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Relativity, or gravitational physics, has widely entered geodetic modelling and parameter determination. This concerns, first
of all, the fundamental reference systems used. The Barycentric Celestial Reference System (BCRS) has to be distinguished
carefully from the Geocentric Celestial Reference System (GCRS), which is the basic theoretical system for geodetic modelling
with a direct link to the International Terrestrial Reference System (ITRS), simply given by a rotation matrix. The relation
to the International Celestial Reference System (ICRS) is discussed, as well as various properties and relevance of these
systems. Then the representation of the gravitational field is discussed when relativity comes into play. Presently, the so-called
post-Newtonian approximation to GRT (general relativity theory) including relativistic effects to lowest order is sufficient
for practically all geodetic applications. At the present level of accuracy, space-geodetic techniques like VLBI (Very Long
Baseline Interferometry), GPS (Global Positioning System) and SLR/LLR (Satellite/Lunar Laser Ranging) have to be modelled
and analysed in the context of a post-Newtonian formalism. In fact, all reference and time frames involved, satellite and
planetary orbits, signal propagation and the various observables (frequencies, pulse travel times, phase and travel-time differences)
are treated within relativity. This paper reviews to what extent the space-geodetic techniques are affected by such a relativistic
treatment and where—vice versa—relativistic parameters can be determined by the analysis of geodetic measurements. At the
end, we give a brief outlook on how new or improved measurement techniques (e.g., optical clocks, Galileo) may further push
relativistic parameter determination and allow for refined geodetic measurements. 相似文献
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Susanna D. Vergani 《Astrophysics and Space Science》2007,311(1-3):197-201
The huge energies involved in gamma-ray bursts (GRBs) coupled with the short emission time scales unavoidably imply that the
emitting source is moving relativistically, with a speed close to that of light. Here we present the REM telescope observations
of the early-time near-infrared light curves of the GRB 060418 and GRB 060607A afterglows. The detection of the afterglow
peak provides for the first time a direct measurement of the initial Lorentz factor Γ0 of the radiating material. We find that the emitting region was indeed highly relativistic in the first seconds after the
explosions, with Γ0∼400. Comparison with the Lorentz factor as determined at later epochs provides direct evidence that the emitting shell is
decelerating and confirms that the afterglow emission is powered by the dissipation of bulk kinetic energy. The deceleration
radius was inferred to be R
dec≈1017 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence
for a different origin of the prompt and afterglow stages of the GRB.
Susanna D. Vergani on behalf of the REM collaboration. 相似文献
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Philip Hardee Yosuke Mizuno Ken-Ichi Nishikawa 《Astrophysics and Space Science》2007,311(1-3):281-286
A new general relativistic magnetohydrodynamics (GRMHD) code “RAISHIN” used to simulate jet generation by rotating and non-rotating
black holes with a geometrically thin Keplarian accretion disk finds that the jet develops a spine-sheath structure in the
rotating black hole case. Spine-sheath structure and strong magnetic fields significantly modify the Kelvin-Helmholtz (KH)
velocity shear driven instability. The RAISHIN code has been used in its relativistic magnetohydrodynamic (RMHD) configuration
to study the effects of strong magnetic fields and weakly relativistic sheath motion, c/2, on the KH instability associated with a relativistic, γ=2.5, jet spine-sheath interaction. In the simulations sound speeds up to
and Alfvén wave speeds up to ∼0.56c are considered. Numerical simulation results are compared to theoretical predictions from a new normal mode analysis of the
RMHD equations. Increased stability of a weakly magnetized system resulting from c/2 sheath speeds and stabilization of a strongly magnetized system resulting from c/2 sheath speeds is found. 相似文献
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Robert L. Oldershaw 《Astrophysics and Space Science》2007,311(4):431-433
The possibility that global discrete dilation invariance is a fundamental symmetry principle of nature is explored. If the
discrete self-similarity observed in nature is exact, then the Principle of General Covariance needs to be broadened in order
to accommodate this form of discrete conformal invariance, and a further generalization of relativity theory is required. 相似文献
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Yoshiaki Kato 《Astrophysics and Space Science》2007,307(1-3):11-15
In spite of the large number of global three-dimensional (3-D) magnetohydrodynamic (MHD) simulations of accretion disks and
astrophysical jets, which have been developed since 2000, the launching mechanisms of jets is somewhat controversial. Previous
studies of jets have concentrated on the effect of the large-scale magnetic fields permeating accretion disks. However, the
existence of such global magnetic fields is not evident in various astrophysical objects, and their origin is not well understood.
Thus, we study the effect of small-scale magnetic fields confined within the accretion disk. We review our recent findings
on the formation of jets in dynamo-active accretion disks by using 3-D MHD simulations. In our simulations, we found the emergence
of accumulated azimuthal magnetic fields from the inner region of the disk (the so-called magnetic tower) and also the formation
of a jet accelerated by the magnetic pressure of the tower. Our results indicate that the magnetic tower jet is one of the
most promising mechanisms for launching jets from the magnetized accretion disk in various astrophysical objects. We will
discuss the formation of cosmic jets in the context of the magnetic tower model. 相似文献
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尤正元 《成都信息工程学院学报》1996,(Z1)
“同时”的相对性是爱因斯坦狭义相对论中一个非常重要的概念。本文用洛仑兹变换式对＂同时＂的相对性作出数学表达，以阐发不同惯性系中时钟不同步的意义，并由此来讨论钟慢效应和尺缩效应的相对性。 相似文献
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R. A. Krikorian 《Astrophysics》2003,46(4):496-501
It is shown that the equation of motion Du
j/Ds = (e/mc
2)F
ji
u
i , a natural generalization to the curved spacetime of the Heaviside-Lorentz law of ponderomotive force, is equivalent to the metric independent and covariant Van Dantzig's equations of motion dx
j [jpi] = 0 or L
v
p
i = 0, where p
i is the conjugate momentum 4-vector and v a vector determined by the condition p
i
v
i, only with respect to holonomic coordinates. With respect to an anholonomic system, the Heaviside-Lorentz equation is a particular case of the VD equations valid for a privileged class of anholonomic frames, those consisting of orthogonal unit vectors. 相似文献
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