Is quantum theory a microscopic theory?

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Demystifier
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Your answer seems to imply that only detections are macroscopic (macroscopic = detection).
No, I imply that all detections are macroscopic. But the converse is not true, some macro objects may not be detections.
 
Demystifier
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There is no classical/quantum cut to be well defined within quantum theory.
True, but there is a measurement/no-measurement cut within the minimal instrumental view of QM.
 
Demystifier
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There's only a measurement problem, if you insist on an ontic interpretation. The very success of QT in describing all known observables disproves the existence of any "measurement problem". QT precisely describes all results of measurement in the real world, and thus there's no measurement problem in any scientific sense.
1. As long as there is no measurement problem in QT, there is a problem of whether QT is about the microscopic world.
2. As long as there is no problem of whether QT is about the microscopic world, there is a measurement problem in QT.
3. As long as there is neither measurement problem in QT nor a problem of whether QT is about the microscopic world, there is a problem with logical consistency of QT.
 
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Demystifier
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Why should a theory not use classical objects in its formulation?
A theory can use classical objects in its formulation, but not if that theory (like QM) claims that it can derive the classical objects. Using classical objects in formulation of a theory more fundamental than classical physics would be like using mathematical analysis in ZFC axioms of set theory.
 
Lord Jestocost
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I say that the minimal instrumental form of QM does not say anything about the micro world.
That's indeed the point. Cord Friebe, Holger Lyre, Manfred Stöckler, Meinard Kuhlmann, Oliver Passon and Paul M. Näger in “The Philosophy of Quantum Physics”:

“If one tries to proceed systematically, then it is expedient to begin with an interpretation upon which everyone can agree, that is with an instrumentalist minimal interpretation. In such an interpretation, Hermitian operators represent macroscopic measurement apparatus, and their eigenvalues indicate the measurement outcomes which can be observed, while inner products give the probabilities of obtaining particular measured values. With such a formulation, quantum mechanics remains stuck in the macroscopic world and avoids any sort of ontological statement about the (microscopic) quantum-physical system itself.”
 
Demystifier
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That's indeed the point. Cord Friebe, Holger Lyre, Manfred Stöckler, Meinard Kuhlmann, Oliver Passon and Paul M. Näger in “The Philosophy of Quantum Physics”:

“If one tries to proceed systematically, then it is expedient to begin with an interpretation upon which everyone can agree, that is with an instrumentalist minimal interpretation. In such an interpretation, Hermitian operators represent macroscopic measurement apparatus, and their eigenvalues indicate the measurement outcomes which can be observed, while inner products give the probabilities of obtaining particular measured values. With such a formulation, quantum mechanics remains stuck in the macroscopic world and avoids any sort of ontological statement about the (microscopic) quantum-physical system itself.”
Yes, that's exactly what I say, nice to see that I am not alone. :smile:
 
Mentz114
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I do not question the success of quantum theory. I question that quantum theory is about the microscopic world. Or more precisely, I question that one particular view of QM is about the microscopic world.
The core formalism of QT allows us to make predictions about microscopic things by observing macroscopic outcomes of experiments. For instance the behaviour of atoms interacting with the EM field in cavities.
I think I miss the point of your question. It is true that we can only predict probabilities, and that imposes a limit what we can infer.
 
Lord Jestocost
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to make predictions about microscopic things by observing macroscopic outcomes of experiments.
The “microscopic things” are merely mental concepts which one uses to “describe” the behavior of measuring instruments in a given experimental context.
 
Mentz114
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The “microscopic things” are merely mental concepts which one uses to “describe” the behavior of measuring instruments in a given experimental context.
No. Atoms really do exist ! The only mental concept involved is probability which does not share the same kind of existence.
 
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But we reveal the "existence" of microscopic atoms via macroscopic devices...
No. Atoms really do exist !
 
Mentz114
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But we reveal the "existence" of atoms via macroscopic devices...
If that is the case then QT does tell us something about the microscopic world and the philosophical doubts are proved meaningless.
 
Lord Jestocost
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No. Atoms really do exist !
The question is: Does an atom “exist” on its own in the full, common-sense notion of the word so that it can be given a description in its own right or is it only a phenomenon in case it is an observed/registered phenomenon?
 
Jimster41
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So what?
Maybe there is some theory that is.
Though the question this thread poses... has me scratching my head... is there some Godel-like proof that there can never be? I mean any theory we could point to is going to be an invention starting in a classical experience (ours) of the frustratingly in-naccessible (in the classical sense) quantum realm so... what hope is there?

Is that the point of the OP?
 
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Mentz114
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The question is: Does an atom “exist” on its own in the full, common-sense notion of the word so that it can be given a description in its own right or is it only a phenomenon in case it is an observed/registered phenomenon?
I would apply that definition of existence to anything that is claimed to exist. A Rydberg atom in a microwave resonant cavity is a vey tiny thing in a relatively huge volume.. We cannot hope to affirm its existence as we might do for a baseball. But experiment shows that there is something there which is interacting with the EM field - as predicted. So that atom existed. That atom made headlines in the 1960's !
 
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This discussion reminds me of the day, many years ago, when my physics teacher (a Nobel prize nominee) handed back an exam paper on the Dirac equation and sternly said, "Fred, the neutron, she's not a little potato!"
 
DarMM
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Well QM in the typical Copenhagen reading is a theory of the statistics of impressions microscopic systems leave in macroscopic objects. So it does concern the microscopic, but it's not a representational theory telling you what they are like in and of themselves.
 
martinbn
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A theory can use classical objects in its formulation, but not if that theory (like QM) claims that it can derive the classical objects. Using classical objects in formulation of a theory more fundamental than classical physics would be like using mathematical analysis in ZFC axioms of set theory.
Well, can QM derive that? May be people think too highly of QM. If you admit that it isn't the most fundamental, the last word, then there wouldn't be "problems" that need interpretations to "solve" them.
 
DarMM
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Well, can QM derive that? May be people think too highly of QM. If you admit that it isn't the most fundamental, the last word, then there wouldn't be "problems" that need interpretations to "solve" them.
The problem to many is that no-go theorems imply QM is the last word unless you're willing to have multiple worlds, retrocausality or nonlocality. A theory which is the last word and leaves in place classical objects outside it is unsatisfying to many.
 
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No, since we would like to say the classical measurement apparatus is made of electrons, which are quantum. However, quantum mechanics does not seem to allow us to say that.
Can you push it to the nucleus too. About your "Is the measurement apparatus (classical macroscopic) made of electrons (quantum microscopic)?". Can you also ask ""Is the measurement apparatus (classical macroscopic) made of atomic nuclei (quantum microscopic)?"? (and the answer not necessarily being yes)
 
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That's indeed the point. Cord Friebe, Holger Lyre, Manfred Stöckler, Meinard Kuhlmann, Oliver Passon and Paul M. Näger in “The Philosophy of Quantum Physics”:

“If one tries to proceed systematically, then it is expedient to begin with an interpretation upon which everyone can agree, that is with an instrumentalist minimal interpretation. In such an interpretation, Hermitian operators represent macroscopic measurement apparatus, and their eigenvalues indicate the measurement outcomes which can be observed, while inner products give the probabilities of obtaining particular measured values. With such a formulation, quantum mechanics remains stuck in the macroscopic world and avoids any sort of ontological statement about the (microscopic) quantum-physical system itself.”
Our regards of the world only could be in terms of human experiences.

/Patrick
 
vanhees71
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No. Atoms really do exist ! The only mental concept involved is probability which does not share the same kind of existence.
Why? As far as we know, nature, as far as we can observe the phenomena with present means, behaves statistically on a fundamental level. So, as far as we know today, that's the way nature is. Why should this feature not "share the same kind of existence" as atoms? What else are atoms than what we can observe about them?
 
A. Neumaier
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What else are atoms than what we can observe about them?
They existed already before there were observers.
 
vanhees71
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That doesn't matter. When there were no observers, there were also no more or less stupid theories about phenomena. There haven't even been phenomena at all, but that's now really too philosophical for a science forum ;-))).
 
martinbn
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The problem to many is that no-go theorems imply QM is the last word unless you're willing to have multiple worlds, retrocausality or nonlocality. A theory which is the last word and leaves in place clasical objects outside it is unsatisfying to many.
Which theorems?
 

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