Časlav Brukner, Richard Healey, and von Weizsäcker on the wave function

[gentzen]

For those who don't want to clutter the other thread, but still want to comment on the references given by Lord Jestocost.

Probably, a new paper by Časlav Brukner, entitled “Schrödinger’s Equation at 100: The Wave Picture That Helped and Possibly Hurt” (https://arxiv.org/abs/2604.26325), gives some hints to understand Feynman. One reads from the abstract:

"Schrödinger’s equation gave early quantum theory a visual language that looked like physics again: a wave evolving by a linear differential equation. This essay argues that the same success also seeded a recurring impulse to keep quantum theory “classical-looking” by treating the wave function as a physical wave. Schrödinger quickly realized that, for many-particle systems, the wave function is naturally defined on configuration space rather than ordinary physical space, blocking any straightforward reading of it as a literal classical wave. Read through Mach and Boltzmann, who shaped his intellectual outlook most deeply, his achievement appears double-edged: it provided an extraordinarily powerful picture for calculation and discovery, while also warning against taking that picture too literally.
I argue that this tension never fully disappeared. It still reappears in modern physics whenever the wave function, or in quantum field theory the field itself, is treated as ontology rather than as part of a representation tied to measurement and observational context, a point sharpened by Bell-type no-go theorems. The centenary moral is: use pictures boldly, but demote them ontologically."

In his book “The Structure of Physics” (the book is a newly arranged and revised English version of "Aufbau der Physik" by Carl Friedrich von Weizsäcker) Carl Friedrich von Weizsäcker remarks:

“But we can now see that the name "state" for the wavefunction |ψ⟩ is misleading. |ψ⟩ is nothing but a catalog of knowledge that follows from one observed fact and which determines the probabilities for possible future events.”

But we have a pre-interpretational problem. Before a wavefunction can be interpreted, it must be correctly written down.

Maybe, I don’t get the point.
I recommend that you read Richard Healey’s paper “Quantum Theory: A Pragmatist Approach”!

 

[gentzen]

I recommend that you read Richard Healey’s paper “Quantum Theory: A Pragmatist Approach”!

https://arxiv.org/abs/1008.3896

And another reference given by Lord Jestocost:

“In physics, a theory has three defining constituents: the physical phenomena, the mathematical formalism, and the interpretation. The phenomena are the empirical evidence about physical objects gathered by passive observation, typical for astronomy and meteorology, or by active experimentation in the laboratory. The formalism provides the adequate mathematical description of the phenomena and enables the physicist to make precise quantitative predictions about the results of future experiments. The interpretation is the link between the formalism and the phenomena.” [Bold by Lord Jestocost]

Berthold-Georg Englert, On Quantum Theory, Eur. Phys. J. D, volume 67, article number 238 (2013)

https://arxiv.org/abs/1308.5290

 

[Matterwave]

I will have to read the paper~ It looks quite interesting!

The centenary moral is: use pictures boldly, but demote them ontologically.

I wonder how this plays with the PBR theorem. Which unfortunately I also have not read. I only saw a presentation of it.

 

[Matterwave]

This demonstrated, in a precise sense, that measurement, or
more generally observation, is an integral and fundamental part of quantum theory.

I am inclined to push back on this unless Brukner is willing and able to provide a precise definition of measurement and observation "fundamentally". Otherwise I will always be stuck pondering how an operational notion became a fundamental part of a foundational theory.

 

[Sambuco]

I wonder how this plays with the PBR theorem. Which unfortunately I also have not read. I only saw a presentation of it.

PBR theorem rules out ontological models in which the wave function plays an epistemic role. Common $\psi$-epistemic interpretations (Copenhagen, QBism, RQM) do not correspond to ontological models, since they presuppose that the wave function is the most complete possible representation of a quantum system.

Lucas.

 

[Sambuco]

I am inclined to push back on this unless Brukner is willing and able to provide a precise definition of measurement and observation "fundamentally". Otherwise I will always be stuck pondering how an operational notion became a fundamental part of a foundational theory.

To learn about Brukner's point of view, you can read this paper.

Lucas.

 

[Matterwave]

To learn about Brukner's point of view, you can read this paper.

Lucas.

I read the abstract and the first section. Unfortunately, it is not convincing to me personally.

He points out 3 assumptions, one of which is:

(ii) measurements have definite
outcomes in the sense that only one outcome occurs;

And at the end of the abstract he states:

The solution lies in understanding that “facts” can only exist relative
to the observer.

If facts exist only relative to the observer, but measurements have only 1 outcome, then at face value, either there is only ever 1 observer or measurements aren't facts. I don't see how else to read this in plain English. "Occurs" doesn't mean "exists"? What other possibilities?

Would you have a pointer to a part of the paper where he most precisely defines observation and measurement?

 

[Sambuco]

If facts exist only relative to the observer, but measurements have only 1 outcome, then at face value, either there is only ever 1 observer or measurements aren't facts. I don't see how else to read this in plain English. "Occurs" doesn't mean "exists"? What other possibilities?

I'm not entirely sure I understand what you're saying, but in any case, I think an example might help. Let's take the Wigner's friend experiment. Suppose that when the friend measures the spin of the particle in the z-direction, he observes spin up. Therefore, spin up is a fact relative to the friend, but not relative to Wigner, for whom the friend-particle system evolves unitarily toward the state $\ket{\psi}_{SF} = \frac{1}{\sqrt{2}} (\ket{\uparrow}_S \ket{\uparrow}_F + \ket{\downarrow}_S \ket{\downarrow}_F)$ and no fact has yet occurred (relative to him).

Would you have a pointer to a part of the paper where he most precisely defines observation and measurement?

On page 4, he discusses the possibility of introducing measurement/observation as a primitive concept, not deducible from others:

"If quantum theory is understood as a fundamental theory of observations and observers’ actions upon these observations, then measurement can be introduced as a primitive notion, which cannot be subject to a complete analysis, not even in principle (...) Such a view is consistent and self-contained, but in my opinion, it is not the whole story. It is silent about the question: what makes a photon counter a better device for detecting photons than a beam splitter? Yet the question is scientifically well posed and has an unambiguous answer (which manufacturers of photodetectors do know!)."

Later, he points to the appearance of distinguishable macroscopic states under POVM-type measurements as the requirement for the emergence of a new fact:

"Detection devices, such as photographic plates or photo-diodes, consist of a large number of constituents in a certain “metastable state”. Their interaction with the observed quantum systems brings them into a “stable state” that can be distinguished from the initial one even under coarse-grained observations. This transition is signified by the “click” in the detector or a new position of the pointer label."

Lucas.

 

[Matterwave]

I'm not entirely sure I understand what you're saying

The plain English reading (at least my reading) of assumption (ii) and the final conclusion in the abstract contradict each other. I realize there could be a more generous reading wherein they don't contradict each other, but I don't believe that reading was made clear in the abstract.

Let's take the Wigner's friend experiment. Suppose that when the friend measures the spin of the particle in the z-direction, he observes spin up. Therefore, spin up is a fact relative to the friend, but not relative to Wigner, for whom the friend-particle system evolves unitarily toward the state $\ket{\psi}_{SF} = \frac{1}{\sqrt{2}} (\ket{\uparrow}_S \ket{\uparrow}_F + \ket{\downarrow}_S \ket{\downarrow}_F)$ and no fact has yet occurred (relative to him).

Does assumption (ii), single definite outcomes, encompass the even more basic fact "whether a measurement occurred or not"? Or is assumption (ii) only making a statement about, e.g., the numerical values of measurement results and whether different people will agree on measurement results if they compare notes?

"If quantum theory is understood as a fundamental theory of observations and observers’ actions upon these observations, then measurement can be introduced as a primitive notion, which cannot be subject to a complete analysis, not even in principle (...) Such a view is consistent and self-contained, but in my opinion, it is not the whole story.

I read this, but thought he was explicitly saying it was not his position in this paper. But upon re-read, I suppose he must use this as a baseline since he says "whole story".

In that case, I can only say that I am more sympathetic to Bell's view, e.g. as presented in "Against Measurement" (1990), that such words as "observation" and "measurement" should not be found in our fundamental physics.

"Detection devices, such as photographic plates or photo-diodes, consist of a large number of constituents in a certain “metastable state”. Their interaction with the observed quantum systems brings them into a “stable state” that can be distinguished from the initial one even under coarse-grained observations.

If this is to serve as the definition of measurement for a fundamental theory, it would be nice if the bolded terms could be more precisely defined.

This transition is signified by the “click” in the detector or a new position of the pointer label."

This is an operational definition, and my objection in post #4 would still stand.

 

[Lord Jestocost]

I am inclined to push back on this unless Brukner is willing and able to provide a precise definition of measurement and observation "fundamentally". Otherwise I will always be stuck pondering how an operational notion became a fundamental part of a foundational theory.

Beforehand, you need to know what an event is as a concept of quantum theory.

Berthold-Georg Englert in “On Quantum Theory” (Eur. Phys. J. D, volume 67, article number 238 (2013)) (https://arxiv.org/abs/1308.5290), section "2 Events":

“The formalism of quantum theory can be used to predict probabilities – probabilities for events…..
.
.
Born’s rule tells us these probabilities — the probability for the ionization event, the probability for the photon-emission event, and the no-event probability for the molecule remaining unaffected. For that to have any meaning, the existence of events must be accepted in the first place. In this sense, then, the event is a preexisting concept of quantum theory. We cannot formulate the theory without this concept.” [Bold by LJ]

 

[Sambuco]

The plain English reading (at least my reading) of assumption (ii) and the final conclusion in the abstract contradict each other. I realize there could be a more generous reading wherein they don't contradict each other, but I don't believe that reading was made clear in the abstract.

Does assumption (ii), single definite outcomes, encompass the even more basic fact "whether a measurement occurred or not"? Or is assumption (ii) only making a statement about, e.g., the numerical values of measurement results and whether different people will agree on measurement results if they compare notes?

Oh, I think I understand what you're saying now. Assumption (ii) about "definite outcomes" is a way of saying that it's a single-world interpretation. That is, when the friend measures the spin of the particle, he obtains a definitive result (spin up, for example) and that's the end of it, there is no other parallel world where a friend might observed spin down. On the other hand, the "facts are only relative to the observer" means that the spin taking a definite spin value is only a "fact" for the friend, whereas for Wigner, his friend's "measurement" is only an interaction that he can model in a unitary way, so no "fact" has yet occurred for him (Wigner).

I read this, but thought he was explicitly saying it was not his position in this paper. But upon re-read, I suppose he must use this as a baseline since he says "whole story".

I interpret it the same way you do!

In that case, I can only say that I am more sympathetic to Bell's view, e.g. as presented in "Against Measurement" (1990), that such words as "observation" and "measurement" should not be found in our fundamental physics.

Yes, I think the article you mention is the most representative of the "other camp".

If this is to serve as the definition of measurement for a fundamental theory, it would be nice if the bolded terms could be more precisely defined.

This is an operational definition, and my objection in post #4 would still stand.

In a way, these types of interpretations (like Brukner's) assume that facts/events are the most fundamental elements of the theory, but at the same time, these events are not absolute/objective, but only certain "for all practical purposes", based on the evolution of a wave function that is also not fundamental. In that sense, there is a certain circularity, and it seems as if nothing is truly fundamental. Some people can live with it, but I understand it's hard to digest. I can't imagine someone like Tim Maudlin thinking this way.

Lucas.

 

[Matterwave]

Beforehand, you need to know what an event is as a concept of quantum theory.

Berthold-Georg Englert in “On Quantum Theory” (Eur. Phys. J. D, volume 67, article number 238 (2013)) (https://arxiv.org/abs/1308.5290)

Thanks for the reference! I don't think I will finish it, but I believe I read the relevant sections (mostly 1 and 2).

As an aside, it does appear like Englert makes some rather contentious claims in section 7 and 8 where he directly calls out:

...experimental data contradict Bell’s theorem [22,23], which implies that — as a statement about physical systems — the theorem is wrong.

As for accepting the existence of events -- it does appear to me that its "definition" (or maybe "conception" is better) by Englert already includes, to me, rather imprecise concepts that seem to just move the problem from "a precise definition of measurement" to "a precise definition of event". Especially point two:

Second, an event is irreversible, it leaves a document behind, a definite trace.

 

[Matterwave]

Oh, I think I understand what you're saying now. Assumption (ii) about "definite outcomes" is a way of saying that it's a single-world interpretation.

Thank you for the clarification. I had a feeling this is what Brukner was talking about, but was unsure since he didn't (in the abstract) call it out directly.

In a way, these types of interpretations (like Brukner's) assume that facts/events are the most fundamental elements of the theory, but at the same time, these events are not absolute/objective, but only certain "for all practical purposes", based on the evolution of a wave function that is also not fundamental.

Bell even has an abbreviation for this -- FAPP. I suppose if Brukner's view is there literally is nothing more fundamental and the formalism given is the best you can do, then that is certainly one view to take. It seems very similar to Bohr's (Copenhagen's) view on the QM formalism in this respect.

In that sense, there is a certain circularity, and it seems as if nothing is truly fundamental. Some people can live with it, but I understand it's hard to digest. I can't imagine someone like Tim Maudlin thinking this way.

I also could not imagine it haha.

 

[gentzen]

As an aside, it does appear like Englert makes some rather contentious claims in section 7 and 8 where he directly calls out:

...experimental data contradict Bell’s theorem [22,23], which implies that — as a statement about physical systems — the theorem is wrong.

I have now carefully read section 7 and 8, and then also section 6 which gets referenced there.
In my opinion, the stuff that Englert writes there can be defended, even if you could argue that he should have used friendlier words and not create even more confusion by giving a different meaning to words typically used in that context.

Englert's point in using the words like that is to show what he is objecting to in Bell's writing, by way of example.

 

[Matterwave]

In my opinion, the stuff that Englert writes there can be defended, even if you could argue that he should have used friendlier words and not create even more confusion by giving a different meaning to words typically used in that context.

I disagree with several things Englert writes, but I suppose the most egregious one is claiming "...the [Bell's] theorem is wrong".

He follows it up with:

Since there is no error in the reasoning that establishes the theorem from its assumptions, the flaw must be in the assumptions.

Bell's theorem is a mathematical theorem. From the assumptions and premises (e.g. locality, factorizability, statistical independence) he deduces an inequality which any locally causal theory (by Bell's own definition) needs to obey. The only way that it could be that the "theorem is wrong" is if the reasoning is wrong.

I can give an analogy. The Hairy Ball theorem says that on a (2-D) sphere there is no continuous vector field that is non-zero everywhere. It assumes you are on a sphere. I view Englert's claim as something akin to saying "we are not 2 dimensional creatures living on a sphere, therefore the hairy ball theorem is wrong."

Englert continues:

Specifically, it is the assumption that a mechanism exists that determines which detector will click for the next photon registered by an apparatus of the kind depicted in Fig. 1. There is no such
deterministic mechanism — quantum processes are fundamentally probabilistic, events are randomly realized...

I don't know what to make of this tbh. It's hard to parse even what Englert is precisely criticising here.

1. Bell's theorem is precisely about what a probabilistic local hidden variable theory can and can not predict. If Englert is saying Bell's theorem can only model a purely deterministic local hidden variable theory then he is just blatantly wrong so I will assume that's not what he means.
2. It is perhaps possible that Englert is criticizing outcome determinism. In that case, in the original Bell paper, outcome determinism is not an assumption but a deduction based on locality and his singlet state exhibiting perfect anti correlations. The CHSH inequality, which does not rely on the exact singlet state, also does not rely on outcome determinism. There's a discussion of that here: https://plato.stanford.edu/entries/bell-theorem/#Intr

 

[gentzen]

I disagree with several things Englert writes, but I suppose the most egregious one is claiming "...the [Bell's] theorem is wrong".

The point of Englert is that Bell's theorem only applies to theories "similar to Bohmian mechanics" in suitable ways:

Recall, for instance, the abstract of a recent paper, which begins with these words:

“Bell’s 1964 theorem, which states that the predictions of quantum theory cannot be accounted for by any local theory, represents one of the most profound developments in the foundations of physics.”

We submit: Doesn’t quantum theory itself, which is a local theory, account for its own predictions?
As the authors of this quote know very well, experimental data contradict Bell’s theorem [22, 23], which implies that — as a statement about physical systems — the theorem is wrong.

In such theories, it makes sense to think about locality in the way Bell did.

However, you should not ignore that in quantum theory, different ways to think about locality or causality might be more appropriate. Here is an example, how such a more appropriate way can look like:

10.2.1 Causality​

Causality is an extremely important postulate for quantum theory which nevertheless has an extremely simple interpretation:

If the output of a process is discarded, it may as well have never happened.​

10.4 Historical notes and references​

[...]
Causality, although it plays a very central role in this book, was the last one to enter the picture. Its importance became clear from the information theoretic axiomatization of Chiribella et al. (2010, 2011).

Chiribella, G., D’Ariano, G. M., and Perinotti, P. 2010. Probabilistic theories with purification. Physical Review A, 81(6), 062348.
Chiribella, G., D’Ariano, G. M., and Perinotti, P. 2011. Informational derivation of quantum theory. Physical Review A, 84(1), 012311.

From
Coecke B, Kissinger A. Picturing Quantum Processes: A First Course in Quantum Theory and Diagrammatic Reasoning. Cambridge University Press; 2017.

---

This example is by far not the only possible way. Here is another example:

Nicolas Gisin’s short book Quantum Chance nicely explains how the paradox arises that quantum mechanics is local and nonlocal at the same time: The randomness itself is nonlocal, and it must be really random, because otherwise this non-locality could be used for instantaneous signal transmission. At the same time, however, the randomness also becomes less problematic, because it is now clear in the sense of which idealization it must be perfect. After all, there is probably no such thing as mathematically perfect true randomness.

With respect to Gisin, there is reference
[4] B.-G. Englert et al., Z. Naturforsch. 47a, 1175-1186 (1992)
(Surrealistic Bohm Trajectories by Berthold-Georg Englert, Marian O. Scully, Georg Süssmann, and Herbert Walther)
in
Why Bohmian Mechanics? one and two-time position measurements, Bell inequalities, philosophy and physics (2015) by Nicolas Gisin

So both Englert and Gisin have thought deeply about Bohmian mechanics, so they earned the right to make such points.

 

[Lord Jestocost]

I disagree with several things Englert writes, but I suppose the most egregious one is claiming "...the [Bell's] theorem is wrong".

He follows it up with: Since there is no error in the reasoning that establishes the theorem from its assumptions, the flaw must be in the assumptions.

Sometimes a more extensive quote might be helpful!

"Since there is no error in the reasoning that establishes the theorem from its assumptions, the flaw must be in the assumptions. Specifically, it is the assumption that a mechanism exists that determines which detector will click for the next photon registered by an apparatus of the kind depicted in Figure 1. There is no such deterministic mechanism – quantum processes are fundamentally probabilistic, events are randomly realized – and the violation of Bell’s theorem by actual data confirms that." [Bold by LJ]

Berthold-Georg Englert in "On Quantum Theory", Eur. Phys. J. D, volume 67, article number 238 (2013) (https://arxiv.org/abs/1308.5290)

 

[Matterwave]

Sometimes a more extensive quote might be helpful!

See the remainder of my post where I quote most of the rest. :)

and the violation of Bell’s theorem by actual data confirms that.

Englert seems to confuse violations of Bell's inequalities with "violation of Bell's theorem". This seems a baffling take indeed.

 

[gentzen]

Englert continues:

Specifically, it is the assumption that a mechanism exists that determines which detector will click for the next photon registered by an apparatus of the kind depicted in Fig. 1. There is no such deterministic mechanism — quantum processes are fundamentally probabilistic, events are randomly realized...

I don't know what to make of this tbh. It's hard to parse even what Englert is precisely criticising here.

Englert has a good intuition on how "quantum processes" behave. Bell didn't think in terms of "processes", he thought in terms of "states" evolving in time. So for Bell, there has to be a "state" which determines "which detector will click for the next photon registered by an apparatus". Only then would you need a mechanism like Bell envisages, and any such mechanism able to reproduce the predictions of QM turns out to be non-local.

But Englert's inutition tells him that this is not a good way to understand quantum theory. Therefore, he writes unfriendly words like:

In a derivation of Bell’s theorem, ..., one relies on common-sense arguments of a certain plausibility in an attempt at describing quantitatively the properties of the joint probabilities in experiments similar to that of Fig. 3. Part of that common sense is a certain notion of locality: If the beam splitter in Fig. 1 is replaced by a mirror that the experimenter may or may not put in place, the free-will decisions by Alice should have no influence on the click frequencies that Bob records in his experiment, and vice versa.
The actual joint probabilities — ... — do not obey the restrictions that follow from that common-sensible adhockery, and why should they? Findings of an inadequate nonquantum formalism are irrelevant for quantum physics.

So when you take offense at his unfriendly words:

and the violation of Bell’s theorem by actual data confirms that.

Englert seems to confuse violations of Bell's inequalities with "violation of Bell's theorem". This seems a baffling take indeed.

his reaction is:

If the findings are at variance with the experimental data, as is the case here, we are reminded of the inappropriateness of the reasoning.

So while you accuse Englert to confuse Bell's inequality with Bell's theorem, he takes offense at how people (including Bell himself) interpret Bell's results:

Quantum theory makes the distinctive prediction that non-local correlations are instant: for example, a measurement of the polarization of one of a pair of quantum-entangled photons should immediately set the polarization of the other, no matter how far the photons are apart, without either photon’s polarization being in any way predetermined.

Bell’s 1964 theorem, which states that the predictions of quantum theory cannot be accounted for by any local theory, represents one of the most profound developments in the foundations of physics.

 

[gentzen]

I am inclined to push back on this unless Brukner is willing and able to provide a precise definition of measurement and observation "fundamentally". Otherwise I will always be stuck pondering how an operational notion became a fundamental part of a foundational theory.

Beforehand, you need to know what an event is as a concept of quantum theory.

Berthold-Georg Englert in “On Quantum Theory” (Eur. Phys. J. D, volume 67, article number 238 (2013)) (https://arxiv.org/abs/1308.5290), section "2 Events":

Brukner neither references Englert, nor uses the word "event". And I guess neither does he implicitly use that concept. Englert himself refers to Rudolf Haag, more precisely to [8,9]:

[8] Fundamental irreversibility and the concept of events (1990)

It is proposed that the transmutation from possibilities to facts should be introduced as an essential element in fundamental theory. This has no bearing on TCP-invariance. If indeterminism is accepted it leads to a picture of an evolving history formed by individual events and causal ties. In the low density regime it can be compared with the treatment of multiple collisions in quantum field theory.

[9] On the Sharpness of Localization of Individual Events in Space and Time (2013)

The concept of event provides the essential bridge from the realm of virtuality of the quantum state to real phenomena in space and time. We ask how much we can gather from existing theory about the localization of an event and point out that decoherence and coarse graining—though important—do not suffice for a consistent interpretation without the additional principle of random realization.

Brukner doesn't refer to Rudolf Haag either. So either he has to provide a (sufficiently) precise definition of measurement himself, or else refer to somebody who provided one. Without that, I don't see how he can be defended against the pushback from Matterwave.

 

[Sambuco]

Although I'm fully on board with the "operationalist" view, I agree that some passages in Englert's text are somewhat problematic, beyond what @Matterwave mentions about "violation of Bell's theorem" not being a good way to refer to "violation of Bell's inequalities." For example:

"And just like the other local theories do, quantum theory predicts nonlocal correlations that originate in local processes. It is true that joint probabilities that result from quantum processes can have stronger correlations than those available by nonquantum simulation."

The second sentence is precisely a consequence of the first one not being true. All other "local" theories, such as Maxwell's electromagnetism, explain correlations in a locally causal way by satisfying Reichenbach's principle of common causes. Quantum mechanics does not satisfy it, and in fact, that is one of the usual ways in which operationalists approach Bell's theorem.

Lucas.

 

[PeterDonis]

Englert seems to confuse violations of Bell's inequalities with "violation of Bell's theorem".

Could it be a language or translation issue? Could what we are reading as "violation of Bell's theorem" have been meant to say "violations of Bell's inequalities"? After all, the fact that we observe violations of Bell's inequalities experimentally does mean that in our actual world, at least one of the assumptions of Bell's theorem must be violated, which seems to be Englert's main point.

 

[gentzen]

If I remember correctly, the views of Rudolf Haag on locality are not too widespread, but not without merit.
[8] Fundamental irreversibility and the concept of events (1990)
[9] On the Sharpness of Localization of Individual Events in Space and Time (2013)
Since Englert refers to him, his positions are probably not overly widespread either. I guess I would have to read the above two references, to better understand how they relate to more standard Copenhagen views. (But first I want to finish reading Englert's paper.)

 

[PeterDonis]

the views of Rudolf Haag on locality are not too widespread

I think Haag's views on QFT in general are not widespread:

https://en.wikipedia.org/wiki/Haag's_theorem#Quantum_field_theorists'_conflicting_reactions

 

[Matterwave]

There are a few different threads of discussion coalescing here, so let me try to address each of them.

So either he has to provide a (sufficiently) precise definition of measurement himself, or else refer to somebody who provided one. Without that, I don't see how he can be defended against the pushback from Matterwave.

I agree. But I'm also ok to drop this if it distracts from the more interesting discussions. We can't discuss everything all at once. I don't mind it if you want to keep discussing it, but I won't push this one.

Englert has a good intuition on how "quantum processes" behave.

I don't doubt this. My complaint is in what he is claiming in this paper. Even people with good intuition can be wrong sometimes. See below.

But Englert's inutition tells him that this is not a good way to understand quantum theory. Therefore, he writes unfriendly words...

So when you take offense at his unfriendly words...

No worries, I don't take offense. :) He is free to say what he wants. I don't begrudge his perspective / rhetoric. However, I do think he is wrong on several technical points which I will try to elaborate on a bit more below.

Could it be a language or translation issue? Could what we are reading as "violation of Bell's theorem" have been meant to say "violations of Bell's inequalities"? After all, the fact that we observe violations of Bell's inequalities experimentally does mean that in our actual world, at least one of the assumptions of Bell's theorem must be violated, which seems to be Englert's main point.

I suppose it certainly could be a language/translation issue!

My most serious objection was indeed Englert's blatant use and reuse of:

violation of Bell’s theorem by actual data

...experimental data contradict Bell’s theorem [22,23], which implies that —

And indeed these would read better if he had simply instead said "Bell's inequalities". It does seem to me, though, that Englert's argument is somewhat predicated on "Bell is wrong" and he uses these provocative statements as rhetoric. I get the sense, and I could be wrong, that he is using a strawman of Bell to argue for the position that QM is strictly local even by Bell's own standard.

But even if it is rhetoric, then so be it. I won't belabor the point any further.

I will focus on a few technical errors that I see.

From Englert:

Specifically, it is the assumption that a mechanism exists that determines which detector will click for the next photon registered by an apparatus of the kind depicted in Fig. 1. There is no such
deterministic mechanism — quantum processes are fundamentally probabilistic, events are randomly realized...

I addressed this in my post #15, at the end. But I will expand. Bell is quite clear in his 1964 paper that (outcome) determinism is an implication of perfect anti-correlation and locality, it is not an assumption. Here's a quote from that paper "On the Einstein-Podolsky-Rosen Paradox":

Now we make the hypothesis, and it seems one at least worth considering, that if the two measurements are made at places remote from one another the orientation of one magnet does not influence the result obtained with the other. Since we can predict in advance the result of measuring any chosen component of $\vec{\sigma}_2$, by previously measuring the same component of $\vec{\sigma}_1$, it follows that the result of any such measurement must actually be predetermined.

This outcome determinism is manifest in Eqn (1) of Bell 1964:
$$
A(\vec{a},\lambda) = \pm 1, \, B(\vec{b},\lambda)= \pm 1
$$
I will assume that Englert is discussing this outcome determinism (or else, see my point 1. in post #15). Even if you grant that the 1964 Bell paper used this outcome determinism as an assumption (it's not, it's an implication, but for the sake of argument let's just say that it is an assumption), the later CHSH (version of Bell's) inequality does not need it to be proven. And this was shown by Bell himself in the paper "Introduction to the Hidden-Variable Question" (1971). See the constraint has changed from Eq. (1) of the 1964 paper to the relaxed form of Eq. (8) in the 1971 paper:
$$
|\bar{A}|\leq 1, \, |\bar{B}| \leq 1
$$
From there he derives the CHSH inequality. Another place Bell explicitly discusses this point about somehow "Bell assumed determinism, and that's where he's wrong" is in his essay Bertlmann's Socks and the Nature of Reality (1980).

Another objection.
Englert writes:

In a derivation of Bell’s theorem, or any of the many variants on record, the formalism of quantum theory is not used.

It seems that Englert completely misses the point. I don't see any other way to read this sentence, even if I substitute "inequality" for theorem here. If Englert means "theorem" then the statement is just wrong -- the content of Bell's theorem is that QM violates his inequalities. He necessarily needs to show that violation by using quantum theory. If "inequality" is meant here, then of course but it's a vacuous point. The point of a Bell Inequality is show a bound on local hidden variable (or as Bell would say, locally causal) theory.

 

[Sambuco]

It does seem to me, though, that Englert's argument is somewhat predicated on "Bell is wrong" and he uses these provocative statements as rhetoric.

I have the same feeling.

Bell is quite clear in his 1964 paper that (outcome) determinism is an implication of perfect anti-correlation and locality, it is not an assumption.

There is much discussion about this, since the "locality" assumption that Bell defines in his 1964 paper is closely related to the idea of "parameter independence" or "non-signaling", rather than to his later "local causality". The problem is that the notion of locality in 1964 paper is not sufficient to demonstrate predetermination, so many consider that his first version of the theorem in 1964 actually proves that QM predictions are not compatible with locality and predetermination. However, his second theorem (1976) introduces the notion of "local causality," which is sufficient in itself to demonstrate the incompatibility with QM predictions, as you said, i.e. predetermination is not needed.

Lucas.

 

[Matterwave]

However, his second theorem (1976) introduces the notion of "local causality," which is sufficient in itself to demonstrate the incompatibility with QM predictions, as you said, i.e. predetermination is not needed.

Lucas.

Could you reference the paper title?

 

[Sambuco]

Could you reference the paper title?

J. S. Bell, "The theory of local beables", Epistemological Lett. 9 (1976).

Lucas.

 

[Matterwave]

J. S. Bell, "The theory of local beables", Epistemological Lett. 9 (1976).

Lucas.

Ah, I've read this one, but I still need to sit with it a while longer.

 

[Sambuco]

I'm sharing a link to a paper by Wiseman & Cavalcanti that compares the two versions of Bell's theorem and how the two different camps ("realists" and "operationalists") react to them. An earlier work by one of the authors (Wiseman), although very similar, delves much deeper into Bell's papers and how they compare with other works, such as EPR paper.

Lucas.