701 research outputs found
The Gibbs paradox, Black hole entropy and the thermodynamics of isolated horizons
This letter presents a new, solely thermodynamical argument for considering
the states of the quantum isolated horizon of a black hole as distinguishable.
We claim that only if the states are distinguishable, the thermodynamic entropy
is an extensive quantity and can be well-defined. To show this, we make a
comparison with a classical ideal gas system whose statistical description
makes only sense if an additional 1/N!-factor is included in the state counting
in order to cure the Gibbs paradox. The case of the statistical description of
a quantum isolated horizon is elaborated, to make the claim evident.Comment: 8 pages, closest to the published version; taken from the author's
diploma thesi
Helicity, polarization, and Riemann-Silberstein vortices
Riemann-Silberstein (RS) vortices have been defined as surfaces in spacetime
where the complex form of a free electromagnetic field given by F=E+iB is null
(F.F=0), and they can indeed be interpreted as the collective history swept out
by moving vortex lines of the field. Formally, the nullity condition is similar
to the definition of "C-lines" associated with a monochromatic electric or
magnetic field, which are curves in space where the polarization ellipses
degenerate to circles. However, it was noted that RS vortices of monochromatic
fields generally oscillate at optical frequencies and are therefore
unobservable while electric and magnetic C-lines are steady. Here I show that
under the additional assumption of having definite helicity, RS vortices are
not only steady but they coincide with both sets of C-lines, electric and
magnetic. The two concepts therefore become one for waves of definite frequency
and helicity. Since the definition of RS vortices is relativistically invariant
while that of C-lines is not, it may be useful to regard the vortices as a
wideband generalization of C-lines for waves of definite helicity.Comment: 5 pages, no figures. Submitted to J of Optics A, special issue on
Singular Optics; minor changes from v.
Landscape science: a Russian geographical tradition
The Russian geographical tradition of landscape science (landshaftovedenie) is analyzed with particular reference to its initiator, Lev Semenovich Berg (1876-1950). The differences between prevailing Russian and Western concepts of landscape in geography are discussed, and their common origins in German geographical thought in the late nineteenth and early twentieth centuries are delineated. It is argued that the principal differences are accounted for by a number of factors, of which Russia's own distinctive tradition in environmental science deriving from the work of V. V. Dokuchaev (1846-1903), the activities of certain key individuals (such as Berg and C. O. Sauer), and the very different social and political circumstances in different parts of the world appear to be the most significant. At the same time it is noted that neither in Russia nor in the West have geographers succeeded in specifying an agreed and unproblematic understanding of landscape, or more broadly in promoting a common geographical conception of human-environment relationships. In light of such uncertainties, the latter part of the article argues for closer international links between the variant landscape traditions in geography as an important contribution to the quest for sustainability
From Spinor Geometry to Complex General Relativity
An attempt is made of giving a self-contained (although incomplete)
introduction to holomorphic ideas in general relativity, following work over
the last thirty years by several authors. The main topics are complex
manifolds, two-component spinor calculus, conformal gravity, alpha-planes in
Minkowski space-time, alpha-surfaces and twistor geometry, anti-self-dual
space-times and Penrose transform, spin-3/2 potentials, heaven spaces and
heavenly equations.Comment: With kind permission from Springer Science and Business Media to use
material in the first 5 sections taken from the 1995 Kluwer book "Complex
General Relativity" by G. Esposito. In the revised version, 11 References
have been adde
The Economic Adjustment Program for Portugal : assessing welfare impact in a heterogeneous-agent framework
The sovereign debt crisis, triggered by the 2007-08 global financial cri- sis, has affected several European Union (EU) countries, leading to unprecedented financial assistance programs. In May 2011, the Portuguese Government set an agreement with the Troika (a supranational institution composed by the European Commission (EC), the European Central Bank (ECB) and the International Monetary Fund (IMF)), through which, in exchange for external help, the Portuguese author- ities committed to an Economic Adjustment Program (EAP). In order to assess the impacts of the EAP on welfare and, in particular, on inequality, this paper simulates the debt consolidation strategy proposed by the Troika using a general equilibrium model with heterogeneous agents. The model enables to explore the impacts of the fiscal adjustment on the endogenous cross-section distribution of income, wealth and welfare. Our results predict a positive net welfare gain, despite the existence of sig- nificant transition costs in terms of output losses and inequality, especially during the first years of implementation. Overall, the net positive welfare gains are biased towards the poorer, which means that the consolidation plan will be, in the end, equality-enhancing. These results reflect the instruments involved in the consolida- tion strategy: productive and unproductive expenditure cuts combined with a slight increase in social transfers. Furthermore, the simulation predicts a positive impact on the Portuguese net foreign asset (NFA) position. Assuming this prediction is correct, this strongly supports the motivation for the adoption of the Economic Adjustment Program which considers the large external indebtedness of Portugal as a central issue in the economic diagnosis.info:eu-repo/semantics/publishedVersio
Twistors and Black Holes
Motivated by black hole physics in N=2, D=4 supergravity, we study the
geometry of quaternionic-Kahler manifolds M obtained by the c-map construction
from projective special Kahler manifolds M_s. Improving on earlier treatments,
we compute the Kahler potentials on the twistor space Z and Swann space S in
the complex coordinates adapted to the Heisenberg symmetries. The results bear
a simple relation to the Hesse potential \Sigma of the special Kahler manifold
M_s, and hence to the Bekenstein-Hawking entropy for BPS black holes. We
explicitly construct the ``covariant c-map'' and the ``twistor map'', which
relate real coordinates on M x CP^1 (resp. M x R^4/Z_2) to complex coordinates
on Z (resp. S). As applications, we solve for the general BPS geodesic motion
on M, and provide explicit integral formulae for the quaternionic Penrose
transform relating elements of H^1(Z,O(-k)) to massless fields on M annihilated
by first or second order differential operators. Finally, we compute the exact
radial wave function (in the supergravity approximation) for BPS black holes
with fixed electric and magnetic charges.Comment: 47 pages, v2: typos corrected, reference added, v3: minor change
Serous cystic neoplasm of the pancreas: A multinational study of 2622 patients under the auspices of the International Association of Pancreatology and European Pancreatic Club (European Study Group on Cystic Tumors of the Pancreas)
OBJECTIVES:
Serous cystic neoplasm (SCN) is a cystic neoplasm of the pancreas whose natural history is poorly known. The purpose of the study was to attempt to describe the natural history of SCN, including the specific mortality.
DESIGN:
Retrospective multinational study including SCN diagnosed between 1990 and 2014.
RESULTS:
2622 patients were included. Seventy-four per cent were women, and median age at diagnosis was 58\u2005years (16-99). Patients presented with non-specific abdominal pain (27%), pancreaticobiliary symptoms (9%), diabetes mellitus (5%), other symptoms (4%) and/or were asymptomatic (61%). Fifty-two per cent of patients were operated on during the first year after diagnosis (median size: 40\u2005mm (2-200)), 9% had resection beyond 1\u2005year of follow-up (3\u2005years (1-20), size at diagnosis: 25\u2005mm (4-140)) and 39% had no surgery (3.6\u2005years (1-23), 25.5\u2005mm (1-200)). Surgical indications were (not exclusive) uncertain diagnosis (60%), symptoms (23%), size increase (12%), large size (6%) and adjacent organ compression (5%). In patients followed beyond 1\u2005year (n=1271), size increased in 37% (growth rate: 4\u2005mm/year), was stable in 57% and decreased in 6%. Three serous cystadenocarcinomas were recorded. Postoperative mortality was 0.6% (n=10), and SCN's related mortality was 0.1% (n=1).
CONCLUSIONS:
After a 3-year follow-up, clinical relevant symptoms occurred in a very small proportion of patients and size slowly increased in less than half. Surgical treatment should be proposed only for diagnosis remaining uncertain after complete workup, significant and related symptoms or exceptionally when exists concern with malignancy. This study supports an initial conservative management in the majority of patients with SCN
The read-across hypothesis and environmental risk assessment of pharmaceuticals
This article is made available through the Brunel Open Access Publishing Fund. Copyright © 2013 American Chemical Society.Pharmaceuticals in the environment have received increased attention over the past decade, as they are ubiquitous in rivers and waterways. Concentrations are in sub-ng to low μg/L, well below acute toxic levels, but there are uncertainties regarding the effects of chronic exposures and there is a need to prioritise which pharmaceuticals may be of concern. The read-across hypothesis stipulates that a drug will have an effect in non-target organisms only if the molecular targets such as receptors and enzymes have been conserved, resulting in a (specific) pharmacological effect only if plasma concentrations are similar to human therapeutic concentrations. If this holds true for different classes of pharmaceuticals, it should be possible to predict the potential environmental impact from information obtained during the drug development process. This paper critically reviews the evidence for read-across, and finds that few studies include plasma concentrations and mode of action based effects. Thus, despite a large number of apparently relevant papers and a general acceptance of the hypothesis, there is an absence of documented evidence. There is a need for large-scale studies to generate robust data for testing the read-across hypothesis and developing predictive models, the only feasible approach to protecting the environment.BBSRC Industrial Partnership Award BB/
I00646X/1 and BBSRC Industrial CASE Partnership Studentship
BB/I53257X/1 with AstraZeneca Safety Health and
Environment Research Programme
Multicenter experience from the UK and Ireland of use of lumen-apposing metal stent for transluminal drainage of pancreatic fluid collections
Background and study aims: Pancreatic fluid collection (PFC) is a common complication of pancreatitis for which endoscopic ultrasound-guided drainage is first-line treatment. A new single-device, lumen-apposing, covered self-expanding metal stent (LAMS) has been licensed for PFC drainage. We therefore present our multicenter experience with the LAMS for PFC drainage in a multicenter prospective case series to assess success and complication rates.
Patients and methods: All adult patients from 11 tertiary centers who had LAMS placement for PFC from July 2015 to July 2016 were included. Data including indications, technical success, clinical success, collection resolution, stent removal, early and late adverse events (AEs), mortality and recurrence at 6 months were collected.
Results: 116 patients, median age 52.5 years (range 16 – 80) and 67 % male, were treated with a single LAMS in each case. The indication was walled off necrosis (WON) in 70 and pseudocyst in 46. Median size of the PFC was 11 cm (5 – 21 cm) and the estimated median necrotic volume in WON was 30 % (5 % – 90 %). Stent insertion was technically successful in 115 (99.1 %) and clinically successful in 109 (94 %). Early serious AEs (SAEs): n = 7 sepsis, n = 1 stent blockage with food, n = 1 stent migration requiring laparotomy, n = 1 stent dislodgement and n = 1 bleeding requiring emboliZation. Late AEs: n = 1 buried stent and n = 1 esophageal fistula. Non-procedure-related deaths: n = 3 (2.5 %).
Conclusion: This multicenter case series demonstrates that use of the new LAMS is feasible, effective and relatively safe in draining PFC with a technical success rate of 99 % and cumulative SAE rate of 11.2 %
Evidence for hydrogen oxidation and metabolic plasticity in widespread deep-sea sulfur-oxidizing bacteria
Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 110 (2013): 330-335, doi:10.1073/pnas.1215340110.Hydrothermal vents are a well-known source of energy that powers chemosynthesis in the
deep sea. Recent work suggests that microbial chemosynthesis is also surprisingly pervasive
throughout the dark oceans, serving as a significant CO2 sink even at sites far-removed
from vents. Ammonia and sulfur have been identified as potential electron donors for this
chemosynthesis, but they do not fully account for measured rates of dark primary
production in the pelagic water column. Here we use metagenomic and metatranscriptomic
analyses to show that deep-sea populations of the SUP05 group of uncultured sulfur
oxidizing Gammaproteobacteria, which are abundant in widespread and diverse marine
environments, contain and highly express genes encoding group 1 Ni-Fe hydrogenase
enzymes for H2 oxidation. Reconstruction of near-complete genomes of two co-occurring
SUP05 populations in hydrothermal plumes and deep waters of the Gulf of California
enabled detailed population-specific metatranscriptomic analyses, revealing dynamic
patterns of gene content and transcript abundance. SUP05 transcripts for genes involved in
H2 and sulfur oxidation are most abundant in hydrothermal plumes where these electron
donors are enriched. In contrast, a second hydrogenase has more abundant transcripts in
background deep sea samples. Coupled with results from a bioenergetic model that suggest
that H2 oxidation can contribute significantly to the SUP05 energy budget, these findings
reveal the potential importance of H2 as a key energy source in the deep ocean. This study
also highlights the genomic plasticity of SUP05, which enables this widely distributed group
to optimize its energy metabolism (electron donor and acceptor) to local geochemical
conditions.This project is funded in part by the Gordon and Betty Moore Foundation
and the National Science Foundation (OCE 1029242)
- …