Dyson's View Of Wavefunction Collapse

[bhobba]

I came across an interesting quote from Freeman Dyson: (start of quote):

The Collapse Of The Wave-Function
Four and seven years ago, Erwin Schrödinger invented wave functions to describe the behaviour of atoms and other small objects. According to the rules of quantum mechanics, the motions of objects are unpredictable. The wave-function tells us only the probabilities of the possible motions. When an object is observed, the observer sees where it is, and the uncertainty of the motion disappears. Knowledge removes uncertainty. There is no mystery here.

Unfortunately, people writing about quantum mechanics often use the phrase "collapse of the wave-function" to describe what happens when an object is observed. This phrase gives a misleading idea that the wave-function itself is a physical object. A physical object can collapse when it bumps into an obstacle. But a wave-function cannot be a physical object. A wave-function is a description of a probability, and a probability is a statement of ignorance. Ignorance is not a physical object, and neither is a wave-function. When new knowledge displaces ignorance, the wave-function does not collapse; it merely becomes irrelevant. (end of quote)'

I must say for years now, I have held the same view suggested by Gleason's Theorem. But Dyson is saying it is different to my musings.

If his (and my view) are correct, then what is the concern of collapse? I realise it is an interpretation issue. What confuses me is that introducing collapse seems to make things more complex. You see articles, papers, and discussions about it. I find it a bit strange.

 

[Demystifier]

If you say that wave function is just a description of probability, problems don't go away. It begs question, if wave function is not physical, then what is? The PBR theorem says that if there is some objective physical reality at all, some $\lambda$ which is not just probability, then wave function is also a part of $\lambda$, and not just a description of probability.

See also footnote 2 in my https://arxiv.org/abs/2205.05986.

 

[bhobba]

Anyway, if you say that wave function is just a description of probability, problems don't go away. It begs question, if wave function is not physical, then what is?

The result of the observation. I also believe the quantum field is physical, but a paper I have linked to in the past found - that in the non-relativistic limit, the quantum field is not exactly the same as the state.

Not that it makes much difference, really - it just changes the question to why the outcome is probabilistic. Gleason would suggest it is because the outcomes are the observables' eigenvalues. But then, why is that so? Really - has anything been resolved?

Thanks
Bill

 

[martinbn]

In another place I saw Dyson advocating the many worlds interpretation. Anyway, if you say that wave function is just a description of probability, problems don't go away. It begs question, if wave function is not physical, then what is? The PBR theorem says that if there is some objective physical reality at all, some $\lambda$ which is not just probability, then wave function is also a part of $\lambda$, and not just a description of probability.

See also footnote 2 in my https://arxiv.org/abs/2205.05986.

And if there isn't any $\lambda$?

 

[javisot20]

Not that it makes much difference, really - it just changes the question to why the outcome is probabilistic. Gleason would suggest it is because the outcomes are the observables' eigenvalues. But then, why is that so? Really - has anything been resolved?

It is a complicated question, it is likely that a mechanic based on UP (uncertainty principle) would not be expected to be the same as other types of mechanics not based on UP.

What do we understand by physical?
Physical reality is the set of all physical truths. A physical truth is something that I can construct theoretically and confirm experimentally. Therefore, physical reality is the set of everything that we can construct theoretically and confirm experimentally.

So is the wave function and collapse a physical object or not? -Well, it is an object that we can build theoretically and confirm experimentally, but it is not the only object capable of performing the same function. (I have heard Spanish-speaking physicists talk about collapse as an obsolete concept in modern physics)

 

[Demystifier]

And if there isn't any $\lambda$?

Then the PBR theorem is not applicable.

 

[Demystifier]

The result of the observation.

What do you mean by observation? Does detection by an apparatus count as observation, or is it necessary that a conscious being is involved? What is your take on recent extended Wigner friend thought experiments?

 

[martinbn]

Then the PBR theorem is not applicable.

Then why did you bring it up!

 

[Demystifier]

Then why did you bring it up!

Because people who argue that wave function is not physical often tacitly assume that something else is physical.

 

[martinbn]

Because people who argue that wave function is not physical often tacitly assume that something else is physical.

There is nothing in the quote to suggest that Dyson makes such an assumption.

 

[Demystifier]

There is nothing in the quote to suggest that Dyson makes such an assumption.

I read between the lines.

 

[Morbert]

What do you mean by observation? Does detection by an apparatus count as observation, or is it necessary that a conscious being is involved? What is your take on recent extended Wigner friend thought experiments?

Asher Peres (in "Quantum Theory: Concepts and Methods") says an apparatus is well-described if i) It is appropriately correlated with the system of interest and ii) It is well-described by a Liouville density derived from its Wigner function. An observation is a test carried out by a reliable apparatus.

If an apparatus is only somewhat reliable, then a physicist will only be able to somewhat reproduce the predictions of QM.

[edit] - Expanded on a reliable apparatus

 

[bhobba]

What do you mean by observation? Does detection by an apparatus count as observation, or is it necessary that a conscious being is involved? What is your take on recent extended Wigner friend thought experiments?

I take it as decoherence, an example of detection by an apparatus. I have never understood the conscious being ideas. It sounds far too much like solipsism for my taste. Not familiar with Extended Wigners, friend - I must look into it.

Thanks
Bill

 

[WernerQH]

(I have heard Spanish-speaking physicists talk about collapse as an obsolete concept in modern physics)

That precisely was Dyson's point. (What Scientific Idea Is Ready For Retirement?)

What confuses me is that introducing collapse seems to make things more complex. You see articles, papers, and discussions about it. I find it a bit strange.

I share Dyson's view and used the same quote in what was probably my first post on this forum. And I agree that it is puzzling (and sad!) that such discussions are still going on in the International Quantum Year.

The root of the problem, in my opinion, is the exaggerated role of the time-dependent wave function and Schrödinger's equation. There is an almost irrepressible urge to describe a quantum object in a pseudo-Markovian (quasi-Newtonian?) fashion as having at all times some definite state evolving continuously and even deterministically. It simply doesn't square with the discontinuities and randomness that experiments seem to reveal in the real world. There's a reason why von Neumann introduced "measurement" (collapse) as a separate process besides unitary evolution. But it's an uneasy combination and, for some physicists, has created a new "measurement problem". I think Schrödinger's equation is but one piece of the mathematical apparatus of quantum mechanics, and it must not be pulled apart from the other essential component of the machinery, namely Born's rule.

It is said that the Heisenberg picture is completely equivalent to the Schrödinger picture. But this is true only if one considers the full picture, and the wave function by itself does not provide the full picture! What is the place of wave function collapse in the Heisenberg picture? In the Heisenberg picture the state (wave function) just remains constant!

I think there is real graininess (discontinuities) in the real world, and that it is correctly described by quantum theory. But it's not caused by collapsing wave functions. In my view, quantum theory must be seen as a stochastic theory.

 

[gentzen]

@bhoppa (or any other mentor): Please move this thread into the Quantum Interpretations and Foundations subforum. At least WernerQH's post above dives deep into interpretation territory, and it isn't really surprising that this question provoked this sort of reaction.

 

[PeterDonis]

And if there isn't any $\lambda$?

Then how are we even having this conversation in the first place? Surely at the very least those of us who are having this conversation are physical. So there is something that's physical. And it seems evident that, since we can communicate, there is some common medium we all live in that is physical as well.

 

[PeterDonis]

Please move this thread into the Quantum Interpretations and Foundations subforum.

Done.

 

[martinbn]

Then how are we even having this conversation in the first place? Surely at the very least those of us who are having this conversation are physical. So there is something that's physical. And it seems evident that, since we can communicate, there is some common medium we all live in that is physical as well.

I am not questioning the existence of the physical reality. But $\lambda$ is not the reality. It is a part of the discription of reality.

 

[PeterDonis]

I am not questioning the existence of the physical reality. But $\lambda$ is not the reality. It is a part of the discription of reality.

Ok, fine, but the question is still there: what would it even mean for there to be no $\lambda$?

 

[martinbn]

Ok, fine, but the question is still there: what would it even mean for there to be no $\lambda$?

I suppose to take QM as it is. There is no lambda in QM.

 

[Peter Morgan]

I came across an interesting quote from Freeman Dyson: (start of quote):

The Collapse Of The Wave-Function
Four and seven years ago, Erwin Schrödinger invented wave functions to describe the behaviour of atoms and other small objects. According to the rules of quantum mechanics, the motions of objects are unpredictable. The wave-function tells us only the probabilities of the possible motions. When an object is observed, the observer sees where it is, and the uncertainty of the motion disappears. Knowledge removes uncertainty. There is no mystery here.

Unfortunately, people writing about quantum mechanics often use the phrase "collapse of the wave-function" to describe what happens when an object is observed. This phrase gives a misleading idea that the wave-function itself is a physical object. A physical object can collapse when it bumps into an obstacle. But a wave-function cannot be a physical object. A wave-function is a description of a probability, and a probability is a statement of ignorance. Ignorance is not a physical object, and neither is a wave-function. When new knowledge displaces ignorance, the wave-function does not collapse; it merely becomes irrelevant. (end of quote)'

I must say for years now, I have held the same view suggested by Gleason's Theorem. But Dyson is saying it is different to my musings.

If his (and my view) are correct, then what is the concern of collapse? I realise it is an interpretation issue. What confuses me is that introducing collapse seems to make things more complex. You see articles, papers, and discussions about it. I find it a bit strange.

I think of Dirac here as somewhat threading the needle between the positions identified by Don Howard in his article in Philosophy of Science 2004, Who Invented the “Copenhagen Interpretation”? A Study in Mythology (DOI, link to PDF on author's website). I take that article to argue that Bohr takes measurements to affect the results of subsequent measurements, which is mathematically the same as but otherwise rather different from the idea that the state collapses, as advocated by Heisenberg and others.
I cite Don Howard's article as part of the argument in my article in JPhysA 2022, "The collapse of a quantum state as a joint probability construction" (arXiv, DOI), which follows Bohr's idea. I think this is also a reasonable way to take seriously an idea that the quantum state is mostly for generating probabilities, which I take to be approximately Dirac's idea here and which presumably should include joint probabilities. The Deferred Measurement Principle is an idea that I have only recently realized is very similar.
From this perspective, Dirac is too precipitate when he says that "When new knowledge displaces ignorance, the wave-function does not collapse; it merely becomes irrelevant", because new knowledge only removes some uncertainty; two consecutive pieces of knowledge removes more uncertainty; and so on. When we have a dataset that contains many pairs of results of consecutive measurements, we typically want to construct a joint probability distribution as a model for that data (and for the expected results of similar future experiments, whether or not we use noncommutative operators as models for those measurements.
In talks since 2022, I have argued that Naimark's Dilation Theorem is an effective alternative to Decoherence. The argument is elementary enough that I present it in a few slides:

 

[PeterDonis]

I suppose to take QM as it is. There is no lambda in QM.

But that's no answer at all to someone who is not just taking QM as it is. Which pretty much includes anyone who is considering the question Dyson is considering in the quote given in the OP. If your answer is "just take QM as it is", then you're saying this thread is pointless. Of course you're entitled to your opinion, but other people posting here don't seem to agree.

 

[martinbn]

But that's no answer at all to someone who is not just taking QM as it is. Which pretty much includes anyone who is considering the question Dyson is considering in the quote given in the OP. If your answer is "just take QM as it is", then you're saying this thread is pointless. Of course you're entitled to your opinion, but other people posting here don't seem to agree.

No, that is not my point of view. That is my understanding of Dyson's. @Demystifier said that Dyson's view still has problems and his argument was the PBR theorem. That's why I asked him, what if Dyson doesn't think that there is a lambda?

 

[bhobba]

Because people who argue that wave function is not physical often tacitly assume that something else is physical.

I assume the quantum field is physical. My concern is (I will give a link to the paper this time):

https://arxiv.org/abs/1712.06605

The conclusion is (with a few minor editorial changes suggested by Grammarly):

'I examine this limit in several approaches ( e.g., Hamiltonian dynamics, Lagrangian and Hamiltonian path integrals, field theoretic description, etc.) and identify the precise issues that arise when one attempts to obtain NRQM from QFT in each of these approaches. The dichotomy of description between NRQM and QFT does not originate just from the square root in the Hamiltonian or the demand of Lorentz invariance, as it is sometimes claimed. The real difficulty arises in the necessary existence of antiparticles to ensure a particular notion of relativistic causality. Because of these conceptual issues, it turns out that one cannot obtain some of the popular descriptions of NRQM by any sensible limiting procedure applied to QFT. To obtain NRQM from QFT seamlessly, it is necessary to work with NRQM expressed in a language closer to that of QFT.'

It would seem ordinary QM is only an 'effective theory', so the reality of the state is questionable - it suggests it is just an aid to calculations - not physically real (whatever that is - I take a common-sense Feynman-type view). We know it's wrong anyway because QFT (or at least a very clever use of ordinary QM) is needed to explain spontaneous emission:

https://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

A separate thread examining this might be interesting. When I have time, I will see if I can post one.

As an aside, I am nearly 70, and after a discussion with the police, I decided to stop driving. Using Uber to go everywhere is a bit time-consuming.

Thanks
Bill

 

[Demystifier]

I take it as decoherence, an example of detection by an apparatus. I have never understood the conscious being ideas. It sounds far too much like solipsism for my taste. Not familiar with Extended Wigners, friend - I must look into it.

The problem with decoherence is that, if you are agnostic about interpretations of QM, it cannot explain the origin of a definite outcome. The extended Wigner friend thought experiments make this problem more explicit.

 

[Demystifier]

I suppose to take QM as it is. There is no lambda in QM.

But QM as it is has collapse. And some people, as this thread shows, want to move collapse away. If there is no collapse, and no lambda, and no any specific interpretation of QM, what remains? Something a'la Ballentine perhaps? But even he needs lambda in the chapter on Bell's theorem of his book.

 

[Demystifier]

No, that is not my point of view. That is my understanding of Dyson's. @Demystifier said that Dyson's view still has problems and his argument was the PBR theorem. That's why I asked him, what if Dyson doesn't think that there is a lambda?

In my opinion, there is no coherent view of QM that does not contain at least one thing from the following list:
1) collapse
2) some kind of lambda
3) some controversial philosophical interpretation far beyond the minimal textbook QM

So if Dyson rejects collapse, he is either incoherent, or accepts some kind of lambda, or accepts some controversial interpretation, or some combination of those.

 

[Demystifier]

Note also that I deleted my previous claims that Dyson advocated many worlds. It was Coleman.

 

[martinbn]

But QM as it is has collapse. And some people, as this thread shows, want to move collapse away. If there is no collapse, and no lambda, and no any specific interpretation of QM, what remains? Something a'la Ballentine perhaps? But even he needs lambda in the chapter on Bell's theorem of his book.

I am guessing that he needs lambda because the version of Bell's theorem he proves needs it. But he doesn't need lambda for QM.

But the topic of this thread is Dyson. And to me it seems that he doesn't think that there is lambda. He says that looking at the collapse in the way he dose makes it not problematic. Of course you can disagree with that because there is still a measurement problem. I don't know what his view is on that.

In my opinion, there is no coherent view of QM that does not contain at least one thing from the following list:
1) collapse
2) some kind of lambda
3) some controversial philosophical interpretation far beyond the minimal textbook QM

So if Dyson rejects collapse, he is either incoherent, or accepts some kind of lambda, or accepts some controversial interpretation, or some combination of those.

I don't think he rejects collapse. He rejects the terminology and the claim that it is a physical process of a physical object. I think he is OK with state reduction.

 

[Demystifier]

I don't think he rejects collapse. He rejects the terminology and the claim that it is a physical process of a physical object. I think he is OK with state reduction.

Then I'm fine with his view, but still I would be interested to see what he thinks that a physical process is.