87,127 research outputs found
Cosmic Strings and Chronology Protection
A space consisting of two rapidly moving cosmic strings has recently been
constructed by Gott that contains closed timelike curves. The global structure
of this space is analysed and is found that, away from the strings, the space
is identical to a generalised Misner space. The vacuum expectation value of the
energy momentum tensor for a conformally coupled scalar field is calculated on
this generalised Misner space. It is found to diverge very weakly on the
Chronology horizon, but more strongly on the polarised hypersurfaces. The
divergence on the polarised hypersurfaces is strong enough that when the proper
geodesic interval around any polarised hypersurface is of order the Planck
length squared, the perturbation to the metric caused by the backreaction will
be of order one. Thus we expect the structure of the space will be radically
altered by the backreaction before quantum gravitational effects become
important. This suggests that Hawking's `Chronology Protection Conjecture'
holds for spaces with non-compactly generated Chronology horizon.Comment: 15 pages, plain TeX, 2 figures (not included), DAMTP-R92/3
A Spinorial Hamiltonian Approach to Gravity
We give a spinorial set of Hamiltonian variables for General Relativity in
any dimension greater than 2. This approach involves a study of the algebraic
properties of spinors in higher dimension, and of the elimination of
second-class constraints from the Hamiltonian theory. In four dimensions, when
restricted to the positive spin-bundle, these variables reduce to the standard
Ashtekar variables. In higher dimensions, the theory can either be reduced to a
spinorial version of the ADM formalism, or can be left in a more general form
which seems useful for the investigation of some spinorial problems such as
Riemannian manifolds with reduced holonomy group. In dimensions ,
the theory may be recast solely in terms of structures on the positive
spin-bundle , but such a reduction does not seem possible in
dimensions , due to algebraic properties of spinors in these
dimensions.Comment: 20 pages, Latex 2e. Published versio
Lsdiff M and the Einstein Equations
We give a formulation of the vacuum Einstein equations in terms of a set of
volume-preserving vector fields on a four-manifold . These vectors
satisfy a set of equations which are a generalisation of the Yang-Mills
equations for a constant connection on flat spacetime.Comment: 5 pages, no figures, Latex, uses amsfonts, amssym.def and amssym.tex.
Note added on more direct connection with Yang-Mills equation
The ADHM construction and non-local symmetries of the self-dual Yang-Mills equations
We consider the action on instanton moduli spaces of the non-local symmetries
of the self-dual Yang-Mills equations on discovered by Chau and
coauthors. Beginning with the ADHM construction, we show that a sub-algebra of
the symmetry algebra generates the tangent space to the instanton moduli space
at each point. We explicitly find the subgroup of the symmetry group that
preserves the one-instanton moduli space. This action simply corresponds to a
scaling of the moduli space.Comment: AMSLatex, 19 pages, no figures. Some discussions clarified, and
citations made more accurate. I am grateful to the referee for detailed
comments. Version to appear in Communications in Mathematical Physic
The emergence of magnetic flux through a partially ionised solar atmosphere
We present results from 2.5D numerical simulations of the emergence of magnetic flux from the upper convection zone through the photosphere and chromosphere into the corona. Certain regions of the solar atmosphere are at suïŹciently low temperatures to be only partially ionised, in particular the lower chromosphere. This leads to Cowling resistivities orders of magnitude larger than the Coulomb values, and thus to anisotropic dissipation in Ohmâs law. This also leads to localised low magnetic Reynolds numbers (R m < 1). We find that the rates of emergence of magnetic field are greatly increased by the partially ionised regions of the model atmosphere, and the resultant magnetic field is more diïŹuse. More importantly, the only currents associated with the magnetic field to emerge into the corona are aligned with the field, and thus the newly formed coronal field is force-free
Web Single Sign-On Authentication using SAML
Companies have increasingly turned to application service providers (ASPs) or Software as a Service (SaaS) vendors to offer specialized web-based services that will cut costs and provide specific and focused applications to users. The complexity of designing, installing, configuring, deploying, and supporting the system with internal resources can be eliminated with this type of methodology, providing great benefit to organizations. However, these models can present an authentication problem for corporations with a large number of external service providers. This paper describes the implementation of Security Assertion Markup Language (SAML) and its capabilities to provide secure single sign-on (SSO) solutions for externally hosted applications
Key pedagogic thinkers: Jean Baudrillard
Jean Baudrillard was born in Reims, France, in 1929, and completed his undergraduate work at the Sorbonne, taking a degree in German. Upon graduation, he taught high school. In the early 1960s, he began graduate studies at the University of Paris, Nanterre, earning his doctorate in sociology in 1966. Baudrillard published 30 books in which he examined various facets of modern society: gender, race, consumerism, politics, the media, and so forth. His focus was semiologicalâhow objects and signs reflect the current human condition. Although Baudrillard did not write about education, his work is nevertheless relevant if we recognize that our educational system is a reflection of society. A Baudrillardian perspective raises the following question: What effect has consumerism had on education? To address this question, we offer some background information related to Baudrillardâs philosophical inquiries. This is followed by our brief analysis of how Baudrillardâs work may provide some potential answers to the above question and of how it can help us interpret the changes that have occurred in education during the modern period. We give special emphasis to The Consumer Society and Simulacra and Simulation
A positive mass theorem for low-regularity Riemannian metrics
We show that the positive mass theorem holds for continuous Riemannian
metrics that lie in the Sobolev space for manifolds of
dimension less than or equal to or spin-manifolds of any dimension. More
generally, we give a (negative) lower bound on the ADM mass of metrics for
which the scalar curvature fails to be non-negative, where the negative part
has compact support and sufficiently small norm. We show that a
Riemannian metric in for some with
non-negative scalar curvature in the distributional sense can be approximated
locally uniformly by smooth metrics with non-negative scalar curvature. For
continuous metrics in , there exist smooth approximating
metrics with non-negative scalar curvature that converge in for all
.Comment: 21 pages. The results on the positive mass theorem were announced in
arxiv:1205.1302, with a sketch of the proo
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