2,398 research outputs found
Field Theory as Free Fall
It is shown that the classical field equations pertaining to gravity coupled
to other bosonic fields are equivalent to a single geodesic equation,
describing the free fall of a point particle in superspace. Some implications
for quantum gravity are discussed.Comment: 18 pages, plain late
Quantization in black hole backgrounds
Quantum field theory in a semiclassical background can be derived as an
approximation to quantum gravity from a weak-coupling expansion in the inverse
Planck mass. Such an expansion is studied for evolution on "nice-slices" in the
spacetime describing a black hole of mass M. Arguments for a breakdown of this
expansion are presented, due to significant gravitational coupling between
fluctuations, which is consistent with the statement that existing calculations
of information loss in black holes are not reliable. For a given fluctuation,
the coupling to subsequent fluctuations becomes of order unity by a time of
order M^3. Lack of a systematic derivation of the weakly-coupled/semiclassical
approximation would indicate a role for the non-perturbative dynamics of
gravity, and possibly for the proposal that such dynamics has an essentially
non-local quality.Comment: 28 pages, 4 figures, harvmac. v2: added refs, minor clarification
High energy QCD scattering, the shape of gravity on an IR brane, and the Froissart bound
High-energy scattering in non-conformal gauge theories is investigated using
the AdS/CFT dual string/gravity theory. It is argued that strong-gravity
processes, such as black hole formation, play an important role in the dual
dynamics. Further information about this dynamics is found by performing a
linearized analysis of gravity for a mass near an infrared brane; this gives
the far field approximation to black hole or other strong-gravity effects, and
in particular allows us to estimate their shape. From this shape, one can infer
a total scattering cross-section that grows with center of mass energy as ln^2
E, saturating the Froissart bound.Comment: 27 pages, 1 fig, harvmac. v2: references added, typos corrected v3:
typo correcte
A qualitative study of 2Create: A mental health service user-led art group
Background: 2Create is a mental health service user-led art group in the UK established by graduates of Open Arts, a community arts and mental health project. The study aimed to explore group members’ experiences over its first year.
Methods: Semi-structured interviews were conducted with five current and one former member of 2Create.
Results: Key themes related to organisation (evolving; flexibility; finance; leadership challenges), the studio environment, personal gains (social inclusion; self-esteem; well-being) and future plans (increasing membership; exhibitions; funding applications; social events).
Conclusion: The gains reported indicate that 2Create is beneficial to its members. Although a number of challenges were identified, all participants identified personal and group-wide gains and emphasised that challenges are to be expected when setting up a new group. The key implication for independent mental health user-led arts groups is that support is needed in the early stages and that independence can then be achieved with time
Comments on information loss and remnants
The information loss and remnant proposals for resolving the black hole
information paradox are reconsidered. It is argued that in typical cases
information loss implies energy loss, and thus can be thought of in terms of
coupling to a spectrum of ``fictitious'' remnants. This suggests proposals for
information loss that do not imply planckian energy fluctuations in the low
energy world. However, if consistency of gravity prevents energy
non-conservation, these remnants must then be considered to be real. In either
case, the catastrophe corresponding to infinite pair production remains a
potential problem. Using Reissner-Nordstrom black holes as a paradigm for a
theory of remnants, it is argued that couplings in such a theory may give
finite production despite an infinite spectrum. Evidence for this is found in
analyzing the instanton for Schwinger production; fluctuations from the
infinite number of states lead to a divergent stress tensor, spoiling the
instanton calculation. Therefore naive arguements for infinite production fail.Comment: 30 pages (harvmac l mode) UCSBTH-93-35 (minor reference and typo
corrections
Numerical Analysis of Black Hole Evaporation
Black hole formation/evaporation in two-dimensional dilaton gravity can be
described, in the limit where the number of matter fields becomes large, by
a set of second-order partial differential equations. In this paper we solve
these equations numerically. It is shown that, contrary to some previous
suggestions, black holes evaporate completely a finite time after formation. A
boundary condition is required to evolve the system beyond the naked
singularity at the evaporation endpoint. It is argued that this may be
naturally chosen so as to restore the system to the vacuum. The analysis also
applies to the low-energy scattering of -wave fermions by four-dimensional
extremal, magnetic, dilatonic black holes.Comment: 10 pages, 9 figures in separate uuencoded fil
Tracking the evolution of alternatively spliced exons within the Dscam family
BACKGROUND: The Dscam gene in the fruit fly, Drosophila melanogaster, contains twenty-four exons, four of which are composed of tandem arrays that each undergo mutually exclusive alternative splicing (4, 6, 9 and 17), potentially generating 38,016 protein isoforms. This degree of transcript diversity has not been found in mammalian homologs of Dscam. We examined the molecular evolution of exons within this gene family to locate the point of divergence for this alternative splicing pattern. RESULTS: Using the fruit fly Dscam exons 4, 6, 9 and 17 as seed sequences, we iteratively searched sixteen genomes for homologs, and then performed phylogenetic analyses of the resulting sequences to examine their evolutionary history. We found homologs in the nematode, arthropod and vertebrate genomes, including homologs in several vertebrates where Dscam had not been previously annotated. Among these, only the arthropods contain homologs arranged in tandem arrays indicative of mutually exclusive splicing. We found no homologs to these exons within the Arabidopsis, yeast, tunicate or sea urchin genomes but homologs to several constitutive exons from fly Dscam were present within tunicate and sea urchin. Comparing the rate of turnover within the tandem arrays of the insect taxa (fruit fly, mosquito and honeybee), we found the variants within exons 4 and 17 are well conserved in number and spatial arrangement despite 248–283 million years of divergence. In contrast, the variants within exons 6 and 9 have undergone considerable turnover since these taxa diverged, as indicated by deeply branching taxon-specific lineages. CONCLUSION: Our results suggest that at least one Dscam exon array may be an ancient duplication that predates the divergence of deuterostomes from protostomes but that there is no evidence for the presence of arrays in the common ancestor of vertebrates. The different patterns of conservation and turnover among the Dscam exon arrays provide a striking example of how a gene can evolve in a modular fashion rather than as a single unit
The information paradox and the locality bound
Hawking's argument for information loss in black hole evaporation rests on
the assumption of independent Hilbert spaces for the interior and exterior of a
black hole. We argue that such independence cannot be established without
incorporating strong gravitational effects that undermine locality and
invalidate the use of quantum field theory in a semiclassical background
geometry. These considerations should also play a role in a deeper
understanding of horizon complementarity.Comment: 21 pages, harvmac; v2-3. minor corrections, references adde
Soluble models in 2d dilaton gravity
A one-parameter class of simple models of two-dimensional dilaton gravity,
which can be exactly solved including back-reaction effects, is investigated at
both classical and quantum levels. This family contains the RST model as a
special case, and it continuously interpolates between models having a flat
(Rindler) geometry and a constant curvature metric with a non-trivial dilaton
field. The processes of formation of black hole singularities from collapsing
matter and Hawking evaporation are considered in detail. Various physical
aspects of these geometries are discussed, including the cosmological
interpretation.Comment: 15 pages, harvmac, 3 figure
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