Other Topics in the Everett Interpretation

Deutsch's "Quantum Theory as a Universal Physical Theory" (2023)

International Journal of Theoretical Physics 62 (2023), 215.

This is an invited essay, forming part of a series discussing influential papers published by International Journal of Theoretical Physics. In this article, I consider the content, context, and later significance of David Deutsch’s 1985 defense of the Everett interpretation, ‘Quantum Theory as a Universal Physical Theory’.

The Everett Interpretation (2011)

In R. Batterman (ed.), The Oxford Handbook of Philosophy of Physics (OUP, 2013)

The Everett interpretation of quantum mechanics - better known as the Many- Worlds Theory - has had a rather uneven reception. Mainstream philosophers have scarcely heard of it, save as science fiction. In philosophy of physics it is well known but has historically been fairly widely rejected. Among physicists (at least, among those concerned with the interpretation of quantum mechanics in the first place), it is taken very seriously indeed, arguably tied for first place in popularity with more traditional operationalist views of quantum mechanics. In this article, I provide a fairly short (15,000 words) and self-contained introduction to the Everett interpretation as it is currently understood. I use little technical machinery, although I do assume the reader has encountered the measurement problem already (at about the level of the well-known discussions by Penrose or Albert).

A prolegomenon to the ontology of the Everett interpretation (2011)

In Alyssa Ney and David Z. Albert (eds.), The Wave Function: Essays in the Metaphysics of Quantum Mechanics (OUP, 2013)

In this article, I briefly explain the quantum measurement problem and the Everett (Many-Worlds) interpretation, in a way that is faithful to modern physics and yet accessible to readers without any physics training. I then consider the metaphysical lessons for ontology from quantum mechanics under the Everett interpretation. My conclusions are largely negative: I argue that very little can be said in full generality about the ontology of quantum mechanics, because quantum mechanics, like abstract classical mechanics, is a framework within which we can consider different physical theories which have very little in common at the level of ontology. Along the way I discuss, and criticise, several positive ontological proposals that have been made in the context of the Everett interpretation: ontologies based on the so-called "eigenstate-eigenvalue link", ontologies based on taking the "many-worlds" language seriously at the fundamental level, and ontologies that treat the wavefunction as a complex field on a high-dimensional space.

Decoherence and Ontology: or, How I Learned to Stop Worrying and Love FAPP (2009)

In Simon Saunders, Jon Barrett, Adrian Kent, and David Wallace (eds.), Many Worlds? Everett, Quantum Theory, and Reality (OUP, 2010)

I make the case that the Universe according to unitary (no-collapse) quantum theory has a branching structure, and so can literally be regarded as a "many-worlds" theory. These worlds are not part of the fundamental ontology of quantum theory - instead, they are to be understood as structures, or patterns, emergent from the underlying theory, through the dynamical process of decoherence. That they are structures in this sense does not mean that they are in any way unreal: indeed, pretty much all higher-level ontology in science, from tables to phonons to tigers, is likewise emergent. Unitary quantum theory is therefore a "many-worlds" theory without any modification of the mathematical structure of the theory: the Everett interpretation does not consist in adding worlds to the formalism, but in realising that they are there already. Our grounds for accepting the reality of those worlds is no more, but no less, than our grounds for accepting any other not-directly observable consequence of an empirically very successful theory.

This paper is effectively an updated version of "Everett and Structure" (below), albeit aimed slightly more towards a physics audience.

Everett and Structure (2001)

Studies in the History and Philosophy of Modern Physics 34 (2003), pp. 86-105.

I address the problem of indefiniteness in quantum mechanics: the problem that the theory, without changes to its formalism, seems to predict that macroscopic quantities have no definite values. The Everett interpretation is often criticised along these lines and I shall argue that much of this criticism rests on a false dichotomy: that the macroworld must either be written directly into the formalism or be regarded as somehow illusory. By means of analogy with other areas of physics, I develop the view that the macroworld is instead to be understood in terms of certain structures and patterns which emerge from quantum theory (given appropriate dynamics, in particular decoherence). I extend this view to the observer, and in doing so make contact with functionalist theories of mind.

Worlds in the Everett Interpretation (2001)

Studies in the History and Philosophy of Modern Physics 33 (2002), pp. 637-661.

This is a discussion of how we can understand the world-view given to us by the Everett interpretation of quantum mechanics, and in particular the role played by the concept of `world'. The view presented is that we are entitled to use `many-worlds' terminology even if the theory does not specify the worlds in the formalism; this is defended by means of an extensive analogy with the concept of an `instant' or moment of time in relativity, with the lack of a preferred foliation of spacetime being compared with the lack of a preferred basis in quantum theory. Implications for identity of worlds over time, and for relativistic quantum mechanics, are discussed.