568 research outputs found
The development of accounts for private households to c.1500 A.D.
SIGLEAvailable from British Library Document Supply Centre- DSC:D95504 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Effective refractive index tensor for weak field gravity
Gravitational lensing in a weak but otherwise arbitrary gravitational field
can be described in terms of a 3 x 3 tensor, the "effective refractive index".
If the sources generating the gravitational field all have small internal
fluxes, stresses, and pressures, then this tensor is automatically isotropic
and the "effective refractive index" is simply a scalar that can be determined
in terms of a classic result involving the Newtonian gravitational potential.
In contrast if anisotropic stresses are ever important then the gravitational
field acts similarly to an anisotropic crystal. We derive simple formulae for
the refractive index tensor, and indicate some situations in which this will be
important.Comment: V1: 8 pages, no figures, uses iopart.cls. V2: 13 pages, no figures.
Significant additions and clarifications. This version to appear in Classical
and Quantum Gravit
A uniqueness theorem for the adS soliton
The stability of physical systems depends on the existence of a state of
least energy. In gravity, this is guaranteed by the positive energy theorem.
For topological reasons this fails for nonsupersymmetric Kaluza-Klein
compactifications, which can decay to arbitrarily negative energy. For related
reasons, this also fails for the AdS soliton, a globally static, asymptotically
toroidal spacetime with negative mass. Nonetheless, arguing from
the AdS/CFT correspondence, Horowitz and Myers (hep-th/9808079) proposed a new
positive energy conjecture, which asserts that the AdS soliton is the unique
state of least energy in its asymptotic class. We give a new structure theorem
for static spacetimes and use it to prove uniqueness of the AdS
soliton. Our results offer significant support for the new positive energy
conjecture and add to the body of rigorous results inspired by the AdS/CFT
correspondence.Comment: Revtex, 4 pages; Matches published version. More detail in Abstract
and one equation corrected. For details of proofs and further results, see
hep-th/020408
Unravelling social constructionism
Social constructionist research is an area of rapidly expanding influence that has brought together theorists from a range of different disciplines. At the same time, however, it has fuelled the development of a new set of divisions. There would appear to be an increasing uneasiness about the implications of a thoroughgoing constructionism, with some regarding it as both theoretically parasitic and politically paralysing. In this paper I review these debates and clarify some of the issues involved. My main argument is that social constructionism is not best understood as a unitary paradigm and that one very important difference is between what Edwards (1997) calls its ontological and epistemic forms. I argue that an appreciation of this distinction not only exhausts many of the disputes that currently divide the constructionist community, but also takes away from the apparent radicalism of much of this work
New Five Dimensional Black Holes Classified by Horizon Geometry, and a Bianchi VI Braneworld
We introduce two new families of solutions to the vacuum Einstein equations
with negative cosmological constant in 5 dimensions. These solutions are static
black holes whose horizons are modelled on the 3-geometries nilgeometry and
solvegeometry. Thus the horizons (and the exterior spacetimes) can be foliated
by compact 3-manifolds that are neither spherical, toroidal, hyperbolic, nor
product manifolds, and therefore are of a topological type not previously
encountered in black hole solutions. As an application, we use the
solvegeometry solutions to construct Bianchi VI braneworld cosmologies.Comment: LaTeX, 20 pages, 2 figures Typographical errors corrected, and
references to printed matter added in favour of preprints where possibl
Scraping the Social? Issues in live social research
What makes scraping methodologically interesting for social and cultural research? This paper seeks to contribute to debates about digital social research by exploring how a ‘medium-specific’ technique for online data capture may be rendered analytically productive for social research. As a device that is currently being imported into social research, scraping has the capacity to re-structure social research, and this in at least two ways. Firstly, as a technique that is not native to social research, scraping risks to introduce ‘alien’ methodological assumptions into social research (such as an pre-occupation with freshness). Secondly, to scrape is to risk importing into our inquiry categories that are prevalent in the social practices enabled by the media: scraping makes available already formatted data for social research. Scraped data, and online social data more generally, tend to come with ‘external’ analytics already built-in. This circumstance is often approached as a ‘problem’ with online data capture, but we propose it may be turned into virtue, insofar as data formats that have currency in the areas under scrutiny may serve as a source of social data themselves. Scraping, we propose, makes it possible to render traffic between the object and process of social research analytically productive. It enables a form of ‘real-time’ social research, in which the formats and life cycles of online data may lend structure to the analytic objects and findings of social research. By way of a conclusion, we demonstrate this point in an exercise of online issue profiling, and more particularly, by relying on Twitter to profile the issue of ‘austerity’. Here we distinguish between two forms of real-time research, those dedicated to monitoring live content (which terms are current?) and those concerned with analysing the liveliness of issues (which topics are happening?)
The Generalized Second Law implies a Quantum Singularity Theorem
The generalized second law can be used to prove a singularity theorem, by
generalizing the notion of a trapped surface to quantum situations. Like
Penrose's original singularity theorem, it implies that spacetime is null
geodesically incomplete inside black holes, and to the past of spatially
infinite Friedmann--Robertson--Walker cosmologies. If space is finite instead,
the generalized second law requires that there only be a finite amount of
entropy producing processes in the past, unless there is a reversal of the
arrow of time. In asymptotically flat spacetime, the generalized second law
also rules out traversable wormholes, negative masses, and other forms of
faster-than-light travel between asymptotic regions, as well as closed timelike
curves. Furthermore it is impossible to form baby universes which eventually
become independent of the mother universe, or to restart inflation. Since the
semiclassical approximation is used only in regions with low curvature, it is
argued that the results may hold in full quantum gravity. An introductory
section describes the second law and its time-reverse, in ordinary and
generalized thermodynamics, using either the fine-grained or the coarse-grained
entropy. (The fine-grained version is used in all results except those relating
to the arrow of time.) A proof of the coarse-grained ordinary second law is
given.Comment: 46 pages, 8 figures. v2: discussion of global hyperbolicity revised
(4.1, 5.2), more comments on AdS. v3: major revisions including change of
title. v4: similar to published version, but with corrections to plan of
paper (1) and definition of global hyperbolicity (3.2). v5: fixed proof of
Thm. 1, changed wording of Thm. 3 & proof of Thm. 4, revised Sec. 5.2, new
footnote
Non-Existence of Black Holes in Certain Spacetimes
Assuming certain asymptotic conditions, we prove a general theorem on the
non-existence of static regular (i.e., nondegenerate) black holes in spacetimes
with a negative cosmological constant, given that the fundamental group of
space is infinite. We use this to rule out the existence of regular negative
mass AdS black holes with Ricci flat scri. For any mass, we also rule out a
class of conformally compactifiable static black holes whose conformal infinity
has positive scalar curvature and infinite fundamental group, subject to our
asymptotic conditions. In a limited, but important, special case our result
adds new support to the AdS/CFT inspired positive mass conjecture of Horowitz
and Myers.Comment: 17 pages, Latex. Typos corrected, minor changes to the text. Accepted
for publication in Classical and Quantum Gravit
Histological changes in intra-oral skin flaps
This review outlines the histological features of intra-oral skin flaps and their pathology as encountered in a routine head and neck diagnostic histopathology service. Problems in recognising and interpreting unfamiliar and complex appearances, and areas of diagnostic and prognostic uncertainty, are highlighted
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