Bell's theorem and local realism

[gill1109]

Whether we say QM violates locality (or local realism) or not depends on our definitions. It seems nowadays *conventional* to say that Bell's theorem shows us that QM is in conflict with locality+realism+no-conspiracy. So if you want to stick with QM (and in particular, if Nature shows that she follows QM in a decisive experiment) we have to reject locality OR realism OR no-conspiracy (aka freedom).

This is just the present-day main-stream way of saying things. It is explained very nicely by Boris Tsirelson in the following encyclopedia article:
http://en.citizendium.org/wiki/entanglement_(physics )

One can say that it is then a matter of taste whether one should reject locality, realism, or freedom. I mean - it is completely optional. Cannot be decided by experiment. Is therefore a matter of taste or of philosophy. It's meta-physics.

Boris does explain very clearly in his article why he thinks that it is wise to keep locality and no-conspiracy but to reject realism. I agree with him; I find his arguments very pleasing. But sure - it is a matter of taste, of philosophy. It is not decidable by experiment. However philosophy is also important in physics since (I submit) the right philosophy generates the right frame of mind for uncovering exciting new physics.

To illustrate this remark: there was a generation of quantum physicists who were kind of brain-washed to think that you can kind of understand QM by simple classical physical notions. e.g. disturbing a system by observing it - nothing weird in that. However the really exciting experiments like Aspect's happened when people took QM seriously, ie took the amazing formalism seriously, and did not try to "explain away" by classical analogy what seemed at first revolutionary in the theory. Instead they embraced what seemed revolutionary in the theory, ie in the formalism, followed it up, and designed daring experiments which showed that it was "for real".

 

[TrickyDicky]

I'm not sure what all you are lumping into the concept of atomism. I also don't understand where you think that atomism comes into play in discussions of Bell's theorem. What Bell's local realism amounts to--as described already by Richard Gill--is basically the idea that any fact about the universe can be factored into facts about tiny little regions of the universe, together with facts about how neighboring regions fit together. Facts about each tiny region can either be continuous (the values of fields) or discrete (the locations, momenta, angular momenta, charges, etc. of particles within the region). There is a second component to local realism that is added by relativity, which is that the evolution of one little region cannot depend on facts about distant regions.

The violation of Bell's inequality implies (in one way of looking at, at least) that there are facts about the universe that don't factor into facts about the little regions making up the universe. I don't see the connection with atomism, though.

I thought I explained clearly that I was introducing atomism as a contextual element not related to the theorem itself but added to it since it is carried as a moreless implicit assumption by most physicist. Adding the two elements(theorem plus atomism) is what leads to what I concluded. Not the theorem by itself. Is it clearer now?

There are actually contrived ways to avoid this conclusion, for instance Bohmian mechanics, but they are usually considered to be basically ad hoc constructions.

 

[bhobba]

Whether we say QM violates locality (or local realism) or not depends on our definitions..

That I STRONGLY agree with.

Thanks
Bill

 

[gill1109]

I thought I explained clearly that I was introducing atomism as a contextual element not related to the theorem itself but added to it since it is carried as a moreless implicit assumption by most physicist. Adding the two elements(theorem plus atomism) is what leads to what I concluded. Not the theorem by itself. Is it clearer now?

There are actually contrived ways to avoid this conclusion, for instance Bohmian mechanics, by they are usually considered to be basically ad hoc constructions.

I would not call Bohmian mechanics contrived or ad hoc; it is wonderfully neat and very satisfying from several points of view ... but it is non-local, *and* it requires an "ether" (preferred reference frame) yet the predictions it makes about reality are independent of what that preferred reference frame is. And it predicts no more and no less than ordinary QM so one could say that it is superfluous. However it can provide mathematical tricks for getting the right answer faster. Just like we can prove things about the real numbers by embedding them in the complex numbers.

 

[bhobba]

it requires an "ether" (preferred reference frame) yet the predictions it makes about reality are independent of what that preferred reference frame is. And it predicts no more and no less than ordinary QM so one could say that it is superfluous.

My view exactly - I couldn't care less about locality - but that aether - that really bothers me.

Thanks
Bill

 

[bohm2]

I would not call Bohmian mechanics contrived or ad hoc; it is wonderfully neat and very satisfying from several points of view ... but it is non-local, *and* it requires an "ether" (preferred reference frame).

Hrvoje Nikolic has published a Bohmian model compatible with relativity. He does it by treating time on an equal footing with space and his model does not involve a preferred Lorenz frame. Some of his stuff can be found here:

Slide Presentation:
Making Bohmian Mechanics compatible with Relativity and Quantum Field Theory
http://www.tcm.phy.cam.ac.uk/~mdt26/tti_talks/deBB_10/nikolic_tti2010.pdf

Relativistic Quantum Mechanics and Quantum Field Theory
http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/nikolic_2010d.pdf

Making nonlocal reality compatible with relativity
http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/nikolic_2010a.pdf

 

[gill1109]

Hrvoje Nikolic has published a Bohmian model compatible with relativity. He does it by treating time on an equal footing with space and his model does not involve a preferred Lorenz frame. Some of his stuff can be found here:

Slide Presentation:
Making Bohmian Mechanics compatible with Relativity and Quantum Field Theory
http://www.tcm.phy.cam.ac.uk/~mdt26/tti_talks/deBB_10/nikolic_tti2010.pdf

Relativistic Quantum Mechanics and Quantum Field Theory
http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/nikolic_2010d.pdf

Making nonlocal reality compatible with relativity
http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/nikolic_2010a.pdf

Nice! Recently also the CSL model has been made relativistically invariant and this means that the same can be done for Belavkin's "eventum mechanics". So the apparent defects of the first versions of these three classes of models were not fundamental, they were just "first rough guesses" which needed careful refinement. I will find the reference later (guy at Imperial college, London).

 

[morrobay]

In this paper: Correlation Functions, Bell's Inequality and Fundamental Conservation Laws.
They are equating the Bell's test experimental outcomes with particles that are realistic objects.
P(a,b)_QM = P(a,b)_C = - cosø

arxiv.org/pdf/quant-ph/0407041.pdf

 

[gill1109]

In this paper: Correlation Functions, Bell's Inequality and Fundamental Conservation Laws.
They are equating the Bell's test experimental outcomes with particles that are realistic objects.
P(a,b)_QM = P(a,b)_C = - cosø

arxiv.org/pdf/quant-ph/0407041.pdf

Here's the abstract:

Correlation functions, Bell's inequalities and the fundamental conservation laws

C. S. Unnikrishnan (Tata Institute, Mumbai)

I derive the correlation function for a general theory of two-valued spin variables that satisfy the fundamental conservation law of angular momentum. The unique theory-independent correlation function is identical to the quantum mechanical correlation function. I prove that any theory of correlations of such discrete variables satisfying the fundamental conservation law of angular momentum violates the Bell's inequalities. Taken together with the Bell's theorem, this result has far reaching implications. No theory satisfying Einstein locality, reality in the EPR-Bell sense, and the validity of the conservation law can be constructed. Therefore, all local hidden variable theories are incompatible with fundamental symmetries and conservation laws. Bell's inequalities can be obeyed only by violating a conservation law. The implications for experiments on Bell's inequalities are obvious. The result provides new insight regarding entanglement, and its measures.

Europhys.Lett. 69 (2005) 489-495
arXiv:quant-ph/0407041

De Raedt has jus done something similar: symmetry + some information principle implies quantum correlation hence incompatible with local realism.

One can't have all attractive fundamental principles at same time. My personal choice: accept fundamental (irreducible) quantum randomness as "real"; reject "realism" = the reality of outcomes of unperformed measurements (rather idealistic, isn't it!?). In particular, give up looking for a LHV theory.

 

[stevendaryl]

Hrvoje Nikolic has published a Bohmian model compatible with relativity. He does it by treating time on an equal footing with space and his model does not involve a preferred Lorenz frame. Some of his stuff can be found here:

Slide Presentation:
Making Bohmian Mechanics compatible with Relativity and Quantum Field Theory
http://www.tcm.phy.cam.ac.uk/~mdt26/tti_talks/deBB_10/nikolic_tti2010.pdf

Relativistic Quantum Mechanics and Quantum Field Theory
http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/nikolic_2010d.pdf

Making nonlocal reality compatible with relativity
http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/nikolic_2010a.pdf

Here's a question that occurred to me while reading the second paper. The author points out that nonlocal interactions are consistent with relativity and causality, provided that the notion of causality is with respect to the scalar parameter $s$ rather than coordinate time. My question is this: What is the difference, conceptually, between (1) N particles moving through 4 dimensional spacetime, and (2) 1 particle moving through 4N dimensional spacetime? It seems to me that locality is the only difference. A single particle through 4N dimensional spacetime can be subject to forces that depend on 4N numbers $x^\mu_a$, (where $a$ ranges over the particles), while in the case of local interactions, N particles in 4 dimensional spacetime, each particle is subject to a force that depends only on 4 coordinates, its own location in spacetime. If you generalize to allow nonlocal forces, then it seems to me that the number of spacetime dimensions is no longer particularly meaningful. In a sense, it is locality that determines (or at least, gives significance to) the number of dimensions of spacetime.

 

[billschnieder]

De Raedt has jus done something similar: symmetry + some information principle implies quantum correlation hence incompatible with local realism.

Are you referring to this article:

http://arxiv.org/abs/1303.4574
Annals of Physics 347, 45 (2014)

It is shown that the basic equations of quantum theory can be obtained from a straightforward application of logical inference to experiments for which there is uncertainty about individual events and for which the frequencies of the observed events are robust with respect to small changes in the conditions under which the experiments are carried out.

...

In the present paper, we demonstrate that the basic equations of quantum theory directly follow from logical inference applied to experiments in which there is
(i) uncertainty about individual events,
(ii) the stringent condition that certain properties of the collection of events are reproducible, meaning that they are robust with respect to small changes in the conditions under which the experiments are carried out.

I did not see a claim by them that their results were incompatible with realism. Unless it's a different paper.

 

[gill1109]

Are you referring to this article:

http://arxiv.org/abs/1303.4574
Annals of Physics 347, 45 (2014)


I did not see a claim by them that their results were incompatible with realism. Unless it's a different paper.

Yes this is the paper I meant. No they don't claim that. Bell's theorem says that. They don't say that Bell was wrong. Bell's theorem (which is a bit of elementary calculus and probability theory) has stood up for more than 50 years now.

Note: de Raedt and Michielsen's many papers on event based simulations of famous experiments do not contradict Bell's theorem, because so far, no experimentalist has actually done (was able to do) the experiment which needs to be done. But they are now at last getting close.

 

[gill1109]

My question is this: What is the difference, conceptually, between (1) N particles moving through 4 dimensional spacetime, and (2) 1 particle moving through 4N dimensional spacetime? It seems to me that locality is the only difference.

Good question. I think your answer is correct. What we mean by locality determines the difference between (1) and (2).

 

[billschnieder]

My question is this: What is the difference, conceptually, between (1) N particles moving through 4 dimensional spacetime, and (2) 1 particle moving through 4N dimensional spacetime? It seems to me that locality is the only difference.

Uhm, the difference is that you have one particle in one case and 4 in the other? 4 separate particles have 4 separate N-dimensional states, while 1 particle will have a single joint 4N-dimentional state. Surely you may have the exact same number of parameters, but the relationships between the parameters and the degrees of freedom involved will be wildly different.

 

[gill1109]

Nice! Recently also the CSL model has been made relativistically invariant and this means that the same can be done for Belavkin's "eventum mechanics". So the apparent defects of the first versions of these three classes of models were not fundamental, they were just "first rough guesses" which needed careful refinement. I will find the reference later (guy at Imperial college, London).

D. Beddingham (2011). Relativistic State Reduction Dynamics. Foundations of Physics 41, 686–704. arXiv:1003.2774

 

[gill1109]

A recently published paper on classical optics seems to make similar suggestions, if I understand correctly what the authors are saying:
" [..] we have presented the first study of nonlocal correlations in classical optical beams with topological singularities. These nonlocal correlations between two different light modes are manifested through the violation of a Bell inequality using the Wigner function for this system of classical vortex beams. [..]
Clearly, the violation of the Bell inequality for classical light fields and the existence of nonlocal correlations bring out totally new statistical features of the optical beams. [..] "
Phys. Rev. A 88, 013830 (2013) - [PLAIN]http://arxiv.org/abs/1307.29...of different continuous outcome measurements.

 

[TrickyDicky]

The problem with BM and the rest of interpretations of QM, is that they are just that, interpretations, one may like one or another based on personal tastes but it doesn't make any difference in the end. BM and many-worlds are sometimes preferred over the rest on the grounds that they are more "realistic" than textbook QM because at least they claim that the wavefunction is something real. But quantum scholars such as Matzkin and Nurock("The Bohmian interpretation of quantum mechanics : a pitfall for realism") make a very good case that i.e BM is as antirealist as Copenhagen. And I see the same antirealism encrusted in many-worlds in the form of basic unfalsifiability of the existence of the other worlds.

All this is hardly surprising as they are just epistemological interpretations of a theory that is as far from scientific realism as they come.

Here is where Bell's theorem powerful tool enters telling us that any theory that explains the outcomes of quantum correlation experiments must be nonlocal. It narrows the possible theories one must consider.

 

[gill1109]

Here is what Bell's theorem powerful tool enters telling us that any theory that explains the outcomes of quantum correlation experiments must be nonlocal. It narrows the possible theories one must consider.

... if one does indeed want a theory which *explains* the outcomes in a "mechanistic" way. One can also choose not to explain the outcomes at all, but accept quantum randomness as a fundamental feature of nature. Not an emergent feature.

 

[TrickyDicky]

... if one does indeed want a theory which *explains* the outcomes in a "mechanistic" way. One can also choose not to explain the outcomes at all, but accept quantum randomness as a fundamental feature of nature. Not an emergent feature.

Sure, that is the usual non-realist "there is no quantum world" camp "a la Bohr".
The zillions of forum threads dedicated to interpretations of the quantum world are testimony that this view leaves many people unsatisfied, which in itself is not a compelling reason to think that it is not the correct way to view it.

 

[stevendaryl]

Uhm, the difference is that you have one particle in one case and 4 in the other? 4 separate particles have 4 separate N-dimensional states, while 1 particle will have a single joint 4N-dimentional state. Surely you may have the exact same number of parameters, but the relationships between the parameters and the degrees of freedom involved will be wildly different.

I don't think they are wildly different if you don't have locality. Let's do things classically, rather than quantum-mechanically. For simplicity, let's just consider 1-D space (so 2-D spacetime) and just two particles. Also, for simplicity, assume that the masses are equal. So the equations of motion are something like:

$m \dfrac{d^2 x_1}{dt^2} = F_1(x_1, x_2)$
$m \dfrac{d^2 x_2}{dt^2} = F_2(x_1, x_2)$

where $x_1$ is the position of particle 1 and $x_2$ is the position of particle 2, and $F_1$ is the force on particle 1, and $F_2$ is the force on particle 2.

Now, that pair of equations is exactly equivalent to a problem in 2-D space (3D spacetime) involving just one particle:

$m \dfrac{d^2 \vec{x}}{dt^2} = \vec{F}(\vec{x})$

where $\vec{x} = (x_1, x_2)$ and $\vec{F}(\vec{x}) = (F_1(x_1, x_2), F_2(x_1, x_2)$.

I don't see any difference at all. It's just a regrouping of parameters, and such a regrouping can't possibly have any physical significance.

If we insist on locality, then there is a big difference, because the force on particle 1 cannot depend on the location of particle 2 (unless they are co-located), and vice-verse. With that restriction, the equations for two particles in 1D space are:

$m \dfrac{d^2 x_1}{dt^2} = F_1(x_1)$
$m \dfrac{d^2 x_2}{dt^2} = F_2(x_2)$

which is not equivalent to a problem in 2D space. So I think that it's really locality that makes the dimensionality of spacetime meaningful.

 

[gill1109]

Sure, that is the usual non-realist "there is no quantum world" camp "a la Bohr".
The zillions of forum threads dedicated to interpretations of the quantum world are testimony that this view leaves many people unsatisfied, which in itself is not a compelling reason to think that it is not the correct way to view it.

Exactly. I think the reason for the dissatisfaction is biological and evolutionary. Our brains are built to *know* that every effect has a cause,"true" randomness does not exist. It scares us deeply or we attribute it to Gods. We do have no problem with action at a distance: Gods can, and do, do that. We know it when we are born. I wrote a small passage on this in my http://arxiv.org/abs/1207.5103 . I'm checking proofs right now for "Statistical Science", it's an invited paper in a special issue on causality.

 

[gill1109]

If we insist on locality, then there is a big difference, So I think that it's really locality that makes the dimensionality of spacetime meaningful.

Exactly. Locality allows a big problem with many dimensions to be decomposed, separated, into many small problems with few.

 

[stevendaryl]

Exactly. I think the reason for the dissatisfaction is biological and evolutionary. Our brains are built to *know* that every effect has a cause,"true" randomness does not exist. It scares us deeply or we attribute it to Gods. We do have no problem with action at a distance: Gods can, and do, do that. We know it when we are born. I wrote a small passage on this in my http://arxiv.org/abs/1207.5103 . I'm checking proofs right now for "Statistical Science", it's an invited paper in a special issue on causality.

I don't think that the dissatisfaction with interpretations of quantum mechanics is really about rejection of determinism. To me, it's not that hard to imagine incorporating nondeterminism into your laws of physics. Rather than having laws describing the state at time $t_1$ to be a deterministic function of the state at time $t_0$, you instead have a probability distribution $P(S_1, t_1, S_0, t_0)$ giving the probability of being in state $S_1$ at time $t_1$ conditional on being in state $S_0$ at time $t_0$. I don't think that would be a huge challenge, conceptually, to make that transition from deterministic Newtonian physics.

But what's confounding about QM is that there doesn't seem to be any good notion of "What is the state at time $t_0$?" There's the wave function, or the density matrix, but that seems to be not a description of the universe, but a description of our subjective information about the universe. I think it's the lack of any coherent notion of what the universe is really doing (as opposed to what experimenters are doing) that is so confounding about QM. Nondeterminism isn't the real problem (although if QM were deterministic, then we would be able to understand the real state of the universe at any time to be the sum total of the information necessary to predict future measurements, so I guess nondeterminism is involved, indirectly).

 

[gill1109]

I don't think that the dissatisfaction with interpretations of quantum mechanics is really about rejection of determinism. To me, it's not that hard to imagine incorporating nondeterminism into your laws of physics. Rather than having laws describing the state at time $t_1$ to be a deterministic function of the state at time $t_0$, you instead have a probability distribution $P(S_1, t_1, S_0, t_0)$ giving the probability of being in state $S_1$ at time $t_1$ conditional on being in state $S_0$ at time $t_0$. I don't think that would be a huge challenge, conceptually, to make that transition from deterministic Newtonian physics.

But what's confounding about QM is that there doesn't seem to be any good notion of "What is the state at time $t_0$?" There's the wave function, or the density matrix, but that seems to be not a description of the universe, but a description of our subjective information about the universe. I think it's the lack of any coherent notion of what the universe is really doing (as opposed to what experimenters are doing) that is so confounding about QM. Nondeterminism isn't the real problem (although if QM were deterministic, then we would be able to understand the real state of the universe at any time to be the sum total of the information necessary to predict future measurements, so I guess nondeterminism is involved, indirectly).

Well if it were just a question of allowing Nature to toss local dice from time to time, no one would have a problem with it. And whether that were random or deterministic would be a matter of taste. One can imagine all the outcomes of all the tosses of all the dice which are going to be needed, being done in advance and stored "inside" the particles or whatever for later use. The trouble is that Bell tells us Nature doesn't do it this way. If Nature is tossing quantum dice, then the probabilities of the different joint outcomes concerning something going on both at A and at B need to depend on information which is only available at A but not at B, and vice versa. There is no way to have separate dice at separate places, the probabilities of the different outcomes for each die only depending on local information. In other words, a die which is "locally manufactured".

Whether such a local die is "truly random" or only "pseudo-random" ... makes no difference.

 

[stevendaryl]

Well if it were just a question of allowing Nature to toss local dice from time to time, no one would have a problem with it. And whether that were random or deterministic would be a matter of taste. One can imagine all the outcomes of all the tosses of all the dice which are going to be needed, being done in advance and stored "inside" the particles or whatever for later use. The trouble is that Bell tells us Nature doesn't do it this way. If Nature is tossing quantum dice, then the probabilities of the different joint outcomes concerning something going on both at A and at B need to depend on information which is only available at A but not at B, and vice versa. There is no way to have separate dice at separate places, the probabilities of the different outcomes for each die only depending on local information. In other words, a die which is "locally manufactured".

Whether such a local die is "truly random" or only "pseudo-random" ... makes no difference.

Of course, QM actually has an interpretation that is similar to your "all the outcomes..being done in advance". You could imagine an enumeration of all possible macroscopic histories of the universe, and at the beginning of time, one is chosen. The information about the chosen history would be embedded in a hidden variable in every single particle, and then each particle just carries out its predetermined program. Such a superdeterministic theory is consistent with QM (or with absolutely any theory of physics), but smacks of being a conspiracy.

 

[gill1109]

Of course, QM actually has an interpretation that is similar to your "all the outcomes..being done in advance". You could imagine an enumeration of all possible macroscopic histories of the universe, and at the beginning of time, one is chosen. The information about the chosen history would be embedded in a hidden variable in every single particle, and then each particle just carries out its predetermined program. Such a superdeterministic theory is consistent with QM (or with absolutely any theory of physics), but smacks of being a conspiracy.

Yep. There's nothing wrong with determinism. But there's a lot wrong with conspiratorial superdeterminism. It explains everything but in a very "cheap" way. It has no predictive power. The smallest description of how the universe works is the history of the whole universe.

 

[TrickyDicky]

Quantum mechanics is not in conflict with locality. There is no action at a distance, no "Bell telephone", no way to use the quantum correlations to communicate instantaneously over some distance. It is only when one hypothesizes an otherwise invisible hidden layer which "explains" those correlations in a classical (mechanistic, deterministic) way that one runs into locality issues.

I disagree, based on Bell's theorem, not on later misconstructions of it.
The theorem rejects locality, period. The subsequent addition of the concept "local realism" that allowed to keep locality if one gave up realistic descriptions of what was going on in order to get the probabilistic outcomes in experiments, was an ad hoc retelling, probably to avoid problems with relativistic QM.

See for instance:
Foundations of Physics, Vol. 37 No. 3, 311-340 (March 2007)
"Against 'realism'" Norsen T

 

[stevendaryl]

I disagree, based on Bell's theorem, not on later misconstructions of it. The theorem rejects locality, period.

It depends on exactly how "locality" is defined. If you define it in terms of interactions--that nothing happening at one event can have a causal influence on something happening at a distant (spacelike separated) event, then QM is perfectly local. Alternatively, you can define it in terms of "local beables" (Bell's term): a theory is local if the most complete description of the state of the world factors into descriptions of what's going on in tiny, localized regions of the world. By that definition, QM is not local, because entanglement means that there are facts about what's going on in distant parts of the world that don't factor into facts about each part separately.

 

[TrickyDicky]

It depends on exactly how "locality" is defined. If you define it in terms of interactions--that nothing happening at one event can have a causal influence on something happening at a distant (spacelike separated) event, then QM is perfectly local. Alternatively, you can define it in terms of "local beables" (Bell's term): a theory is local if the most complete description of the state of the world factors into descriptions of what's going on in tiny, localized regions of the world. By that definition, QM is not local, because entanglement means that there are facts about what's going on in distant parts of the world that don't factor into facts about each part separately.

Your first definition enters in the causes of the nonlocality to avoid confrontation with relativity disallowance of ftl signals, but the theorem works irrespective of the causes, treats them like a black Box.
So it is obvious that is not a valid definition of locality regarding Bells theorem.

 

[gill1109]

Your first definition enters in the causes of the nonlocality to avoid confrontation with relativity disallowance of ftl signals, but the theorem works irrespective of the causes, treats them like a black Box.
So it is obvious that is not a valid definition of locality regarding Bells theorem.

A *reasonable* definition of locality depends on what you take to be *real* hence located in space-time, and what you don't take to be real. Most people find it reasonable to let detector clicks be part of reality (according to MWI they are not real since only the set of possible outcomes is real; one particular branch is imagination). Whether or not the wave function is real and whether or not outcomes of unperformed measurements are real etc etc are questions of metaphysics.

So the definition of *locality* is not absolute, but relative.

 

[gill1109]

Your first definition enters in the causes of the nonlocality to avoid confrontation with relativity disallowance of ftl signals, but the theorem works irrespective of the causes, treats them like a black Box.
So it is obvious that is not a valid definition of locality regarding Bells theorem.

PS I remind you that Boris Tsirelson, who may certainly be regarded as an authority in this field, states that Bell's theorem says that QM is incompatible with locality+realism+no-conspiracy and that the choice of which of those three to reject (taking QM to be true or close to true) is a matter of *taste* or if you prefer *philosophy*.

Sure, there are other authorities who say different things; and perhaps they have different definitions of locality, or perhaps are not so sharp in philosophy as they are in physics. I think that there is presently a consensus among experts on Bell's theorem that Tsirelson's statement is correct, but maybe there is a different broad consensus among physicists at large. So everyone can choose what is the "official line" and indeed according to Tsirelson everyone can choose what they like to believe.

 

[TrickyDicky]

A *reasonable* definition of locality depends on what you take to be *real* hence located in space-time, and what you don't take to be real. Most people find it reasonable to let detector clicks be part of reality (according to MWI they are not real since only the set of possible outcomes is real; one particular branch is imagination). Whether or not the wave function is real and whether or not outcomes of unperformed measurements are real etc etc are questions of metaphysics.

So the definition of *locality* is not absolute, but relative.

I don't think the theorem is about realism, it is an exercise in logic, and it is concerned with locality in a quite specific and well defined way. Insisting in the definition being "relative" or in whether you take the term local as real or not seems to render the theorem totally useless. Like saying: well the conclusion of the theorem depends on the meaning you may want to give to the central concept being proved(since its definition is relative) so that you can make the theorem conclude whatever you like just by adding conditions or that they depend on whether you give a real significance to that concept.
In a theorem the definitions can't be relative in that sense, they better be specifically defined or it is not a theorem.

 

[stevendaryl]

I don't think the theorem is about realism, it is an exercise in logic, and it is concerned with locality in a quite specific and well defined way.

Bell's theorem is an answer to the question: "Can the correlations in EPR be explained by supposing that there are hidden local variables shared by the two particles?" The answer to that question is "no". It's not purely a question about locality, it's a question about a particular type of local model of correlations. The fact that it isn't purely about locality is proved by the possibility of superdeterministic local explanations for the EPR. (On the other hand, if you're going to allow superdeterminism, then the distinction between local and nonlocal disappears, I guess.)

 

[atyy]

I disagree, based on Bell's theorem, not on later misconstructions of it.
The theorem rejects locality, period. The subsequent addition of the concept "local realism" that allowed to keep locality if one gave up realistic descriptions of what was going on in order to get the probabilistic outcomes in experiments, was an ad hoc retelling, probably to avoid problems with relativistic QM.

See for instance:
Foundations of Physics, Vol. 37 No. 3, 311-340 (March 2007)
"Against 'realism'" Norsen T

PS I remind you that Boris Tsirelson, who may certainly be regarded as an authority in this field, states that Bell's theorem says that QM is incompatible with locality+realism+no-conspiracy and that the choice of which of those three to reject (taking QM to be true or close to true) is a matter of *taste* or if you prefer *philosophy*.

Sure, there are other authorities who say different things; and perhaps they have different definitions of locality, or perhaps are not so sharp in philosophy as they are in physics. I think that there is presently a consensus among experts on Bell's theorem that Tsirelson's statement is correct, but maybe there is a different broad consensus among physicists at large. So everyone can choose what is the "official line" and indeed according to Tsirelson everyone can choose what they like to believe.

bohm2 has pointed out on these forums that Wiseman argues that there are two theorems and two definitions of locality, so that it depends on what one is talking about. http://arxiv.org/abs/1402.0351.

 

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I don't think the theorem is about realism, it is an exercise in logic, and it is concerned with locality in a quite specific and well defined way. Insisting in the definition being "relative" or in whether you take the term local as real or not seems to render the theorem totally useless. Like saying: well the conclusion of the theorem depends on the meaning you may want to give to the central concept being proved(since its definition is relative) so that you can make the theorem conclude whatever you like just by adding conditions or that they depend on whether you give a real significance to that concept.
In a theorem the definitions can't be relative in that sense, they better be specifically defined or it is not a theorem.

How about this method of arguing that reality is at least assumed in using a Bell test to disprove nonlocality? The Bell inequality is about the correlation between definite results. In quantum mechanics, we can put the Heisenberg cut however we want. So Bob can deny the reality that Alice had a result at spacelike separation. Bob is entitled to say that he had a result that Alice claimed a result at spacelike separation, but this result is about Alice's claim, which Bob obtained at non-spacelike separation. So there is no spacelike separation, and no Bell test.