5,690 research outputs found
Does a relativistic metric generalization of Newtonian gravity exist in 2+1 dimensions?
It is shown that, contrary to previous claims, a scalar tensor theory of
Brans-Dicke type provides a relativistic generalization of Newtonian gravity in
2+1 dimensions. The theory is metric and test particles follow the space-time
geodesics. The static isotropic solution is studied in vacuum and in regions
filled with an incompressible perfect fluid. It is shown that the solutions can
be consistently matched at the matter vacuum interface, and that the Newtonian
behavior is recovered in the weak field regime.Comment: 6 pages, no figures, Revtex4. Some discussions on the physical nature
of the interior solution and on the omega->infinity limit and some references
added. Version to appear in Phys. Rev.
The Two-Dimensional Analogue of General Relativity
General Relativity in three or more dimensions can be obtained by taking the
limit in the Brans-Dicke theory. In two dimensions
General Relativity is an unacceptable theory. We show that the two-dimensional
closest analogue of General Relativity is a theory that also arises in the
limit of the two-dimensional Brans-Dicke theory.Comment: 8 pages, LaTeX, preprint DF/IST-17.9
Collapsing shells of radiation in anti-de Sitter spacetimes and the hoop and cosmic censorship conjectures
Gravitational collapse of radiation in an anti-de Sitter background is
studied. For the spherical case, the collapse proceeds in much the same way as
in the Minkowski background, i.e., massless naked singularities may form for a
highly inhomogeneous collapse, violating the cosmic censorship, but not the
hoop conjecture. The toroidal, cylindrical and planar collapses can be treated
together. In these cases no naked singularity ever forms, in accordance with
the cosmic censorship. However, since the collapse proceeds to form toroidal,
cylindrical or planar black holes, the hoop conjecture in an anti-de Sitter
spacetime is violated.Comment: 4 pages, Revtex Journal: to appear in Physical Review
Conformal entropy from horizon states: Solodukhin's method for spherical, toroidal, and hyperbolic black holes in D-dimensional anti-de Sitter spacetimes
A calculation of the entropy of static, electrically charged, black holes
with spherical, toroidal, and hyperbolic compact and oriented horizons, in D
spacetime dimensions, is performed. These black holes live in an anti-de Sitter
spacetime, i.e., a spacetime with negative cosmological constant. To find the
entropy, the approach developed by Solodukhin is followed. The method consists
in a redefinition of the variables in the metric, by considering the radial
coordinate as a scalar field. Then one performs a 2+(D-2) dimensional
reduction, where the (D-2) dimensions are in the angular coordinates, obtaining
a 2-dimensional effective scalar field theory. This theory is a conformal
theory in an infinitesimally small vicinity of the horizon. The corresponding
conformal symmetry will then have conserved charges, associated with its
infinitesimal conformal generators, which will generate a classical Poisson
algebra of the Virasoro type. Shifting the charges and replacing Poisson
brackets by commutators, one recovers the usual form of the Virasoro algebra,
obtaining thus the level zero conserved charge eigenvalue L_0, and a nonzero
central charge c. The entropy is then obtained via the Cardy formula.Comment: 21 page
The Three-Dimensional BTZ Black Hole as a Cylindrical System in Four-Dimensional General Relativity
It is shown how to transform the three dimensional BTZ black hole into a four
dimensional cylindrical black hole (i.e., black string) in general relativity.
This process is identical to the transformation of a point particle in three
dimensions into a straight cosmic string in four dimensions.Comment: Latex, 9 page
Visual Cortex Plasticity Following Peripheral Damage To The Visual System: fMRI Evidence
Over the last two decades, functional magnetic resonance imaging (fMRI) has become a powerful research method to investigate cortical visual plasticity. Abnormal fMRI response patterns have been occasionally detected in the visually deprived cortex of patients with bilateral retinal diseases. Controversy remains whether these observations indicate structural reorganization of the visual cortex or unmasking of previously silent cortico-cortical connections. In optic nerve diseases, there is weak evidence showing that early visual cortex seems to lack reorganization, while higher-order visual areas undergo plastic changes which may contribute to optimise visual function. There is however accumulating imaging evidence demonstrating trans-synaptic degeneration of the visual cortex in patients with disease of the anterior visual pathways. This may preclude the use of restorative treatments in these patients. Here, we review and update the body of fMRI evidence on visual cortical plasticity.info:eu-repo/semantics/publishedVersio
Thermodynamics of the two-dimensional black hole in the Teitelboim-Jackiw theory
The two-dimensional theory of Teitelboim and Jackiw has constant and negative
curvature. In spite of this, the theory admits a black hole solution with no
singularities. In this work we study the thermodynamics of this black hole
using York's formalism.Comment: 16 pages, Late
Probing time orientability of spacetime
In general relativity, cosmology and quantum field theory, spacetime is
assumed to be an orientable manifold endowed with a Lorentz metric that makes
it spatially and temporally orientable. The question as to whether the laws of
physics require these orientability assumptions is ultimately of observational
or experimental nature, or the answer might come from a fundamental theory of
physics. The possibility that spacetime is time non-orientable lacks
investigation, and so should not be dismissed straightaway. In this paper, we
argue that it is possible to locally access a putative time non-orientability
of Minkowski empty spacetime by physical effects involving quantum vacuum
electromagnetic fluctuations. We set ourselves to study the influence of time
non-orientability on the stochastic motions of a charged particle subject to
these electromagnetic fluctuations in Minkowski spacetime equipped with a time
non-orientable topology and with its time orientable counterpart. To this end,
we introduce and derive analytic expressions for a statistical time
orientability indicator. Then we show that it is possible to pinpoint the time
non-orientable topology through an inversion pattern displayed by the
corresponding orientability indicator, which is absent when the underlying
manifold is time orientable.Comment: 21 pages, 3 figure
Relativistic Static Thin Disks with Radial Stress Suport
New solutions for static non-rotating thin disks of finite radius with
nonzero radial stress are studied. A method to introduce either radial pressure
or radial tension is presented. The method is based on the use of conformal
transformations.Comment: 19 pages, LaTeX, 7 figures, submitted to Class. Quan. Gra
Testing Born-Infeld electrodynamics in waveguides
Waveguides can be employed to test non-linear effects in electrodynamics. We
solve Born-Infeld equations for TE waves in a rectangular waveguide. We show
that the energy velocity acquires a dependence on the amplitude, and harmonic
components appear as a consequence of the non-linear behavior.Comment: 3 pages. To appear in PR
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