Title:

The emergent multiverse : quantum theory according to the Everett interpretation

This thesis attempts a full defence of the Everett ("manyworlds") interpreta tion of quantum mechanics. Its purpose is to show that unitary quantum mechanics, interpreted just as we interpret other physical theories  that is, literally and re alistically  is a perfectly coherent physical theory and tells us that the observed universe is one in an indefinitely large number of quasiclassical universes. These universes are not part of the fundamental formalism of quantum mechanics, but are emergent from it in the usual way in which macroontology emerges from underlying microphysics  hence, "emergent multi verse". The thesis is divided into two parts. In the first part I expound and motivate the Everett interpretation, explain the gen eral framework for thinking about emergence which I use, and show qualitatively how this framework, applied to unitary quantum mechanics, leads to the existence of 'many worlds'. I then consider the technical aspects of this process in detail, and show exactly what part environmentinduced decoherence has to play. In the second part, I address the problem of probability. I first argue that essentially all extant strategies for understanding probability make as much (or as little!) sense in (Everettian) quantum mechanics as they do in nonbranching physics, so that probability is not a problem specific to the Everett interpretation. I then develop a decisiontheoretic framework for thinking about probability in quantum mechanics, within which it is possible to prove a very general representation theorem, which tells us that rational agents should treat quantummechanical weights exactly as objective probabilities, and connect that framework to previous work by myself and others. I conclude that unitary quantum mechanics is fully satisfactory as a physical theory  just as satisfactory, in fact, as classical theories like Newtonian particle mechanics or general relativity. The Everett interpretation, therefore, provides a dissolution of the problem of measurement. 1 This is the second DPhil thesis I have submitted to Oxford University. My previous thesis, submitted in fulfilment of the requirements of the degree of DPhil in Atomic and Laser Physics, was examined and passed in 2002. I submit this thesis in fulfilment of the requirements of the degree of DPhil in Philosophy.
