3,218 research outputs found
Particle creation by moving spherical shell in the dynamical Casimir effect
The creation of massless scalar particles from the quantum vacuum by
spherical shell with time varying radius is studied. In the general case of
motion the equations are derived for the instantaneous basis expansion
coefficients. The examples are considered when the mean number of particles can
be explicitly evaluated in the adiabatic approximation.Comment: 9 pages, LaTeX, no figures, typos corrected, discussion added.
Journal-ref adde
Black hole entropy as T-duality invariant
We study the Euler numbers and the entropies of the non-extremal intersecting
D-branes in ten-dimensions. We use the surface gravity to constrain the
compactification radii. We correctly obtain the integer valued Euler numbers
for these radii. Moreover, the entropies are found to be invariant under the
T-duality transformation. In the extremal limit, we obtain the finite entropies
only for two intersecting D-branes. We observe that these entropies are
proportional to the product of the charges of each D-brane. We further study
the entropies of the boosted metrics. We find that their entropies can be
interpreted in term of the microscopic states of D-branes.Comment: 15 pages, Revte
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Fetal midline anomalies: Diagnosis and counselling part 2: Septal anomalies.
Irreducible modules over finite simple Lie conformal superalgebras of type K
We construct all finite irreducible modules over Lie conformal superalgebras
of type KComment: Accepted for publication in J. Math. Phys
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Fetal midline anomalies: Diagnosis and counselling Part 1: Corpus callosum anomalies.
Midline anomalies encompasses a heterogeneous group of conditions caused by an abnormal process of ventral induction after the end of primary neurulation. Advances in prenatal imaging techniques have led to an increase in the detection rate of such anomalies since the first trimester of pregnancy although a significant proportion of them remain undiagnosed until birth. Ultrasound is the primary technique in detecting such anomalies while fetal magnetic resonance imaging (MRI) is commonly performed to confirm the diagnosis and detect additional anomalies, especially those involving the cortical surface of the brain, which may potentially impact post-natal outcome. Neurodevelopmental outcome of cerebral anomalies involving the midline is directly related to the type of anomaly, cause and presence of associated anomalies. However, even in case of isolated anomalies prenatal counselling is challenging. The aim of this review is to provide an up to date on the diagnosis, counselling and management of the most common supra-tentorial anomalies involving the midline and diagnosed on prenatal ultrasound
Field equations from a surface term
As is well known, in order for the Einstein--Hilbert action to have a well
defined variation, and therefore to be used for deriving field equation through
the stationary action principle, it has to be amended by the addition of a
suitable boundary term. It has recently been claimed that, if one constructs an
action by adding this term to the matter action, the Einstein field equations
can be derived by requiring this action to be invariant under active
transformations which are normal to a null boundary. In this paper we
re-examine this approach both for the case of pure gravity and in the presence
of matter. We show that in the first case this procedure holds for more general
actions than the Einstein-Hilbert one and trace the basis of this remarkable
attribute. However, it is also pointed out the when matter is rigorously
considered the approach breaks down. The reasons for that are thoroughly
discussed.Comment: Typos corrected, minor changes to match published versio
Optimal Control of Industrial Assembly Lines
This paper discusses the problem of assembly line control and introduces an optimal control formulation that can be used to improve the performance of the assembly line, in terms of cycle time minimization, resources' utilization, etc. A deterministic formulation of the problem is introduced, based on mixed-integer linear programming. A simple numerical simulation provides a first proof of the proposed concept
Sonoluminescence as a QED vacuum effect. II: Finite Volume Effects
In a companion paper [quant-ph/9904013] we have investigated several
variations of Schwinger's proposed mechanism for sonoluminescence. We
demonstrated that any realistic version of Schwinger's mechanism must depend on
extremely rapid (femtosecond) changes in refractive index, and discussed ways
in which this might be physically plausible. To keep that discussion tractable,
the technical computations in that paper were limited to the case of a
homogeneous dielectric medium. In this paper we investigate the additional
complications introduced by finite-volume effects. The basic physical scenario
remains the same, but we now deal with finite spherical bubbles, and so must
decompose the electromagnetic field into Spherical Harmonics and Bessel
functions. We demonstrate how to set up the formalism for calculating Bogolubov
coefficients in the sudden approximation, and show that we qualitatively retain
the results previously obtained using the homogeneous-dielectric (infinite
volume) approximation.Comment: 23 pages, LaTeX 209, ReV-TeX 3.2, five figure
Theory of gravitation theories: a no-progress report
Already in the 1970s there where attempts to present a set of ground rules,
sometimes referred to as a theory of gravitation theories, which theories of
gravity should satisfy in order to be considered viable in principle and,
therefore, interesting enough to deserve further investigation. From this
perspective, an alternative title of the present paper could be ``why are we
still unable to write a guide on how to propose viable alternatives to general
relativity?''. Attempting to answer this question, it is argued here that
earlier efforts to turn qualitative statements, such as the Einstein
Equivalence Principle, into quantitative ones, such as the metric postulates,
stand on rather shaky grounds -- probably contrary to popular belief -- as they
appear to depend strongly on particular representations of the theory. This
includes ambiguities in the identification of matter and gravitational fields,
dependence of frequently used definitions, such as those of the stress-energy
tensor or classical vacuum, on the choice of variables, etc. Various examples
are discussed and possible approaches to this problem are pointed out. In the
course of this study, several common misconceptions related to the various
forms of the Equivalence Principle, the use of conformal frames and equivalence
between theories are clarified.Comment: Invited paper in the Gravity Research Foundation 2007 special issue
to be published by Int. J. Mod. Phys.
Black Hole Graybody Factor and Black Hole Entropy
We have proposed the entropy formula of the black hole which is constructed
by the intersecting D1-brane and D5-brane with no momentum, whose
compactification radii are constrained by the surface gravities in
ten-dimensions. We interpret the entropy of the black hole as the statistical
entropy of the effective string living on the D5-brane. We further study the
behavior of the absorption cross-section of the black hole using our entropy
formula.Comment: 15 pages, RevTex, version to appear in PR
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