398 research outputs found
Spacetime: Arena or Reality?
For small values of the mass (in relation to the angular momentum and
electric charge), the Kerr-Newman (KN) solution of Einstein equation reduces to
a naked singularity of circular shape. By considering the Hawking and Ellis
extended interpretation of the KN spacetime, as well as Wheeler's idea of
"charge without charge", the non-trivial topological structure of the extended
KN spatial section is found to represent gravitational states with
half-integral angular momentum. As a consequence, it can be consistently
interpreted as a model for the electron-positron system, in which the concepts
of mass, charge and spin emerge from the spacetime geometry. According to this
model, therefore, instead of a simple arena, spacetime must have a concrete
existence, being responsible -- through its highly non-trivial topological
structures -- for the building blocks of (at least some of) the existing matter
in the universe.Comment: Chapter in the book "Relativity and the Dimensionality of the World",
Springer series "Fundamental Theories of Physics", Vol. 153 (2007). Volume
Editor: Vesselin Petko
Kerr-Newman solution as a Dirac particle
For m^2 < a^2 + q^2, with m, a, and q respectively the source mass, angular
momentum per unit mass, and electric charge, the Kerr--Newman (KN) solution of
Einstein's equation reduces to a naked singularity of circular shape, enclosing
a disk across which the metric components fail to be smooth. By considering the
Hawking and Ellis extended interpretation of the KN spacetime, it is shown
first that, similarly to the electron-positron system, this solution presents
four inequivalent classical states. Next, it is shown that due to the
topological structure of the extended KN spacetime it does admit states with
half-integral angular momentum. This last property is corroborated by the fact
that, under a rotation of the space coordinates, those inequivalent states
transform into themselves only after a 4pi rotation. As a consequence, it
becomes possible to naturally represent them in a Lorentz spinor basis. The
state vector representing the whole KN solution is then constructed, and its
evolution is shown to be governed by the Dirac equation. The KN solution can
thus be consistently interpreted as a model for the electron-positron system,
in which the concepts of mass, charge and spin become connected with the
spacetime geometry. Some phenomenological consequences of the model are
explored.Comment: 19 pages, 6 figures. References added, section 2 enhanced, an
appendix and one figure adde
Torsion and Gravitation: A new view
According to the teleparallel equivalent of general relativity, curvature and
torsion are two equivalent ways of describing the same gravitational field.
Despite equivalent, however, they act differently: whereas curvature yields a
geometric description, in which the concept of gravitational force is absent,
torsion acts as a true gravitational force, quite similar to the Lorentz force
of electrodynamics. As a consequence, the right-hand side of a
spinless-particle equation of motion (which would represent a gravitational
force) is always zero in the geometric description, but not in the teleparallel
case. This means essentially that the gravitational coupling prescription can
be minimal only in the geometric case. Relying on this property, a new
gravitational coupling prescription in the presence of curvature and torsion is
proposed. It is constructed in such a way to preserve the equivalence between
curvature and torsion, and its basic property is to be equivalent with the
usual coupling prescription of general relativity. According to this view, no
new physics is connected with torsion, which appears as a mere alternative to
curvature in the description of gravitation. An application of this formulation
to the equations of motion of both a spinless and a spinning particle is madeComment: To appear on IJMP
Determination of pulsation periods and other parameters of 2875 stars classified as MIRA in the All Sky Automated Survey (ASAS)
We have developed an interactive PYTHON code and derived crucial ephemeris
data of 99.4% of all stars classified as 'Mira' in the ASAS data base,
referring to pulsation periods, mean maximum magnitudes and, whenever possible,
the amplitudes among others. We present a statistical comparison between our
results and those given by the AAVSO International Variable Star Index (VSX),
as well as those determined with the machine learning automatic procedure of
Richards et al. 2012. Our periods are in good agreement with those of the VSX
in more than 95% of the stars. However, when comparing our periods with those
of Richards et al, the coincidence rate is only 76% and most of the remaining
cases refer to aliases. We conclude that automatic codes require still more
refinements in order to provide reliable period values. Period distributions of
the target stars show three local maxima around 215, 275 and 330 d, apparently
of universal validity, their relative strength seems to depend on galactic
longitude. Our visual amplitude distribution turns out to be bimodal, however
1/3 of the targets have rather small amplitudes (A 2.5) and could
refer to semi-regular variables (SR). We estimate that about 20% of our targets
belong to the SR class. We also provide a list of 63 candidates for period
variations and a sample of 35 multiperiodic stars which seem to confirm the
universal validity of typical sequences in the double period and in the
Petersen diagramsComment: 14 pages, 14 figures, and 8 tables. Accepted to The Astrophysical
Journal Supplement Series, September 201
Consistent Gravitationally-Coupled Spin-2 Field Theory
Inspired by the translational gauge structure of teleparallel gravity, the
theory for a fundamental massless spin-2 field is constructed. Accordingly,
instead of being represented by a symmetric second-rank tensor, the fundamental
spin-2 field is assumed to be represented by a spacetime (world) vector field
assuming values in the Lie algebra of the translation group. The flat-space
theory naturally emerges in the Fierz formalism and is found to be equivalent
to the usual metric-based theory. However, the gravitationally coupled theory,
with gravitation itself described by teleparallel gravity, is shown not to
present the consistency problems of the spin-2 theory constructed on the basis
of general relativity.Comment: 16 pages, no figures. V2: Presentation changes, including addition of
a new sub-section, aiming at clarifying the text; version accepted for
publication in Class. Quantum Grav
Analytical solutions for radiation-driven winds in massive stars - II: The δ-slow regime
Accurate mass-loss rates and terminal velocities from massive stars winds are essential to obtain synthetic spectra from radiative transfer calculations and to determine the evolutionary path of massive stars. From a theoretical point of view, analytical expressions for the wind parameters and velocity profile would have many advantages over numerical calculations that solve the complex non-linear set of hydrodynamic equations. In a previous work, we obtained an analytical description for the fast wind regime. Now, we propose an approximate expression for the line-force in terms of new parameters and obtain a velocity profile closed-form solution (in terms of the Lambert W function) for the δ-slow regime. Using this analytical velocity profile, we were able to obtain the mass-loss rates based on the m-CAK theory. Moreover, we established a relation between this new set of line-force parameters with the known stellar and m-CAK line-force parameters. To this purpose, we calculated a grid of numerical hydrodynamical models and performed a multivariate multiple regression. The numerical and our descriptions lead to good agreement between their values.Fil: Araya, I.. Universidad Mayor; ChileFil: Christen, A.. Universidad de Valparaíso; ChileFil: Cure, M.. Universidad de Valparaíso; ChileFil: Cidale, Lydia Sonia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Venero, Roberto Oscar José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Arcos, C.. Universidad de Valparaíso; ChileFil: Gormaz Matamala, A.. Universidad de Valparaíso; Chile. Universidad Adolfo Ibañez; ChileFil: Haucke, Maximiliano. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Escarate, P.. Universidad Austral de Chile; ChileFil: Clavería, H.. Pontificia Universidad Católica de Valparaíso; Chil
Critical flux pinning and enhanced upper-critical-field in magnesium diboride films
We have conducted pulsed transport measurements on c-axis oriented magnesium
diboride films over the entire relevant ranges of magnetic field 0 \alt H \alt
H_{c2} (where \hcu is the upper critical field) and current density 0 \alt j
\alt j_{d} (where is the depairing current density). The intrinsic
disorder of the films combined with the large coherence length and
three-dimensionality, compared to cuprate superconductors, results in a
six-fold enhancement of and raises the depinning current density
to within an order of magnitude of . The current-voltage
response is highly non-linear at all fields, resulting from a combination of
depinning and pair-breaking, and has no trace of an Ohmic free-flux-flow
regime.
Keywords: pair, breaking, depairing, superconductor, superconductivity, flux,
fluxon, vortex, mgb
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