Getting rid of nonlocality from quantum physics

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  • #101
vanhees71
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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.
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.
 
  • #102
vanhees71
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Are you saying that in the said examples we can consider the electron as "prepared".
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.
 
  • #103
RUTA
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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".
From the first paper (p. 11), Andrei says:
In fact, (2) is just the transformation that connects probability distributions related to different contexts.
The idea is that we have interference between classical contexts for quantum probability while no such interference exists for classical probability. Johann replies:
Any given experimental situation is always only one context.
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.
 
  • #104
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In which examples?
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.
 
  • #105
martinbn
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Given that, do you think that Bohmian mechanics is less plausible than the standard QM? If you do, why do you think so?
I don't know, I would say equally plausible. I assume that by plausible you mean the true description of nature.
 
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  • #106
vanhees71
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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.
You cannot predict anything about electrons you don't know the initial state about. That's also true in classical physics. I don't see, where this is specifically a quantum problem.
 
  • #107
Demystifier
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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.
So? Similar objections were made by Mach on the Boltzmann theory of atoms.
 
  • #108
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I don't know, I would say equally plausible. I assume that by plausible you mean the true description of nature.
Would you be able to identify the relative strengths and weaknesses of the standard and Bohmian QM?
 
  • #109
martinbn
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Would you be able to identify the relative strengths and weaknesses of the standard and Bohmian QM?
Oh, I have some knowledge of the standard QM, but not enough to evaluate. With BQM even less.
 
  • #110
DrChinese
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...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.
Amen! :smile:
 
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  • #111
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110 posts from the beginning of the thread can someone from the realist camp please evaluate what exactly exists prior to measurement? And how. Thanks! 👍
 
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  • #112
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You cannot predict anything about electrons you don't know the initial state about. That's also true in classical physics. I don't see, where this is specifically a quantum problem.
I am not sure how that answers my question. I am just questioning the ironic situation whereby the same equations that predict the hydrogen atom (that exists all the time) "properties" we then say that particles don't have defined properties until measured, the ultimate in contradiction if you ask me, or the fanciest trick of all times. :smile:
 
  • #113
martinbn
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I am not sure how that answers my question. I am just questioning the ironic situation whereby the same equations that predict the hydrogen atom (that exists all the time) "properties" we then say that particles don't have defined properties until measured, the ultimate in contradiction if you ask me, or the fanciest trick of all times. :smile:
For you exists and observables have definite values seem to be the same thing. Why?
 
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  • #114
vanhees71
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So? Similar objections were made by Mach on the Boltzmann theory of atoms.
Are you saying, Bohmian trajectories are observable? If so, why has nobody observed them yet?
 
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  • #115
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Amen! :smile:
Suppose you see some guy down the street talking to himself and making nonsensical comments to by passers you take him to a doctor after some psychological investigation and applying some psychology theories he is determined to have some named mental disease. That is the model described the observation. Now, even doctors are not satisfied with these general behavioral/historical analysis, today they try to study how the brain is actually being affected by MRI and chemical analysis and what not. To solve the problem they want to know WHY as to all the details of the brain processes that led to malfunction. I hope you understand what I am saying.
 
  • #116
vanhees71
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I am not sure how that answers my question. I am just questioning the ironic situation whereby the same equations that predict the hydrogen atom (that exists all the time) "properties" we then say that particles don't have defined properties until measured, the ultimate in contradiction if you ask me, or the fanciest trick of all times. :smile:
That's not what I say. QT says that observable don't take determined values if it is not prepared to take one and that you cannot prepare all observables to be determined at once. This is not identical with the claim that something doesn't exist, because nobody is observing it.
 
  • #117
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For you exists and observables have definite values seem to be the same thing. Why?
You tell me of anything that does not have a definite value and makes sense to you, that is outside of QM.

That's not what I say. QT says that observable don't take determined values if it is not prepared to take one and that you cannot prepare all observables to be determined at once. This is not identical with the claim that something doesn't exist, because nobody is observing it.
Indeed to me is the same thing. Well maybe I have become the guy in my example to DRchinese.:smile:

Ok, I have the answer, but give me sometime so I can present it in an acceptable way.
 
  • #118
Lord Jestocost
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110 posts from the beginning of the thread can someone from the realist camp please evaluate what exactly exists prior to measurement? And how. Thanks!
Those, who follow the De Broglie–Bohm theory, can!
Regarding QM: Claims that something exists or doesn't exist between measurements cannot be proven by the scientific method which is based upon observations. Based upon observations, you cannot give a judgement about such claims. Thus, I think that nobody can answer your question (the realist's camp tries it since decades and decades without any success).
That's an instrumentalist's point of view: It doesn't matter whether something exists or doesn't exist between measurements. Observations can be used to construct and test models (do the models work and allow to make accurate predictions about what will happen next). That's it.
 
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  • #119
PeroK
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In post #31 you declare from the nowhere: “Electron is the name for the thing that have objective existence.”

Isn't this a reality hypothesis? One can see this in a different way. For example, as Paul Davies puts it: "Thus an electron or an atom cannot be regarded as a little thing in the same sense that a billiard ball is a thing. One cannot meaningfully talk about what an electron is doing between observations because it is the observations alone that create the reality of the electron. Thus a measurement of an electron's position creates an electron-with-a-position; a measurement of its momentum creates an electron-with-a-momentum. But neither entity can be considered already to be in existence prior to the measurement being made."
If the electron really doesn't exist until you measure it, then why do you get an electron? Let's say you prepare silver atoms as per SG.

We have preparation (oven) where we claim we produce silver atoms. If the atoms coming out of the oven do not exist, then what does it mean to "prepare silver atoms"? Either something comes out of the oven or nothing comes out of the oven. If you say nothing exists until measurement, then a) there is no silver atom and b) there is no gold atom and c) there is no anything else. Until when?

If nothing exists until the interaction with the SG magnet then why does a silver atom come into existence? Why not a gold atom?

If an electron does not exist, then in what sense is there ever an electron in an experiment? As opposed to a proton, or a carbon atom or a sausage? Why doesn't the measurement intended to measure the spin of an electron find a sausage instead?

Why do we observe properties consistent with a silver atom if the silver atom did not exist until we measured it? The SG appartus does not look specifically for silver atoms. You could fire anything through the magnetic field. Why doesn't the SG apparatus bring a gold atom into existence that presumably goes straight through?
 
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  • #120
RUTA
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If the electron really doesn't exist until you measure it, then why do you get an electron? Let's say you prepare silver atoms as per SG.

We have preparation (oven) where we claim we produce silver atoms. If the atoms coming out of the oven do not exist, then what does it mean to "prepare silver atoms"? Either something comes out of the oven or nothing comes out of the oven. If you say nothing exists until measurement, then a) there is no silver atom and b) there is no gold atom and c) there is no anything else. Until when?

If nothing exists until the interaction with the SG magnet then why does a silver atom come into existence? Why not a gold atom?

If an electron does not exist, then in what sense is there ever an electron in an experiment? As opposed to a proton, or a carbon atom or a sausage? Why doesn't the measurement intended to measure the spin of an electron find a sausage instead?

Why do we observe properties consistent with a silver atom if the silver atom did not exist until we measured it? The SG appartus does not look specifically for silver atoms. You could fire anything through the magnetic field. Why doesn't the SG apparatus bring a gold atom into existence that presumably goes straight through?
That’s a great question and let me answer it in the vein of contextuality. To say, ”I have a gas of silver atoms in this oven for my SG experiment” means something about the context you put the oven in before placing it in the context of the SG measurement. And, that previous context has elements that are defined in contexts previous to that context, etc. Ultimately, everything you mean about a context must be observable or derive from a context of observables. Otherwise, repeatable experimental science is impossible.
 
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  • #121
Lord Jestocost
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I merely want to point out that everyone has a story which is influenced by personal ideological beliefs. That's why I - as a working physicist - adopt an instrumentalist's point of view.
My personal story could be, for example: The mind is fundamental and the objects only exist as illusions in my mind. Neither your story nor my story can be proved or disproved by means of the scientific method.
 
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  • #122
PeroK
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@PeroK

I merely want to point out that everyone has a story which is influenced by personal ideological beliefs. That's why I - as a working physicist - adopt an instrumentalist's point of view.
My personal story could be, for example: The mind is fundamental and the objects only exist as illusions in my mind. Neither your story nor my story can be proved or disproved by means of the scientific method.
This is why I rarely reply to posts in this sub-forum.

Whenever anyone is confronted with a hard question, they can retreat into an argument barely discernible from Zen Buddhism!

It seems to me a huge unnecessary burden on orthodox QM to explain how specific things (things with specific properties) come into existence, as opposed to other things with other properties. The simplest explanation is that something with the specific properties exists and measurements are compatible with that thing and those properties.

Explaining how properties can be not well-defined until measurement is bad enough. But, that the thing itself comes in and out of existence is asking too much, for no theoretical benefit that I can see.
 
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  • #123
RUTA
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I merely want to point out that everyone has a story which is influenced by personal ideological beliefs. That's why I - as a working physicist - adopt an instrumentalist's point of view.
My personal story could be, for example: The mind is fundamental and the objects only exist as illusions in my mind. Neither your story nor my story can be proved or disproved by means of the scientific method.
Read Becker’s book https://www.google.com/books/edition/What_Is_Real/X4QqDwAAQBAJ?hl=en&gbpv=1&printsec=frontcover He makes a good case for the relevance of going beyond instrumentalism.
 
  • #124
PeterDonis
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The OP appears to be gone and the discussion seems to have covered every possible viewpoint. Thread closed.
 
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