Getting rid of nonlocality from quantum physics

[Vaxjo]

TL;DR Summary

The wide use of the notion of quantum nonlocality overshadows the real output of quantum theory, mystifies it, generates unjustified expectations and speculative interviews for mass-media of otherwise very respectable scientists. The aim of this post is to decouple nonlocality from quantum theory.

The main message of this post is that quantum theory is local, that "``spooky action at a distance'' was just shicky Einstein's slogan from a letter to Born in 1947. Einstein directed it against the individual interpretation of a quantum state. This interpretation is often referred as the Copenhagen interpretation} of QM. From his viewpoint, one should either reject this interpretation or confront with spooky action at a distance . This viewpoint was especially clearly presented in letters' exchange between Einstein and Schrödinger.

One of complications in getting rid of nonlocality from QM is that so-called ``"quantum nonlocality'' is two faced Janus. People freely refer to his different faces, mix them, and often cannot distinguish them. Two faces of nonlocality Janus are

L\"uders nonlocalty: apparent nonlocality of QM based on the projection postulate and discussed in the EPR-paper.

Bell nonlocality:} subquantum nonlocality based on misleading interpretation of violation of the Bell inequalities

Typically by saying "``quantum nonlocality'' one does not specify whether this is Luders or Bell nonlocalty (often a debater even does not understand the difference between these nonlocalities). So, the first step to elimination of nonlocality from quantum theory is learning its Janus-like structure.

 

[Demystifier]

What does the Vaxjo interpretation say about ontology? What is the microscopic world made of when we don't observe it?

 

[Vaxjo]

The Växjö interpretation as an interpretation of QM does not say anything about ontology. In the line with Bohr, QM is considered as a theory for observations, it does not describe reality as it is; QM is an epistemological theory, about extracting of knowledge from nature, but not nature as it is, when nobody looks at it.
The essence of the Växjö interpretation is elevation of the role of experimental context, once again I follow Bohr, who said something as the whole experimental arrangement should be taken into account. And contextuality (in Växjö interpretation) is understood in Bohr's sense, even for one observable the measurement is contextual, so it differs from contextuality in Bell's discussions. If one nevertheless wants ontology, then it is minimal as by Bohr, atoms, electrons, and quantum fields exist. Ontology has to be represented in ontological models, they are of course different from Qm (I remind that the latter is epistemological model). Thanks for your question!

 

[Demystifier]

The Växjö interpretation as an interpretation of QM does not say anything about ontology.

Fine. But I think you would agree that the Bell theorem implies that a certain class of ontological models (compatible with predictions of QM) must be nonlocal. My question for you is: Is there a plausible class of local ontological models (compatible with predictions of QM)? If yes, then what that class is? If no, then can we say that ontology is nonlocal?

Or to quote from https://arxiv.org/abs/2001.02977:
"Thus, the whole story is not about physics, but about the rules for update of the given probability distribution on the basis of measurement output."
If QM is not about physics, then which theory is about physics? And is that theory (the one which is about physics) local or nonlocal?

 

[vanhees71]

What does the Vaxjo interpretation say about ontology? What is the microscopic world made of when we don't observe it?

The "microscopic" world as far as we know is made of quarks, leptons, gauge bosons, and a Higgs boson. It's with quite some probability incomplete, because I think it's very likely that there really is "dark matter" in terms of other non-standard-model particles rather than MOND-like modifications of gravity due to the fact that one observes "dark-matter free" galaxies.

Whether or not you observe it the conservation laws tell you that if there's something at one time (where you observed it) it's still there when leaving it unobserved. You can even know the probabilities of any observable at any later time, if you have observed the system well enough at one time.

According to the minimal interpretation there's not more to know about the system than what's in the state operator (statistical operator), and these are probabilities (or probability distributions).

In relativistic standard formulations of QFT all interactions are local. There is no spooky-action at a distance, as Einstein thought when taking the Copenhagen collapse interpretation literally. Imho Einstein was right with this criticism of the Copenhagen collapse interpretation but wrong in the assumption it's an argument against QT as a whole. Just taking the Born rule seriously and assuming that Nature is intrinsically probabilistic is enough to make it a consistent theory about Nature (though Einstein wouldn't like this view too, because he thought obviously that a physical theory should be deterministic, which QT is no in this statistical interpretation).

What Einstein was also wrong about was his criticism against non-separability, which is what's described by entanglement which can lead to correlations (and only correlations!) between far-distant parts of a quantum system, and that's what's tested in Bell experiments, which all show that QT is incompatible with the assumption of any kind of local deterministic hidden-variable model. Since the violation of Bell's inequality is observed and the predictions of local relativistic QFT (about e.g., the observations made with entangled photon pairs in zillions of Bell experiments, demonstrating quantum teleportation, entanglement swapping, and the violation of Bell's inequality) are correct, there's no need for any "expalanations" of apparent paradoxes concerning the measurement process whatsoever. The only prize you have to pay is to abandon the hope for a deterministic worldview and accept the intrinsic randomness of nature.

Last but not least, a physicist doesn't care about ontology but only about observable facts, and a good theory should simply describe and predict observable facts, which QT obviously does with great success, including the prediction of intrinsic randomness of nature, which in the above stated sense is one of the best established observational facts ever.

 

[Demystifier]

...

Well, I don't think that one should take too seriously any non-mathematical statement on quantum interpretations too seriously.

 

[vanhees71]

Right, and the math delivers probabilities for the outcome of measurements, no more and no less, and this seems to describe the observational facts. Case closed! No need for further non-mathematical debates. The more I discuss about it the wisest advice one can students gives concerning QT: "Shut up and calculate!"

 

[Lord Jestocost]

...atoms, electrons, and quantum fields exist.

EXIST??
Notions of convenience which represent nothing but an abstract encodement of a set of potentialities or possible outcomes of measurements.

 

[Demystifier]

The more I discuss about it the wisest advice one can students gives concerning QT: "Shut up and calculate!"

Physicist to philosopher: Shut up and calculate!
Philosopher to physicist: Calculate and shut up!

 

[martinbn]

What does the Vaxjo interpretation say about ontology? What is the microscopic world made of when we don't observe it?

What do you mean by the second questions?! What does it matter if we observe it or not, it is made of the same things no matter what.

 

[Demystifier]

What do you mean by the second questions?! What does it matter if we observe it or not, it is made of the same things no matter what.

Some physicists from the Copenhagen spectrum would disagree.

 

[martinbn]

Some physicists from the Copenhagen spectrum would disagree.

Why do you think so?!

 

[Demystifier]

Why do you think so?!

I've already explained it to you several times before, but you didn't agree with my arguments.

Anyway, how would you interpret a denial of realism in local interpretations of the Bell theorem?

 

[martinbn]

I've already explained it to you several times before, but you didn't agree with my arguments.

Here it is not the case if I agree with you or not. It just doesn't make sense to me. Or may be I should say, that I don't understand what your statement means. I would be happy if I knew what it meant, no matter whether I agree with it or not.

 

[martinbn]

I'll try to explain what I don't understand. Your question implies that the answer to the question "what is the world made of?" depends on whether it's been measured or not. How can that be! If you measure, it is made of electrons, protons,... And if you don't measure, of course the same, what else? And I don't think that anyone, Copenhagen or not, has anything different to say, or can you quote someone?

 

[Lord Jestocost]

Heisenberg, for example:

"In the experiments about atomic events we have to do with things and facts, with phenomena that are just as real as any phenomena in daily life. But the atoms or the elementary particles themselves are not as real; they form a world of potentialities or possibilities rather than one of things or facts."

Paul Davies, for example:

"What, then, is an electron, according to this point of view [Copenhagen, LJ]? It is not so much a physical thing as an abstract encodement of a set of potentialities or possible outcomes of measurements. It is a
shorthand way of referring to a means of connecting different observations via the quantum mechanical formalism. But the reality is in the observations, not in the electron."Bernard d'Espagnat, for example:

"What quantum mechanics tells us, I believe, is surprising to say the least. It tells us that the basic components of objects – the particles, electrons, quarks etc. – cannot be thought of as "self-existent". The reality that they, and hence all objects, are components of is merely "empirical reality"."

 

[vanhees71]

Heisenberg! Yes, he and Bohr were those making the whole thing ununderstandable. Fortunately there's also Dirac, Born, Schrödinger, and Pauli who made it understandable so that it can be used to describe the atomic (and subatomic) events, which are things and facts, even according to the formalism of QT. The only thing, which many people seem to have still difficulties with is that it's not deterministic, but why should it be? Nature doesn't care about our prejudices.

 

[Lord Jestocost]

What agenda are you following with this Heisenberg/Bohr "bashing"?

That you don't understand Heisenberg or Bohr, is your personal problem.

Other physicists, who have some background in philosophy, do!

 

[EPR]

And if you don't measure, of course the same, what else? And I don't think that anyone, Copenhagen or not, has anything different to say, or can you quote someone?

If you don't measure, electrons have definite positions? If not, what is this 'same' you refer to, when compared with measured electrons?

 

[martinbn]

If you don't measure, electrons have definite positions? If not, what is this 'same' you refer to, when compared with measured electrons?

No, they don't have positions if you don't measure their positions, but how does that change the fact that electrons are part of the composition of the world whether you measure anything or not?

 

[PeroK]

If you don't measure, electrons have definite positions? If not, what is this 'same' you refer to, when compared with measured electrons?

In single-particle QM at least, the electron must still exist (between measurements). If it did not, then why would a second measurement detect an electron and not something else? Or nothing?

If you say that the wave-function exists, then it's the wave-function associated with an electron. Practically that amounts to the same thing. There must be an electron with probability $1$.

The philosophical argument that the electron itself may not exist between measurements is not (IMHO) demanded by Copenhagen; only that its dynamic properties are described by probabilities - and not well defined except by measurement.

PS I guess it boils down to whether you consider the conservation of probablity to imply continuous existence.

 

[Lord Jestocost]

...but how does that change the fact that electrons are part of the composition of the world whether you measure anything or not?

What fact? That's your personal belief, nothing more!
Why do so many mistake their mental images, which are in their mind (and not in the external world) for the "reality"?

Bertrand Russell:

"We cannot find out what the world looks like from a place where there is nobody, because if we go to look there will be somebody there; the attempt is as hopeless as trying to jump on one's own shadow."

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

Thread reopened. Please bear in mind that this is the QM foundations and interpretations forum and many statements will of necessity be people's opinions. This is an area where there are no known "right" answers.

 

[PeterDonis]

What fact? That's your personal belief, nothing more!
Why do so many mistake their mental images, which are in their mind (and not in the external world) for the "reality"?

The term "electron" is not just a personal belief or a mental image. It refers to an abundance of experimental evidence, not to mention everyday experience (at least, it used to be when CRT televisions and monitors were common), that is parsimoniously explained by a particular theoretical construct. @martinbn is simply pointing out that that theoretical construct does not just say "electrons are there when you measure them"; it says that there is always this thing called "electron" (or more precisely "electron field" since QFT is the most fundamental version of QM we currently have), and the experimental phenomena are particular manifestations of this thing. QFT certainly does not say that the electron field is only there when we measure it.

Please be charitable when interpreting what others are saying, particularly here when, as I mentioned in my previous post, there are no known "right" answers to the questions under discussion.

 

[bhobba]

it does not describe reality as it is;

Well physics, including QM, is a mathematical model. Mathematical models relationship to reality, whatever that is, is up for grabs.

My view of QM is its simply how we make sense out of complex probability vectors. Write the matrix of the Markov Chain for turning a coin over once a second. Now we can ask - what is the 'state' at half a second. You take the 'square root' of the matrix and low and behold you get complex states. Using things like Gleason's Theorem QM is just a way of making sense out of such musings. Does it apply to nature - well that is an experimental matter - but QM itself is just a model using novel mathematical ideas that follow from other mathematical ideas in the same way mathematics often progresses, by asking interesting questions.

Of course that is not its history, nor how it's usually taught, but IMHO a much clearer way of looking at it that leads to less confusion. It's not just my view:
https://www.scottaaronson.com/democritus/lec9.html

In my view we have theorems that shed light on the issue of locality in QM, but really QM is silent on the matter. There is one 'exception' however, as is made clear in Ballentine, chapter 3, Schrodinger's equation etc depends of the Galilean transformations that are fundamentally non-local, but that is not what is usually meant in discussions of quantum locality.

Thanks
Bill

 

[Demystifier]

I'll try to explain what I don't understand. Your question implies that the answer to the question "what is the world made of?" depends on whether it's been measured or not. How can that be! If you measure, it is made of electrons, protons,... And if you don't measure, of course the same, what else? And I don't think that anyone, Copenhagen or not, has anything different to say, or can you quote someone?

I cannot explain it to you if I don't understand how do you think about this stuff. So I have a question for you. The Bell theorem is often stated in the form that local reality is incompatible with QM. So either locality or reality (or both) are supposd to be wrong. Copenhagenish interpretations often claim that locality is fine but reality is wrong. My question to you is: How do you interpret the idea that reality is wrong? What does it mean to you?

In addition, what do you mean by electron? A click in the electron detector? A wave function of the electron? A mental picture of an electron by a physicist? Something else? What?

 

[PeterDonis]

So either locality or reality (or both) are supposd to be wrong. Copenhagenish interpretations often claim that locality is fine but reality is wrong. My question to you is: How do you interpret the idea that reality is wrong? What does it mean to you?

Before answering this question, we should first be precise about exactly what "locality" and "reality" mean in the context of Bell's Theorem.

"Locality" means that the joint probability function for the two measurements factorizes, i.e., schematically, $P(A, B|a, b, \lambda) = P(A|a, \lambda) P(B|b, \lambda)$, where $A, B$ are the measurement results, $a, b$ are the measurement settings, and $\lambda$ are the hidden variables. In Bell's original paper [1], the locality assumption corresponds to equation (2).

"Reality" (or "realism" as I think it is more often called) means that one can treat mathematical expressions for measurement settings and results different from those that actually occurred as well-defined. In Bell's original paper, this corresponds to introducing the third vector $c$ between equations (14) and (15), and treating it as well-defined.

[1] https://cds.cern.ch/record/111654/files/vol1p195-200_001.pdf

 

[Demystifier]

"Reality" (or "realism" as I think it is more often called) means that one can treat mathematical expressions for measurement settings and results different from those that actually occurred as well-defined.

This particular definition of realism (which is not the only definition of realism used, but is perhaps more precise than other definitions) is called counterfactual definiteness.

 

[Demystifier]

No, they don't have positions if you don't measure their positions, but how does that change the fact that electrons are part of the composition of the world whether you measure anything or not?

The right question is what they do have even when we don't measure it? Clearly they have a name (an "electron"), but what else? And if there is nothing specific which they have (except the name), then how is that different from the claim that they don't exist?

 

[martinbn]

My question to you is: How do you interpret the idea that reality is wrong? What does it mean to you?

This is where you refuse to understand that realism in quantum interpretations is used differently than it is in philosophy. Realism means that all observables have definite values at all times. Antirealism rejects that proposition. It has nothing to do with the existence of say an electron, it is about whether the electron's position, momentum etc have definite values.

In addition, what do you mean by electron? A click in the electron detector? A wave function of the electron? A mental picture of an electron by a physicist? Something else? What?

Electron is the name for the thing that have objective existence.

 

[bhobba]

This particular definition of realism (which is not the only definition of realism used, but is perhaps more precise than other definitions) is called counterfactual definiteness.

That's one reason Bell made progress, he gave more precise definitions of things that are often quite 'fuzzy' like reality etc. We need to somehow overcome what Feynman very profoundly said in his famous lectures: “We can't define anything precisely. If we attempt to, we get into the paralysis of thought that comes to philosophers... one saying to the other: you don't know what you are talking about! The second one says: what do you mean by talking? What do you mean by you? What do you mean by know?” This is a very hard part, maybe even the hardest, of investigating such questions.

Thanks
Bill

 

[martinbn]

The right question is what they do have even when we don't measure it? Clearly they have a name (an "electron"), but what else? And if there is nothing specific which they have (except the name), then how is that different from the claim that they don't exist?

The electron has charge for example, doesn't it?

 

[Demystifier]

Realism means that all observables have definite values at all times. Antirealism rejects that proposition.

By that definition, Bohmian interpretation is an antirealist interpretation. And yet it is nonlocal. Moreover, the Bell theorem proves that all "antirealist" interpretations (in the sense in which the Bohmian interpretation is "antirealist") must be nonlocal. How about that?

Electron is the name for the thing that have objective existence.

Can we make any mathematical statement about properties of those objectively existing entities? Yes we can. The Bell theorem is one such statement, it says that objective properties must obey nonlocal laws.

 

[Demystifier]

The electron has charge for example, doesn't it?

Actually it doesn't. The charge $e$ is just a coupling "constant" describing the interaction between two electrons. It is not a property of one electron itself. In fact it is not even a constant, but depends on the distance (or momentum transfer) between the electrons.

Or perhaps by charge you meant the operator $\hat{Q}$? It is an observable like any other (energy, momentum, etc) and hence amenable to Heisenberg uncertainty relations.