10,870 research outputs found
Non-equilibrium Thermodynamics of Spacetime: the Role of Gravitational Dissipation
In arXiv:gr-qc/9504004 it was shown that the Einstein equation can be derived
as a local constitutive equation for an equilibrium spacetime thermodynamics.
More recently, in the attempt to extend the same approach to the case of
theories of gravity, it was found that a non-equilibrium setting is indeed
required in order to fully describe both this theory as well as classical GR
(arXiv:gr-qc/0602001). Here, elaborating on this point, we show that the
dissipative character leading to a non-equilibrium spacetime thermodynamics is
actually related -- both in GR as well as in gravity -- to non-local
heat fluxes associated with the purely gravitational/internal degrees of
freedom of the theory. In particular, in the case of GR we show that the
internal entropy production term is identical to the so called tidal heating
term of Hartle-Hawking. Similarly, for the case of gravity, we show that
dissipative effects can be associated with the generalization of this term plus
a scalar contribution whose presence is clearly justified within the
scalar-tensor representation of the theory. Finally, we show that the allowed
gravitational degrees of freedom can be fixed by the kinematics of the local
spacetime causal structure, through the specific Equivalence Principle
formulation. In this sense, the thermodynamical description seems to go beyond
Einstein's theory as an intrinsic property of gravitation.Comment: 13 pages, 1 figur
Entropy and Topology for Gravitational Instantons
In this work a relation between topology and thermodynamical features of
gravitational instantons is shown. The expression for the Euler characteristic,
through the Gauss-Bonnet integral, and the one for the entropy of gravitational
instantons are proposed in a form that makes the relation between them
self-evident. A new formulation of the Bekenstein-Hawking formula, where the
entropy and the Euler characteristic are related by , is obtained.
This formula provides the correct results for a wide class of gravitational
instantons described by both spherically and axially symmetric metrics.Comment: 25 pages, RevTeX, accepted for publication in Phys. Rev.
Dynamic distributed clustering in wireless sensor networks via Voronoi tessellation control
This paper presents two dynamic and distributed clustering algorithms for Wireless Sensor Networks (WSNs). Clustering approaches are used in WSNs to improve the network lifetime and scalability by balancing the workload among the clusters. Each cluster is managed by a cluster head (CH) node. The first algorithm requires the CH nodes to be mobile: by dynamically varying the CH node positions, the algorithm is proved to converge to a specific partition of the mission area, the generalised Voronoi tessellation, in which the loads of the CH nodes are balanced. Conversely, if the CH nodes are fixed, a weighted Voronoi clustering approach is proposed with the same load-balancing objective: a reinforcement learning approach is used to dynamically vary the mission space partition by controlling the weights of the Voronoi regions. Numerical simulations are provided to validate the approaches
Sonoluminescence: Two-photon correlations as a test of thermality
In this Letter we propose a fundamental test for probing the thermal nature
of the spectrum emitted by sonoluminescence. We show that two-photon
correlations can in principle discriminate between real thermal light and the
quasi-thermal squeezed-state photons typical of models based on the dynamic
Casimir effect. Two-photon correlations provide a powerful experimental test
for various classes of sonoluminescence models.Comment: 6 pages, revtex 3; revised to include more discussion of finite
volume effects; physics conclusions unchanged; to appear in Physics Letters
On pseudo-bialgebras
We study pseudoalgebras from the point of view of pseudo-dual of classical
Lie coalgebra structures. We define the notions of Lie H-coalgebra and Lie
pseudo-bialgebra. We obtain the analog of the CYBE, the Manin triples and
Drinfeld's double for Lie pseudo-bialgebras. We also get a natural description
of the annihilation algebra associated to a pseudoalgebra as a convolution
algebra, clarifying this constructions in the theory of pseudoalgebras.Comment: arXiv admin note: substantial text overlap with arXiv:math/000712
Perturbative superluminal censorship and the null energy condition
We argue that ``effective'' superluminal travel, potentially caused by the
tipping over of light cones in Einstein gravity, is always associated with
violations of the null energy condition (NEC). This is most easily seen by
working perturbatively around Minkowski spacetime, where we use linearized
Einstein gravity to show that the NEC forces the light cones to contract
(narrow). Given the NEC, the Shapiro time delay in any weak gravitational field
is always a delay relative to the Minkowski background, and never an advance.
Furthermore, any object travelling within the lightcones of the weak
gravitational field is similarly delayed with respect to the minimum traversal
time possible in the background Minkowski geometry.Comment: 5 pages. Uses AIP proceedings style (aipproc.sty). To appear in the
Proceedings of the Eighth Canadian Conference on General Relativity and
Relativistic Astrophysics. (McGill University, Montreal, June 1999). To be
published by AIP Pres
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