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Alfredo Macias

Publications and source records attributed to Alfredo Macias.

17 recordsLinked to original sources
arXiv2018

Effects of non-linear vacuum electrodynamics on the polarization plane of light

We consider the Pleba{ń}ski class of nonlinear theories of vacuum electrodynamics, i.e., Lagrangian theories that are Lorentz invariant and gauge invariant. Our main goal is to derive the transport law of the polarization plane in such a theory, on an unspecified general-relativistic spacetime and with an unspecified electromagnetic background field. To that end we start out from an approximate-plane-harmonic-wave ansatz that takes the generation of higher harmonics into account. By this ansatz, the electromagnetic field is written as an asymptotic series with respect to a parameter $α$, where the limit $α\to 0$ corresponds to sending the frequency to infinity. We demonstrate that by solving the generalized Maxwell equations to zeroth and first order with respect to $α$ one gets a unique transport law for the polarization plane along each light ray. We exemplify the general results with the Born-Infeld theory.

gr-qc↗
arXiv2008

Temperature measurements of a Bose--Einstein condensate by ultra--intense light pulses

Experimentally the temperature in a Bose--Einstein condensate is always deduced resorting to the comparison between the Maxwell--Boltzmann velocity distribution function and the density profile in momentum space. Though a successful method it is an approximation, since it also implies the use of classical statistical mechanics at temperatures close to the condensation temperature where quantal effects play a relevant role. The present work puts forward a new method in which we use an ultra--intense light pulse and a nonlinear optical material as detectors for differences in times--of--flight. This experimental value shall be compared against the result here calculated, using the Bose--Einstein distribution function, which is a temperature--dependent variable, and in this way the temperature of the condensate is obtained.

cond-mat.stat-mech↗
arXiv2008

Midisuperspace Supersymmetric Quantum Cosmology

We investigate the canonical quantization in the framework of N=1 simple supergravity for the case of a very simple gravitational midisuperspace described by Gowdy $T^3$ cosmological models. We consider supersymmetric quantum cosmology in the mentioned midisuperspace, where a matrix representation for the gravitino covector--spinor is used. The full Lorentz constraint and its implications for the wave function of the universe are analyzed in detail. We found that there are indeed physical states in the midisuperspace sector of the theory in contrast to the case of minisuperspace where there exist no physical states.

gr-qc↗
arXiv2006

Time paradox in Quantum Gravity

The aim of this work is to review the concepts of time in quantum mechanics and general relativity to show their incompatibility. We show that the absolute character of Newtonian time is present in quantum mechanics and also partially in quantum field theories which consider the Minkowski metric as the background spacetime. We discuss the problems which this non-dynamical concept of time causes in general relativity that is characterized by a dynamical spacetime.

gr-qc↗
arXiv2005

On the local Lorentz invariance in N=1 supergravity

We discuss the local Lorentz invariance in the context of N=1 supergravity and show that a previous attempt to find explicit solutions to the Lorentz constraint in terms of $γ-$matrices is not correct. We improve that solution by using a different representation of the Lorentz operators in terms of the generators of the rotation group, and show its compatibility with the matrix representation of the fermionic field. We find the most general wave functional that satisfies the Lorentz constraint in this representation.

gr-qc↗
arXiv2005

Gowdy $T^3$ Cosmological Models in N=1 Supergravity

We investigate the canonical quantization of supergravity N=1 in the case of a midisuperspace described by Gowdy $T^3$ cosmological models. The quantum constraints are analyzed and the wave function of the universe is derived explicitly. Unlike the minisuperspace case, we show the existence of physical states in midisuperspace models. The analysis of the wave function of the universe leads to the conclusion that the classical curvature singularity present in the evolution of Gowdy models is removed at the quantum level due to the presence of the Rarita-Schwinger field.

gr-qc↗
arXiv2005

Lorentz invariance violation and charge (non--)conservation: A general theoretical frame for extensions of the Maxwell equations

All quantum gravity approaches lead to small modifications in the standard laws of physics which lead to violations of Lorentz invariance. One particular example is the extended standard model (SME). Here, a general phenomenological approach for extensions of the Maxwell equations is presented which turns out to be more general than the SME and which covers charge non--conservation (CNC), too. The new Lorentz invariance violating terms cannot be probed by optical experiments but need, instead, the exploration of the electromagnetic field created by a point charge or a magnetic dipole. Some scalar--tensor theories and higher dimensional brane theories predict CNC in four dimensions and some models violating Special Relativity have been shown to be connected with CNC and its relation to the Einstein Equivalence Principle has been discussed. Due to this upcoming interest, the experimental status of electric charge conservation is reviewed. Up to now there seem to exist no unique tests of charge conservation. CNC is related to the precession of polarization, to a modification of the $1/r$--Coulomb potential, and to a time-dependence of the fine structure constant. This gives the opportunity to describe a dedicated search for CNC.

gr-qc↗
arXiv2004

New experimental proposals for testing Dirac equation

The advent of phenomenological quantum gravity has ushered us in the search for experimental tests of the deviations from general relativity predicted by quantum gravity or by string theories, and as a by--product of this quest the possible modifications that some field equations, for instance, the motion equation of spin--1/2--particles, have already been considered. In the present work a modified Dirac equation, whose extra term embraces a second--order time derivative, is taken as mainstay, and three different experimental proposals to detect it are put forward. The novelty in these ideas is that two of them do not fall within the extant approaches in this context, to wit, red--shift, atomic interferometry, or Hughes--Drever type--like experiments.

hep-th↗
arXiv2003

Exact vacuum solution of a (1+2)-dimensional Poincare gauge theory: BTZ solution with torsion

In (1+2)-dimensional Poincaré gauge gravity, we start from a Lagrangian depending on torsion and curvature which includes additionally {\em translational} and {\em Lorentzian} Chern-Simons terms. Limiting ourselves to to a specific subcase, the Mielke-Baekler (MB) model, we derive the corresponding field equations (of Einstein-Cartan-Chern-Simons type) and find the general vacuum solution. We determine the properties of this solution, in particular its mass and its angular momentum. For vanishing torsion, we recover the BTZ-solution. We also derive the general conformally flat vacuum solution with torsion. In this framework, we discuss {\em Cartan's} (3-dimensional) {\em spiral staircase} and find that it is not only a special case of our new vacuum solution, but can alternatively be understood as a solution of the 3-dimensional Einstein-Cartan theory with matter of constant pressure and constant torque.

gr-qc↗
arXiv2003

Space--time torsion contribution to quantum interference phases

From the latest experimental readouts in this context an intriguing discrepancy has been elicited. Indeed, theory and experiment dissent by one per cent, and though this fact could be a consequence of the mounting of the experimental device, it might also embody a difference between the way in which gravity behaves in classical and quantum mechanics. In this work the effects, upon the interference pattern, of space--time torsion will be analyzed heeding its coupling with the spin of the neutron beam. It will be proved that, even with this contribution, there is enough leeway for a further discussion of the validity of the equivalence principle in nonrelativistic quantum mechanics.

gr-qc↗
arXiv2000

A class of colliding waves in metric-affine gravity, nonmetricity and torsion shock waves

By using our recent generalization of the colliding waves concept to metric-affine gravity theories, and also our generalization of the advanced and retarded time coordinate representation in terms of Jacobi functions, we find a general class of colliding wave solutions with fourth degree polynomials in metric-affine gravity. We show that our general approach contains the standard second degree polynomials colliding wave solutions as a particular case.

gr-qc↗
arXiv2000

Plane-fronted waves in metric-affine gravity

We study plane-fronted electrovacuum waves in metric-affine gravity theories (MAG) with cosmological constant. Their field strengths are, on the gravitational side, curvature $R_α{}^β$, nonmetricity $Q_{αβ}$, torsion $T^α$ and, on the matter side, the electromagnetic field strength $F$. Our starting point is the work by Ozsváth, Robinson, and Rózga on type N gravitational fields in general relativity as coupled to null electromagnetic fields.

gr-qc↗
arXiv1999

Chiral supergravity and anomalies

Similarily as in the Ashtekar approach, the translational Chern-Simons term is, as a generating function, instrumental for a chiral reformulation of simple (N=1) supergravity. After applying the algebraic Cartan relation between spin and torsion, the resulting canonical transformation induces not only decomposition of the gravitational fields into selfdual and antiselfdual modes, but also a splitting of the Rarita-Schwinger fields into their chiral parts in a natural way. In some detail, we also analyze the consequences for axial and chiral anomalies.

gr-qc↗
arXiv1998

Class of colliding plane waves in terms of Jacobi functions

We present a general class of noncolinear colliding wave solutions of the Einstein-Maxwell equations given in terms of fourth order polynomials, which in turn can be expressed through Jacobi functions depending on generalized advanced and retarded time coordinates. The solutions are characterized by six free parameters. The parameters can be chosen in such a way to avoid the generic focusing singularity

gr-qc↗
arXiv1998

Computer algebra in gravity: Programs for (non-)Riemannian spacetimes. I

Computer algebra programs are presented for application in general relativity, in electrodynamics, and in gauge theories of gravity. The mathematical formalism used is the calculus of exterior differential forms, the computer algebra system applied Hearn's Reduce with Schruefer's exterior form package Excalc. As a non-trivial example we discuss a metric of Plebanski & Demianski (of Petrov type D) together with an electromagnetic potential and a triplet of post-Riemannian one-forms. This whole geometrical construct represents an exact solution of a metric-affine gauge theory of gravity. We describe a sample session and verify by computer that this exact solution fulfills the appropriate field equations.-- Computer programs are described for the irreducible decomposition of (non-Riemannian) curvature, torsion, and nonmetricity.

gr-qc↗
arXiv1996

Generalized Gross--Perry--Sorkin--Like Solitons

In this paper, we present a new solution for the effective theory of Maxwell--Einstein--Dilaton, Low energy string and Kaluza--Klein theories, which contains among other solutions the well known Kaluza--Klein monopole solution of Gross--Perry--Sorkin as special case. We show also the magnetic and electric dipole solutions contained in the general one.

hep-th↗