Probability and branching in Everettian Quantum Physics

Abstract

Everettians often claim that their approach is nothing more than quantum mechanics with unitary time evolution. This thesis focuses on Everettian Quantum Mechanics (EQM) over other interpretations because it can apply more broadly to the whole of quantum physics. However, EQM is only a viable interpretation if it can explain probability as well as branching. The probability problem is one of the biggest obstacles to EQM and the popular solution via decision theory faces significant problems. Here, other solutions are analysed and found to recover probability but at the price of additional postulates to EQM. Looking beyond decision theory, these different approaches also help resolve problems relating to decoherence and probability. The importance of probability in quantum physics requires Everettians to admit new principles if they can be physically motivated. This does have the consequence of making EQM more than just quantum mechanics with unitary dynamics but allows it to be consistent.

Questions of branching and a preferred basis require a more rigorous approach using quantum field theory (QFT). An Everettian Quantum Field Theory (EQFT) requires the branching formalism and decoherence to carry across to QFT. While the framework is available, the interpretational jump to multiple branches can only be grounded in metaphysical claims beyond standard quantum theory. Analysis of possible ontologies from which to build the emergent structure of branches raises many questions for Everettians with no clear solution. Overall, EQFT is an option available for scientific realists, but it is wrong to claim that it just takes the physics as it is.

Date of Award	3 Oct 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	James A C Ladyman (Supervisor) & Richard G Pettigrew (Supervisor)