B Why is it assumed communication through entanglement would be FTL?

[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.

 

[DarMM]

Rejecting a common sample space clearly permits violations of Bell's inequality, it's what QM actually does where there is no Gelfand homomorphism mapping all four CHSH variables into one sample space.

 

[zonde]

Essentially, it boils down to Mermin's tongue-in-cheek statement (American Journal of Physics 66, 753-767 (1998) , https://arxiv.org/abs/quant-ph/9801057):

"My complete answer to the late 19th century question “what is electrodynamics trying to tell us” would simply be this:
Fields in empty space have physical reality; the medium that supports them does not.

Having thus removed the mystery from electrodynamics, let me immediately do the same for quantum mechanics:
Correlations have physical reality; that which they correlate does not."

I don't see how this statement can be taken seriously.
In Bell experiments correlations correlate detection events given measurement settings. Would you say following Mermin that either or both detection events and measurement settings do not have physical reality?

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.

What do you mean by "joint choice of measurement bases for Alice and Bob"? Choices of measurement bases are made by Alice and Bob at two spacelike separated events. You need nonlocality influencing choice of measurment settings (!) or superdeterminism to have something like that.
Well, if you know the choice of measurement basis for either Alice or Bob at the moment when entangled particles are produced you can replicate correlations with LHV, no doubt about that.

 

[zonde]

Rejecting a common sample space clearly permits violations of Bell's inequality, it's what QM actually does where there is no Gelfand homomorphism mapping all four CHSH variables into one sample space.

Basically you are saying that Many local Worlds permits violations of Bell's inequality? Or I didn't understood you correctly?

 

[Demystifier]

Neither formulation has states for spatially extended systems

The Schrodinger picture does have a state $|\psi(t)\rangle$ for spatially extended system. It is not manifestly Lorentz invariant, but there is a Lorentz-invariant version based on many-time formalism.

 

[Demystifier]

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.

But at least Minkowski spacetime has an analytic continuation to an Euclidean space, right? This means that path integral QFT in the absence of gravity is well defined. QFT in curved spacetime has other problems too, but the full theory of quantum gravity is expected to solve those one day.

 

[Demystifier]

**Our own @Demystifier published an article

Nice to see that there is a popular journalist exposition of my work. Thanks!

 

[Cthugha]

I don't see how this statement can be taken seriously.
In Bell experiments correlations correlate detection events given measurement settings. Would you say following Mermin that either or both detection events and measurement settings do not have physical reality?

Huh? experimental correlations on detectors do not fall from the sky. You can measure spin correlations, momentum correlations, OAM correlations time-bin correlations or whatever. The message is quite clear. Having a state with well-defined correlations does not imply that the individual correlated quantites are well defined. Well-defined values of spin correlations do not imply that the individual spin values are well-defined. In fact, quite the opposite is true as these are usually complementary quantities. See, e.g. Phys. Rev. A 62, 043816 (2000) or Phys. Rev. A 63, 063803 (2001).

What do you mean by "joint choice of measurement bases for Alice and Bob"? Choices of measurement bases are made by Alice and Bob at two spacelike separated events. You need nonlocality influencing choice of measurment settings (!) or superdeterminism to have something like that.
Well, if you know the choice of measurement basis for either Alice or Bob at the moment when entangled particles are produced you can replicate correlations with LHV, no doubt about that.

I do not get your post. Why do you bring LHV models into play? This thread is not about LHV models. By joint choice I mean just that: a set of detector settings. Ordinary standard QM already gives the correct predictions for every possible detector setting Alice and Bob might use. It does so irrespective of which interpretation of QM you might use as long as it is consistent with QM, which means that it cannot be local realistic. QFT as the more demanding theory contains ordinary QM in the non-relativistic limit and of course also already gives the correct predictions for every possible detector setting Alice and Bob may use. I do not see how this could be even controversial. The math of QFT also does so irrespective of how you want to interpret QFT as long as your interpretation is consistent with QFT. This again means no local realism. However, as QFT is relativistically invariant already, it would be somewhat awkward to sacriface this feature by going for a non-local interpretation. At least unless you assume that there is some need for potentials with an infinite number of derivatives in some even deeper theory or something like that.

 

[DarMM]

But at least Minkowski spacetime has an analytic continuation to an Euclidean space, right? This means that path integral QFT in the absence of gravity is well defined. QFT in curved spacetime has other problems too, but the full theory of quantum gravity is expected to solve those one day.

Definitely it's well defined in Minkowski spacetime. It's just an interesting fact as such.

 

[DarMM]

Basically you are saying that Many local Worlds permits violations of Bell's inequality? Or I didn't understood you correctly?

No, I'm saying the lack of a common sample space/context permits violation of Bell's inequalities. This isn't really anything to do with Many Worlds.

 

[atyy]

He is also saying QFT is local microcausal.

It is standard and correct to say that relativistic QFT is "local microcausal". "Microcausality" means no superluminal communication.

No superluminal communication is a distinct concept from the classical relativistic causality addressed by Bell's theorem. Thus the violation of Bell's inequalities disallows classical relativistic causality, but it allows no superluminal communication or microcausality.

The funny thing about microcausality, is that a technical trick to impose it in QFT is to pretend the observables are real, and to follow steps that are pretty much the same as imposing classical relativistic causality. Thus some people mistake microcausality for classical relativistic causality, and mistakenly say that QFT has microcausality (true) and therefore it has classical relativistic causality (false). I believe this is the mistake that @vanhees71 is making when he says that the nonlocality of collapse is at odds with microcausality.

BTW, I should note that even Weinberg occasionally uses sloppy language that makes this mistake. See the comments of @Demystifier and @humanino in this thread: https://www.physicsforums.com/threads/cluster-decomposition-and-epr-correlations.409861/.

 

[DrChinese]

It is standard and correct to say that relativistic QFT is "local microcausal". "Microcausality" means no superluminal communication.

Thanks for clarifying that.

I don't fully understand why that label "microcausal" would be a useful distinction in this thread. After all: the issue here is "spooky action at a distance" via entanglement, which does not offer FTL signalling. On the other hand, I guess it makes sense to point out that a relativistic formulation of QM explicitly requires signal locality. Which also answers the OP.

 

[Cthugha]

After all: the issue here is "spooky action at a distance" via entanglement, which does not offer FTL signalling. On the other hand, I guess it makes sense to point out that a relativistic formulation of QM explicitly requires signal locality. Which also answers the OP.

Just to add to that, it should be said that we already know that "no signalling" is an assumption that is too weak to represent reality. It is well known that in theory one can consider states that are non-signaling as defined by relativistic causality and still not realizable in quantum mechanics. We know that e.g. from the seminal paper by Popescu and Rohrlich.
We also know that it is locality applied to uncertainty relations (or rather: a general formulation of uncertainty and a certain version of locality termed relativistic independence that in a nutshell boils down to being unable to tinker with local uncertainty relations from a distance) that exactly give the bounds: https://advances.sciencemag.org/content/5/4/eaav8370 (should be open access).

 

[zonde]

Huh? experimental correlations on detectors do not fall from the sky. You can measure spin correlations, momentum correlations, OAM correlations time-bin correlations or whatever. The message is quite clear. Having a state with well-defined correlations does not imply that the individual correlated quantites are well defined. Well-defined values of spin correlations do not imply that the individual spin values are well-defined. In fact, quite the opposite is true as these are usually complementary quantities. See, e.g. Phys. Rev. A 62, 043816 (2000) or Phys. Rev. A 63, 063803 (2001).

Well, you was the one who quoted Mermin who said correlations fall from the sky. If the quote is viewed in context it might become clear Mermin meant that correlated quantities are not well defined, that way it makes more sense actually.
But still this approach does not resolve Bell inequality question as Bell type inequality is provable without any reference to hypothetical quantities relying only on measurement settings and detections: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.47.R747 (see part II Bell inequalities for n<100%)

However, as QFT is relativistically invariant already, it would be somewhat awkward to sacriface this feature by going for a non-local interpretation.

And the alternative in your viewpoint is ... ?

 

[bobob]

Weinberg phrases the answer as: "Of course, according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem..."

But that cannot be correct. The measurements are spacelike separated, so they are made simultaneously. Neither measurement can occur "before" the other and create a cause and effect relationship because they cannot be time ordered. You would have to reject relativity. Apparently, what people must be arguing about is whether relativity is correct.

 

[zonde]

No, I'm saying the lack of a common sample space/context permits violation of Bell's inequalities. This isn't really anything to do with Many Worlds.

Can you explain what do you mean by "sample space/context" because it seems that you attach different meaning to "sample space" than the one used in probability theory.
Wikipedia: In probability theory, the sample space of an experiment or random trial is the set of all possible outcomes or results of that experiment.