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

#### DarMM

Gold Member
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

Gold Member
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

Gold Member
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

Mentor
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

Mentor
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

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

#### DarMM

Gold Member
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

Mentor
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

Mentor
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

Gold Member

I found the last the best, but others have learned from the former ones.

#### DarMM

Gold Member
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

Mentor
it transforms between frames correctly
How does the transformation law work? Is it something like the ADM or Hamiltonian formulation of General Relativity?

#### DarMM

Gold Member
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:

It's Lorentz invariant due to how distributional subspaces map into each other.

#### PeterDonis

Mentor
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

Gold Member
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

Gold Member
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

Gold Member
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

Mentor
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

Gold Member
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

Gold Member
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

Mentor
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

Gold Member
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

Mentor
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".