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I've no clue, what C. F. von Weizsäcker was after ;-).
Maybe, the following might be of help.vanhees71 said:I've no clue, what C. F. von Weizsäcker was after ;-).
If the knowledge is merely knowledge of a possible location of the particle, then we have conceptualised a property of the particle independent from a measurement context. To avoid the charge of introducing hidden variables (see common critiques of psi-epistemic interpretations), the knowledge represented by the state has to be re-examined. Two approaches:Lord Jestocost said:In contrast, there is never a paradox if we realize that the wave function is just an encoded mathematical representation of our knowledge of the system. When the state of a quantum system has a non-zero value at some position in space at some particular time, it does not mean that the system is physically present at that point, but only that our knowledge (or lack of knowledge) of the system allows the particle the possibility of being present at that point at that instant.
Not at all, I am fine with any constraints and rules that helps in any model. I see no issue with micro-causality. But there is a specific person on this forum that clearly does not understand its domain of application, and makes wrong claims about its domain of application.PeterDonis said:Ah, ok. But you seem to be thinking that this feature is somehow unique to QFT and raises issues.
Thank you for this summary. It's actually one of the best I've read.PeterDonis said:Actually, that's not the case. Commuting measurements where different observables are being measured are the norm--the natural case. Classical measurements of different observables always commute. The new feature that QM (and QFT, which just makes clearer how relativity plays into it all) introduces is non-commuting measurements--measurements of different observables (like spin-z and spin-x) that do not commute.
But surely you don't infer that this restriction means those measurement are not possible. It mean those measurement are not in the domain of QFT.PeterDonis said:The QFT constraint just makes clear that the domain in which such non-commuting measurements are possible is restricted by the requirements of relativistic causality.
Maybe it is, so I'll assume micro-causality never enter the equation to predict entanglements results, or "the speed at which" collapse occurs ... if there is such a thing.[/B]PeterDonis said:To the extent that this is the case, as above, the predictions are the same as those in non-relativistic QM, and indeed in classical physics.
Again, this is way too strong and you need to stop making this claim.Simple question said:there is a specific person on this forum that clearly does not understand its domain of application, and makes wrong claims about its domain of application
Of course not. Nobody is claiming that.Simple question said:surely you don't infer that this restriction means those measurement are not possible.
It means no such thing. Commuting measurements are just as much in the domain of QFT as non-commuting measurements.Simple question said:It mean those measurement are not in the domain of QFT.
I do not see @vanhees71 making any such claim. You are seriously misinterpreting his posts if you think this is what he is saying.Simple question said:@vanhees71 thinks that the theory comes first and that implies that those experiment results (since Aspect, 40 years ago !!!) should be dismissed.
You cannot assume any such thing. All you can assume is what I explicitly said: that for the case of spacelike separated measurements, the predictions of QFT are the same as those of non-relativistic QM. (It is true that for the case where such measurements are made on entangled particles, the predictions will not be the same as those of classical physics, since the latter will never predict violations of the Bell inequalities. I did not intend to state otherwise, but on reading my previous post I see how it could have been interpreted that way. Sorry for the ambiguity on my part.)Simple question said:I'll assume micro-causality never enter the equation to predict entanglements results
There is no such thing except in "objective collapse" interpretations, which, as I think has already been noted in this thread, actually become different theories (i.e., different math from the standard math of QM, making different experimental predictions) when developed fully.Simple question said:"the speed at which" collapse occurs
Nice ! You finally walked-back your claim that "nature is fundamentally random". Because if the theory is not, you then have no mathematical way to prove it.vanhees71 said:QM is described by usual partial differential equations like the Schrödinger equation for the wave function. It's not a stochastic differential equation
So is game theory or statistical mechanics. Still, it does not make those "fundamentally random" nor "mysterious".vanhees71 said:The meaning of the state is probabilistic.
You yourself provided those hints. You are just stuck in the past and you cannot accept that Nature behaves as she does and not as you want.vanhees71 said:That's true. There are attempts to extend the quantum formalism with some stochastic collapse mechanism, but that's not QM anymore but a new theory. There's, however, not the slightest hint that such an alteration is needed anywhere.
Everyone knows that QFT is unrealistic it has been mathematically proven. You just don't understand what that means, which is: you cannot make claim about NATURE using it. You can barely describe the most basic setup with it (see **)vanhees71 said:Don't interpret something into what I'm saying, which I never said. QFT as any QT is not realistic, i.e., within this theory not all observables always take determined values.
Another falsehood. You would be hard pressed to quote any such "claim" from anyone on this thread.vanhees71 said:Also it's weird to claim that standard local relativsitic QFT were wrong
"class ?" "successful ?". Unsubstantiated opinion holds no water on scientific forum.vanhees71 said:while in fact it's the most successful class of theories ever discovered!
Experiment within its domain only.vanhees71 said:Experiment shows that also local relativistic QFT is correct
No it does not. Experiment show that spooky correlation at a distance exist. So the theory does not "exclude" that, because it would mean the theory is wrong or incomplete.vanhees71 said:and this excludes spooky actions at a distance by construction.
So stop that gibberish. Mathematical property are just that, not fact about Nature.vanhees71 said:That's a mathematical property of the theory and cannot be argued away by some "interpretation" gibberish.
Cool. So micro-causality has its purpose. I am really not surprised. So how to use it in the simplest setup (see **) ? or in entanglement cases, swapping maybe ?vanhees71 said:It's among THE key features, and it predicts from the start very well established facts about nature like the CPT symmetry and the relation between spin and statistics.
** It doesn't surprise me. You are not interested by experiments nor how those can be described by a theory. Not even in principle. You wrote:vanhees71 said:I've no clue what you mean in #71. Whether I use one equipment to run an experiment 10000 times or whether I build 10000 different equipments doesn't make any difference. It's just preparing large enough ensembles to have a high significance in my statistical tests of the probabilistic predictions of Q(F)T.
It's just a contradiction. Those 10000 labs are "prepared" in different light cone. Your own idiosyncratic miss-use of QFT explicitly forfeit its predictive power because you assume micro-causality and local interaction apply. Always everywhere.The detectors don't negotiate anything. It's just the interaction of the photon with the material around it. Where it will be detected is random,
And if one want to shoot only one photon some place, or entangle of few QBit in a QComputer, it is not worthy of "science". The least is the best it can do.Natural science is not for explaining the world, and especially not describing at best as possible with mathematical models. You don't aim to describe nature because it is un-real. And just shut-up and calculate probabilities, even though nature deal in events, not probability of platonic ensemble.
But that is not the point ! I agree with that perfectly: QFT have no additional claim to make about entanglement of spacelike measurement. @vanhees71 is not agreeing with this. So why do you say *I* made too strong claim ?PeterDonis said:You cannot assume any such thing. All you can assume is what I explicitly said: that for the case of spacelike separated measurements, the predictions of QFT are the same as those of non-relativistic QM.
I though so. I doubt I would ever be able to understand how this would explain entanglement "speed". But that's for another thread.PeterDonis said:There is no such thing except in "objective collapse" interpretations, which, as I think has already been noted in this thread, actually become different theories (i.e., different math from the standard math of QM, making different experimental predictions) when developed fully.
vanhees71 said:and this excludes spooky actions at a distance by construction.
Because "spooky action at a distance" (or the German equivalent) was coined by Einstein, its meaning is much more technical and fixed, than you seem to assume. This meaning simply does not include nonlocal correlations. (If you want, include the Bohmian type of nonlocality in it, and all the theories/interpretations with an explicit collapse of the wavefunction, but mere "correlations" are too "passive" as to be termed "action" by Einstein.)Simple question said:No it does not. Experiment show that spooky correlation at a distance exist. So the theory does not "exclude" that, because it would mean the theory is wrong or incomplete.
Science prefer experiment over theory, by construction
Simple question said:"class ?" "successful ?". Unsubstantiated opinion holds no water on scientific forum.
I get the impression that you are trolling. But maybe my impression is wrong, and this is just your way to have a lively discussion.Simple question said:So stop that gibberish. Mathematical property are just that, not fact about Nature.
An off topic subthread that was more or less spawned by this comment has been deleted. Please keep discussion in this thread focused on the specific topic of how "locality" is defined in QM. More general discussion of what science is "for" and how it should be done belongs in a separate thread in General Discussion if anyone wants to pursue it.vanhees71 said:Natural science is not for explaining the world
This claim, of course, requires that you adopt the particular interpretation you describe. It should be obvious to you by now that not everyone accepts that interpretation. And the guidelines for this forum make clear that no particular interpretation can be asserted to be "correct". Everyone in the discussion must accept that there are different interpretations of QM that say different, sometimes incompatible things, and that that fact is not going to change as a result of any discussion here.vanhees71 said:Brukner and Zeilinger just set the record straight by using the minimal statistical information. That solves all pseudo-problems.
This is the kind of claim that the guidelines for this forum do not permit. The fact that you prefer a particular interpretation does not allow you to claim that anyone who doesn't accept it is not accepting how Nature behaves. The only things we know about how Nature behaves are the things we see in experiments, and all QM interpretations agree on all experimental predictions.vanhees71 said:The only problem that remains is that some philosophers cannot accept that Nature behaves as she does and not as they want. They are still confined in their "classical worldview". That's all that's left.
This is an utter nonsense, at least 3 levels.vanhees71 said:realistic (which means that all observables always take determined values).
Demystifier said:Observable is a self-adjoint operator
Yes, but then the post by @vanhees71 would make even less sense, because then "observables" would always commute, not only at spacelike separations.weirdoguy said:Some people (e.g. Ballentine, if I recall correctly) differentiate between observables as quantities that can be measured in an experiment, and operators that are associated with them.
I think everyone does. Demystifier just continues with his sophistry.weirdoguy said:Some people (e.g. Ballentine, if I recall correctly) differentiate between observables as quantities that can be measured in an experiment, and operators that are associated with them.
Demystifier said:because then "observables" would always commute, not only at spacelike separations.
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 ;-).Demystifier said:Yes, but then the post by @vanhees71 would make even less sense, because then "observables" would always commute, not only at spacelike separations.
As the commutator under multiplication, ##AB-BA##. I guess I don't need to explain what is multiplication of operators, and what is multiplications of real and complex numbers.weirdoguy said:How do you define commutator of two quantities?
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 said: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 ;-).
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/0605250vanhees71 said: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.
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 said:I don't know anything about string theory. What do you mean when you say "Bell-nonlocality"?
I mean the thing you call nonseparability.vanhees71 said:I ask you what you mean by Bell nonlocality
Yes, that's a part of the problem.vanhees71 said: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.
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.vanhees71 said: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".
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 said:Entanglement is used for engineering purposes nowadays (quantum cryptography, quantum computing, and all that).
Surely it's more than that. When it is used in cosmology, does it mean that the Universe is just an experiment?vanhees71 said:QFT is just a theory predicting the probabilities for the outcome experiments.
To my mind, the debates are merely about the question:vanhees71 said:Entanglement is a very specific kind of correlations. That's why we have all these debates about them!
So what is your objection to observationally equivalent theories/interpretations?vanhees71 said: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
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:vanhees71 said: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 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 said:So what is your objection to observationally equivalent theories/interpretations?
I don't think it's quite so simple...vanhees71 said:If they make the same predictions they are the same theory.
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.vanhees71 said: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.vanhees71 said: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?vanhees71 said: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.
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.AndreasC said:Suitable Lorentz ether theories are also observationally equivalent to special relativity, but the shift in perspective was important!
This is not what you meant. You really meant this:vanhees71 said: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.
Because it contradicts the very foundation of the Lorentz ether theory it pretends to interpret.vanhees71 said:Why should I oppose Einstein's no-ether interpretation?