14,782 research outputs found
Life, Death and Preferential Attachment
Scientific communities are characterized by strong stratification. The highly
skewed frequency distribution of citations of published scientific papers
suggests a relatively small number of active, cited papers embedded in a sea of
inactive and uncited papers. We propose an analytically soluble model which
allows for the death of nodes. This model provides an excellent description of
the citation distributions for live and dead papers in the SPIRES database.
Further, this model suggests a novel and general mechanism for the generation
of power law distributions in networks whenever the fraction of active nodes is
small.Comment: 5 pages, 2 figure
Correlations in Networks associated to Preferential Growth
Combinations of random and preferential growth for both on-growing and
stationary networks are studied and a hierarchical topology is observed. Thus
for real world scale-free networks which do not exhibit hierarchical features
preferential growth is probably not the main ingredient in the growth process.
An example of such real world networks includes the protein-protein interaction
network in yeast, which exhibits pronounced anti-hierarchical features.Comment: 4 pages, 4 figure
Spatial distribution of podoconiosis in relation to environmental factors in Ethiopia: a historical review
BACKGROUND
An up-to-date and reliable map of podoconiosis is needed to design geographically targeted and cost-effective intervention in Ethiopia. Identifying the ecological correlates of the distribution of podoconiosis is the first step for distribution and risk maps. The objective of this study was to investigate the spatial distribution and ecological correlates of podoconiosis using historical and contemporary survey data.
METHODS
Data on the observed prevalence of podoconiosis were abstracted from published and unpublished literature into a standardized database, according to strict inclusion and exclusion criteria. In total, 10 studies conducted between 1969 and 2012 were included, and data were available for 401,674 individuals older than 15 years of age from 229 locations. A range of high resolution environmental factors were investigated to determine their association with podoconiosis prevalence, using logistic regression.
RESULTS
The prevalence of podoconiosis in Ethiopia was estimated at 3.4% (95% CI 3.3%-3.4%) with marked regional variation. We identified significant associations between mean annual Land Surface Temperature (LST), mean annual precipitation, topography of the land and fine soil texture and high prevalence of podoconiosis. The derived maps indicate both widespread occurrence of podoconiosis and a marked variability in prevalence of podoconiosis, with prevalence typically highest at altitudes >1500 m above sea level (masl), with >1500 mm annual rainfall and mean annual LST of 19-21°C. No (or very little) podoconiosis occurred at altitudes 24°C.
CONCLUSION
Podoconiosis remains a public health problem in Ethiopia over considerable areas of the country, but exhibits marked geographical variation associated in part with key environmental factors. This is work in progress and the results presented here will be refined in future work
Keck Hires Observations of the QSO First J104459.6+365605: Evidence for a Large Scale Outflow
This paper presents an analysis of a Keck HIRES spectrum of the QSO FIRST
J104459.6+365605. The line of sight towards the QSO contains two clusters of
outflowing clouds that give rise to broad blue shifted absorption lines. The
outflow velocities of the clouds range from -200 to -1200 km/s and from -3400
to -5200 km/s, respectively. The width of the individual absorption lines
ranges from 50 to more than 1000 km/s. The most prominent absorption lines are
those of Mg II, Mg I, and Fe II. The low ionization absorption lines occur at
the same velocities as the most saturated Mg II lines, showing that the Fe II,
Mg I and Mg II line forming regions must be closely associated. Many absorption
lines from excited states of Fe II are present, allowing a determination of the
population of several low lying energy levels. From this we determine an
electron density in the Fe II line forming regions of 4000 per cubic cm.
Modelling the ionization state of the absorbing gas with this value of the
electron density as a constraint, we find that the distance between the Fe II
and Mg I line forming region and the continuum source is of order 700 parsec.
From the correspondence in velocity between the Fe II, Mg I and Mg II lines
we infer that the Mg II lines must be formed at the same distance. The Mg II
absorption fulfills the criteria for Broad Absorption Lines defined by Weymann
et al. (1991). This large distance is surprising, since BALs are generally
thought to be formed in outflows at a much smaller distance from the nucleus.Comment: 34 pages, 11 figures. Accepted by The Astrophysical Journa
From Einstein's Theorem to Bell's Theorem: A History of Quantum Nonlocality
In this Einstein Year of Physics it seems appropriate to look at an important
aspect of Einstein's work that is often down-played: his contribution to the
debate on the interpretation of quantum mechanics. Contrary to popular opinion,
Bohr had no defence against Einstein's 1935 attack (the EPR paper) on the
claimed completeness of orthodox quantum mechanics. I suggest that Einstein's
argument, as stated most clearly in 1946, could justly be called Einstein's
reality-locality-completeness theorem, since it proves that one of these three
must be false. Einstein's instinct was that completeness of orthodox quantum
mechanics was the falsehood, but he failed in his quest to find a more complete
theory that respected reality and locality. Einstein's theorem, and possibly
Einstein's failure, inspired John Bell in 1964 to prove his reality-locality
theorem. This strengthened Einstein's theorem (but showed the futility of his
quest) by demonstrating that either reality or locality is a falsehood. This
revealed the full nonlocality of the quantum world for the first time.Comment: 18 pages. To be published in Contemporary Physics. (Minor changes;
references and author info added
The Stretched Horizon and Black Hole Complementarity
Three postulates asserting the validity of conventional quantum theory,
semi-classical general relativity and the statistical basis for thermodynamics
are introduced as a foundation for the study of black hole evolution. We
explain how these postulates may be implemented in a ``stretched horizon'' or
membrane description of the black hole, appropriate to a distant observer. The
technical analysis is illustrated in the simplified context of 1+1 dimensional
dilaton gravity. Our postulates imply that the dissipative properties of the
stretched horizon arise from a course graining of microphysical degrees of
freedom that the horizon must possess. A principle of black hole
complementarity is advocated. The overall viewpoint is similar to that
pioneered by 't~Hooft but the detailed implementation is different.Comment: (some misprints in equations have been fixed), 48 pages (including
figures), SU-ITP-93-1
Two massive star-forming regions at early evolutionary stages
We report sensitive ATCA radio-continuum observations toward IRAS 15596-5301
and 16272-4837, two luminous objects (> 2x10^4 Lsun) thought to represent
massive star-forming regions in early stages of evolution (due to previously
undetected radio emission at the 1-sigma level of 2 mJy per beam). Also
reported are 1.2-millimeter continuum and a series of molecular-line
observations made with the SEST telescope. For IRAS 15596-5301, the
observations reveal the presence of three distinct compact radio-continuum
sources associated with a dense molecular core. We suggest that this core
contains a cluster of B stars which are exciting compact HII regions that are
in pressure equilibrium with the dense molecular surroundings. No radio
continuum emission was detected from IRAS 16272-4837 (3-sigma limit of 0.2
mJy). However, a dense molecular core has been detected. The high luminosity
and lack of radio emission from this massive core suggests that it hosts an
embedded young massive protostar that is still undergoing an intense accretion
phase. This scenario is supported by the observed characteristics of the line
profiles and the presence of a bipolar outflow detected from observations of
the SiO emission. We suggest that IRAS 16272-4837 is a bona fide massive star-
forming region in a very early evolutionary stage, being the precursor of an
ultra compact HII region.Comment: 25 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Bell Correlations and the Common Future
Reichenbach's principle states that in a causal structure, correlations of
classical information can stem from a common cause in the common past or a
direct influence from one of the events in correlation to the other. The
difficulty of explaining Bell correlations through a mechanism in that spirit
can be read as questioning either the principle or even its basis: causality.
In the former case, the principle can be replaced by its quantum version,
accepting as a common cause an entangled state, leaving the phenomenon as
mysterious as ever on the classical level (on which, after all, it occurs). If,
more radically, the causal structure is questioned in principle, closed
space-time curves may become possible that, as is argued in the present note,
can give rise to non-local correlations if to-be-correlated pieces of classical
information meet in the common future --- which they need to if the correlation
is to be detected in the first place. The result is a view resembling Brassard
and Raymond-Robichaud's parallel-lives variant of Hermann's and Everett's
relative-state formalism, avoiding "multiple realities."Comment: 8 pages, 5 figure
Quasinormal Modes of AdS Black Holes and the Approach to Thermal Equilibrium
We investigate the decay of a scalar field outside a Schwarzschild anti de
Sitter black hole. This is determined by computing the complex frequencies
associated with quasinormal modes. There are qualitative differences from the
asymptotically flat case, even in the limit of small black holes. In
particular, for a given angular dependence, the decay is always exponential -
there are no power law tails at late times. In terms of the AdS/CFT
correspondence, a large black hole corresponds to an approximately thermal
state in the field theory, and the decay of the scalar field corresponds to the
decay of a perturbation of this state. Thus one obtains the timescale for the
approach to thermal equilibrium. We compute these timescales for the strongly
coupled field theories in three, four, and six dimensions which are dual to
string theory in asymptotically AdS spacetimes.Comment: 25 pages, 9 figures extended discussion of horizon boundary
conditions, added note on higher l mode
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