Assumptions of the Bell theorem

  • #776
Lynch101
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Well yes. The problem is that the disagreement is about philosophy and not about physics. The indication for that is that obviously we still have not a clear agreement on the meaning of the words, particularly locality. For me locality is simply microcausality. For you obviously it has a different meaning. The same holds for "reality", which is even harder to define. For me reality is objective, reproducible observability, i.e., what can be tested by experiments.
A word that seems to cause less consternation is the word 'universe' or 'nature'. We can define it as 'that which physics seeks to probe', 'that which physics seeks to describe', 'the subject of investigation of physics', or something along those lines. Even if we strictly define 'physics' as 'reproducible observability' it might be the case that there are limits to how far we can probe nature.

The universe itself is not, or at least does not appear to be, reproducible. To what extent the entirety of the universe is observable is a matter of debate.
 
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  • #777
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we still have not a clear agreement on the meaning of the words, particularly locality. For me locality is simply microcausality. For you obviously it has a different meaning.
Did it ever occur to you that locality might be a normal word present in nearly all human languages, and that this is an indication that its meaning is actually pretty clear? And that microcausality might simply one way in which a theory can be local?

I guess microcausality actually implies that there is no faster than light signaling, but the absence of faster than light signaling all by itself is also a way in which a theory can be local. And since more people are able to grasp that meaning, it might even be used more often than microcausality.

And to go further, there appears to be randomness in Bell type experiments, and that randomness seems to include correlations between spatially separated events. And this is one way in which the predictions of quantum mechanics (that have been confirmed in many experiments) seem to not be local. And therefore many people say that quantum mechanics is nonlocal. Can you accept that this meaning does not contradict microcausality, and that therefore both you are right when you point out that quantum theory satisfies locality, but the many other people who say that quantum mechanics is nonlocal are not wrong either?

But more importantly, did it ever occur to you that this is not the fault of philosophy? Philosophy did not invent the concept of locality, because locality is simply an intuivite concept that has been present in human thinking all along. And it is a totally unproblematic concept, if you ask me!
 
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  • #778
Lynch101
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the many other people who say that quantum mechanics is nonlocal are not wrong either?
I think there is an important distinction to be made here. From my reading of discussions on here and elsewhere, it seems that those you refer to are not necessarily saying that QM is nonlocal rather that nature is nonlocal (or has some form of nonlocal mechanism).

Again, it seems to be bound up in the issue of 'completeness', since the contention - to my mind - appears to be that statistical interpretations are incomplete descriptions of the system and a more complete description would require either:
- nonlocal causal influence
- superdeterminsm
- anti-realism (in the sense of the system not existing until it is measured)
- [possibly others?]
 
  • #779
Lord Jestocost
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And therefore many people say that quantum mechanics is nonlocal.
It's clearly an error in thinking. Murray Gell-Mann puts it in “The Quark and the Jaguar” in the following way:

The label ‘nonlocal’ applied by some physicists to quantum-mechanical phenomena like the EPRB effect is thus an abuse of language. What they mean is that if interpreted classically in terms of hidden variables, the result would indicate nonlocality, but of course such a classical interpretation is wrong.” [bold by LJ]

One should thus avoid the term “quantum non-locality”. “Quantum non-separability” is the correct term in this context. Quantum non-separabilty is indeed rooted in the way the quantum formalism represents systems and sub-systems. Franck Laloë in “Do We Really Understand Quantum Mechanics?”:

The idea is that different quantum systems, when they have interacted in the past, no longer have in general their own physical properties; they are both part of a larger system, which is the only one possessing physical properties. One should then not try to separate (conceptually) the whole system into two smaller physical systems and attribute them properties; the whole system is non-separable.”
 
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  • #780
vanhees71
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Did it ever occur to you that locality might be a normal word present in nearly all human languages, and that this is an indication that its meaning is actually pretty clear? And that microcausality might simply one way in which a theory can be local?
In science we cannot use everyday language but we have to clearly define what we mean. Microcausality for sure is a meaning of locality nobody has in mind when using the word in everyday language.
I guess microcausality actually implies that there is no faster than light signaling, but the absence of faster than light signaling all by itself is also a way in which a theory can be local. And since more people are able to grasp that meaning, it might even be used more often than microcausality.
As I said, you have to define what's meant by locality, because it has not a well-defined meaning. Microcausality is a clear property of relativistic QFTs and thus has a well-defined meaning, and it seems to me the meaning most physicists and textbook writers interpret the meaning in Bell's HV model, though one cannot always be sure, because all too often the meaning is not explicitly defined by the authors.
And to go further, there appears to be randomness in Bell type experiments, and that randomness seems to include correlations between spatially separated events. And this is one way in which the predictions of quantum mechanics (that have been confirmed in many experiments) seem to not be local. And therefore many people say that quantum mechanics is nonlocal. Can you accept that this meaning does not contradict microcausality, and that therefore both you are right when you point out that quantum theory satisfies locality, but the many other people who say that quantum mechanics is nonlocal are not wrong either?
That's cause of a lot of confusion (not only in quantum theory). A statistical correlation does not necessarily imply a causal connection, and that is the case for the correlations of observables on far-distant parts of an entangled quantum system. Einstein introduced the much more precise word "inseparability" for this. Of course, this does not locality (in the sense of microcausality), because it's consistently described by local relativistic QFT. That's why I reserve the word "locality" to mean microcausality and talk about "long-ranged correlations" or "inseparability" rather than (non)locality. Definitions are made to make language as simple and concise as possible and thus one should use different words for different things.
But more importantly, did it ever occur to you that this is not the fault of philosophy? Philosophy did not invent the concept of locality, because locality is simply an intuivite concept that has been present in human thinking all along. And it is a totally unproblematic concept, if you ask me!
Obviously it's totally problematic, because I have to repeatedly make clear what I understand using this word as well as you have to make clear what you understand. It would be better not to use the word anymore within physics, but this is of course impossible, because it's all too well established in the literature, including it's fuzzy meaning.

Again, as particularly quantum theory has taught us, intuitive concepts in human thinking is not a sufficient way to talk about the natural sciences.
 
  • #781
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That's why I reserve the word "locality" to mean microcausality and talk about "long-ranged correlations" or "inseparability" rather than (non)locality.
If you have to tell me personally that you redefined "locality" to mean "microcausality", then this does not seem to be helpful from my perspective. If most introductory textbooks on quantum mechanics would make such a redefinition for some words with good reason, then maybe it could be helpful.

But I have not yet seen any introductory textbook on quantum mechanics that even defined microcausality. Some do talk about absence of faster than light signaling, and I do find it helpful when they explain to me that this is one sense in which QM can be made to respect special relativity and locality.

Fine with me if you want to use the word "inseparability". But please do take care to still explain the importance of absence of faster than light signaling. This an important concept, and no redefinition of the word locality or nonlocality or use of a different word will substitute a proper explanation of that concept. And an advanced technical concept like microcausality is no proper substitute either.

Obviously it's totally problematic, because I have to repeatedly make clear what I understand using this word as well as you have to make clear what you understand.
The negation of the word locality might be problematic, because the negation of a positive property can depend on the context. But trying to forbid the use of a perfectly clear and understandable word is unreasonable, if the only reason for that move is that its negation started to get used in somewhat confusing ways.
 
  • #782
vanhees71
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Introductory QM books are about non-relativistic QT, and thus of course you don't find microcausality discussed in them. Within non-relativsitic QT there's of course also no problem with nonlocality to begin with. Of course non-relativistic QT has a much more limited realm of validity than relativistic QFT.

Microcausality is at the heart of the conception of local relativistic QFT and thus contained in any introductory or advanced textbook about it, though not always with the careful emphasis this important concept deserves. It's most clearly described in Weinberg, The Quantum Theory of Fields vol. 1.
 
  • #783
Lynch101
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One should thus avoid the term “quantum non-locality”. “Quantum non-separability” is the correct term in this context. Quantum non-separabilty is indeed rooted in the way the quantum formalism represents systems and sub-systems. Franck Laloë in “Do We Really Understand Quantum Mechanics?”
When talking about “Quantum non-separability” is there the implication that the system is spatially extended?
 
  • #784
Lord Jestocost
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When talking about “Quantum non-separability” is there the implication that the system is spatially extended?
Franck Laloë in “Do We Really Understand Quantum Mechanics?”:

In general, separability is a notion that is conceptually distinct from locality. It is not necessarily related to space: two physical systems could occupy the same region of space and remain distinct with their own physical properties (separable is not the same thing as separate).
.......
Quantum non-separability is rooted in the way the quantum formalism describes systems and sub-systems, and clearly related to the notion of entanglement (§6.1): a perfect description of the whole does not contain a perfect description of the parts. We mentioned earlier that Schrödinger considered entanglement as one of the most fundamental properties of quantum mechanics. Entanglement drastically restricts the number of physical properties that can be attributed to the sub-systems; this number may even vanish in some cases. In other words, the ‘best possible description’ (with a state vector) is not available to the sub-systems; they have an additional level of indeterminacy, which never occurs in classical mechanics.
” [bold and bold/red by LJ]
 
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  • #785
Lynch101
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Franck Laloë in “Do We Really Understand Quantum Mechanics?”:

In general, separability is a notion that is conceptually distinct from locality. It is not necessarily related to space: two physical systems could occupy the same region of space and remain distinct with their own physical properties (separable is not the same thing as separate).
Thanks LJ.

The emboldened part seems to be a different scenario to where we have the single [entangled] system measured in spatially separated locations. Can we infer, from the spatially separated detection events, that the quantum system is also spatially extended? Or do such concepts not apply to the quantum system?

I'm asking because it would seem to have similar implications for FTL-nonlocality if we can.
 
  • #786
Lord Jestocost
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Can we infer, from the spatially separated detection events, that the quantum system is also spatially extended? Or do such concepts not apply to the quantum system.
What means a "spatially extended quantum systems"?
 
  • #787
Lynch101
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What means a "spatially extended quantum systems"?
I'm asking if we can infer that the quantum system is spatially extended by virtue of the fact that measurements of it occur in spatially separated locations?

So, the measurement events are spatially separated, does this imply that the quantum system is extended in space?
 
  • #789
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Murray Gell-Mann puts it in “The Quark and the Jaguar” in the following way:
...
Franck Laloë in “Do We Really Understand Quantum Mechanics?”:
...
Both are certainly nice references, and the quoted parts are relevant to the discussed topic and the points were confusion can arise. I wasn't even aware of Franck Laloë's book, and I am a huge fan of Quantum Mechanics: Volume III: Fermions, Bosons, Photons by Claude Cohen-Tannoudji, Bernard Diu, and Frank Laloë. I was aware of Gell-Mann's book, but I never made any serious effort to read it. I did read (what I believe to be) the last paper coauthored by Gell-Mann, and it has had a huge impact on my thinking about probability. I am quite familiar with the consistent histories framework (from articles and books by Roland Omnès and Robert Griffiths), but less familiar with the related decoherent histories interpretation by Gell-Mann and Hartle.

It's clearly an error in thinking.
I assume that you did read carefully what Gell-Mann wrote before quoting him. But I have the impression that you did not read carefully what I have written. Or maybe you cared most about giving a relevant reference, and less about whether it supported your statement.

The quoted passage from Gell-Mann (and also the wider context in which it appeared in his book) doesn't contradict what I wrote. I highlighted the importance of "the absence of faster than light signaling", and so does Gell-Mann. The "error in thinking" would be to deduce that type of nonlocality from "correlations between spatially separated events". So his complaint about "an abuse of language" for that sort of misconception is fully compatible with my claim that the word "local" is not the problem.
 
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  • #790
Lynch101
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Sure, why not?
Instead of thinking in terms of two separate systems we can think of a single spatially extended system. If the measurement outcomes on either 'end' of the system are not pre-determined but measurement on one 'end' instantly determines the measurement outcome at the other 'end', this would equate to an FTL influence. Wouldn't it?

This is assuming we can consider the system spatially extended.
 
  • #791
Lord Jestocost
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Both are certainly nice references, and the quoted parts are relevant to the discussed topic and the points were confusion can arise. I wasn't even aware of Franck Laloë's book, and I am a huge fan of Quantum Mechanics: Volume III: Fermions, Bosons, Photons by Claude Cohen-Tannoudji, Bernard Diu, and Frank Laloë. I was aware of Gell-Mann's book, but I never made any serious effort to read it. I did read (what I believe to be) the last paper coauthored by Gell-Mann, and it has had a huge impact on my thinking about probability. I am quite familiar with the consistent histories framework (from articles and books by Roland Omnès and Robert Griffiths), but less familiar with the related decoherent histories interpretation by Gell-Mann and Hartle.


I assume that you did read carefully what Gell-Mann wrote before quoting him. But I have the impression that you did not read carefully what I have written. Or maybe you cared most about giving a relevant reference, and less about whether it supported your statement.

The quoted passage from Gell-Mann (and also the wider context in which it appeared in his book) doesn't contradict what I wrote. I highlighted the importance of "the absence of faster than light signaling", and so does Gell-Mann. The "error in thinking" would be to deduce that type of nonlocality from "correlations between spatially separated events". So his complaint about "an abuse of language" for that sort of misconception is fully compatible with my claim that the word "local" is not the problem.
Maybe, there is some misunderstanding.
To my mind, words like "local" or "non-local" are problematic in conjuction with quantum theory. They can over and over again trigger people to think about quantum phenomena with classical ideas (this I meant with "error in thinking").
 
  • #792
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To my mind, words like "local" or "non-local" are problematic in conjuction with quantum theory. They can over and over again tirigger people to think about quantum phenomena with classical ideas.
Interesting! Can you substitute them with better words?

And can you really talk about quantum phenomena without classical ideas? For example, what about macroscopic measurement outcomes?
 
  • #793
vanhees71
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Instead of thinking in terms of two separate systems we can think of a single spatially extended system. If the measurement outcomes on either 'end' of the system are not pre-determined but measurement on one 'end' instantly determines the measurement outcome at the other 'end', this would equate to an FTL influence. Wouldn't it?

This is assuming we can consider the system spatially extended.
Indeed, two entangled photons are a single system by definition. They are not separable. Nevertheless there are two photons which can be detected at far distant places.
 
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  • #794
Lord Jestocost
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And can you really talk about quantum phenomena without classical ideas? For example, what about macroscopic measurement outcomes?
To be honest: I personally try avoid to think about quantum phenomena with classical ideas and concepts. I have the feeling that it merely leads down a rabbit hole.

Regarding observations (measurement outcomes): We can never be certain whether appearances in our mind can be thought "classically" as experiences of an outer world and are not mere imagining. The possibility of thinking of appearances as experiences of “something outer” allows us to talk about measurement outcomes in a conventional classical way.
 
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  • #795
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Indeed, two entangled photons are a single system by definition. They are not separable. Nevertheless there are two photons which can be detected at far distant places.
Can we therefore conclude that this single system is spatially extended?
 
  • #796
Lynch101
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To be honest: I personally try avoid to think about quantum phenomena with classical ideas and concepts. I have the feeling that it merely leads down a rabbit hole.
If all our measurements of quantum systems are at the classical level, are are we not then forced to at least consider classical ideas? Surely we have to explain how quantum systems give rise to classical observables?

Also, wasn't it consideration of classical ideas that led to the EPR paper, which in turn led to Bell's theorem, so there can be some benefit to doing it, no?

Regarding observations (measurement outcomes): We can never be certain whether appearances in our mind can be thought "classically" as experiences of an outer world and are not mere imagining. The possibility of thinking of appearances as experiences of “something outer” allows us to talk about measurement outcomes in a conventional classical way.
It's possible to follow the implications of both scenarios. In general we tend to start with the assumption that there is an 'outer world', but we could equally explore the idea that there isn't. I don't think it would change much however, because ultimately it all boils down to describing our observations.
 
  • #797
vanhees71
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Can we therefore conclude that this single system is spatially extended?
Sure, why not? All there is are, however, the probabilities or probability distributions for the outcome of measurements.
 
  • #798
Lynch101
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Sure, why not? All there is are, however, the probabilities or probability distributions for the outcome of measurements.
It's more to do with the use of the term 'quantum non-separability' instead of 'quantum non-locality' (FTL-nonlocality).
It's clearly an error in thinking. Murray Gell-Mann puts it in “The Quark and the Jaguar” in the following way:

The label ‘nonlocal’ applied by some physicists to quantum-mechanical phenomena like the EPRB effect is thus an abuse of language. What they mean is that if interpreted classically in terms of hidden variables, the result would indicate nonlocality, but of course such a classical interpretation is wrong.” [bold by LJ]
If we can infer the spatial extension of the quantum system then it isn't necessarily a classical interpretation in terms of hidden variables that indicates FTL-nonlocality. If the measurement on one 'end' of the system immediately determines the outcome at the other, spatially separated 'end' of the system, this too would imply FTL-nonlocality.
 
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All there is are, however, the probabilities or probability distributions for the outcome of measurements.
But measurements are macroscopic. So on the microscopic level, where measurements don't exist, there are no even probabilities. In a theoretical universe containing only one hydrogen atom there would be nothing at all, not even probabilities. Is it what you are saying?

What I am asking is, are the probabilities of measurement outcomes there when there are no measurements?
 
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  • #800
vanhees71
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If there were only a single hydrogen atom there'd be nobody to bother about its state and the meaning of this state.

Of course the probabilities are there when nobody measures. If the measurement is done you don't need any probabilities anymore.
 

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