30,000 research outputs found
Energy-momentum and angular momentum densities in gauge theories of gravity
In the \bar{\mbox{\rm Poincar\'{e}}} gauge theory of gravity, which has
been formulated on the basis of a principal fiber bundle over the space-time
manifold having the covering group of the proper orthochronous Poincar\'{e}
group as the structure group, we examine the tensorial properties of the
dynamical energy-momentum density and the ` `
spin" angular momentum density of the
gravitational field. They are both space-time vector densities, and transform
as tensors under {\em global} - transformations. Under {\em local}
internal translation, is invariant, while
transforms inhomogeneously. The dynamical
energy-momentum density and the ` ` spin"
angular momentum density of the matter field
are also examined, and they are known to be space-time vector densities and to
obey tensorial transformation rules under internal \bar{\mbox{\rm
Poincar\'{e}}} gauge transformations. The corresponding discussions in
extended new general relativity which is obtained as a teleparallel limit of
\bar{\mbox{\rm Poincar\'{e}}} gauge theory are also given, and
energy-momentum and ` ` spin" angular momentum densities are known to be well
behaved. Namely, they are all space-time vector densities, etc. In both
theories, integrations of these densities on a space-like surface give the
total energy-momentum and {\em total} (={\em spin}+{\em orbital}) angular
momentum for asymptotically flat space-time. The tensorial properties of
canonical energy-momentum and ` ` extended orbital angular momentum" densities
are also examined.Comment: 18 page
On the teleparallel limit of Poincare gauge theory
We will address the question of the consistency of teleparallel theories in
presence of spinning matter which has been a controversial subject of
discussion over the last twenty years. We argue that the origin of the problem
is not simply the symmetry or asymmetry of the stress-energy tensor of the
matter fields, which has been recently analyzed by several authors, but arises
at a more fundamental level, namely from the invariance of the field equatins
under a frame change, a problem that has been discussed long time ago by
Kopczynski in the framework of the teleparallel equivalent of general
relativity. More importantly, we show that the problem is not only confined to
the purely teleparallel theory but arises actually in every Poincare gauge
theory that admits a teleparallel geometry in the absence of spinning sources,
i.e. in its classical limit.Comment: 4 pages, RevTe
Structure and Stability of Magnetic Fields in Solar Active Region12192 Based on Nonlinear Force-Free Field Modeling
We analyze a three-dimensional (3D) magnetic structure and its stability in
large solar active region(AR) 12192, using the 3D coronal magnetic field
constructed under a nonlinear force-free field (NLFFF) approximation. In
particular, we focus on the magnetic structure that produced an X3.1-class
flare which is one of the X-class flares observed in AR 12192. According to our
analysis, the AR contains multiple-flux-tube system, {\it e.g.}, a large flux
tube, both of whose footpoints are anchored to the large bipole field, under
which other tubes exist close to a polarity inversion line (PIL). These various
flux tubes of different sizes and shapes coexist there. In particular, the
later are embedded along the PIL, which produces a favorable shape for the
tether-cutting reconnection and is related to the X-class solar flare. We
further found that most of magnetic twists are not released even after the
flare, which is consistent with the fact that no observational evidence for
major eruptions was found. On the other hand, the upper part of the flux tube
is beyond a critical decay index, essential for the excitation of torus
instability before the flare, even though no coronal mass ejections (CMEs) were
observed. We discuss the stability of the complicated flux tube system and
suggest the reason for the existence of the stable flux tube. In addition, we
further point out a possibility for tracing the shape of flare ribbons, on the
basis of a detailed structural analysis of the NLFFF before a flare.Comment: 24 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Relation between microstructural heterogeneous surface layer and nitrogen pressure during sintering in Si3N4-Ml2O3 ceramics
The effects of N2 pressure (0.1 to 50 MPa) during sintering on the thickness of the microstructurally heterogeneous layer (MHL) formed near the surface of the compact, transverse-rupture strength, were investigated for Si3N4-(10 to 20) mol % MgO-5.5 mol % Al2O3 ceramics. The sintering temperature and time were 1973 K and 3.6 ks, respectively. The N2 gas was introduced into the furnace at about 1273 K. When the compacts were sintered under a certain N2 pressure, for example, about 20 and 7 MPa for 10 and 15 mol% MgO, respectively, the evolutions of N and Si were suppressed. The thickness of the MHL became very small and at the same time the strength of the surface layer of the compact (which was normally less than that of the inside in the case of 0.1 MPa) became nearly the same value as that of the inside. At higher pressure, the strength of both surface layer and the inside decreased considerably. Some discussion was made on these results
Nonlinear cancellation of the parametric resonance in elastic beams: theory and experiment
A non-linear control strategy is applied to a simply supported uniform elastic beam subjected to an axial end force at the principal-parametric resonance frequency of the first skew-symmetric mode. The control input consists of the bending couples applied by two pairs of piezoceramic actuators attached onto both sides of the beam surfaces and symmetrically with respect to the midspan, driven by the same voltage, thus resulting into symmetric control forces. This control architecture has zero control authority, in a linear sense, onto skew-symmetric vibrations. The non-linear transfer of energy from symmetric motions to skew-symmetric modes, due to non-linear inertia and curvature effects, provides the key physical mechanism for channelling suitable control power from the actuators into the linearly uncontrollable mode. The reduced dynamics of the system, constructed with the method of multiple scales directly applied to the governing PDE’s and boundary conditions, suggest effective forms of the control law as a two-frequency input in sub-combination resonance with the parametrically driven mode. The performances of different control laws are investigated. The relative phase and frequency relationships are designed so as to render the control action the most effective. The control schemes generate non-linear controller forces which increase the threshold for the activation of the parametric resonance thus resulting into its annihilation. The theoretical predictions are compared with experimentally obtained results
Massive Spin-2 fields of Geometric Origin in Curved Spacetimes
We study the consistency of a model which includes torsion as well as the
metric as dynamical fields and has massive spin-2 particle in its spectrum. The
massive spin-2 mode resides in the torsion, rather than in the metric. It is
known that this model is tachyon- and ghost-free in Minkowski background. We
show that this property remains valid and no other pathologies emerge in de
Sitter and anti-de Sitter backgrounds, with some of our results extending to
arbirary Einstein space backgrounds. This suggests that the model is
consistent, at least at the classical level, unlike, e.g., the Fierz--Pauli
theory.Comment: 17 pages, Clarifying remarks added in section 5, minor changes,
version to be published in the Phys. Rev.
Universal approximation of multi-copy states and universal quantum lossless data compression
We have proven that there exists a quantum state approximating any multi-copy
state universally when we measure the error by means of the normalized relative
entropy. While the qubit case was proven by Krattenthaler and Slater (IEEE
Trans. IT, 46, 801-819 (2000); quant-ph/9612043), the general case has been
open for more than ten years. For a deeper analysis, we have solved the
mini-max problem concerning `approximation error' up to the second order.
Furthermore, we have applied this result to quantum lossless data compression,
and have constructed a universal quantum lossless data compression
Basic Properties of a Vortex in a Noncentrosymmetric Superconductor
We numerically study the vortex core structure in a noncentrosymmetric
superconductor such as CePt3Si without mirror symmetry about the xy plane.
A single vortex along the z axis and a mixed singlet-triplet Cooper pairing
model are considered.
The spatial profiles of the pair potential, local density of states,
supercurrent density, and radially-textured magnetic moment density around the
vortex are obtained in the clean limit on the basis of the quasiclassical
theory of superconductivity.Comment: 6 pages; submitted to Proc. of VORTEX I
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