2,292 research outputs found
How can exact and approximate solutions of Einstein's field equations be compared?
The problem of comparison of the stationary axisymmetric vacuum solutions
obtained within the framework of exact and approximate approaches for the
description of the same general relativistic systems is considered. We suggest
two ways of carrying out such comparison: (i) through the calculation of the
Ernst complex potential associated with the approximate solution whose form on
the symmetry axis is subsequently used for the identification of the exact
solution possessing the same multipole structure, and (ii) the generation of
approximate solutions from exact ones by expanding the latter in series of
powers of a small parameter. The central result of our paper is the derivation
of the correct approximate analogues of the double-Kerr solution possessing the
physically meaningful equilibrium configurations. We also show that the
interpretation of an approximate solution originally attributed to it on the
basis of some general physical suppositions may not coincide with its true
nature established with the aid of a more accurate technique.Comment: 32 pages, 5 figure
Remarks on the Charged, Magnetized Tomimatsu-Sato Solution
The full metric describing a charged, magnetized generalization of the
Tomimatsu-Sato (TS) solution is presented in a concise explicit
form. We use it to investigate some physical properties of the solution; in
particular, we point out the existence of naked ring singularities in the
hyperextreme TS metrics, the fact previously overlooked by the researchers, and
we also demonstrate that the ring singularities can be eliminated by
sufficiently strong magnetic fields in the subextreme case, while in the
hyperextreme case the magnetic field can move singularities to the equatorial
plane.Comment: 12 pages, 3 ps figures, LaTeX with epsf style (necessary files are
included
Bell's inequalities in the tomographic representation
The tomographic approach to quantum mechanics is revisited as a direct tool
to investigate violation of Bell-like inequalities. Since quantum tomograms are
well defined probability distributions, the tomographic approach is emphasized
to be the most natural one to compare the predictions of classical and quantum
theory. Examples of inequalities for two qubits an two qutrits are considered
in the tomographic probability representation of spin states.Comment: 11 pages, comments and references adde
Frame dragging, vorticity and electromagnetic fields in axially symmetric stationary spacetimes
We present a general study about the relation between the vorticity tensor
and the Poynting vector of the electromagnetic field for axially symmetric
stationary electrovacuum metrics. The obtained expressions allow to understand
the role of the Poynting vector in the dragging of inertial frames. The
particular case of the rotating massive charged magnetic dipole is analyzed in
detail. In addition, the electric and magnetic parts of the Weyl tensor are
calculated and the link between the later and the vorticity is established.
Then we show that, in the vacuum case, the necessary and sufficient condition
for the vanishing of the magnetic part is that the spacetime be static.Comment: 16 pages Latex. Some minor changes in the text and typos correcte
Evaluation of RGB-D Multi-Camera Pose Estimation for 3D Reconstruction
Advances in visual sensor devices and computing power are revolutionising the interaction of robots with their environment. Cameras that capture depth information along with a common colour image play a significant role. These devices are cheap, small, and fairly precise. The information provided, particularly point clouds, can be generated in a virtual computing environment, providing complete 3D information for applications. However, off-the-shelf cameras often have a limited field of view, both on the horizontal and vertical axis. In larger environments, it is therefore often necessary to combine information from several cameras or positions. To concatenate multiple point clouds and generate the complete environment information, the pose of each camera must be known in the outer scene, i.e., they must reference a common coordinate system. To achieve this, a coordinate system must be defined, and then every device must be positioned according to this coordinate system. For cameras, a calibration can be performed to find its pose in relation to this coordinate system. Several calibration methods have been proposed to solve this challenge, ranging from structured objects such as chessboards to features in the environment. In this study, we investigate how three different pose estimation methods for multi-camera perspectives perform when reconstructing a scene in 3D. We evaluate the usage of a charuco cube, a double-sided charuco board, and a robot’s tool centre point (TCP) position in a real usage case, where precision is a key point for the system. We define a methodology to identify the points in the 3D space and measure the root-mean-square error (RMSE) based on the Euclidean distance of the actual point to a generated ground-truth point. The reconstruction carried out using the robot’s TCP position produced the best result, followed by the charuco cuboid; the double-sided angled charuco board exhibited the worst performance
Impact of the «Simbiter» group multiprobiotics on the contents of monomers of bindweb structures in the periodontal tissues in conditions of prolonged hypoacidity
In conditions of prolonged omeprazole-induced hypoacidity in the periodontal tissues of rats an increased catabolism of collagenous and non- collagenous proteins is occurred, The "Simbiter acidophilic concentrated" and "Simbiter-omega" multiprobiotics prevent depolymerization of collagenous and non-collagenous structures of periodontal connective tissues of rats in conditions of prolonged hypoacidity
Coherent states for Hamiltonians generated by supersymmetry
Coherent states are derived for one-dimensional systems generated by
supersymmetry from an initial Hamiltonian with a purely discrete spectrum for
which the levels depend analytically on their subindex. It is shown that the
algebra of the initial system is inherited by its SUSY partners in the subspace
associated to the isospectral part or the spectrum. The technique is applied to
the harmonic oscillator, infinite well and trigonometric Poeschl-Teller
potentials.Comment: LaTeX file, 26 pages, 3 eps figure
General-relativistic Model of Magnetically Driven Jet
The general scheme for the construction of the general-relativistic model of
the magnetically driven jet is suggested. The method is based on the usage of
the 3+1 MHD formalism. It is shown that the critical points of the flow and the
explicit radial behavior of the physical variables may be derived through the
jet ``profile function."Comment: 12 pages, LaTex, no figure
Energy Release During Disk Accretion onto a Rapidly Rotating Neutron Star
The energy release L_s on the surface of a neutron star (NS) with a weak
magnetic field and the energy release L_d in the surrounding accretion disk
depend on two independent parameters that determine its state (for example,
mass M and cyclic rotation frequency f) and is proportional to the accretion
rate. We derive simple approximation formulas illustrating the dependence of
the efficiency of energy release in an extended disk and in a boundary layer
near the NS surface on the frequency and sense of rotation for various NS
equations of state. Such formulas are obtained for the quadrupole moment of a
NS, for a gap between its surface and a marginally stable orbit, for the
rotation frequency in an equatorial Keplerian orbit and in the marginally
stable circular orbit, and for the rate of NS spinup via disk accretion. In the
case of NS and disk counterrotation, the energy release during accretion can
reach . The sense of NS rotation is a factor that strongly
affects the observed ratio of nuclear energy release during bursts to
gravitational energy release between bursts in X-ray bursters. The possible
existence of binary systems with NS and disk counterrotation in the Galaxy is
discussed. Based on the static criterion for stability, we present a method of
constructing the dependence of gravitational mass M on Kerr rotation parameter
j and on total baryon mass (rest mass) m for a rigidly rotating neutron star.
We show that all global NS characteristics can be expressed in terms of the
function M(j, m) and its derivatives.Comment: 42 pages, 12 figures, to appear in Astronomy Letters, 2000, v.26,
p.69
Strange meson production in Al+Al collisions at 1.9A GeV
The production of K, K and (1020) mesons is studied in Al+Al
collisions at a beam energy of 1.9A GeV which is close or below the production
threshold in NN reactions. Inverse slopes, anisotropy parameters, and total
emission yields of K mesons are obtained. A comparison of the ratio of
kinetic energy distributions of K and K mesons to the HSD transport
model calculations suggests that the inclusion of the in-medium modifications
of kaon properties is necessary to reproduce the ratio. The inverse slope and
total yield of mesons are deduced. The contribution to K production
from meson decays is found to be [17 3 (stat) (syst)]
%. The results are in line with previous K and data obtained for
different colliding systems at similar incident beam energies.Comment: 16 pages, 11 figure
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