A Is quantum theory a microscopic theory?

[atyy]

So does it even make sense to talk about a purely classical microscopic theory?

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.

 

[Mentz114]

Summary: If quantum theory is nothing but a set or rules to compute the probabilities of macroscopic measurement outcomes, then what is microscopic about it?
Does quantum theory really tell us something about the microscopic world?

Daft question. Quantum theory is the most successful theory known to mankind. No prediction has ever been contradicted and mathematical precision is $O(10^{-12})$

 

[martinbn]

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.

Why not! What forbids this?

 

[vanhees71]

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.

I'd recommend to read a modern textbook on condensed-matter physics or attend some talks about condensed-matter physics. Then you'll learn that QT has to tell us a lot, if not everything, about macroscopic systems!

 

[akvadrako]

QT seems to explain the behavior of systems with few independent degrees of freedom. That always applies to microscopic systems but it's not about size. At the extreme, some people are even using QT to explain human cognition; for example the so-called "conjunction falacy" is that most people consider it more likely that “Linda is a feminist and a bank teller” than “Linda is a bank teller”. This can't be explained with classical reasoning but can with QT if feminist and bank teller are incompatible dimensions.

So that would make QT a theory about a class of systems which includes microscopic ones.

 

[vanhees71]

QT also applies to macroscopic systems as its very successful use in condensed-matter physics shows.

 

[DarMM]

I think @atyy is referring to how in Copenhagen presentations (Peres, Landau & Lifshitz, Weinberg 2nd Edition) quantum theory is formulated in terms of observables witnessed by a device that's treated classically. In technical language the classical device constitutes the Boolean frame with respect to which outcomes occur.

Of course one can then treat the device itself quantum mechanically, but this is from the perspective of the presence of a separate larger device that is treated classically which is capable of measuring the first. So everything can be described quantum mechanically, but not everything at once.

It's not as such a micro/macro division or that QM does not apply above a certain scale, just something must lie outside the theory to constitute the outcomes. The above mentioned textbooks discuss this.

 

[martinbn]

I think @atyy is referring to how in Copenhagen presentations (Peres, Landau & Lifshitz, Weinberg 2nd Edition) quantum theory is formulated in terms of observables witnessed by a device that's treated classically. In technical language the classical device constitutes the Boolean frame with respect to which outcomes occur.

Of course one can then treat the device itself quantum mechanically, but this is from the perspective of the presence of a separate larger device that is treated classically which is capable of measuring the first. So everything can be described quantum mechanically, but not everything at once.

It's not as such a micro/macro division or that QM does not apply above a certain scale, just something must lie outside the theory to constitute the outcomes. The above mentioned textbooks discuss this.

Yes, but what is the theorem that states that it must be so? Take for example classical physics (non quantum), you can have a system consisting of a large number of particles obeying Newton's laws that exhibits new emergent behavior. The system may have constant temperature, pressure, and volume although the individual particles are in constant motion, and some of the emergent properties don't even make sense for the individual building blocks. So, what in QT forbits an emergent classical behavior? Why is it impossible for the chair, I am sitting on, to be made of many QM particles obeying QM's laws and having classical properties, that follow from the QM's laws?

 

[vanhees71]

I'd rather say, it's because of QT that we have a stable chair to sit on!

 

[DarMM]

Yes, but what is the theorem that states that it must be so? Take for example classical physics (non quantum), you can have a system consisting of a large number of particles obeying Newton's laws that exhibits new emergent behavior. The system may have constant temperature, pressure, and volume although the individual particles are in constant motion, and some of the emergent properties don't even make sense for the individual building blocks. So, what in QT forbits an emergent classical behavior? Why is it impossible for the chair, I am sitting on, to be made of many QM particles obeying QM's laws and having classical properties, that follow from the QM's laws?

As I said above there's nothing preventing you from treating the device quantum mechanically, thus it's not a problem with obtaining emergent classical behavior. It's a separate problem. It's that when you do model the device with QM you invoke a second device that is treated classically. You could treat this device with qm, but you invoke a third device and so on. This is sometimes known as the Von Neumann chain.

The presence of something not modeled with QM that selects a particular Boolean frame is always assumed in typical Copenhagen presentations of the theory.

This is not the case in classical theories, where the theory is not written with reference to a system lying outside the theory.

 

[martinbn]

As I said above there's nothing preventing you from treating the device quantum mechanically, thus it's not a problem with obtaining emergent classical behavior. It's a separate problem. It's that when you do model the device with QM you invoke a second device that is treated classically. You could treat this device with qm, but you invoke a third device and so on. This is sometimes known as the Von Neumann chain.

The presence of something not modeled with QM that selects a particular Boolean frame is always assumed in typical Copenhagen presentations of the theory.

This is not the case in classical theories, where the theory is not written with reference to a system lying outside the theory.

Why do I need a second device? I have a QM system which is in a state (evolves to a state) that is classical in a certain sense, and I know that because it is a consequence of the theory, I don't need a second devise. Just like the classical physics case. If I have gas in a box, I can talk about its temperature without the need of a thermometer.

 

[DarMM]

It's due to the fact that QM won't give the actual resultant state of the device. The device will end up with terms for each outcome rather than the one it actually displays. Thus in the Copenhagen reading it predicts the chances for a second device to observe the first in its various pointer states.

This is quite an old issue. It's in Von Neumann's book and the ones I mentioned above.

 

[martinbn]

It's due to the fact that QM won't give the actual resultant state of the device. The device will end up with terms for each outcome rather than the one it actually displays. Thus in the Copenhagen reading it predicts the chances for a second device to observe the first in its various pointer states.

This is quite an old issue. It's in Von Neumann's book and the ones I mentioned above.

That's not the issue. I am not talking about any measurements. @atyy said that QM doesn't allow (at least it seems so) to say that a classically behaving object is made out of quantum mechanical particles. My question is how so?

 

[DarMM]

That's not the issue. I am not talking about any measurements. @atyy said that QM doesn't allow (at least it seems so) to say that a classically behaving object is made out of quantum mechanical particles. My question is how so?

I'm only assuming, so perhaps I'm wrong, my guess is that he was referring to the Von Neumann chain where one always has some system present that's not modeled as being made of quantum particles.

 

[DarMM]

That's not the issue. I am not talking about any measurements

Another aspect of the problem is that in QM you're always talking about measurements with respect to some device modeled classically. There have been attempts to remove this and have QM not require a classical device. Such as decoherent histories. However although they achieve much they don't manage this. Weinberg in the 2nd edition of his text has a nice exposition on this.

Of course anything may be modeled quantum mechanically, but you always invoke a classical device.

This is confined to Copenhagen style views.

 

[HallsofIvy]

If quantum theory is nothing but a set or rules to compute the probabilities of macroscopic measurement outcomes, then what is microscopic about it?

I surely wouldn't say quantum theory "is nothing but a set of rules". Perhaps you meant "quantum mechanics"? Quantum mechanics is pretty well understood but there are a number of different "theories" about what is behind the mechanics.

In any case, the only experiments we can do are pretty much "macroscopic" because we are macroscopic. Quantum theory is a theory about how microscopic events can affect macroscopic observations. In that sense it is "microscopic".

 

[martinbn]

Another aspect of the problem is that in QM you're always talking about measurements with respect to some device modeled classically. There have been attempts to remove this and have QM not require a classical device. Such as decoherent histories. However although they achieve much they don't manage this. Weinberg in the 2nd edition of his text has a nice exposition on this.

Of course anything may be modeled quantum mechanically, but you always invoke a classical device.

This is confined to Copenhagen style views.

I never understood why that is a problem. Why should a theory not use classical objects in its formulation?

 

[DarMM]

I never understood why that is a problem. Why should a theory not use classical objects in its formulation?

Practically it's no issue.

It's more that some think a theory should be able to describe the world without requiring objects outside the theory acting as "witnesses" to define events.

For example just describing an electron on its own, not needing to include a measuring device.

 

[Demystifier]

Do you consider atoms, molecules, atomic bonds, etc. to be microscopic?

If the answer to both of those is yes, then quantum theory has told us about features of the microscopic world.

I do not say that QM does not say anything about the micro world. I say that the minimal instrumental form of QM does not say anything about the micro world.

 

[Demystifier]

Daft question. Quantum theory is the most successful theory known to mankind. No prediction has ever been contradicted and mathematical precision is $O(10^{-12})$

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.

 

[Demystifier]

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]

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]

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.

 

[Demystifier]

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]

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]

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.

 

[vanhees71]

So what?

 

[Mentz114]

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]

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]

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.