Macroscopicity and the measurement problem

[EPR]

Assuming the validity of the Heisenberg-von Nuemann cut, does the measurement problem influence the macroscopic nature of the macro world?
Quantum effects, roughly speaking, 'lose' their quantum nature over the many degrees of freedom of many-particles systems(cars, chairs...) via averaging out on their constituents.
Given the cut, is the MP that which produces the definiteness of the macro reality or is it inapplicable to systems much larger than the cut?
I am an MA electrical engineer... bear with me if some intricacy escapes me.

 

[PeterDonis]

Assuming the validity of the Heisenberg-von Nuemann cut

What exactly would this mean? Would it mean that QM simply stops being valid once the cut is passed? Or would it just mean that we have to use different approximation schemes above the cut than below it?

 

[atyy]

Assuming the validity of the Heisenberg-von Nuemann cut, does the measurement problem influence the macroscopic nature of the macro world?
Quantum effects, roughly speaking, 'lose' their quantum nature over the many degrees of freedom of many-particles systems(cars, chairs...) via averaging out on their constituents.
Given the cut, is the MP that which produces the definiteness of the macro reality or is it inapplicable to systems much larger than the cut?

There are two different concepts of "macroscopic" or "classical" in QM.

One concept is fundamental, and takes the existence of "macroscopic" objects such as the observer and measurement apparatus for granted. This concept is used in the Heisenberg cut, which divides the universe into the "macroscopic" part and the quantum part. The definiteness of reality on the "macroscopic" side of the cut is assumed to be intuitively obvious. Thus this concept of "macroscopic" deals with the definiteness of "macroscopic" reality, and has not so much to say about how classical mechanics arises from quantum mechanics under certain conditions or limits. Thus when dealing with a quantum system, one gets definite outcomes on the "macroscopic" side of the cut that are the predictions of quantum mechanics. This first sense of "macroscopic" or "classical" is a little misnamed, although those terms are traditional - the key point here is that here there are definite outcomes, ie. a definite reality. This first concept of "macroscopic" is fundamental, and always present in quantum mechanics, and leads to the measurement problem.

The second concept of "macroscopic" or "classical" is that under certain circumstances, the predictions of quantum mechanics are well approximated by classical mechanics, ie. the full predictions using the quantum side of the cut about what is observed on the "macroscopic" side of the cut is well approximated by quantum mechanics. This second concept of macroscopic is not fundamental in quantum mechanics, and arises only in conditions or limits in which quantum mechanics is well approximated by quantum mechanics. As a specific example, in the Feynman path integral, the classical approximation is the saddle point approximation to the path integral, and is useful when the quantum corrections are small enough for what ever one needs.

These two different concepts of "macroscopic" or "classical" are why Landau and Lifshitz say in their textbook: "Quantum mechanics occupies a very unusual place among physical theories: it contains classical mechanics as a limiting case, yet at the same time, it requires this limiting case for its own formulation."

 

[EPR]

What exactly would this mean? Would it mean that QM simply stops being valid once the cut is passed? Or would it just mean that we have to use different approximation schemes above the cut than below it?

This is a tough question that i wanted to clear up for myself.

https://www.semanticscholar.org/pap...rndt/1b4ab0aead781f18765e5ed66cd4893b060dc2e2

The authors establish the 'cut' which is gradual from between 2 nm(about 20 atoms across) all the way up to 3-4 micrometers(30 000 atoms across) where classical mechanics is a more useful description(the test was done with C70 mollecules).
The processor industry has been dealing with quantum issues(tunneling, rough edges, etc) for at least a decade. But this is the practical side which agrees with the predictions established by the HUP.
My question regards the treatement of the world in a quantum mechanical fashion and extending and interpreting reality beyond the cut. I read threads where this was addressed briefly as a side-issue and most seem to agree Bohr was mistaken in assuming the double slit setup should be treated as 2 separate domains - the system as qunatum mechanical and the apparatus as classical.
At least from a practical point, such treatment should be quite valid and obvious given the graph and the scales where quantum effects start to become overwhelming. It could be that the current framework of quantum theory should not be taken as a literal explanation of macro reality, given the cut(whose nature still appears be unknown, though much research appears to be focused on mesoscopic scales 10 to 1000nm). To me it makes more sense to treat the apparatus as classical ala Bohr, esp. given the cut which was likely not experimentally known in the 1940's(though it was predicted theoretically even earlier).
A side issue is whether it could be that the MP(a quantum phenomenon) plays no role at macro scales and how classical reality emerges(i was unable to find literature on this issue from the point of view of the cut)?

 

[Lord Jestocost]

...and most seem to agree Bohr was mistaken in assuming the double slit setup should be treated as 2 separate domains - the system as qunatum mechanical and the apparatus as classical.

Such a nonsense might be spread by people who are not able to understand Bohr's reasoning. On Bohr's point of view regarding the "cut", N.P. Landsman writes the following in section "3.2 Object and apparatus: the Heisenberg cut” in his paper "Between classical and quantum" (https://arxiv.org/abs/quant-ph/0506082):

"The first step of this resolution that he [Bohr] and Heisenberg proposed is to divide the system whose description is sought into two parts: one, the object, is to be described quantum-mechanically, whereas the other, the apparatus, is treated as if it were classical. Despite innumerable claims to the contrary in the literature (i.e. to the effect that Bohr held that a separate realm of Nature was intrinsically classical), there is no doubt that both Bohr and Heisenberg believed in the fundamental and universal nature of quantum mechanics, and saw the classical description of the apparatus as a purely epistemological move without any counterpart in ontology, expressing the fact that a given quantum system is being used as a measuring device.⁶⁷ For example: ‘The construction and the functioning of all apparatus like diaphragms and shutters, serving to define geometry and timing of the experimental arrangements, or photographic plates used for recording the localization of atomic objects, will depend on properties of materials which are themselves essentially determined by the quantum of action’ (Bohr, 1948), as well as: ‘We are free to make the cut only within a region where the quantum mechanical description of the process concerned is effectively equivalent with the classical description’ (Bohr, 1935).⁶⁸"

[italics in original, bold by LJ]

 

[PeterDonis]

The authors establish the 'cut'

They do no such thing.

First, the authors are being inconsistent. On the one hand, they label part of the chart as "classical regime"; but on the other hand, they claim that this experiment is a confirmation of the uncertainty principle. There is no uncertainty principle in classical physics, so in order to interpret the experiment as confirming the uncertainty principle, you have to interpret the entire regime using quantum mechanics. You can't arbitrarily label part of it as "classical".

Second, to establish a "cut" anywhere, the authors would have to show both of the following:

(1) That the part labeled "quantum regime" cannot be modeled correctly by classical physics. But this is wrong because classical diffraction theory predicts the same widening of the beam width for narrow enough slits that is observed. So this regime does not require quantum physics for its explanation.

(2) That the part labeled "classical regime" cannot be modeled correctly by quantum physics. But this is wrong because the predictions of quantum physics are pretty much the same as the predictions of classical physics in the part labeled "classical regime"; they have to be since quantum physics is well approximated by classical physics in this regime.

 

[EPR]

One would expect someone like Anton Zeilinger to know that classical diffraction can explain the sudden widening of the beam for narrowing slits.

Why is this experiment cited so ubiquitously in the literature and peer reviewed journals if its interpretation can be made via classical defraction?

 

[Lord Jestocost]

First, the authors are being inconsistent. On the one hand, they label part of the chart as "classical regime"; but on the other hand, they claim that this experiment is a confirmation of the uncertainty principle.

The figure caption makes it clear what the authors want to express:

FIG. 3. The experimental molecular beam width (full circle) is compared with the quantum prediction (continuous line) as a function of the slit opening Δx. The agreement is excellent across the whole range of slit openings (70nm...20 μm). A purely classical shadow model predicts the dotted line and is in marked disagreement with the data for Δx < 4μm. The latter is therefore designated as the quantum regime and magnified in the inset of fig 3.

 

[PeterDonis]

A purely classical shadow model

Which is not the only possible classical model. A classical wave model predicts diffraction.

 

[PeterDonis]

One would expect someone like Anton Zeilinger to know that classical diffraction can explain the sudden widening of the beam for narrowing slits.

One would also expect that, if he knows this, he would recognize it in the paper. But apparently he doesn't, since the only classical model the paper mentions is the "shadow model".

Why is this experiment cited so ubiquitously in the literature and peer reviewed journals if its interpretation can be made via classical defraction?

I couldn't say, but my guess would be because the paper doesn't treat a classical wave model and nobody else bothered to consider that.

 

[PeterDonis]

The experimental molecular beam width (full circle) is compared with the quantum prediction (continuous line) as a function of the slit opening Δx. The agreement is excellent across the whole range of slit openings (70nm...20 μm)

Which in itself means this experi`ment can't be evidence for a classical-quantum "cut", since the quantum prediction matches the experimental result over the entire experimental regime.

 

[PeterDonis]

Why is this experiment cited so ubiquitously in the literature

Does anyone cite it as support for a classical-quantum "cut" being an actual, physical thing? The paper itself does not claim that; it only claims that the experiment confirms the Heisenberg uncertainty principle, which it does since the quantum prediction matches the observed result.

 

[EPR]

If figure 3 is correct, something as big as a virus will likely not be found in a coherent state. Furthermore, all of you tacitly treat the apparatus as 'classical' for scientific consistency(if not, how can you probe anything at all?). I don't see why this 'cut' is an issue in the practical sense.

 

[kith]

Which is not the only possible classical model. A classical wave model predicts diffraction.

The experiment is done with fullerene molecules, i.e. matter. There isn't a classical wave theory of matter.

 

[PeterDonis]

There isn't a classical wave theory of matter.

If that's true, then the label "classical regime" on the graph is wrong, because there is no "classical regime". Which I agree with: classical physics predicts that "matter" as we know it should not exist--atoms should collapse. But then, as I've said, this paper is not evidence for a "classical-quantum cut", since there can't be any such thing if classical physics can't explain the existence of matter.

 

[PeterDonis]

If figure 3 is correct, something as big as a virus will likely not be found in a coherent state.

What do you mean by "a coherent state"?

 

[EPR]

What do you mean by "a coherent state"?

In a macroscopically coherent state.

 

[PeterDonis]

In a macroscopically coherent state.

That doesn't help. What is a "macroscopically coherent state"? (Hint: you should not be giving me an answer in words, you should be giving me an answer in math.)

 

[kith]

If that's true, then the label "classical regime" on the graph is wrong, because there is no "classical regime".

I think by "classical regime" they mean the regime were classical mechanics can be used as an approximation to explain the results while in the "quantum regime", QM is needed.

I agree that the labeling is a bit misleading and that this paper doesn't intend to show that there's a "regime" where QM is false and classical mechanics is true instead. The correct notion of a "cut" is the Heisenberg cut and this cut is shiftable and is present only in some interpretations.

 

[EPR]

This clears it up for me about the 'cut'. The classical regime should be labelled 'classical-like regime' as all systems at all scales are always in a superposition of states. It's an abuse of terms, as there is no classical theory of matter(or atoms would collapse).

 

[PeterDonis]

There isn't a classical wave theory of matter.

There isn't any classical particle theory of matter either. Such a theory would have to explain why the particles are stable, and it can't.

To be clear, the main claim of the paper is not affected by this. We already know on other grounds that classical physics can't be a correct fundamental theory. So it's perfectly fine for the paper to use a quantum model and show that it matches the data through the entire regime.

But once we recognize that classical physics is wrong, I don't think we can say that a classical wave model is somehow more wrong than a classical particle model. Buckyballs show interference in the double slit experiment, so they have wave properties as well as particle properties. So as an approximation for this particular case, I see no justification for not considering a classical wave model, which, as noted, matches the data better in this case.

 

[EPR]

The gradual 'cut' must be irrelevant to the question whether the measurement problem determines the definitness of macro reality(chairs, apparati, etc.) as all quantum observables become definite only through measurement/observation. This should hold at all scales regardless of size as quantum theory is clearly a more comprehensive theory than CM. I stand corrected that a virus/bacteria will not be found to manifest quantum properties. I was misled by the labelling of the graph and the perculiar case of classical mechanics as a limiting case of quantum theory, which at the same time requires this limiting case for its own formulation.

 

[kith]

There isn't any classical particle theory of matter either. Such a theory would have to explain why the particles are stable, and it can't.

By the same token there isn't a classical wave theory of light because it would have to explain the ultraviolet catastrophe. I agree that this use of terminology is consistent but I don't like it. The term "classical" was invented as an umbrella term for pre-quantum theories of matter and light in the first place. These theories both describe our everyday world very well and have severe problems. Part of the experiment in question is to look exactly how far one of these theories (classical mechanics) can be used to correctly model the behaviour of fullerene molecules.

 

[PeterDonis]

By the same token there isn't a classical wave theory of light because it would have to explain the ultraviolet catastrophe.

Yes.

The term "classical" was invented as an umbrella term for pre-quantum theories of matter and light in the first place.

Yes, but that umbrella includes wave theories as well as particle theories.

Part of the experiment in question is to look exactly how far one of these theories (classical mechanics) can be used to correctly model the behaviour of fullerene molecules.

And my point is that, if all you are trying to do is model the experiment as best you can using a classical theory, a classical wave theory does a better job, and I see no justifiable reason for excluding it on the logic you are using. However, as I said, it doesn't affect the paper's main claim anyway, and as you agree, it doesn't justify any claims about a "cut" being real, so it's tangential to the main topic of this thread.

 

[atyy]

Yes, but that umbrella includes wave theories as well as particle theories.

Maybe they did not consider classical wave theories because those don't give particle-like discrete "hits" on the screen.

 

[PeterDonis]

Maybe they did not consider classical wave theories because those don't give particle-like discrete "hits" on the screen.

In the experiment described in the paper linked to in post #4, as far as I can tell, there aren't any; all that is detected is a continuous beam.

 

[kith]

Yes, but that umbrella includes wave theories as well as particle theories.

Which wave theories do you have in mind here?

 

[Lord Jestocost]

In the experiment described in the paper linked to in post #4, as far as I can tell, there aren't any; all that is detected is a continuous beam.

Typical count rates in such a type of experiments can be found in Fig. 2 of https://www.nature.com/articles/44348?draft=journal (Ref. [9] in the paper linked to in post #4).

 

[PeterDonis]

Which wave theories do you have in mind here?

Classical wave theory in general. I'm not saying there's a classical wave theory of fullerenes sitting on the shelf. I'm saying that making a classical wave model of what's going on in the particular experiment described in the paper would be roughly similar in scope to making the classical particle model that the paper used.

 

[PeterDonis]

Typical count rates in such a type of experiments

Hm, yes, the paper you reference includes a key part of the apparatus that the paper linked to in post #4 does not: the "ion detection unit" after the scanning laser. If individual ions are being counted, that of course does rule out a classical wave model.