12,747 research outputs found
Construction of N = 2 Chiral Supergravity Compatible with the Reality Condition
We construct N = 2 chiral supergravity (SUGRA) which leads to Ashtekar's
canonical formulation. The supersymmetry (SUSY) transformation parameters are
not constrained at all and auxiliary fields are not required in contrast with
the method of the two-form gravity. We also show that our formulation is
compatible with the reality condition, and that its real section is reduced to
the usual N = 2 SUGRA up to an imaginary boundary term.Comment: 16 pages, late
String dynamics and ejection along the axis of a spinning black hole
Relativistic current carrying strings moving axisymmetrically on the
background of a Kerr black hole are studied. The boundaries and possible types
of motion of a string with a given energy and current are found. Regions of
parameters for which the string falls into the black hole, or is trapped in a
toroidal volume, or can escape to infinity, are identified, and representative
trajectories are examined by numerical integration, illustrating various
interesting behaviors. In particular, we find that a string can start out at
rest near the equatorial plane and, after bouncing around, be ejected out along
the axis, some of its internal (elastic or rotational kinetic) energy having
been transformed into translational kinetic energy. The resulting velocity can
be an order unity fraction of the speed of light. This process results from the
presence of an outer tension barrier and an inner angular momentum barrier that
are deformed by the gravitational field. We speculatively discuss the possible
astrophysical significance of this mechanism as a means of launching a
collimated jet of MHD plasma flux tubes along the spin axis of a gravitating
system fed by an accretion disk.Comment: 14 pages, 11 figures; v.2: minor edits, references added, typos
corrected, published versio
Comment on "Accelerated Detectors and Temperature in (Anti) de Sitter Spaces"
It is shown how the results of Deser and Levin on the response of accelerated
detectors in anti-de Sitter space can be understood from the same general
perspective as other thermality results in spacetimes with bifurcate Killing
horizons.Comment: 5 pages, LaTe
Black hole entropy: inside or out?
A trialogue. Ted, Don, and Carlo consider the nature of black hole entropy.
Ted and Carlo support the idea that this entropy measures in some sense ``the
number of black hole microstates that can communicate with the outside world.''
Don is critical of this approach, and discussion ensues, focusing on the
question of whether the first law of black hole thermodynamics can be
understood from a statistical mechanics point of view.Comment: 42 pages, contribution to proceedings of Peyresq
General relativity and cosmology derived from principle of maximum power or force
The field equations of general relativity are shown to derive from the
existence of a limit force or of a limit power in nature. The limits have the
value of c^4/4G and c^5/4G. The proof makes use of a result by Jacobson. All
known experimental data is consistent with the limits. Applied to the universe,
the limits predict its darkness at night and the observed scale factor. Some
experimental tests of the limits are proposed. The main counter-arguments and
paradoxes are discussed, such as the transformation under boosts, the force
felt at a black hole horizon, the mountain problem, and the contrast to
scalar--tensor theories of gravitation. The resolution of the paradoxes also
clarifies why the maximum force and the maximum power have remained hidden for
so long. The derivation of the field equations shows that the maximum force or
power plays the same role for general relativity as the maximum speed plays for
special relativity.Comment: 24 pages, 1 figure, LaTeX, published versio
Measurements of the magnetic field induced by a turbulent flow of liquid metal
Initial results from the Madison Dynamo Experiment provide details of the
inductive response of a turbulent flow of liquid sodium to an applied magnetic
field. The magnetic field structure is reconstructed from both internal and
external measurements. A mean toroidal magnetic field is induced by the flow
when an axial field is applied, thereby demonstrating the omega effect.
Poloidal magnetic flux is expelled from the fluid by the poloidal flow.
Small-scale magnetic field structures are generated by turbulence in the flow.
The resulting magnetic power spectrum exhibits a power-law scaling consistent
with the equipartition of the magnetic field with a turbulent velocity field.
The magnetic power spectrum has an apparent knee at the resistive dissipation
scale. Large-scale eddies in the flow cause significant changes to the
instantaneous flow profile resulting in intermittent bursts of non-axisymmetric
magnetic fields, demonstrating that the transition to a dynamo is not smooth
for a turbulent flow.Comment: 9 pages, 11 figures, invited talk by C. B. Forest at 2005 APS DPP
meeting, resubmitted to Physics of Plasma
Preferred foliation effects in Quantum General Relativity
We investigate the infrared (IR) effects of Lorentz violating terms in the
gravitational sector using functional renormalization group methods similar to
Reuter and collaborators. The model we consider consists of pure quantum
gravity coupled to a preferred foliation, described effectively via a scalar
field with non-standard dynamics. We find that vanishing Lorentz violation is a
UV attractive fixed-point of this model in the local potential approximation.
Since larger truncations may lead to differing results, we study as a first
example effects of additional matter fields on the RG running of the Lorentz
violating term and provide a general argument why they are small.Comment: 12 pages, no figures, compatible with published versio
Reductions for the Stable Set Problem
One approach to finding a maximum stable set (MSS) in a graph is to try to reduce the size of the problem by transforming the problem into an equivalent problem on a smaller graph. This paper introduces several new reductions for the MSS problem, extends several well-known reductions to the maximum weight stable set (MWSS) problem, demonstrates how reductions for the generalized stable set problem can be used in conjunction with probing to produce powerful new reductions for both the MSS and MWSS problems, and shows how hypergraphs can be used to expand the capabilities of clique projections. The effectiveness of these new reduction techniques are illustrated on the DIMACS benchmark graphs, planar graphs, and a set of challenging MSS problems arising from Steiner Triple Systems
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