I The notion of locality in (Quantum) Physics should be clearly defined

[Demystifier]

Why that? Of course there are no self-adjoint operators in the lab nor Hilbert spaces and all that. That's the mathematical description. In the lab you have accelerators, detectors, lasers, and all that theoreticians don't want to get their hands dirty with ;-).

Exactly. So if we reserve the name "observable" for operators, then the quantities in the laboratory should not be called "observables". You certainly agree that in science we need precise language, so we should not use the word "observable" for two different things. It is exactly for this purpose that Bell introduced the word "beable", to distinguish it from the "observable".

 

[vanhees71]

Newton's theory of gravitation cannot be local in the here discussed sense, because there's no relativstic spacetime. The Wheeler-Feynman theory hasn't come to anything useful. It was a dead end. In condensed matter physics your field theories are usually not relativistic either and thus also there the microcausality principle cannot be formulated nor does it hold in any sense. Of course, Newtonian approximations are valid in their domain of applicability.

 

[WernerQH]

So, does this mean that your notion of locality must be a fundamental truth about nature?
Or even a fundamental feature of QFT? (There appears to be agreement that QFT is not (yet) in form that would satisfy mathematicians.)

 

[vanhees71]

The funddamental feature is causality in Minkowski space. The assumption of the microcausality condition for local self-adjoint operators that represent observables is a sufficient condition for the relativistic causality condition, and this can be realized in terms of local relativistic QFTs. For the details see Weinberg, QT of Fields, vol.1.

Whether there are other possibilities to construct relativistic QTs that obey the causality constraints of Minkowski spacetime, I don't know. At least I've never found any attempts in this direction in the literature.

 

[Demystifier]

Whether there are other possibilities to construct relativistic QTs that obey the causality constraints of Minkowski spacetime, I don't know. At least I've never found any attempts in this direction in the literature.

There is another possibility, it's string theory. Interestingly, string theory also violates a certain kind of "locality", which is different from both QFT definition of locality and Bell locality. In one paper, I argued that this intrinsic stringy-nonlocality can be avoided, at the expense of making Bell-nonlocality more explicit. https://arxiv.org/abs/hep-th/0605250

 

[vanhees71]

I don't know anything about string theory. What do you mean when you say "Bell-nonlocality"?

 

[Demystifier]

I don't know anything about string theory. What do you mean when you say "Bell-nonlocality"?

Bell nonlocality in string theory means the same as in all other quantum theories. I hope you are not asking me what Bell nonlocality means in quantum theory.

 

[vanhees71]

I ask you what you mean by Bell nonlocality, since locality or nonlocality seems to have completely different meanings when ever these words are used. For me the usual inconsistent lingo of the quantum-foundations community it simply means the violation of Bell's inequalities. We are again back at my plead to make clear definitions of what's meant when the word "locality" or "non-locality" are used!

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

Some more off topic posts have been deleted. Please keep discussion here on the specific topic of locality in QM. Discussion of fhe philosophy of science is off limits in this thread, and I have issued several zero point warnings for posts that violate that constraint. If you see someone else posting about philosophy of science, or anything else off the thread topic, please do not respond. Use the Report button if you think someone else's post is off limits.

 

[PeterDonis]

Thread reopened.

 

[Demystifier]

I ask you what you mean by Bell nonlocality

I mean the thing you call nonseparability.

 

[vanhees71]

So it simply means entanglement. Why then not saying entanglement instead of using non-locality with an altered meaning. I think the entire "foundational issues" are simply plagued by inprecise language, and that's why it never comes to any conclusion but discusses the same pseudo-problems over and over again. Once even the most stuborn philosophers should realize that on the scientific level the case is closed: It's QT that describes Nature correctly and not "local realistic hidden-variable theories". Science has moved on in the meantime: Entanglement is used for engineering purposes nowadays (quantum cryptography, quantum computing, and all that). Thus QT now becomes part of the engineers' curriculum at the universities of applied sciences!

The true open question in foundational physics is the understanding of the quantum theory of spacetime and/or the gravitational interaction! It'll of course not solved by philosophy but through better and better (astronomical) observations and a new idea from it by theorists.

 

[Demystifier]

I think the entire "foundational issues" are simply plagued by inprecise language, and that's why it never comes to any conclusion but discusses the same pseudo-problems over and over again.

Yes, that's a part of the problem.

Once even the most stuborn philosophers should realize that on the scientific level the case is closed: It's QT that describes Nature correctly and not "local realistic hidden-variable theories".

But a part of the problem is that even scientists are not always sufficiently precise. For example, by "observable" sometimes they mean the operator, and sometimes a thing in the laboratory. Philosophers are motivated to make such things more precise, but in this attempt they produce new imprecisions. In my opinion, a better precision can be achieved by a cooperation between scientists and philosophers.

 

[WernerQH]

Entanglement is used for engineering purposes nowadays (quantum cryptography, quantum computing, and all that).

Why not use the less mysterious term "correlations"? If you would, as I do, see QFT as a statistical theory describing the correlations between isolated events distributed in spacetime, you would find "locality" a very strange starting assumption.

 

[vanhees71]

Entanglement is a very specific kind of correlations. That's why we have all these debates about them!

I don't mean what you mean by "isolated events distributed in spacetime". QFT is just a theory predicting the probabilities for the outcome experiments.

 

[WernerQH]

QFT is just a theory predicting the probabilities for the outcome experiments.

Surely it's more than that. When it is used in cosmology, does it mean that the Universe is just an experiment?
Some people are inclined to think like that, but I did not expect you among them.

 

[Lord Jestocost]

Entanglement is a very specific kind of correlations. That's why we have all these debates about them!

To my mind, the debates are merely about the question:
Why do we experience these correlations in our experiential reality?

 

[vanhees71]

We experience these correlations in our experiential reality (what other reality should be?), because obviously QT is a correct description of Nature and not something invented by EPR what they think should be the right description. That's, how the natural sciences work under the best of all circumstances: You have two well-defined models about how Nature is described (this was of course not given by EPR but by Bell about 30 years later for the model "local, realistic HV theory", while it was established for modern QT already in 1926 ;-)), and you can thus objectively decide which of the models describe the observations better, and that's clearly QT. It's even better: There's not the slightest hint that QT delivers any wrong predictions for the outcome of experiments yet!

 

[AndreasC]

That's, how the natural sciences work under the best of all circumstances: You have two well-defined models about how Nature is described (this was of course not given by EPR but by Bell about 30 years later for the model "local, realistic HV theory", while it was established for modern QT already in 1926 ;-)), and you can thus objectively decide which of the models describe the observations better, and that's clearly QT

So what is your objection to observationally equivalent theories/interpretations?

 

[Lord Jestocost]

We experience these correlations in our experiential reality (what other reality should be?), because obviously QT is a correct description of Nature and not something invented by EPR what they think should be the right description. That's, how the natural sciences work under the best of all circumstances: You have two well-defined models about how Nature is described (this was of course not given by EPR but by Bell about 30 years later for the model "local, realistic HV theory", while it was established for modern QT already in 1926 ;-)), and you can thus objectively decide which of the models describe the observations better, and that's clearly QT. It's even better: There's not the slightest hint that QT delivers any wrong predictions for the outcome of experiments yet!

I completely agree. The heart of the problem in all these disputes is quantum probability and randomness. Andrei Khrennikov and Karl Svozil/1/ put it – to my mind – in a nutshell:

“It might not be totally unreasonable to claim that, already starting from some of the earliest (in hindsight) indications of quanta in the 1902 Rutherford–Soddy exponential decay law and the small aberrations predicted by Schweidler [6], the tide of indeterminism [7,8] was rolling against chartered territories of fin de siécle mechanistic determinism. Riding the waves were researchers like Exner, who already in his 1908 inaugural lecture as rector magnificus [9] postulated that irreducible randomness is, and probability theory therefore needs to be, at the heart of all sciences; natural as well as social. Exner [10] was forgotten but cited in Schrödinger’s alike “Zürcher Antrittsvorlesung” of 1922 [11]. Not much later Born expressed his inclinations to give up determinism in the world of the atoms [12], thereby denying the existence of some inner properties of the quanta which condition a definite outcome for, say, the scattering after collisions.

Von Neumann [13] was among the first who emphasized this new feature which was very different from the “in principle knowable unknowns” grounded in epistemology alone. Quantum randomness was treated as individual randomness; that is, as if single electrons or photons are sometimes capable of behaving acausally and irreducibly randomly. Such randomness cannot be reduced to a variability of properties of systems in some ensemble. Therefore, quantum randomness is often considered as irreducible randomness.

Von Neumann understood well that it is difficult, if not outright impossible in general, to check empirically the randomness for individual systems, say for electrons or photons. In particular, he proceeded with the statistical interpretation of probability based on the mathematical model of von Mises [14,15] based upon relative frequencies after admissible place selections.” [Bold by LJ]

/1/ Khrennikov, A., Svozil, K.: Quantum probability and randomness. Entropy 21(1), 35 (2019)

 

[vanhees71]

So what is your objection to observationally equivalent theories/interpretations?

I have no objection against equivalent theories. If they make the same predictions they are the same theory. I only have strong objections against interpretations that contradict the very foundation of the theory they pretend to interpret.

 

[AndreasC]

If they make the same predictions they are the same theory.

I don't think it's quite so simple...

I only have strong objections against interpretations that contradict the very foundation of the theory they pretend to interpret.

What do you mean exactly? The point is that you can have theories with different foundational postulates that deliver the same observational predictions. That's what, say, Bohmian mechanics does. It's not really the same theory, but you can't rule it out observationally.

Suitable Lorentz ether theories are also observationally equivalent to special relativity, but the shift in perspective was important!

 

[Fra]

I only have strong objections against interpretations that contradict the very foundation of the theory they pretend to interpret.

That a theory is corroborated theory doesn't mean its foundations or constraints are impeccable.

Some object to precisely to the QM foundations(but for different reasons), but doesn't dispute predictions as an effective theory.

/Fredrik

 

[Demystifier]

I have no objection against equivalent theories. If they make the same predictions they are the same theory. I only have strong objections against interpretations that contradict the very foundation of the theory they pretend to interpret.

What if two theories have different foundations (so they contradict the foundations of each other), but make the same measurable predictions?

I've asked you this question so many times, in different forms, but never got a self-consistent answer.

One example is quantum theory with and without the collapse. They contradict the foundations of each other, but make the same measurable predictions. How do you decide which of the two is right? (And please, don't repeat your mantra that collapse only describes the special case of projective experiments, because all POVM measurements can be described by a generalized collapse rule, see the book by Nielsen and Chuang, which is a book about practical applications of QM, not about interpretations.)

Another example is Bohmian and standard quantum theory. What if the Bohmian version was developed first, but then later someone developed what we call "standard" version, would you be against the standard version because it contradicts foundations of the Bohmian theory?

@PeterDonis, don't delete it because collapse and Bohmian mechanics have a lot to do with quantum nonlocality.

 

[Demystifier]

Suitable Lorentz ether theories are also observationally equivalent to special relativity, but the shift in perspective was important!

Excellent point! The Einstein special theory of relativity was formulated as an interpretation of Maxwell equations and Lorentz theory of ether, but it contradicted the very foundations of the ether theory. Einstein didn't base his interpretation on the Michelson-Morley experiment. @vanhees71 , according to his own principles, should be the first to oppose the Einstein no-ether interpretation.

 

[vanhees71]

What an utter nonsense. Why should I oppose Einstein's no-ether interpretation? To the contrary: Something that's no observable nor necessary to formulate the theory isn't needed and complicates the issues. It's as with Bohmian trajectories, which are just superfluous complications of math and neither needed to formulate QT nor does it provide anything observable.

 

[Demystifier]

I have no objection against equivalent theories. If they make the same predictions they are the same theory. I only have strong objections against interpretations that contradict the very foundation of the theory they pretend to interpret.

This is not what you meant. You really meant this:

"I have no objection against equivalent theories. If they make the same predictions and accept the same foundations, they are the same theory. I only have strong objections against interpretations based on other foundations, different from foundations of my favored interpretation. My favored interpretation is the best because it is the minimal interpretation, which means that it makes the smallest number of assumptions, where, of course, the foundations of the theory are not counted as assumptions, because they are the right foundations that every theory compatible with present experiments should be based on."

 

[Demystifier]

Why should I oppose Einstein's no-ether interpretation?

Because it contradicts the very foundation of the Lorentz ether theory it pretends to interpret.

 

[vanhees71]

Einstein didn't intepret Lorentz ether theory but introduced an entirely new concept, i.e., introducing a new description of space and time and a new realization of inertial frames.