. Meanwhile, y-spin is doing the same thing independently of x-spin (that is, -x can be found with +y or -y). Given that, it seems perfectly normal that Alice measuring a result of +x leaves Bob with no reason to suppose +y or -y and that he'll get oth results in a ration of 50:50. There appears to be no action at a distance here, just a conservation of spin law operating at the point where the particles initially constitute a single system before flying off around the universe. In essense the particles are acting just as a pair of normal billiard balls would, assuming that they were launched under the condition that the pair's total angular momentum along each axis was always zero. And at no point was Alice able to directly measure x and y spins; she can only deduce them from Bob's results, which apparently match up just fine. Where's the paradox? 213.78.235.176 (talk) 16:45, 20 November 2009 (UTC)

I believe that the source of confusion is that you have assumed that the spins along 2 axes can be known simultaneously (read your second sentence). It is true that angular momentum is conserved. So if Bob measures the spin of his electron along the z-axis to be up, then Alice would find the the spin along the z-axis of her electron to be down. I believe this much everyone can agree on. The paradox is this: If Bob measures the spin to be up along the z axis, and Alice measures the the spin to up along the x axis, does this mean that Bob's electron also has spin down along the x axis (and by the same reasoning Alice's electron has spin down along the z axis)? If so, this would imply that you can know the spin of an electron simultaneously along both the x and z axes. But the Copenhagen interpretation of QM does not permit this knowledge. Hence, Alice's measurement of spin up along the x axis has magically affected Bob's electron.

Here is another way to look at it: Suppose that the electrons are created (using a Stern-Gerlach apparatus) with Bob's spin-up along the z axis, and Alice's with spin-down along the z. So as the electrons are traveling towards Bob and Alice we know that they have well-defined spin states along the z axis, and only along that axis. But if Bob makes a measurement along the x axis and determines it to be spin up, Alice's electron will instantly find itself in a spin down state along the x axis. This is the so-called collapse of the wave function and is a necessary consequence of the Copenhagen interpretation. Ty8inf (talk) 03:41, 3 December 2009 (UTC)

There is no paradox if you do not ask "why questions". Quantum mechanics yields perfect descriptions of many features of microscopic physics, but it does not yield explanations. It in general does not explain why a certain measurement result is found in a particular individual measurement (in the singlet state it might be spin-up or spin-down). It actually is the Kochen-Specker theorem that makes it impossible to assume that each quantum mechanical measurement result was possessed by the microscopic object as an objective property (Einstein's element of physical reality). Hence, it is not possible to attribute to the initial state of the object well-defined values of spins in different directions, thus preventing explanation of the strict correlations between measurement results in the singlet state. Such an explanation is often thought to be provided by von Neumann's projection postulate (only to replace the problem of explanation by a problem of nonlocality). Another solution -in my view a better one- is to replace Einstein's elements of physical reality (conceived as quantum mechanical quantities) by subquantum quantities, not described by quantum mechanics but by some subquantum theory, comparable with but different from Bohm's theory.WMdeMuynck (talk) 18:07, 22 November 2009 (UTC)

There seems to be some considerable confusion between Bell's inequality and EPR paradox. Bell's inequality is linked to EPR but not as directly as some here seem to think. The EPR paradox is simply about two particles momentum and position, not spin (which is about Bell's inequality the violation of which indirectly proves the the seeming paradox of the following to be true). In EPR it is postulated that, we are "allowed" to measure precisely the combined momentum of the two particles and the distance between them. The point is then that later we can choose to measure the momentum of one particle thus telling us the momentum of the other or measure the position of one and know the position of the other. Einstein Podolsky and Rosen then argued that its one thing to say that physical measurement of the first particle's momentum affects uncertainty in its own position, but that to say that measuring the first particle's momentum affects the uncertainty in the position of the other is another thing all together. They asked how can the second particle "know" to have precisely defined momentum and uncertain position? As this implies that one particle is communicating with the other instantaneously across space and this is the "paradoxical" nature of the experiment. N.B. The idea works the other way round as well measuring position affecting momentum. —Preceding unsigned comment added by Shroedinger101 (talk • contribs) 19:00, 8 November 2010 (UTC)

Misunderstanding of Einstein, Podolsky, & Rosen on "spooky action at a distance"?

The third paragraph of the opening section of the article currently contains the following sentence: "The 'spooky action at a distance' that so disturbed the authors of EPR consistently occurs in numerous and widely replicated experiments, though the validity of these experiments does remain in debate." This might reflect a very widespread misunderstanding of the structure of the argument of the original EPR paper.

Einstein, Podolsky & Rosen did not dispute the prediction that entangled particles would show such correlation, no matter how far apart they might become. On the contrary, the truth of this prediction is a key premise in their argument. What they argue is that the reality of such correlations, when combined with the assumption of local causation and the assumption that QM is complete, yield a contradiction. The thrust of the argument is that BECAUSE OF the reality of such correlations (which on their view are simply the result of entanglement, and not of non-local causation or "spooky action") one must reject either local causation or the completeness of QM. They opt for the latter option, rejecting the completeness of QM. They purport to show that proponents of the completeness of QM must reject local causation and embrace "spooky action at a distance," which they consider absurd. But the reality of correlation between the entangled particles is not in question in the paper. At issue is whether non-local causation needs to be invoked at all to explain the correlations.

It is worth asking, though, how Einstein, Podolsky & Rosen would have reacted to Bell's Theorem, as well as to the actual experimental results obtained in Bell test experiments. Those results show a curious "anti-correlation correlation" rather than the more straightforward correlations originally considered by Einstein, Podolsky & Rosen. (Do I have this right?) Bell claimed to have derived an empirically testable difference between the sorts interpretations of EPR-type scenarios favored by Einstein, Podolsky & Rosen and friends (i.e., local realist interpretations according to which EPR-type experiments force an admission of QM's incompleteness) on the one hand, and interpretations according to which such experiments force an admission non-locality (with or without an admission of the incompleteness of QM) on the other. What would Einstein, Podolsky & Rosen themselves have to say about the Bell Inequality and its apparent violation? —Preceding unsigned comment added by 71.117.230.43 (talk):71.117.230.43|71.117.230.43]] 07:46, 19 October 2007‎ (UTC)

God doesn't play dice?

Source: Mark Leyner, Einstein calling; A genius brought down to earth, The New Republic, 21 Nov 1994, 211(21); referenced at www.SkewsMe.com/albert_einstein.html

Freedom of Information Act file number: 2951B 992

location: Phone booth on the corner of Nassau Street and Murray Place in Princeton, New Jersey.

time: November 3, 1935, 10:15 a.m.

participants: Albert Einstein, mobster Meyer Lansky

Lansky: So what you’re trying to say is that the universe is too complex to make a book on?

Einstein: No, that’s not what I’m saying at all. Look – the universe is like a craps game, O.K., except instead of a Euclidian three dimensional table, you got a four dimensional space time curvature.

Lansky: I thought you said: “Gott wurfelt nicht,” – God doesn’t play dice.

Einstein: No, Meyer. I said: “God plays dice, it’s just that the dice are loaded.” You know what I’m saying? The fix is in. That’s the Unified Field Theory in a nutshell, my friend.

Lansky: So this things . . . (inaudible) This thing of ours . . . it’s gonna last forever?

Einstein: Oh yes, Meyer. Gott crap out nicht.

—Preceding unsigned comment added by 97.113.163.184 (talk) 00:39, 10 May 2009 (UTC)

I'm not really sure what you're getting at here, but there was something I wanted to say about this quote, so this is as good a place as any. While Einstein is commonly quoted as saying "God does not play dice" there actually isn't any evidence to support that he said so in exactly those words. While he did say something amounting to that if the article is going to try to quote him it should be exact. He once wrote this in a letter of 1926: "Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the ‘old one’. I, at any rate, am convinced that He does not throw dice." 1

He has also wrote in another letter in 1942: "It seems hard to sneak a look at God’s cards. But that He plays dice and uses ‘telepathic’ methods...is something that I cannot believe for a single moment." --Dekker451 (talk) 13:14, 5 July 2010 (UTC)

Einstein also said something like: Don't look at what they say, but at what they do. If this maxim is applied to Einstein himself you see that what he did was precisely what he said: he assumed that measurement results of quantum mechanical measurements exist as so-called elements of physical reality prior to measurement, thus assuming determinism. In 1935 this assumption was not unreasonable. Whereas at that time it could neither be proven nor disproven, its impossibility was established much later by the Kochen-Specker theorem. Note, however, that this eliminates just one particular form of elements of physical reality. There are other possibilities.WMdeMuynck (talk) 15:04, 5 July 2010 (UTC)

Proposal for Layman’s Description

WHY THIS PROPOSAL

Some confusion exists among us non-physicists as to what exactly is being described here. I would like to clear this up by painting a graphic analogy in macroscopic terms and then explain where the analogy works and where it doesn’t. I hope the explanations themselves will further illustrate what is happening at the quantum level. I leave it to the real physicists editing this article to explain why it’s happening. My only interest is to explain what is happening.

Note that 2 different phenomena are being described here. Thus far in the article, they’re being mixed together in a confusing way, at least to us non-physicists. I therefore separate them and deal with each in turn. Phenomenon 1 is where Alice and Bob both measure the x-axis of their particles. Phenomenon 2 is where Alice measures the x axis of her electron and Bob measures the z-axis of his positron. In Phenomenon 2, what happens next follows is really unclear from the article, but below I try to glean what is being described and then translate it into laymen’s terms. If it’s incorrect, please educate all of us and correct the analogy as needed.

Here are the models:

PHENOMENA ONE

Suppose I take two pictures of a coin, one of the heads side and one of the tails side. I put them in separate envelopes and give one to Alice and one to Bob. Neither Alice nor Bob know which picture they have. I ask Alice to predict which picture Bob has. Her odds of guessing correctly are 50/50. Now I ask Alice to open her envelope. She sees she has heads. Her odds of predicting what’s in Bob’s envelope suddenly jump to 100%, i.e. she knows he has tails. Further, she can tell this to Bob so that Bob will know without ever having to open his envelope.

That doesn’t appear too amazing, because the outcome was predetermined before the picture taking ever took place—by my assigning the photos to Alice or Bob. The analogy works like this: Suppose I ask Alice to measure the electron along the x-axis only. If Alice gets the heads picture it’s analogous to Alice measuring up spin; if she gets the tails picture it’s like she measures a down spin. In either case, after Alice’s measurement, Bob’s measurement along the X-axis can now be predicted with 100% certainty.

Now in some interpretations of quantum mechanics, the analogy has nothing wrong with it—the electron and positron really do have spins that were already determined when they split apart from each other. However in the other set of interpretations, the analogy falls apart because there is no me to take such a picture, there is no camera, and there is no coin that is already heads or tails. This lack of measurement is crucial to this interpretation because it would say that the picture in Bob’s envelope is neither heads nor tails, but “indeterminate.” “Indeterminate” is considered an actual state of being. This state of being changes to “determined” as soon as anyone or anything finds out what’s in Bob’s envelope. So, in this interpretation of quantum physics, when Alice opens her envelope, Bob’s photo changes from “indeterminate” to “determinate.” That’s “spooky action at a distance.”

Another way this analogy is flawed is that under this interpretation you can’t use a coin because it is larger than a submicroscopic particle and thus quantum mechanical effects are too negligible to measure. But the point was to aid understanding among us low-brows by painting a picture in broad terms and then stating where the picture does and doesn’t work as an analogy.

PHENOMENA TWO

Suppose instead of 2 pictures of one coin, I take a picture of the heads and tails of 3 different coins-a penny a dime and a quarter. I then divide them up between Alice and Bob. For simplicity suppose I give Alice all the heads and Bob all the tails. The envelopes are labeled with which coin is pictured inside, but not whether it’s a heads or tails picture. I ask Alice to open her penny envelope and it comes up heads. Of course she and Bob can now predict that Bob’s penny envelope will come up tails without opening his envelope. If he does open the envelope, no surprise, it comes up tails.

Suppose Alice opens her penny envelope as before and it shows heads. Now instead of asking Bob to open his penny envelope, I ask Bob to open his dime envelope. It shows tails as you would expect.

Here’s where things get really surprising to most people: I next ask Alice to open her dime envelope. What’s inside? Not being a physicist, I have a 50/50 chance of being quite surprised. So surprised that I run the whole experiment again 100 times. It turns out that somehow, about half the time Alice sees a picture of the heads side of the dime AND THE OTHER HALF THE TIME SHE SEES THE TAILS PICTURE.

Why? Because she opened the penny picture first. Now obviously that doesn’t really happen in the world of real coins and real pictures of coins. That’s where the analogy predicts nonsense, because the analogy doesn’t work anymore. But the purpose of the analogy is just to create a picture description of what we’re dealing with at the quantum level. Because what appears to be nonsense at the macroscopic level actually does happen at the submicroscopic level where we enter the world of quantum mechanics.

Here’s how the analogy works: Suppose all the pictures of the coins represent the up or down spin states of each axis. The penny represents the x-axis, the dime the y-, and the quarter the z-axis. Heads represents an up state and tails the down state. Remember that Alice only measures electrons and Bob only positrons. So Alice getting a penny-heads picture means she’s measured her electron along the x-axis and got the result “up”. Now, let’s have the dime represent the y-axis, and the quarter represent the z-axis. In both the coin experiment and the electron/positron experiment, if Alice just measures the x-axis, then Alice’s measurement determines Bob’s subsequent measurement 100% of the time. In fact, if you just measure one axis, it doesn’t matter who does the measurement first or which axis it is-the first person to measure any axis determines the other person’s outcome for that axis, exactly as if we were dealing with photos of coins.

Now here’s where the analogy fails: if you try to measure two axes, the second measurement DOES NOT predict the outcome of the other person’s second measurement. In other words, as soon as Alice does her first measurement along the x-axis, the correlations between the two particles along the y- and z-axes cease to exist. After Alice measures along the x-axis and can predict what Bob will measure along the x-axis, if Alice or Bob measures either particle along the y- or z- axis, it will no longer predict what the other person will measure along y- or z- axis of the other particle. That is what is meant by the term “quantum-disentaglement.” It works at the quantum level, but when you describe it in the analogy, you get both Bob and Alice with pictures of tails of the dime, which suggests the analogy doesn’t really match the reality at the quantum level.

Don’t worry if this whole thing seems unbelievable, you’re in good company. You’re scratching your head and asking “where did the dime-head picture go, and how did we wind up with two copies of the dime-tails picture.” Well apparently, Einstein also did a lot of head scratching and had some analogous questions like and problems with the quantum mechanical descriptions.

---

(Note to physicists: for the Phenomena 2 analogy, I’m making some assumptions about what you physicists are trying to describe to us low-brows by “disentanglement.” If it’s wrong, please, please try to correct it using a macroscopic analogy like the coin/photo analogy. Also, please take care to paint a picture of what’s happening before explaining why it’s happening).

75.168.250.230 (talk) —Preceding undated comment added 19:17, 5 October 2009 (UTC).

Where is the paradox?

I have some trouble seeing the classical view from which there is a paradox. I had to look at Principle of locality to find: "Local realism is the combination of the principle of locality with the 'realistic' assumption that all objects must objectively have pre-existing values for any possible measurement before these measurements are made." Is that a good definition? Why didn't I see the definition here? Of course qm violates that, the measurement results are classical! An electron doesn't really have any classical properties.

From another point of view, if one believes that wavefunction collapse is a physical process, then there seems to be action at a distance, because Bob's positron collapses when Alice measures her electron. But that is not a reasonable assumption either. Schrodinger already said in his cat article that it is a mental process.

You open Pandora's box by asking these questions. Indeed, local realism as defined above comes close to Einstein's interpretation of quantum mechanics: observables as objective properties of the microscopic object, the wave function as an objective description of a statistical ensemble. However, the term "local realism" is used in quite a few different ways, each having its proponents.

As to wave function collaps, Einstein did not consider it to be a physical process but rather to describe a selection of a subensemble of Bob's particles triggered by readings of measurement results obtained by Alice. Once again we have different forms of "realism" here, some of them even nonlocal.WMdeMuynck (talk) 21:57, 2 December 2009 (UTC)

I now see qualitatively what local realism is and why we don't want to lose it. It is here the question of how does an electron who only has a little two by two Pauli matrix know its spins in x, y and z directions.

This, indeed, is the key question. If you do not like the (nonlocal) von Neumann collapse solution there seems to be only one alternative, viz. assume the existence of elements of physical reality. Since the quantum mechanical ones, employed by Einstein, do not work (because of the Kochen-Specker theorem) it seems necessary to rely on subquantum elements of physical reality. Thus, the electron knows its spin because it is in possession of a subquantum element of physical reality determining that spin, if measured (compare: the atomic constitution of a billiard ball determines the rigidity of the ball observed at the macroscopic level of observation). Problem: this solution opens Pandora's box, releasing a host of criticisms by believers in all kinds of alternative solutions.WMdeMuynck (talk) 11:34, 4 December 2009 (UTC)

Schroedinger's way out was or would be that if the experimenters counted spins by killing cats, instead of writing in a notebook, they would end up with exactly one dead cat, because neither cat had a unique classical description until they were brought together and counted. So I want to add "Large objects always have unique classical discriptions" to the list of assumptions that make it a paradox, but that is too long to fit in the sentence. Does anyone know a short name for that? David R. Ingham (talk) 02:57, 4 December 2009 (UTC)

Physical non-neutrality

I don't think this is a neutral article. It mainly covers obly the Copenhagen Interpretation and is direct on "debunking" the EPR paradox in favor of it's postulates. It does not include (or even mention?) other interpretations such as the De Broglie–Bohm theory which postulates a nonlocal hidden variable, or the Consistent Histories approach, that says that classical logic can be mantained in most aspects of quantum physics. Furthermore, this article contradicts the Ensemble Interpretation article, which states that Einstein's view "is identical in all of its predictions to the standard interpretations" (here it is stated otherwise). Please edit the article to be more neutral and include all points of view and theories and not just the most mainstream one. --190.174.64.243 (talk) 13:30, 3 March 2010 (UTC)

There are no appointed editors for a wikipedia page. Anyone can edit this page. If you have a problem with the article I suggest you make relevant changes and motivate them here. —Preceding unsigned comment added by 130.242.128.41 (talk) 15:27, 17 May 2010 (UTC)

Quantum Mysteries for Anyone

What about the simplified versions of paradox, e.g. produced by Mermin based on Bell inequalities. Should they be mentioned in the encyclopedia? --Javalenok (talk) 17:00, 8 July 2010 (UTC)

Mermin's article asserts that "we live in a world in which such a device can be built". The reality is that many experimenters have tried to build a device of this kind, but none have succeeded in building one that simultaneously satisfies all the hypotheses that Mermin demands. Thus Mermin's article is a statement of hope rather than of fact. Since the denial of local reality is such a radical one, it calls for radically reliable proof. Mermin's argument is not radically reliable enough to do the job, and should not be trumpeted as if it did. The idea that it is safe to "simplify" the EPR paradox is enthusiastic but does not recognise the difficulties in the way. Many people, Mermin included, are enthusiastic to see the overthrow of local reality, but their enthusiasm is only a point of view, not a certainty. It would be lovely to think one was so much cleverer than silly old Einstein, but I for one do not think I am so. The best brains have better things to do with their time than to tangle with this difficult and possibly futile argumentation. Putting a reference to Mermin's article into the Wikipedia article here would be further pushing a point of view that is already pushed repeatedly and very hard by the article as it stands.Chjoaygame (talk) 23:57, 9 July 2010 (UTC)

It is not bad if the simplification does demonstrate EPR. We are in EPR entry after all. Do you want to say that the Wikipedia is created for the "brains better than" the modern quantum computation leading experts rather than for the general population? --Javalenok (talk) 12:32, 19 July 2010 (UTC)

The problem here is that belief in the validity of the Mermin interpretation of the EPR and Bell papers is very much a point of view. It is not appropriate to put in a simplification that does not make it clear to the naive reader, whom you are trying to protect, that it is very much a point of view. You might as well just say: "Bell is right to say that QM proves violations of causality: don't argue." That is the ultimate simplification. The simplification in Mermin's article is logically like that, but it is cleverly disguised by Mermin to look as if it is scientifically impeccable. The simplification in Mermin's article inevitably will not convey to the reader who would like a simplification, whom you are trying to protect, that the simplification is very much a point of view; that is part of the meaning of the words 'simplification' and 'naive reader'. Putting in a citation of Mermin's article would not protect, but would mislead, the reader whom you are trying to protect.Chjoaygame (talk) 23:13, 19 July 2010 (UTC)

1. Have you abandoned your point that Wikipedia is not intended for general population? 2. Denying a simplified explanation based on that "Einstein was clever" is funny. Einstein was not that clever because all he proposed was the informal idea. Those who succeeded in devising the formalization and experimentally proving him wrong were "more clever" in this sense. Like Godel was more clever than Gilbert. 3. When you say "I'm trying to protect", you suggest that I'm biased. I'm not. Neither Mermin is. I see him stating what has been proposed by EPR and proven in Bell inequalities in other words. This is what 'simplification' means. Naiive reader is saved from understanding spins, entangled states, wave functions and other quantum mechanical stuff where he is lost (like me) and forgets what is the problem and what can QM do that classical machine cannot. Also, the article says "According to present view of the situation, quantum mechanics simply contradicts Einstein's philosophical postulate that any acceptable physical theory should fulfill 'local realism'". Why must Mermin build his machine to illustrate the accepted concept? 4. Want to attack the Bohr's followers? - Attack the Bell inequalities and Alain Aspect experiments. All the consequences, e.g. Mermin's interpretation, will fade. No problem. Right? --Javalenok (talk) 10:11, 20 July 2010 (UTC)

Your comments seem to show that you are entirely convinced that Mermin's point of view is the only valid one, and you are so sure of it that you say your position is not biased. These matters all arise from counts from particle detectors, which pertain to spins, entangled states, wave functions and other quantum mechanical stuff, which you tell me lose the general population. These matters do not pertain to macroscopic processes, which are not understandable directly in terms of particle detector counts, but which the general population nevertheless understand. It is misleading to treat these matters as if they pertain to macroscopic processes. The Mermin article makes out that these matters pertain to macroscopic processes and is thus misleading to members of the general population. It is part of the function of the Wikipedia to speak to the general population; but not to mislead them.Chjoaygame (talk) 14:12, 20 July 2010 (UTC)

I have said nothing that implying the "Mermin's point of view is the only valid one". Blaming this on me is especially crazy after his article is recognized as "a popular exposition of Bell's theorem" by recognized scientists and published in peer-reviewed journal. According to Wiki standards, peer-reviewed source has the top unreliability, which is a clear sign of not NPOV. Right? If a physicist and a leading quantum computation teacher can understand how his picture simplifies the EPR paradox, if publishers and readers including me can understand it and only you who does not, I think that something is abnormal with you. --Javalenok (talk) 17:35, 20 July 2010 (UTC)

This is an article about the EPR paradox. Bell's work and its consequences are closely related but are not the exact titled target of the article. Reading the original EPR paper, one sees that Einstein, Podolsky, and Rosen presented what they judged to be a fairly formal theoretical argument. Bohm changed the basis of discussion by moving from EPR's example of a free particle with a continuous spectrum to his own example of the spin of a particle which has a discrete spectrum. This is already a formal theoretical move away from the original EPR argument. The logical significance of this move is hardly recognized in the professional literature, and would be utterly lost on a member of the general population. Following Bohm's move, though still referring to EPR, Bell's 1964 argument is hardly formal, as one may see by reading it carefully.

You write "Those who succeeded in devising the formalization and experimentally proving him [Einstein] wrong" and "you suggest that I'm biased. I'm not. Neither Mermin is. I see him stating what has been proposed by EPR and proven in Bell inequalities in other words." This seems to be a point of view that Mermin's point of view, expressed in his "simplification", is the only valid one.

As I understand Wikipedia protocol, secondary sources are considered more reliable, and primary sources such as Mermin's article are not considered to be of such high reliability. WP:RS. "Articles should rely on secondary sources whenever possible. When relying on primary sources, extreme caution is advised: Wikipedians should never interpret the content of primary sources for themselves." In reality, in this area, even secondary, and indeed even tertiary, sources are often very tendentious. This is a very difficult area. It would be misleading to give an impression that it is not, which would be the effect of citing Mermin's "simplification".

Are you getting personal when you write "If a physicist and a leading quantum computation teacher can understand how his picture simplifies the EPR paradox, if publishers and readers including me can understand it and only you who does not, I think that something is abnormal with you."?Chjoaygame (talk) 23:03, 20 July 2010 (UTC)

Wiki explains that primary sources are undesirable when a) authors are the only witnesses and b) the article is based only on PS. Yet, a) the Einstein's and Mermin's experiments are thought experiments, which can be logically checked and thus allowed in Wiki and b) the reference is illustrative. I have encountered the Mermin's exposition made by Penrose reviewing the EPR for the general population. This is definitely a second source. Telling that C regards B that refers A is stupid when you want to give a simple illustration. --Javalenok (talk) 14:39, 22 July 2010 (UTC)

Instead of going for personal comments and Wiki lawyering, it is better to go for careful consideration of the reasons that relate to the question of whether the Mermin article is suitable for the Wikipedia page. Till now you have mostly avoided doing the latter. I suggest you read over and re-consider the above comments. The Mermin article is not suitable for the Wikipedia page.Chjoaygame (talk) 03:53, 24 July 2010 (UTC)

I don´t get it

I really don´t see the problem. If either particle has two characteristics (call them spins or whatever) that are always opposites of the other particle´s, it makes complete sense to me that if one characteristic is measured for one particle, the probability of any value will be 50%. If then the other characteristic is measured on the other particle, the probability for any value is of course also 50%. Any other result would scare me. Obviously there is something fundamental about quantum mechanics I don´t grasp because of either stupidity or ignorance. However, this article does not mention why I should be scared. —Preceding unsigned comment added by 90.230.90.243 (talk) 20:58, 2 August 2010 (UTC)

See the dispute above where I propose to include a simpler explanation devised by a prominent scientist exactly to reveal this problem to general population. --Javalenok (talk) 08:16, 3 August 2010 (UTC)

Idk, but i think quantum mechanics states that a particle has no spin until it is measured... 173.183.69.134 (talk) 01:28, 3 November 2010 (UTC)

Reason for undo of two edits

The edits that I have undone are editorializing of own research. They are quite unsuitable as introductions to the article. They are expressions of what the editor would like to believe, but are not accurate statements about the EPR paper. The version that I have reverted to was carefully written to show what Einstein Podolsky and Rosen wrote. What people make of that is a matter of opinion, and such opinion is not appropriate for an introduction. True, the edits that I have undone express deep feelings and hopes by many people, but this does not make them suitable introductory material.Chjoaygame (talk) 19:30, 22 August 2010 (UTC)

When you wrote, "are not accurate statements about the EPR paper", could you explain what you mean in more detail? Thanks. --Bob K31416 (talk) 20:08, 22 August 2010 (UTC)

For reference, here's the summary that was reverted.

"The EPR paradox (or Einstein–Podolsky–Rosen paradox) refers to a thought experiment in which a measurement of a physical quantity in one system affects the measurement of a physical quantity in another system that is isolated from it."

--Bob K31416 (talk) 20:16, 22 August 2010 (UTC)

Bob, your proposed edit clearly supposes that somehow the Einstein Podolsky Rosen 1935 paper asserts or supposes that "a measurement of a physical quantity in one system affects the measurement of a physical quantity that is isolated from it". On the contrary, the paper takes it as entirely reliable that such an imagined effect is not physically possible. The paper states: "This makes the reality of P and Q depend upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this." Yet your edit clearly supposed that this is already a reality. It is the opinion or belief of Bell theorists, and it seems apparently also of yours, that experiments have supported or verified this supposition, but that is an opinion, derived by Bell theory, and is not suitable as an introductory assertion in an article about the EPR paper. There are significant physical differences between the Einstein Podolsky Rosen thought experiment and the Bohm-Bell thought experiment, that are not apparenty recognized by Bell theorists. Bob, to explain in more detail I would need to rely on your having read the Einstein, Podolsky, Rosen 1935 paper, and the Bell 1964 paper. At present I do not see that I can rely on that. What you wrote suggests, though of course does not prove, that you have not read the EPR paper. If you want more explanation, please let me know about your reading of the papers.Chjoaygame (talk) 23:26, 22 August 2010 (UTC)

The edit that I quoted in my previous message was a summary of the thought experiment in the original EPR article. The summary did not make any assertions or suppositions. It states the heart of the paradox as derived in the article by EPR, i.e that a measurement of one system can affect another system that isn't interacting with it. The authors then use the thought experiment to show that the wave function obtained by the theory of quantum mechanics is not a complete description of reality. --Bob K31416 (talk) 02:40, 23 August 2010 (UTC)

Bob, the original Einstein Podolsky Rosen 1935 article states: "no real change can take place in the second system in consequence of anything that may be done to the first system". Your summary means, in your words: "It states the heart of the paradox as derived in the article by EPR, i.e that a measurement of one system can affect another system that isn't interacting with it." The authors use the fact that no real change takes place to prove their point. Thus your summary contradicts the original paper. Can you find in the original paper, and tell us about, something that you read as an admission that "a measurement of a physical quantity in one system affects the measurement of a physical quantity that is isolated from it"? Chjoaygame (talk) 06:04, 23 August 2010 (UTC)

I have to agree with Chjoaygame. The paper's basis was not that they proposed:

"a thought experiment in which a measurement of a physical quantity in one system affects the measurement of a physical quantity in another system that is isolated from it."

It was more of an either/or statement, not an assertion of the existence of quantum entanglement. When you try to simplify things too far, at some point they are no longer true. Like the common statement that trees breath CO2. I believe that is the problem we have with this edit. Phancy Physicist (talk) 10:08, 23 August 2010 (UTC)

Please note that my edit merely describes the basic thought experiment of the EPR paper. I'll try to explain by discussing the thought experiment and its use several times in the EPR paper.

Preface to thought experiment

Consider the last paragraph of p. 778 of the EPR paper and its continuation on p. 779.

"In quantum mechanics it is usually assumed that the wave function does contain a complete description of the physical reality of the system in the state to which it corresponds. ... We shall show, however, that this assumption, together with the criterion of reality given above, leads to a contradiction."

In other words, the paper is going to use a thought experiment based on quantum mechanics to derive a result that is a contradiction, which brings into question the assumption that the wave function is a complete description of the system.

Thought experiment with measured quantity A

The paper begins section 2 on p. 779 with a thought experiment.

"For this purpose let us suppose that we have two systems, I and II, which we permit to interact from the time t=0 to t=T, after which time we suppose that there is no longer any interaction."

Then the paper considers the wave function $\Psi$ for the combined system I+II after time t=T when the two systems are no longer interacting.

$\Psi (x_{1},x_{2})=\sum _{n=1}^{\infty }\psi _{n}(x_{2})u_{n}(x_{1})$

It then considers a measurement of the quantity A for system I. At the bottom of the left column of p. 779 is the following.

"Suppose now that the quantity A is measured and it is found that it has the value a_k. It is then concluded that after the measurement the first system is left in the state given by the wave function $u_{k}(x_{1})$, and the second system is left in the state given by the wave function $\psi _{k}(x_{2})$."

In other words, a measurement of A for system I when the two systems are not interacting has resulted in system II being in the state corresponding to $\psi _{k}(x_{2})$. This is a paradox since a measurement of system I has determined the state of system II even though the two systems are not interacting. Later in the paper, after this thought experiment is repeated with a different measurement, this point is mentioned.

Repeat of thought experiment, except with measured quantity B

At the top of the right column of p. 779, the thought experiment is repeated, except the physical quantity B is measured instead of A. This time the measurement of system I resulted in system II being in a different state which corresponds to the different wave function $\phi _{k}(x_{2})$.

The paper then says,

"We see therefore that, as a consequence of two different measurements performed upon the first system, the second system may be left in states with two different wave functions. On the other hand, since at the time of measurement the two systems no longer interact, no real change can take place in the second system in consequence of anything that may be done to the first system."

Again, this is another statement of the same paradox since two separate measurements of system I, i.e. measurements of A and B, has determined two separate states of system II, even though systems I and II are not interacting.

Repeat of thought experiment with system II having eigenfunctions of non-commuting operators

Beginning with the last paragraph of p. 779, the paper repeats the thought experiment, except with the wave functions $\psi _{k}$ and $\phi _{r}$ of system II being specified as eigenfunctions of two non-commuting operators, respectively. This is mentioned at the bottom of the left column of p. 780 along with a reminder that quantities A and B were measured for system I.

"... we assume that $\psi _{k}$ and $\phi _{r}$ are indeed eigenfunctions of some non-commuting operators P and Q, corresponding to the eigenvalues p_k and q_r, respectively. Thus, by measuring either A or B we are in a position to predict with certainty, and without in any way disturbing the second system, either the value of the quantity P (that is p_k) or the value of the quantity Q (that is q_r').

The paper then uses this result to conclude that the wave function does not provide a complete description of reality.

                 

Please note that this is a form of reasoning called reductio ad absurdum that starts with a premise that the wave function is a complete description of reality, then uses a basic thought experiment several times to show that contradictory results occur, and thus shows that the premise is incorrect.

As I mentioned at the beginning of this message, my edit merely describes the thought experiment that is used several times in the EPR paper. --Bob K31416 (talk) 16:56, 23 August 2010 (UTC)

I have followed this discussion with some interest. Phancy Physicist is right when thinking that the EPR discussion does not purport to prove that entanglement is due to nonlocal influences in an EPR experiment. In the EPR paper EPR state an alternative explanation: "One could object to this conclusion on the grounds that our criterion of reality is not sufficiently restrictive. Indeed, one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted. On this point of view, since either one or the other, but not both simultaneously, of the quantities P and Q can be predicted, they are not simultaneously real. This makes the reality of P and Q depend upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this."

Unfortunately, EPR associate this alternative explanation once again with nonlocal influences. Today we have a far more convincing argument against the simultaneous reality of P and Q, viz. the Kochen-Specker theorem, basing such non-existence on incompatibility of observables P and Q (which refers to local mutual disturbance of these observables in a simultaneous measurement of these very observables), not basing it on a disturbance of either P or Q by a measurement of an observable of a distant particle (which observables are compatible observables, since belonging to different particles). Note that simultaneous measurement of incompatible observables is possible (and experimentally realized) if inaccuracy of the measurement results is permitted in agreement with the Heisenberg uncertainty relations. Anyway, the notion of entanglement would not be interesting if all observables were compatible (then one single basis would exist for all observables), entanglement being interesting because of non-existence of such a basis. The upshot of these considerations is that the standard treatment already is a simplification because by ignoring the possibility of joint nonideal measurements of incompatible observables the explanation alternative to nonlocality remains out of sight.WMdeMuynck (talk) 18:53, 23 August 2010 (UTC)

Thank you WmdeMuynck for your comment, which I recognise as that of a serious expert. I note that you also cite the original paper as saying "This makes the reality of P and Q depend upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this."

To focus back on the present rather narrow problem, the wording of a brief leader. Bob, you comment at some considerable length, but your commentary does not directly deal with the relevant question: does the EPR paper admit what your proposed edit asserts, in your words: "that a measurement of one system can affect another system that isn't interacting with it." or in the words of your proposed edit: "The EPR paradox (or Einstein–Podolsky–Rosen paradox) refers to a thought experiment in which a measurement of a physical quantity in one system affects the measurement of a physical quantity in another system that is isolated from it." ? The answer is no. The considerable length of your commentary here betrays the fact that you do not have a direct answer to the question of does the EPR paper admit a real effect without an interaction. You try to argue that the measurement of one system affects the other system on the basis of the words "the second system may be left in states with two different wave functions"; to say that the wave functions are different is not to say that the system has been affected, for the wave function is a quantum mechanical description, which is under question here, and cannot be assumed to be a valid description for the purposes of this argument; to infer the latter, that the system has been affected, requires an assumption that the wave function tells about real effects on the system, which is under question by Einstein Podolsky and Rosen. To use another Latin tag, by making that assumption, you are engaging in petitio principii. It stands that your proposed edit contradicts the original Einstein Podolsky Rosen 1935 paper, and thus does not "summarize" it as you say it does, nor does it "merely describe" it in the terms that the original paper describes it. I note that I have the explicit support of Phancy Physicist on this point. Thus your proposed edit is not a suitable one for a brief leader.Chjoaygame (talk) 22:19, 23 August 2010 (UTC)

Chjoaygame, it was not my intention to summarize the EPR paper. On the contrary, it was my intention to convey that the paper should be read as a product of its time of publication, not being aware of later theoretical developments that are relevant to present-day experimental developments. I do not think that your suggestion of a petitio principii is relevant, as you may be able to see when consulting my website [[3]].WMdeMuynck (talk) 14:40, 24 August 2010 (UTC)

WMdeMuynck, I think the the second half of the post you are responding to is directed at Bob K31416's proposed edit and not your post. Also, I agree with your point about not including future developments when trying to describe the content of the paper. But to do the paper justice, we should also have discussions about the response to the paper and how it fits into the current state. Phancy Physicist (talk) 21:39, 24 August 2010 (UTC)

Chjoaygame replying to WMdeMuynck. Sorry, it seems I crossed wires. My comment about petitio principii did not refer to your post,WMdeMuynck. My comment about petitio principii referred to the above explanation by Bob. My sentence "To focus back on the present rather narrow problem, the wording of a brief leader." was intended to change the subject of my comments, from being about your post, to being about Bob's above explanation and its relevance to the editorial task.Chjoaygame (talk) 21:55, 24 August 2010 (UTC)

Actually, I thought my above explanation was quite good and clarified the EPR paper. I'm sorry that you didn't profit from reading it. Regards, --Bob K31416 (talk) 00:16, 24 August 2010 (UTC)

Dear Bob, For what it is worth, my opinon is that your above explanation was good, and I profited from reading it. I didn't say your above explanation was not good, nor that I did not profit from reading it. I said that it did not answer a particular question, that relates to the task of editing the leader of the article.Chjoaygame (talk) 11:17, 24 August 2010 (UTC)

Please see my response of 15:51, 25 August 2010 below. --Bob K31416 (talk) 15:22, 26 August 2010 (UTC)

Bob K31416, I also find no problems with the actual statements you made. I believe that the issue me and Chjoaygame have with your edit is just that the wording seems to make it sound like the point of the paper was "a thought experiment in which a measurement of a physical quantity in one system affects the measurement of a physical quantity in another system that is isolated from it." I agree that the body of the paper and the vehicle for its point was the though experiment but the thought experiment was not the focus of the paper. We just don't want the article to sound like it was the focus to someone "not in the know". Phancy Physicist (talk) 20:53, 24 August 2010 (UTC)

Please note that the present article is EPR paradox and the first sentence of the present form of the article begins by stating, "The EPR paradox (or Einstein–Podolsky–Rosen paradox) refers to a thought experiment ...". And goes on to say that the thought experiment is in the EPR paper. This article should state at the beginning, as I put it in my edit, what the EPR paradox is, i.e. what the thought experiment in the EPR paper is. Regards, --Bob K31416 (talk) 15:51, 25 August 2010 (UTC)

You are right about the first sentence in the article. I have changed it to talk about the EPR Paradox and not the thought experiment itself. As I said before I have no major problem with your actual statements about the thought experiment, just the emphasis it gets over the actual statement of the paradox itself. Phancy Physicist (talk) 10:45, 4 September 2010 (UTC)

AFAICT, the thought experiment is the paradox. Please note that this is consistent with the article about the Twin paradox where the thought experiment is also the paradox. --Bob K31416 (talk) 15:56, 5 September 2010 (UTC)

Recent edit 05:33, 25 August 2010

I understand User:Chjoaygame's concern with the original text of the EPR page (which probably is not very different from Phancy Physicist's), but his last edit of the EPR page does not belong to that page if it is meant to convey the gist of the 1935 EPR paper without being mixed up with all later discussions inspired by the subject. The 1935 paper was actually directed against the Copenhagen empiricist interpretation as at that time it was experienced and generally (apart from a few dissidents like de Broglie and Schrödinger) accepted as the standard account of quantum mechanics. User:Chjoaygame's edit is inspired by more recent discussions based on the possibility of hidden variables theories that in 1935 were pretty much anathema, and for that reason not addressed in the EPR paper. The EPR `elements of physical reality' do not refer to hidden variables but to values of quantum mechanical observables. Therefore the whole discussion remains inside the quantum mechanical (standard) formalism. The authors of the 1935 EPR paper have no other aspiration than establishing a paradox within that formalism and its standard interpretation. They agree that this paradox can be solved by making a choice between `completeness of quantum mechanics' and `locality of measurement influences', a choice in favour of completeness making locality impossible. Their preference of the latter is admitted by them to be liable to doubt, a doubt that has been considerably increased by more recent considerations based on the results of experiments performed by Aspect in the 1980s.WMdeMuynck (talk) 10:45, 25 August 2010 (UTC)

Perhaps WMdeMuynck can read what inspires my mind (by spooky action at a distance?) better than I can read it myself.

I had in mind that the term Copenhagen interpretation was ambiguous, and I wanted to be more explicit. According to Dugald Murdoch (1987, ISBN 0521333202, page 179) "In the aftermath [of the EPR paper], Bohr's construal of quantum mechanics established itself as the basis of the orthodox interpretation -- the 'Copenhagen intertpretation', as it came to be called." According to Kristian Camilleri (2009, ISBN 9780521884846, page 2) "The lack of clarity about precisely what constitutes the Copenhagen interpretation stems partly from the fact that none of the founding fathers of quantum mechanics ever set out in a clear fashion the basic tenets of the orthodox interpretation."

I did not have any idea of hidden variables in mind, though I cannot speak for my unconscious inspiration. So far as I know Einstein did not write about adding 'hidden variables' to the quantum mechanical formalism. "the possibility of hidden variables theories that in 1935 were pretty much anathema". According to John von Neumann (1932/1955, page 209) "Whether or not an explanation of this type, by means of hidden parameters, is possible for quantum mechanics, is a much discussed question. The view that it will sometime be answered in the affirmative has at present prominent representatives. If it were correct, it would brand the present form of the theory as provisional, since then the description of the states would be essentially incomplete." von Neumann then produced an argument to settle the question, an argument that authorized the anathema. Perhaps Einstein was afraid to utter the anathema so authorized? Perhaps the EPR paper was an attempt to contradict the authority of the anathema? If Einstein entertained the hypothesis that quantum mechanics as it then stood could be 'incomplete', then he must have thought that there might exist some valid theory other than quantum mechanics as it then stood; otherwise what does 'incomplete' mean?

"The EPR `elements of physical reality' do not refer to hidden variables but to values of quantum mechanical observables." I think Einstein, Podolsky, and Rosen (1935) were rather coy about what were really the 'elements of reality'. They wrote: "A comprehensive definition of reality is, however, unnecessary for our purpose."

It is not apparent to me that "Their preference of the latter is admitted by them to be liable to doubt". Einstein later wrote: "But on one supposition we should, in my opinion, hold absolutely fast: the real factual situation of the system S₂ is independent of what is done with the system S₁, which is spatially separated from the former." (as translated in Albert Einstein, Philosopher-Scientist, edited by Schilpp, P.A., Open Court, La Salle IL, part 1, 1949, page 85).Chjoaygame (talk) 15:03, 25 August 2010 (UTC)

My remark "Their preference of the latter is admitted by them to be liable to doubt" was based on the EPR paper (last page): "One could object to this conclusion on the grounds that our criterion of reality is not sufficiently restrictive. Indeed, one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted. On this point of view, since either one or the other, but not both simultaneously, of the quantities P and Q can be predicted, they are not simultaneously real. This makes the reality of P and Q depend upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this." This objection is evidently foreseeing Bohr's answer to the EPR paper, highlighting that in the EPR experiment P and Q are not measured simultaneously. Here EPR seem to admit the logical (not experimental) possibility of nonlocal effects during a measurement of either P or Q. This possibility has later been made popular as a result of hidden variables theories like the Bohm one, and by the results of the Aspect experiments as interpreted in terms of subquantum (hidden) variables. Most of this is discussed on my web site, which can be found via my User page.WMdeMuynck (talk) 04:08, 27 August 2010 (UTC)

Thought experiment

In the first paragraph of the lead is the sentence, "Einstein, Podolsky, and Rosen introduced the thought experiment in a 1935 paper to argue that the description of reality given by a wave function is not complete."

Would anyone care to summarize the above mentioned thought experiment of the EPR paper? Thanks. --Bob K31416 (talk) 09:38, 24 August 2010 (UTC)

Comment on the edits of 26 Aug 2010 at 07:48 and 08:03

Bob, as far as I am concerned, you should feel free to do whatever takes your fancy here. It is clear that you want to make your mark. Go for it, I say. The article is mostly rubbish and I doubt that you will be able to make it much worse. I don't have any more time to spare for this; I have scientific work to do.Chjoaygame (talk) 15:45, 26 August 2010 (UTC)

New and Neutral Lead

I have changed the lead to the article to:

The EPR paradox (or Einstein–Podolsky–Rosen paradox) refers to a dichotomy, where either the measurement of a physical quantity in one system affects the measurement of a physical quantity in another, spatially separated system or the description of reality given by a wave function is not complete. This challenge was made through the use of a thought experiment and resulted in what seemed to be a contradiction in what has become known as the Copenhagen interpretation.

Einstein, Podolsky, and Rosen introduced this debate in a 1935 paper. In the words of the authors, the thought experiment yields a dichotomy which states, "Either (1) the quantum-mechanical description of reality given by the wave function is not complete or (2) when the operators corresponding to two physical quantities do not commute the two quantities cannot have simultaneous reality."

I have tried to change the first few sentences to represent the paradox in a quantum mechanical interpretation neutral way. By this, I mean not favoring one over another and just stating the paradox.

If there is dispute over this change, can we please debate it on the talk page before any major changes are made.Phancy Physicist (talk) 11:03, 4 September 2010 (UTC)

I think the above proposal by Phancy Physicist is yielding a good rendition of the content of the EPR paper. I like to add that according to a historian of science (I don't remember who, it might be Gerald Holton) Einstein was not particularly happy with the overly logical way of putting the dilemma, which seems to be attributable to Podolsky. A better rendition of Einstein's views can be found in Einstein A., Dialectica 2, 320 (1948), Quanten-Mechanik und Wirklichkeit.WMdeMuynck (talk) 16:01, 4 September 2010 (UTC)

Re "The EPR paradox (or Einstein–Podolsky–Rosen paradox) refers to a dichotomy, where either the measurement of a physical quantity in one system affects the measurement of a physical quantity in another, spatially separated system or the description of reality given by a wave function is not complete." - Phancy Physicist, What is the source and excerpt from the source that states this is what is known as the "paradox". Thanks. --Bob K31416 (talk) 13:40, 5 September 2010 (UTC)

WMdeMuynck, Could you give here the excerpt from your webpage where you have specified what the EPR paradox is? Thanks.

P.S. I looked on your webpage here and here and I didn't see any statement that specifies what the "EPR paradox" is. Regards, --Bob K31416 (talk) 13:57, 5 September 2010 (UTC)

Bob K31416, I don't think your request to be a good idea. Everyone can read my rendition of the EPR paradox here: [4]. There is no need to reproduce it. That there is no mention of a paradox in my rendition, is because there is no paradox as far as the EPR paper is concerned, which paper deals with a proof of incompleteness of quantum mechanics based on certain well-defined (although questionable) presuppositions. WMdeMuynck (talk) 16:53, 5 September 2010 (UTC)

Re "That there is no mention of a paradox in my rendition, is because there is no paradox as far as the EPR paper is concerned" - Does that mean that you feel that there is no such thing as the EPR paradox? Is that the problem here in these discussions, that there is a controversy outside of Wikipedia about whether or not there is a paradox, and your opinion is that there is no paradox? I'm just trying to understand what's going on here so that we can better develop the article. Regards, --Bob K31416 (talk) 18:28, 5 September 2010 (UTC)

From a paper on the EPR problem that can be found here [[5]] I quote the following: "In his (Einstein's, WMdM) writings subsequent to EPR, Einstein probes an incompatibility between affirming locality and separability, on the one hand, and completeness in the description of individual systems by means of state functions, on the other. His argument is that we can have at most one of these but never both. He frequently refers to this dilemma as a “paradox”." Hence, Einstein referred to an incompatibility as a paradox, in disagreement with a notion of paradox as an apparent inconsistency. In my opinion it is better to evade the use of the term `paradox' because it will inevitably drag you into a pointless quarrel about the meaning of that term.WMdeMuynck (talk) 21:19, 5 September 2010 (UTC)

Thanks for the link to the Arthur Fine paper in the Stanford Encyclopedia of Philosophy.

Re "In my opinion it is better to evade the use of the term `paradox' because it will inevitably drag you into a pointless quarrel about the meaning of that term." - From my experience here, it does appear that it is a contentious issue and that is unfortunate. Since the article is titled EPR paradox, it would seem inappropriate to not use the term paradox. I would suggest changing the name of the article, except that the term "EPR paradox" seems to be widely used in discussing this subject in the literature.

I have the impression that I am interacting with editors here that support the Einstein side of the Bohr-Einstein debates, or rather the evolvement of that side to the present day. I also have the impression that the Einstein side of the debate is held by a minority in the physics community, but I'm certainly amenable to having these impressions corrected if they are false ones. I hope that you haven't perceived me as having taken a side in the debate because I haven't. I think I'm pretty open minded regarding it.

But in any case, we should go by the sources. Perhaps the best way to proceed is to accumulate sources that give information about the meaning of the term EPR paradox, e.g. the Fine article that you mentioned may be one of them. Do you know of other sources that give information that would lead to other interpretations of what the EPR paradox is? Thanks. --Bob K31416 (talk) 16:27, 6 September 2010 (UTC)

In my opinion the change doesn't make sense. It mentions "spatially separated" but the point is that they are non-interacting, as described in the EPR paper. What distance is considered "spatially separated"? This needs to have the proper context, otherwise it is unclear.

There is mention of a "challenge". How is the reader to know what this means?

There is mention of a thought experiment but no mention of what the thought experiment is. Isn't the thought experiment the paradox, whereby two systems that are not interacting affect each other in the experiment? Please note the definition of paradox.[6]

"a : a statement that is seemingly contradictory or opposed to common sense and yet is perhaps true"

The new beginning of this article looks more like a discussion than a description of a paradox. Again, the paradox is that thought experiment where non-interacting systems seem to affect each other and this seems to be a contradiction, and is a paradox according to the above definition.

Regarding being neutral, I don't understand what the sides are. Frankly, I've encountered some pretty weird comments on this Talk page, beginning with the "greeting" I got when I first came here. I think some of the editors are carrying some kind of battle from elsewhere to this article and it's disrupting the development of this article IMO. Please note that I don't have any baggage in that regard and I am only trying to be an editor on Wikipedia and report what I have found in sources, to the best of my ability. --Bob K31416 (talk) 14:57, 5 September 2010 (UTC)

Please note that for the above reasons I reverted to the version that resulted after considerable discussion, after which the above editors pulled out and were silent for over a week. --Bob K31416 (talk) 15:47, 5 September 2010 (UTC)

What editors are you talking about pulling out? If you look back WMdeMuynck and I (Phancy Physicist (talk)) have been here for the entire debate. Chjoaygame and I do not support your edit to the lead and we said so. Many times. And if you look at the edit history for this article, You will see that you changed it back on 2010-08-26T19:08:12, only two days after the last post in that debate. Clearly, Chjoaygame was disgusted with your lack of respect for our input on this subject in the post on this talk page and to make it perfectly clear so am I.

Even so, I didn't just simply revert your change. I tried to rewrite the lead, giving more emphasis, which is justified, to the thought experiment. As well as changing the tone from an EPR versus Quantum Mechanics tone to EPR versus the Copenhagen interpretation which is more consistent with what the paradox is about. Your response to this was is a bunch of rapid fire posts and reverting it yet again without even letting me respond to your posts. Your revert goes against: my request, WMdeMuynck's support for my edit and the guiding principles of wikipedia.

I am reverting your change to the lead. Please, if you at all honor the spirit of wikipedia, allow time to address your objections before you make changes that others have stated are controversial.

Phancy Physicist (talk) 09:21, 6 September 2010 (UTC)

Answers to Bob K31416's Objections From Phancy Physicist (talk)

0) As a general comment about the lead, I want to remind everyone who reads this the article we are talking about is called, EPR paradox. It is not an article just about the article published on this subject by Einstein, Podolsky and Rosen. There have been many changes and refinements though the years to the original argument. Even Einstein wrote many different formulations after the paper was originally published. Please consider that.

1) In my opinion the change doesn't make sense. It mentions "spatially separated" but the point is that they are non-interacting, as described in the EPR paper. What distance is considered "spatially separated"? This needs to have the proper context, otherwise it is unclear.

I wanted to avoid saying non-interacting because the idea is that particles are non-locally effecting each other or the wave form theory is incomplete. This non-local interaction is at the heart of the paradox. In the original paper, when they say the particles are non interacting, they are talking about local interactions. I realize that the ideas of local and non-local interactions where not defined as they are today when the paper was written. The term "spatially separated" is not something I made up. It is a term that is frequently used when talking about these interactions. Would changing it somehow to "greatly separated systems" fix your objection?

Phancy Physicist (talk) 11:38, 6 September 2010 (UTC)

I thought that the term interaction was used for: electromagnetism, weak, strong, or gravity. If there is some other use in physics of the term "interaction" could you give me a source that would validate that other use of the term? --Bob K31416 (talk) 20:13, 8 September 2010 (UTC)

2) There is mention of a "challenge". How is the reader to know what this means?

I agree. The wording is bad here. How about something like:

This challenge to the Copenhagen interpretation originated from the consequences of a thought experiment introduced in 1935 by Einstein, Podolsky, and Rosen and resulted in what seemed to be a contradiction in the interpretation.

I feel this wording more properly gives emphasis to the original paper without confining the article to it.

Phancy Physicist (talk) 11:38, 6 September 2010 (UTC)

3) There is mention of a thought experiment but no mention of what the thought experiment is. Isn't the thought experiment the paradox, whereby two systems that are not interacting affect each other in the experiment? Please note the definition of paradox.[4]

"a : a statement that is seemingly contradictory or opposed to common sense and yet is perhaps true"

The new beginning of this article looks more like a discussion than a description of a paradox. Again, the paradox is that thought experiment where non-interacting systems seem to affect each other and this seems to be a contradiction, and is a paradox according to the above definition.

Again, No. The paradox is not the thought experiment. The thought experiment is logical and not debated. It is the conclusions that they reached that are debated. The thought experiment proved that it is an either/or situation. EPR came to the conclusion that the Copenhagen interpretation was incomplete due the thought experiment's result and choosing one of the either/ors to be true with their provided reasoning. The paradox was that the Copenhagen interpretation seemed opposed to common sense provided by EPR yet is perhaps true. In this case the thought experiment and their assumptions are the common sense. Einstein, Podolsky and Rosen believed they proved the Copenhagen interpretation incomplete.

Phancy Physicist (talk) 11:38, 6 September 2010 (UTC)

In any case, I think we need to give a summary in the lead of the thought experiment of the EPR paper, e.g. like the summary that was in my edit. Although, I have since rewrote a draft of the summary using "separated" and "interaction" that might be more acceptable:

The thought experiment involves two systems that interact with each other and are then separated so that they presumably don't interact any more. Then in the thought experiment, a property of one of the systems is measured and the possible effect of this measurement on the other separated system is considered.

--Bob K31416 (talk) 20:13, 8 September 2010 (UTC)

4) Regarding being neutral, I don't understand what the sides are. Frankly, I've encountered some pretty weird comments on this Talk page, beginning with the "greeting" I got when I first came here. I think some of the editors are carrying some kind of battle from elsewhere to this article and it's disrupting the development of this article IMO. Please note that I don't have any baggage in that regard and I am only trying to be an editor on Wikipedia and report what I have found in sources, to the best of my ability.

As I stated in my original post about this edit that:

I have tried to change the first few sentences to represent the paradox in a quantum mechanical interpretation neutral way. By this, I mean not favoring one over another and just stating the paradox.

The original text seemed to make it EPR versus Quantum Mechanics and, as WMdeMuynck pointed out, this is not the case. There is no external battle and this has nothing to do with you personally.

Phancy Physicist (talk) 11:38, 6 September 2010 (UTC)

5) I do not believe myself to be the ultimate authority on the EPR paradox and I don't claim that what I have written is the best form for the lead. Please comment on these points and provide arguments for your rebuttals to and agreements with these points. We can come to a consensus by proceeding respectfully in this way. Phancy Physicist (talk) 11:38, 6 September 2010 (UTC)

I've done some googling and have given it more thought and, unless something comes up, I won't be pursuing my version. However, I think we need a better opening for the article. I'll try to think about it and if I come up with some suggestions, I don't expect that it will result in any discord between us. At least I don't expect to be pushing anything if there is resistance. It could be that there is no specific definition of the EPR paradox and the term just refers to the subject area consisting of its unusual thought experiment phenomena and conclusions. --Bob K31416 (talk) 01:50, 7 September 2010 (UTC)

@Bob K31416: I agree the article needs work and I don't want in anyway to stop you from contributing to wikipedia. Everyone that posts on the talk page just wants the article to be a good resource on the topic. Please continue to express your view and reasoning so that we can all get at the truth and help wikipedia become better.

Phancy Physicist (talk) 07:28, 7 September 2010 (UTC)

Perhaps we should add " EPR argument " as an alternate name for the topic of this article? Just a thought. --Bob K31416 (talk) 09:26, 7 September 2010 (UTC)

Orphaned text

The following text seemed redundant in its current placing but still seems like it should have a place in the article.

In the words of the authors, the thought experiment yields a dichotomy which states, "Either (1) the quantum-mechanical description of reality given by the wave function is not complete or (2) when the operators corresponding to two physical quantities do not commute the two quantities cannot have simultaneous reality."

So I put it here to hold on to it.

Phancy Physicist (talk) 09:04, 7 September 2010 (UTC)

Proposed addition to opening sentence

Here's a proposed addition to the opening sentence that is oriented towards readers who have less knowledge of the field. (In the following, I put the existing text in small font and the proposed text in normal font.)

The EPR paradox (or Einstein–Podolsky–Rosen paradox) is a topic in quantum physics and the philosophy of science regarding measurements of microscopic systems (such as individual photons, electrons or atoms) and the description of those systems by the methods of quantum physics. It refers to ...

--Bob K31416 (talk) 14:29, 8 September 2010 (UTC)

citation needed

Michael, 2 1/2 weeks ago I put in 2 {{citation needed}}'s because there were two unsourced parts of the article that seemed to contradict each other.[7]. Yesterday, one of the {{citation needed}}'s was removed.[8] From the edit summary, I thought the editor simply made an oversight so I reverted with an explanation in my edit summary. Then today I see that my edit was reverted,[9] so I came here to discuss because maybe I'm overlooking something! Here's the subject excerpt from the version of the article with the {{citation needed}}.

The most prominent opponent of the Copenhagen interpretation was Albert Einstein. In his view, quantum mechanics is incomplete and, commenting on this, other writers (such as John von Neumann and David Bohm) though not Einstein himself, have suggested that consequently there would have to be 'hidden' variables responsible for random measurement results.

AFAICT, the phrase "though not Einstein himself" makes the claim that Einstein did not suggest that there would have to be hidden variables. Whereas earlier in the article there is the excerpt,

"Before delving into the complicated logic that leads to the 'paradox', it is perhaps worth mentioning the simple version of the argument, as described by Greene and others, which Einstein used to show that 'hidden variables' must exist.

which apparently contradicts the above because it indicates that Einstein not only suggested that there would have to be hidden variables but he showed that hidden variables must exist, according to that excerpt. So I requested sources for both statements.

It seems reasonable to request a source that backs up a claim made in an article that someone didn't suggest something. For example, suppose in another article there was the statement that Obama didn't suggest that Iran stop trying to develop nuclear weapons. Wouldn't a source be needed for such a claim about Obama? Regards, --Bob K31416 (talk) 22:41, 25 September 2010 (UTC)

I may have been too hasty in removing the tag - apologies.

One thing I have been mulling over since removing the tag is that hidden variables only became viable in 1952 with Bohm's 2 articles. Einstein died in 1955, which gives him a rather short window in which to comment specifically on hidden variables. This leads me to suspect that Einstein probably never expressed any views on HVs (especially since old people do not readily take on board new ideas in their declining years). Yes, he was sceptical about QM - but up to date about Bohm? I doubt it, but I doubt there will be many (any?) sources that accurately testify either way. What I have found are articles that conflate Einstein's QM scepticism with belief in HVs, but I suspect we all know enough here to realise that one does not follow from the other.

--Michael C. Price ^talk 23:05, 25 September 2010 (UTC)

A comment by Einstein on Bohm's hidden variables theory can be found in a paper by Einstein in Scientific papers presented to Max Born, Oliver and Boyd, Edinburgh, 1953, p. 33. This reference is given in an account of Bohm's theory in chapter 10 of my book (Willem M. de Muynck, Foundations of quantum mechanics, an empiricist approach, Fundamental theories of physics, vol. 127, Kluwer Academic Publishers, Dordrecht, Boston, London, 2002). In chapter 4 of this book you may also find a discussion of the distinction between two notions of completeness, viz. completeness in a wider sense (addressing the problem of hidden variables) and completeness in a restricted sense (being a strictly quantum mechanical notion, endorsed by the Copenhagen interpretation). It should be noted that in the EPR paper only the latter kind of completeness is attacked. A brief account of these subjects can also be found on my web site at [[10]] and [[11]].WMdeMuynck (talk) 09:35, 26 September 2010 (UTC)

Can you give the actual Einstein quotation here? --Michael C. Price ^talk 10:06, 26 September 2010 (UTC)

I have to retrieve the book in which the quotation can be found from another university. As soon as I have it I hope to be able to put the quotation on your talk page.WMdeMuynck (talk) 22:00, 2 October 2010 (UTC)

The given statement "other writers (...) though not Einstein himself,[citation needed] have suggested that" may be contradicted and even proven to be wrong but NOT POSSIBLY confirmed by any citation. ---- Sintermerte ---- (typing tildes wouldn't work).

Sentence in lead re paradox and hidden variables

The following sentence begins the last paragraph of the lead.

"The EPR paradox is a paradox in the following sense: if one adds to quantum mechanics some seemingly reasonable but apparently otherwise hidden variables that supposedly express the concept of local realism (not to be confused with philosophical realism; see Bell inequality, Bell test experiments, and counterfactual definiteness) — then one obtains a contradiction."

This notion that the paradox is some contradiction from adding hidden variables does not seem to be in any sources and may be incorrect. It's my impression that adding hidden variables was supposed to eliminate a paradox, not create one. --Bob K31416 (talk) 16:58, 1 October 2010 (UTC)

Deleted above sentence and rest of paragraph which is predicated on it. --Bob K31416 (talk) 17:03, 16 October 2010 (UTC)

[corrected my own comment] Actually you are right, Bell describes it as EPR's proposed solution to the apparent spooky action-at-a-distance. However, the lead may still need more rework. Harald88 (talk) 10:56, 15 December 2010 (UTC)

Monster Lead

The current lead to this article is:

The EPR paradox (or Einstein–Podolsky–Rosen paradox) is a topic in quantum physics and the philosophy of science regarding measurements of microscopic systems (such as individual photons, electrons or atoms) and the description of those systems by the methods of quantum physics. It refers to a dichotomy, where either the measurement of a physical quantity in one system affects the measurement of a physical quantity in another, spatially separated system or the description of reality given by a wave function is not complete.

This challenge to the Copenhagen interpretation originated from the consequences of a thought experiment introduced in 1935 by Einstein, Podolsky, and Rosen and resulted in what seemed to be a contradiction in the interpretation. The thought experiment involves two systems that interact with each other and are then separated so that they presumably interact no longer. Then, the position or momentum of one of the systems is measured, and due to the known relationship between the measured value of the first particle and the value of the second particle, the observer is aware of the value in the second particle. A measurement of the second value is made on the second particle, and again, due to the relationship between the two particles, this value can then be known in the first particle. This outcome seems to violate the uncertainty principle, since both the position and momentum of a single particle would be known with certainty.

Einstein never accepted quantum mechanics as a "real" and complete theory as he could not believe that measurement of the second particle invalidates the measurement of the first as this requires "information" to travel faster than light between the two (although it is not actually possible to transfer information in this manner as the results would be uncontrollable). Einstein struggled to the end of his life for a theory that could better comply with causality, protesting against the view that there exists no objective physical reality other than that which is revealed through measurement as it is interpreted in terms of the quantum mechanical formalism.

However since Einstein's death experiments analogous to that of the EPR paradox were carried out, starting in 1976 by French scientists at the Saclay Nuclear Research Centre, which appeared to show that the measurement of one does indeed affect the other and that a local realistic view of the world is false.

That is one monster of a lead-in. Leads should never be this long. Can we work to chop up this beast?

Phancy Physicist (talk) 20:37, 16 December 2010 (UTC)

Maybe the last two paragraphs could be in a separate section called "Einstein's Views of EPR" or something. Einstein's views are an important aspect of this but not necessary to give a quick overview of the topic.

Phancy Physicist (talk) 20:52, 16 December 2010 (UTC)