12,029 research outputs found
Angular momentum conservation for uniformly expanding flows
Angular momentum has recently been defined as a surface integral involving an
axial vector and a twist 1-form, which measures the twisting around of
space-time due to a rotating mass. The axial vector is chosen to be a
transverse, divergence-free, coordinate vector, which is compatible with any
initial choice of axis and integral curves. Then a conservation equation
expresses rate of change of angular momentum along a uniformly expanding flow
as a surface integral of angular momentum densities, with the same form as the
standard equation for an axial Killing vector, apart from the inclusion of an
effective energy tensor for gravitational radiation.Comment: 5 revtex4 pages, 3 eps figure
Enabling occupational therapy students to take a fresh approach to psychosis
This practice evaluation describes the implementation of a 2-day workshop on
psychosis with third-year undergraduate occupational therapy students at
Brunel University. The work was undertaken by the teaching team at Brunel
University, a clinical psychologist working in assertive outreach and an
occupational therapist working in community mental health. The background
to the project and the way in which the 2-day workshop was adapted to
accommodate the university timetable are outlined. An evaluation of the
workshop, its place in the occupational therapy programme and the feedback
from students are presented
Fate of the first traversible wormhole: black-hole collapse or inflationary expansion
We study numerically the stability of Morris & Thorne's first traversible
wormhole, shown previously by Ellis to be a solution for a massless ghost
Klein-Gordon field. Our code uses a dual-null formulation for spherically
symmetric space-time integration, and the numerical range covers both universes
connected by the wormhole. We observe that the wormhole is unstable against
Gaussian pulses in either exotic or normal massless Klein-Gordon fields. The
wormhole throat suffers a bifurcation of horizons and either explodes to form
an inflationary universe or collapses to a black hole, if the total input
energy is respectively negative or positive. As the perturbations become small
in total energy, there is evidence for critical solutions with a certain
black-hole mass or Hubble constant. The collapse time is related to the initial
energy with an apparently universal critical exponent. For normal matter, such
as a traveller traversing the wormhole, collapse to a black hole always
results. However, carefully balanced additional ghost radiation can maintain
the wormhole for a limited time. The black-hole formation from a traversible
wormhole confirms the recently proposed duality between them. The inflationary
case provides a mechanism for inflating, to macroscopic size, a Planck-sized
wormhole formed in space-time foam.Comment: 10 pages, RevTeX4, 11 figures, epsf.st
Unified first law of black-hole dynamics and relativistic thermodynamics
A unified first law of black-hole dynamics and relativistic thermodynamics is
derived in spherically symmetric general relativity. This equation expresses
the gradient of the active gravitational energy E according to the Einstein
equation, divided into energy-supply and work terms. Projecting the equation
along the flow of thermodynamic matter and along the trapping horizon of a
blackhole yield, respectively, first laws of relativistic thermodynamics and
black-hole dynamics. In the black-hole case, this first law has the same form
as the first law of black-hole statics, with static perturbations replaced by
the derivative along the horizon. There is the expected term involving the area
and surface gravity, where the dynamic surface gravity is defined as in the
static case but using the Kodama vector and trapping horizon. This surface
gravity vanishes for degenerate trapping horizons and satisfies certain
expected inequalities involving the area and energy. In the thermodynamic case,
the quasi-local first law has the same form, apart from a relativistic factor,
as the classical first law of thermodynamics, involving heat supply and
hydrodynamic work, but with E replacing the internal energy. Expanding E in the
Newtonian limit shows that it incorporates the Newtonian mass, kinetic energy,
gravitational potential energy and thermal energy. There is also a weak type of
unified zeroth law: a Gibbs-like definition of thermal equilibrium requires
constancy of an effective temperature, generalising the Tolman condition and
the particular case of Hawking radiation, while gravithermal equilibrium
further requires constancy of surface gravity. Finally, it is suggested that
the energy operator of spherically symmetric quantum gravity is determined by
the Kodama vector, which encodes a dynamic time related to E.Comment: 18 pages, TeX, expanded somewhat, to appear in Class. Quantum Gra
Construction and enlargement of traversable wormholes from Schwarzschild black holes
Analytic solutions are presented which describe the construction of a
traversable wormhole from a Schwarzschild black hole, and the enlargement of
such a wormhole, in Einstein gravity. The matter model is pure radiation which
may have negative energy density (phantom or ghost radiation) and the
idealization of impulsive radiation (infinitesimally thin null shells) is
employed.Comment: 22 pages, 7 figure
The Magnetization of Cu_2(C_5H_{12}N_2)_2Cl_4 : A Heisenberg Spin Ladder System
We study the magnetization of a Heisenberg spin ladder using exact
diagonalization techniques, finding three distinct magnetic phases. We consider
the results in relation to the experimental behaviour of the new copper
compound Cu_2(C_5H_{12}N_2)_2Cl_4 and deduce that the compound is well
described by such a model with a ratio of `chain' to `rung' bond strengths
(J/J^\prime) of the order of 0.2, consistent with results from the magnetic
susceptibility. The effects of temperature, spin impurities and additional
diagonal bonds are presented and we give evidence that these diagonal bonds are
indeed of a ferromagnetic nature.Comment: Latex file (4 pages), related figures (encapsulated postscript)
appende
Energy of gravitational radiation in plane-symmetric space-times
Gravitational radiation in plane-symmetric space-times can be encoded in a
complex potential, satisfying a non-linear wave equation. An effective energy
tensor for the radiation is given, taking a scalar-field form in terms of the
potential, entering the field equations in the same way as the matter energy
tensor. It reduces to the Isaacson energy tensor in the linearized,
high-frequency approximation. An energy conservation equation is derived for a
quasi-local energy, essentially the Hawking energy. A transverse pressure
exerted by interacting low-frequency gravitational radiation is predicted.Comment: 7 REVTeX4 page
Dilatonic wormholes: construction, operation, maintenance and collapse to black holes
The CGHS two-dimensional dilaton gravity model is generalized to include a
ghost Klein-Gordon field, i.e. with negative gravitational coupling. This
exotic radiation supports the existence of static traversible wormhole
solutions, analogous to Morris-Thorne wormholes. Since the field equations are
explicitly integrable, concrete examples can be given of various dynamic
wormhole processes, as follows. (i) Static wormholes are constructed by
irradiating an initially static black hole with the ghost field. (ii) The
operation of a wormhole to transport matter or radiation between the two
universes is described, including the back-reaction on the wormhole, which is
found to exhibit a type of neutral stability. (iii) It is shown how to maintain
an operating wormhole in a static state, or return it to its original state, by
turning up the ghost field. (iv) If the ghost field is turned off, either
instantaneously or gradually, the wormhole collapses into a black hole.Comment: 9 pages, 7 figure
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