A Assumptions of the Bell theorem

[gentzen]

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.

 

[vanhees71]

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.

 

[Lynch101]

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?

 

[Lord Jestocost]

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]

 

[Lynch101]

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.

 

[Lord Jestocost]

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"?

 

[Lynch101]

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?

 

[vanhees71]

Sure, why not?

 

[gentzen]

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.

 

[Lynch101]

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.

 

[Lord Jestocost]

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").

 

[Demystifier]

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?

 

[vanhees71]

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.

 

[Lord Jestocost]

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.

 

[Lynch101]

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?

 

[Lynch101]

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.

 

[vanhees71]

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.

 

[Lynch101]

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.

 

[Demystifier]

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?

 

[vanhees71]

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.

 

[Demystifier]

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.

So probabilities of measurement outcomes are only relevant when there are no measurement outcomes?

 

[gentzen]

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").

I see. I never had to teach students, so the problem that my words would trigger ideas that make it harder for them to learn quantum theory never happened to me. For me personally, it was rather the absence of the concept of density matrix that initially prevented me from understanding quantum mechanics (in my QM course at university).

After I learned a similar concept in statistical optics later in my job, I guessed that it was this concept that had been missing for me before. Much later a new job forced me to really learn and understand QM. Today I have the impression that most of classical physics remains valid, and the tricky part is rather to convince others that taking quantum corrections (like exchange effects, quantum surface transmission, channeling contrast, quantum moment conservation) into account is both possible and required for reproducing certain effects seen in experimental data, despite the fact that Monte Carlo simulations seem to be based entirely on classical concepts. A correction for exchange effects or for channeling contrast can feel badly non-local. To convince others, it helps to dig a bit into where the non-locality came from. Typically two or more electrons became indistinguishable for some specific reason. I don't think that the word "non-local" itself ever played a role is such discussions, neither positive nor negative.

 

[physika]

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.

...for others, have pre-defined values

 

[physika]

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.

not necessarily.

 

[vanhees71]

I see. I never had to teach students, so the problem that my words would trigger ideas that make it harder for them to learn quantum theory never happened to me. For me personally, it was rather the absence of the concept of density matrix that initially prevented me from understanding quantum mechanics (in my QM course at university).

After I learned a similar concept in statistical optics later in my job, I guessed that it was this concept that had been missing for me before. Much later a new job forced me to really learn and understand QM. Today I have the impression that most of classical physics remains valid, and the tricky part is rather to convince others that taking quantum corrections (like exchange effects, quantum surface transmission, channeling contrast, quantum moment conservation) into account is both possible and required for reproducing certain effects seen in experimental data, despite the fact that Monte Carlo simulations seem to be based entirely on classical concepts. A correction for exchange effects or for channeling contrast can feel badly non-local. To convince others, it helps to dig a bit into where the non-locality came from. Typically two or more electrons became indistinguishable for some specific reason. I don't think that the word "non-local" itself ever played a role is such discussions, neither positive nor negative.

Sure, in "bread-and-butter physics" dealing with the description of observable phenomena, there's only one meaning of "locality", namely the impossibility to transmit information with any "faster-than-light signal" within any theory which is consistent with any theory within the (special-)relativistic (!) spacetime model. In relativistic QFT this is implemented from the very beginning by the microcausality principle for local observables, from which all the fundamental properties derivable from the so realized local relativsitic QFTs follow: unitarity and Poincare invariance of the S-matrix/optical theorem/dispersion relations, relation between spin and statistics (half-integer spin=fermions; integer spin=bosons), CPT symmetry.

What's often confusingly called "non-locality" in the more quantum-foundations inclined community refers to long-ranged correlations between "entangled parts" of a quantum system. It would help tremendously to call this "inseparability" as Einstein did. The trouble seems to be that Einstein's much clearer written paper of 1948 has been mostly ignored in comparison to the unfortunate EPR paper of 1935, and thus the confusing lingo of the EPR paper and the even more confusing answer by Bohr prevailed.

 

[Demystifier]

Much later a new job forced me to really learn and understand QM.

May I ask what's your job? It sounds as if it is not an academic job, so I'm curious what kind of non-academic job requires good understanding of QM.

 

[Lynch101]

not necessarily.

If two 'ends' are spatially separated and an action performed one one end instantaneously affects the other end, then, by my reasoning, this would imply an FTL causal influence. Is there any alternative?

 

[vanhees71]

There are no FTL causal influences within local relativistic QFT. The experimentally confirmed violations of Bell's inequality, consistent with the predictions of local relativistic QFT (usually QED since most experiments are done with entangled photons) are thus still consistent with locality.

 

[gentzen]

May I ask what's your job? It sounds as if it is not an academic job, so I'm curious what kind of non-academic job requires good understanding of QM.

If you work in the semiconductor industry, then you have a good chance to encounter tasks where a good understanding of physics is helpful. This physics can also reach into the domain of QM, sometimes more, sometimes less. Let me elaborate on what I wrote in the New Members Introductions Forum:

I am an applied mathematician working in semiconductor manufacturing. This means stuff like optical lithography, ebeam lithography, resist development processes, etching processes, optical metrology, scanning electron beam metrology, and other related physical or chemical processes.

My QM tasks are about the interaction of electron beams with matter. At the beginning (February 2013) it was about the energy range 10 keV - 100 keV, i.e. the simulation of electron beam lithography. An understanding of QM was still unimportant here, it was enough to use databases with (differential) scattering cross-sections (for atoms) provided by others, and there were also existing simulators against which I could verify our simulator. (The simulator already existed, only it provided significantly different results than existing simulators. It was my job to find the causes and fix them.)

From the beginning it was clear that we also wanted to be able to simulate scanning electron microscopy (SEM). Internal prototypes, external simulators and code "licensed" from research institutes existed for this as well. But that was a tragedy because each simulator calculated totally different results and there was no chance of differentiating right from wrong. Well, for some simulators you could explain why they were definitely wrong. But it wasn't bad either, because after "directed self assembly" (DSA) went out of fashion again, the end customers' need for the simulation of the SEM also decreased. (And there were and are many other tasks for me.) With DSA, defects can arise under the surface, and the task of the SEM simulation would have been to determine from what depth on which defects (size, material) can still be seen, what the signal-to-noise ratio is, and which SEM settings (energy of the electrons, which type of detectors, ...) would be helpful.

Then a student did his master's thesis with us extending the simulator with charging effects and doing clean room SEM measurements to somewhat verify the stuff. He was really enthusiastic, and then also finished a PhD thesis on related topics later. ... And customers for the simulator also emerged: the manufacturers of the SEM machines.

 

[Lynch101]

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.

With words like 'reality', there is a danger of descending into a philosophical rabbit hole in trying to define the terms with impossible precision. It is possible, however, to avoid such tangents by defining it in contrast to other well defined terms.

For example, with regard to 'the physical reality' referred to in the EPR paper (which Bell took as the basis for his own paper), 'the physical reality' can be defined in contrast to 'the mathematical model' of the physical experimental set-up. So, 'the physical reality' simply refers to the what is happening in the lab.

Sure, in "bread-and-butter physics" dealing with the description of observable phenomena, there's only one meaning of "locality", namely the impossibility to transmit information with any "faster-than-light signal" within any theory which is consistent with any theory within the (special-)relativistic (!) spacetime model.

Is there not also the possibility to interpret it as the impossibility of causal influences propagating FTL? But, as long as FTL causal influences cannot be used for signaling they would not violate relativity. Some might say that it violates the 'spirit' of relativity, but that is a separate matter. (I'm not arguing that it does violate relativity, just that there is another possible interpretation of 'locality'.)

What's often confusingly called "non-locality" in the more quantum-foundations inclined community refers to long-ranged correlations between "entangled parts" of a quantum system. It would help tremendously to call this "inseparability" as Einstein did.

At this stage, you are probably right. The term 'inseparability' might be better because too much time seems to go into discussing the meaning of the word 'non-local'.

From my reading of the literature and from discussions on here about the literature, my reasoning leads me to conclude that there are those who us the term 'non-locality' not simply to refer to the observed correlations, rather about the possible mechanisms which could explain the observed correlations. They seem to be talking about causal influences propagating FTL or, more accurately, instantaneously.

There seems to be others then who use the term 'non-local' to simply refer to the observed correlations themselves.

But, if we do choose to use the term 'inseparability' - where we talk about a single system - we can ask if the system is spatially separated, given that it is measured in spatially separated laboratories. We can then ask if measurement on one 'end' has an instantaneous (or FTL) causal influence on the other, spatially separated 'end'.

By my reasoning, the underlying issue is whether or not there are FTL causal influences, regardless of whether we use the terms 'non-locality' or 'inseparability'.