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arXiv:hep-th/0509122 · Advances in Theoretical and Mathematical Physics

General boundary quantum field theory: Foundations and probability interpretation

Robert Oeckl

Published Jan 1, 2008• 103 citations
arXivHTMLPDF

Abstract

We elaborate on the proposed general boundary formulation as an extension of standard quantum mechanics to arbitrary (or no) backgrounds.Temporal transition amplitudes are generalized to amplitudes for arbitrary space-time regions.State spaces are associated to general (not necessarily spacelike) hypersurfaces.We give a detailed foundational exposition of this approach, including its probability interpretation and a list of core axioms.We explain how standard quantum mechanics arises as a special case.We include a discussion of probability conservation and unitarity, showing how these concepts are generalized in the present framework.We formulate vacuum axioms and incorporate space-time symmetries into the framework.We show how the Schrödinger-Feynman approach is a suitable starting point for casting quantum field theories into the general boundary form.We discuss the role of operators.

DOI
10.4310/atmp.2008.v12.n2.a3

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We elaborate on the proposed general boundary formulation as an extension of standard quantum mechanics to arbitrary (or no) backgrounds.Temporal transition amplitudes are generalized to amplitudes for arbitrary space-time regions.State spaces are associated to general (not necessarily spacelike) hypersurfaces.We give a detailed foundational exposition of this approach, including its probability interpretation and a list of core axioms.We explain how standard quantum mechanics arises as a special case.We include a discussion of probability conservation and unitarity, showing how these concepts are generalized in the present framework.We formulate vacuum axioms and incorporate space-time symmetries into the framework.We show how the Schrödinger-Feynman approach is a suitable starting point for casting quantum field theories into the general boundary form.We discuss the role of operators.
We show that the real massive Klein-Gordon theory admits a description in terms of states on various timelike hypersurfaces and amplitudes associated with regions bounded by them. This realizes crucial elements of the general boundary framework for quantum field theory. The hypersurfaces considered are hyperplanes on the one hand and timelike hypercylinders on the other hand. The latter lead to the first explicit examples of amplitudes associated with finite regions of space, and admit no standard description in terms of ``initial'' and ``final'' states. We demonstrate a generalized probability interpretation in this example, going beyond the applicability of standard quantum mechanics.
We give a brief introduction to the so-called general boundary formulation (GBF) of quantum theory. This new axiomatic formulation provides a description of the quantum dynamics which is manifestly local and does not rely on a metric background structure for its definition. We present the basic ingredients of the GBF, in particular we review the core axioms that assign algebraic structures to geometric ones, the two quantisation schemes so far developed for the GBF and the probability interpretation which generalizes the standard Born rule. Finally we briefly discuss some of the results obtained studying specific quantum field theories within the GBF.
This chapter introduces the quantum mechanics. It explores the formulation in terms of the operator calculus. The chapter shows how nonrelativistic quantum kinematics emerges from the general theory. It discusses how physical quantities are related to symmetries through unitary transformations, and consider translations. The chapter also introduces the statistical operator and the density matrix. It introduces the consistent history interpretation of quantum mechanics. The true core of quantum mechanics is the rule for assigning the amplitude to an event that can be effected in indistinguishable ways. The chapter examines the presentation of Feynman's space-time approach since it offers the most intuitive formulation of quantum mechanics. A physical quantity is said to be conserved if it for any state of the system stays constant in time. In quantum mechanics this therefore means that its probability distribution in any state is independent of time.
Familiar textbook quantum mechanics assumes a fixed background spacetime to define states on spacelike surfaces and their unitary evolution between them. Quantum theory has changed as our conceptions of space and time have evolved. But quantum mechanics needs to be generalized further for quantum gravity where spacetime geometry is fluctuating and without definite value. This paper reviews a fully four-dimensional, sum-over-histories, generalized quantum mechanics of cosmological spacetime geometry. This generalization is constructed within the framework of generalized quantum theory. This is a minimal set of principles for quantum theory abstracted from the modern quantum mechanics of closed systems, most generally the universe. In this generalization, states of fields on spacelike surfaces and their unitary evolution are emergent properties appropriate when spacetime geometry behaves approximately classically. The principles of generalized quantum theory allow for the further generalization that would be necessary were spacetime not fundamental. Emergent spacetime phenomena are discussed in general and illustrated with the example of the classical spacetime geometries with large spacelike surfaces that emerge from the `no-boundary' wave function of the universe. These must be Lorentzian with one, and only one, time direction. The essay concludes by raising the question of whether quantum mechanics itself is emergent.
Abstract The book starts with a description of classical mechanics then discusses the quantum phenomena that require us to give up our commonsense classical intuitions. We consider the physical and conceptual arguments that led to the standard von Neumann-Dirac formulation of quantum mechanics and how the standard theory explains quantum phenomena. This includes a discussion of how the theory’s two dynamical laws work with the standard interpretation of states to explain determinate measurement records, quantum statistics, interference effects, entanglement, decoherence, and quantum nonlocality. A careful understanding of how the standard theory works ultimately leads to the quantum measurement problem. We consider how the measurement problem threatens the logical consistency of the standard theory then turn to a discussion of the main proposals for resolving it. This includes collapse formulations of quantum mechanics like Wigner’s extension of the standard theory and the GRW approach and no-collapse formulations like pure wave mechanics, the various many-worlds theories, and Bohmian mechanics. In discussing alternative formulations of quantum mechanics we pay particular attention to the explanatory role played by each theory’s empirical ontology and associated metaphysical commitments and the conceptual trade-offs between theoretical options.
I present a derivation of quantum probabilities (Born rule) from a minimal set of spacetime composition axioms. By treating boundary states on spacetime slices as fundamental and imposing linearity under sequential composition, tensor factorization for spacelike-separated regions, and unitary invariance, I show that the quadratic probability measure arises uniquely. My derivation interprets quantum amplitudes as intrinsic features of spacetime: probabilities naturally emerge from how amplitudes on boundaries glue together, without introducing additional operators or postulates. This submission establishes priority for a specific, elegant argument linking the Born rule to the symmetries of spacelike factorization and foliation invariance, providing a first-principles foundation for quantum probabilities from spacetime considerations.