‘Interpretations’ of quantum mechanics

I’m going to alternate properly new posts with short extracts from my thesis. So here’s the first such extract, on why Everettian quantum mechanics is not just one ‘interpretation’ among many:

Often EQM is presented as an interpretation of quantum mechanics, for purposes of comparison with other ‘interpretations’; examples usually given are pilot-wave theory, the ‘Copenhagen interpretation’, and dynamical collapse theories such as that of Ghirardi, Rimini and Weber (Ghirardi, Rimini, and Weber 1986). I think that that this way of conceiving the foundational issues, where the equations of quantum mechanics are common to the different approaches and they are distinguished only by a quasi-metaphysical layer placed on top of the equations, is badly misleading.

The main reason for this is that that most ‘interpretations’ impose extra dynamical structure of their own onto the basic skeleton of the quantum mechanical formalism. For example, in addition to the unitary evolution of the quantum state, (non-relativistic) pilot-wave theory postulates point-like particles with definite trajectories, and a ‘guidance equation’ which tells the particles how to move based on the structure of the state.  As such, it is strictly speaking not an interpretation of quantum mechanics at all; it is an autonomous theory which happens to share much of the theoretical structure of quantum mechanics. (In recognition of this point, the name ‘Bohmian mechanics’ is often adopted by enthusiasts of pilot-wave theory.)

The Copenhagen interpretation, as preached by Bohr (Bohr 1934), (Bohr 1958), (Bohr 1963) is different from these explicit modifications of quantum mechanics; it takes the equations of quantum mechanics as they stand but accounts for their link with our experience of the macroscopic world in an non-standard way. In this respect it has much in common with EQM. The difference is that EQM is naturally thought of as a realist theory; it interprets the quantum state as a description of the physical properties of a system. The Copenhagen interpretation (at least as it is ordinarily understood[1]) is an instrumentalist theory; it tells us how to use quantum mechanics to predict behaviour in the macroscopic world (antecedently understood in terms of the theories and concepts of classical physics), but rejects any of talk of ‘correspondence’ between the quantum-mechanical description and any macroscopic reality. It is thus natural to group EQM and the Copenhagen interpretation together, treating the former as a form of scientific realism about quantum mechanics and the latter as a form of anti-realism[2] about quantum mechanics; we can then contrast them both with those theories which explicitly modify quantum mechanics.

The reasons for preferring EQM to Copenhagen are essentially instances of our more general reasons for preferring scientific realism to instrumentalism. EQM provides us with a picture of fundamental reality, albeit a strange one, while the Copenhagen interpretation rejects any such demands. According to the Copenhagen interpretation, questions about the fundamental nature of microscopic reality without reference to experimental context are simply misguided – there can be no informative answer to such questions.

EQM and the Copenhagen interpretation do not just differ in the metaphysical picture they give us. EQM can form of a coherent and unified theory of nature in a way in which the Copenhagen interpretation cannot. EQM is a theory of closed systems, and thus can be applied to the entire universe, unlike the Copenhagen interpretation which restricts quantum mechanics to describing the behaviour of quantum systems embedded in classical environments. This is reflected in the way that quantum cosmology invariably proceeds, implicitly or explicitly, with EQM as a background assumption.

[1] Bohr himself can be interpreted rather differently, as a realist who took classical mechanics to be fundamental. See (Saunders 2005).

[2] By ‘anti-realism’ here I mean something stronger than van Fraassen’s constructive empiricism (Van Fraassen 1980), which takes physical theories about unobservable entities literally but advises a restrained epistemic stance towards them. The kind of anti-realism about the microscopic exemplified by the Copenhagen interpretation is semantic rather than epistemic in nature.

‘Interpretations’ of quantum mechanics

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