# Is quantum theory a microscopic theory?

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bhobba
Mentor
I meant that I found it weird to use the word “microscopic” to refer only to objects on the quantum side of the Heisenberg cut
This may require another thread but can you perhaps expand on what you think of as the Heisenberg Cut? Conventionally it dates back to the early days of QM, and Von--Neumann shows it can really be placed anywhere which led him to place it at consciousnesses which has led to, IMHO, much confusion in 'pop-sci' literature and a lot of misconception correcting here from 'lay' posters. But we now have interpretations without that cut - so it would seem an interpretational thing rather than something inherent in QM. I have to say my favored interpretation us Ensemble - but only applied after decoherence - I call it the ignorance ensemble interpretation) so I still use it as a concept. But has it now outlived its usefulness? If it hasn't then it should be part of the QM formalism - which of course it is not.

Thanks
Bill

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atyy
The weirdness I was referring to had nothing to do with the measurement problem or even any interpretation of quantum mechanics. I meant that I found it weird to use the word “microscopic” to refer only to objects on the quantum side of the Heisenberg cut—hence my example of classical statistical mechanics. It seemed like a really good example of a theory with a microscopic/macroscopic divide: ensemble averages are emergent from collective dynamics of constituent particles (and moreover, the motions of the constituent particles are largely unobservable). But if everything classical is automatically macroscopic, then classical statistical mechanics is purely macroscopic.
It is intimately related to the measurement problem. The quesstion remains even if one is able to use terms like classical microscopic, classical macroscopic, quantum microscopic, quantum macroscopic. Is the measurement apparatus (classical macroscopic) made of electrons (quantum microscopic)?

vanhees71
Gold Member
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.

There are philosophical quibbles, but they don't belong to physics and with a probability close to 1 pondering them won't solve the true fundamental problem of contemporary phsyics, namely a consistent description of quantum theory and gravitation.

TeethWhitener
Gold Member
This may require another thread but can you perhaps expand on what you think of as the Heisenberg Cut?
I’m with you: I think the Heisenberg cut is a matter of interpretation and not fundamental to the quantum formalism. But I’m more of a “shut up and calculate” kind of guy—I don’t really have a dog in the QM interpretation fight.

This thread seems to be asking where different interpretations land on the micro/macro question. That can be solved with suitable definitions. In the line of questions I was pursuing, I was trying to figure out if @Demystifier and @atyy shared a definition of micro/macroscopic. I’m still not sure. If it’s just about detection, then Demystifier might allow classical microscopic theories, whereas if it’s about wavefunctions/Born rule/etc., clearly quantum mechanics must be involved in any microscopic theory.
It is intimately related to the measurement problem. The quesstion remains even if one is able to use terms like classical microscopic, classical macroscopic, quantum microscopic, quantum macroscopic. Is the measurement apparatus (classical macroscopic) made of electrons (quantum microscopic)?
So does it even make sense to talk about a purely classical microscopic theory?

ftr
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?

Sure, the minimal quantum formalism does contain objects, such as particle position operator or field operator, that are in a certain sense microscopic objects. But they are merely tools to compute the probabilities of macroscopic measurement outcomes. In this sense minimal quantum theory is not about local objects such as position or field operators. The minimal quantum theory is about macroscopic measurement outcomes, while the local objects above only make sense if they can be somehow used to predict the properties of macroscopic measurement outcomes. Hence the microscopic objects by themselves have no purpose, and hence no meaning at all, if the minimal instrumental view of quantum theory is adopted.
Obviously if the formulas that describe the microscopic world lead to correct prediction for the macro that obviously means that there is definitely something right about the micro theory. BUT also obviously something is missing, it seems these so called interpretations that suppose to tell us more about the micro, yet they themselves are the victims( and I may add the perpetuate) since they are so proud that they do not contradict standard QM, i.e. they add nothing fundamentally new.

In the the same sense it does not matter if we can "measure/see" these terms in the equations AS LONG AS it leads to a complete prediction like the mass of the electrons, proton and any other fundamental parameter (including gravity, it is a must). If such theory can do it then IT IS the correct micro theory. Had string theory , for example, predicted the exact parameters and not the huge landscape it would have turned the table on the whole of physics, and nobody would have cared to take a peek in the small dimensions, we would have accepted them just the same.

Now, as everybody know there are three major QM "pictures" and each can solve certain problems more easily than others. My conclusion is that there must be another formulation that expands on the other pictures i.e. the TRUE picture.

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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
Gold Member
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
Gold Member
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!

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

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

DarMM
Gold Member
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 modelled 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.

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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 modelled 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
Gold Member
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
Gold Member
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 modelled as being made of quantum particles.

DarMM
Gold Member
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 modelled 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 modelled quantum mechanically, but you always invoke a classical device.

This is confined to Copenhagen style views.

HallsofIvy
Homework Helper
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 modelled 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 modelled 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
Gold Member
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.

2018 Award
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.

2018 Award
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.