Confused by nonlocal models and relativity

[DarMM]

I've read the book and don't really agree with it, but that would be another thread.

However note the object I'm talking about is not the quantum field $\hat{\phi}\left(x,t\right)$ but rather the wave functional $\Psi\left[\phi\left(x\right),t\right)$. It's the latter that has to be a real propagating object in a Many Worlds view of QFT.

Since classical fields are real, and they are simply a limit of quantum fields, quantum fields must be real

That's a much more subtle issue than one would think. There's no real limit where a quantum field becomes a classical field. Classical fields are more a certain limit of a quantum field's expectation values in certain states.

Since classical fields are real, and they are simply a limit of quantum fields, quantum fields must be real. Normal QM is QFT when particles are dilute - but since the fields are real that means normal QM is real

Again it's a complex issue the exact form the relation between QM and QFT takes. Again it's a limit along a certain subfamily of states. Complicated by the fact that in QFT we have that particle number $N$ is not well defined in general.

Also generally actual operators like $\phi$ are taken as "real" even in QM, since they can be measured, it's more a question about the "reality" of the quantum state $\psi$. Even for operators though one has the interesting effect that most POVMs don't correspond to the quantization of any classical variable.

 

[A. Neumaier]

If you can read between the lines, you can find it in item 1 at page 84.

I cannot read between the lines. Eigenstates of position form a continuum and cannot be split in the way (5.17) requires; supports are closed sets and disjoint supports therefore require a gap in the spectrum!

In that discussion before, it seems that you missed my post #70, where I explained mathematically why the ideas work even without the nondemolition assumption.

Yes, I had missed that. But your analysis there is not convincing; see my answer in that thread.

 

[DarMM]

Just to come back to this.

So what I understand this view to be is it is Copenhagen, and it starts off with the wave function is just a catalogue of probabilities, but it adds: "the wave function is just a catalogue of probabilities because of some obvious absurdities like collapse if it is real. Nonetheless, from an operational point of view we don't need to decide on the reality of the wave function to proceed with using the formalism, so we set up the cut and state and quantum formalism. At the point of usage, since we believe it makes no operational difference whether the wave function is real or not, we can use the mental picture of a real wave function to help with calculations, ie. reality is a tool to predict the probabilities of measurement outcomes." Since the reality of the wave function only enters at the last stage, I think it has enough of Copenhagen to be protected from the extended Wigner friend scenarios.

I haven't been able to find much on this view. Typically the cut is taken to be a subjective. This idea that the wave function is real, but there is also an objective cut doesn't seem to have much discussion in the literature. It seems hard to see how such an objective cut could be Lorentz invariant. It seems to be MWI, but with a length scale where the worlds "stop".

Do you have a link to a paper that lays out this view?

 

[vanhees71]

So far there is no objective cut found. Just recently the limits were pushed even further into the "mesoscopic" realm of very large molecules:

https://doi.org/10.1038/s41567-019-0663-9

 

[jk22]

Coming back to Bell's CHSH inequality, I'm confused that the formula : $$\int A(\vec{a},\lambda)B(\vec{b},\lambda)\rho(\lambda)$$ is considered local. Like how can the "element" lambda be at the same time both in A and B ? It seems lambda is non local there, at the measurement time. Or am I misunderstanding the physical setup described by the formula ?

 

[PeterDonis]

how can the "element" lambda be at the same time both in A and B ?

$\lambda$ refers to what Bell calls "hidden variables" that are in the causal past of both A and B. "Locality" in this context includes things in the causal past. It just doesn't include things outside the causal past, i.e., outside the past light cone. A and B are outside each other's past light cone, which is why, to satisfy locality, measurement A can only depend on the settings $\vec{a}$, not on $\vec{b}$, and vice versa. But both can depend on $\lambda$ because $\lambda$ is in the causal past of both.

 

[jk22]

$\lambda$ refers to what Bell calls "hidden variables" that are in the causal past of both A and B. "Locality" in this context includes things in the causal past. It just doesn't include things outside the causal past, i.e., outside the past light cone. A and B are outside each other's past light cone, which is why, to satisfy locality, measurement A can only depend on the settings $\vec{a}$, not on $\vec{b}$, and vice versa. But both can depend on $\lambda$ because $\lambda$ is in the causal past of both.

That's ok if lambda is information, but let suppose it is a physical particle. In a spacetime diagram lambda is first at the source, then it goes with at most speed of light in both directions ? For then arriving in A and B ?

This seems physically impossible, or that formula is a mixture of local a and nonlocal lambda, like a sweet-sour tasting dish.

I would suppose an example of completely local formula $\frac{1}{\lambda_1}\int_0^{\lambda_1} \cos(a-s_1)ds_1\frac{1}{\lambda_2}\int_0^{\lambda_2} \cos(a-s_2)ds_2$

Then by noting that : $\max_x\{A(x)+B(x)\}\leq\max_x\{A(x)\}+\max_x\{B(x)\}$ and computing the maximum of each covariance completely locally, the value 3.16 should appear ?

 

[PeterDonis]

That's ok if lambda is information, but let suppose it is a physical particle. In a spacetime diagram lambda is first at the source, then it goes with at most speed of light in both directions ? For then arriving in A and B ?

This seems physically impossible

Of course it is. And Bell's formulation rules this out anyway. $\lambda$ has to be something that can causally affect both measurements. Obviously it can't if it is a particle that can only go in one direction.

 

[PeterDonis]

I would suppose an example of completely local formula $\frac{1}{\lambda_1}\int_0^{\lambda_1} \cos(a-s_1)ds_1\frac{1}{\lambda_2}\int_0^{\lambda_2} \cos(a-s_2)ds_2$

Then by noting that : $\max_x\{A(x)+B(x)\}\leq\max_x\{A(x)\}+\max_x\{B(x)\}$ and computing the maximum of each covariance completely locally, the value 3.16 should appear ?

I don't understand what you're trying to do here.

 

[jk22]

I don't understand what you're trying to do here.

I'm just considering that a particle s1 is going to A and another particle s2 to B measurement places. But I'm still mixing information (math) and the physics described by the symbols. This I call local in the sense that "every element of physical reality (particles 1 and 2, each flying in one direction) has a counterpart in the theory (s1,s2)"[EPR quote].

Nevertheless, the particle s1 carries an information like a polarisation angle and arrives at A and interacts with the measurement apparatus which in turn contains information about the angle of measurement "a". The hidden variable is here lambda1, the bound of integration. It is found by extremising the value of the average. This is to say that extremal values are like stable measurement results.

 

[DrChinese]

1. I'm just considering that a particle s1 is going to A and another particle s2 to B measurement places. But I'm still mixing information (math) and the physics described by the symbols. This I call local in the sense that "every element of physical reality (particles 1 and 2, each flying in one direction) has a counterpart in the theory (s1,s2)"[EPR quote].

2. Nevertheless, the particle s1 carries an information like a polarisation angle and arrives at A and interacts with the measurement apparatus which in turn contains information about the angle of measurement "a".

3. ... lambda1, the bound of integration. It is found by extremising the value of the average. This is to say that extremal values are like stable measurement results.

1. The EPR quote is leading to the idea that there must have been a separable physical reality for both s1 and s2 prior to measurements at A and B. And since QM does not present such a picture, it must be incomplete. Of course, EPR made an important assumption - that such physical reality extends to counterfactual measurements that could have been made at A and B.

2. The statistical results, as shown by Bell, do not support this model. It is the relationship of the angles of the measurements at A and at B - and nothing else - that dictates those.

3. I don't understand any of this terminology, or what it means to this subject.

 

[PeterDonis]

I'm just considering that a particle s1 is going to A and another particle s2 to B measurement places. But I'm still mixing information (math) and the physics described by the symbols. This I call local in the sense that "every element of physical reality (particles 1 and 2, each flying in one direction) has a counterpart in the theory (s1,s2)"[EPR quote].

Nevertheless, the particle s1 carries an information like a polarisation angle and arrives at A and interacts with the measurement apparatus which in turn contains information about the angle of measurement "a". The hidden variable is here lambda1, the bound of integration. It is found by extremising the value of the average. This is to say that extremal values are like stable measurement results.

I still don't understand what you're trying to do here. If you're trying to construct your own hidden variable model, personal speculations are off limits here.

 

[Nugatory]

Like how can the "element" lambda be at the same time both in A and B ?

$\lambda$ isn’t a single “element”, it’s all the values of all the hidden variables whether relevant to the interaction at A or at B. But look again at that integral $$\int A(\vec{a},\lambda)B(\vec{b},\lambda)\rho(\lambda)$$

$A$ is a function of $\lambda$ and $\vec{a}$ but not $\vec{b}$. Therefore the value of $A$ is independent of $\vec{b}$, and likewise $B$ is independent of $\vec{a}$. That’s sufficient to establish locality even if $\lambda$ is not “local”.

Suppose that we were working with the classical pair of gloves: put each glove in a box, give us each a box, I open my box and see the handedness of that glove and I immediately know the handedness of the glove in your box. In this situation $\lambda$ would just be an array of length two initialized at pair creation time: $\lambda[0]$ is the handedness of the glove in my box, $\lambda[1]$ is the handedness of the glove in your box, and both of our measurements are unquestionably local even though we have the same $\lambda$.

 

[jk22]

Thanks for all the explanations. Even if this mixing of math I have done seems local, there remain to calculate $\max A(\lambda_1)B(\lambda_2)$ which is non locsl, like if the computer or the consciousness would modify the wanted results.

 

[PeterDonis]

Even if this mixing of math I have done seems local, there remain to calculate $\max A(\lambda_1)B(\lambda_2)$

No. There is no $\lambda_1$ and $\lambda_2$. There is just $\lambda$. The whole point of $\lambda$, as I have already said, is that it denotes hidden variables that are in the causal past of both measurements and therefore can be taken to be common to both measurements. There is no such thing as some pieces of $\lambda$ traveling to measurement $A$ and other, different pieces of $\lambda$ traveling to measurement $B$. There is just $\lambda$. That is what appears in Bell's math.

 

[PeterDonis]

it’s all the values of all the hidden variables whether relevant to the interaction at A or at B.

I would say it is hidden variables that are in the causal past of both A and B. That is why the same $\lambda$ appears in both terms in Bell's integral. You don't have one $\lambda$ for A and another, different one for B. You just have $\lambda$.

In this situation $\lambda$ would just be an array of length two initialized at pair creation time: $\lambda[0]$ is the handedness of the glove in my box, $\lambda[1]$ is the handedness of the glove in your box, and both of our measurements are unquestionably local even though we have the same $\lambda$.

But here you are saying we don't have the same $\lambda$; you have $\lambda[0]$ and I have $\lambda[1]$. That's not how things appear in Bell's equation. In Bell's equation, as above, there is just one $\lambda$. In your scenario $\lambda$ would be the state of the pair of gloves in the causal past of both measurements, which would include the fact that they are of opposite handedness. The locality is guaranteed by the fact that the gloves were made by a local process that enforced their opposite handedness. The result of that one local process, in the causal past of both A and B, is what is contained in $\lambda$.

 

[Nugatory]

But here you are saying we don't have the same $\lambda$; you have $\lambda[0]$ and I have $\lambda[1]$. That's not how things appear in Bell's equation. In Bell's equation, as above, there is just one $\lambda$

From Bell's original paper:

Let this more complete specification be effected by means of parameters $\lambda$. It is a matter of indifference in the following whether $\lambda$ denotes a single variable or a set, or even a set of functions, and whether the variables are discrete or continuous.
.... our $\lambda$ can then be thought of as the initial values of these variables at some suitable instant

And that is pretty much what the initial specification of the handedness of the gloves in the two boxes as an array of two variables is.

 

[PeterDonis]

that is pretty much what the initial specification of the handedness of the gloves in the two boxes as an array of two variables is.

As initial values, yes (and in the causal past of both measurements, which IIRC is mentioned elsewhere in the paper). @jk22 seems to be thinking of them as values at the measurement events and then wondering how they can be "local". But of course they are "local" in the causal past of both measurements, and are assumed to be deterministically "transported" to both measurements to whatever extent is necessary.

 

[vanhees71]

No. There is no $\lambda_1$ and $\lambda_2$. There is just $\lambda$. The whole point of $\lambda$, as I have already said, is that it denotes hidden variables that are in the causal past of both measurements and therefore can be taken to be common to both measurements. There is no such thing as some pieces of $\lambda$ traveling to measurement $A$ and other, different pieces of $\lambda$ traveling to measurement $B$. There is just $\lambda$. That is what appears in Bell's math.

$\lambda$ can be any number of "hidden variables". The strong point of Bell's derivation of his inequalities is the generality.

The problem is the very confusing language involved, which is due to EPR. Einstein himself was not happy with the EPR paper, and he wrote a much more concise paper in 1948. The point he was uneasy with was what he called "inseparability", i.e., the fact that with entanglement in QT there are systems, where far distant parts (or more precisely where observations on far distant parts) of a quantum system are strongly correlated, but the measured observables showing these correlations are maximally indetermined. The most simple example, going back to Bohm, is the entanglement between spin components for two particles from a decay of a spin-0 particle, which are in a spin-singlet state. If nothing disturbes this particles you can wait until they are arbitrarily far away from each other, and although the spin-$z$ components are maximally indetermined (i.e., each of the particles is completely unpolarized) there's a strong anti-correlation in the outcomes of measurements, i.e., if particle 1 is found with $\sigma_z=+1/2$, the other is found with $\sigma_z=-1/2$ and vice versa. Measuring certain combinations of spin components (which is of course possible only on ensembles of equally prepared particles) demonstrates the violation of Bell's inequalities.

In Bell's "local, realistic" hidden-variable theories you have the assumptions

(a) all observables always take determined values. The probabilistic description is necessary because of our ignorance about the hidden variable(s), $\lambda$. There is a probability distribution $P(\lambda)$ for these variables. This is "realism".

(b) local observables of far distant parts are independent, i.e., the outcome of the measurement of an observable $A$ at one place and of an observable $B$ at another far distant place are independent of each other, i.e., they are determined by the setup of the measurement devices, $a$ and $b$ (e.g., in the above mentioned Bohm spin-1/2 example the choice of the measured spin components on particles at $A$ and $B$) and the hidden variables only: $A=A(a,\lambda)$ and $B=B(b,\lambda)$, i.e., $B$ doesn't depend on $a$ and $A$ doesn't depend on $B$. Both, however, depend on $\lambda$. That's what somewhat unfortunately is called "locality", although it is in fact "separability".

The mathemically clear description of a "local, realistic HV theory" simply is that the correlation function is given by
$$\langle A B \rangle = \int \mathrm{d} \lambda A(a,\lambda) B(b,\lambda) P(\lambda).$$
As far as I know the literature the realization of "inseparability" is usually considered fulfilled with certainty precisely by the realization of the local measurments on the far distant parts of the system in such a way that the "measurement events" are space-like separated, i.e., that there cannot be any causal influences of the measurement on B by the measurement on A and vice versa. This of course hinges on the fufillment of relativistic causality constraints, and indeed all relativistic theories are by construction fulfill this requirement.

This is realized in both the classical as well as the quantum realm by the paradigm of locality, i.e., the field concept. In classical relativistic physics this means that interactions between far distant parts (e.g., particles) of a system is due to local field actions. E.g., a particle "feels" the electromagnetic interaction with another particle due to the field due to this other particle's electromagnetic field at the spacetime point, $q$, of the former particle, $\vec{K}^{\mu}=q F^{\mu \nu}(x)$, and $F^{\mu \nu}$ is the retarded solution of the Maxwell equations with the other particle's electric-charge four-current as the source.

For QT this classical paradigm of "local" field interactions is realized by formulating it as local relativistic QFT, i.e., the local observable operators (densities of energy, momentum, angular momentum, charge-current densities) are commuting at space-like distances of their arguments, which includes the Hamilton density of the system, which ensures that there cannot be any causal influences between space-like separated measurement events.

In this sense local relativistic QFTs are indeed "local" but, of course, not "realistic", and that's why at least within this interpretation of QFT (statistical/ensemble interpretation), where there's nothing else than the probabilities, including the strong correlations described by entanglement, predicted by QFT, the quibble of EPR concerning relativistic causality is resolved.

 

[PeterDonis]

$\lambda$ can be any number of "hidden variables".

Yes, but they are not partitioned so that some "belong" to measurement A and some to measurement B, as @jk22 appears to be thinking. They are all in the causal past of both measurements.

 

[vanhees71]

That's of course true.

 

[jk22]

I would say that the usage of $\lambda$ is neither local nor temporal. I.e. all covariances in CHSH are simultaneously measured, I don't know if this is experimentally the case. It is also at all places simultaneously.

 

[vanhees71]

The important point is that you can choose what's measured and register the outcome at far distant parts of an entangled quantum system such that the choice and measurement registrations are space-like separated, i.e., they are not temporally ordered according to the relativistic spacetime description and thus cannot be in any way mutually causally connected. The outcome is as predicted by local, microcausal Q(F)T and not as by any kind of local hidden-variable theory.

 

[jk22]

I don't know which prediction for CHSH is made by QFT ?

QM predicts $2\sqrt2$ and HV 2, whereas experimental values are inbetween, 2.67, 2.25, 2.47, 2.73, 2.07, 2.22 which is on average nearer to 2 than to QM ?

 

[vanhees71]

What are you referring to? Everything concerning photons is of course a prediction of QED, i.e., the QFT dealing with the interaction of charged particles with the electromagnetic fields.

 

[DrChinese]

...they are not temporally ordered according to the relativistic spacetime description... The outcome is as predicted by local, microcausal Q(F)T...

Relativistic considerations have absolutely nothing to do with the quantum description (predictions) of entangled systems. Time ordering is never a factor, and that has nothing to do with concepts of simultaneity or reference frame (or Einsteinian locality). Obviously distance is not a factor either. I'm not sure if you were implying otherwise by what you said, so if I am reading your words wrong please let me know.

See for example this description of GHZ entanglement by Pan & Zeilinger:

"First, note that the [quantum] predictions are independent of the spatial separation of the photons and independent of the relative time order of the measurements. Let us thus consider the experiment to be performed such that the three measurements are performed simultaneously in a given reference frame, say, for conceptual simplicity, in the reference frame of the source. Thus we can employ the notion of Einstein locality, which implies that no information can travel faster than the speed of light. Hence the specific measurement result obtained for any photon must not depend on which specific measurement is performed simultaneously on the other two or on the outcome of these measurements. [And yet the result does depend on the specific choice of measurement.]"

https://www.drchinese.com/David/Bell-MultiPhotonGHZ.pdf

 

[vanhees71]

Relativistic considerations have absolutely nothing to do with the quantum description (predictions) of entangled systems. Time ordering is never a factor, and that has nothing to do with concepts of simultaneity or reference frame (or Einsteinian locality). Obviously distance is not a factor either. I'm not sure if you were implying otherwise by what you said, so if I am reading your words wrong please let me know.

If you want to discuss locality, then you must use a relativsitic description, because it's a relativistic concept.

See for example this description of GHZ entanglement by Pan & Zeilinger:

"First, note that the [quantum] predictions are independent of the spatial separation of the photons and independent of the relative time order of the measurements. Let us thus consider the experiment to be performed such that the three measurements are performed simultaneously in a given reference frame, say, for conceptual simplicity, in the reference frame of the source. Thus we can employ the notion of Einstein locality, which implies that no information can travel faster than the speed of light. Hence the specific measurement result obtained for any photon must not depend on which specific measurement is performed simultaneously on the other two or on the outcome of these measurements. [And yet the result does depend on the specific choice of measurement.]"

https://www.drchinese.com/David/Bell-MultiPhotonGHZ.pdf

Precisely that's the point. Locality in the here discussed context in all scientific papers on the subject means what Pan and Zeilinger write, i.e., "Einstein locality". That's why you have to use relativistic QFT to discuss these experiment. That's anyway inevitable, because the discussed experiment is done with photons, which are as relativistic as anything can get.

As I stressed very often in this forum, it is very unfortunate that at the same time in the quantum-foundations community one uses the standard notion of "Einstein locality" and at the same time talks about "quantum non-locality", which however has nothing to do with a violation of "Einstein locality" by QT. The only known realization of QT compatible with the relativistic space-time model and the associated notion of causality is in terms of local relativistic QFT, implementing "Einstein locality" in it's construction in terms of the "microcausality condition" on local observable-operators.

The long-ranged correlations described by entanglement are due to the preparation of the three-photon state in this specific GHZ state and not due to some superluminal locality-violating influences of the local measurements on the three photons with each other. This is clearly emphasized in the chapter you quote. The correct citation is Chpt. 16 in

R. A. Bertlmann, A. Zeilinger, Quantum [Un]speakables, Spektrum (2002)

 

[DrChinese]

1. If you want to discuss locality, then you must use a relativsitic description, because it's a relativistic concept.

2. Precisely that's the point. Locality in the here discussed context in all scientific papers on the subject means what Pan and Zeilinger write, i.e., "Einstein locality".

3. That's why you have to use relativistic QFT to discuss these experiment. That's anyway inevitable, because the discussed experiment is done with photons, which are as relativistic as anything can get.

4. The long-ranged correlations described by entanglement are due to the preparation of the three-photon state in this specific GHZ state and not due to some superluminal locality-violating influences of the local measurements on the three photons with each other. This is clearly emphasized in the chapter you quote.

5. The correct citation is Chpt. 16 in

R. A. Bertlmann, A. Zeilinger, Quantum [Un]speakables, Spektrum (2002)

1. No, there is no relativistic element in the quantum description of GHZ. Good ol' fashioned quantum mechanics does the trick. Simultaneous measurements (within the bounds of very small timing errors/differences) are essentially always nonlocal.

2. Exactly. They say exactly what I say, and nothing like what you are asserting. They say "[quantum] predictions are independent of the spatial separation of the photons" and therefore they violate all notions of locality, whatever they may be. Again, relativity has nothing to do with this.

3. Repeating myself: relativistic QFT is not required. Makes no difference whether you use the essential QM ideas of the early days or more recent relativistic iterations. Check the GHZ proof and you will see there is no mention of QFT or relativistic concepts.

4. This is a meaningless statement as you have presented it. Of course entangled states must be prepared. But that is not why there are correlations. The correlations are strictly due to (and calculated from) the future measurement context - which is not related to the preparation. A future nonlocal context provides the quantum expectation.

5. It was excerpted from the book you mention. And it is written by Pan & Zeilinger, as I said. I have presented their portion (chapter 16) in its entirety for the readers' benefit. Considering your usual refusal to provide any quotes or specific references to anything you say, I am somewhat surprised to see you "correcting" one of my references.

 

[PeterDonis]

They say "[quantum] predictions are independent of the spatial separation of the photons" and therefore they violate all notions of locality, whatever they may be.

No, they don't violate the notion of "locality" which says that spacelike separated measurements must commute. (I know you don't like using the term "locality" to refer to this; I don't either, as we have both said in prior discussions. But the term is used that way in some places in the literature.) In fact these measurements commute regardless of whether they are spacelike, null, or timelike separated. They also don't violate signal locality--information cannot be transmitted FTL.

 

[vanhees71]

1. No, there is no relativistic element in the quantum description of GHZ. Good ol' fashioned quantum mechanics does the trick. Simultaneous measurements (within the bounds of very small timing errors/differences) are essentially always nonlocal.

Without the relativistic argument you cannot guarantee the causal independence between the local measurements on the different parts of the systems (here single-photon polarization states), because in Newtonian physics you can have actions at a distance.

2. Exactly. They say exactly what I say, and nothing like what you are asserting. They say "[quantum] predictions are independent of the spatial separation of the photons" and therefore they violate all notions of locality, whatever they may be. Again, relativity has nothing to do with this.

But that's precisely what I say: The spatial separation of the photon measurement events guarantees, given microcausality of QED, that the measurements can not causally influence each other, and this argument is brought by Pan and Zeilinger in your quote from their paper.

3. Repeating myself: relativistic QFT is not required. Makes no difference whether you use the essential QM ideas of the early days or more recent relativistic iterations. Check the GHZ proof and you will see there is no mention of QFT or relativistic concepts.

It is required. The GHZ proof is very simple, but for the argument that space-like separated local measurements on distant parts of the three-photon system guarantee the impossibility of mutual causal influence of the local measurements on the single photons, prepared in the GHZ state, you need "Einstein causality", as stated by Pan and Zeilinger in your quote.

4. This is a meaningless statement as you have presented it. Of course entangled states must be prepared. But that is not why there are correlations. The correlations are strictly due to (and calculated from) the future measurement context - which is not related to the preparation. A future nonlocal context provides the quantum expectation.

No! The correlations are due to the preparation in an entangled state. You contradict yourself. If you accept, as all physicists seem to do, Einstein causality, then there cannot be causal influences between the local single-photon measurements, which are spacelike separated.

5. It was excerpted from the book you mention. And it is written by Pan & Zeilinger, as I said. I have presented their portion (chapter 16) in its entirety for the readers' benefit. Considering your usual refusal to provide any quotes or specific references to anything you say, I am somewhat surprised to see you "correcting" one of my references.

I don't need to quote anything else, because your quote says precisely what I wrote above!

 

[vanhees71]

No, they don't violate the notion of "locality" which says that spacelike separated measurements must commute. (I know you don't like using the term "locality" to refer to this; I don't either, as we have both said in prior discussions. But the term is used that way in some places in the literature.) In fact these measurements commute regardless of whether they are spacelike, null, or timelike separated. They also don't violate signal locality--information cannot be transmitted FTL.

The problem indeed is, that locality is not clearly defined, and even in that book chapter Pan and Zeilinger use "non-locality" not as opposite of "locality" but simply as the violation of the prediction of local realistic HV models as predicted by Q(F)T. I don't know, why this confusion occured in the literature, but we can't help it. It would be much better to follow Einstein in calling the long-ranged correlations described by entanglement "inseparability" rather than "non-locality".

 

[DrChinese]

No, they don't violate the notion of "locality" which says that spacelike separated measurements must commute. (I know you don't like using the term "locality" to refer to this; I don't either, as we have both said in prior discussions. But the term is used that way in some places in the literature.) In fact these measurements commute regardless of whether they are spacelike, null, or timelike separated. They also don't violate signal locality--information cannot be transmitted FTL.

No one is saying this mechanism transmits information (they say signal locality is respected, as I have also said above). No one is saying the measurements don't commute (since time ordering doesn't matter, as they say). What Pan and Zeilinger are saying is that the choice of measurement basis here dictates a result at distant there. That's quantum nonlocality, as mentioned in their title. There is no reference there to any relativistic consideration in determining the quantum expectation.

They apparently thought you would understand: That by experimentally demonstrating such nonlocality, it was obvious that Einsteinian locality was violated precisely because simultaneous measurements must be distant relative to the speed of light. Why duck this? Why pretend that their conclusion doesn't mean what they say? We don't need to debate the definition of "locality" here to see that GHZ is not only proof of quantum nonlocality, it does so without reference to a statistical ensemble (or QFT).

 

[PeterDonis]

The problem indeed is, that locality is not clearly defined

No, the problem, as we have already discussed ad nauseam in previous threads, is that different sources in the literature use "locality" to mean different things. All of those things are perfectly well-defined; they're just different. Which means that in any discussion of this general topic, one needs to say which meaning of "locality" one is using. It does not mean you can discount someone else's arguments because they are using "locality" with one of the other meanings besides the one you prefer.

 

[PeterDonis]

No one is saying this mechanism transmits information (they say signal locality is respected, as I have also said above). No one is saying the measurements don't commute (since time ordering doesn't matter, as they say).

In other words, these concepts of "locality" are not violated--contrary to your statement that any concept of locality whatever is violated. Which was my point.

What Pan and Zeilinger are saying is that the choice of measurement basis here dictates a result at distant there.

Yes.

That's quantum nonlocality

No, that is their claimed mechanism for producing quantum nonlocality. In other words, it is their preferred interpretation. It is not fact.

There is no reference there to any relativistic consideration in determining the quantum expectation.

Yes. But that just means this is one of those cases where the relativistic calculation gives the same answer.

They apparently thought you would understand: That by experimentally demonstrating such nonlocality, it was obvious that Einsteinian locality was violated

Again, this is their preferred interpretation. It is not fact.

GHZ is not only proof of quantum nonlocality, it does so without reference to a statistical ensemble (or QFT).

And this, as I have already said, is your preferred interpretation. It is not fact. I have already given references for how a proponent of the ensemble interpretation (Ballentine) uses that intepretation with these experiments. And since all interpretations make the same predictions for actual experimental results, it is impossible to use any experimental results to disprove any interpretation. The most you can do is to express your opinion about which interpretation you prefer. (All this is spelled out in the guidelines for this subforum.) Which you have done.

 

[bhobba]

Maybe Bell's famous paper on the definitions of the words used in discussing these issues can help clarify some things:
https://www.informationphilosopher.com/solutions/scientists/bell/Against_Measurement.pdf

I was recently reminded of the paper in a lecture by Tim Maudlin. He concluded QM must be non-local, and thought there were only two viable interpretations - DBB and GRW. I have issues with DBB in generalising to QFT. I am not aware of any problems with GRW.

We can't speak of separate particles when they are entangled. Yet we talk of particles moving away from each other. There seem to be inherent semantic difficulties.

Thanks
Bill