Is Quantum Mechanics Truly Underdetermined by Evidence?

[A. Neumaier]

TL;DR

A new preprint by David Wallace

David Wallace, The sky is blue, and other reasons quantum mechanics is not underdetermined by evidence, Manuscript (2022). arXiv:2205.00568.

From the Abstract:
''I argue that there as yet no empirically successful generalization of''
[Bohmian Mechanics and dynamical-collapse theories like the GRW or CSL theories]
''to interacting quantum field theory [...] The class of quantum experiments reproducible by either is much smaller than is commonly recognized and excludes many of the most iconic successes of quantum mechanics, including the quantitative account of Rayleigh scattering that explains the color of the sky.''

 

[Demystifier]

When this paper appeared, I've sent an e-mail to David Wallace pointing to my recent paper on Bohmian QFT (in which, by the way, I cite two other Wallace's papers). His reply started with "If I understood your paper correctly, ..." and then wrote something which had absolutely nothing to do with my paper.

To make the long story short, it seems that Wallace is completely unfamiliar with a vast literature that extends Bohmian mechanics to QFT.

 

[A. Neumaier]

When this paper appeared, I've sent an e-mail to David Wallace pointing to my recent paper on Bohmian QFT (in which, by the way, I cite two other Wallace's papers). His reply started with "If I understood your paper correctly, ..." and then wrote something which had absolutely nothing to do with my paper.

To make the long story short, it seems that Wallace is completely unfamiliar with a vast literature that extends Bohmian mechanics to QFT.

Please post here a link to the paper where Bohmian mechanics is extended to interacting QED, which is what is needed for the quantitative account of Rayleigh scattering.

 

[Demystifier]

Please post here a link to the paper where Bohmian mechanics is extended to interacting QED, which is what is needed for the quantitative account of Rayleigh scattering.

I can point to a paper explaining how, in principle, Bohmian mechanics is extended to any interacting QFT, but that's not what you ask for. People who don't believe that it is possible usually give general reasons why they think it is impossible. When Bohmians explain them why those general reasons are wrong, then people don't want to read this and some of them ask for a paper with details. But there is no point in writing those details (which would be straightforward but boring) if people don't understand the general principles.

 

[A. Neumaier]

there is no point in writing those details

Your post gives the best justification I can think of for skipping Bohmian mechanics as being irrelevant for quantum physics. One doesn't need to be familar with a vast literature on Bohmian mechanics to realize this.

I think many people would take interest in a paper where Bohmian mechanics is used to explain why the sky is blue.

Unless it is only of the kind 'bla bla bla. Now you can apply the standard textbook reasoning'. For stating only this much will be interpreted by those who would be interested in a true treatment as 'we don't need the bla bla bla since it does not shed any light on the problem'.

 

[Demystifier]

I think many people would take interest in a paper where Bohmina mechanics is used to explain why the sky is blue.

Maybe they would take interest, but they would not understand it. They would understand the 95% of the paper which would be a copy-paste from a standard QFT textbook, but they would not understand the 5% that makes it different from standard QFT. They would not understand it, because they don't understand how Bohmian mechanics works in general. The existing Bohmian literature concentrates on explaining this 5%, because this is all what really matters in this context, but people don't get it. They don't get the general principles of Bohmian mechanics, so explaining details of why the sky is blue doesn't help. Just like explaining why the sky is blue with standard QED doesn't help to someone who doesn't understand the general principles of standard QM.

 

[A. Neumaier]

They would not understand it, because they don't understand how Bohmian mechanics works in general.

You severely underrate their capabilities. It is not the case that quantum physicists are dummies, except for the few who match your understanding.

Bohmian mechanics is not hard to understand - except in places where they sweep things under the rug of details that their masters think don't need to be spelt out.

 

[berkeman]

Thread closed temporarily for Moderation...

EDIT -- Thread reopened.

 

[Demystifier]

Bohmian mechanics is not hard to understand - except in places where they sweep things under he rug of details that their masters think don't need to be spelt out.

This refers to how the macroscopic apparatus works according to BM. This indeed is hard to understand rigorously, even for Bohmians, because it involves $10^{23}$ particles, which is why some details are swept under the rug. But Bohmians at least understand it qualitatively, while many others don't understand it even qualitatively.

The point is, the problem is not why the sky is blue, or even QFT. The problem is to understand how the macroscopic apparatus works, and that's a problem existing even in non-relativistic QM. If Bohmians cannot explain convincingly how that works for non-relativistic QM, it's illusory that they could explain how that works for QFT. But the irony is that it is not what people object. Instead, they say something like: OK, you explained how BM works for non-relativistic QM; now tell me how that works for QFT. It doesn't make sense to explain them how it works for QFT if they haven't really understood how that works for QM.

 

[PeterDonis]

I can point to a paper explaining how, in principle, Bohmian mechanics is extended to any interacting QFT

Didn't you post a link to this paper in a PF thread some time ago? Can you give a pointer to the thread? (Or if not, to the paper?)

 

[PeterDonis]

Maybe they would take interest, but they would not understand it.

This is not a valid response. Basically @A. Neumaier is asking you for a reference to back up your claims. You've been here long enough to know the PF rules for that. Providing a link to previous PF threads where the topic has already been discussed is fine. But refusing to provide anything because you claim nobody will understand it is not.

 

[mitchell porter]

The real joke here is that Wallace's favorite (Everettian many worlds) can't explain interacting QFT either. (Believers are welcome to provide a reference where they think this *has* been achieved.)

Or maybe the real joke is that, although everyone disagrees severely about what has and has not been shown, regarding "interpretation", we all meta-agree in cynically expecting disappointment from any attempt to discuss it rationally. :-)

 

[Demystifier]

This is not a valid response. Basically @A. Neumaier is asking you for a reference to back up your claims. You've been here long enough to know the PF rules for that. Providing a link to previous PF threads where the topic has already been discussed is fine. But refusing to provide anything because you claim nobody will understand it is not.

I'm not refusing to give a reference. I'm explaining why such a reference doesn't exist. He knows very well what has and what has not been done in existing references, so he asks me to give the kind of reference he knows that doesn't exist. In every branch of physics there are things that haven't been solved rigorously, he knows what hasn't been solved rigorously in this branch for physics, so he asks me to give the reference where this is solved rigorously only because he knows that it doesn't exist. I've been here long enough not only to know the PF rules, but also to know his tactic tricks for (mis)using this rules in his favor. His goal in this case is not to learn something from the reference (because he knows it doesn't exist), his goal is to win the argument by using those tactic tricks.

 

[PeterDonis]

I'm not refusing to give a reference. I'm explaining why such a reference doesn't exist.

You said in post #2 that you had published a paper on Bohmian QFT. As I said in my previous posts, I'm pretty sure I recall this paper being discussed in past PF threads. If you have a link to either the paper or one of those threads, you should post it here since it's relevant to this thread's discussion. If that is the only relevant reference you have, that's fine, but it still exists and is relevant and it needs to be posted here.

His goal in this case is not to learn something from the reference (because he knows it doesn't exist), his goal is to win the argument by using those tactic tricks.

I don't care what you think someone else's goal is, I care that whatever reference does exist and is relevant to this thread is given here.

 

[PeterDonis]

I've sent an e-mail to David Wallace pointing to my recent paper on Bohmian QFT (in which, by the way, I cite two other Wallace's papers). His reply started with "If I understood your paper correctly, ..." and then wrote something which had absolutely nothing to do with my paper.

Are you able to post the relevant content of these emails here? If you have to ask Wallace for permission, that's fine. But without seeing the actual content of the emails, no one else here has any way of independently evaluating what you're claiming.

 

[Demystifier]

Didn't you post a link to this paper in a PF thread some time ago? Can you give a pointer to the thread? (Or if not, to the paper?)

Sure, in the meantime it has been accepted for publication. https://arxiv.org/abs/2205.05986

 

[PeterDonis]

in the meantime it has been accepted for publication

Glad to hear it!

 

[Demystifier]

The real joke here is that Wallace's favorite (Everettian many worlds) can't explain interacting QFT either.

Good point! An additional joke is that even standard QED cannot explain why the sky is blue with a rigor @A. Neumaier asks of Bohmian QED. Even standard QED uses a projection postulate (or some replacement for it) associated with measurement, and this cannot be explained rigorously because measurement involves a complex interaction with a macroscopic apparatus containing a huge number of degrees of freedom. But A. Neumaier puts this problem with standard QED under the rug, while at the same time he's strongly worried about the analogous problem with Bohmian QED.

 

[Demystifier]

You severely underrate their capabilities. It is not the case that quantum physicists are dummies, except for the few who match your understanding.

I'm not saying that they are dummies. Many of them are smarter than me. The main problem, I believe, are their prejudices acquired by over-training in the standard interpretation. I identified many of those prejudices in the paper I linked above.

 

[Demystifier]

Bohmian mechanics is not hard to understand - except in places where they sweep things under the rug of details that their masters think don't need to be spelt out.

If Bohmian mechanics sweeps things under the rug, then standard QM hides the same things under the tiles covered by the rug.

Let me explain the metaphor. There is no doubt that measuring apparatus is an important part of understanding how measurement outcomes appear and why they obey the Born rule. But realistic measuring apparatus is too complex to describe it precisely and rigorously. Bohmian mechanics (as does the many world interpretation, decoherence theory and von Neumann theory of measurement) nevertheless gives its best to describe it at least approximately and qualitatively, by writing some explicit equations that describe it, so that the non-rigorous nature of this analysis is not difficult to spot (it's swept under the rug, where it can be found easily). On the other hand, the standard textbook QM avoids explicit talk about the measuring apparatus as much as it can, without ever writing any equation that refers to the quantum state of the apparatus, thus creating a very convincing illusion that everything is rigorous and hides the problem so well that it is very difficult to spot (hides it under the tiles covered by the rug).

But the greatest irony is that this same standard textbook QM says than only measurable quantities make sense. It teaches us to talk only about the measurable, but not to talk about what happens in the measuring apparatus.

 

[Fra]

To continue the metaphors, what i find objectionable with decoherence is that they literally swep things further and further out to infinifty. With such strategy it should be no surprise that the only observables we get are scattering matrixes. This does not solve the measurement problem, it just pushes it out of reach where some seems think it is solved.

/Fredrik

 

[gentzen]

David Wallace, The sky is blue, and other reasons quantum mechanics is not underdetermined by evidence, Manuscript (2022). arXiv:2205.00568.

From the Abstract:
''I argue that there as yet no empirically successful generalization of''
[Bohmian Mechanics and dynamical-collapse theories like the GRW or CSL theories]
''to interacting quantum field theory [...] ... that explains the color of the sky.''

But then the verbal expression ''quantum state of its particles'' in the article on the Many Worlds Interpretation is meaningless. (I am not surprised, since I find most of what those explaining MWI write to be meaningless. But here it is particularly obvious.)

So you "find most of what those explaining MWI write to be meaningless". Still you defended that paper which claims that MWI works for interacting quantum field theories, while Bohmian Mechanics does not.

I have no problem with the claim that Bohmian Mechanics does not work for that "asymptotic states only," renormalized, perturbative interacting QFT as currently used, which can explain the color of the sky. But honestly, I don't understand the claim that MWI would work for that QFT. I have never seen any math trying to back-up that claim. In fact, I have not even seen vague philosophical words trying to defend that claim.

Did you? Do you have any opinion on that question at all? Did you just enjoy that Bohmian Mechanics got bashed?

 

[A. Neumaier]

So you "find most of what those explaining MWI write to be meaningless". Still you defended that paper which claims that MWI works for interacting quantum field theories, while Bohmian Mechanics does not.

No. I criticised Bohmian mechanics for being promoted as a theory of QFT (which it isn't - it is only a superfical theory of lattice approximations or free fields). But I did not defend MWI.

Did you just enjoy that Bohmian Mechanics got bashed?

No. It is very unfortunate that Bohmian mechanics needs to be bashed because, based on bla, bla, bla only, it makes repeated unfounded claims about what it is capable to achieve. I'd enjoy much more if their proponents would apply the amount of modesty typical for good science.