Operator ordering for generally covariant systems

The constraint operators belonging to a generally covariant system are found out within the framework of the BRST formalism. The result embraces quadratic Hamiltonian constraints whose potential can be factorized as a never null function times a gauge invariant function. The building of the inner product between physical states is analyzed for systems featuring either intrinsic or extrinsic time.Comment: 4 pages. Talk given at the Third Conference on "Constrained Dynamics and Quantum Gravity" held in Sardinia (Italy), September 1999. Journal reference:Nucl. Phys. B (Proc. Suppl.) 88 (2000) 322-32

Cuantificación de sistemas con covariancia general

En esta tesis se estudia la cuantificación de sistemas con covariancia general. En el marco de los formalismos BRST y de cuantificación canónica de Dirac, se estudian modelos de dimensión finita que emulan la estructura de vínculos de la Relatividad General. Se comienza con el estudio de un sistema sujeto a un vínculo cuadrático en los momentos y un conjunto de vínculos lineales en los momentos (correspondientes respectivamente a los vínculos "super-Hamiltoniano" y de "supermomentos" de Relatividad General). El punto de partida es discernir que las contribuciones no hermíticas de los fantasmas a los vínculos de supermomento pueden leerse en términos del volumen natural inducido por los vínculos sobre las órbitas. Este volumen juega luego un papel fundamental en la construcción del sector cuadrático de la carga BRST nilpotente. A nivel cuántico, la teoría permanece invariante ante transformaciones de escala del vínculo super-Hamiltoniano. En el caso en que el sistema posee un tiempo intrínseco, esta propiedad se traduce en una contribución del potencial al término cinético. En este aspecto el resultado difiere sustancialmente del tratamiento usual, donde la invariancia ante transformaciones de escala se fuerza con la introducción de un acoplamiento con la curvatura. Dicha contribución lejos de ser antinatural, se justifica elegantemente a la luz del principio variacional de Jacobi. Luego, el tratamiento se extiende al caso de sistemas con tiempo extrínseco. En este caso, dado que la métrica posea un vector de Killing temporal conforme y el potencial se comporte de manera adecuada respecto al mismo, el rol jugado por el potencial en el caso con tiempo intrínseco aquí es tomado por el módulo del vector de Killing de la teoría. Finalmente, los resultados obtenidos se extienden para un sistema con dos vínculos super-Hamiltonianos. Este paso es sumamente importante ya que la Relatividad General posee una infinidad de tales vínculos, con un álgebra entre ellos no trivial.In this Thesis it is studied the quantization of generally covariant systems. Finite dimensional models which mimic the constraint structure of Einstein's General Relativity theory are studied in the framework of BRST formalism and Dirac's canonical quantization. First, it is studied a system featuring a quadratic constraint in the momenta and a set of linear constraints in the momenta (the "super-Hamiltonian" and "supermomentum" constraints of General Relativity respectively). The starting point is to realize that the ghost contributions to the supermomentum constraint operators can be read in terms of the natural volume induced by the constraints in the orbits. This volume plays a fundamental role in the construction of the quadratic sector of the nilpotent BRST charge. It is shown that the quantum theory is invariant under scaling of the super-Hamiltonian constraint. As long as the system has an intrinsic time, this property translates in a contribution of the potential to the kinetic term. In this aspect, the results substantially differ from other works where the scaling invariance is forced by introducing a coupling to the curvature. The contribution of the potential, far from being unnatural, is beautifully justified in light of the Jacobi's principle. Then, it is shown that the obtained results can be extended to systems with extrinsic time. In this case, if the metric has a conformal temporal Killing vector and the potential exhibits a suitable behavior with respect to it, the role played by the potential in the case of intrinsic time is now played by the norm of the Killing vector. Finally, the results for the previous cases are extended to a system featuring two super- Hamiltonian constraints. This step is extremely important due to the fact that General Relativity features an infinite number of such constraints satisfying a non trivial algebra among themselves.Fil:Sforza, Daniel Marcelo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Changes in global gene expression in rat myometrium in transition from late pregnancy to parturition

The process of parturition involves the complex interplay of factors that change the excitability and contractile activity of the uterus. We have compared the relative gene expression profile of myometrium from rats before parturition (21 days pregnant) and during delivery, using high-density DNA microarray. Of 8,740 sequences available in the array, a total of 3,782 were detected as present. From the sequences that were significantly altered, 59 genes were upregulated and 82 genes were downregulated. We were able to detect changes in genes described to have altered expression level at term, including connexin 43 and 26, cyclooxygenase 2, and oxytocin receptor, as well as novel genes that have been not previously associated with parturition. Quantitative real-time PCR on selected genes further confirmed the microarray data. Here we report for the first time that aquaporin5 (AQP5), a member of the aquaporin water channel family, was dramatically downregulated during parturition (approximately 100-fold by microarray and approximately 50-fold by real-time PCR). The emerging profile highlights biochemical cascades occurring in a period of approximately 36 h that trigger parturition and the initiation of myometrium reverse remodeling postpartum. The microarray analysis uncovered genes that were previously suspected to play a role in parturition. This regulation involves genes from immune/inflammatory response, steroid/lipid metabolism, calcium homeostasis, cell volume regulation, cell signaling, cell division, and tissue remodeling, suggesting the presence of multiple and redundant mechanisms altered in the process of birth.Fil: Helguera, Gustavo Fernando. University of California at Los Angeles; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay; ArgentinaFil: Eghbali, Mansoureh. University of California at Los Angeles; Estados UnidosFil: Sforza, Daniel. University of California at Los Angeles; Estados UnidosFil: Minosyan, Tamara Y.. University of California at Los Angeles; Estados UnidosFil: Toro, Ligia. University of California at Los Angeles; Estados Unidos. University of California at Los Angeles. School of Medicine; Estados UnidosFil: Stefani, Enrico. University of California at Los Angeles; Estados Unidos. University of California at Los Angeles. School of Medicine; Estados Unido

Genetic and Genomic Strategies in Learning and Memory

Abstract: Learning and memory is a property of central importance in the nervous system, yet many of the molecular mechanisms for this behavior remain enshrouded in mystery. Despite the daunting nature of the problem, a number of complementary strategies have been employed to unravel the complexities of learning and memory, ranging from genetics to biochemistry. One of the most recent tools brought to bear in this area is genomics. Here, we review some of the most significant insights that have been so far obtained in learning and memory, and we suggest possible areas of future progress

Hemodynamic Analysis of Intracranial Aneurysms with Moving Parent Arteries: Basilar Tip Aneurysms

The effects of parent artery motion on the hemodynamics of basilar tip saccular aneurysms and its potential effect on aneurysm rupture were studied. The aneurysm and parent artery motions in two patients were determined from cine loops of dynamic angiographies. The oscillatory motion amplitude was quantified by registering the frames. Patient‐specific computational fluid dynamics (CFD) models of both aneurysms were constructed from 3D rotational angiography images. Two CFD calculations were performed for each patient, corresponding to static and moving models. The motion estimated from the dynamic images was used to move the surface grid points in the moving model. Visualizations from the simulations were compared for wall shear stress (WSS), velocity profiles, and streamlines. In both patients, a rigid oscillation of the aneurysm and basilar artery in the anterio‐posterior direction was observed and measured. The distribution of WSS was nearly identical between the models of each patient, as well as major intra‐aneurysmal flow structures, inflow jets, and regions of impingement. The motion observed in pulsating intracranial vasculature does not have a major impact on intra‐aneurysmal hemodynamic variables. Parent artery motion is unlikely to be a risk factor for increased risk of aneurysmal rupture

A Weld Defects Detection System Based on a Spectrometer

Improved product quality and production methods, and decreased production costs are important objectives of industries. Welding processes are part of this goal. There are many studies about monitoring and controlling welding process. This work presents a non-intrusive on-line monitoriment system and some algorithms capable of detecting GTAW weld defects. Some experiments were made to simulate weld defects by disturbing the electric arc. The data comes from a spectrometer which captures perturbations on the electric arc by the radiation emission of chosen lines. Algorithms based on change detection methods are used to indicate the presence and localization of those defects

Arago (1810): the first experimental result against the ether

95 years before Special Relativity was born, Arago attempted to detect the absolute motion of the Earth by measuring the deflection of starlight passing through a prism fixed to the Earth. The null result of this experiment gave rise to the Fresnel's hypothesis of an ether partly dragged by a moving substance. In the context of Einstein's Relativity, the sole frame which is privileged in Arago's experiment is the proper frame of the prism, and the null result only says that Snell's law is valid in that frame. We revisit the history of this premature first evidence against the ether theory and calculate the Fresnel's dragging coefficient by applying the Huygens' construction in the frame of the prism. We expose the dissimilar treatment received by the ray and the wave front as an unavoidable consequence of the classical notions of space and time.Comment: 16 pages. To appear in European Journal of Physic