I A very interesting paper on orthodox quantum mechanics

[Peter Morgan]

In his textbook, he gives basically the same description you quoted, but he also says that an equivalent way of looking at the quantum state is as representing an ensemble of preparations all done according to the same preparation process. Since the preparation process is describable in ordinary classical terms, it seems like a better place to anchor the meaning of the quantum state than in a "quantum system" that we can't observe directly anyway.

Thank you. I didn't have a copy of Ballentine available yesterday. I associate a focus on preparation as distinct from measurement with Margenau, but there is surely room for Ballentine's take on that approach in general to be different in details.
I much like his postulate 1 on page 43,

That wording delicately avoids introducing the idea of a 'system' as orthodox interpretations usually do, but leaves space for people reading it from an orthodox perspective to think there's an implied idea of an ensemble of systems. Unfortunately, from my perspective, Ballentine then has a lengthy discussion that goes back and forth between this more abstract idea —which obviously I prefer— and a much more concrete idea that he presents on page 47, "The quantum state description may be taken to refer to an ensemble of similarly prepared systems", hewing close to his position in RevModPhys 1970, immediately followed by a discussion of how to back away from that. It's all masterly at the same time as being stuck firmly in QM instead of moving towards QFT, which I think we must. It's a textbook, so I shouldn't expect what I'm asking for almost 30 years later, but I suppose infiltrating small qualifications into the work of past masters is part of the nature of things.

On page 49, Ballentine introduces a modified Postulate 1a, "To each dynamical variable there is a Hermitian operator whose eigenvalues are the possible values of the dynamical variable", which is almost right for me, except that In my version I would replace "dynamical observable", giving
Postulate 1b: To each dataset there is a Hermitian operator whose eigenvalues include all the entries that occur in the dataset.
One point of this kind of formulation is to make no assertion that we are discussing particles or fields. It is too operational for most people —and also for me— so that the task from this point on is to rebuild the world we experience from this much too abstract idea of what the interplay between theory and experiment is about: the comparison of expected statistics, predicted by a theory, with the statistics we can compute for actually recorded datasets, together with the decision problem that we face when there is inevitably a mismatch between actual and expected. The decision problem is subtle insofar as statistical significance is subtle: a single new data entry will only very rarely change our future predictions and even large differences of expected and actual statistics for large datasets may for decades be thought by orthodoxy to be not statistically significant enough or otherwise flawed, often in hard to pin down ways and subject to personal choice.
I keep trying to pull my comments here back to the question of orthodoxy in the original post, but I have to apologize for my thinking being on a different continent from much of the current orthodoxy. I could go on and on about what I love about Ballentine and where I think we can helpfully adjust his take, but I will stop now.

 

[Lord Jestocost]

The quote from the conclusion. This one

Reads like the typical pop science QM exposition of the wave particle duality.

Regarding the quote from the conclusion of G. Beck's paper "How to be an orthodox quantum mechanic" (https://arxiv.org/abs/2504.20597).

One must carefully distinguish between what happens mathematically in the quantum mechanical formalism and what one is allowed to interpret physically. Such 'overinterpretations' as in the quote continue to plague the foundations of quantum physics to these days.

 

[pines-demon]

See:

Thanks, I prefer reading so I found

• S. Weinberg, "The Trouble of Quantum Mechanics" (2017), The New York Review

It might be found easily on the web. It is basically an edited transcript of the lecture.

Basically Weinberg says that he is not convinced by many-worlds interpretation or the instrumentalist approach, and then he rambles on the promise of generalized Linbladians. If I ever have more time, I will try to listen to the talk to see if he gives more insight into that but I am not very hopeful.

 

[gentzen]

Basically Weinberg says that he is not convinced by many-worlds interpretation or the instrumentalist approach, and then he rambles on the promise of generalized Linbladians. If I ever have more time, I will try to listen to the talk to see if he gives more insight into that but I am not very hopeful.

There is a Q&A at the end, starting at 49:19, which contains stuff not contained in that transcript.

Below, I collected how I tried to understand where Weinberg was coming from in his attack:
Sep 14, 2021 (Is Relational Quantum Mechanics the Key to Understanding Quantum Interactions?)

Steven Weinberg in "Lectures on Quantum Mechanics" in section "8.3 Broken Symmetry" seems to "suggest" that often even molecules won't be in strange superpositions of states (even if that superposition would constitute the minimal energy eigenstate), if some related timeinterval far exceeds the lifetime of the universe. Well, a molecule normally has no elaborate mechanics copying a superposition over to those states, so it is not directly comparable to Schrödinger's thought experiment. But the point is that you don't even need to go to a macroscopic cat to come into that conflict between "naive prediction" and actual observation.

Oct 20, 2022 (Nature Physics on quantum foundations)

... Steven Weinberg's attack on quantum interpretations and specifically instrumentalism appeared, which I found totally unacceptable back then. At some later point I read something which temporarily made me understand his attack (it had something to do with being able to explain such stuff in a textbook in a satisfying way), but I forgot it again in the meantime.

Aug 31, 2023 (QFT made Bohmian mechanics a non-starter: missed opportunities?)

I guess in the end I am still an instrumentalist, just like I was in Jan '17 when I reacted with huge anger against Steven Weinberg's "The Trouble With Quantum Mechanics" and his unfair characterization of instrumentalism.
...
After the death of Steven Weinberg, I learned more about his motivation(s) for his attack against instrumentalism, and that he even had the audacity to compute "relaxation times" for specific scenarios in his QM textbook, and suggest that them being much bigger than the age of the universe had consequences like ... well, I don't want to dive into that sort of discussion here.
Let me just say that he cared deeper about that stuff than I do.

 

[bhobba]

More generally, for any interpretation of QM there is a corresponding generalization to QFT.

True.

But we have the EFT view of QFT, which suggests we really do need to know more about regions beyond which we can currently probe to resolve 'issues' with QM:

https://en.wikipedia.org/wiki/Effective_field_theory

The standard model, including gravity as an EFT, is really a framework where we put in all the constants for which we have no explanation. Therein lies the central mystery.

Thanks
Bill