2,558 research outputs found
Dimensionally continued Oppenheimer-Snyder gravitational collapse II: solutions in odd dimensions
The Lovelock gravity extends the theory of general relativity to higher
dimensions in such a way that the field equations remain of second order. The
theory has many constant coefficients with no a priori meaning. Nevertheless it
is possible to reduce them to two, the cosmological constant and Newton's
constant. In this process one separates theories in even dimensions from
theories in odd dimensions. In a previous work gravitational collapse in even
dimensions was analysed. In this work attention is given to odd dimensions. It
is found that black holes also emerge as the final state of gravitational
collapse of a regular dust fluid.Comment: 16 pages, 3figures, latex Journal: to appear in Journal of
Mathematical Physic
Gravitational magnetic monopoles and Majumdar-Papapetrou stars
A large amount of work has been dedicated to studying general relativity
coupled to non-Abelian Yang-Mills type theories. It has been shown that the
magnetic monopole, a solution of the Yang-Mills-Higgs equations can be coupled
to gravitation. For a low Higgs mass there are regular solutions, and for a
sufficiently massive monopole the system develops an extremal magnetic
Reissner-Nordstrom quasi-horizon. These solutions, called quasi-black holes,
although non-singular, are arbitrarily close to having a horizon. However, at
the critical value the quasi-black hole turns into a degenerate spacetime. On
the other hand, for a high Higgs mass, a sufficiently massive monopole develops
also a quasi-black hole, but it turns into an extremal true horizon, with
matter fields outside. One can also put a small Schwarzschild black hole inside
the magnetic monopole, an example of a non-Abelian black hole. Surprisingly,
Majumdar-Papapetrou systems, Abelian systems constructed from extremal dust,
also show a resembling behavior. Previously, we have reported that one can find
Majumdar-Papapetrou solutions which can be arbitrarily close of being a black
hole, displaying quasi-black hole behavior. With the aim of better
understanding the similarities between gravitational monopoles and
Majumdar-Papapetrou systems, we study a system composed of two extremal
electrically charged spherical shells (or stars, generically) in the
Einstein--Maxwell--Majumdar-Papapetrou theory. We review the gravitational
properties of the monopoles, and compare with the properties of the double
extremal electric shell system. These quasi-black holes can help in the
understanding of true black holes, and can give insight into the nature of the
entropy of black holes in the form of entanglement.Comment: 38 pages,9 Figures, minor change
New concepts in global wargaming
This report addresses resurgence in wargaming and new techniques used to address complex political-military issues. Relationships between gaming and analysis are examined and benefits, opportunities, and drawbacks of gaming addressed. Requirements for Control Team in modern computer-assisted State and Path games and seminar games are discussed. Authors conclude with the value of modern simulation techniqueshttp://archive.org/details/newconceptsinglo00tritN
Exposure to violence and PTSD symptoms among Somali women
Posttraumatic stress disorder (PTSD) symptoms, exposure to traumatic stressors, and health care utilization were examined in 84 women attending a primary health care clinic in Mogadishu, Somalia. The Somalia-Posttraumatic Diagnostic Scale was used in this active warzone to measure symptoms. Nearly all women reported high levels of confrontations with violence; half described being exposed to a potentially traumatizing event. Nearly one third had significant PTSD symptoms. Compared to those who did not, women who reported exposure to a traumatic stressor reported more confrontations with violence (7.1 vs. 3.3; p < . 001), health complaints (3.8 vs. 2.9; p = .03), and nearly 3 times as much (p = .03) health service utilization. A potentially traumatizing event was found to be a simplified proxy for assessing mental health distress in women attending a primary health care facility in highly insecure, unpredictable, resource-limited settings
Quantum matter wave dynamics with moving mirrors
When a stationary reflecting wall acting as a perfect mirror for an atomic
beam with well defined incident velocity is suddenly removed, the density
profile develops during the time evolution an oscillatory pattern known as
diffraction in time. The interference fringes are suppressed or their
visibility is diminished by several effects such as averaging over a
distribution of incident velocities, apodization of the aperture function,
atom-atom interactions, imperfect reflection or environmental noise. However,
when the mirror moves with finite velocity along the direction of propagation
of the beam, the visibility of the fringes is enhanced. For mirror velocities
below beam velocity, as used for slowing down the beam, the matter wave splits
into three regions separated by space-time points with classical analogues. For
mirror velocities above beam velocity a visibility enhancement occurs without a
classical counterpart. When the velocity of the beam approaches that of the
mirror the density oscillations rise by a factor 1.8 over the stationary value.Comment: 5.2 pages, 6 figure
Four-path interference and uncertainty principle in photodetachment microscopy
We study the quantal motion of electrons emitted by a pointlike monochromatic
isotropic source into parallel uniform electric and magnetic fields. The
two-path interference pattern in the emerging electron wave due to the electric
force is modified by the magnetic lens effect which periodically focuses the
beam into narrow filaments along the symmetry axis. There, four classical paths
interfere. With increasing electron energy, the current distribution changes
from a quantum regime governed by the uncertainty principle, to an intricate
spatial pattern that yields to a semiclassical analysis.Comment: submitted to Europhysics Letter
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