Why is it assumed communication through entanglement would be FTL?

[DarMM]

What's excluded in fact IS Bell's local realistic HV theories. What's confirmed by the experiments is QT and particularly also relativistic local (microcausal) QFT! I think this is the point, where we cannot agree upon, but the experimental facts speak for themselves

I don't think @DrChinese is disagreeing with any of that, he clearly says he's not saying local hidden variables are true. As a third party this all reads as a discussion spun from confusing terminology.

You are using "local" to mean microlocality, @DrChinese is using it to mean classical correlations. Thus to him your denial of nonlocality appears as denying CHSH violation and to you saying QFT is nonlocal (by which he only means "contains nonclassical correlation") appears to be a rejection of microlocality and cluster decomposition.

 

[vanhees71]

I agree that it seems like there must be some underlying mechanism that produces the correlations that violate the Bell inequalities, and "nonlocal connection" is as good a name for this unknown mechanism as any. But the fact remains that the mechanism is unknown (and even our belief that there must be some such mechanism might possibly be wrong).

I couldn't agree more with what you said in the part I deleted. Thanks for telling @DrChinese what I'm telling him for ages in other words. Maybe it helps!

The only point is that you don't need any "unknown mechanism". It's all described by standard QT (and of course relativistic QFT, which is just a special case of QT with the special feature that it's in accordance with the SRT space-time structure and all causality constraints implied by this space-time structure) entanglement.

Entanglement occurs naturally when you have two separable parts of a system, e.g., two particles. Here separable means that you can prepare product states $|\psi_1 \rangle \otimes |\psi_2 \rangle$, e.g., when you have to dinstinguishable (sic!) particles. Then the common Hilbert space is $\mathcal{H}_1 \otimes \mathcal{H}_2$, which consists of course not only of product states but all linear combinations thereof. Of course, the product states are part of the Hilbert space, and thus it is possible to prepare the two-particle system in such product states. These are by definition the states, for which the particles are NOT entangled. Einstein called this very nicely "separability", i.e., there are states.

But as is easy to see, the separable states (i.e., the product states) are quite special, and indeed interactions between the two particles lead to linear combinations which cannot be written as product states, and then you call the parts of the system "entangled". That's the origin of the violation of the Bell inequalities and thus the stronger-than-classical correlations of far-distant parts on an entangled quantum system.

To start with you always use particles created in some local process and usually the entanglement is due to some conservation law:

E.g., in the original EPR argument you can think of two (asymptotic) free particles originating from a particle decay. The original particle has a pretty well defined momentum. Now suppose it decays at some time $t=0$ in some region (e.g., take an $\alpha$-decaying nucleus in a cloud chamber, decaying to the $\alpha$ particle and the daughter nucleus). The resulting asymptotic free state is a free $\alpha$ particle and the doughter nucleus with pretty well defined total momentum and a pretty well defined relative position. Note that these to observables are compatible, and you can prepare the two-particle system in a product state of these two observables,
$$\Psi(r,P)=\psi_{\text{rel}}(r) \tilde{\psi}_{\text{CM}}(P).$$
This you can Fourier transform to the product state
$$\Psi(r,R)=\psi_{\text{rel}}(r) \psi_{\text{CM}}(R),$$
where $\psi_{\text{CM}}(R)$ is a pretty broad distribution.

Now it's easy to transform this to the single-particle positions. You simply need to set
$$r=x_1-x_2, \quad R=\frac{m_1 x_1+m_2 x_2}{m_1+m_2}.$$
Wrt. to these observables you end up in the entangled state
$$\Phi(x_1,x_2)=\Psi(x_1-x_2,(m_1 x_1+m_2 x_2)/(m_1+m_2))=\psi_{\text{rel}}(x_1-x_2) \psi_{\text{cm}}[(m_1 x_1+m_2 x_2)/(m_1+m_2)].$$
Through the usual manipulations it's easy to see that the probability distributions for both $x_1$ and $x_2$ alone are broad, i.e., both positions are pretty much indetermined. The same holds true for the momenta (which you get by Fourier transforming $\Phi$ wrt. $x_1$ and $x_2$ of course).

If you determine by a measurement, which can be as accurate as you wish, the position of particle 1, then you get also a narrow distribution for the position of particle 2 (since $\psi_{\text{rel}}(x_1-x_2)$ is narrowly peaked). The same is true in momentum space: Determining $p_1$ well also determines $p_2$. Of course you can never determine both $x_1$ and $p_1$ well at the same time, because the precise position measurement prevents the momentum to be determined well too and vice versa.

As you see, there's no need for any further "mechanism" to describe the correlations due to entanglement (i.e., "inseparability") than the known quantum-dynamical rules, and entanglement is the rule rather than the exception.

If it comes to indistinguishable particles, it's even difficult to define separable states. For bosons the two-boson state with the two particles in the same state is an example. All other two-particle states are entangled in the one or the other observable due to the necessity of Bose symmetrization (most conveniently taken account of by using creation and annhilation field operators).

 

[DarMM]

Sure it does. The authors talking about quantum nonlocality all talk about the Schrodinger Equation. The Schrodinger Equation is not QFT. It's non-relativistic QM

Technically it is in QFT, though not often used.

Usually the nonlocality in quantum foundations is defined in the ontological models framework which is a general framework that applies equally to discussing QM and QFT.

There is no confirmed experimental fact that "Alice steers Bob"

I think @DrChinese may be referring to Quantum Steering which is a term in Quantum Information.

 

[DrChinese]

1. I do not see how the assumption that QFT as outlined by @vanhees71 is classically local causal is warranted.

2. I also somewhat disagree with the following point:

DrChinese said:
My point is the other way: experimental fact, plus virtually any assumption about quantum theory (QM or QFT or whatever that includes the HUP) shows us that Alice's choice of measurement basis casts Bob's particle into a pure state determined solely by Alice (from an infinite number of such).

I see no reason for this claim. There is a joint choice of measurement bases for Alice and Bob and QFT (and every correct theory) yields the correct results for this combination of measurements. Within this framework it does not matter which measurement comes first and one does not have to assume any causal influence. It of cause does not rule out such an influence either, but there is no need to assume one.

1. Naturally I agree. 2. I mostly agree with your statement, except you skip the situation in which one measurement unambiguously occurs first. So here's how I would summarize:

a. In a Bell test in which the order of Alice and Bob's measurements is NOT well defined: there is no clear underlying statement about causality that can be made. We agree on this.

b. In a Bell test in which the order of Alice and Bob's measurements IS well defined with Alice acting first: Alice can be said to steer Bob. (Of course the outcomes are themselves random and are not determined by any known factor.) This is what Weinberg means when he states: "according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem.. " Of course he is referring to the original EPR paradox as the basis for this statement, where ordering was assumed. But he also means that nothing currently prevents us from executing the experiment so that ordering is in fact clear.

c. Even in the b. case, there are quantum interpretations which there is no causality; i.e. the decisions of both Alice and Bob are part of the overall context. In these, the action at a distance cannot be said to be caused by anything and there is no direction of action. Relational Blockworld is such a theory, for example. So even though we say there is steering (which implies causal direction), this is more of a linguistic aid than anything else.NOTE: Just in case you would like a specific reference, here is an incredible experiment that demonstrates not only steering; it demonstrates ONE-WAY steering! That is: Alice can steer Bob but Bob cannot steer Alice! Of course, I am simplifying somewhat as this is a very complex setup.

https://arxiv.org/abs/1206.4446
The distinctive non-classical features of quantum physics were first discussed in the seminal paper by A. Einstein, B. Podolsky and N. Rosen (EPR) in 1935. In his immediate response E. Schrödinger introduced the notion of entanglement, now seen as the essential resource in quantum information as well as in quantum metrology. Furthermore he showed that at the core of the EPR argument is a phenomenon which he called steering. In contrast to entanglement and violations of Bell's inequalities, steering implies a direction between the parties involved. Recent theoretical works have precisely defined this property. Here we present an experimental realization of two entangled Gaussian modes of light by which in fact one party can steer the other but not conversely. The generated one-way steering gives a new insight into quantum physics and may open a new field of applications in quantum information.

 

[DarMM]

Figure 1 in this paper has a nice diagram of the correlation hierarchy:
https://www.nature.com/articles/s41598-018-34463-y

 

[DarMM]

But as is easy to see, the separable states (i.e., the product states) are quite special, and indeed interactions between the two particles lead to linear combinations which cannot be written as product states, and then you call the parts of the system "entangled". That's the origin of the violation of the Bell inequalities and thus the stronger-than-classical correlations of far-distant parts on an entangled quantum system.

Not quite. Nonclassical correlations are strictly stronger than entanglement. The hierarchy is:
Coherence -> Discord -> Entanglement -> Steering -> Nonclassical Correlations

Local hidden variable models can replicate entanglement and even steering. It's only the final level of the hierarchy they cannot replicate.

 

[DrChinese]

I will respectfully repeat my many earlier requests:

1. I have provided repeated quotes, references, books, etc. that support everything I have said. At every turn, I have been either had the quote marginalized as if the author meant something else, or didn't know what they were talking about, or it was ignored. PeterDonis even dissed me for saying it was an argument from authority, when in fact forum rules require me to be able to back up what I say with suitable references. And I have been giving relevant quotes from the best. Please, quit marginalizing my proper support for my position. Which is:

Quantum Nonlocality (spooky action at a distance) is a generally accepted feature of Quantum Mechanics in all of its forms (from QM to QFT), as indicated by thousands of experiments and their respected authors.2. I have asked for quotes, references, etc supporting any position other than the above (especially the idea that QFT is local realistic or local noncontextual; or the idea that quantum interpretations are rendered unneeded because QFT answers everything). At no time has a single such reference been provided. (The most I have received is "read any book on QFT" which is absurd on the face of it.)

When anyone is challenged here, the protocol is to provide adequate specific clear references. So please: produce. Fair is fair. What I am saying (see bold above) is orthodox within the scientific community, and should be shielded if anything for that reason (although I have supported it many times over). The other position is far outside the norm, a position I have never read in a thousand papers on the subject (although I am ready and willing to stand corrected).

It makes no sense to tell newbies - or well-advanced readers for that matter - that current scientific consensus is that Quantum Nonlocality is not experimental fact, or that QFT explains Bell experiments by a purely local mechanism. No one knows the mechanism, that is where the interpretations come in - and why we have ongoing threads about these.

-DrC

 

[DrChinese]

There is no confirmed experimental fact that "Alice steers Bob". The confirmed experimental fact is that their measurement results show correlations that violate the Bell inequalities. You are confusing experimental facts with theory-dependent claims.

It seems obvious that since the Alice and Bob measurements commute, neither can "steer" the other, since the results do not depend on the order in which the measurements are made. And the experimental evidence does not show that either one "steers" the other; all it shows is correlations that violate the Bell inequalities.

You appear to agree that nobody knows what underlying mechanism produces those correlations; but your claim about "steering" is a claim that we do know what the mechanism is--"steering" is precisely such a mechanism.

I agree that it seems like there must be some underlying mechanism that produces the correlations that violate the Bell inequalities, and "nonlocal connection" is as good a name for this unknown mechanism as any. But the fact remains that the mechanism is unknown (and even our belief that there must be some such mechanism might possibly be wrong).

As I have said many times (and agreeing with you): no one knows the mechanism. As for steering: of course there are plenty of experiments where Alice "steers" Bob because Alice acts first. But that word ("steers") is a linguistic artifact, precisely because the mechanism is unknown. In fact, the idea that the future influences the past is a well accepted possibility within quantum mechanics (and I stress the word "possibility"). The point is that the well-documented effect is called Quantum Nonlocality (your "nonlocal connection"), whatever it is and however it happens. That is the case even though it APPEARS that Alice is steering Bob. I certainly don't dispute that if Alice measures first, that perhaps it is actually Bob steering Alice; or that neither steers either. So again, agreeing with you.

If you thought I was saying otherwise, my apologies as I was not clear. There is an effect called Quantum Nonlocality (also called spooky action at a distance); it is well documented by experiment; and it can be measured by correlations that cannot be explained by actions limited to a light cone.

And note that nowhere is QFT required for this discussion. QFT being an enhanced relativistic QM, the state of the art. Still it adds nothing substantive to resolve things - there certainly are no few interpretations today than 50 years ago. Many mysteries were well identified by 1935, and subsequent theory and experiment take us little farther than confirming this statement from EPR*:

"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." * Of course they immediately dismissed this conclusion as unreasonable.

 

[Cthugha]

1. Naturally I agree.

I am not sure, you got me right. ;) Just to make sure: my opinion is that the standard QFT version described in detail by @vanhees71 is not classically local causal. I do not actually see how one gets the impression that he describes it as such.

2. I mostly agree with your statement, except you skip the situation in which one measurement unambiguously occurs first. So here's how I would summarize:

a. In a Bell test in which the order of Alice and Bob's measurements is NOT well defined: there is no clear underlying statement about causality that can be made. We agree on this.

Yes, no problem here.

b. In a Bell test in which the order of Alice and Bob's measurements IS well defined with Alice acting first: Alice can be said to steer Bob. (Of course the outcomes are themselves random and are not determined by any known factor.) This is what Weinberg means when he states: "according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem.. " Of course he is referring to the original EPR paradox as the basis for this statement, where ordering was assumed. But he also means that nothing currently prevents us from executing the experiment so that ordering is in fact clear.

c. Even in the b. case, there are quantum interpretations which there is no causality; i.e. the decisions of both Alice and Bob are part of the overall context. In these, the action at a distance cannot be said to be caused by anything and there is no direction of action. Relational Blockworld is such a theory, for example. So even though we say there is steering (which implies causal direction), this is more of a linguistic aid than anything else.NOTE: Just in case you would like a specific reference, here is an incredible experiment that demonstrates not only steering; it demonstrates ONE-WAY steering! That is: Alice can steer Bob but Bob cannot steer Alice! Of course, I am simplifying somewhat as this is a very complex setup.

Well, quantum steering is a nasty beast, especially as the term goes back to Schrödinger, but was more or less ill-defined until Wiseman's seminal paper came out (PRL 98, 140402 (2007), https://arxiv.org/abs/quant-ph/0612147). I fully agree that quantum steering can be made to be one-way, but this does not depend on the temporal order of events, but on the "quality" of the states given to Alice and Bob. Bob's state is contaminated with additional vacuum, which is obviously uncorrelated with Alice's state. Thus, starting from a certain degree of "contamination", the space of available joint states depends more strongly on Alice's measurement than on Bob's and this is called one-way-steering. This does not change by adding long delay lines on either the side of Alice or Bob. Otherwise interpretations such as QBism would have been ruled out already.

It makes no sense to tell newbies - or well-advanced readers for that matter - that current scientific consensus is that Quantum Nonlocality is not experimental fact, or that QFT explains Bell experiments by a purely local mechanism. No one knows the mechanism, that is where the interpretations come in - and why we have ongoing threads about these.

I do not really see your point here. Maybe I am missing something simple. The consensus is that local realism is not a viable option. I fully agree with that. There is no consensus whether non-locality or non-realism/contextuality is more suitable (or both). Essentially, all @vanhees71 does, is to merge the minimal statistical interpretation with QFT and by doing so, QFT of course reproduces what is expected in Bell-type experiments. It just has the standard drawback of the minimal interpretation that some people find it lacking in terms of ontology. In a nutshell it is "shut up and calculate", which obviously does not require collapse or ontological non-locality and of course is fully described by knowing the state preparation procedures. However, QFT is of course silent on how to interpret the math.

 

[PeterDonis]

Then you didn't read my example completely. The B particle is measured well after Alice steers.

But, unless I'm misunderstanding, the QM prediction for the correlations for this case is exactly the same as for the case where the measurements are spacelike separated, so one would expect the same underlying mechanism, whatever it might be, to be involved in both. So any such mechanism cannot be one that only makes sense if the measurements are timelike separated as they are in what you describe.

Assuming no retrocausality (an easy assumption when we are debating quantum nonlocality), then either Alice steers Bob - or Bob steers Alice.

I understand that this is your favored interpretation. I do not think it is justified to claim that it is an experimental fact. The experimental fact is correlations that violate the Bell inequalities.

 

[PeterDonis]

Technically it is in QFT, though not often used.

Huh? The Schrodinger Equation is a non-relativistic equation.

If you want to say it appears in a non-relativistic approximation that can be derived from QFT, then yes, I have already mentioned that. But that's not the same as saying the Schrodinger Equation is relativistic. It isn't Lorentz invariant, so it isn't relativistic.

 

[PeterDonis]

Usually the nonlocality in quantum foundations is defined in the ontological models framework

What is a good reference to learn more details about this framework?

 

[DarMM]

Huh? The Schrodinger Equation is a non-relativistic equation.

If you want to say it appears in a non-relativistic approximation that can be derived from QFT, then yes, I have already mentioned that. But that's not the same as saying the Schrodinger Equation is relativistic. It isn't Lorentz invariant, so it isn't relativistic.

No I am saying it occurs in QFT not as a nonrelativistic approximation.

If $\phi$ is a generic field then we have:
$$i\frac{\partial}{\partial t}\Psi\left[\phi,t\right) = \hat{H}\left(\hat{\phi},\hat{\pi}\right)\Psi\left[\phi,t\right)\\
\phi \in \mathcal{S}^{'}\left(\mathbb{R}^{d-1}\right)
$$

With $\mathcal{S}^{'}\left(\mathbb{R}^{d-1}\right)$ the space of tensor and Group rep valued tempered distributions on a spacelike hypersurface.

 

[PeterDonis]

There is an effect called Quantum Nonlocality (also called spooky action at a distance); it is well documented by experiment; and it can be measured by correlations that cannot be explained by actions limited to a light cone.

I would say that the correlations violating the Bell inequalties is "quantum nonlocality"; the question is what "effect" or "mechanism" is going on behind the scenes to produce the correlations, and as you say, nobody knows the answer to that at this point.

nowhere is QFT required for this discussion

QFT is not required to model the experimental situations under discussion, no--at least it isn't in the sense that non-relativistic QM makes accurate predictions of the results and using QFT to make the predictions doesn't significantly change them.

However, QFT has a very different ontology from non-relativistic QM. In fact, as my exchange with @Demystifier earlier in the thread shows, it's not entirely clear what that ontology is, since things look very different in the path integral formulation than they do in the canonical formulation. But whatever that ontology is, it isn't quantum states assigned to spatially extended systems. And all of the discussion about foundations that I've seen uses an ontology of quantum states assigned to spatially extended systems. That seems like an obvious issue to me.

Possibly the ontological models framework that @DarMM mentioned addresses this.

 

[PeterDonis]

I am saying it occurs in QFT not as a nonrelativistic approximation.

Don't you have to pick a preferred frame for this to work?

 

[DarMM]

What is a good reference to learn more details about this framework?

https://arxiv.org/abs/1501.03202https://arxiv.org/abs/1409.5041https://arxiv.org/abs/quant-ph/0406166https://arxiv.org/abs/1409.1570
I found the last the best, but others have learned from the former ones.

 

[DarMM]

Don't you have to pick a preferred frame for this to work?

Yes, but it transforms between frames correctly, i.e. it's true in all frames. It's not a non-relativistic approximation.

It can require more renormalizations than the Heisenberg picture though.

 

[PeterDonis]

it transforms between frames correctly

How does the transformation law work? Is it something like the ADM or Hamiltonian formulation of General Relativity?

 

[DarMM]

How does the transformation law work? Is it something like the ADM or Hamiltonian formulation of General Relativity?

It's not manifestly Lorentz invariant so the transformation is quite complex and doesn't take the form of a simple law.

The book:
K.O. Friedrichs, Mathematical aspects of the quantum theory of fields (Interscience, New York, 1953)

Includes comments on it, as does the work of Luscher and Symanzik beginning with this paper:
https://www.sciencedirect.com/science/article/pii/055032138590210X?via=ihub
It's Lorentz invariant due to how distributional subspaces map into each other.

 

[PeterDonis]

It's not manifestly Lorentz invariant so the transformation is quite complex and doesn't take the form of a simple law.

The book:
K.O. Friedrichs, Mathematical aspects of the quantum theory of fields (Interscience, New York, 1953)

Includes comments on it, as does the work of Luscher and Symanzik beginning with this paper:
https://www.sciencedirect.com/science/article/pii/055032138590210X?via%3Dihub
It's Lorentz invariant due to how distributional subspaces map into each other.

Ah, ok; that makes me feel better that at least I wasn't missing something obvious.

 

[DarMM]

I don't think "ontology" is the right word here. In particular, in the path-integral formulation of QFT there are no field operators at all, but ontology should not depend on the formulation.

it's not entirely clear what that ontology is, since things look very different in the path integral formulation than they do in the canonical formulation

Note the path integral is only well-defined in a Riemannian space, not in Lorentzian spacetimes. Since some spacetimes have no analytic continuation to a Riemannian space there is no path integral in general.

 

[DrChinese]

1. ... my opinion is that the standard QFT version described in detail by @vanhees71 is not classically local causal. I do not actually see how one gets the impression that he describes it as such.

2. I do not really see your point here. Maybe I am missing something simple. The consensus is that local realism is not a viable option. I fully agree with that. There is no consensus whether non-locality or non-realism/contextuality is more suitable (or both). Essentially, all @vanhees71 does, is to merge the minimal statistical interpretation with QFT and by doing so, QFT of course reproduces what is expected in Bell-type experiments. It just has the standard drawback of the minimal interpretation that some people find it lacking in terms of ontology. In a nutshell it is "shut up and calculate", which obviously does not require collapse or ontological non-locality and of course is fully described by knowing the state preparation procedures. However, QFT is of course silent on how to interpret the math.

1. Per Vanhees71: "Under the assumption of a non-local deterministic theory there's be the violation to the space-time model of special relativity, but that contradicts the empirical facts about its very validity, particularly the universality of the speed of light in vacuum. The only conclusion from this experiment (as from many others) thus can be that non-local deterministic models contradict fundamental physics, which is not the case for local (microcausal) relativistic QFT, which in turn describes the observed results of all Bell tests known today. "

He is flat out saying that a non-local deterministic theory (Bohmian Mechanics being one) is excluded as a viable option. That is certainly far from consensus, even if there is not a relativistic version of Bohmian Mechanics at this time.

He is also saying QFT is local microcausal. I admittedly do not follow the distinction between "local causal" and "local microcausal". However, if I don't follow that distinction, I doubt many others do either unless they are knee deep in QFT. The term "microcausal" does not show up in papers on entanglement, ergo I assume it is not relevant. In fact, I would say as a rule that elements of QFT (as opposed to older QM) are not usually referenced in papers on entanglement.

2. I agree with everything you say here. So apparently the point missed is: whether it is non-locality or non-realism/contextuality that rules, the effect is called Quantum Nonlocality in the literature and it is a generally accepted feature in the quantum world. Attempting to mask this by calling it "nonlocal correlations that result from local microcausality" goes against the grain of almost any publication, either lay or scientific. Just this year, an entire book was written on this so I guess we should call them up and tell them to retitle it "Local Microcausality". So I would say it is very misleading to label it "local microcausality" when the Bell options are to reject locality or to reject realism/contextuality. I can't even get Vanhees71 to acknowledge that QFT is either nonlocal or contextual. So obviously he is trying to have his cake and eat it too.

@Cthugha the rest of this below is not directed at you, but to all.

----------------------------------------------------

How are we supposed to get a useful message across in our many threads if we are not using standard arguments and terminology? We can't be publishing book-long arguments to answer straight-forward questions. The OPs won't be able to interpret them.

If Steven Weinberg published a graduate level book in 2012 on Quantum Mechanics saying the following 2 statements, and I am getting flak for stating these exact words as my position: something is seriously wrong. I don't think it's with me. And this is not Weinberg being sloppy with language either (which a couple of posters here have accused him of being, unfairly and in my opinion insultingly).

"There is a troubling weirdness about quantum mechanics. Perhaps its weirdest feature is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas. "

"...according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem ..."

or from Vaidman (2019):

"It is important to understand what the meaning of nonlocality is in quantum theory. Quantum theory does not have the strongest and simplest concept of nonlocality, which is the possibility of making an instantaneous observable local change at a distance. However, all single-world interpretations do have actions at a distance. The quantum nonlocality also has an operational meaning for us, local observers, who can live only in a single world. Given entangled particles placed at a distance, a measurement on one of the particles instantaneously changes the quantum state of the other, from a density matrix to a pure state. It is only in the framework of the many-worlds interpretation, considering all worlds together, where the measurement causes no change in the remote particle, and it remains to be described by a density matrix."

If anyone here is afraid to make these statements because they are not suitably detailed or accurate enough, lord help us.

-DrC

 

[DrChinese]

DrChinese said:
There is an effect called Quantum Nonlocality (also called spooky action at a distance); it is well documented by experiment; and it can be measured by correlations that cannot be explained by actions limited to a light cone.

I would say that the correlations violating the Bell inequalties is "quantum nonlocality"; the question is what "effect" or "mechanism" is going on behind the scenes to produce the correlations, and as you say, nobody knows the answer to that at this point.DrChinese said:
nowhere is QFT required for this discussion

QFT is not required to model the experimental situations under discussion, no--at least it isn't in the sense that non-relativistic QM makes accurate predictions of the results and using QFT to make the predictions doesn't significantly change them.

So we agree on every essential. Actions are happening that cannot be bounded by a light cone, and we call that Quantum Nonlocality (replacing the now out-dated* phrase "spooky action at a distance"**).

We could call the effect "Quantum Locality", but I hope it is obvious why that would not be a good label. I don't think it's suitable to label it "Local Microcausality" for the same reason. The word LOCAL is completely misleading in both cases, and does not match common usage. So I strenuously object to its usage alongside descriptions of entanglement. Obviously, entangled systems have spatial extent; so that should preclude any description as local.* Although apparently it is not as outdated as I thought: China Shatters “Spooky Action at a Distance” Record, Preps for Quantum Internet (2017)

**Our own @Demystifier published an article saying not only that Einstein used this phrase originally in 1935, he actually had the basic idea for entanglement earlier, in 1930.

 

[PeterDonis]

Perhaps its weirdest feature is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas. "

No problem with this at all.

"...according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem ..."

At least on the evidence of many threads here at PF, I think this is a very unfortunate choice of terminology since, when you actually dig into the details, it doesn't mean what the plain words taken at face value appear to a lay person to mean. The plain words taken at face value appear to mean that you can transmit signals FTL by measuring one of a pair of entangled particles; but you can't. How many threads have we had here where we've had to explain that exact point to newbies? Often many times in the same thread because they simply can't wrap their minds around the fact that a quantum physicist would use such language to describe something that can't be used to send information.

this is not Weinberg being sloppy with language either (which a couple of posters here have accused him of being, unfairly and in my opinion insultingly)

Apart from the substantive issues, I do not agree with the claim that it is insulting to point out what seems to be an obvious issue with a particular choice of language, such as the issue I have explained in a bit more detail just above. Even Nobel Prize winning physicists can make mistakes. And given what I have read of Weinberg's writings, I think he would agree that any claim made in a scientific text is fair game for questioning. Science is not done by making or accepting authoritative pronouncements. The issue I am pointing out above is one I would be perfectly happy to point out to Weinberg directly if I were in a classroom or lecture or conference with him.

 

[DarMM]

Actions are happening that cannot be bounded by a light cone, and we call that Quantum Nonlocality

I would say the correlations cannot be explained by local future directed single-valued dynamical processes.

 

[DrChinese]

DrChinese said:
Assuming no retrocausality (an easy assumption when we are debating quantum nonlocality), then either Alice steers Bob - or Bob steers Alice.

I understand that this is your favored interpretation. I do not think it is justified to claim that it is an experimental fact. The experimental fact is correlations that violate the Bell inequalities.

I said ASSUMING no retrocausality. I am not attempting to push an interpretation, but certainly that could be an "out" for bringing back locality.

But my real point is this: There is good reason to use the term "Quantum Nonlocality" - rather than just saying "correlations that violate the Bell inequalities". You have waltzed over a key fact here: the existence of perfect correlations! So these 2 things together are much more stringent:

1. Perfect correlations from entangled pairs.
2. Violation of Bell inequalities from entangled pairs.

If you had only the first, you could assert "local hidden variables" (although you'd need a lot). If you had only the second, you could talk about "nonlocal correlations". But to have both forces us to acknowledge that the connection between 2 entangled particles is something that acts very tightly, in each and every pair. It's not simply a statistical tendency.

 

[PeterDonis]

How are we supposed to get a useful message across in our many threads if we are not using standard arguments and terminology?

As a point of information, the Mentors are working on guidelines for separating out discussions on QM interpretations and foundations into a separate forum. This would also include guidelines for what the ground rules would be for QM threads outside that separate forum, including things like what the accepted standard terminology would be. I expect that we'll be running those guidelines by the advisors for review and comment before going live.

 

[DrChinese]

As a point of information, the Mentors are working on guidelines for separating out discussions on QM interpretations and foundations into a separate forum. This would also include guidelines for what the ground rules would be for QM threads outside that separate forum, including things like what the accepted standard terminology would be. I expect that we'll be running those guidelines by the advisors for review and comment before going live.

Ya'll are so good, I should have guessed that would be coming.

 

[PeterDonis]

There is good reason to use the term "Quantum Nonlocality" - rather than just saying "correlations that violate the Bell inequalities". You have waltzed over a key fact here: the existence of perfect correlations!

Yes, this is a fair point. "Quantum Nonlocality" certainly is easier to say and type than "correlations that violate the Bell inequalities, plus perfect correlations at certain measurement angles".

 

[DrChinese]

I would say the correlations cannot be explained by local future directed single-valued dynamical processes.

No disagreement, but I hope we don't have to say that every time...

 

[DrChinese]

Yes, this is a fair point.

PeterDonis: 25,872
DrChinese: 1

 

[akvadrako]

No disagreement, but I hope we don't have to say that every time...

But what else could you say? You could say they can't be explained by Bell-local theories, but that's a tautology. Bell's theorem has quite a few assumptions, as does the ontological models framework.

 

[DrChinese]

But what else could you say?

Well, I guess my vote would be... Quantum Nonlocality.

(Please forgive me for that...)

 

[Cthugha]

Just a short response as it is getting late here.

He is also saying QFT is local microcausal. I admittedly do not follow the distinction between "local causal" and "local microcausal". However, if I don't follow that distinction, I doubt many others do either unless they are knee deep in QFT. The term "microcausal" does not show up in papers on entanglement, ergo I assume it is not relevant. In fact, I would say as a rule that elements of QFT (as opposed to older QM) are not usually referenced in papers on entanglement.

Well, we have Witten, who wrote quite a detailed review paper on entanglement in QFT (Rev.Mod.Phys. 90, 045003 (2018), https://arxiv.org/abs/1803.04993). Reinhard Werner and others also frequently emphasize that the story is more complicated than one usually assumes. Indeed semi-popular papers rarely make use of anything more complicated. They would be pretty dumb to do so. Let me give more details in the next response.

2. I agree with everything you say here. So apparently the point missed is: whether it is non-locality or non-realism/contextuality that rules, the effect is called Quantum Nonlocality in the literature and it is a generally accepted feature in the quantum world. Attempting to mask this by calling it "nonlocal correlations that result from local microcausality" goes against the grain of almost any publication, either lay or scientific. Just this year, an entire book was written on this so I guess we should call them up and tell them to retitle it "Local Microcausality". So I would say it is very misleading to label it "local microcausality" when the Bell options are to reject locality or to reject realism/contextuality.

Well, let me put it this way: Physics is the art of making models that make predictions that match reality (as quantified by experiments). So of course any effect should be considered within its model or framework. If one uses standard QM, which is not relativistically invariant anyway, it is quite natural to consider nonlocality and consider entanglement as a property of the states. It is the natural way of looking at entanglement in QM. In QFT, entanglement is already a property of the algebra of observables (see e.g. Witten's review above) and not just of the states. It is quite natural to consider different mechanisms and terminology.

I find it perfectly reasonable to talk about non-locality in the sense used within this thread, if both author and reader are aware that they are having a discussion on the QM level. This is the framework most publications use. I just think it is good practice to keep in mind that there are more complete theoretical frameworks out there.

 

[zonde]

Although I think what's sometimes missing in these accounts is that dropping determinism is not enough to get nonclassical correlations, you also have to drop the existence of countertfactuals.

The standard meaning in Quantum Foundations, that variables unmeasured have values.

Chapter 6 of Peres's monograph "Quantum Theory: Concepts and Methods" discusses it and it is used very explicitly in his proof of Bell's theorem. It's not an assumption called out in the original Bell proof, but it is the assumption Copenhagen rejects so it is important to recognize. He says famously "Unperformed experiments have no results"

I do not mean (and I want to empasize this as it is what people seemed to think it means in previous discussions) the trivial fact that unperformed experiments did not happen.

Rejecting assumptions about unmeasured variables goes not give way out of Bell type inequalities.
There is Eberhard's proof that is not assuming any mechanism behind detection events. Well it considers only models that take choice of measurement settings as an external variables (no superdeterminism) and detection events as experimental facts (single world), but then any scientific model has to do that.
I reproduced Eberhard's proof here as paper containing the proof is behind paywall.