That's nonsense. All the observations are also in accordance with Bohmian mechanics (because it makes the same measurable predictions as standard quantum theory, including QFT), and yet you will not say that "it's known" that nature obeys nonlocal laws just because it does according to Bohmian mechanics.vanhees71 said:It's known, because all observations are in accordance with this description at a very high level of accuracy.
I am not talking about nonlocality at all in this thread. My original qustion and the following discussion was only about the existence. There is no disagreement about the nonlocality except about the terminology.Demystifier said:I agree with all the above, but it seems to me that you are missing my point. The simplest model we have (QM with the collapse postulate) is nonlocal. The simplest model without the collapse (many worlds) is nonlocal. The simplest model without the collapse and without the many worlds (Bohmian mechanics) is nonlocal. The very general class of models (defined by the Bell theorem) are nonlocal.
So is there a local model at all? No there isn't. There is only a local interpretation (Copenhagen), but such an interpretation is not a model. (By a model I mean a set of equations, by an interpretation I mean a set of words in plain English that explain what those equations mean.) In particular, any model containing a nonlocal wave function such as ##\psi({\bf x}_1,{\bf x}_2)## is a nonlocal model. A local interpretation of such a nonlocal model may contain a statement such as: "But this ##\psi## is not real, it's only our description.", but it doesn't change the fact that the model is nonlocal.
Now you may argue that the reality itself is local, but how then would you interpret the Bell theorem that no model of reality (under very general assumptions) can be local?
That's typical for Copenhagenians to use such a vague language.martinbn said:Yes, and he says that they are not as real. He doesn't say that they don't exist, does he? He could mean that that they don't have coordinates if not measured.
Yes, it does prove that. I only dislike the choice of terminology.Demystifier said:That's typical for Copenhagenians to use such a vague language.
But I think you still didn't tell me what I really want to here from you. What is your interpretation of the Bell theorem? Does it prove nonlocality of QM?
Well, I am. See the title of the thread.martinbn said:I am not talking about nonlocality at all in this thread.
Good! Now do you agree that some physicists don't agree that the Bell theorem proves "nonlocality" (or whatever you want to call it)? And if you do, how do they justify it? It seems to me that they justify it by denying the existence of things in the absence of measurement, but maybe I'm wrong, maybe they justify it by denying something else.martinbn said:Yes, it does prove that. I only dislike the choice of terminology.
I know, but the OP doesn't seem interested in any discussion. I started in post #10.Demystifier said:Well, I am. See the title of the thread.
Concerning "realism", I only talk about it to the extent it is relevant to nonlocality.
I don't think that they disagree with the theorem. They (and I) disagree with the conclusions some physicists jump to.Demystifier said:Good! Now do you agree that some physicists don't agree that the Bell theorem proves "nonlocality" (or whatever you want to call it)? And if you do, how do they justify it? It seems to me that they justify it by denying the existence of things in the absence of measurement, but maybe I'm wrong, maybe they justify it by denying something else.
So what's your conclusion regarding the Bell theorem?martinbn said:I don't think that they disagree with the theorem. They (and I) disagree with the conclusions some physicists jump to.
martinbn said:No, it doesn't. The quotes are missing the context, and they don't say that things don't exist.
Again one should really be specific what's meant by "nonlocal"! Relativistic QFT is local by construction, i.e., any local observables commute at space-like-separated arguments. This implies the cluster-decomposition principle and thus there are no nonlocal causal influences. There's inseparability described by entanglement, but that's nothing that contradicts relativistic causality as the collapse postulate implies, and I still have not seen any convincing argument, what the collapse is good for. I also don't understand in how far "many worlds" helps to solve any physical problem. Bohmian mechanics adds some mathematical formalism on top of standard QM and describes "trajectories". That's of course in a sense non-local, but it's just an addition in the math which doesn't add anything to what's observable according to standard QT, i.e., probablities to find a particle at some position.Demystifier said:I agree with all the above, but it seems to me that you are missing my point. The simplest model we have (QM with the collapse postulate) is nonlocal. The simplest model without the collapse (many worlds) is nonlocal. The simplest model without the collapse and without the many worlds (Bohmian mechanics) is nonlocal. The very general class of models (defined by the Bell theorem) are nonlocal.
So is there a local model at all? No there isn't. There is only a local interpretation (Copenhagen), but such an interpretation is not a model. (By a model I mean a set of equations, by an interpretation I mean a set of words in plain English that explain what those equations mean.) In particular, any model containing a nonlocal wave function such as ##\psi({\bf x}_1,{\bf x}_2)## is a nonlocal model. A local interpretation of such a nonlocal model may contain a statement such as: "But this ##\psi## is not real, it's only our description.", but it doesn't change the fact that the model is nonlocal.
Now you may argue that the reality itself is local, but how then would you interpret the Bell theorem that no model of reality (under very general assumptions) can be local?
No Bell's theorem does not prove "nonlocality" it only proves that there's no local deterministic theory that reproduces the outcome of quantum theory and the corresponding results of their real-lab tests, which always agree with QT rather than local deterministic hidden-variable theories. Relativistic QFT has only local interactions and describes the Bell tests correctly. So there is a local model, namely simply relativistic QFT! It's of course not deterministic. Some people say it's "non-realistic" and they say that one gives up "realism". But if you simply consider as realistic what's objectively and reproducibly observed, there's nothing non-realistic with QT.Demystifier said:Good! Now do you agree that some physicists don't agree that the Bell theorem proves "nonlocality" (or whatever you want to call it)? And if you do, how do they justify it? It seems to me that they justify it by denying the existence of things in the absence of measurement, but maybe I'm wrong, maybe they justify it by denying something else.
That it proves Bell non-locality (which is a misnomer), not non-locality in the usual sense of the word.Demystifier said:So what's your conclusion regarding the Bell theorem?
I don't know what to say. This seems to have absolutely no connection to science.Lord Jestocost said:To merely claim the existence, or reality, of something is no great challenge. A realist has to provide more: a specific set of claims concerning what exists and, especially, how it exists, viz. the character of its existence.
“The separate images that we form of the quantum world (wave, particle, for example) from different experimental viewpoints do not combine into one comprehensive whole” (Nick Herbert). Thus, we cannot simply think of an “electron” or an “atom” as an object which can be given a description in its own right: There exists no single image that corresponds to an electron or an atom: The character of their existence is missing. What is within this context the meaning of the term "things"? It's an empty notion. That’s the reasoning behind Heisenberg’s "Atoms are not things".
martinbn said:I don't know what to say. This seems to have absolutely no connection to science.
Fine. And which name (instead of nonlocality) would be better? And how would you explain (interpret) what it means in plain English?martinbn said:That it proves Bell non-locality (which is a misnomer), not non-locality in the usual sense of the word.
And how does this relate to what I wrote? I never claimed that QM needs a reality hypothesis.Lord Jestocost said:The connection is simple. Quantum mechanics is in no need for any reality hypothesis.
The mathematical formalism says that quantum fields can extend indefinitely(mathematically, this isn't an issue, right?). There is always some non-zero probability that particles with definite momentum may be completely delocalized(and found on Mars, theoretically). What about conservation laws in the scenario that involved my conundrum( quantum fields which permeate all of space-time)?martinbn said:The answer is that the terminology isn't perfect. Quantum fields refer both to the things that exist in space-time and to some of the objects in the mathematical model.
Any name that is not already in use. I am sure you agree that it is confusing to call it red, and claim that Bell proved that QM is red! Plain English is not often suitable for these things.Demystifier said:Fine. And which name (instead of nonlocality) would be better? And how would you explain (interpret) what it means in plain English?
I don't know, it seems like a valid question about quantum field theory. But it is not related to existence.EPR said:The mathematical formalism says that quantum fields can extend indefinitely(mathematically, this isn't an issue, right?). There is always some non-zero probability that particles with definite momentum may be completely delocalized(and found on Mars, theoretically). What about conservation laws in the scenario that involved my conundrum( quantum fields which permeate all of space-time)?
This point never became clear to me.
OK, one (hopefully last) question for you. Given that Bohmian mechanics makes the same measurable predictions as standard quantum theory, in your opinion which of the following is true? (Mutiple choices are allowed.)martinbn said:Any name that is not already in use. I am sure you agree that it is confusing to call it red, and claim that Bell proved that QM is red! Plain English is not often suitable for these things.
I really don't understand this issue at all. Two electrons will interact as predicted even when they are are not "measured", correct? an electron in the hydrogen atom stays put(i.e. has all the properties that binds it to the proton at all times) even when it is not "measured", correct?. So it seems there is a serious misunderstanding somewhere in QM.martinbn said:But it is not related to existence.
I don't know. I think 2. possibly 1.Demystifier said:OK, one (hopefully last) question for you. Given that Bohmian mechanics makes the same measurable predictions as standard quantum theory, in your opinion which of the following is true? (Mutiple choices are allowed.)
1. Bohmian mechanics is nonlocal in the ordinary sense.
2. Bohmian mechanics is "nonlocal" in the Bell sense.
3. Bohmian mechanics is "nonlocal" in some third sense.
Why is there a misunderstanding?ftr said:I really don't understand this issue at all. Two electrons will interact as predicted even when they are are not "measured", correct? an electron in the hydrogen atom stays put(i.e. has all the properties that binds it to the proton at all times) even when it is not "measured", correct?. So it seems there is a serious misunderstanding somewhere in QM.
because QM claims that particles have undefined properties when not "observed".martinbn said:Why is there a misunderstanding?
BM claims that fundamental objects (particles or fields) have some properties (positions or field configurations in space) always defined. That's what it means, according to BM, that things exist even without measurements.vanhees71 said:QM claims that particles have undefined properties when they are not prepared to have them defined. Further it claims that it is not possible to prepare all possible properties (determining the values of all observables) at once, i.e., that some properties are undefined.
That's what QM claims and all observations so far have confirmed with high accuracy.
martinbn said:And how does this relate to what I wrote? I never claimed that QM needs a reality hypothesis.
Given that, do you think that Bohmian mechanics is less plausible than the standard QM? If you do, why do you think so?martinbn said:I don't know. I think 2. possibly 1.
But if particles have trajectories, I should be able to precisely predict where they are at any time, but that's not the case. I cannot observe the Bohmian trajectories in, e.g. the notorious double-slit experiment. All observable I get are probability distributions, which can be observed with ensembles of particles. The Bohmian trajectories are just a fiction to claim some determinism which is not observable though. So they do not change the observational fact of randomness predicted by QT and thus BM is just QT with the additional task to calculate non-observable trajectories.Demystifier said:BM claims that fundamental objects (particles or fields) have some properties (positions or field configurations in space) always defined. That's what it means, according to BM, that things exist even without measurements.
That's what BM claims and all observations so far have confirmed the measurable predictions of BM with high accuracy.
In which examples? If we observe electrons, of course we prepare them first to be observed, and be it simply like Thomson as "cathode rays" of a gas-discharge tube.ftr said:Are you saying that in the said examples we can consider the electron as "prepared".
From the first paper (p. 11), Andrei says:Vaxjo said:The earliest presentation of the contextual (Bohr-like) viewpoint on quantum theory was presented in the debate with Johann Summhammer, see https://arxiv.org/abs/quant-ph/0111130, see also https://arxiv.org/pdf/quant-ph/0401072.pdf for formulation in the form of an interpretation, and it was summarized in the book by Springer, "Contextual approach to quantum formalism".
The idea is that we have interference between classical contexts for quantum probability while no such interference exists for classical probability. Johann replies:In fact, (2) is just the transformation that connects probability distributions related to different contexts.
But, he misses Andrei’s point — the single context (C12) Johann is referencing can be viewed as a combination of C1 and C2. So, why not P1 + P2 = P as in classical probability? Quantum probability says the classical possibilities interfere to create P (add amplitudes then square). It makes perfect sense to use a “Born rule” for, say, the photon density associated with monochromatic radiation. Normalizing that over some volume of space then gives the probability density. In that case, the number of photons per unit volume is just ##\frac{\epsilon_o E^2}{2hf}##, i.e., energy density divided by energy per photon. So, first one must add the electric field contributions ##\vec{E}_1 + \vec{E}_2 + ...## from all sources then square to produce ##E^2##. You don’t have simply ##E^2 = E_1^2 + E_2^2 + ...##. Likewise, quantum probability theory is telling us the various classical contextual possibilities, C1 and C2 in the twin-slit case, must first be added then squared to produce the probability distribution at the detector screen.Any given experimental situation is always only one context.
vanhees71 said:In which examples?
ftr said:I really don't understand this issue at all. Two electrons will interact as predicted even when they are are not "measured", correct? an electron in the hydrogen atom stays put(i.e. has all the properties that binds it to the proton at all times) even when it is not "measured", correct?. So it seems there is a serious misunderstanding somewhere in QM.
I don't know, I would say equally plausible. I assume that by plausible you mean the true description of nature.Demystifier said:Given that, do you think that Bohmian mechanics is less plausible than the standard QM? If you do, why do you think so?