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Journal of Computer Engineering and Information Technology

Causal and Acausal Sets Based on a Quotient Lattice

Gunji YP, Taichi Haruna

Published Jan 1, 2016• 2 citations
Publisher

Abstract

Causal and Acausal Sets Based on a Quotient Lattice The spacetime viewed from the inside by using a lattice is here described as a causal set, and a quotient lattice is described as a causal history. It is shown that this description is consistent with general relativity and quantum theory. In particular, the influence of measurement on a causal set is considered and the interaction between a causal set and history is defined. The transitions from a causal set to a causal history and vice versa are both defined in a category theory and they are shown to be an adjunction. This adjunction leads to the idea of a relative casual set, and it results in the evolution and superposition of causal sets and histories. It is shown that the superposed casual set is a bicontinuous poset equipped with a compact interval. Relative causal sets are also equipped with the operation of self-embedding. A causal set is continually transformed into a degenerate distributive lattice due to the self-embedding. When an acausal set in a causal set can be transformed into a tensor product of underlying Hilbert spaces, the distributive lattice reveals a unitary evolution of composite states that is consistent with quantum theory.

DOI
10.4172/2324-9307.1000143

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Causal and Acausal Sets Based on a Quotient Lattice The spacetime viewed from the inside by using a lattice is here described as a causal set, and a quotient lattice is described as a causal history. It is shown that this description is consistent with general relativity and quantum theory. In particular, the influence of measurement on a causal set is considered and the interaction between a causal set and history is defined. The transitions from a causal set to a causal history and vice versa are both defined in a category theory and they are shown to be an adjunction. This adjunction leads to the idea of a relative casual set, and it results in the evolution and superposition of causal sets and histories. It is shown that the superposed casual set is a bicontinuous poset equipped with a compact interval. Relative causal sets are also equipped with the operation of self-embedding. A causal set is continually transformed into a degenerate distributive lattice due to the self-embedding. When an acausal set in a causal set can be transformed into a tensor product of underlying Hilbert spaces, the distributive lattice reveals a unitary evolution of composite states that is consistent with quantum theory.
We provide a precise definition and analysis of quantum causal histories (QCHs). A QCH consists of a discrete, locally finite, causal pre-spacetime with matrix algebras encoding the quantum structure at each event. The evolution of quantum states and observables is described by completely positive maps between the algebras at causally related events. We show that this local description of evolution is sufficient and that unitary evolution can be recovered wherever it should actually be expected. This formalism may describe a quantum cosmology without an assumption of global hyperbolicity; it is thus more general than the Wheeler–De Witt approach. The structure of a QCH is also closely related to quantum information theory and algebraic quantum field theory on a causal set.
The Causal Set hypothesis for Quantum Gravity asserts that the smooth Lorentzian spacetime manifold of General Relativity is only an approximation to a fundamental microscopic discrete structure: a locally-finite partially-ordered set. The points of this set are akin to the point-events of the spacetime manifold, and the order-relations among these points are akin to the causal-relations among these point-events. The local-finiteness condition implies a fundamental discreteness and provides an important [conformal] factor needed for the continuum approximations. In this dissertation, we address two aspects toward the formulation of a quantum dynamics for causal sets. The first aspect concerns the dynamics of a zero-mass classical scalar field on a background causal set, Poisson-sampled from an Alexandrov interval in (1 + 1)-dimensional Minkowski spacetime. Our numerical simulations suggest that actions can be defined for such a scalar field, expressed solely in terms of the causal set and the scalar field, which approximates the corresponding classical action on the continuum Minkowski spacetime. This result may help us describe matter and gauge fields in a local and Lorentz-invariant way on a causal set, a discrete structure now being considered as the arena for spacetime physics. More importantly, it may help us formulate an action for the causal set itself, which would be used in a sum-over-histories approach to describe the quantum dynamics of causal sets. The second aspect concerns Quantum Measure Theory, a new approach to quantum dynamics inspired by the sum-over-histories approach, which is better suited to formulating the dynamics of causal sets and of other alternative structures for spacetime. To better understand the mathematical structure underlying this new approach, we derive some algebraic identities involving the sum-rules of the Quantum Measure and its generalizations. We conclude with a discussion on possible applications and open problems concerning these aspects.
Abstract In this paper, we construct a tensor network representation of quantum causal histories, as a step towards directly representing states in quantum gravity via bulk tensor networks. Quantum causal histories are quantum extensions of causal sets in the sense that on each event in a causal set is assigned a Hilbert space of quantum states, and the local causal evolutions between events are modeled by completely positive and trace-preserving maps. Here we utilize the channel-state duality of completely positive and trace-preserving maps to transform the causal evolutions to bipartite entangled states. We construct the matrix product state for a single quantum causal history by projecting the obtained bipartite states onto the physical states on the events. We also construct the two dimensional tensor network states for entangled quantum causal histories in a restricted case with compatible causal orders. The possible holographic tensor networks are explored by mapping the quantum causal histories in a way analogous to the exact holographic mapping. The constructed tensor networks for quantum causal histories are exemplified by the non-unitary local time evolution moves in a quantum system on temporally varying discretizations, and these non-unitary evolution moves are shown to be necessary for defining a bulk causal structure and a quantum black hole. Finally, we comment on the limitations of the constructed tensor networks, and discuss some directions for further studies aiming at applications in quantum gravity.
Isham's topos-theoretic perspective on the logic of the consistent-histories theory is extended in two ways. First, the presheaves of consistent sets of history propositions in the topos proposed by Isham are endowed with a Vietoris-type of topology and subsequently they are sheafified with respect to it. The category resulting from this sheafification procedure is the topos of sheaves of sets varying continuously over the Vietoris-topologized base poset category of Boolean subalgebras of the universal orthoalgebra of quantum history propositions. The second extension of Isham's topos consists in endowing the stalks of the aforementioned sheaves, which were originally inhabited by structureless sets, with further algebraic structure, that also enjoys a quantum causal interpretation, so as to arrive at the topos of consistent-histories of quantum causal sets. The resulting quantum causal histories topos is compared with Markopoulou's quantum causal histories, with Mallios and this author's finitary spacetime sheaves of quantum causal sets, as well as with Butterfield and Isham's topos perspective on the Kochen-Specker theorem of quantum logic.
Quantum theory challenges classical causal explanations for physical phenomena, calling for a quantum generalisation of causal structure. This thesis advances the study of causal decompositions, a research programme that aims to bridge two prominent, yet conceptually distinct types of structural assumption found across the quantum causality literature. The first is in terms of the global dependency relations between input and output systems of quantum transformations, which in the unitary case we shall refer to as causal structure; the second is in terms of the compositional structure of quantum circuits or networks. Concretely, a causal decomposition is a circuit representation of a unitary transformation in which no-influence relations that hold between its inputs and outputs are made manifest in the absence of paths through the circuit connecting them. Applications include translating between different quantum causal modelling frameworks; equipping a framework based on unitary causal structure with a local, bottom-up description via generalised autonomous causal mechanisms; and relating notions of compatibility between quantum transformations and relativistic causality. Whether causal decompositions always exist remains an open problem. The contributions of this thesis are threefold. First, we develop an order-theoretic account of circuit syntax and demonstrate that every circuit with a given input-output connectivity can be rewritten into a one of a canonical form given by the concept lattice construction from lattice theory. This systematises the search for causal decompositions by providing the shape of the quantum circuit one should look for. Second, we establish a combinatorial condition that fully characterises when causal decompositions into unitary quantum circuits exist without requiring the generalisation to routed quantum circuits prompted by earlier research. This solves a well-defined subproblem of the broader quest, and our proof, an interplay of order theory and operator algebra, lays a foundation for future extensions. Third, we detail the application to relativistic causality. Within a multipartite framework generalising existing literature, we emphasise that the mere lack of superluminal signalling through a quantum channel is not sufficient to conclude its realisability by dynamics that are locally consistent with relativity theory. We conjecture that, on the other hand, a lack of superluminal causal influences through its unitary dilations is sufficient—at least up to arbitrarily small errors. We demonstrate this in various special cases and show that the general statement reduces to the existence of a weak version of causal decompositions.