8,728 research outputs found
Airborne bacterial populations above desert soils of the McMurdo Dry Valleys, Antarctica
Bacteria are assumed to disperse widely via aerosolized transport due to their small size and resilience. The question of microbial endemicity in isolated populations is directly related to the level of airborne exogenous inputs, yet this has proven hard to identify. The ice-free terrestrial ecosystem of Antarctica, a geographically and climatically isolated continent, was used to interrogate microbial bio-aerosols in relation to the surrounding ecology and climate. High-throughput sequencing of bacterial ribosomal RNA (rRNA) genes was combined with analyses of climate patterns during an austral summer. In general terms, the aerosols were dominated by Firmicutes, whereas surrounding soils supported Actinobacteria-dominated communities. The most abundant taxa were also common to aerosols from other continents, suggesting that a distinct bio-aerosol community is widely dispersed. No evidence for significant marine input to bio-aerosols was found at this maritime valley site, instead local influence was largely from nearby volcanic sources. Back trajectory analysis revealed transport of incoming regional air masses across the Antarctic Plateau, and this is envisaged as a strong selective force. It is postulated that local soil microbial dispersal occurs largely via stochastic mobilization of mineral soil particulates
Scaling laws for the photo-ionisation cross section of two-electron atoms
The cross sections for single-electron photo-ionisation in two-electron atoms
show fluctuations which decrease in amplitude when approaching the
double-ionisation threshold. Based on semiclassical closed orbit theory, we
show that the algebraic decay of the fluctuations can be characterised in terms
of a threshold law as with exponent
obtained as a combination of stability exponents of the triple-collision
singularity. It differs from Wannier's exponent dominating double ionisation
processes. The details of the fluctuations are linked to a set of infinitely
unstable classical orbits starting and ending in the non-regularisable triple
collision. The findings are compared with quantum calculations for a model
system, namely collinear helium.Comment: 4 pages, 1 figur
Why Nature has made a choice of one time and three space coordinates?
We propose a possible answer to one of the most exciting open questions in
physics and cosmology, that is the question why we seem to experience four-
dimensional space-time with three ordinary and one time dimensions. We have
known for more than 70 years that (elementary) particles have spin degrees of
freedom, we also know that besides spin they also have charge degrees of
freedom, both degrees of freedom in addition to the position and momentum
degrees of freedom. We may call these ''internal degrees of freedom '' the
''internal space'' and we can think of all the different particles, like quarks
and leptons, as being different internal states of the same particle. The
question then naturally arises: Is the choice of the Minkowski metric and the
four-dimensional space-time influenced by the ''internal space''?
Making assumptions (such as particles being in first approximation massless)
about the equations of motion, we argue for restrictions on the number of space
and time dimensions. (Actually the Standard model predicts and experiments
confirm that elementary particles are massless until interactions switch on
masses.)
Accepting our explanation of the space-time signature and the number of
dimensions would be a point supporting (further) the importance of the
''internal space''.Comment: 13 pages, LaTe
On the distribution of career longevity and the evolution of home run prowess in professional baseball
Statistical analysis is a major aspect of baseball, from player averages to
historical benchmarks and records. Much of baseball fanfare is based around
players exceeding the norm, some in a single game and others over a long
career. Career statistics serve as a metric for classifying players and
establishing their historical legacy. However, the concept of records and
benchmarks assumes that the level of competition in baseball is stationary in
time. Here we show that power-law probability density functions, a hallmark of
many complex systems that are driven by competition, govern career longevity in
baseball. We also find similar power laws in the density functions of all major
performance metrics for pitchers and batters. The use of performance-enhancing
drugs has a dark history, emerging as a problem for both amateur and
professional sports. We find statistical evidence consistent with
performance-enhancing drugs in the analysis of home runs hit by players in the
last 25 years. This is corroborated by the findings of the Mitchell Report [1],
a two-year investigation into the use of illegal steroids in major league
baseball, which recently revealed that over 5 percent of major league baseball
players tested positive for performance-enhancing drugs in an anonymous 2003
survey.Comment: 5 pages, 5 figures, 2-column revtex4 format. Revision has change of
title, a figure added, and minor changes in response to referee comment
Condensation and Clustering in the Driven Pair Exclusion Process
We investigate particle condensation in a driven pair exclusion process on
one- and two- dimensional lattices under the periodic boundary condition. The
model describes a biased hopping of particles subject to a pair exclusion
constraint that each particle cannot stay at a same site with its pre-assigned
partner. The pair exclusion causes a mesoscopic condensation characterized by
the scaling of the condensate size and the number of
condensates with the total number of sites .
Those condensates are distributed randomly without hopping bias. We find that
the hopping bias generates a spatial correlation among condensates so that a
cluster of condensates appears. Especially, the cluster has an anisotropic
shape in the two-dimensional system. The mesoscopic condensation and the
clustering are studied by means of numerical simulations.Comment: 4 pages, 5 figure
Self-Attracting Walk on Lattices
We have studied a model of self-attracting walk proposed by Sapozhnikov using
Monte Carlo method. The mean square displacement
and the mean number of visited sites are calculated for
one-, two- and three-dimensional lattice. In one dimension, the walk shows
diffusive behaviour with . However, in two and three dimension, we
observed a non-universal behaviour, i.e., the exponent varies
continuously with the strength of the attracting interaction.Comment: 6 pages, latex, 6 postscript figures, Submitted J.Phys.
Collaborative multidisciplinary learning : quantity surveying students’ perspectives
The construction industry is highly fragmented and is known for its adversarial culture, culminating
in poor quality projects not completed on time or within budget. The aim of this study is thus to
guide the design of QS programme curricula in order to help students develop the requisite
knowledge and skills to work more collaboratively in their multi-disciplinary future workplaces.
A qualitative approach was considered appropriate as the authors were concerned with gathering an
initial understanding of what students think of multi-disciplinary learning. The data collection
method used was a questionnaire which was developed by the Behaviours4Collaboration (B4C)
team.
Knowledge gaps were still found across all the key areas where a future QS practitioner needs to be
collaborative (either as a project contributor or as a project leader) despite the need for change
instigated by the multi-disciplinary (BIM) education revolution.
The study concludes that universities will need to be selective in teaching, and innovative in
reorienting, QS education so that a collaborative BIM education can be effected in stages, increasing
in complexity as the students’ technical knowledge grows. This will help students to build the
competencies needed to make them future leaders. It will also support programme currency and
delivery
Numerical simulation of strongly nonlinear and dispersive waves using a Green-Naghdi model
We investigate here the ability of a Green-Naghdi model to reproduce strongly
nonlinear and dispersive wave propagation. We test in particular the behavior
of the new hybrid finite-volume and finite-difference splitting approach
recently developed by the authors and collaborators on the challenging
benchmark of waves propagating over a submerged bar. Such a configuration
requires a model with very good dispersive properties, because of the
high-order harmonics generated by topography-induced nonlinear interactions. We
thus depart from the aforementioned work and choose to use a new Green-Naghdi
system with improved frequency dispersion characteristics. The absence of dry
areas also allows us to improve the treatment of the hyperbolic part of the
equations. This leads to very satisfying results for the demanding benchmarks
under consideration
The UV Upturn in Elliptical Galaxies as an Age Indicator
We show that the UV flux of old stellar systems can tell us about their ages.
Two independent populations synthesis groups that have had wildly different
views have here worked together and generated two solutions that can be easily
tested using space telescopes. Proposed tests will constrain the ages of giant
Es, that are often considered the oldest populations in the universe, and thus
cosmology.Comment: LaTeX and 11 eps figures Accepted for publication in Ap
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