14,554 research outputs found
Electric field formulation for thin film magnetization problems
We derive a variational formulation for thin film magnetization problems in
type-II superconductors written in terms of two variables, the electric field
and the magnetization function. A numerical method, based on this formulation,
makes it possible to accurately compute all variables of interest, including
the electric field, for any value of the power in the power law current-voltage
relation characterizing the superconducting material. For high power values we
obtain a good approximation to the critical state model solution. Numerical
simulation results are presented for simply and multiply connected films, and
also for an inhomogeneous film.Comment: 15 p., submitte
Feynman diagams coupled to three-dimensional quantum gravity
A framework for quantum field theory coupled to three-dimensional quantum
gravity is proposed. The coupling with quantum gravity regulates the Feynman
diagrams. One recovers the usual Feynman amplitudes in the limit as the
cosmological constant tends to zero.Comment: 7 pages. v2: minor corrections, added re
Engineering analysis and design of a mechanism to simulate a sonic boom
Mechanism simulating vibrational and acoustic properties of sonic boom
Holonomy observables in Ponzano-Regge type state sum models
We study observables on group elements in the Ponzano-Regge model. We show
that these observables have a natural interpretation in terms of Feynman
diagrams on a sphere and contrast them to the well studied observables on the
spin labels. We elucidate this interpretation by showing how they arise from
the no-gravity limit of the Turaev-Viro model and Chern-Simons theory.Comment: 15 pages, 2 figure
Spin Foam Models of Matter Coupled to Gravity
We construct a class of spin foam models describing matter coupled to
gravity, such that the gravitational sector is described by the unitary
irreducible representations of the appropriate symmetry group, while the matter
sector is described by the finite-dimensional irreducible representations of
that group. The corresponding spin foam amplitudes in the four-dimensional
gravity case are expressed in terms of the spin network amplitudes for
pentagrams with additional external and internal matter edges. We also give a
quantum field theory formulation of the model, where the matter degrees of
freedom are described by spin network fields carrying the indices from the
appropriate group representation. In the non-topological Lorentzian gravity
case, we argue that the matter representations should be appropriate SO(3) or
SO(2) representations contained in a given Lorentz matter representation,
depending on whether one wants to describe a massive or a massless matter
field. The corresponding spin network amplitudes are given as multiple
integrals of propagators which are matrix spherical functions.Comment: 30 pages, 9 figures, further remarks and references added. Version to
appear in Class. Quant. Gra
Quantum Gravity as a Deformed Topological Quantum Field Theory
It is known that the Einstein-Hilbert action with a positive cosmological
constant can be represented as a perturbation of the SO(4,1) BF theory by a
symmetry-breaking term quadratic in the B field. Introducing fermionic matter
generates additional terms in the action which are polynomial in the tetrads
and the spin connection. We describe how to construct the generating functional
in the spin foam formalism for a generic BF theory when the sources for the B
and the gauge field are present. This functional can be used to obtain a path
integral for General Relativity with matter as a perturbative series whose the
lowest order term is a path integral for a topological gravity coupled to
matter.Comment: 7 pages, talk presented at the QG05 conference, 12-16 September 2005,
Cala Gonone, Ital
Current research in oxidation-resistant carbon-carbon composites at NASA. Langley Research Center
The significant potential of carbon-carbon composites for high-temperature structural applications is well established. For hypersonic vehicle applications, desirable properties include low density, high specific strength and stiffness, low coefficients of thermal expansion, and retention of mechanical properties above 3000 F. A significant problem associated with carbon materials, however, is that they oxidize rapidly in air at temperatures above about 800 F, and therefore must be protected from oxidation. Successful development of effective methods of oxidation protection is key to the eventual utilization of carbon-carbon composites on hypersonic vehicles such as NASP. In this presentation, the basic elements of an oxidation-protection system are described. Results from oxidation-performance evaluations of state-of-the-art ACC-4 type material in simulated airframe vehicle environments (temperature, pressure, and time) conducted at NASA Langley are also presented. NASA Langley has an active research effort to improve the oxidation resistance of carbon-carbon materials for airframe structural and vehicle thermal protection applications. Conversion coating and sealant development research is highlighted
Observables in 3-dimensional quantum gravity and topological invariants
In this paper we report some results on the expectation values of a set of
observables introduced for 3-dimensional Riemannian quantum gravity with
positive cosmological constant, that is, observables in the Turaev-Viro model.
Instead of giving a formal description of the observables, we just formulate
the paper by examples. This means that we just show how an idea works with
particular cases and give a way to compute 'expectation values' in general by a
topological procedure.Comment: 24 pages, 47 figure
Area Regge Calculus and Discontinuous Metrics
Taking the triangle areas as independent variables in the theory of Regge
calculus can lead to ambiguities in the edge lengths, which can be interpreted
as discontinuities in the metric. We construct solutions to area Regge calculus
using a triangulated lattice and find that on a spacelike hypersurface no such
discontinuity can arise. On a null hypersurface however, we can have such a
situation and the resulting metric can be interpreted as a so-called refractive
wave.Comment: 18 pages, 1 figur
Lorentzian spin foam amplitudes: graphical calculus and asymptotics
The amplitude for the 4-simplex in a spin foam model for quantum gravity is
defined using a graphical calculus for the unitary representations of the
Lorentz group. The asymptotics of this amplitude are studied in the limit when
the representation parameters are large, for various cases of boundary data. It
is shown that for boundary data corresponding to a Lorentzian simplex, the
asymptotic formula has two terms, with phase plus or minus the Lorentzian
signature Regge action for the 4-simplex geometry, multiplied by an Immirzi
parameter. Other cases of boundary data are also considered, including a
surprising contribution from Euclidean signature metrics.Comment: 30 pages. v2: references now appear. v3: presentation greatly
improved (particularly diagrammatic calculus). Definition of "Regge state"
now the same as in previous work; signs change in final formula as a result.
v4: two references adde
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