What would a consistent instrumentalism about quantum mechanics be? Or, why Wigner's friendly after all.

          @misc{ pirsa_PIRSA:09090029,
            doi = {10.48660/09090029},
            url = {https://pirsa.org/09090029},
            author = {Timpson, Christopher},
            keywords = {Quantum Foundations},
            language = {en},
            title = {What would a consistent instrumentalism about quantum mechanics be? Or, why Wigner{\textquoteright}s friendly after all.},
            publisher = {Perimeter Institute},
            year = {2009},
            month = {sep},
            note = {PIRSA:09090029 see, \url{https://pirsa.org}}
          }
          

Abstract

Instrumentalism about the quantum state is the view that this mathematical object does not serve to represent a component of (non-directly observable) reality, but is rather a device solely for making predictions about the results of experiments. One honest way to be such an instrumentalist is a) to take an ensemble view (= frequentism about quantum probabilities), whereby the state represents predictions for measurement results on ensembles of systems, but not individual systems and b) to assign some specific level for the quantum/classical cut. But what happens if one drops (b), or (a), or both, as some have been inclined to? Can one achieve a consistent view then? A major worry is illustrated by the Wigner's friend scenario: it looks as if it should make a measurable difference where one puts the cut, so how can it be consistent to slide it around (as, e.g., Bohr was wont to)? I'll discuss two main cases: that of Asher Peres' book, which adopts (a) but drops (b); and that of the quantum Bayesians Caves, Fuchs and Shack, which drops both. A view of Peres' sort can I, think, be made consistent, though may look a little strained; the quantum Bayesians' can too, though there are some subtleties (which I shall discuss) about how one should handle Wigner's friend.