The purpose of this thesis is to examine different conceptions of decoherence and their significance within interpretations of quantum mechanics. I set out three different conceptions of decoherence found in the literature and examine the relations between them. I argue that only the weakest of these conceptions is empirically well supported, and that the other two rely on claims about the structure of the histories (robust patterns within the wavefunction) which we occupy which require justification. \n \nI also examine the ways in which conceptions of decoherence are used to solve aspects of the quantum measurement problem and support modern interpretations of quantum mechanics. I focus particularly on Wallace's Everettian interpretation of quantum mechanics. I argue that while decoherence is generally successful in supporting this interpretation in a variety of ways, the very strong conception of decoherence on which he relies is itself difficult to justify. \n \nI consider a variety of possible approaches to justifying the use of this strong conception of decoherence and argue that many of them are either unconvincing, or rely on controversial cosmological claims. Finally, I suggest that the best way to justify the use of this strong conception of decoherence is by appealing directly to its indispensability to an otherwise very attractive interpretation of quantum mechanics.
The purpose of this thesis is to examine different conceptions of decoherence and their significance within interpretations of quantum mechanics. I set out three different conceptions of decoherence found in the literature and examine the relations between them. I argue that only the weakest of these conceptions is empirically well supported, and that the other two rely on claims about the structure of the histories (robust patterns within the wavefunction) which we occupy which require justification. \n \nI also examine the ways in which conceptions of decoherence are used to solve aspects of the quantum measurement problem and support modern interpretations of quantum mechanics. I focus particularly on Wallace's Everettian interpretation of quantum mechanics. I argue that while decoherence is generally successful in supporting this interpretation in a variety of ways, the very strong conception of decoherence on which he relies is itself difficult to justify. \n \nI consider a variety of possible approaches to justifying the use of this strong conception of decoherence and argue that many of them are either unconvincing, or rely on controversial cosmological claims. Finally, I suggest that the best way to justify the use of this strong conception of decoherence is by appealing directly to its indispensability to an otherwise very attractive interpretation of quantum mechanics.
The complex relation between quantum mechanics and classical mechanics is crucial in the philosophy of modern physics, and it cuts across current quantum physics. This paper is divided in two parts. In the first part I will offer a critical analysis of the role that decoherence plays in the account of the quantum-classical limit. In the second part I will mention three ways in which philosophers are engaging with the realist interpretation of quantum mechanics in light of the assessment that the problem of the quantum-classical limit is still open to debate. My main claim is that the problem of the quantum-classical limit is overrated and it receives too much attention for the realist who looks at quantum mechanics. The question that the realist wants to focus on is the crucial interpretation question: what is a quantum system?
The central part of Everett's formulation of quantum mechanics is a quantum mechanical model of memory and of observation as the recording of information in a memory. To use this model as an answer to the measurement problem, Everett has to assume that a conscious observer can be in a superposition of such memory states and be unaware of it. This assumption has puzzled generations of readers. The fundamental aim of this dissertation is to find a set of simpler assumptions which are sufficient to show that Everett's model is empirically adequate. I argue that Everett's model needs three assumptions to account for the process of observation: an assumption of decoherence of observers as quantum mechanical systems; an assumption of supervenience of mental states (qualities) over quantum mechanical properties; and an assumption about the interpretation of quantum mechanical states in general: quantum mechanical states describe ensembles of states of affairs coexisting in the same system. I argue that the only plausible understanding of such ensembles is as ensembles of possibilities, and that all standard no-collapse interpretations agree in this reading of quantum mechanical states. Their differences can be understood as different theories about what marks the real state within this ensemble, and Everett's theory as the claim that no additional 'mark of reality' is necessary. Using the three assumptions, I argue that introspection cannot determine the objective quantum mechanical state of an observer. Rather, the introspective qualities of a quantum mechanical state can be represented by a (classical) statistical ensemble of subjective states. An analysis of these subjective states and their dynamics leads to the conclusion that they suffice to give empirically correct predictions. The argument for the empirical adequacy of the subjective state entails that knowledge of the objective quantum mechanical state is impossible in principle. Empirical reality for a conscious observer is not described by the objective state, but by a Everettian relative state conditional on the subjective state, and no theoretical 'mark of reality' is necessary for this concept of reality. I compare the resulting concept of reality to Kant's distinction between empirical and transcendental reality.
Quantum decoherence is a dynamical process whereby a system's phase relations become delocalized due to interaction and subsequent entanglement with its environment. This delocalization, or decoherence, forces the quantum system into a state that is apparently classical (or apparently an eigenstate) by prodigiously suppressing features that typically give rise to so-called quantum behavior. Thus it has been frequently proposed by physicists and philosophers alike that decoherence explains the dynamical transition from quantum behavior to classical behavior. Statements like this assume the existence of distinct realms, however, and the present thesis is an exploration of the metaphysical consequences of quantum decoherence motivated by the question of the quantum-to-classical transition and interlevel relations: if there are in-principle ``classical' and ``quantum' levels, what are the relations between them? And if there are no such levels, what follows? Importantly, the following philosophical investigations are carried out by intentionally leaving aside the measurement problem and concerns about particular interpretations of quantum mechanics. Good philosophical work, it is argued, can be done without adopting a specific interpretational framework and without recourse to the measurement problem. After introducing the physics of decoherence and exploring the four canonical models applied to system-environment interactions, it is argued that, ontologically speaking, there exist no levels. This claim--called the ``nontological thesis'--exposes as ill-posed questions regarding the transition from the quantum regime to the classical regime and reveals the inappropriateness of interlevel relations (like reduction, supervenience and emergence) operating within metaphysical frameworks. The nontological thesis has further important consequences regarding intralevel relations: not only are there no meaningful ways to carve the world into levels, but there are no meaningful ways to carve the world into parts and wholes either. These conclusions, supported by quantum decoherence and the empirical success of its models, drastically alter the philosophical terrain--not just in physics or in the philosophy of physics, but in traditional metaphysics as well.
D. Wallace has tried to use decoherence to solve the preferred basis problem of Everettian Quantum Mechanics, and this solution lays the foundation for his proof of the Born rule. But this is a circular argument, as approximations used in decoherence usually rely on the probabilistic interpretation of the Hilbert space norm. He claims the norm can measure approximations even without probabilities, but this assumption has not been properly justified. Without it, the combination of the Everettian and decoherent histories formalisms leads to strange consequences, such as a proliferation of small amplitude histories with lots of macroscopic quantum jumps. Still, this erratic behavior may provide a way to justify the approximations, in a new histories formalism, in which macroscopic causal relations play a central role. Small histories, suffering too much interference, may lose causality, being thus discarded as invalid. The remaining branches can present some small interference, opening the possibility of experimental verification.
Quantum mechanics has engendered some philosophical disputes surrounding its conceptual foundation.To solve these disputes,some alternative interpretations which are different from orthodox(and Copenhagen) interpretation have been engendered.In the recent twenty years,the idea of decoherence has aroused attention among physicists and physical philosophers.Decoherence effects alone cannot solve the measurement problem.But decoherence including environmental interactions not only poses new conceptual questions but also suggests solutions to fundamental problems.
Decoherence is widely felt to have something to do with the quantum measurement problem, but getting clear on just what is made difficult by the fact that the 'measurement problem', as traditionally presented in foundational and philosophical discussions, has become somewhat disconnected from the conceptual problems posed by real physics. This, in turn, is because quantum mechanics as discussed in textbooks and in foundational discussions has become somewhat removed from scientific practice, especially where the analysis of measurement is concerned. This paper has two goals: firstly (§§1-2), to present an account of how quantum measurements are actually dealt with in modern physics (hint: it does not involve a collapse of the wave function) and to state the measurement problem from the perspective of that account; and secondly (§§3-4), to clarify what role decoherence plays in modern measurement theory and what effect it has on the various strategies that have been proposed to solve the measurement problem.