3,858 research outputs found
Quantum Black Holes
Static solutions of large- quantum dilaton gravity in dimensions are
analyzed and found to exhibit some unusual behavior. As expected from previous
work, infinite-mass solutions are found describing a black hole in equilibrium
with a bath of Hawking radiation. Surprisingly, the finite mass solutions are
found to approach zero coupling both at the horizon and spatial infinity, with
a ``bounce'' off of strong coupling in between. Several new zero mass solutions
-- candidate quantum vacua -- are also described.Comment: 14 pages + 6 figure
Dirichlet Boundary State in Linear Dilaton Background
Dirichlet-branes have emerged as important objects in studying
nonperturbative string theory. It is important to generalize these objects to
more general backgrounds other than the usual flat background. The simplest
case is the linear dilaton condensate. The usual Dirichlet boundary condition
violates conformal invariance in such a background. We show that by switching
on a certain boundary interaction, conformal invariance is restored. An
immediate application of this result is to two dimensional string theory.Comment: 6 pages, harvmac, some remarks are modified and one reference is
added, formulas remain the sam
Loop Variables and Gauge Invariance in (Open) Bosonic String Theory
We give a simplified and more complete description of the loop variable
approach for writing down gauge invariant equations of motion for the fields of
the open string. A simple proof of gauge invariance to all orders is given. In
terms of loop variables, the interacting equations look exactly like the free
equations, but with a loop variable depending on an extra parameter, thus
making it a band of finite width. The arguments for gauge invariance work
exactly as in the free case. We show that these equations are Wilsonian RG
equations with a finite world-sheet cutoff and that in the infrared limit,
equivalence with the Callan-Symanzik -functions should ensure that they
reproduce the on-shell scattering amplitudes in string theory. It is applied to
the tachyon-photon system and the general arguments for gauge invariance can be
easily checked to the order calculated. One can see that when there is a finite
world sheet cutoff in place, even the U(1) invariance of the equations for the
photon, involves massive mode contributions. A field redefinition involving the
tachyon is required to get the gauge transformations of the photon into
standard form.Comment: 20 pages, Late
Active Gel Model of Amoeboid Cell Motility
We develop a model of amoeboid cell motility based on active gel theory.
Modeling the motile apparatus of a eukaryotic cell as a confined layer of
finite length of poroelastic active gel permeated by a solvent, we first show
that, due to active stress and gel turnover, an initially static and
homogeneous layer can undergo a contractile-type instability to a polarized
moving state in which the rear is enriched in gel polymer. This agrees
qualitatively with motile cells containing an actomyosin-rich uropod at their
rear. We find that the gel layer settles into a steadily moving, inhomogeneous
state at long times, sustained by a balance between contractility and filament
turnover. In addition, our model predicts an optimal value of the
gel-susbstrate adhesion leading to maximum layer speed, in agreement with cell
motility assays. The model may be relevant to motility of cells translocating
in complex, confining environments that can be mimicked experimentally by cell
migration through microchannels.Comment: To appear in New Journal of Physic
The Virtual Black Hole in 2d Quantum Gravity and its Relevance for the S-matrix
As shown recently 2d quantum gravity theories -- including spherically
reduced Einstein-gravity -- after an exact path integral of its geometric part
can be treated perturbatively in the loops of (scalar) matter. Obviously the
classical mechanism of black hole formation should be contained in the tree
approximation of the theory. This is shown to be the case for the scattering of
two scalars through an intermediate state which by its effective black hole
mass is identified as a "virtual black hole". We discuss the lowest order tree
vertex for minimally and non-minimally coupled scalars and find a non-trivial
finite S-matrix for gravitational s-wave scattering in the latter case.Comment: 4 pages, Talk given at the Fifth Workshop on "Quantum Field Theory
under the Influence of External Conditions" in Leipzig, Sept. 200
S-Wave Scattering of Charged Fermions by a Magnetic Black Hole
We argue that, classically, -wave electrons incident on a magnetically
charged black hole are swallowed with probability one: the reflection
coefficient vanishes. However, quantum effects can lead to both electromagnetic
and gravitational backscattering. We show that, for the case of extremal,
magnetically charged, dilatonic black holes and a single flavor of low-energy
charged particles, this backscattering is described by a perturbatively
computable and unitary -matrix, and that the Hawking radiation in these
modes is suppressed near extremality. The interesting and much more difficult
case of several flavors is also discussed.Comment: 9p
CT-duality as a local property of the world-sheet
In the present article, we study the local features of the world-sheet in the
case when probe bosonic string moves in antisymmetric background field. We
generalize the geometry of surfaces embedded in space-time to the case when the
torsion is present. We define the mean extrinsic curvature for spaces with
Minkowski signature and introduce the concept of mean torsion. Its orthogonal
projection defines the dual mean extrinsic curvature. In this language, the
field equation is just the equality of mean extrinsic curvature and extrinsic
mean torsion, which we call CT-duality. To the world-sheet described by this
relation we will refer as CT-dual surface.Comment: Latex, 15 pages, 2 Figure
A class of non-singular gravi-dilaton backgrounds
We present a class of static, spherically symmetric, non-singular solutions
of the tree-level string effective action, truncated to first order in
. In the string frame the solutions approach asymptotically (at and ) two different anti-de Sitter configurations, thus
interpolating between two maximally symmetric states of different constant
curvature. The radial-dependent dilaton defines a string coupling which is
everywhere finite, with a peak value that can be chosen arbitrarily small so as
to neglect quantum-loop corrections. This example stresses the possible
importance of finite-size corrections, typical of string theory, in
avoiding space-time singularities.Comment: 9 pages, LATEX, four figure included using EPSFIG. Essay written for
the 1997 Awards of the Gravity Research Foundation, and selected for
"Honorable Mention". To appear in Mod. Phys. Lett.
Exact C=1 Boundary Conformal Field Theories
We present a solution of the problem of a free massless scalar field on the
half line interacting through a periodic potential on the boundary. For a
critical value of the period, this system is a conformal field theory with a
non-trivial and explicitly calculable S-matrix for scattering from the
boundary. Unlike all other exactly solvable conformal field theories, it is
non-rational ({\it i.e.} has infinitely many primary fields). It describes the
critical behavior of a number of condensed matter systems, including
dissipative quantum mechanics and of barriers in ``quantum wires''.Comment: harvmac, 10 pages, PUPT-1432/IASSNS-HEP-93/7
Information Loss and Anomalous Scattering
The approach of 't Hooft to the puzzles of black hole evaporation can be
applied to a simpler system with analogous features. The system is
dimensional electrodynamics in a linear dilaton background. Analogues of black
holes, Hawking radiation and evaporation exist in this system. In perturbation
theory there appears to be an information paradox but this gets resolved in the
full quantum theory and there exists an exact -matrix, which is fully
unitary and information conserving. 't Hooft's method gives the leading terms
in a systematic approximation to the exact result.Comment: 18 pages, 3 figures (postscript files available soon on request),
(earlier version got corrupted by mail system
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