Sorkin has introduced a new, observer independent, interpretation of quantum mechanics that can give a successful realist account of the ‘quantum micro-world’ as well as explaining how classicality emerges at the level of observable events for a range of systems including single time ‘Copenhagen measurements’. This ‘co-event interpretation’ presents us with a new ontology, in which a single ‘co-event’ is real. A new ontology necessitates a review of the dynamical and predictive mechanism of a theory, and in this paper we begin the process by exploring means of expressing the dynamical and predictive content of histories theories in terms of co-events.
Sorkin has introduced a new, observer independent, interpretation of quantum mechanics that can give a successful realist account of the ‘quantum micro-world’ as well as explaining how classicality emerges at the level of observable events for a range of systems including single time ‘Copenhagen measurements’. This ‘co-event interpretation’ presents us with a new ontology, in which a single ‘co-event’ is real. A new ontology necessitates a review of the dynamical and predictive mechanism of a theory, and in this paper we begin the process by exploring means of expressing the dynamical and predictive content of histories theories in terms of co-events.
Quantum measure theory can be introduced as a histories based reformulation (and generalisation) of Copenhagen quantum mechanics in the image of classical stochastic theories. These classical models lend themselves to a simple interpretation in which a single history (a single element of the sample space) is deemed to be 'real'; we require only that this real history should not be ruled out by the dynamics, the axioms of which ensure that not all histories are precluded. However, applying this interpretation naively to quantum measure theory we can find experimentally realisable systems (notably the Peres-Kochen-Specker system) in which every history is ruled out by the dynamics, challenging us to formulate a deeper realist framework. Our first response is to hold on to our existing interpretative framework and attempt a revision of the dynamics that would reduce quantum measure theory to a classical dynamics. We explore this approach by examining the histories formulation of a stochastic-collapse model on a simple (discrete) null-lattice, concluding that the drawbacks of this approach outweigh the benefits. Our second response is to abandon our classically inspired interpretation in favour of Sorkin's 'co-events', a more general ontology that still allows for strict realism. In this case the 'potentially real' objects of the theory (the 'beables' in Bell's language) are not individual histories but truth valuation maps, or co-events. We develop & evaluate various co-event schemes that have been suggested to date, finally adopting the multiplicative scheme; the current working model of co-event theory and a promising interpretation of quantum measure theory, though still a work in progress. We conclude by exploring the expression of the dynamics & predictions in this new framework.
In this thesis we will explore the development of a realist quantum theory based on the decoherence functional using the co-event interpretation of Quantum Measure Theory. The Sum-Over-Histories theory of quantum mechanics will provide the bedding for a Hilbert-space-free stochastic-like theory that can accommodate spacetime-like objects, and can therefore be applied to quantum gravity and cosmology, as well as give an alternative perspective on quantum phenomena. The primitive objects of the theory are histories, which give different accounts of a system’s evolution, and the decoherence functional, which sums the quantum interference between these histories. Quantum Measure Theory and Generalised Quantum Mechanics (a theory close to Decoherent Histories) then give alternative interpretations of the decoherence functional’s relation to reality. In these theories, the decoherence functional is mathematically constrained in analogue to probability measures. However, one of the conditions, called weak positivity, can be lost under composition of isolated systems. We will extend this composition argument to take the case for a stronger condition of strong positivity for decoherence functionals. The bulk of the report will then focus on the co-event interpretation of Quantum Measure Theory, where co-events give full accounts of which events do or do not occur for a given system. The quantum nature of reality is expressed through the breaking of classical logic within these co-event descriptions. We will focus on evolving co-event schemes, which dynamically construct co-events to describe the reality of an evolving system in tandem with its progression. The evolving co-event schemes will be shown to reproduce classical logic when they are applied to classical systems. Moreover, similar to classical stochastic theory, these schemes will be shown to be invariant under the inclusion or exclusion of non-interfering histories. We will also explore a number of outstanding problems for these schemes, and will propose some potential modifications.
We begin by discussing ``What exists?'', i.e. ontology, in Classical Physics which provided a description of physical phenomena at the macroscopic level. The microworld however necessitates a introduction of Quantum ideas for its understanding. It is almost certain that the world is quantum mechanical at both microscopic as well as at macroscopic level. The problem of ontology of a Quantum world is a difficult one. It also depends on which interpretation is used. We first discuss some interpretations in which Quantum Mechanics does not provide a complete framework but has to be supplemented by extra ingredients e.g. (i) Copenhagen group of interpretations associated with the names of Niels Bohr, Heisenberg, von-Neumann, and (ii) de-Broglie-Bohm interpretations. We then look at some interpretations in which Quantum mechanics is supposed to provide the entire framework such as (i) Everett-deWitt many world, (ii) quantum histories interpretations. We conclude with some remarks on the rigidity of the formalism of quantum mechanics, which is sharp contrast to it's ontological fluidity.
This book discusses the ontology in the quantum world and argues that it should be interpreted as a historicized ontology. This perspective is advocated by the quantum histories interpretation. Inspired by the nomological interpretation from Bohmian mechanics and the corresponding understanding of the law of nature and logic, the ontology will be useful to solve the difficulties of confusing ontology and perplexing logical inference. Furthermore, this book proposes that the historicized ontology can be generalized to more complicated structures and fit naturally into temporal correlation. Core philosophical issues such as the law of nature, the ontology of events, history, and processes, and the metaphysics of history are coherently discussed, revealing the profound meaning behind the formalism of quantum histories. By providing a new framework for a historicized picture of ontology, it will effectively promote the debates in the philosophy of physics and the fundamental research in quantum mechanics. This book will appeal to scholars and students of the philosophy of science, metaphysics, and quantum theory.
This chapter discusses the ontological and logical issues involved in criticisms. The ontology of quantum histories interpretation is based on two core contents. One is the stochastic evolution dynamics, which is similar to standard quantum mechanics; the other is the reasoning principles (especially the single-framework rule) that must be followed if the quantum process is expressed in the histories formalism, which makes the interpretation deviate from previous rules of classical propositional reasoning. Although in those cases where classical mechanics can be regarded as an approximation of quantum mechanics, the “quantum logic” proposed by histories interpretation is enough to reduce to the classical propositional logic, there are still unresolved challenges regarding the relationship between the new and old logics, the new and old ontologies. In particular, there is a certain anti-realism in the expressions of the founders of the quantum histories interpretation, which causes mutual influence between the characterization of the properties of quantum histories and the logical inference of quantum histories propositions.
We introduce and explore Rafael Sorkin's \textit{evolving co-event scheme}: a theoretical framework for determining completely which events do and do not happen in evolving quantum, or indeed classical, systems. The theory is observer-independent and constructed from discrete histories, making the framework a potential setting for discrete quantum cosmology and quantum gravity, as well as ordinary discrete quantum systems. The foundation of this theory is Quantum Measure Theory, which generalises (classical) measure theory to allow for quantum interference between alternative histories; and its co-event interpretation, which describes whether events can or can not occur, and in what combination, given a system and a quantum measure. In contrast to previous co-event schemes, the evolving co-event scheme is applied in stages, in the stochastic sense, without any dependence on later stages, making it manifestly compatible with an evolving block view. It is shown that the co-event realities produced by the basic evolving scheme do not depend on the inclusion or exclusion of zero measure histories in the history space, which follows non-trivially from the basic rules of the scheme. It is also shown that this evolving co-event scheme will reduce to producing classical realities when it is applied to classical systems.