902 research outputs found
Links between different analytic descriptions of constant mean curvature surfaces
Transformations between different analytic descriptions of constant mean
curvature (CMC) surfaces are established. In particular, it is demonstrated
that the system descriptive of CMC surfaces within the
framework of the generalized Weierstrass representation, decouples into a
direct sum of the elliptic Sh-Gordon and Laplace equations. Connections of this
system with the sigma model equations are established. It is pointed out, that
the instanton solutions correspond to different Weierstrass parametrizations of
the standard sphere
Phemeranthus rugospermus (Holz.) Kiger
https://thekeep.eiu.edu/herbarium_specimens_byname/20844/thumbnail.jp
Metric of a tidally perturbed spinning black hole
We explicitly construct the metric of a Kerr black hole that is tidally
perturbed by the external universe in the slow-motion approximation. This
approximation assumes that the external universe changes slowly relative to the
rotation rate of the hole, thus allowing the parameterization of the
Newman-Penrose scalar by time-dependent electric and magnetic tidal
tensors. This approximation, however, does not constrain how big the spin of
the background hole can be and, in principle, the perturbed metric can model
rapidly spinning holes. We first generate a potential by acting with a
differential operator on . From this potential we arrive at the metric
perturbation by use of the Chrzanowski procedure in the ingoing radiation
gauge. We provide explicit analytic formulae for this metric perturbation in
spherical Kerr-Schild coordinates, where the perturbation is finite at the
horizon. This perturbation is parametrized by the mass and Kerr spin parameter
of the background hole together with the electric and magnetic tidal tensors
that describe the time evolution of the perturbation produced by the external
universe. In order to take the metric accurate far away from the hole, these
tidal tensors should be determined by asymptotically matching this metric to
another one valid far from the hole. The tidally perturbed metric constructed
here could be useful in initial data constructions to describe the metric near
the horizons of a binary system of spinning holes. This perturbed metric could
also be used to construct waveforms and study the absorption of mass and
angular momentum by a Kerr black hole when external processes generate
gravitational radiation.Comment: 17 pages, 3 figures. Final PRD version, minor typos, etc corrected.
v3: corrected typo in Eq. (35) and (57
Effective Values of Komar Conserved Quantities and Their Applications
We calculate the effective Komar angular momentum for the Kerr-Newman (KN)
black hole. This result is valid at any radial distance on and outside the
black hole event horizon. The effcetive values of mass and angular momentum are
then used to derive an identity () which relates the Komar
conserved charge () corresponding to the null Killing vector
() with the thermodynamic quantities of this black hole. As an
application of this identity the generalised Smarr formula for this black hole
is derived. This establishes the fact that the above identity is a local form
of the inherently non-local generalised Smarr formula.Comment: v3, minor modifications over v2; LaTex, 9 pages, no figures, to
appear in Int. Jour. Theo. Phy
Gravitational collapse in asymptotically Anti-de Sitter/de Sitter backgrounds
We study here the gravitational collapse of a matter cloud with a
non-vanishing tangential pressure in the presence of a non-zero cosmological
term. Conditions for bounce and singularity formation are derived for the
model. It is also shown that when the tangential pressures vanish, the bounce
and singularity conditions reduce to that of the dust case studied earlier. The
collapsing interior is matched with an exterior which is asymptotically de
Sitter or anti de Sitter, depending on the sign of cosmological constant. The
junction conditions for matching the cloud to exterior are specified. The
effect of the cosmological term on apparent horizons is studied in some detail,
and the nature of central singularity is analyzed. We also discuss here the
visibility of the singularity and implications for the cosmic censorship
conjecture.Comment: 11 pages, 1 figure, Revtex
The optical system of the H.E.S.S. imaging atmospheric Cherenkov telescopes, Part II: mirror alignment and point spread function
Mirror facets of the H.E.S.S. imaging atmospheric Cherenkov telescopes are
aligned using stars imaged onto the closed lid of the PMT camera, viewed by a
CCD camera. The alignment procedure works reliably and includes the automatic
analysis of CCD images and control of the facet alignment actuators. On-axis,
80% of the reflected light is contained in a circle of less than 1 mrad
diameter. The spot widens with increasing angle to the telescope axis. In
accordance with simulations, the spot size has roughly doubled at an angle of
1.4 degr. from the axis. The expected variation of spot size with elevation due
to deformations of the support structure is visible, but is completely
non-critical over the usual working range. Overall, the optical quality of the
telescope exceeds the specifications.Comment: 23 pages, 13 figure
de Sitter special relativity
A special relativity based on the de Sitter group is introduced, which is the
theory that might hold up in the presence of a non-vanishing cosmological
constant. Like ordinary special relativity, it retains the quotient character
of spacetime, and a notion of homogeneity. As a consequence, the underlying
spacetime will be a de Sitter spacetime, whose associated kinematics will
differ from that of ordinary special relativity. The corresponding modified
notions of energy and momentum are obtained, and the exact relationship between
them, which is invariant under a re-scaling of the involved quantities,
explicitly exhibited. Since the de Sitter group can be considered a particular
deformation of the Poincar\'e group, this theory turns out to be a specific
kind of deformed (or doubly) special relativity. Some experimental
consequences, as well as the causal structure of spacetime--modified by the
presence of the de Sitter horizon--are briefly discussed.Comment: V2: Some presentation changes; a new section introduced, with a
discussion about possible phenomenological consequences; new references
added; version to be published in Classical and Quantum Gravit
On the equivalence principle and gravitational and inertial mass relation of classical charged particles
We show that the locally constant force necessary to get a stable hyperbolic
motion regime for classical charged point particles, actually, is a combination
of an applied external force and of the electromagnetic radiation reaction
force. It implies, as the strong Equivalence Principle is valid, that the
passive gravitational mass of a charged point particle should be slight greater
than its inertial mass. An interesting new feature that emerges from the
unexpected behavior of the gravitational and inertial mass relation, for
classical charged particles, at very strong gravitational field, is the
existence of a critical, particle dependent, gravitational field value that
signs the validity domain of the strong Equivalence Principle. For electron and
proton, these critical field values are
and , respectively
Using eye-tracking in applied linguistics and second language research
With eye-tracking technology the eye is thought to give researchers a window into the mind. Importantly, eye-tracking has significant advantages over traditional online processing measures: chiefly that it allows for more ‘natural’ processing as it does not require a secondary task, and that it provides a very rich moment-to-moment data source. In recognition of the technology’s benefits, an ever increasing number of researchers in applied linguistics and second language research are beginning to use it. As eye-tracking gains traction in the field, it is important to ensure that it is established in an empirically sound fashion. To do this it is important for the field to come to an understanding about what eye-tracking is, what eye-tracking measures tell us, what it can be used for, and what different eye-tracking systems can and cannot do. Further, it is important to establish guidelines for designing sound research studies using the technology. The goal of the current review is to begin to address these issues
Supporting the game construction process: development of artefacts in the context of a Toolkit to Game Design
The Gamers4Nature project aims to deliver a set of strategies to empower and en-courage youngsters (upper-secondary and undergraduate students) to actively participate in games creation while raising knowledge about environmental preservation and biodiversity conservation. To accomplish these goals, a Toolkit to Game Design is being created, containing a set of resources and tools aiming to help in the creation of mobile digital games, namely a Game Construction Cards Set. This paper presents the creation process of the project’s general identi-ty and its concept adaptation to the Game Construction Cards Set, which required a deep understanding of the target audience and the development of a strategy to engage the participants in its activities. A general identity was defined, where a specific “language” to reach this audience was designed and a brand to support it was created. This “language” was applied in all graphic materials developed since day one, namely to the Game Construction Cards Set – focus of this paper. The validation of the prototypes was made through focus groups, using an iterative design approach. The focus groups participants’ inputs were integrated in the de-sign and helped with the graphic elements’ evolution, allowing the project to maintain a coherent strategy in all its representations that exceeds its graphic lan-guage and a deeper identification with the developed artefacts.publishe
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