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showing 10 items of 586 documents
Kinematic relative velocity with respect to stationary observers in Schwarzschild spacetime
2013
We study the kinematic relative velocity of general test particles with respect to stationary observers (using spherical coordinates) in Schwarzschild spacetime, obtaining that its modulus does not depend on the observer, unlike Fermi, spectroscopic and astrometric relative velocities. We study some fundamental particular cases, generalizing some results given in other work about stationary and radial free-falling test particles. Moreover, we give a new result about test particles with circular geodesic orbits: the modulus of their kinematic relative velocity with respect to any stationary observer depends only on the radius of the circular orbit, and so, it remains constant.
Mirror, Mirror on the Wall
2014
Our man stands with his face turned towards bygone times. He must spin around to catch a glimpse of the new days waiting to rise. The tornithologist has also arrived at the very essence of his own being, the point of embarkation, from which he can set off in search of the roots of each present moment. For the Torni, being a tower involves a twin-layered perceptual dimension. The height of the tower allows views to far-off places. From the top the gaze of an observer can pick out distant landscapes. Rising into the heavens also means standing out from one’s surroundings and this makes it impossible for a tower to conceal itself. In other words, it is gazed from and gazed upon. Even though th…
A note on the computation of geometrically defined relative velocities
2011
We discuss some aspects about the computation of kinematic, spectroscopic, Fermi and astrometric relative velocities that are geometrically defined in general relativity. Mainly, we state that kinematic and spectroscopic relative velocities only depend on the 4-velocities of the observer and the test particle, unlike Fermi and astrometric relative velocities, that also depend on the acceleration of the observer and the corresponding relative position of the test particle, but only at the event of observation and not around it, as it would be deduced, in principle, from the definition of these velocities. Finally, we propose an open problem in general relativity that consists on finding intr…
On the Bel radiative gravitational fields
2011
We analyze the concept of intrinsic radiative gravitational fields defined by Bel and we show that the three radiative types, N, III and II, correspond with the three following different physical situations: {\it pure radiation}, {\it asymptotic pure radiation} and {\it generic} (non pure, non asymptotic pure) {\it radiation}. We introduce the concept of {\em observer at rest} with respect to the gravitational field and that of {\em proper super-energy} of the gravitational field and we show that, for non radiative fields, the minimum value of the relative super-energy density is the proper super-energy density, which is acquired by the observers at rest with respect to the field. Several {…
Gravito-magnetic vacuum spacetimes: kinematic restrictions
2003
We show that there are no vacuum solutions with a purely magnetic Weyl tensor with respect to an observer submitted to kinematic restrictions involving first order differential scalars. This result generalizes previous ones for the vorticity-free and shear-free cases. We use a covariant approach which makes evident that only the Bianchi identities are used and, consequently, the results are also valid for non vacuum solutions with vanishing Cotton tensor.
Observer for a thick layer of solid deuterium-tritium using backlit optical shadowgraphy and interferometry.
2007
Our work is in the context of the French "laser megajoule" project, about fusion by inertial confinement. The project leads to the problem of characterizing the inner surface, of the approximately spherical target, by optical shadowgraphy techniques. Our work is entirely based on the basic idea that optical shadowgraphy produces "caustics" of systems of optical rays, which contain a great deal of 3D information about the surface to be characterized. We develop a method of 3D reconstruction based upon this idea plus a "small perturbations" technique. Although computations are made in the special "spherical" case, the method is in fact general and may be extended to several other situations.
Monitoring serologic response to single in ovo vaccination with an immune complex vaccine against infectious bursal disease in broilers
2021
Este artículo se encuentra disponible en la siguiente URL: https://www.sciencedirect.com/science/article/pii/S003257912100033X The infectious bursal disease (IBD) virus is one of the most resistant and prevalent virus worldwide in the poultry industry, being vaccination the main tool to control the disease. For this reason, consistent and uniform immunization of broiler flocks against IBD is necessary to avoid the disease spreading. The aim of this study was to apply and assess an epidemiologic mapping tool focused on the immunization by in ovo single broiler vaccination using an immune complex IBD vaccine. With this regard, 7,576 serum samples were collected from 603 broiler flocks raised …
Datu noliktavām specifisku īpašību salīdzināšana dažādās izstrādes vidēs
2016
Datu noliktavas pielietošana uzņēmumu vai organizāciju darbā ir efektīvs rīks datu izpētei un secinājumu veikšanai, kas ir nozīmīgi darba efektivitātes uzlabošanai. Taču kāda izstrādes vide labāk nodrošina datu noliktavām nepieciešamo funkcionalitāti? Šajā darbā tiek pētīts, kādas datu noliktavām specifiskas īpašības tiek atbalstītas izstrādes vidēs Microsoft SQL Server, Oracle, PostgreSQL un MySQL. Darbā tiek aplūkots, kādas ir datu noliktavām specifiskas īpašības, kas ir būtiskas datu noliktavas izstrādes vidē, kā arī tiek pētīts, kādas no šīm īpašībām un kādas papildus īpašības tiek atbalstītas katrā no aplūkotajām izstrādes vidēm. Uzdevuma veikšanai tiek pielietota viena datu noliktava,…
Comparative evaluation of three semi-quantitative radiographic grading techniques for knee osteoarthritis in terms of validity and reproducibility in…
2008
OBJECTIVE: The objective of this work was to compare the measurement properties of three categorical X-ray scoring methods of knee osteoarthritis (OA), both on semiflexed and extended views. METHODS: In data obtained from trials and cohorts, X-rays were graded using Kellgren and Lawrence (KL), the OA Research Society International (OARSI) joint space narrowing score, and measurement of joint space width (JSW). JSW was analyzed as a categorical variable. Construct validity was assessed through logistic regression between X-ray stages and Western Ontario and McMaster Universities OA Index. Inter-observer reliability was assessed in 50 subjects for extended views by weighted kappa. Intra-obser…
On temporal deixis and cognitive models in early Indo-European
2013
Crosslinguistic evidence suggest that there are two different (often coexistent) basic cognitive models for time, on the basis of which the world’s languages express time in terms of conceptual metaphor from the source spatial domain to the target temporal domain: i) the Time-based (Time-Reference-Point) model, in which time is conceptualized in terms of sequentially arrayed objects moving in space, so that a temporal event is relative to another earlier or later temporal event; ii) the Ego-based (Ego-Reference-Point) model, which is considered to have a more complex structure in which times are conceptualized as objects relative to a canonical deictic observer (Ego) located at the hic et n…