[n4r9]

So... quantum interpretations can broadly be categorised thus.

1) Psi-ontic: the wave function is a real objective property of a system. This splits further depending on what is said to happen during collapse:

1a) Collapse is real (I don't know if this interpretation has a name but I think a lot of practising physicists think in these terms): this leads to the measurement and Wigner's friends problems I alluded to above

1b) Collapse is apparent (many worlds, de broglie-bohm): this is somewhat more satisfactory but usually raises other issues (e.g. the emergence of the Born rule).

2) Psi-epistemic: the wave function is a representation of some subjective state regarding a system and an agent. This splits further depending on how subjective you're prepared to go.

2a) Weakly psi-epistemic: the wave function represents an agent's state of information/ignorance regarding some true underlying objective ontic state of the system. This type of interpretation is (more or less) demolished by PBR.

2b) Strongly psi-epistemic (QBism): denies the existence of an underlying objective ontic state of a system. The wave function merely represents an agent's beliefs regarding the outcome of future interactions with the system.

I agree QBism is pretty controversial and that it lacks a certain satisfying explanatory mechanism. However I don't think you can deny that it is more than simply "standard QM", or that there are some good reasons for preferring it.

[kgwgk]

QBism doesn't seem "more" than standard QM to me, it seems "less" because it's just standard QM without pretending that the quantum state describes the physical state. There may be some reasons to throw the towel on realism and just shut up and calculate, but it's not like that's a new idea.

The operations done to establish from the available information what is the quantum state of a system and to calculate predictions are exactly the same in QBism and standard QM. All the positive-operator-valued-measures stuff works just the same, as far as I can tell, when quantum states are considered representations of reality. If our knowledge is sharp we have a wave function which describes the physical state, otherwise we have a density matrix and we ignore the precise state but we can make predictions about physics. In QBism we have the same predictions (and the same wave function / density matrix but devoid of meaning).

I don't completely understand the split 2a/2b and how 2b is more tenable. What does "the outcome of future interactions with the system" mean if there is no "true underlying objective ontic state of the system"? I understand that according to QBism quantum states do not represent reality. But does physical reality exist at all or not? Is there a "physical state of the system", even though it cannot be described using QM? If there is no state of the system, what does "system" mean? How is the "outcome of the interactions with the system" determined?

[n4r9]

This thread is getting a lot longer and more involved than I anticipated. Apologies but for the sake of my own free time and sanity I may have to slow down and make more use of citations. The main point I was trying to get across was that QBism does say these things about quantum theory, regardless of whether or not you find it tenable.

As I say the mathematics is invariant across interpretations, therefore the Born rule and POVMs have a use to an Everettian just as they do a QBist. The difference is what they think of the physical meaning of such a thing. [0][1]

Interpreting quantum theory with a radical, personalist Bayesian perspective of the quantum state is novel, although it has historical precedents starting with Bohr and continuing with Jaynes, Wheeler. [2]

QBists broadly speaking tend to assume a pragmatist view of realism, at least in relation to quantum theory. Fuchs describes it as "participatory realism". There is a physical reality but we are ourselves entwined with it and cannot assume that the models we construct are objective and observer-independent. I think Fuchs has argued that the measurement problem, Wigner's friend, Bell's theorem and Kochen-Specker all point us irrevocably towards a radical Bayesian perspective of the quantum state. The task remaining is to disentangle the subjective from the objective:

> The professed goal is to strip away all those elements of quantum theory that can be interpreted in subjective, agent-dependent terms. The hope is that whatever remains will hint at something essential and objective about nature. [3]

[0] https://arxiv.org/pdf/quant-ph/0104088.pdf

[1] https://arxiv.org/pdf/quant-ph/0205039.pdf

[2] https://arxiv.org/pdf/1705.03483.pdf

[3] https://arxiv.org/pdf/1405.2390.pdf

[kgwgk]

I don't deny that QBism says things, but they don't seem so interesting to me. At least not as much as I expected when I approached the subject some time ago. I'm a fan of Jaynes and his work on information theory and statistical mechanics and I very much like the Bayesian angle here but it can be applied exactly the same (even better, I'd say) in a "realist" setting.

I think the problems in QM may not be problems after all if we could understand the physics better. Standard QM also consists in doing "as if" the quantum state represented physical reality: we (should) know that QM cannot be right and it's just an approximation to something else (QFT or whatever unification with GR) and even the way we apply QM cannot be right because it's also full of approximations (isolated systems do not exist, etc.). The "problems" that QBism "solves" may be artifacts due to those approximations and not fundamental problems. I find that all the (highly-speculative) physical theories trying to explain why things are "as if" QM was true are more promising than the metaphysical proposals of QBism or MWI (and as much as the MWI is metaphysical at least it seems better defined!).

I lack the time and patience to read the thousands of pages that Fuchs has written on the subject and I don't expect you to explain them to me either. So long, and thanks for all the fish^H^H^H^H discussion.