12,952 research outputs found
String dynamics and ejection along the axis of a spinning black hole
Relativistic current carrying strings moving axisymmetrically on the
background of a Kerr black hole are studied. The boundaries and possible types
of motion of a string with a given energy and current are found. Regions of
parameters for which the string falls into the black hole, or is trapped in a
toroidal volume, or can escape to infinity, are identified, and representative
trajectories are examined by numerical integration, illustrating various
interesting behaviors. In particular, we find that a string can start out at
rest near the equatorial plane and, after bouncing around, be ejected out along
the axis, some of its internal (elastic or rotational kinetic) energy having
been transformed into translational kinetic energy. The resulting velocity can
be an order unity fraction of the speed of light. This process results from the
presence of an outer tension barrier and an inner angular momentum barrier that
are deformed by the gravitational field. We speculatively discuss the possible
astrophysical significance of this mechanism as a means of launching a
collimated jet of MHD plasma flux tubes along the spin axis of a gravitating
system fed by an accretion disk.Comment: 14 pages, 11 figures; v.2: minor edits, references added, typos
corrected, published versio
Comment on "Accelerated Detectors and Temperature in (Anti) de Sitter Spaces"
It is shown how the results of Deser and Levin on the response of accelerated
detectors in anti-de Sitter space can be understood from the same general
perspective as other thermality results in spacetimes with bifurcate Killing
horizons.Comment: 5 pages, LaTe
Construction of N = 2 Chiral Supergravity Compatible with the Reality Condition
We construct N = 2 chiral supergravity (SUGRA) which leads to Ashtekar's
canonical formulation. The supersymmetry (SUSY) transformation parameters are
not constrained at all and auxiliary fields are not required in contrast with
the method of the two-form gravity. We also show that our formulation is
compatible with the reality condition, and that its real section is reduced to
the usual N = 2 SUGRA up to an imaginary boundary term.Comment: 16 pages, late
Non-equilibrium Thermodynamics of Spacetime
It has previously been shown that the Einstein equation can be derived from
the requirement that the Clausius relation dS = dQ/T hold for all local
acceleration horizons through each spacetime point, where dS is one quarter the
horizon area change in Planck units, and dQ and T are the energy flux across
the horizon and Unruh temperature seen by an accelerating observer just inside
the horizon. Here we show that a curvature correction to the entropy that is
polynomial in the Ricci scalar requires a non-equilibrium treatment. The
corresponding field equation is derived from the entropy balance relation dS
=dQ/T+dS_i, where dS_i is a bulk viscosity entropy production term that we
determine by imposing energy-momentum conservation. Entropy production can also
be included in pure Einstein theory by allowing for shear viscosity of the
horizon.Comment: 4 pages. Dedicated to Rafael Sorkin on the occasion of his 60th
birthda
Flood Wave-Tide Wave Interaction on the James River During the Agnes Flood
During the Agnes flood hourly tidal height data were collected at seven locations along the tidal James River and currents were measured at two transects in the lower James. A comparison between actual tides and currents and the predicted tidal features as given by the tide and tidal current tables of NOAA was made. Results of this comparison show that Agnes did significantly affect water levels in the upper portion of the tidal James, especially near Richmond. However in the lower portion of the James no discernible rise was evident due to the passage of the flood crest. A small storm surge (\u3c2 \u3efeet) was noted on the day of the passage of Agnes, 21 June, throughout the tidal James. A phase shift in times of high and low water due to the interaction of the two wave systems was not observed. In the freshwater portion of the tidal James currents continually ebbed during the passage of the flood crest. In the saline portion of the system, the flood effect on the currents was limited to the surface portion of the channel.https://scholarworks.wm.edu/vimsbooks/1065/thumbnail.jp
General covariance, and supersymmetry without supersymmetry
An unusual four-dimensional generally covariant and supersymmetric SU(2)
gauge theory is described. The theory has propagating degrees of freedom, and
is invariant under a local (left-handed) chiral supersymmetry, which is half
the supersymmetry of supergravity. The Hamiltonian 3+1 decomposition of the
theory reveals the remarkable feature that the local supersymmetry is a
consequence of Yang-Mills symmetry, in a manner reminiscent of how general
coordinate invariance in Chern-Simons theory is a consequence of Yang-Mills
symmetry. It is possible to write down an infinite number of conserved
currents, which strongly suggests that the theory is classically integrable. A
possible scheme for non-perturbative quantization is outlined. This utilizes
ideas that have been developed and applied recently to the problem of
quantizing gravity.Comment: 17 pages, RevTeX, two minor errors correcte
A Covariant Approach To Ashtekar's Canonical Gravity
A Lorentz and general co-ordinate co-variant form of canonical gravity, using
Ashtekar's variables, is investigated. A co-variant treatment due to Crnkovic
and Witten is used, in which a point in phase space represents a solution of
the equations of motion and a symplectic functional two form is constructed
which is Lorentz and general co-ordinate invariant. The subtleties and
difficulties due to the complex nature of Ashtekar's variables are addressed
and resolved.Comment: 18 pages, Plain Te
Reality Conditions and Ashtekar Variables: a Different Perspective
We give in this paper a modified self-dual action that leads to the
-ADM formalism without having to face the difficult second class
constraints present in other approaches (for example if one starts from the
Hilbert-Palatini action). We use the new action principle to gain some new
insights into the problem of the reality conditions that must be imposed in
order to get real formulations from complex general relativity. We derive also
a real formulation for Lorentzian general relativity in the Ashtekar phase
space by using the modified action presented in the paper.Comment: 22 pages, LATEX, Preprint CGPG-94/10-
The association among diet, dietary fiber, and bowel preparation at colonoscopy
BACKGROUND AND AIMS: Pre-colonoscopy dietary restrictions vary widely and lack evidence-based guidance. We investigated whether fiber and various other foods/macronutrients consumed during the 3 days before colonoscopy are associated with bowel preparation quality.
METHODS: This was a prospective observational study among patients scheduled for outpatient colonoscopy. Patients received instructions including split-dose polyethylene glycol, avoidance of vegetables/beans 2 days before colonoscopy, and a clear liquid diet the day before colonoscopy. Two 24-hour dietary recall interviews and 1 patient-recorded food log measured dietary intake on the 3 days before colonoscopy. The Nutrition Data System for Research was used to estimate dietary exposures. Our primary outcome was the quality of bowel preparation measured by the Boston Bowel Preparation Scale (BBPS).
RESULTS: We enrolled 201 patients from November 2015 to September 2016 with complete data for 168. The mean age was 59 years (standard deviation, 7 years), and 90% of colonoscopies were conducted for screening/surveillance. Only 17% and 77% of patients complied with diet restrictions 2 and 1 day(s) before colonoscopy, respectively. We found no association between foods consumed 2 and 3 days before colonoscopy and BBPS scores. However, BPPS was positively associated with intake of gelatin, and inversely associated with intake of red meat, poultry, and vegetables on the day before colonoscopy.
CONCLUSIONS: Our findings support recent guidelines encouraging unrestricted diets >1 day before colonoscopy if using a split-dose bowel regimen. Furthermore, we found no evidence to restrict dietary fiber 1 day before colonoscopy. We also found evidence to promote consumption of gelatin and avoidance of red meat, poultry, and vegetables 1 day before colonoscopy.Dr Jacobson has acted as a consultant for MOTUS GI and Remedy Partners. All other authors disclosed no financial relationships relevant to this publication. Supported by NIH/NIDDK R21DK105476. (R21DK105476 - NIH/NIDDK)Accepted manuscrip
Brain Differently Changes Its Algorithms in Parallel Processing of Visual Information
Feedback from the visual cortex (Vl) to the Lateral Geniculate Nucleus (LGN) in macaque monkey increase contrast gain of LGN neurons for black and white (B&W) and for color (C) stimuli. LGN parvocellular cells responses to B&W gratings are enhanced by feedback multiplicatively and in contrast independent manner. However, in magnocellular neurons corticofugal pathways enhance cells responses in a contrast~dependent non-linear manner. For C stimuli cortical feedback enhances parvocellular neurons responses in a very strong contrast-dependent manner. Based on these results [13] we propose a model which includes excitatory and inhibitory effects on cells activity (shunting equations) in retina and LGN while taking into account the anatomy of cortical feedback connections. The main mechanisms related to different algorithms of the data processing in the visual brain are differences in feedback properties from Vl to parvocellular (PC) and to magnocellular (MC) neurons. Descending pathways from Vl change differently receptive field (RF) structure of PC and MC cells. For B&W stimuli, in PC cells feedback changes gain similarly in the RF center and in the RF surround, leaving PC RF structure invariant. However, feedback influence MC cells in two ways: directly and through LGN interneurons, which together changes gain and sizes of their RF center differently than gain and size of the RF surround. For C stimuli PC cells operate like MC cells for B&W. The first mechanism extracts from the stimulus an important features in a independent way from other stimulus parameters, whereas the second channel changes its tuning properties as a function of other stimulus attributes like contrast and/or spatial extension. The model suggests novel idea about the possible functional role of PC and MC pathways
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