| 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 |
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.