637 research outputs found
The Effective Lorentzian and Teleparallel Spacetimes Generated by a Free Electromagnetic Field
In this paper we show that a free electromagnetic field living in Minkowski
spacetime generates an effective Weitzenbock or an effective Lorentzian
spacetime whose properties aredetermined in details. These results are possible
because we found using the Clifford bundle formalism the noticeable result that
the energy-momentum densities of a free electromagnetic field are sources of
the Hodge duals of exact 2-form fields which satisfy Maxwell like equations.Comment: A missing term in Eq.(14) has been inserted and some misprints
correcte
Algebraic And Dirac-hestenes Spinors And Spinor Fields
Almost all presentations of Dirac theory in first or second quantization in physics (and mathematics) textbooks make use of covariant Dirac spinor fields. An exception is the presentation of that theory (first quantization) offered originally by Hestenes and now used by many authors. There, a new concept of spinor field (as a sum of nonhomogeneous even multivectors fields) is used. However, a careful analysis (detailed below) shows that the original Hestenes definition cannot be correct since it conflicts with the meaning of the Fierz identities. In this paper we start a program dedicated to the examination of the mathematical and physical basis for a comprehensive definition of the objects used by Hestenes. In order to do that we give a preliminary definition of algebraic spinor fields (ASF) and Dirac-Hestenes spinor fields (DHSF) on Minkowski space-time as some equivalence classes of pairs (ξu, ψu), where ξu is a spinorial frame field and ψu is an appropriate sum of multivectors fields (to be specified below). The necessity of our definitions are shown by a careful analysis of possible formulations of Dirac theory and the meaning of the set of Fierz identities associated with the bilinear covariants (on Minkowski space-time) made with ASF or DHSF. We believe that the present paper clarifies some misunderstandings (past and recent) appearing on the literature of the subject. It will be followed by a sequel paper where definitive definitions of ASF and DHSF are given as appropriate sections of a vector bundle called the left spin-Clifford bundle. The bundle formulation is essential in order to be possible to produce a coherent theory for the covariant derivatives of these fields on arbitrary Riemann-Cartan space-times. 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Causal explanation for observed superluminal behavior of microwave propagation in free space
In this paper we present a theoretical analysis of an experiment by Mugnai
and collaborators where superluminal behavior was observed in the propagation
of microwaves. We suggest that what was observed can be well approximated by
the motion of a superluminal X wave. Furthermore the experimental results are
also explained by the so called scissor effect which occurs with the
convergence of pairs of signals coming from opposite points of an annular
region of the mirror and forming an interference peak on the intersection axis
traveling at superluminal speed. We clarify some misunderstandings concerning
this kind of electromagnetic wave propagation in vacuum.Comment: 9 pages, 3 figures, accepted for publication in Physics Letters
Electronic Correlations In Narrow-band Solids
The Hubbard Hamiltonian is rederived from the full many-body Hamiltonian with the assumption that only intrasite correlations are important. It is shown to be exact in both the Hartree-Fock and narrow-band limits, provided the appropriate linearization procedure is adopted in the former case. Real-time and imaginary-time Green's functions are derived for the cases intermediate between the Hartree-Fock and narrow-band limits, and a long-standing puzzle with regard to the number of electrons in the upper pseudoparticle band is cleared up. The chemical potential and total energy of the system are calculated in the narrow-band limit and are shown to be identical with those derived from an effective one-electron representation. It is shown that because these quantities depend only on the number of doubly occupied sites, important transport parameters such as electrical conductivity and thermoelectric power can be calculated from the effective one-electron representation, without the necessity of evaluating the two-particle Green's function. For finite bands, the total energy in the Hubbard model is shown to give the exact result, in contradiction to a previous calculation. It is shown that thermodynamic quantities such as the total energy and chemical potential which depend only on derivatives of the grand partition function are independent of the presence or absence of magnetic ordering, but that the entropy is not. Thus a study of the insulatormetal phase transitions is very sensitive to magnetic ordering. © 1979 The American Physical Society.1921203121
Ictiofauna Dos Enclaves De Floresta úmida Nos Planaltos Da Ibiapaba E Do Araripe, Nordeste Do Brasil
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Humid highland forest enclaves are remnants of Atlantic Forest found in tablelands within the Caatinga biome (Northeastern Brazil), which emerged during interglacial periods in the Pleistocene. These ecosystems have a highly diverse and endemic fish fauna. Most earlier surveys have focused on the tableland of Borborema (Pernambuco and Paraíba States). In this study we surveyed the fish fauna of the humid forest enclaves in the tablelands of Ibiapaba and Araripe, based on samples collected in the rainy season (March and April) between 2009 and 2014. The 45 sampling points covered rivers, streams and reservoirs in five river basins belonging to three ecoregions. The species were listed according to drainage divide, and endemism was determined for each ecoregion and for the Caatinga. Our area was more species-rich (n=59) than Borborema (n=27). The samples included five introduced species and 29 species endemic to the Caatinga (49.1% of the sampled species). The distribution of Parotocinclus haroldoi was expanded to the Mid-Northeastern Caatinga ecoregion (Timonha river basin, Ceará State). Our study intends to make a significant contribution to current knowledge of the ichthyofauna in humid highland forest enclaves of semiarid Northeastern Brazil, identified as a priority in the conservation of the biodiversity in the Caatinga. © 2016, Universidade Estadual de Campinas UNICAMP. All rights reserved.164457463/2012-0, CNPq, Conselho Nacional de Desenvolvimento Científico e Tecnológico552009/2011-3, CNPq, Conselho Nacional de Desenvolvimento Científico e Tecnológico552086/2011-8, CNPq, Conselho Nacional de Desenvolvimento Científico e TecnológicoConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq
Magnetic Monopoles Without String In The Kähler-clifford Algebra Bundle: A Geometrical Interpretation
In substitution for Dirac monopoles with string (and for topological monopoles), "monopoles without string" have recently been introduced on the basis of a generalized potential, the sum of a vector A, and a pseudovector γ5 B potential. By making recourse to the Clifford bundle script c sign(τM,g) [ ( TxM,g) = ℝ1,3; script c sign(TxM,g) = ℝ1,3 ], which just allows adding together for each x∈M tensors of different ranks, in a previous paper a Lagrangian and Hamiltonian formalism was constructed for interacting monopoles and charges that can be regarded as satisfactory from various points of view. In the present article, after having "completed" the formalism, a purely geometrical interpretation of it is put forth within the Kähler-Clifford bundle script K(τ*M,ĝ) of differential forms, essential ingredients being a generalized curvature and the Hodge decomposition theorem. Thus the way is paved for the extension of our "monopoles without string" to non-Abelian gauge groups. The analogy with supersymmetric theories is apparent. © 1990 American Institute of Physics.31250250
On the relation of Thomas rotation and angular velocity of reference frames
In the extensive literature dealing with the relativistic phenomenon of
Thomas rotation several methods have been developed for calculating the Thomas
rotation angle of a gyroscope along a circular world line. One of the most
appealing concepts, introduced in \cite{rindler}, is to consider a rotating
reference frame co-moving with the gyroscope, and relate the precession of the
gyroscope to the angular velocity of the reference frame. A recent paper
\cite{herrera}, however, applies this principle to three different co-moving
rotating reference frames and arrives at three different Thomas rotation
angles. The reason for this apparent paradox is that the principle of
\cite{rindler} is used for a situation to which it does not apply. In this
paper we rigorously examine the theoretical background and limitations of
applicability of the principle of \cite{rindler}. Along the way we also
establish some general properties of {\it rotating reference frames}, which may
be of independent interest.Comment: 14 pages, 2 figure
Effect of metformin therapy and dietary supplements on semen parameters in hyperinsulinaemic males
Previous reports indicated that hyperinsulinaemic men may exhibit a higher
percentage of poorly compacted DNA in their spermatozoa and less success in
an IVF programme (Andrologia, 45, 2003, 18; Andrologia, 2014, doi: 10.1111/
and.12227). The aim of this study was to investigate the effect of metformin
(Glucophage ) and antioxidant treatment (StaminoGro ) on the semen parameters
of hyperinsulinaemic men. Nineteen hyperinsulinaemic male patients were
treated for 3 months with metformin alone (Group A), and fifteen patients used
metformin in combination with the nutritional supplement (Group B). Combined
data of the two groups (pre- and post-treatment) differ significantly
regarding sperm morphology (P = 0.0003) and CMA3 (P < 0.0001) values. The
improvement in sperm morphology after treatment was similar for the two
respective groups (P < 0.05). The morphological normal sperm forms increased
from the mean percentage of 3.9 to 5.5% and from 4.2 to 5.5% for Group A
and B respectively. Where a combination of metformin and the supplement
were used (Group B), the combination treatment proved to be superior in
obtaining enhanced chromatin packaging quality although not statistically significant
(P = 0.5929) when compared with the metformin (Group A) group.
The chromatin packaging quality in Group B improved with 10% while the
improvement in Group A was approximately 8.3%. Therefore, infertile hyperinsulinaemic
men can benefit from metformin treatment and should be advised
on the use of nutritional supplements with antioxidant properties.http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1439-02722016-11-30hb2016Clinical Epidemiolog
Superluminal Localized Solutions to Maxwell Equations propagating along a waveguide: The finite-energy case
In a previous paper of ours [Phys. Rev. E64 (2001) 066603, e-print
physics/0001039] we have shown localized (non-evanescent) solutions to Maxwell
equations to exist, which propagate without distortion with Superluminal speed
along normal-sized waveguides, and consist in trains of "X-shaped" beams. Those
solutions possessed therefore infinite energy. In this note we show how to
obtain, by contrast, finite-energy solutions, with the same localization and
Superluminality properties. [PACS nos.: 41.20.Jb; 03.50.De; 03.30.+p; 84.40.Az;
42.82.Et. Keywords: Wave-guides; Localized solutions to Maxwell equations;
Superluminal waves; Bessel beams; Limited-dispersion beams; Finite-energy
waves; Electromagnetic wavelets; X-shaped waves; Evanescent waves;
Electromagnetism; Microwaves; Optics; Special relativity; Localized acoustic
waves; Seismic waves; Mechanical waves; Elastic waves; Guided gravitational
waves.]Comment: plain LaTeX file (12 pages), plus 10 figure
Primakoff effect in eta-photoproduction off protons
We analyse data on forward eta-meson photoproduction off a proton target and
extract the eta to gamma gamma decay width utilizing the Primakoff effect. The
hadronic amplitude that enters into our analysis is strongly constrained
because it is fixed from a global fit to available gamma p to p eta data for
differential cross sections and polarizations. We compare our results with
present information on the two-photon eta-decay from the literature. We provide
predictions for future PrimEx experiments at Jefferson Laboratory in order to
motivate further studies.Comment: 5 pages, 6 figures, gamma-gamma*-eta form factor included, version to
appear in Eur. Phys. J. A
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