QM Assumptions Regarding Entanglement Properties

[zonde]

EPR refers to the..... "value of a physical quantity" that can be "predicted with certainty"........ . In other words the (unspecified) physical quantity has a value which would be revealed ......"for any possible measurement" (WIKI)

In other words if we can "predict with certainty the value" That value would be known (pre-exist) when and if we confirm the prediction by making suitable observations (any possible measurements)

I would like to ask you: according to EPR should we assume that physical quantity has preexisting value when we can't predict it with certainty (but we can measure it)?

 

[Jilang]

It seems clear that hidden variables cannot be coded in the probability, however they can be in the probability amplitudes, see here as SD demonstrates at post #66.
https://www.physicsforums.com/threa...-bell-correlations.930853/page-4#post-5885421
The issue we all have, as I see it, is that probality amplitudes don’t need to be be “real” and can be complex. I would suggest that this should be interpreted a quantum reality (as opposed to a classical one) that the maths describes beautifully, but that we don’t quite understand it yet. (There is no need to dispense with locality or reality right now).

 

[Mentz114]

I would like to ask you: according to EPR should we assume that physical quantity has preexisting value when we can't predict it with certainty (but we can measure it)?

If I could add a comment on this tricky subject. Surely it depends on which physical quantity you consider ?
The Hamiltonian ($H$)requires that there be values for its components at all times. So if there is a potental term and and a kinetic term in $H$ then $p$ and $x$ must have values even when not measured. The confusion comes if $H$ has an angular momentum term. $H$ does not tell us about the orientation of the angular momentum which is something we can measure. In order to measure it we construct ( or assume) a coordinate system and use apparatus whose alignment determines the direction of the measurement. The measurement is projective and some objects whose spin alignment is not exactly along and axis will be re-aligned so they are. This is all well known and understood and it is the origin of the assertion that we cannot sensibly say that an object has spin alignment in this or that direction until the measurement is carried out. There's nothing strange here. Until we go through the mental and physical processes of setting up and measuring it is meaningless to ascribe a value to spin orientation.

 

[Jilang]

If I could add a comment on this tricky subject. Surely it depends on which physical quantity you consider ? This is all well known and understood and it is the origin of the assertion that we cannot sensibly say that an object has spin alignment in this or that direction until the measurement is carried out. There's nothing strange here. Until we go through the mental and physical processes of setting up and measuring it is meaningless to ascribe a value to spin orientation.

Just so. But are we able to describe a hidden variable that will predict the probability of what will happen when such a measurement is made? It seems that on some level we can, but the description is not “real” in the way classical physics would describe it.

 

[Mentz114]

Just so.
But are we able to describe a hidden variable that will predict the probability of what will happen when such a measurement is made?

The Bell experiment is conducted on prepared (pre-projected) states and we can say that this state is a superposition $a|\uparrow\downarrow\rangle + a|\downarrow\uparrow\rangle,\ \ 2 |a|^2=1$. My own view is that the value of the preparation is already chosen and is actually fixed ( pre-projected) to $|\uparrow\downarrow\rangle$ or $|\downarrow\uparrow\rangle$. It is the 'hidden' variable. It is said that this violates EPR/Bell but I don't see how.

 

[Jilang]

The Bell experiment is conducted on prepared (pre-projected) states and we can say that this state is a superposition $a|\uparrow\downarrow\rangle + a|\downarrow\uparrow\rangle,\ \ 2 |a|^2=1$. My own view is that the value of the preparation is already chosen and is actually fixed ( pre-projected) to $|\uparrow\downarrow\rangle$ or $|\downarrow\uparrow\rangle$. It is the 'hidden' variable. It is said that this violates EPR/Bell but I don't see how.

Like I said. It is encoded in the amplitude rather than the probability and only probabilities are real.

 

[Mentz114]

Like I said. It is encoded in the amplitude rather than the probability and only probabilities are real.

Referring to @stevendaryl s post, he concludes

I don't know physically what it means that amplitudes, rather than probabilities factor, but it shows that quantum problems are often a lot simpler in terms of amplitudes.

It seems to support my own inclinations ( as if that matters ).
But whether this means we can have a hidden variable - I can't answer that now or maybe ever. I mean non-local HVs have not been ruled out have they ?

 

[Dadface]

To my mind, these papers need no further clarification.

But I think they were clarified. According to the Stanford Encyclopaedia of Philosophy the original Bell paper was "relaxed" in later years. By Bell himself in 71, 85 and 87 and also by others including Clauser, Horne , Mermin Aspect and others. I don't know if these later works give greater insights.

 

[Dadface]

I would like to ask you: according to EPR should we assume that physical quantity has preexisting value when we can't predict it with certainty (but we can measure it)?

This is the sort of question I have been trying to get other peoples opinions on. See the opening question in post one. I have my own opinion about the answer but am not yet convinced that what i think is correct. As you probably know personal opinions can change for various reasons for example reading more about the subject. And I am in the process of doing just that, the trouble is finding the time to do it.

 

[zonde]

This is the sort of question I have been trying to get other peoples opinions on. See the opening question in post one. I have my own opinion about the answer but am not yet convinced that what i think is correct.

My answer is that EPR argument does not say (assume) anything about measurements that can not be predicted with certainty. Basically it is irrelevant to EPR argument.

In a nutshell I think that in local realistic theories it is assumed that:

Each entangled object has definite properties at all times, even when not observed.

I know the assumption is proved to be incorrect but is that an assumption actually made in such theories?

We can only speak about hypothetical local realistic theories of QM phenomena. Apart from that in What Bell Did Maudlin criticizes viewpoint that Bell inequality violations falsify only local hidden variable theories. His argument is that EPR argument show inconsistency between QM and local indeterministic models and Bell extends the argument to local deterministic models. So that EPR+Bell covers all local models that could reproduce QM predictions and show them inconsistent with QM.

But what assumptions about properties, if any, are made in QM? Are either of the following assumptions made?

When not observed each object has the property of existing in all possible states simultaneously but observations reveal one state only for each object.

Each object cannot be described as having properties at all, until and unless an observation is made.

Are there other assumptions and do the assumptions made depend on what interpretation of QM is used?

I would say that minimal QM gives only statistical prediction about measurements and does not assume anything about individual objects. So assumptions about individual objects should be viewed in context of QM interpretations.

 

[Dadface]

My answer is that EPR argument does not say (assume) anything about measurements that can not be predicted with certainty. Basically it is irrelevant to EPR argument.We can only speak about hypothetical local realistic theories of QM phenomena. Apart from that in What Bell Did Maudlin criticizes viewpoint that Bell inequality violations falsify only local hidden variable theories. His argument is that EPR argument show inconsistency between QM and local indeterministic models and Bell extends the argument to local deterministic models. So that EPR+Bell covers all local models that could reproduce QM predictions and show them inconsistent with QM.I would say that minimal QM gives only statistical prediction about measurements and does not assume anything about individual objects. So assumptions about individual objects should be viewed in context of QM interpretations.

Thank you. I will try to read the Maudlin paper despite the fact that two things put me off.
1. 28 pages!
2. Arxiv. Has Maudlin been accepted by a mainstream journal?

 

[zonde]

2. Arxiv. Has Maudlin been accepted by a mainstream journal?

In arxiv abstract page there is a field "Journal reference". So you can check which arxiv papers are published and where.
For this paper it is: "Journal reference: J. Phys. A: Math. Theor.47 424010, 2014"

 

[Simon Phoenix]

But what assumptions about properties, if any, are made in QM? Are either of the following assumptions made?

Sorry for coming to this a bit late. Been a tad busy

My advice for anyone trying to understand Bell's inequality is to completely forget about QM. The inequality itself is absolutely nothing to do with QM - it is a restriction on plain old probabilities.

What is the BI about then? Well we imagine 2 locations - say Alice's Lab, and Bob's Lab. There's some measuring device at each location. Each device has a dial that can be set to various values - and the devices also have a readout to give the result obtained during the measurement.

So nothing quantum, no assumption about anything at all - just settings and measurement results - just data.

We imagine that Alice and Bob do a whole series of runs of this experiment and then look at the data. So they're going to be able to work out (from the data) things like the probability of getting some result. They're also going to be able to work out (from the data) the probability of getting some result $given$ a particular setting that they chose. And if they get together at some later stage they can also pool their data to work out the joint probabilities.

Let's imagine they've got together to look at their joint data. They find that there's some evidence that their data are correlated. They want to explain this - correlation cries out for explanation. Surely there's some connection between the things they've measured if they're seeing a correlation?

So they make the assumption that there are some set of properties (unmeasured in their experiments) that is the underlying cause for the observed correlation in the data.

So experimentally they can work out the probabilities of particular results $given$ particular device settings, $P(A,B | a,b)$, where $A,B$ are the measurement results they get, respectively and $a,b$ are the respective measurement device settings they chose.

Their assumption of some underlying cause means that really, if they could somehow know the underlying properties, they would have $P(A,B | a,b, \lambda, \mu, . . .)$ where $\lambda, \mu, . . .$ are the values of these underlying properties. It turns out that we can lump all of these underlying properties together and just use the single symbol $\lambda$ to represent all of them. So $\lambda$ just means some set of properties.

These properties 'explain' the observed correlation. What does this mean? Well it means that if we've taken account (or we know) $all$ of these properties then any left over fluctuation in the data has to be independent (if it weren't, if there was still some correlation left, then we wouldn't have captured all of the underlying properties). That means we can write $$P(A,B | a,b, \lambda ) = P(A | a,b, \lambda ) P(B | a,b, \lambda )$$Now of course it would be rather strange to assume that the results in Alice's Lab depend in some way on the $settings$ in Bob's Lab (and vice versa). If there was some dependence we'd have to explain that - there'd have to be some connection, some difference to Alice's set-up when Bob turned his dial to another setting - colloquially we might say that Alice's experimental set-up would 'know' about any changes made to Bob's configuration. So it's very natural to assume that no such connection exists. This is the 'locality' assumption - and it's very reasonable, as you can see!

The upshot is that the conditional joint probability can now (with this locality assumption) be written as $$P(A,B | a,b, \lambda ) = P(A | a, \lambda ) P(B | b, \lambda )$$The last piece is the 'realism' bit - this gets used later on in the derivation where an assumption is made in the math. This assumption is tantamount to saying that properties exist independently of measurement. This is given the fancy name of 'counterfactual definiteness' - but it's really nothing more than a cornerstone of classical physics - in a nutshell it's saying that if I have an object I can measure its position, but I could have measured it's momentum instead an I'd have gotten such and such a value. If you think about it - it's pretty much an underlying assumption of all classical physics. The term 'counterfactual definiteness' just makes it sound like something mysterious and intellectual.

With these entirely reasonable assumptions it can then be shown that there exist constraints on the probability functions - not all choices of function will be consistent (this kind of result, in a totally different context, was derived by Boole a century before Bell - so it's known in classical probability theory). It simply says that given joint distributions of random variables the marginal distributions are constrained. The constraint for our experimental set-up above is, of course, simply the Bell inequality.

No QM here so far - no assumptions of any mechanisms, no 'fields', no 'particles', just measurement results and the probabilities that can be worked out from them and some very natural assumptions about what might be causing any correlation between the data.

The thing is, as we know, there are physical systems we can examine - and when we do the experiments we find the probabilities we work out from the data are not constrained as we expect from the analysis. Therefore at least one of the assumptions we've made in the analysis can't be correct. They might all be incorrect, but at least one has to be decidedly iffy.

So as far as QM is concerned (which does predict the right experimental result) we're saying that QM cannot be wholly replaced by any theory which makes all of these natural assumptions. That's Bell's theorem.

Don't know whether this answers your question or not - but hope it helps frame things a bit.

 

[zonde]

The last piece is the 'realism' bit - this gets used later on in the derivation where an assumption is made in the math. This assumption is tantamount to saying that properties exist independently of measurement. This is given the fancy name of 'counterfactual definiteness' - but it's really nothing more than a cornerstone of classical physics - in a nutshell it's saying that if I have an object I can measure its position, but I could have measured it's momentum instead an I'd have gotten such and such a value. If you think about it - it's pretty much an underlying assumption of all classical physics. The term 'counterfactual definiteness' just makes it sound like something mysterious and intellectual.

Counterfactual thinking is post factum "what if?" type of analysis. But Bell theorem is not talking about reality, but about hypothetical models (of reality) instead that could explain entanglement and satisfy locality assumption. And obviously any scientific model represents ante factum "what if?" type of analysis (as it has to make predictions). So claiming that Bell theorem assumes 'counterfactual definiteness' is just red herring.

 

[Lord Jestocost]

I think I have the general idea about entanglement, Bell and Bell tests but I'm stuck on what I think are very relevant assumptions made by local realists and by QM adherents.

Regarding the term "local realism" in conjunction with Bell's theorem it might be of interest to have a look at Travis Norsen's paper "Against ‘Realism’ " (https://arxiv.org/abs/quant-ph/0607057).

 

[DrChinese]

Regarding the term "local realism" in conjunction with Bell's theorem it might be of interest to have a look at Travis Norsen's paper "Against ‘Realism’ " (https://arxiv.org/abs/quant-ph/0607057).

Although I would comment that paper is selling a very specific version of that term, one which fits Travis' Bohmian world view. His view is that Bell leads us to reject Locality, that there is no option to reject Realism alone.

So I would say the paper is not a mainstream view, although it does have its believers. The mainstream view is that either Locality or Realism, or both, are incompatible with Quantum Mechanics.

 

[Nugatory]

Regarding the term "local realism" in conjunction with Bell's theorem it might be of interest to have a look at Travis Norsen's paper "Against ‘Realism’ " (https://arxiv.org/abs/quant-ph/0607057).

By the way, Travis Norsen is a well-regarded member here, although these days he visits more often than he posts.

 

[DrChinese]

By the way, Travis Norsen is a well-regarded member here, although these days he visits more often than he posts.

Well regarded certainly. Though I'm afraid I might be the cause of him not posting more often, precisely due to that paper. Not that it's not a good paper, it is.

 

[jerromyjon]

So I would say the paper is not a mainstream view, although it does have its believers.

He specifies in that paper that it isn't a very popular view at all, that most just accept realism at face value. I found his logic quite convincing!

 

[Nameless_Paladin]

This thread seems to be going in different directions which is fine by me. I would however appreciate it if anyone could come up with the following.

A rigorous yet simple (one that can be understood by an interested amateur) description of what exactly it is that Bell's theory disproves.
Thank you

Unfortunately some of the comments on this thread, though not necessarily wrong, are quite vague and can easily be misinterpreted by someone who isn't well-versed in the history of the Bell-EPR debate. So let me just summarise the argument of Bell's theorem for you with clarity.

The question that Einstein, Podolsky and Rosen raised with their argument for the incompleteness of Quantum Mechanics was this - Is it possible for an appropriately constructed classical (statistical) theory, built on the foundations of classical probability theory, to replicate all of the experimental predictions of a Quantum Theory without having to resort to abstract and unintuitive notions involving non-classical probabilities, non-commutativity of observables and so on? Such a classical theory may even rely on hidden-variables which may be out of reach of the experimenter but whose knowledge would, in principle, allow for deterministic predictions for the outcomes of all experiments.

Although the EPR argument itself (on the possibility of measuring observables that aren't simultaneously diagonalizable) is built on false assumptions, because of a misunderstanding of entanglement, this question of whether it is possible to recast Quantum theories as Classical theories remained unanswered for another 30 years or so.

Bell's theorem is just one (particularly strong) restriction (there are several others) on the kinds of classical theories that have any chance of replicating the predictions of Quantum Mechanics. It says "no local and counter-factual definite Classical theory, that is built on the foundations of classical probability theory, can ever replicate the predictions of Quantum Mechanics". In other words, you cannot think of the intrinsic probabilities that arise in Quantum Mechanics as arising from ignorance of local pre-existing properties in a classical sense. That is what Bell's theorem proves.

 

[DrChinese]

Unfortunately some of the comments on this thread, though not necessarily wrong, are quite vague and can easily be misinterpreted by someone who isn't well-versed in the history of the Bell-EPR debate.
...
Bell's theorem is just one (particularly strong) restriction (there are several others) on the kinds of classical theories that have any chance of replicating the predictions of Quantum Mechanics. It says "no local and counter-factual definite Classical theory, that is built on the foundations of classical probability theory, can ever replicate the predictions of Quantum Mechanics". In other words, you cannot think of the intrinsic probabilities that arise in Quantum Mechanics as arising from ignorance of local pre-existing properties in a classical sense. That is what Bell's theorem proves.

Nicely stated. My own view is nearly diametrically opposite to Norsen's. I believe that no realistic theory - local or not - can replicate the predictions of QM. This is not "mainstream" any more than Norsen's view is. But it is probably equally popular if not more so. My point being that subtle changes in definitions can change your conclusions. The mainstream view, though, is defensible in every way. Which is why Bell is such a good paper.

 

[Simon Phoenix]

Counterfactual thinking is post factum "what if?" type of analysis. But Bell theorem is not talking about reality, but about hypothetical models (of reality) instead that could explain entanglement and satisfy locality assumption. And obviously any scientific model represents ante factum "what if?" type of analysis (as it has to make predictions). So claiming that Bell theorem assumes 'counterfactual definiteness' is just red herring.

I disagree : Bell's theorem states that the predictions of QM cannot wholly be replicated by any locally realistic theory.

The term 'local' means that results 'here' do not depend on configuration settings 'there'.

The term 'realism' is a bit harder to pin down but conceptually the crucial ingredient is the assumption that things have properties independent of measurement - which is more or less equivalent to counterfactual definiteness. This assumption is absolutely necessary for the derivation of the Bell inequality; so a long way from being some kind of 'red herring'.

I can't match Paladin's beautifully concise elegant and clear description above - so I'll just re-post one of the key statements there :

In other words, you cannot think of the intrinsic probabilities that arise in Quantum Mechanics as arising from ignorance of local pre-existing properties in a classical sense

If the 'realism', or more correctly 'counterfactual definiteness', is just a red herring, as you suggest, then we wouldn't be able make these kinds of strong statements.

Obviously Bell undertook his original analysis to try to pin down something about entanglement, but I feel it's important to underline that the Bell inequality itself is a constraint on theories that have very natural and reasonable properties we expect from classical thinking - it's saying that any theory we construct that looks like the kind of theory we expect from classical thinking is going to have to satisfy some constraints.

Of course, Bell had his eye on the prize, so to speak, because it was clear that it was possible to violate these constraints with the predictions of QM. Trying to replace QM with a theory that says "it's all classical really, it's just that we don't know the proper underlying bits and pieces" is doomed to failure. And Bell's great achievement was to pin this down in a way that could be experimentally tested.

In a way this was already well-known. In the introductory chapter of his classic textbook Dirac mentions the problems of specific heat capacity where he points out that whatever the (assumed) underlying variables are they cannot behave like traditional classical variables because they don't contribute in the correct thermodynamic way. In other words - adopt a model with some unknown (but classical) hidden degrees of freedom and you predict the wrong specific heat capacities. It's probably the first general argument against classical 'hidden variables'. As I said, Bell eventually pinned this down in a rather breath-taking way, and more importantly in a way that could be tested.

I believe that no realistic theory - local or not - can replicate the predictions of QM

I'm with Dr Chinese on this one. The 'local' bit means that we don't theoretically allow some configuration change in Bob's lab to affect results observed in Alice's lab. If we dispense with this condition and opt for 'realism' we're still in big trouble. If we make the measurements in the 2 labs spacelike separated we get backed into a very tight box - if we want realism we have to accept some kind of FTL effects. However, classical theories are already struggling without this. Even if the measurements are not spacelike separated, so we have the possibility that some information about configuration changes in one lab is accessible in the other without having to assume some FTL signal we're still in a position of having to develop some theory that looks nothing like any classical theory we've ever seen before! So we can have 'realism' - but we're still needing to completely re-wire our understanding of physics to make it work.

 

[zonde]

I disagree : Bell's theorem states that the predictions of QM cannot wholly be replicated by any locally realistic theory.

Is your disagreement semantic in nature? It seems so to me.
And btw did you look at Norsen's paper (given by Lord Jestocost in post #51)? One of the main points in this paper is that term 'realism' in Bell inequality contexts have many different meanings. And I completely agree with this point so I consider it pointless to argue about "locally realistic theories" if you do not state what do you mean by 'realistic' and differentiate if from other meanings of 'realism' as to avoid confusion and possible false references.

The term 'realism' is a bit harder to pin down but conceptually the crucial ingredient is the assumption that things have properties independent of measurement - which is more or less equivalent to counterfactual definiteness.

Ok so you have defined your understanding of 'realism'. Norsen in his paper offers term "Non-Contextual Hidden Variable Theory" and "Naive Realism". I would offer to use "non-contextual HV" because "Naive Realism" often has different meaning more in line with Norsen's "Perceptual Realism" and "Metaphysical Realism".

This assumption is absolutely necessary for the derivation of the Bell inequality; so a long way from being some kind of 'red herring'.

Non-contextual HV are not assumed in Bell's original derivation of inequalities. What is taken as granted is Local Hidden Variables (contextual or non-contextual it does not matter). But still they are not assumed but inferred from locality and perfect correlations as in EPR.

If the 'realism', or more correctly 'counterfactual definiteness', is just a red herring, as you suggest, then we wouldn't be able make these kinds of strong statements.

I still insist on my argument that the term 'counterfactual' is inappropriate in the context of Bell theorem and what is left then is just 'definiteness' that can be equated with 'determinism' or HVs.

 

[zonde]

My own view is nearly diametrically opposite to Norsen's.

One of the main points of Norsen is that term 'realism' in Bell inequality contexts have many different meanings. Is your view opposite to that particular point of Norsen too?

 

[Simon Phoenix]

Is your disagreement semantic in nature? It seems so to me.

Quite possibly

One of the things EPR/Bell stuff seems to generate is a whole ton of philosophical verbiage and nitpicking about the 'precise' meanings of terms. I confess I don't have too much interest in all of that. The seemingly infinite 'nuances' of the term 'realism' that folk dream up are of mild interest at best, to me at any rate.

Ultimately I have a rather plain, and quite possibly over-simplistic, approach. Bell's theorem tells us that you can't replace QM with any natural classical physical theory - where the term 'natural' is (I believe) self-evident to everyone except those of a philosophical persuasion

 

[zonde]

One of the things EPR/Bell stuff seems to generate is a whole ton of philosophical verbiage and nitpicking about the 'precise' meanings of terms. I confess I don't have too much interest in all of that. The seemingly infinite 'nuances' of the term 'realism' that folk dream up are of mild interest at best, to me at any rate.

But please tell me, when you look at some mathematical formula are you interested that all the variables in formula are defined or described? And if someone refers to some formula without doing this would you say that discussion about what variable is what is philosophical verbiage?

 

[Simon Phoenix]

But please tell me, when you look at some mathematical formula are you interested that all the variables in formula are defined or described?

Of course, but in Bell's original work and the later CHSH generalized versions, the math and the terms are all well-defined - what the issue is is that this seems to generate endless speculation on what this implies for things like 'realism'.

Counterfactuals, non-contextuality, etc etc - all very nice grand-sounding phrases that lead to essentially the same thing in the context of EPR/Bell - QM ain't any kind of natural classical theory.

As Paladin and Dr Chinese have pointed out - the EPR/Bell stuff is only one (very nice) realization of this fundamental fact. We could then indulge in trying to pin down to the nth degree of precision exactly how quantum mechanics differs from our classical view of 'reality', but honestly what's the point? It is (for me) a law of diminishing returns.

 

[Lord Jestocost]

The mainstream view is that either Locality or Realism, or both, are incompatible with Quantum Mechanics.

The point is: "Locality" or the classical notion of "Realism", or both, are incompatible with "Quantum Mechanics".

 

[DrChinese]

One of the main points of Norsen is that term 'realism' in Bell inequality contexts have many different meanings. Is your view opposite to that particular point of Norsen too?

People have different interpretations/definitions of Bell realism. But I believe Bell himself had but one meaning in mind in his paper. And that was the Realism of EPR. Specifically, if a physical observable could be predicted with certainty in advance, then it constitutes an "element of reality". Adding the EPR explicit assumption that elements of reality need not be simultaneously predictable to maintain that designation: the collection of those are, as a whole, labeled "realistic". Hence Realism. It can also be labeled equivalently as "Objective Reality" (as this is just another label). "Indeed, one would not arrive at our conclusion if one insisted that two or more physical realities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted."

Bell exploited this point as the basis for his proof, i.e. he assumed EPR's position that they ARE simultaneous elements of reality. It is this kind of Realism which is ruled out by Bell (of course with Locality added by Bell's explicit assumption regarding "a mechanism whereby the setting of one measuring device can influence the reading of another instrument, however remote").

I recognize Norsen's talent and extensive knowledge of the source material, such as Bell. But no, he is adding his own coloring to what Bell wrote - and regardless of what Bell later came to believe.

a) I don't believe in Objective Reality a la EPR. I think most physicists agree with this position as well. We live in a universe in which the choice of measurement basis shapes reality. I.e. there is "Subjective Reality". So you can label me in the "Non-realistic" camp.
b) I believe that an entangled system of 2 spacelike separated particles does not consist of 2 independent particles. I think most physicists agree with this as well. If this meets one's definition of non-locality, then you can label me a believer in non-locality as well.

 

[DrChinese]

The point is: "Locality" or the classical notion of "Realism", or both, are incompatible with "Quantum Mechanics".

I thought that was what I said.

 

[Jilang]

I didn’t know that you had to give up both! I thought it was one or the other...

 

[jerromyjon]

I didn’t know that you had to give up both! I thought it was one or the other...

Everyone is free to do as they wish! It doesn't mean your choice of physical determinism sets the path your thoughts lead... I think that is a bit deeper territory where your realistic, non-local universe sets super-determinism in stone.

 

[Simon Phoenix]

The point, for me at any rate, is the following :

Classical physics, as we know it, is a non-starter for an explanation of QM. And perhaps more generally, local theories built upon variables that have the same properties as those of classical physics aren't good enough either.

Whatever our nuanced definition of a realistic theory might be we can safely say that classical physics (as we know it) is included as one of the theories that doesn't work. Indeed, we have an example of a realistic theory that reproduces QM - Bohmian mechanics - but it's not anything like classical physics as we know it with its unexplained complex guiding potential and its hugely non-local character. In one sense BM is just a trivial mathematical trick; you assume QM and just shuffle the maths a bit. In another sense it's quite profound

So whilst it might be of interest to pin down precisely the exact meaning of 'realism' implied by the Bell/CHSH analyses we can definitely say that classical physics as we know it isn't going to be on the medal podium when the grand prix is over. Classical physics crashed sometime on the 2nd lap and the car can't be rebuilt.

 

[zonde]

People have different interpretations/definitions of Bell realism. But I believe Bell himself had but one meaning in mind in his paper. And that was the Realism of EPR. Specifically, if a physical observable could be predicted with certainty in advance, then it constitutes an "element of reality". Adding the EPR explicit assumption that elements of reality need not be simultaneously predictable to maintain that designation: the collection of those are, as a whole, labeled "realistic". Hence Realism. It can also be labeled equivalently as "Objective Reality" (as this is just another label). "Indeed, one would not arrive at our conclusion if one insisted that two or more physical realities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted."

EPR goes on after that sentence and explain their position what it means to insist on that point: "On this point of view, since either one or the other, but not both simultaneously, of the quantities P and Q can be predicted, they are not simultaneously real. This makes the reality of P and Q depend upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this."
So what is this un-reasonable definition of reality that could permit this? It seems to me that this is superdeterministic reality.
And we can step away from philosophical arguments about reality and could ask: what scientific model (of reality) could allow this? And it seems that there is none at all because superdeterministic models are not scientific.

a) I don't believe in Objective Reality a la EPR. I think most physicists agree with this position as well. We live in a universe in which the choice of measurement basis shapes reality. I.e. there is "Subjective Reality". So you can label me in the "Non-realistic" camp.

I don't think that you are right about most physicists. I think that most physicists do not subscribe to superdeterminism and even less to solipsism. I think that most physicists just do whatever they do and draw comfort from those that say that there still is a gap between superdeterminism or solipsism and non-locality that nature could take. Or alternatively are waiting for someone to come and propose a new theory on which they could work on.

b) I believe that an entangled system of 2 spacelike separated particles does not consist of 2 independent particles. I think most physicists agree with this as well. If this meets one's definition of non-locality, then you can label me a believer in non-locality as well.

Then I will rather stick to this option. Not sure about most physicists however.

 

[zonde]

Whatever our nuanced definition of a realistic theory might be we can safely say that classical physics (as we know it) is included as one of the theories that doesn't work.

Only don't throw the baby out with the bathwater. There are things that are fundamental to science in general. Are sure you can tell apart those essential things from non essential in the body of knowledge you call "classical physics"?