1,505 research outputs found
Particle motion in a rotating dust spacetime: the Bonnor solution
We investigate the geometrical properties, spectral classification,
geodesics, and causal structure of the Bonnor's spacetime [Journal of Physics A
Math. Gen., \textbf{10}, 1673 (1977)], i.e., a stationary axisymmetric solution
with a rotating dust as a source. This spacetime has a directional singularity
at the origin of the coordinates (related to the diverging vorticity field of
the fluid there), which is surrounded by a toroidal region where closed
timelike curves (CTCs) are allowed, leading to chronology violations. We use
the effective potential approach to provide a classification of the different
kind of orbits on the symmetry plane as well as to study the motion parallel to
the symmetry axis. In the former case we find that as a general feature test
particles released from a fixed space point and directed towards the
singularity are repelled and scattered back as soon as they approach the CTC
boundary, without reaching the central singularity.Comment: 11 pages, 5 figures (10 eps files
Design of high gradient, high repetition rate damped C-band rf structures
The gamma beam system of the European Extreme Light Infrastructure–Nuclear Physics project
foresees the use of a multibunch train colliding with a high intensity recirculated laser pulse. The linac
energy booster is composed of 12 traveling wave C-band structures, 1.8 m long with a field phase
advance per cell of 2Ï€=3 and a repetition rate of 100 Hz. Because of the multibunch operation, the
structures have been designed with a dipole higher order mode (HOM) damping system to avoid beam
breakup (BBU). They are quasiconstant gradient structures with symmetric input couplers and a very
effective damping of the HOMs in each cell based on silicon carbide (SiC) rf absorbers coupled to each
cell through waveguides. An optimization of the electromagnetic and mechanical design has been done to
simplify the fabrication and to reduce the cost of the structures. In the paper, after a review of the beam
dynamics issues related to the BBU effects, we discuss the electromagnetic and thermomechanic design
criteria of the structures. We also illustrate the criteria to compensate the beam loading and the rf
measurements that show the effectiveness of the HOM damping
Limitations of Radar Coordinates
The construction of a radar coordinate system about the world line of an
observer is discussed. Radar coordinates for a hyperbolic observer as well as a
uniformly rotating observer are described in detail. The utility of the notion
of radar distance and the admissibility of radar coordinates are investigated.
Our results provide a critical assessment of the physical significance of radar
coordinates.Comment: 12 pages, revtex and pictex macros, 3 pictex figures, 1 eps figure.
Expanded versio
A null frame for spacetime positioning by means of pulsating sources
We introduce an operational approach to the use of pulsating sources, located
at spatial infinity, for defining a relativistic positioning and navigation
system, based on the use of four-dimensional bases of null four-vectors, in
flat spacetime. As a prototypical case, we show how pulsars can be used to
define such a positioning system. The reception of the pulses for a set of
different sources whose positions in the sky and periods are assumed to be
known allows the determination of the user's coordinates and spacetime
trajectory, in the reference frame where the sources are at rest. We describe
our approach in flat Minkowski spacetime, and discuss the validity of this and
other approximations we have considered.Comment: 19 pages, revised to match the version accepted for publication in
Advances in Space Researc
Corrigendum to a new neurocognitive interpretation of shoulder position sense during reaching:unexpected competence in the measurement of extracorporeal space
[This corrects the article DOI: 10.1155/2016/9065495.
Correlates of spinal deforming index (SDI) in HIV-positive patients naive and on treatment
Methods HIV-infected subjects naive or on stable HAART were included. Vertebral deformities were identified using SDI (according to semiquantitative method by Genant), calculated by summing the deformity grades of all vertebrae (T4 to L4); pathological deformities are defined as follow: grade 1 between 20–25%, grade 2 between 26–40%, and grade 3 > >40%. According to WHO criteria, osteopenia and osteoporosis were diagnosed in patients having spine BMD calculated as -1 << T-score << -2.5 and T-score ≤≤2.5, respectively. The correlation between SDI and spine BMD was evaluated by univariate and multivariate linear regression. [Other variables considered: gender, age, current CD4 count, CD4 nadir, BMI, lipid parameters, alcohol intake, smoking habit, physical activity, family history for bone fracture, months of ARV exposure, and co-infection with hepatitis viruses; only the variables with p <<0.2 in univariate analyses were included in the final model.
A new neurocognitive interpretation of shoulder position sense during reaching: unexpected competence in the measurement of extracorporeal space
Background.The position sense of the shoulder joint is important during reaching. Objective. To examine the existence of additional
competence of the shoulder with regard to the ability to measure extracorporeal space, through a novel approach, using the shoulder
proprioceptive rehabilitation tool (SPRT), during reaching. Design. Observational case-control study. Methods. We examined 50
subjects: 25 healthy and 25 with impingement syndrome with a mean age [years] of 64.52 +/− 6.98 and 68.36 +/− 6.54, respectively.
Two parameters were evaluated using the SPRT: the integration of visual information and the proprioceptive afferents of the
shoulder (Test 1) and the discriminative proprioceptive capacity of the shoulder, with the subject blindfolded (Test 2). These tasks
assessed the spatial error (in centimeters) by the shoulder joint in reaching movements on the sagittal plane. Results. The shoulder
had proprioceptive features that allowed it to memorize a reaching position and reproduce it (error of 1.22 cm to 1.55 cm in healthy
subjects). This ability was lower in the impingement group, with a statistically significant difference compared to the healthy group
( < 0.05 by Mann–Whitney test). Conclusions. The shoulder has specific expertise in the measurement of the extracorporeal space
during reaching movements that gradually decreases in impingement syndrome
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