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indeed I have no problem with fuzzy assumptions, if you are honest about them. I would also clearly not object to Bohmians using arguments from QM. It is a different thing though, when the claim is that from “exact” BM you can derive QM, when that derivation is full of decoherence assumptions about the measuring devices. You could say that the claim of BM=>QM, and with it the claim that BM has non-zero empirical content, logically requires a solution of the FAPP measurement problem. You guys consider that a triviality (Bell coined the FAPP phrase to belittle all practical purposes), to me it is a pretty tall order. But without spelling these things out (at least a bit better) you can hardly speak of a “derivation”.
Maybe I should remind you that operational QM does not have a measurement problem. The theory works just fine without setting up wave functions for the measuring apparatus (Nobody gets anywhere with that anyhow). If you ever came to analyze a concrete experiment you would be well-advised to the same, even as a Bohmian. In fact, you wouldn’t do your job right if you didn’t work from a macroscopic description of the devices. You would need to show that the particular choice of wave functions for the devices is irrelevant (like the device trajectories), because that stability is part of the definition of an experiment. So to answer your question: I do not assume a wave function for the apparatus. It may have one (whatever that means), and at some meta-level describing the apparatus in many body quantum terms (i.e., by statistical mechanics) may be an interesting problem. But that is not part of quantum mechanics as I know it.