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Why are Bell's inequalities violated?

by JK423
Tags: bell, inequalities, violated
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harrylin
#37
Jan17-13, 02:25 AM
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Quote Quote by DrChinese View Post
[..]
In my book, you pick and choose what evidence you accept, in order to be consistent with your pre-ordained conclusion.
Why would you do that? Or was it meant as a personal attack?
kith
#38
Jan17-13, 04:12 AM
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Quote Quote by nanosiborg View Post
I agree with this. The assumption of superdeterminism is useless for understanding why Bell's lhv formulation is inappropriate (ie., why it produces incorrect predictions) for modeling Bell tests.
Do you think the observer obeys the laws of nature, i.e. can be considered as a physical system?
nanosiborg
#39
Jan17-13, 04:52 AM
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Quote Quote by kith View Post
Do you think the observer obeys the laws of nature, i.e. can be considered as a physical system?
Sure. But the OP is concerned with why Bell's inequalities are violated. How will an ad hoc metaphysical assumption, such as superdeterminism, inform regarding that?
kith
#40
Jan17-13, 06:23 AM
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Quote Quote by nanosiborg View Post
Sure.
Do you agree that in a deterministic theory, the behaviour of a physical system is determined by its past or current state? Do you agree that in such a theory, the behaviour of the observer is determined by its past or current state if we treat him as a physical system?

Quote Quote by nanosiborg View Post
But the OP is concerned with why Bell's inequalities are violated. How will an ad hoc metaphysical assumption, such as superdeterminism, inform regarding that?
Bell's theorem makes the assumption that the probability distributions for Alice and Bob are independent. If you don't make this assumption, you can't derive the inequality in the first place. So discussing this assumption seems relevant to me. Personally, I haven't completely wrapped my mind around superdeterminism and want to understand the arguments more deeply.
bohm2
#41
Jan17-13, 08:44 AM
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Quote Quote by nanosiborg View Post
Assuming either superdeterminism or nonlocality is not informative.The answer is in the realm of anti-realism, which has to do with modeling restrictions.
Just to be clear, when you are using the term "anti-realism", do you mean: no pre-existing properties (non-counterfactuals). I'm asking because this stuff is a bit confusing as there are problems even with what is meant by "realism" also. For instance Wood and Spekkens write:
It has always been rather unclear what precisely is meant by "realism". Norsen has considered various philosophical notions of realism and concluded that none seem to have the feature that one could hope to save locality by abandoning them. For instance, if realism is taken to be a commitment to the existence of an external world, then the notion of locality-that every causal influence between physical systeems propagates subluminally already presupposes realism.
The lesson of causal discovery algorithms for quantum correlations: Causal explanations of Bell-inequality violations require fine-tuning
http://arxiv.org/pdf/1208.4119v1.pdf

Perhaps, Maccone's definition of "realism" seems as one of the more clearer ones:
Let us define “counterfactual” a theory whose experiments uncover properties that are pre-existing. In other words, in a counterfactual theory it is meaningful to assign a property to a system (e.g. the position of an electron) independently of whether the measurement of such property is carried out. Sometime this counterfactual definiteness property is also called “realism”, but it is best to avoid such philosophically laden term to avoid misconceptions. Bell’s theorem can be phrased as “quantum mechanics cannot be both local and counterfactual”. A logically equivalent way of stating it is “quantum mechanics is either non-local or non-counterfactual”.
Simplest proof of Bell’s inequality
http://arxiv.org/pdf/1212.5214v1.pdf

But I always have trouble understanding this. If something is not pre-existing, would not the Wood and Spekkens argument above hold?
DrChinese
#42
Jan17-13, 09:03 AM
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Quote Quote by kith View Post
Usually, we have a system S and an observer O measuring some observable of the system. As soon as we consider the combined system S+O as a physical system (which may be observed by another observer O'), we acknowledge that the current state of S and O influences the evolution of the combined system. This evolution may of course include interactions between S and O. What's wrong with this kind of thinking?
It can apply to any theory equally! It certainly applies to special relativity, that is it's very reason to exist is to account for observer reference frames. There is absolutely no reason to suspect that there is anything special in that regards about QM. And not the slightest evidence to suspect that the observer's PAST influences the outcome (only the observer's choice of measurement setting).
DrChinese
#43
Jan17-13, 11:17 AM
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Quote Quote by harrylin View Post
Why would you do that? Or was it meant as a personal attack?
I don't attack people, sorry if anything I said implied otherwise.

However, I think it is clear that you value a specific view that is usually excluded by Bell+experiment. You are certainly not the only one. However, it is very difficult to discuss the subject meaningfully when you assume that which you seek to prove.

Of course, for all I know you might say the same thing about my viewpoint.
Dan Fitzgibbon
#44
Jan17-13, 12:56 PM
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Quote Quote by nanosiborg View Post
Unlikely because it's an assumption without evidence. Convenient because, using Bell's formulation, assuming nonlocality allows that the results at one end depend on the analyzer settings at the other end. Too convenient for my taste. Maybe not yours and others. But at this point it is just a matter of taste.

I think that something other than nonlocality will eventually answer the thread question.
Suppose there are two sorts of time, one in which the entangled particles in Aspect's experiment both change state at the same time and one (the usual one) in which their change of state should be separated by the amount of time it takes for the effect to travel between the two events at the speed of light, but is not observed to do so, hence our problem. Would that help?
nanosiborg
#45
Jan17-13, 06:14 PM
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Quote Quote by kith View Post
Do you agree that in a deterministic theory, the behaviour of a physical system is determined by its past or current state? Do you agree that in such a theory, the behaviour of the observer is determined by its past or current state if we treat him as a physical system?
I agree with what DrChinese said. It's irrelevant.

Quote Quote by kith View Post
Bell's theorem makes the assumption that the probability distributions for Alice and Bob are independent. If you don't make this assumption, you can't derive the inequality in the first place. So discussing this assumption seems relevant to me. Personally, I haven't completely wrapped my mind around superdeterminism and want to understand the arguments more deeply.
I believe that superdeterminism is clutching at straws, and that it will not help us to understand the incompatibility between the lhv formalism and experiment. On the other hand, Bell's formulation of the independence assumption (his locality condition) is relevant, and some people (eg., Jarrett) think that a component of it (which, re Jarrett's analysis, doesn't necessarily inform regarding locality/nonocality in nature) might be the effective cause of BI violation.

Here's another way to approach the OP question. What is it about a basic Bell lhv model that produces a linear correlation (which is incompatible with the nonlinear one produced by qm and experiment) between θ and rate of coincidental detection?


Quote Quote by bohm2 View Post
Just to be clear, when you are using the term "anti-realism", do you mean: no pre-existing properties (non-counterfactuals). I'm asking because this stuff is a bit confusing as there are problems even with what is meant by "realism" also.
I should have said that I think the answer has to do with some aspect of the realism of Bell's formulation, which includes the separable functions A and B (which describe individual detection) expressed in terms of λ, and the expression of the independence (locality) assumption in terms of the functions A and B -- with the result of the Bell lhv program being against realism or "anti-realism" in that only nonrealistic models of quantum entanglement, such as in standard qm or MWI, are allowed (unless you assume ftl or instantaneous action at a distance, such as in dBB).

Quote Quote by bohm2 View Post
But I always have trouble understanding this. If something is not pre-existing, would not the Wood and Spekkens argument above hold?
I don't know. I don't understand the Wood and Spekkens article. Maybe you can explain it?

Quote Quote by Dan Fitzgibbon View Post
Suppose there are two sorts of time, one in which the entangled particles in Aspect's experiment both change state at the same time and one (the usual one) in which their change of state should be separated by the amount of time it takes for the effect to travel between the two events at the speed of light, but is not observed to do so, hence our problem. Would that help?
I don't see how it would. There's no particularly compelling reason to posit effects traveling between the two events. Paired detection events aren't correlated with each other except when the analyzer settings are aligned, in which case a local common cause explanation suffices.
bohm2
#46
Jan17-13, 07:51 PM
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Quote Quote by nanosiborg View Post
...only nonrealistic models of quantum entanglement, such as in standard qm or MWI, are allowed.
MWI is a "realistic" interpretation.
nanosiborg
#47
Jan17-13, 09:49 PM
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Quote Quote by bohm2 View Post
MWI is a "realistic" interpretation.
Ok, then change ...
"with the result of the Bell lhv program being against realism or 'anti-realism' in that only nonrealistic models of quantum entanglement, such as in standard qm or MWI, are allowed (unless you assume ftl or instantaneous action at a distance, such as in dBB)"
... to ...
"with the result of the Bell lhv program being against realism or 'anti-realism' in that only models of quantum entanglement which don't employ hidden variables, such as in standard qm or MWI, are allowed (unless you assume ftl or instantaneous action at a distance, such as in dBB)".

So my answer to your question ...
"Just to be clear, when you are using the term "anti-realism", do you mean: no pre-existing properties ... ?"
... would be that when I'm using the term "anti-realism", I mean no hidden variables.
harrylin
#48
Jan18-13, 02:53 AM
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Quote Quote by bohm2 View Post
[..]
Simplest proof of Bell’s inequality
http://arxiv.org/pdf/1212.5214v1.pdf

But I always have trouble understanding this. If something is not pre-existing, would not the Wood and Spekkens argument above hold?
Thanks for that link - regretfully they handle the assumption that it "is meaningful to assign a property to a system (e.g. the position of an electron) independently of whether the measurement of such property is carried out." However I see no reason to believe that this must always be applicable, and I thought that also Bell did not impose such a requirement to "realism" (or did I overlook it somewhere?).
DrChinese
#49
Jan18-13, 09:08 AM
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Quote Quote by harrylin View Post
I thought that also Bell did not impose such a requirement to "realism" (or did I overlook it somewhere?).
Yes, sadly Bell does not highlight this point. I guess he thought it was obvious. It is the spot after his (14) where he says, "It follows that c is another unit vector". The idea is that a, b and c are simultaneously real (i.e. elements of reality). So this is the spot where that assumption takes place, and without it he would not be able to continue to derive his conclusion.
harrylin
#50
Jan18-13, 10:18 AM
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Quote Quote by DrChinese View Post
Yes, sadly Bell does not highlight this point. I guess he thought it was obvious. It is the spot after his (14) where he says, "It follows that c is another unit vector". The idea is that a, b and c are simultaneously real (i.e. elements of reality). So this is the spot where that assumption takes place, and without it he would not be able to continue to derive his conclusion.
I never realised that, and will have to check it out - thanks!
DrChinese
#51
Jan18-13, 10:34 AM
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Quote Quote by harrylin View Post
I never realised that, and will have to check it out - thanks!
Yes, and note that this is the very first equation with a, b and c in it. Without all 3, of course, you don't have realism expressed as a testable assumption.
bohm2
#52
Jan18-13, 10:38 AM
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Quote Quote by nanosiborg View Post
I don't know. I don't understand the Wood and Spekkens article. Maybe you can explain it?
If one argues that something is local, realism is implied as above posts, I think. Analogously, if non-realism, then the issue of locality vs non-locality is kind of pointless since there's no ontological issues. I mean what ontological difference would there be between local vs non-local non-realism? Anyway, that's how I understood it. I think Gisin argues similarily here:
What is surprising is that so many good physicists interpret the violation of Bell’s inequality as an argument against realism. Apparently their hope is to thus save locality, though I have no idea what locality of a non-real world could mean? It might be interesting to remember that no physicist before the advent of relativity interpreted the instantaneous action at a distance of Newton’s gravity as a sign of non-realism (although Newton’s nonlocality is even more radical than quantum nonlocality, as it allowed instantaneous signaling).
Is realism compatible with true randomness?
http://arxiv.org/pdf/1012.2536v1.pdf
harrylin
#53
Jan18-13, 02:14 PM
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Quote Quote by DrChinese View Post
Yes, and note that this is the very first equation with a, b and c in it. Without all 3, of course, you don't have realism expressed as a testable assumption.
I certainly recall that point, which was the subject of many discussions... I just never interpreted it in the sense as expressed in that paper!

Their clarification ("assign a property to a system (e.g. the position of an electron)") plus your explanation allows me to make sense of Neumaier's claim (which he did not prove and with which you probably disagree). He holds that:
"all proofs of Bell type results [..] become invalid when particles have a temporal and spatial extension, with an internal structure that is modified when interacting in a beam splitter."
- http://www.mat.univie.ac.at/~neum/ms/lightslides.pdf

So, he suggests that that is why Bell's inequalities are violated.
DrChinese
#54
Jan18-13, 02:49 PM
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Quote Quote by harrylin View Post
I certainly recall that point, which was the subject of many discussions... I just never interpreted it in the sense as expressed in that paper!

Their clarification ("assign a property to a system (e.g. the position of an electron)") plus your explanation allows me to make sense of Neumaier's claim (which he did not prove and with which you probably disagree). He holds that:
"all proofs of Bell type results [..] become invalid when particles have a temporal and spatial extension, with an internal structure that is modified when interacting in a beam splitter."
- http://www.mat.univie.ac.at/~neum/ms/lightslides.pdf

So, he suggests that that is why Bell's inequalities are violated.
He makes several statements like this that are difficult for me to place in a suitable context. Photons have both temporal and spatial extension in my view. Not sure if they do to a local realist though. And I do not follow his reasoning on how that connects to Bell. So I guess I would say that I disagree. If you are enforcing strict Einsteinian locality as is done in Weihs et al (1998), the spatial extent of a photon is already accounted for.


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