132,454 research outputs found
Canonical quantization of constrained theories on discrete space-time lattices
We discuss the canonical quantization of systems formulated on discrete
space-times. We start by analyzing the quantization of simple mechanical
systems with discrete time. The quantization becomes challenging when the
systems have anholonomic constraints. We propose a new canonical formulation
and quantization for such systems in terms of discrete canonical
transformations. This allows to construct, for the first time, a canonical
formulation for general constrained mechanical systems with discrete time. We
extend the analysis to gauge field theories on the lattice. We consider a
complete canonical formulation, starting from a discrete action, for lattice
Yang--Mills theory discretized in space and Maxwell theory discretized in space
and time. After completing the treatment, the results can be shown to coincide
with the results of the traditional transfer matrix method. We then apply the
method to BF theory, yielding the first lattice treatment for such a theory
ever. The framework presented deals directly with the Lorentzian signature
without requiring an Euclidean rotation. The whole discussion is framed in such
a way as to provide a formalism that would allow a consistent, well defined,
canonical formulation and quantization of discrete general relativity, which we
will discuss in a forthcoming paper.Comment: 18 pages, RevTex, one figur
Degenerate Variational Integrators for Magnetic Field Line Flow and Guiding Center Trajectories
Symplectic integrators offer many advantages for the numerical solution of
Hamiltonian differential equations, including bounded energy error and the
preservation of invariant sets. Two of the central Hamiltonian systems
encountered in plasma physics --- the flow of magnetic field lines and the
guiding center motion of magnetized charged particles --- resist symplectic
integration by conventional means because the dynamics are most naturally
formulated in non-canonical coordinates, i.e., coordinates lacking the familiar
partitioning. Recent efforts made progress toward non-canonical
symplectic integration of these systems by appealing to the variational
integration framework; however, those integrators were multistep methods and
later found to be numerically unstable due to parasitic mode instabilities.
This work eliminates the multistep character and, therefore, the parasitic mode
instabilities via an adaptation of the variational integration formalism that
we deem ``degenerate variational integration''. Both the magnetic field line
and guiding center Lagrangians are degenerate in the sense that their resultant
Euler-Lagrange equations are systems of first-order ODEs. We show that
retaining the same degree of degeneracy when constructing a discrete Lagrangian
yields one-step variational integrators preserving a non-canonical symplectic
structure on the original Hamiltonian phase space. The advantages of the new
algorithms are demonstrated via numerical examples, demonstrating superior
stability compared to existing variational integrators for these systems and
superior qualitative behavior compared to non-conservative algorithms
Weak Form of Stokes-Dirac Structures and Geometric Discretization of Port-Hamiltonian Systems
We present the mixed Galerkin discretization of distributed parameter
port-Hamiltonian systems. On the prototypical example of hyperbolic systems of
two conservation laws in arbitrary spatial dimension, we derive the main
contributions: (i) A weak formulation of the underlying geometric
(Stokes-Dirac) structure with a segmented boundary according to the causality
of the boundary ports. (ii) The geometric approximation of the Stokes-Dirac
structure by a finite-dimensional Dirac structure is realized using a mixed
Galerkin approach and power-preserving linear maps, which define minimal
discrete power variables. (iii) With a consistent approximation of the
Hamiltonian, we obtain finite-dimensional port-Hamiltonian state space models.
By the degrees of freedom in the power-preserving maps, the resulting family of
structure-preserving schemes allows for trade-offs between centered
approximations and upwinding. We illustrate the method on the example of
Whitney finite elements on a 2D simplicial triangulation and compare the
eigenvalue approximation in 1D with a related approach.Comment: Copyright 2018. This manuscript version is made available under the
CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0
The Midpoint Rule as a Variational--Symplectic Integrator. I. Hamiltonian Systems
Numerical algorithms based on variational and symplectic integrators exhibit
special features that make them promising candidates for application to general
relativity and other constrained Hamiltonian systems. This paper lays part of
the foundation for such applications. The midpoint rule for Hamilton's
equations is examined from the perspectives of variational and symplectic
integrators. It is shown that the midpoint rule preserves the symplectic form,
conserves Noether charges, and exhibits excellent long--term energy behavior.
The energy behavior is explained by the result, shown here, that the midpoint
rule exactly conserves a phase space function that is close to the Hamiltonian.
The presentation includes several examples.Comment: 11 pages, 8 figures, REVTe
Quantization of systems with temporally varying discretization I: Evolving Hilbert spaces
A temporally varying discretization often features in discrete gravitational
systems and appears in lattice field theory models subject to a coarse graining
or refining dynamics. To better understand such discretization changing
dynamics in the quantum theory, an according formalism for constrained
variational discrete systems is constructed. While the present manuscript
focuses on global evolution moves and, for simplicity, restricts to Euclidean
configuration spaces, a companion article discusses local evolution moves. In
order to link the covariant and canonical picture, the dynamics of the quantum
states is generated by propagators which satisfy the canonical constraints and
are constructed using the action and group averaging projectors. This projector
formalism offers a systematic method for tracing and regularizing divergences
in the resulting state sums. Non-trivial coarse graining evolution moves lead
to non-unitary, and thus irreversible, projections of physical Hilbert spaces
and Dirac observables such that these concepts become evolution move dependent
on temporally varying discretizations. The formalism is illustrated in a toy
model mimicking a `creation from nothing'. Subtleties arising when applying
such a formalism to quantum gravity models are discussed.Comment: 45 pages, 1 appendix, 6 figures (additional explanations, now matches
published version
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