7 research outputs found
On Gravitational Shock Waves in Curved Spacetimes
Some years ago Dray and 't Hooft found the necessary and sufficient
conditions to introduce a gravitational shock wave in a particular class of
vacuum solutions to Einstein's equations. We extend this work to cover cases
where non-vanishing matter fields and cosmological constant are present. The
sources of gravitational waves are massless particles moving along a null
surface such as a horizon in the case of black holes. After we discuss the
general case we give many explicit examples. Among them are the -dimensional
charged black hole (that includes the 4-dimensional Reissner-Nordstr\"om and
the -dimensional Schwarzschild solution as subcases), the 4-dimensional
De-Sitter and Anti-De-Sitter spaces (and the Schwarzschild-De-Sitter black
hole), the 3-dimensional Anti-De-Sitter black hole, as well as backgrounds with
a covariantly constant null Killing vector. We also address the analogous
problem for string inspired gravitational solutions and give a few examples.Comment: 34 pages, harvmac, THU-94/13 (A few minor corrections are made
(mainly arithmetic factors). Final version to appear in Nucl. Phys. B.
Stochastic Tachyon Fluctuations, Marginal Deformations and Shock Waves in String Theory
Starting with exact solutions to string theory on curved spacetimes we obtain
deformations that represent gravitational shock waves. These may exist in the
presence or absence of sources. Sources are effectively induced by a tachyon
field that randomly fluctuates around a zero condensate value. It is shown that
at the level of the underlying conformal field theory (CFT) these deformations
are marginal and moreover all \a'-corrections are taken into account. Explicit
results are given when the original undeformed 4-dimensional backgrounds
correspond to tensor products of combinations of 2-dimensional CFT's, for
instance SL(2,R)/R \times SU(2)/U(1).Comment: 26 pages, harvmac, no figures. Very minor modifications, and in
addition conditions (B.3) and (B.4) were also obtained using beta-function
equations. Version to appear in Phys. Rev.
Pair Creation of Dilaton Black Holes
We consider dilaton gravity theories in four spacetime dimensions
parametrised by a constant , which controls the dilaton coupling, and
construct new exact solutions. We first generalise the C-metric of
Einstein-Maxwell theory () to solutions corresponding to oppositely
charged dilaton black holes undergoing uniform acceleration for general . We
next develop a solution generating technique which allows us to ``embed" the
dilaton C-metrics in magnetic dilaton Melvin backgrounds, thus generalising the
Ernst metric of Einstein-Maxwell theory. By adjusting the parameters
appropriately, it is possible to eliminate the nodal singularities of the
dilaton C-metrics. For (but not for ), it is possible to further
restrict the parameters so that the dilaton Ernst solutions have a smooth
euclidean section with topology , corresponding to
instantons describing the pair production of dilaton black holes in a magnetic
field. A different restriction on the parameters leads to smooth instantons for
all values of with topology .Comment: 22 pages, EFI-93-51, FERMILAB-Pub-93/272-A, UMHEP-393. (Asymptotics
of Ernst solutions clarified, typos repaired
Low prevalence match and mismatch detection in simultaneous face matching: Influence of face recognition ability and feature focus guidance
Simultaneous face matching to verify identity is key to security and policing. However, matching is error-prone, particularly when target item prevalence is low. Two experiments examined whether superior face recognition ability and the use of internal or external facial feature guidance scales would reduce low prevalence effects. In Experiment 1, super-recognisers (n = 317) significantly outperformed typical-ability controls (n = 452), while internal feature guidance enhanced accuracy across all prevalence conditions. However, an unexpected effect in controls revealed higher accuracy in low prevalence conditions, probably because no low match or low mismatch prevalence information was provided. In Experiment 2, top-end-of-typical range ability participants (n = 841) were informed of their low prevalence condition and demonstrated the expected low prevalence effects. Findings and implications are discussed
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Low prevalence match and mismatch detection in simultaneous face matching: Influence of face recognition ability and feature focus guidance
Simultaneous face matching to verify identity is key to security and policing. However, matching is error-prone, particularly when target-item prevalence is low. Two experiments examined whether superior face recognition ability and the use of internal or external facial feature guidance scales would reduce low prevalence effects. In Experiment 1, super-recognisers (n = 317) significantly outperformed typical-ability controls (n = 452), while internal feature guidance enhanced accuracy across all prevalence conditions. However, an unexpected effect in controls revealed higher accuracy in low prevalence conditions, probably because no low-match or low-mismatch prevalence information was provided. In Experiment 2, top-end-of-typical range ability participants (n = 841) were informed of their low prevalence condition and demonstrated the expected low-prevalence effects. Findings and implications are discussed