2,918 research outputs found
From Stochastic Differential Equations to Quantum Field Theory
Covariant stochastic partial (pseudo-)differential equations are studied in
any dimension. In particular a large class of covariant interacting local
quantum fields obeying the Morchio-Strocchi system of axioms for indefinite
quantum field theory is constructed by solving the analysed equations. The
associated random cosurface models are discussed and some elementary properties
of them are outlined.Comment: 11 pages, Latex, to appear in: Reports On Mathematical Physics No.X
Vol.XX (199X
Jensen-Shannon divergence as a measure of distinguishability between mixed quantum states
We discuss an alternative to relative entropy as a measure of distance
between mixed quantum states. The proposed quantity is an extension to the
realm of quantum theory of the Jensen-Shannon divergence (JSD) between
probability distributions. The JSD has several interesting properties. It
arises in information theory and, unlike the Kullback-Leibler divergence, it is
symmetric, always well defined and bounded. We show that the quantum JSD (QJSD)
shares with the relative entropy most of the physically relevant properties, in
particular those required for a "good" quantum distinguishability measure. We
relate it to other known quantum distances and we suggest possible applications
in the field of the quantum information theory.Comment: 14 pages, corrected equation 1
Notas sobre la concepción de Maxwell acerca de la fisica experimental
El Laboratorio Cavendish fue inaugurado en 1874 y James Clerk Maxwell fue su primer director.
En ese momento Maxwell ocupaba el cargo de Profesor de Física Experimental en la cátedra
Cavendish de la Universidad de Cambridge. La creación de este laboratorio tuvo la intención de
fortalecer la física experimental en el Reino Unido. Se asocia su creación con la "necesidad de
entrenamiento práctico de científicos e ingenieros" tras el éxito de la Gran Exhibición Industrial
de 1851, que dejó claramente expuestos los requerimientos de una sociedad industrial. Hasta ese
momento, la física en Inglaterra significaba física teórica y se la pensaba en el ámbito de las
matemáticas.
Hubo mucha especulación sobre la elección del Profesor de Física Experimental. Tanto
William Thomson (de Glasgow) como John Rayleigh (de Essex) fueron candidatos con grandes
posibilidades, pero ambos rechazaron la oferta Cuando se anunció la designación de Maxwell,
hubo cierto asombro (y malestar) en la comunidad científica londinense. El nuevo profesor
Maxwell era, por aquel entonces, relativamente desconocido. Su nombramiento como profesor
fue anunciado el 8 de marzo de 1871, y más allá de las críticas iniciales, su clase inaugural fue
seguida por una gran cantidad de estudiantes e investigadores de Cambridge. Sus libros más
influyentes, Teoría Cinética ( 1871) y el Tratado de Electricidad y Magnetismo ( 1873), -no habían
sido todavía publicados.
En esta clase, Maxwell dejó claramente expuesta la impronta que él darla unos años
después al Laboratorio Cavendish, cuando fuera su Director. Una de sus primeras acciones al
asumir como Director del laboratorio, fue la construcción de un conjunto de equipos de física
experimental, muchos de los cuales eran producto de sus propios desarrollos y concepciones.
Entre ellos se destaca un modelo mecánico que tenía por objetivo representar la interacción de
dos circuitos eléctricos. El estudio de este modelo es el propósito primordial del presente trabajo.
Para una mejor comprensión de los objetivos perseguidos por Maxwell con este tipo de
desarrollos, haremos, por un lado una breve descripción de las ideas que Maxwell tenía sobre la
física experimental y por el otro, un análisis del modelo desde la concepción mecanicista que él
tenía del electromagnetismo
Domain invariance for local solutions of semilinear evolution equations in Hilbert spaces
A closed set K of a Hilbert space H is said to be invariant under the evolution equation
X'(t) = AX(t) + f(t,X(t)) (t > 0),
whenever all solutions starting from a point of K, at any time t0 0, remain in K as long as
they exist.
For a self-adjoint strictly dissipative operator A, perturbed by a (possibly unbounded)
nonlinear term f, we give necessary and sufficient conditions for the invariance of K, formulated
in terms of A, f, and the distance function from K. Then, we also give sufficient conditions for
the viability of K for the control system
X'(t) = AX(t) + f(t,X(t), u(t)) (t > 0, u(t) ∈ U).
Finally, we apply the above theory to a bilinear control problem for the heat equation in a
bounded domain of RN, where one is interested in keeping solutions in one fixed level set of a
smooth integral functional
Carbon nanotubes and catalysis: the many facets of a successful marriage
Carbon nanotubes have emerged as unique carbon allotropes that bear very interesting prospects in catalysis. Their use is mostly related to that of supports for inorganic metal catalysts, including molecular catalysts, metal nanoparticles, metal oxides or even more complex hierarchical hybrids. However, several reports have shown that they can intriguingly act as metal-free catalysts, with performance often superior to that of other carbon materials, in particular when ad hoc organic functional groups are attached prior to catalytic screening. The range of catalytic reactions is quite wide, and it includes standard organic synthesis, electrocatalysis, photocatalysis as well as other important industrial processes. In the last few years, the energy sector has acquired a dominant role as one of the most sought-after fields of application, given its ever-increasing importance in society
Scaled Gradient Projection Methods for Astronomical Imaging
This book is a collection of 19 articles which reflect the courses given at the Collège de France/Summer school “Reconstruction d'images − Applications astrophysiques“ held in Nice and Fréjus, France, from June 18 to 22, 2012. The articles presented in this volume address emerging concepts and methods that are useful in the complex process of improving our knowledge of the celestial objects, including Earth
Electrocatalytic CO2 reduction: role of the cross-talk at nano-carbon interfaces
The electrocatalytic CO2 reduction reaction (CO2RR) is an interfacial process, involving a minimum of three phases at the contact point of gaseous CO2 with the electrodic surface and the liquid electrolyte. As a consequence, surface chemistry at composite interfaces plays a central role for CO2RR selectivity and catalysis. Each interface defines a functional boundary, where active sites are exposed to a unique environment, with respect to distal sites in the bulk of organic and inorganic domains. While the individual role of each component-type is hardly predictable "a-solo", the interface ensemble works via a strategic interplay of individual effects, including: (i) enhanced electrical conductivity, (ii) high surface area and exposure of the interfacial catalytic sites, (iii) favorable diffusion and feeding of reactants, (iv) complementary interactions for the "on/off" stabilization of cascade intermediates, (v) a secondary sphere assistance to lower the activation energy of bottleneck steps, (vi) a reinforced robustness and long-term operation stability. Selected CO2RR case studies are compared and contrasted to highlight how the organic domains of carbon nanostructures merge with metal and metal-oxide active sites to separate tasks but also to turn them into a cooperative asset of mutual interactions, thus going beyond the classic "Divide et Impera" rule
Regulatory peptides in the urinary bladder of two genera of Antarctic Teleosts
Somatostatin 14, prolactin, atrial natriuretic peptide, galanin and urotensin II were found using immunohistochemistry in the urinary bladders of the Antarctic \uaeshes Trematomus bernacchii (Nototheniidae) and Chionodraco hamatus (Channichthyidae) caught in the Ross Sea. The urinary bladders of the two species showed a dierent histology in the epithelial layer. In T. bernacchii the epithelium comprises a single type of columnar cells, while in C. hamatus the columnar cells are restricted to the ventral portion of the bladder, and the dorso-lateral region is lined by cuboidal cells. No dierence in the intensity of the immunostaining was observed in the two cell types; the only variation was a dierent distribution of the immunoreactions, which were present in the whole cytoplasm in the cuboidal cells and restricted to the apical and/or basal portion of the columnar cells
Star Spot Induced Radial Velocity Variability in LkCa 19
We describe a new radial velocity survey of T Tauri stars and present the
first results. Our search is motivated by an interest in detecting massive
young planets, as well as investigating the origin of the brown dwarf desert.
As part of this survey, we discovered large-amplitude, periodic, radial
velocity variations in the spectrum of the weak line T Tauri star LkCa 19.
Using line bisector analysis and a new simulation of the effect of star spots
on the photometric and radial velocity variability of T Tauri stars, we show
that our measured radial velocities for LkCa19 are fully consistent with
variations caused by the presence of large star spots on this rapidly rotating
young star. These results illustrate the level of activity-induced radial
velocity noise associated with at least some very young stars. This
activity-induced noise will set lower limits on the mass of a companion
detectable around LkCa 19, and similarly active young stars.Comment: ApJ accepted, 27 pages, 12 figures, aaste
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