36,153 research outputs found
Entropy and Mass Bounds of Kerr-de Sitter Spacetimes
We consider Kerr-de Sitter spacetimes and evaluate their mass, angular
momentum and entropy according to the boundary counterterm prescription. We
provide a physicall interpretation for angular velocity and angular momentum at
future/past infinity. We show that the entropy of the four-dimensional Kerr-de
Sitter spacetimes is less than of pure de Sitter spacetime in agreement to the
entropic N-bound. Moreover, we show that maximal mass conjecture which states
any asymptotically de Sitter spacetime with mass greater than de Sitter has a
cosmological singularity is respected by asymptotically de Sitter spacetimes
with rotation. We furthermore consider the possibility of strengthening the
conjecture to state that any asymptotically dS spacetime will have mass greater
than dS if and only if it has a cosmological singularity and find that Kerr-de
Sitter spacetimes do not respect this stronger statement. We investigate the
behavior of the c-function for the Kerr-de Sitter spacetimes and show that it
is no longer isotropic. However an average of the c-function over the angular
variables yields a renormalization group flow in agreement with the expansion
of spacetime at future infinity.Comment: 13 pages, 3 figures, one figure added, typos correcte
Action, Mass and Entropy of Schwarzschild-de Sitter black holes and the de Sitter/CFT Correspondence
We investigate a recent proposal for defining a conserved mass in
asymptotically de Sitter spacetimes that is based on a conjectured holographic
duality between such spacetimes and Euclidean conformal field theory. We show
that an algorithm for deriving such terms in asymptotically anti de Sitter
spacetimes has an asymptotically de Sitter counterpart, and derive the explicit
form for such terms up to 9 dimensions. We show that divergences of the
on-shell action for de Sitter spacetime are removed in any dimension in
inflationary coordinates, but in covering coordinates a linear divergence
remains in odd dimensions that cannot be cancelled by local terms that are
polynomial in boundary curvature invariants. We show that the class of
Schwarzschild-de Sitter black holes up to 9 dimensions has finite action and
conserved mass, and construct a definition of entropy outside the cosmological
horizon by generalizing the Gibbs-Duhem relation in asymptotically dS
spacetimes. The entropy is agreement with that obtained from CFT methods in
. In general our results provide further supporting evidence for a dS/CFT
correspondence, although some important interpretive problems remain.Comment: 16 pages, LaTeX, typos correcte
Quasiclassical Equations of Motion for Nonlinear Brownian Systems
Following the formalism of Gell-Mann and Hartle, phenomenological equations
of motion are derived from the decoherence functional formalism of quantum
mechanics, using a path-integral description. This is done explicitly for the
case of a system interacting with a ``bath'' of harmonic oscillators whose
individual motions are neglected. The results are compared to the equations
derived from the purely classical theory. The case of linear interactions is
treated exactly, and nonlinear interactions are compared using classical and
quantum perturbation theory.Comment: 24 pages, CALT-68-1848 (RevTeX 2.0 macros
A Note on the Topology of Space-time in Special Relativity
We show that a topology can be defined in the four dimensional space-time of
special relativity so as to obtain a topological semigroup for time. The
Minkowski 4-vector character of space-time elements as well as the key
properties of special relativity are still the same as in the standard theory.
However, the new topological structure allows the possibility of an intrinsic
asymmetry in the time evolution of physical systems
Research and development at ORNL/CESAR towards cooperating robotic systems for hazardous environments
One of the frontiers in intelligent machine research is the understanding of how constructive cooperation among multiple autonomous agents can be effected. The effort at the Center for Engineering Systems Advanced Research (CESAR) at the Oak Ridge National Laboratory (ORNL) focuses on two problem areas: (1) cooperation by multiple mobile robots in dynamic, incompletely known environments; and (2) cooperating robotic manipulators. Particular emphasis is placed on experimental evaluation of research and developments using the CESAR robot system testbeds, including three mobile robots, and a seven-axis, kinematically redundant mobile manipulator. This paper summarizes initial results of research addressing the decoupling of position and force control for two manipulators holding a common object, and the path planning for multiple robots in a common workspace
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