6,152 research outputs found
Entropy of an extremal electrically charged thin shell and the extremal black hole
There is a debate as to what is the value of the the entropy of extremal
black holes. There are approaches that yield zero entropy , while there
are others that yield the Bekenstein-Hawking entropy , in Planck
units. There are still other approaches that give that is proportional to
or even that is a generic well-behaved function of . Here
is the black hole horizon radius and is its horizon area.
Using a spherically symmetric thin matter shell with extremal electric charge,
we find the entropy expression for the extremal thin shell spacetime. When the
shell's radius approaches its own gravitational radius, and thus turns into an
extremal black hole, we encounter that the entropy is , i.e., the
entropy of an extremal black hole is a function of alone. We speculate
that the range of values for an extremal black hole is .Comment: 11 pages, minor changes, added references, matches the published
versio
An Object-Based Approach to Modelling and Analysis of Failure Properties
In protection systems, when traditional technology is replaced by software, the functionality and complexity of the system is likely to increase. The quantitative evidence normally provided for safety certification of traditional systems cannot be relied upon in software-based systems. Instead there is a need to provide qualitative evidence. As a basis for the required qualitative evidence, we propose an object-based approach that allows modelling of both the application and software domains. From the object class model of a system and a formal specification of the failure properties of its components, we generate a graph of failure propagation over object classes, which is then used to generate a graph in terms of object instances in order to conduct fault tree analysis. The model is validated by comparing the resulting minimal cut sets with those obtained from the fault tree analysis of the original system. The approach is illustrated on a case study based on a protection system from..
The Two-Dimensional Analogue of General Relativity
General Relativity in three or more dimensions can be obtained by taking the
limit in the Brans-Dicke theory. In two dimensions
General Relativity is an unacceptable theory. We show that the two-dimensional
closest analogue of General Relativity is a theory that also arises in the
limit of the two-dimensional Brans-Dicke theory.Comment: 8 pages, LaTeX, preprint DF/IST-17.9
Conformal entropy from horizon states: Solodukhin's method for spherical, toroidal, and hyperbolic black holes in D-dimensional anti-de Sitter spacetimes
A calculation of the entropy of static, electrically charged, black holes
with spherical, toroidal, and hyperbolic compact and oriented horizons, in D
spacetime dimensions, is performed. These black holes live in an anti-de Sitter
spacetime, i.e., a spacetime with negative cosmological constant. To find the
entropy, the approach developed by Solodukhin is followed. The method consists
in a redefinition of the variables in the metric, by considering the radial
coordinate as a scalar field. Then one performs a 2+(D-2) dimensional
reduction, where the (D-2) dimensions are in the angular coordinates, obtaining
a 2-dimensional effective scalar field theory. This theory is a conformal
theory in an infinitesimally small vicinity of the horizon. The corresponding
conformal symmetry will then have conserved charges, associated with its
infinitesimal conformal generators, which will generate a classical Poisson
algebra of the Virasoro type. Shifting the charges and replacing Poisson
brackets by commutators, one recovers the usual form of the Virasoro algebra,
obtaining thus the level zero conserved charge eigenvalue L_0, and a nonzero
central charge c. The entropy is then obtained via the Cardy formula.Comment: 21 page
Two-dimensional gravitation and Sine-Gordon-Solitons
Some aspects of two-dimensional gravity coupled to matter fields, especially
to the Sine-Gordon-model are examined. General properties and boundary
conditions of possible soliton-solutions are considered. Analytic
soliton-solutions are discovered and the structure of the induced space-time
geometry is discussed. These solutions have interesting features and may serve
as a starting point for further investigations.Comment: 23 pages, latex, references added, to appear in Phys.Rev.
The Three-Dimensional BTZ Black Hole as a Cylindrical System in Four-Dimensional General Relativity
It is shown how to transform the three dimensional BTZ black hole into a four
dimensional cylindrical black hole (i.e., black string) in general relativity.
This process is identical to the transformation of a point particle in three
dimensions into a straight cosmic string in four dimensions.Comment: Latex, 9 page
Documented international enquiry on solid sedimentary fossil fuels; Coal: definitions, classifications, reserves-resources and energy potential
This paper deals with all solid sedimentary fossil fuels, i.e. coal, the main one for geological reserves and resources, peat,
and oil shales. Definitions of coal ( < 50% ash) and coal seam (thickness and depth limits) are examined in view of an
international agreement regarding new concepts for a common reserves and resources evaluation using the same nomenclature.
The 50% ash limit, already adopted by UN-ECE for coal definition, allows the creation of a new category—the organic
shales (50–75% ash)—comprising energetic materials still valuable for thermal use (coal shales) or to be retorted for oil
production (oil shales).
Geological relations between coals, oil shales, solid bitumen, liquid hydrocarbons, natural gas, and coalbed methane are also
examined together with environmental problems.
As a final synthesis of all topics, the paper discusses the problems related with a modern geological classification of all solid
sedimentary fuels based on: various rank parameters (moisture content, calorific value, reflectance), maceral composition, and
mineral matter content (and washability).
Finally, it should be pointed out that the paper is presented as series of problems, some of them old ones, but never resolved
until now. In order to facilitate the next generation of coal geologists to resolve these problems on the basis of international
agreements, all sections begin with documented introductions for further questions opening an international enquiry. The
authors hope that the answers will be abundant enough and pertinent to permit synthetic international solutions, valuable for the
new millennium, with the help of interested consulted authorities, international pertinent organisations, and regional experts.
D 2002 Elsevier Science B.V. All rights reserved
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