Abstract
This chapter comments on that by Chris Fuchs on qBism. It presents some mild criticisms of this view, some based on the EPR and Wigner’s friend scenarios, and some based on the quantum theory of measurement. A few alternative suggestions for implementing a subjectivist interpretation of probability in quantum mechanics conclude the chapter.
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Notes
- 1.
Mr Braque is a very bold young man. […] He despises form, reduces everything, places and figures and houses, to geometric schemes, to cubes. Let us not rail him, since he is in good faith. And let us wait. (My translation from Vauxcelles 1908)
- 2.
With the addition of Sect. 3.1, the material in this note was presented at the Bertinoro conference. My thanks go to Maria Carla Galavotti, Raffaella Campaner, Beatrice Collina and all the colleagues in the ESF network who organised the conference. But most of all I wish to thank Chris Fuchs for the pleasure of endless discussions of quantum Bayesianism over the years, and for removing the prejudice in my mind that a subjective interpretation of quantum probabilities must be a non-starter.
- 3.
Such a view seems to be suggested for instance by Greaves and Myrvold (2010).
- 4.
When Schrödinger introduced his wave functions, he clearly understood them as representing physical states of matter. But also Heisenberg and other advocates of matrix mechanics had a notion of stationary state, both preceding and distinct from Schrödinger’s wave functions. Cf. Bacciagaluppi and Valentini (2009, ch. 3) and Bacciagaluppi (2008).
- 5.
Disregarding notions of extended cognition – which are presumably beside the point here.
- 6.
The situation is quite analogous to that of collapse on the forward light cone. If technically feasible, a theory in which the collapse of the quantum state takes place along the forward light cone of a triggering event would be manifestly Lorentz-invariant. However, in the case of an EPR pair, it would leave unexplained how space-like separated collapses are correlated so as to match up on the overlap of their future light cones. Cf. e.g. the discussion in Bacciagaluppi (2010).
- 7.
See below for the definition of an effect.
- 8.
I cannot resist teasing Chris here by pointing out that pace the remarks in Fuchs (2010, pp. 24–25), the results of decoherence just scream out for an ontic interpretation of the quantum state (so much classical-like structure within the quantum state that could be used to explain the classical world – if only we could avail ourselves to an ontic interpretation of the state). Cf. Bacciagaluppi (2012, esp. Sect. 3.5). The quantum-to-classical transition is here to stay, and qBism ought to incorporate it.
- 9.
- 10.
- 11.
For an accessible reference, see Ghirardi (2011).
- 12.
- 13.
There is a debate about the most natural interpretation of spontaneous collapse theories: whether – in increasing order of resilience against the so-called “tails” problem – it is in terms of wave functions, matter density, or collapse events (so-called “flashes” or “hits”). For details see e.g. Ghirardi (2000), Allori et al. (2008), and Bacciagaluppi (2010).
- 14.
Cf. also my comments in Bacciagaluppi (2013), which reviews the state-of-the-art volume on the Everett theory edited by Saunders et al. (2010). Everett’s complete writings on quantum mechanics, together with a wealth of other original material, have been published and annotated by Barrett and Byrne (2012).
- 15.
Note that, as clearly shown by Vaidman (1996), we do have ignorance of the result of a measurement at least after the measurement has occurred and we are not yet aware of the result. After the branching induced by a measurement there is a genuine question about self-location.
- 16.
For a lively and representative sample of the literature, see the relevant contributions by Albert, Greaves and Myrvold, Kent, Price, Saunders, and Wallace, as well as the transcripts of the discussions, in Saunders et al. (2010). In particular, Price (2010) argues that agents may have global reasons on which to base their decisions, i.e. reasons other than the utilities of their successors. Such arguments of course undermine the idea that there should be compelling rational arguments for adopting the usual quantum probabilities in Everett.
- 17.
In further work, in particular with my graduate students, I hope to elaborate both on the analogy between Everett’s view of probability in his own theory and in classical statistical mechanics – in particular on how it allows one to make statistical inferences in an Everettian universe –, and on the pragmatist reading of typicality, both in Everett and in classical statistical mechanics.
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Bacciagaluppi, G. (2014). A Critic Looks at QBism. In: Galavotti, M., Dieks, D., Gonzalez, W., Hartmann, S., Uebel, T., Weber, M. (eds) New Directions in the Philosophy of Science. The Philosophy of Science in a European Perspective, vol 5. Springer, Cham. https://doi.org/10.1007/978-3-319-04382-1_27
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