3,124 research outputs found
Static and non-static quantum effects in two-dimensional dilaton gravity
We study backreaction effects in two-dimensional dilaton gravity. The
backreaction comes from an term which is a part of the one-loop effective
action arising from massive scalar field quantization in a certain
approximation. The peculiarity of this term is that it does not contribute to
the Hawking radiation of the classical black hole solution of the field
equations. In the static case we examine the horizon and the physical
singularity of the new black hole solutions. Studying the possibility of time
dependence we see the generation of a new singularity. The particular solution
found still has the structure of a black hole, indicating that non-thermal
effects cannot lead, at least in this approximation, to black hole evaporation.Comment: 10 pages, no figure
Timeless path integral for relativistic quantum mechanics
Starting from the canonical formalism of relativistic (timeless) quantum
mechanics, the formulation of timeless path integral is rigorously derived. The
transition amplitude is reformulated as the sum, or functional integral, over
all possible paths in the constraint surface specified by the (relativistic)
Hamiltonian constraint, and each path contributes with a phase identical to the
classical action divided by . The timeless path integral manifests the
timeless feature as it is completely independent of the parametrization for
paths. For the special case that the Hamiltonian constraint is a quadratic
polynomial in momenta, the transition amplitude admits the timeless Feynman's
path integral over the (relativistic) configuration space. Meanwhile, the
difference between relativistic quantum mechanics and conventional
nonrelativistic (with time) quantum mechanics is elaborated on in light of
timeless path integral.Comment: 41 pages; more references and comments added; version to appear in
CQ
Emprego de isotermas de langmuir e freundlich no processo de adsorção de paraquat por hidrogéis de poliacrilamida e metilcelulose.
ASTROD, ASTROD I and their gravitational-wave sensitivities
ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) is a
mission concept with three spacecraft -- one near L1/L2 point, one with an
inner solar orbit and one with an outer solar orbit, ranging coherently with
one another using lasers to test relativistic gravity, to measure the solar
system and to detect gravitational waves. ASTROD I with one spacecraft ranging
optically with ground stations is the first step toward the ASTROD mission. In
this paper, we present the ASTROD I payload and accelerometer requirements,
discuss the gravitational-wave sensitivities for ASTROD and ASTROD I, and
compare them with LISA and radio-wave PDoppler-tracking of spacecraft.Comment: presented to the 5th Edoardo Amaldi Conference (July 6-11, 2003) and
submitted to Classical and Quantum Gravit
Patchy Interspecific Sequence Similarities Efficiently Identify Positive cis-Regulatory Elements in the Sea Urchin
We demonstrate that interspecific sequence conservation can provide a systematic guide to the identification of functional cis-regulatory elements within a large expanse of genomic DNA. The test was carried out on the otx gene of Strongylocentrotus purpuratus. This gene plays a major role in the gene regulatory network that underlies endomesoderm specification in the embryo. The cis-regulatory organization of the otx gene is expected to be complex, because the gene has three different start sites (X. Li, C.-K. Chuang, C.-A. Mao, L. M. Angerer, and W. H. Klein, 1997, Dev. Biol. 187, 253–266), and it is expressed in many different spatial domains of the embryo. BAC recombinants containing the otx gene were isolated from Strongylocentrotus purpuratus and Lytechinus variegatus libraries, and the ordered sequence of these BACs was obtained and annotated. Sixty kilobases of DNA flanking the gene, and included in the BAC sequence from both species, were scanned computationally for short conserved sequence elements. For this purpose, we used a newly constructed software package assembled in our laboratory, “FamilyRelations.” This tool allows detection of sequence similarities above a chosen criterion within sliding windows set at 20–50 bp. Seventeen partially conserved regions, most a few hundred base pairs long, were amplified from the S. purpuratus BAC DNA by PCR, inserted in an expression vector driving a CAT reporter, and tested for cis-regulatory activity by injection into fertilized S. purpuratus eggs. The regulatory activity of these constructs was assessed by whole-mount in situ hybridization (WMISH) using a probe against CAT mRNA. Of the 17 constructs, 11 constructs displayed spatially restricted regulatory activity, and 6 were inactive in this test. The domains within which the cis-regulatory constructs were expressed are approximately consistent with results from a WMISH study on otx expression in the embryo, in which we used probes specific for the mRNAs generated from each of the three transcription start sites. Four separate cis-regulatory elements that specifically produce endomesodermal expression were identified, as well as ubiquitously active elements, and ectoderm-specific elements. We confirm predictions from other work with respect to target sites for specific transcription factors within the elements that express in the endoderm
Does the host match the content? A taxonomical update on online consumption communities
This article proposes a taxonomy of online consumption communities in order to address this rather ambiguously conceptualised research field. Specifically, intercommunity differences are investigated with regard to how content focus (brand vs activity) and its congruency with the type of host (doubled vs mixed) affect consumers’ posting behaviour. Based on an online survey (n = 888), a series of regressions of various benefits on posting behaviour supports the usability of the proposed taxonomy. In particular, social benefits had the strongest effect on consumers’ posting behaviour across all communities, while the effects of functional, altruistic and sharing benefits varied in significance and direction of influence when accounting for the different community characteristics. These findings help marketing managers to design online communities and motivate consumers to contribute. © 2015, Westburn Publishers Ltd
A Matrix Model for the Null-Brane
The null-brane background is a simple smooth 1/2 BPS solution of string
theory. By tuning a parameter, this background develops a big crunch/big bang
type singularity. We construct the DLCQ description of this space-time in terms
of a Yang-Mills theory on a time-dependent space-time. Our dual Matrix
description provides a non-perturbative framework in which the fate of both
(null) time, and the string S-matrix can be studied.Comment: 26 pages, LaTeX; references adde
- …