Abstract
Gao (Synthese, 2017. https://doi.org/10.1007/s11229-017-1476-y) presents a new mentalistic reformulation of the well-known measurement problem affecting the standard formulation of quantum mechanics. According to this author, it is essentially a determinate-experience problem, namely a problem about the compatibility between the linearity of the Schrödinger’s equation, the fundamental law of quantum theory, and definite experiences perceived by conscious observers. In this essay I aim to clarify (i) that the well-known measurement problem is a mathematical consequence of quantum theory’s formalism, and that (ii) its mentalistic variant does not grasp the relevant causes which are responsible for this puzzling issue. The first part of this paper will be concluded claiming that the “physical” formulation of the measurement problem cannot be reduced to its mentalistic version. In the second part of this work it will be shown that, contrary to the case of quantum mechanics, Bohmian mechanics and GRW theories provide clear explanations of the physical processes responsible for the definite localization of macroscopic objects and, consequently, for well-defined perceptions of measurement outcomes by conscious observers. More precisely, the macro-objectification of states of experimental devices is obtained exclusively in virtue of their clear ontologies and dynamical laws without any intervention of human observers. Hence, it will be argued that in these theoretical frameworks the measurement problem and the determinate-experience problem are logically distinct issues.
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Notes
I assume that the reader is familiar with the content of Gao’s and Maudlin’s papers.
In this paper I will concentrate only on Bohmian mechanics and GRW theories since they have a clear primitive ontology of matter, contrary to MWI.
Here \(|\alpha |^2,|\beta |^2\), with \(\alpha ,\beta \in {\mathbb {C}}\), represent the probabilities to find the system in \(|\psi _1\rangle ,|\psi _2\rangle \) respectively. The normalization \(|\alpha |^2+|\beta |^2=1\) means that with certainty we will find the system in one of the possible eigenstates.
For the mathematical justification of this statement the reader should refer to Dürr et al. (2013), Chapter 2, Sects. 4–7. In this regard, it is also worth noting that BM restores also a classical interpretation of quantum probabilities. These are manifestation of our ignorance about the exact positions of the Bohmian particles and our inability to manipulate them. Thus, the maximum knowledge of particles’ configurations at our disposal in BM is provided by \(|\psi |^2\). For the specific treatment of the classical interpretation of probabilities in BM the reader should consult Bohm (1952).
In this version of GRW theory the term ‘particle’ has not to be interpreted literally, since this theory is only about wave functions. We use a particle language only to remain stuck to the usual jargon. Bassi and Ghirardi made this point precise claiming that: “when we speak of “particles” we are simply using the standard, somehow inappropriate, quantum mechanical language. Within dynamical reduction models particles are not point-like objects which move in space following appropriate trajectories according to the forces they are subjected to (as it is the case of, e.g., Bohmian mechanics). In dynamical reduction models, like in standard quantum mechanics, particles are represented just by the wave function which, in general, is spread all over the space. As we will see, the basic property of the models analyzed here is that, when a large number of “particles” interact with each other in appropriate ways [e.g. according to the GRW algorithm], they end up being always extremely well localized in space, leading in this way to a situation which is perfectly adequate for characterizing what we call a “macroscopic object”. Thus, strictly speaking there are no particles in dynamical reduction models at the fundamental level; there is simply a microscopic, quantum, wave-like realm which gives rise to the usual classical realm at the macroscopic level” [Bassi and Ghirardi (2003, p. 299)].
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Acknowledgements
I would like to thank Olga Sarno, Michael Esfeld and Davide Romano for helpful comments on this paper. I am grateful to the Swiss National Science Foundation for financial support (Grant No. 105212-175971).
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Oldofredi, A. Some remarks on the mentalistic reformulation of the measurement problem: a reply to S. Gao. Synthese 198, 1217–1233 (2021). https://doi.org/10.1007/s11229-019-02101-3
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DOI: https://doi.org/10.1007/s11229-019-02101-3