284 research outputs found
Numerical Modelling of Satellite Downlink Signals in a Finslerian-Perturbed Schwarzschild Spacetime
The work presented in this paper aims to contribute to the problem of testing
Finsler gravity theories by means of experiments and observations in the solar
system. Within a class of spherically symmetric static Finsler spacetimes we
consider a satellite with an on-board atomic clock, orbiting in the
Finslerian-perturbed gravitational field of the earth, whose time signal is
transmitted to a ground station, where its receive time and frequency are
measured with respect to another atomic clock. This configuration is realized
by the Galileo 5 and 6 satellites that have gone astray and are now on
non-circular orbits. Our method consists in the numerical integration of the
satellite's orbit, followed by an iterative procedure which provides the
numerically integrated signals, i.e., null geodesics, from the satellite to the
ground station. One of our main findings is that for orbits that are
considerably more eccentric than the Galileo 5 and 6 satellite orbits,
Finslerian effects can be separated from effects of perturbations of the
Schwarzschild spacetime within the Lorentzian geometry. We also discuss the
separation from effects of non-gravitational perturbations. This leads us to
the conclusion that observations of this kind combined with appropriate
numerical modelling can provide suitable tests of Finslerian modifications of
general relativity
A Morse-theoretical analysis of gravitational lensing by a Kerr-Newman black hole
Consider, in the domain of outer communication of a Kerr-Newman black hole, a
point (observation event) and a timelike curve (worldline of light source).
Assume that the worldline of the source (i) has no past end-point, (ii) does
not intersect the caustic of the past light-cone of the observation event, and
(iii) goes neither to the horizon nor to infinity in the past. We prove that
then for infinitely many positive integers k there is a past-pointing lightlike
geodesic of (Morse) index k from the observation event to the worldline of the
source, hence an observer at the observation event sees infinitely many images
of the source. Moreover, we demonstrate that all lightlike geodesics from an
event to a timelike curve in the domain of outer communication are confined to
a certain spherical shell. Our characterization of this spherical shell shows
that in the Kerr-Newman spacetime the occurrence of infinitely many images is
intimately related to the occurrence of centrifugal-plus-Coriolis force
reversal.Comment: 14 pages, 2 figures; REVTEX; submitted to J. Math. Phy
Sollte der Stabilitäts- und Wachstumspakt geändert werden?
Die EU-Kommission hatte im Januar 2002 die Haushaltslage in Deutschland zum Anlass genommen, dem Ecofin-Rat zu empfehlen, gegenüber Deutschland eine Frühwarnung auszusprechen. Der so genannte „Blaue Brief" des Rates konnte auf Intervention der Bundesregierung vermieden werden. Die Glaubwürdigkeit des europäischen Stabilitäts- und Wachstumspaktes wurde damit jedoch beschädigt. Sollte der Pakt geändert werden? --
Gravitational lensing in spherically symmetric static spacetimes with centrifugal force reversal
In Schwarzschild spacetime the value of the radius coordinate is
characterized by three different properties: (a) there is a ``light sphere'',
(b) there is ``centrifugal force reversal'', (c) it is the upper limiting
radius for a non-transparent Schwarschild source to act as a gravitational lens
that produces infinitely many images. In this paper we prove a theorem to the
effect that these three properties are intimately related in {\em any}
spherically symmetric static spacetime. We illustrate the general results with
some examples including black-hole spacetimes and Morris-Thorne wormholes.Comment: 18 pages, 3 eps-figure
The determination of the electron-phonon interaction from tunneling data in the two-band superconductor MgB2
We calculate the tunneling density of states (DOS) of MgB2 for different
tunneling directions, by directly solving the real-axis, two-band Eliashberg
equations (EE). Then we show that the numeric inversion of the standard
single-band EE, if applied to the DOS of the two-band superconductor MgB2, may
lead to wrong estimates of the strength of certain phonon branches (e.g. the
E_2g) in the extracted electron-phonon spectral function alpha^(2)F(omega). The
fine structures produced by the two-band interaction turn out to be clearly
observable only for tunneling along the ab planes in high-quality single
crystals. The results are compared to recent experimental data.Comment: 2 pages, 2 figures, proceedings of M2S-HTSC-VII conference, Rio de
Janeiro (May 2003
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