12,611 research outputs found
Relativistic Acoustic Geometry
Sound wave propagation in a relativistic perfect fluid with a non-homogeneous
isentropic flow is studied in terms of acoustic geometry. The sound wave
equation turns out to be equivalent to the equation of motion for a massless
scalar field propagating in a curved space-time geometry. The geometry is
described by the acoustic metric tensor that depends locally on the equation of
state and the four-velocity of the fluid. For a relativistic supersonic flow in
curved space-time the ergosphere and acoustic horizon may be defined in a way
analogous the non-relativistic case. A general-relativistic expression for the
acoustic analog of surface gravity has been found.Comment: 14 pages, LaTe
New representation and a vacuum state for canonical quantum gravity
A new representation for canonical gravity and supergravity is presented,
which combines advantages of Ashtekar's and the Wheeler~DeWitt representation:
it has a nice geometric structure and the singular metric problem is absent. A
formal state functional can be given, which has some typical features of a
vacuum state in quantum field theory. It can be canonically transformed into
the metric representation. Transforming the constraints too, one recovers the
Wheeler~DeWitt equation up to an anomalous term. A modified Dirac quantization
is proposed to handle possible anomalies in the constraint algebra.Comment: 28 pages, LaTe
Minimal Off-Shell Version of N = 1 Chiral Supergravity
We construct the minimal off-shell formulation of N = 1 chiral supergravity
(SUGRA) introducing a complex antisymmetric tensor field and a
complex axial-vector field as auxiliary fields. The resulting algebra
of the right- and left-handed supersymmetry (SUSY) transformations closes off
shell and generates chiral gauge transforamtions and vector gauge
transformations in addition to the transformations which appear in the case
without auxiliary fields.Comment: 9 pages, late
Supersymmetry algebra in N = 1 chiral supergravity
We consider the supersymmetry (SUSY) transformations in the chiral Lagrangian
for supergravity (SUGRA) with the complex tetrad following the method
used in the usual SUGRA, and present the explicit form of the SUSY
trasformations in the first-order form. The SUSY transformations are generated
by two independent Majorana spinor parameters, which are apparently different
from the constrained parameters employed in the method of the 2-form gravity.
We also calculate the commutator algebra of the SUSY transformations on-shell.Comment: 10 pages, late
Signaling, Entanglement, and Quantum Evolution Beyond Cauchy Horizons
Consider a bipartite entangled system half of which falls through the event
horizon of an evaporating black hole, while the other half remains coherently
accessible to experiments in the exterior region. Beyond complete evaporation,
the evolution of the quantum state past the Cauchy horizon cannot remain
unitary, raising the questions: How can this evolution be described as a
quantum map, and how is causality preserved? What are the possible effects of
such nonstandard quantum evolution maps on the behavior of the entangled
laboratory partner? More generally, the laws of quantum evolution under extreme
conditions in remote regions (not just in evaporating black-hole interiors, but
possibly near other naked singularities and regions of extreme spacetime
structure) remain untested by observation, and might conceivably be non-unitary
or even nonlinear, raising the same questions about the evolution of entangled
states. The answers to these questions are subtle, and are linked in unexpected
ways to the fundamental laws of quantum mechanics. We show that terrestrial
experiments can be designed to probe and constrain exactly how the laws of
quantum evolution might be altered, either by black-hole evaporation, or by
other extreme processes in remote regions possibly governed by unknown physics.Comment: Combined, revised, and expanded version of quant-ph/0312160 and
hep-th/0402060; 13 pages, RevTeX, 2 eps figure
Loop Quantum Gravity Modification of the Compton Effect
Modified dispersion relations(MDRs) as a manifestation of Lorentz invariance
violation, have been appeared in alternative approaches to quantum gravity
problem. Loop quantum gravity is one of these approaches which evidently
requires modification of dispersion relations. These MDRs will affect the usual
formulation of the Compton effect. The purpose of this paper is to incorporate
the effects of loop quantum gravity MDRs on the formulation of Compton
scattering. Using limitations imposed on MDRs parameters from Ultra High Energy
Cosmic Rays(UHECR), we estimate the quantum gravity-induced wavelength shift of
scattered photons in a typical Compton process. Possible experimental detection
of this wavelength shift will provide strong support for underlying quantum
gravity proposal.Comment: 12 pages, 2 eps figures, revised versio
Unitarity and the Hilbert space of quantum gravity
Under the premises that physics is unitary and black hole evaporation is
complete (no remnants, no topology change), there must exist a one-to-one
correspondence between states on future null and timelike infinity and on any
earlier spacelike Cauchy surface (e.g., slices preceding the formation of the
hole). We show that these requirements exclude a large set of semiclassical
spacetime configurations from the Hilbert space of quantum gravity. In
particular, the highest entropy configurations, which account for almost all of
the volume of semiclassical phase space, would not have quantum counterparts,
i.e. would not correspond to allowed states in a quantum theory of gravity.Comment: 7 pages, 3 figures, revtex; minor changes in v2 (version published in
Class. Quant. Grav.
Constant Curvature Black Holes
Constant curvature black holes are constructed by identifying points in
anti-de Sitter space. In n dimensions, the resulting topology is R^{n-1} * S_1,
as opposed to the usual R^2 * S_{n-2} Schwarzschild black hole, and the
corresponding causal structure is displayed by a (n-1)-dimensional picture, as
opposed to the usual 2-dimensional Kruskal diagram. The five dimensional case,
which can be embedded in a Chern-Simons supergravity theory, is analyzed in
detail.Comment: New references added and some improvements in the presentation
introduced, 5 pages, 2 eps figures, REVTe
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