50th anniversary of Bell's theorem

[bohm2]

I don't think the RQM approach contradicts Norsen's view....

I agree. Norsen discusses RQM in his paper. Norsen is willing to accept the view that Bell's theorem does, in fact, assume a type of "realism: metaphysical realism or the existence of an external world (non-solipsism). Norsen holds that RQM approach can evade Bell's by denying metaphysical realism:

It is interesting that Smerlak and Rovelli refer to Metaphysical Realism as “strict Einstein realism” – the implication being that what they are advocating as an alternative is only some less strict form of realism. But, simply put, that is not the case. What they are advocating is the complete rejection of the most fundamental type of realism, i.e., they are endorsing solipsism...Yet, clearly, this is precisely what they do advocate: for example, in their analysis of a simple EPR correlation experiment, it emerges that, when Alice and Bob get together later to compare results, Alice need not hear Bob reporting the same value for the outcome of his experiment that Bob himself believes he saw. If this isn’t an example of each observer’s picture of reality being disconnected from that of other observers, it’s hard to imagine what would be...What’s “relational” in “relational QM” (RQM) is reality itself: there is no such thing as reality simpliciter ; there is only reality-for-X (where X is some physical system or conscious observer). Advocates of RQM thus use the word “reality” to mean what people normally mean by the word “belief”. That some fact is, say, “real-for-Alice” simply means (translating from RQM back to normal English) that Alice believes it. And, crucially, what is real-for-Alice need not be real-for-Bob: “different observers can give different accounts of the same sequence of events.”

Against ‘Realism’
http://arxiv.org/pdf/quant-ph/0607057.pdf

 

[DrChinese]

I don't agree with you that this notion of "elements of reality" is independent of considerations of locality and entanglement. You can imagine a probabilistic interpretation where outcomes of measurements are not determined ahead of time, but that the outcome is created by interaction between the particle and the measuring device. So the question: "What result would Bob get if he measured the spin/polarization along axis \vec{a}?" would simply have no answer unless Bob actually did measure the spin/polarization along that axis. In such a probabilistic model, I don't think you would associate an "element of reality" with each spin direction.

However, that's where entanglement comes in. The fact that Alice measuring the spin along axis \vec{a} implies what Bob would get, if he did measure the spin along axis \vec{a}, means (to EPR) that there is an element of reality associated with the counterfactual "What would Bob get if he measured along axis \vec{a}?", even when Bob doesn't choose to measure along that axis. That element of reality can't be created by Alice's choice of axis (because that choice was made far away). So the conclusion is that the element of reality exists whether or not Bob (or Alice) chooses to measure it. Since \vec{a} was arbitrary in this argument, it should be true for all possible axes.

So, to me, the question of "elements of reality" is not independent of locality and entanglement. Locality and entanglement is what implies that there should be an element of reality associated with spin measurements.

I think a good example is to ask: does ONE photon have definite polarization at 0, 120 and 240 degrees independent of the act of observation? When you ask that question, locality (or non-locality) is not a factor. All 3 are at the same place.

Einstein would have answered the question above as YES (he said that particles have such properties/elements of reality at all times). He postulated QM was incomplete, while you and I now know either there are no such properties independent of observation. Some side of observation affects things in all interpretations.

The issue of locality comes only into play when you attempt to measure *pairs* of particles. So I agree with you on that part. But the fundamental question of "what is reality/realism" has nothing to do with entanglement. I think a better way to envision EPR is to ask: can I use entanglement as a way to probe the elements of reality of a particle? I use a particle's entangled partner to perform that probe.

 

[DrChinese]

Against ‘Realism’
http://arxiv.org/pdf/quant-ph/0607057.pdf

If he's against, I'm FOR. :-) :-) :-)

 

[Demystifier]

So Bohmian is non-local realistic.

Now you are confusing me, because you said so many times that Bohmian is non-realistic. Indeed, according to the definition you agreed above, the macroscopic manifestations of spins in 3 different directions are certainly not all real in Bohmian mechanics, so how can you call it real? Can you be consistent?

 

[atyy]

Now you are confusing me, because you said so many times that Bohmian is non-realistic. Indeed, according to the definition you agreed above, the macroscopic manifestations of spins in 3 different directions are certainly not all real in Bohmian mechanics, so how can you call it real? Can you be consistent?

The definition of "real" is contextual. :D

 

[Demystifier]

The definition of "real" is contextual. :D

Brilliant! :D:D:D:D:D

 

[stevendaryl]

I think a good example is to ask: does ONE photon have definite polarization at 0, 120 and 240 degrees independent of the act of observation? When you ask that question, locality (or non-locality) is not a factor. All 3 are at the same place.

If you only measure one photon, and that photon is produced with a random polarization, then there is no information to distinguish between the two possibilities: (1) The photon polarization is created by interaction with the filter--so there is no pre-existing "element of reality" associated, or (2) the photon has, for each angle, a corresponding, pre-existing element of reality determining whether the photon will pass a filter that is oriented at that angle. It's only the existence of perfect correlations between paired photons that would lead you to reject possibility (1).

Einstein would have answered the question above as YES (he said that particles have such properties/elements of reality at all times). He postulated QM was incomplete, while you and I now know either there are no such properties independent of observation. Some side of observation affects things in all interpretations.

I think that Einstein may have had a predisposition against intrinsically probabilistic theories, and assumed that they were always due to ignorance about the details of the state. However, even without that bias towards determinism, I think that perfect correlations for twin pairs argues against the probabilistic interpretation (because there is no way that the randomness could be resolved in the same way for distant measurements without nonlocality).

The issue of locality comes only into play when you attempt to measure *pairs* of particles. So I agree with you on that part. But the fundamental question of "what is reality/realism" has nothing to do with entanglement. I think a better way to envision EPR is to ask: can I use entanglement as a way to probe the elements of reality of a particle? I use a particle's entangled partner to perform that probe.

Well, the question for clarifying what "realism" means is whether a stochastic theory is "realistic" or not. If instead of a deterministic evolution equation, suppose that the world was described, at the most fundamental level, by a stochastic process, where there were many possible futures consistent with the present. Would that be considered a "realistic" model, or not? In such a model, there might be no element of reality corresponding to "what Bob will get if he measures the polarization using axis \vec{a}".

 

[DrChinese]

Now you are confusing me, because you said so many times that Bohmian is non-realistic. Indeed, according to the definition you agreed above, the macroscopic manifestations of spins in 3 different directions are certainly not all real in Bohmian mechanics, so how can you call it real? Can you be consistent?

Well you got me. :-)

I know it's observer dependent, I think you have even acknowledged as much in our previous discussions. But most Bohmians insist that there are values for a, b and c at all times. So rather than press the point, I thought this would be a reasonable compromise.

If there is an a, b and c at all times, I would see some theoretical problems with that (and would be glad to elaborate). But honestly, I don't know enough about BM to really argue the point one way or the other.

 

[DrChinese]

If you only measure one photon,...

I said "independent of observation". Does a single photon have simultaneous definite polarization at 0/12/240? I am not asking whether you believe one way or the other, I am saying EPR saw this as the essential question. They thought a particle - any particle, entangled or not - had all attributes at all times. And the idea of perfect correlations for entangled pairs supports this idea very strongly.

 

[Ilja]

1. There is NO connection between a, b and c as elements of reality, and locality. These are separate assumptions. Elements of reality are predictable with certainty, and it matters not for this definition whether locality is a constraint or is not.

2. You are confused. Realism is not only the result of experiments that can be performed. It INCLUDES counterfactual questions too.

3. As I said: you can't get a Bell inequality without a, b and c in one formula. You must ASSUME the existence of something which cannot be demonstrated by experiment: the simultaneous existence of a, b and c.

Please answer this question: does a single photon have simultaneous definite polarization values at 0, 120 and 240 degrees? (Probably an unfair question to ask a Bohmian though. :) )

1. There is, because without locality the EPR criterion of reality would give nothing. The results of the measurements could be created by accident once measured, and their result FTL-communicated to the other particle. And the proof would fail.

2. Of course, there may be counterfactual questions, and some realistic theories may answer some of them, but realism is not obliged to answer them. What is the color of the invisible unicorn? A counterfactual question. If one refuses to answer "pink", this does not mean that one rejects realism.

3. Of course, a, b, and c are in one formula. But I have an alternative to ASSUME, namely to DERIVE their existence. This is done by using the EPR criterion of reality and Einstein locality. Which are assumptions, but different from a naive realism about particle spins.

Once I prefer a field ontology instead of a particle ontology, your question seems quite meaningless to me. I do not ask other people about particular properties of particular phonons too, and photons have, for me, the same status as phonons, namely of no fundamental importance.

 

[carllooper]

Entangled particles, a and b, don't have definite values, but they do have a definite relationship, eg. a == not b.

The relationship can be said to be real in terms of the various definitions of realism, ie. as there prior to measurement and/or following measurement. The probability of the values (compared to a single particle) is in terms of a pair of values, rather than a single value.

a=1, b=0
or
a=0, b=1

The relationship between a and b is non-local in the sense that it is independant of where/when measurements are made. It is defined that way. As non-local.

 

[Ilja]

But most Bohmians insist that there are values for a, b and c at all times.

Among Bohmians may be stupid people too. Those who are not, are able to recognize that the consequence would be that Bell's inequality would hold in BM, which would be fatal. As for experimental tests (which tell it is violated) as for mathematics (once there is an equivalence proof with quantum theory, and a prediction of quantum theory that it is violated, thus, there should be an error in the mathematics ;-) ).

 

[bohm2]

I think the problem is in terminology and definitions, because many people do not consider
i) contextuality,
ii) observer-dependent reality, and
iii) non-reality
to be the same.

In fact, almost nobody (but you may be one of rare exceptions) considers i) and iii) to be the same.

This is the part that confuses me. It isn't only DrC who uses sees contextuality as implying non-realism. There are a number of other authors like Nieuwenhuizen, Hess, Krennikov, Accardi, Pitowsky, Rastal , Kupczynski, de Raedt, etc. who see contextuality in a somewhat similar light but draw different conclusions:

Andrei Khrennikov took the position that violations of Bell inequalities occur in Nature, but do not rule out local realism, due to lack of contextuality: the measurements needed to test Bell inequalities (BI) such as the BCHSH inequality cannot be performed simultaneously.

http://dare.uva.nl/document/2/104604

So, whereas DrC sees contextuality as implying some type of "realism" assumption in Bell's along with locality, this other group tries to argue that "no conclusion can be drawn on local realism, since incompatible information cannot be used to draw any conclusion." This is confusing the hell out of me. Even after 50 years there seems to be 3 views on implications of Bell's theorem and violations of BI:

1. A small minority that see no implications on locality nor realism (Krennikov and group)
2. A growing minority that see confirmation of Bell's theorem and violations of BI as implying non-locality irrespective of other "realism" issues (e.g. hidden variables / pre-existing values / counter-factual definiteness issues).
3. The majority who see Bell's as implying a choice between non-locality and "realism" (where realism=hidden variables / pre-existing values / counter-factual definiteness)

 

[atyy]

This is confusing the hell out of me.

There is, I think, very little controversy, as long as one defines what one means.

For example, Gisin http://arxiv.org/abs/0901.4255 and Hall http://arxiv.org/abs/0909.0015 might appear to disagree, with Gisin saying Bell inequality violation implies "no locality" while Hall saying it implies "no local realism". But then you find out that Hall is mainly saying that "realism" is a technical term which is better captured by "determinism".

You can also find Maudlin http://arxiv.org/abs/1408.1826 and Cavalcanti and Wiseman http://arxiv.org/abs/0911.2504 appearing to disagree, with Maudlin saying Bell inequality violation implies "no locality" while Cavalcanti and Wiseman say it implies "no local determinism". But then you find out that Maudlin's "locality" is Cavalcanti and Wiseman's "local causality". So Cavalcanti and Wiseman do say, appearing to support Werner against Maudlin, that the Copenhagen interpretation is local and non deterministic - but it's not clear whether they do, because their "local" seems different from Maudlin's "local". For example, Wiseman http://arxiv.org/abs/1402.0351 agrees with Maudlin that if "locality" is "local causality" then the violation does rule out "locality". Also, Hall's "local determinsim" is not the same as Cavalcanti and Wiseman's "local determinism" because Hall means "signal local" while Calalcanti and Wiseman mean Bell 1964 "local".

Also Norsen http://arxiv.org/abs/quant-ph/0607057 and Cavalcanti and Wiseman http://arxiv.org/abs/0911.2504 might appear to disagree because Norsen says the Bell inequalities are not about "local determinism" while Calvalcanti and Wiseman say it is. But of course they mean different things.

I might not have gotten everything right above, because I do find the terminology confusing. But my main point is that I think there is overall very little disagreement. As carllooper said in a different thread, once you know what they mean, it doesn't matter even if they say the wave function is a chicken. :)

 

[DrChinese]

1. There is, because without locality the EPR criterion of reality would give nothing. The results of the measurements could be created by accident once measured, and their result FTL-communicated to the other particle. And the proof would fail.

What proof? EPR? Or Bell?

EPR's elements of reality mean that you can predict something with certainty. It doesn't matter whether someone is whispering in your ear or what, and it certainly doesn't matter where the source of the answer is (near or far). The question EPR attempted to answer was whether there were more elements of reality than QM could account for. EPR knew that only 1 could be predicted at a time, just like QM. But they also noticed that ANY element could be predicted. So that led them to hypothesize that ALL were essentially predetermined in some fashion. That is, unless the observer's choice was a factor.

Assuming it wasn't: the EPR conclusion was that all elements of reality existed simultaneously. That assumption was what Bell tackled in his proof. It had nothing to do with locality per se. But the problem was: if there is observer dependence, what is the mechanism by which Alice's choice of what to observe seems to influence Bob's outcome? That has something to do with locality, sure, but there is nothing about that which is actually new to QM.

The fact is: the Bell proof does NOT depend on locality so much as separability of a, b and c. Separability of course can be violated if there is instantaneous action at a distance. Which is what quantum non-locality appears to demonstrate. Collapse is immediate, however it happens or whatever it is.

 

[DrChinese]

I might not have gotten everything right above, because I do find the terminology confusing. But my main point is that I think there is overall very little disagreement. As carllooper said in a different thread, once you know what they mean, it doesn't matter even if they say the wave function is a chicken. :)

Pretty good job of it I thought!

We all accept Bell anyway. So we can't be that far off from each other. :-) And in the end, that is actually what this thread is about... IE 50 years of Bell and we are still debating the details!

 

[stevendaryl]

I said "independent of observation".

EPR's "elements of reality" might be independent of observation, in the sense that they exist whether or not we observe anything, but they are not independent of observation in the sense that we posit elements of reality in order to account for our observations. You know, Occam's razor, you don't make up elements of reality above and beyond those necessary to explain what we observe.

So if you ask the question of whether a photon's polarization is an element of reality, whether we hypothesize that there is such a thing as photon polarization depends on what the experimental evidence is. To give an analogy: you flip a coin, and you slap the coin to the ground, and you get a result "heads" or "tails". Its being "heads" might not correspond to any "element of reality" prior to slapping the coin to the ground, because the result might cooperatively be created by the state of the coin and also by the details of how you slap it to the ground. Photon polarization could conceivably be the same sort of thing, that whether the photon passes a filter at angle \vec{a} might depend on details of the filter, as well as details of the photon. A different filter might have produced a different result. But that's where entanglement provides evidence to the contrary: the predictability of a polarization measurements for one photon from measurements on the other photon imply that there is nothing about the details of the detector that are relevant (at least if we ignore exotic causality, such as FTL, back-in-time, superdeterminism, etc).

Does a single photon have simultaneous definite polarization at 0/12/240? I am not asking whether you believe one way or the other, I am saying EPR saw this as the essential question. They thought a particle - any particle, entangled or not - had all attributes at all times. And the idea of perfect correlations for entangled pairs supports this idea very strongly.

Surely, EPR realized the possibility that a measurement result could depend on details of the measuring device, as well as details of the object being measured? That's a possibility classically for things such as the result of a coin flip.

 

[DrChinese]

EPR's "elements of reality" might be independent of observation, in the sense that they exist whether or not we observe anything, but they are not independent of observation in the sense that we posit elements of reality in order to account for our observations. You know, Occam's razor, you don't make up elements of reality above and beyond those necessary to explain what we observe.

So if you ask the question of whether a photon's polarization is an element of reality, whether we hypothesize that there is such a thing as photon polarization depends on what the experimental evidence is. ...

Surely, EPR realized the possibility that a measurement result could depend on details of the measuring device, as well as details of the object being measured? That's a possibility classically for things such as the result of a coin flip.

EPR didn't say what was real, just that something was real IF you could predict with certainty the result of a measurement (and as you say, the measuring device could be a factor too). We all agree that you can predict the result of ANY measurement on an entangled photon by observing its partner. But you can only do that for 1 non-commuting observable at a time. Even in 1935, this point was not in serious doubt (although admittedly it was early on).

So they used what everyone agreed as the basis for their argument, and then asked: if they are separately elements of reality, are they collectively so?

No need to jump ahead and talk about how to explain entanglement or what experiments show. The question EPR & Bell were addressing was specifically: whether a, b and c existed simultaneously (Bell used these 3, that's why I'm so certain of this) and if so, was their relationship consistent with QM's predictions? You do not need to consider locality at all. Locality is an issue to discuss as to mechanism as to explain experimental results. The reference in Bell to Bohmian Mechanics is strictly superfluous, and ditto for any comments about locality. If you assume a, b and c are observer independent (which is the proposition), then you are saying that Alice's outcome will always be independent of Bob's setting.

So Bell proves observer independence false. Even for non-local observers. That is where locality comes in, because non-local observer Bob could potentially change the outcome of a measurement by Alice that would have yielded a different result if Bob had done something else. EPR specified that the prediction (with certainty) had to be made without disturbing the system.

Bohr would object anyway (I guess) that there is no way to have a quantum system of 2 entangled particles and expect a measurement on one NOT to have an effect on the other. In other words, that there are no elements of reality until a measurement is performed. :-)

 

[stevendaryl]

No need to jump ahead and talk about how to explain entanglement or what experiments show. The question EPR & Bell were addressing was specifically: whether a, b and c existed simultaneously (Bell used these 3, that's why I'm so certain of this) and if so, was their relationship consistent with QM's predictions?

But it seems to me that locality is the reason for believing that they all exist simultaneously. The reasoning goes: either (1) the element of reality existed all along (independently of what axis Alice or Bob chooses to measure), or (2) it came into existence at the moment Alice (or Bob) made their respective choices. Case (2) is not consistent with locality: If it came into existence when Alice made her choice, then how could it affect Bob, who is far away?

So, to me, it's not simply a matter of blithely assuming that if they exist separately, then they all must exist simultaneously. The fact that they must exist simultaneously follows from locality (plus the perfect correlations predicted by quantum mechanics).

 

[DrChinese]

But it seems to me that locality is the reason for believing that they all exist simultaneously. The reasoning goes: either (1) the element of reality existed all along (independently of what axis Alice or Bob chooses to measure), or (2) it came into existence at the moment Alice (or Bob) made their respective choices. Case (2) is not consistent with locality: If it came into existence when Alice made her choice, then how could it affect Bob, who is far away?

So, to me, it's not simply a matter of blithely assuming that if they exist separately, then they all must exist simultaneously. The fact that they must exist simultaneously follows from locality (plus the perfect correlations predicted by quantum mechanics).

Locality is not the reason for believing they exist simultaneously. The reason for that is that you can predict ANY attribute in advance (perfect correlations). Sort of a Bertlmann's socks viewpoint led them to your (1). EPR was never suspecting that measuring Bob (to predict Alice) would change Alice's outcome to match anyway - your (2). (Of course entanglement was not well understood at that point anyway so everyone was taking a bit of a leap.)

Note that depending on where you want to place the emphasis (or where I want to), it seems like both of our viewpoints are quite reasonable . :)

What we all are asking is: once you know about Bell's Theorem, where do we go from here? We cannot expect an EPR-like "more complete" solution as this would be ruled out. OK, we all agree on that. Where do we look next? We can dissect Bell a lot of ways, but the important point is once we know about it, our viewpoint is forever changed.

Note that Bohr and others, in 1935, denied the EPR conclusion (QM is incomplete) without the benefit of Bell. And their answer was not "let's talk about locality/nonlocality now". Perhaps someone could have come up with an EPR-like argument that proved QM must be completed by a nonlocal theory. In fact, perhaps all this didn't sit well with Bohm, and so he developed the non-local Mechanics named after him - I don't know much about that. But the overall point is that QM was already "quantum nonlocal" and perhaps it took a while longer for that to become clear.

 

[atyy]

http://dare.uva.nl/document/2/104604

Even here, I don't think there is any big disagreement. The suggestion here is that Bell assumes that the measurement settings and measurement outcomes do not influence the state that is prepared. It is generally agreed that this is an assumption. If we get rid of this assumption, can one come up with a local deterministic model that is consistent with experimental data? The question is discussed in these papers. (The focus here is different from the Maudlin/Werner discussion. Maudlin/Werner are talking about whether quantum mechanics is nonlocal, whereas here the question is whether reality is nonlocal given experimental constraints.)

http://arxiv.org/abs/quant-ph/0110137
Accardi contra Bell (cum mundi): The Impossible Coupling
Richard D. Gill

http://arxiv.org/abs/quant-ph/0301059
Time, Finite Statistics, and Bell's Fifth Position
Richard D. Gill

http://arxiv.org/abs/quant-ph/0205016
Quantum nonlocality, Bell inequalities and the memory loophole
Jonathan Barrett, Daniel Collins, Lucien Hardy, Adrian Kent, Sandu Popescu

http://arxiv.org/abs/1001.1750
The statistical strength of experiments to reject local realism with photon pairs and inefficient detectors
Yanbao Zhang, Emanuel Knill, Scott Glancy

http://arxiv.org/abs/1108.2468
Asymptotically optimal data analysis for rejecting local realism
Yanbao Zhang, Scott Glancy, Emanuel Knill

 

[bohm2]

Even here, I don't think there is any big disagreement.

I see a big difference:

1. One group holds that one can have both locality and realism.
2. Another group holds that nature is fundamentally non-local, irrespective of all other issues (e.g. "realism", determinism, CFD, etc.)
3. Another group holds that we are driven to a choice between non-locality versus non-realism (e.g. local/non-local non-realism versus non-local realism).

I see these 3 views as quite different.

 

[atyy]

I see a big difference:

1. One group holds that one can have both locality and realism.
2. Another group holds that nature is fundamentally non-local, irrespective of all other issues (e.g. "realism", determinism, CFD, etc.)
3. Another group holds that we are driven to a choice between non-locality versus non-realism (e.g. local/non-local non-realism versus non-local realism).

I see these 3 views as quite different.

All three views can be correct in some sense. I'm not sure whether all proponents of the various views will agree with me here, but here are senses in which the three views can all be correct.

1. Local realism of nature is always in play, because real experiments have a finite number of trials, so experiments can only make local realism unlikely, not impossible. Also, it is true that the memory loophole makes local realism of nature less unlikely. It is also true that one cannot rule out that quantum mechanics may have a local explanation in some sense, if one goes beyond theories consistent with Kolmogorov's definition of probability. The references for this are http://arxiv.org/abs/quant-ph/0301059 and others listed in post #111.

2 and 3. If one defines "local" as "local causality" following Bell 1976 and "La Nouvelle Cuisine", then quantum mechanics does not have a local explanation. If one defines "local" following Wiseman's http://arxiv.org/abs/1402.0351 Definition 9 (Eq 2), then the Copenhagen-style interpretation of quantum mechanics is local, and provides a counterexample to the idea that locality is sufficient to derive a Bell inequality. Other references for this are in post #104.

 

[Ilja]

The question EPR & Bell were addressing was specifically: whether a, b and c existed simultaneously (Bell used these 3, that's why I'm so certain of this) and if so, was their relationship consistent with QM's predictions? You do not need to consider locality at all.

You do not need to consider Bell's theorem at all, if you don't like it.

And, without doubt, you do not need to consider the first part of Bell's proof that, given locality and the EPR criterion, they have to exist simultaneously. Ignore the first part, the remaining part remains a valuable contribution to science, suggests experimental tests if the resulting inequalities are really violated.

But the whole really important conclusion - that one has to give up Einstein causality - is, then, forgotten.

I have, by the way, always suggested that "locality" is a very wrong and misleading name. Simply imagine a theory with preferred frame and a maximal speed of information transfer in this preferred frame of $C=10^300 c$. This theory would be unable to create violations of Bell's inequality for pairs of events which are space-like separated in the Minkowski-metric corresponding to C instead of c, but it could be Einstein-causal in the C-metric, and the theory could be realistic and local in any meaningful meaning of these notions but nonetheless in agreement with observation, and approximately (except for pairs of C-spacelime-separated events) in agreement with quantum theory.

Einstein causality is a much better name for what is impossible. Because "signal locality" is only a fact about correlations, not about causation. Causality is something which goes beyond operational observation of correlation, and, thus, closely connected with realism. To use "causality" presupposes the acceptance of Reichenbach's principle of common cause, it implies the idea that there is something like a causal explanation of observable correlations, else it makes no sense.

 

[Demystifier]

Well you got me. :)

It was my intention. By observing you for a long time, I thought you might be an easy prey and designed a logical trap for you, for which I was quite confident you might fall in it. :D

Now seriously.

But most Bohmians insist that there are values for a, b and c at all times.

Actually they don't. All they insist is that there are values for positions of all particles at all times.

So rather than press the point, I thought this would be a reasonable compromise.

By contrast, below I will press my point, without compromise.

But honestly, I don't know enough about BM to really argue the point one way or the other.

I believe I know enough about BM and about all other interpretations, to say the following:
According to your definition of reality, no viable interpretation of QM is realistic, either local or non-local. With such a definition of reality, the Bell theorem can be restated as "QM is non-realistic, period!", without any mentioning of locality or non-locality.

On the other hand, your signature contains a very different formulation of Bell theorem, so your signature is not compatible with your definition of reality.

So if the formulation of Bell theorem "QM is non-realistic, period!" sounds too uncompromising and you want to make a reasonable compromise, then what can you do? My final point is: Abandon your definition of "reality" and adopt a more conventional one! If you do that, the definition of reality you will adopt will become compatible with your signature.

 

[DrChinese]

Einstein causality is a much better name for what is impossible. Because "signal locality" is only a fact about correlations, not about causation. Causality is something which goes beyond operational observation of correlation, and, thus, closely connected with realism.

Yes, I would say that's a goner. :-) A good description indeed.

 

[DrChinese]

1. It was my intention. By observing you for a long time, I thought you might be an easy prey and designed a logical trap for you, for which I was quite confident you might fall in it. :D

Now seriously.

2. Actually they don't. All they insist is that there are values for positions of all particles at all times.3. I believe I know enough about BM and about all other interpretations, to say the following:
According to your definition of reality, no viable interpretation of QM is realistic, either local or non-local. With such a definition of reality, the Bell theorem can be restated as "QM is non-realistic, period!", without any mentioning of locality or non-locality.

4. On the other hand, your signature contains a very different formulation of Bell theorem, so your signature is not compatible with your definition of reality.

1. I trapped you into trapping me. :-)

2. Ah, that sounds more Bohmian-like than what I said.

3. It would not surprise me that ultimately, realism and locality can't be separated into completely distinct concepts - and we rule out both. To explain Bell correlations, you need a non-local mechanism. On the other hand, no interpretation has the ability to postulate more information than the HUP allows (i.e. realism, hidden variables, a/b/c, etc.).

4. My signature is good, certainly a solid representation of Bell's Theorem. It may seem inconsistent to what I have said in this thread in some respects, but I don't think the overall effect of Bell hinges on the meaning of a word. We use Bell to weed out candidate theories left and right. And after all this time, new interpretations and all, QM sits there as it has for a long time. And we still debate things it tells us, just as in 1927 - but with even greater appreciation.

 

[DrChinese]

A timely 50th anniversery item appeared in arxiv today I think you will enjoy:

http://arxiv.org/abs/1411.5322

If you think our views vary a bit but are in essential agreement, here are some quotes from the SAME paper:

1. Amazingly, just by relying on conditions (3) and (4) one can construct so-called No-Go Theorems that arrive at a contradiction. [Note: 3 is
f(A;B;C;...) = 0 and 4 is similar. I.e. these are requirements of realism.]

2. Bell's profound discovery was that the requirement of locality is incompatible with the statistical predictions ofquantum mechanics

3. Local realistic theories are incompatible with quantum mechanics!

All of the above from Bertlmann, who knew Bell quite well. As we know, Bell was also an expert in Bertlmann's socks. This paper covers a lot of ground, and has some cool anecdotes about Bell.

 

[atyy]

2. Another group holds that nature is fundamentally non-local, irrespective of all other issues (e.g. "realism", determinism, CFD, etc.)
3. Another group holds that we are driven to a choice between non-locality versus non-realism (e.g. local/non-local non-realism versus non-local realism).

As Wiseman points out, this depends on definitions. One new way of defining locality that makes locality alone insufficient, and requires one to add determinism is to use signal locality. Signal local theories include Bell local theories, and the Bell local theories can be excluded by requiring "intrinsic randomness". This seems to be an old idea, but apparently only proved by Masanes, Acin and Gisin in http://arxiv.org/abs/quant-ph/0508016.

So this means there are two different ways to define locality so that it is insufficient to define Bell locality. The first definition of local is that local actions cannot affect distant local observations. The second definition is local in the sense of no faster than light communication. Of course there is still another way of defining locality so that locality alone is sufficient is to use the concept of local causality, or Ilja's term "Einstein causality", to derive the mathematical condition of separability. If I were to put it in words, I think local causality or Einstein causality is the idea that nonlocal correlations have local explanations.

Basically, once you reach the mathematical condition of separability you can derive the Bell inequalities. So the question is what are your definitions and assumptions in reaching separability.

 

[atyy]

Einstein causality is a much better name for what is impossible. Because "signal locality" is only a fact about correlations, not about causation. Causality is something which goes beyond operational observation of correlation, and, thus, closely connected with realism. To use "causality" presupposes the acceptance of Reichenbach's principle of common cause, it implies the idea that there is something like a causal explanation of observable correlations, else it makes no sense.

Another group that often mentions Reichenbach's principle are computer scientists and statisticians who use Bayesian networks, causal networks or graphical models. These are very popular in biology, so I instinctively picture the separability condition as a graphical model. It's interesting how similar Bell's description of local causality and Pearl's description of causality in Bayesian networks is. Norsen http://arxiv.org/abs/0707.0401 quotes Bell: "completely shields off", while in Bayesian networks http://en.wikipedia.org/wiki/Markov_blanket one says "The Markov blanket of a node contains all the variables that shield the node from the rest of the network." A paper discussing Bell's theorem using Bayesian networks is http://arxiv.org/abs/1208.4119, which mentions Reichenbach's principle.

I wonder whether Bell and Pearl came up with the language independently, or whether they knew each other's work. Nowadays, Bell is well known in Bayesian networks, and is mentioned in textbooks, but what is the history of it?