Abstract
From the beginnings of quantum theory in the early 20th Century, physicists and philosophers of physics have been struggling to interpret it. The ongoing debate about the measurement problem has lead to the creation of multiple different interpretations intended to solve it. Each interpretation presents quantum theory differently in an attempt to reconcile it with what we see in the world.The measurement problem, and alongside it the more general problem of the quantum to classical transition, are explored in the following chapters in a journey from the first interpretation of quantum mechanics to modern Quantum Field Theory. Chapter 1 examines the basic quantum formalism and the first quantum interpretations. In Chapter 2, a chronological review of influential quantum mechanical thought experiment highlights the central issue of measurement. Chapter 3 introduces the measurement problem and divides it in two; the unique outcomes problem and the preferred basis problem, and examines how each is addressed in different interpretations. Chapter 4 investigates the theory of decoherence. Chapter 5 reexamines the interpretations from Chapter 3 in the light of decoherence as a model for the emergence of classical macrostates from quantum microstates. Chapter 6 evaluates a possible analogy between using the idealisation of an infinite limit in the quantum to classical transition and in phase transitions in statistical mechanics. It argues that the analogy fails due to strange phenomena in the quantum to classical transition which are currently explained by semiclassical mechanics. Chapter 7 considers the issue of realism in interpretations of theories beyond ordinary quantum mechanics, such as Quantum Field Theory.
This examination of quantum theory shows why interpreting it is so hard as to be ongoing after a hundred years of study, considers how we should choose between interpretations  and ultimately asks whether we should have to choose at all.
Date of Award  26 Nov 2020 

Original language  English 
Awarding Institution 

Supervisor  James A C Ladyman (Supervisor) & Karim P Y Thebault (Supervisor) 