10,124 research outputs found
Dynamic wormholes
A new framework is proposed for general dynamic wormholes, unifying them with
black holes. Both are generically defined locally by outer trapping horizons,
temporal for wormholes and spatial or null for black and white holes. Thus
wormhole horizons are two-way traversible, while black-hole and white-hole
horizons are only one-way traversible. It follows from the Einstein equation
that the null energy condition is violated everywhere on a generic wormhole
horizon. It is suggested that quantum inequalities constraining negative energy
break down at such horizons. Wormhole dynamics can be developed as for
black-hole dynamics, including a reversed second law and a first law involving
a definition of wormhole surface gravity. Since the causal nature of a horizon
can change, being spatial under positive energy and temporal under sufficient
negative energy, black holes and wormholes are interconvertible. In particular,
if a wormhole's negative-energy source fails, it may collapse into a black
hole. Conversely, irradiating a black-hole horizon with negative energy could
convert it into a wormhole horizon. This also suggests a possible final state
of black-hole evaporation: a stationary wormhole. The new framework allows a
fully dynamical description of the operation of a wormhole for practical
transport, including the back-reaction of the transported matter on the
wormhole. As an example of a matter model, a Klein-Gordon field with negative
gravitational coupling is a source for a static wormhole of Morris & Thorne.Comment: 5 revtex pages, 4 eps figures. Minor change which did not reach
publisher
On the semiclassical treatment of Hawking radiation
In the context of the semiclassical treatment of Hawking radiation we prove
the universality of the reduced canonical momentum for the system of a massive
shell self gravitating in a spherical gravitational field within the Painlev\'e
family of gauges. We show that one can construct modes which are regular on the
horizon both by considering as hamiltonian the exterior boundary term and by
using as hamiltonian the interior boundary term. The late time expansion is
given in both approaches and their time Fourier expansion computed to reproduce
the self reaction correction to the Hawking spectrum.Comment: 18 pages, LaTeX, Corrected typo
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
Initial Value Problems and Signature Change
We make a rigorous study of classical field equations on a 2-dimensional
signature changing spacetime using the techniques of operator theory. Boundary
conditions at the surface of signature change are determined by forming
self-adjoint extensions of the Schr\"odinger Hamiltonian. We show that the
initial value problem for the Klein--Gordon equation on this spacetime is
ill-posed in the sense that its solutions are unstable. Furthermore, if the
initial data is smooth and compactly supported away from the surface of
signature change, the solution has divergent -norm after finite time.Comment: 33 pages, LaTeX The introduction has been altered, and new work
(relating our previous results to continuous signature change) has been
include
Energy distribution in a BTZ black hole spacetime
We evaluate the energy distribution associated with the (2+1)-dimensional
rotating BTZ black hole. The energy-momentum complexes of Landau-Lifshitz and
Weinberg are employed for this computation. Both prescriptions give exactly the
same form of energy distribution. Therefore, these results provide evidence in
support of the claim that, for a given gravitational background, different
energy-momentum complexes can give identical results in three dimensions, as it
is the case in four dimensions.Comment: 16 pages, LaTeX; v2: comments, clarifications and references added,
version to appear in Int.J.Mod.Phys.
Analysis of ultrasonic transducers with fractal architecture
Ultrasonic transducers composed of a periodic piezoelectric composite are generally accepted as the design of choice in many applications. Their architecture is normally very regular and this is due to manufacturing constraints rather than performance optimisation. Many of these manufacturing restrictions no longer hold due to new production methods such as computer controlled, laser cutting, and so there is now freedom to investigate new types of geometry. In this paper, the plane wave expansion model is utilised to investigate the behaviour of a transducer with a self-similar architecture. The Cantor set is utilised to design a 2-2 conguration, and a 1-3 conguration is investigated with a Sierpinski Carpet geometry
Evidence for Past Subduction Earthquakes at a Plate Boundary with Widespread Upper Plate Faulting: Southern Hikurangi Margin, New Zealand
At the southern Hikurangi margin, New Zealand, we use salt marsh stratigraphy, sedimentology, micropaleontology, and radiocarbon dating to document evidence of two earthquakes producing coseismic subsidence and (in one case) a tsunami over the past 1000 yrs. The earthquake at 520-470 yrs before present (B.P.) produced 0.25 +/- 0.1 m of subsidence at Big Lagoon. The earthquake at 880-800 yrs B.P. produced 0.45 +/- 0.1 m of subsidence at Big Lagoon and was accompanied by a tsunami that inundated >= 360 m inland with a probable height of >= 3.3 m. Distinguishing the effects of upper plate faulting from plate interface earthquakes is a significant challenge at this margin. We use correlation with regional upper plate paleoearthquake chronologies and elastic dislocation modeling to determine that the most likely cause of the subsidence and tsunami events is subduction interface rupture, although the older event may have been a synchronous subduction interface and upper plate fault rupture. The southern Hikurangi margin has had no significant (M > 6.5) documented subduction interface earthquakes in historic times, and previous assumptions that this margin segment is prone to rupture in large to great earthquakes were based on seismic and geodetic evidence of strong contemporary plate coupling. This is the first geologic evidence to confirm that the southern Hikurangi margin ruptures in large earthquakes. The relatively short-time interval between the two subduction earthquakes (similar to 350 yrs) is shorter than in current seismic-hazard models.GNSEQC Biennial ProjectNew Zealand Natural Hazards Research Platform and Foundation for Research Science and TechnologyInstitute for Geophysic
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