39,228 research outputs found
Birds and people in Europe
At a regional scale, species richness and human population size are frequently positively correlated across space. Such patterns may arise because both species richness and human density increase with energy availability. If the species-energy relationship is generated through the 'more individuals' hypothesis, then the prediction is that areas with high human densities will also support greater numbers of individuals from other taxa. We use the unique data available for the breeding birds in Europe to test this prediction.
Overall regional densities of bird species are higher in areas with more people; species of conservation concern exhibit the same pattern. Avian density also increases faster with human density than does avian biomass, indicating that areas with a higher human density have a higher proportion of small-bodied individuals. The analyses also underline the low numbers of breeding birds in Europe relative to humans, with a median of just three individual birds per person, and 4 g of bird for every kilogram of human
Evolution to a singular measure and two sums of Lyapunov exponents
We consider dissipative dynamical systems represented by a smooth
compressible flow in a finite domain. The density evolves according to the
continuity (Liouville) equation. For a general, non-degenerate flow the result
of the infinite time evolution of an initially smooth density is a singular
measure. We give a condition for the non-degeneracy which allows to decide for
a given flow whether the infinite time limit is singular. The condition uses a
Green-Kubo type formula for the space-averaged sum of forward and
backward-in-time Lyapunov exponents. We discuss how the sums determine the
fluctuations of the entropy production rate in the SRB state and give examples
of computation of the sums for certain velocity fields.Comment: 4 pages, published versio
Dynamical Mass Estimates for the Halo of M31 from Keck Spectroscopy
The last few months have seen the measurements of the radial velocities of
all of the dwarf spheroidal companions to the Andromeda galaxy (M31) using the
spectrographs (HIRES and LRIS) on the Keck Telescope. This paper summarises the
data on the radial velocities and distances for all the companion galaxies and
presents new dynamical modelling to estimate the mass of extended halo of M31.
The best fit values for the total mass of M31 are between 7 and 10 x 10^{11}
solar masses, depending on the details of the modelling. The mass estimate is
accompanied by considerable uncertainty caused by the smallness of the dataset;
for example, the upper bound on the total mass is roughly 24 x 10^{11} solar
masses, while the lower bound is about 3 x 10^{11} solar masses. These values
are less than the most recent estimates of the most likely mass of the Milky
Way halo. Bearing in mind all the uncertainties, a fair conclusion is that the
M31 halo is roughly as massive as that of the Milky Way halo. There is no
dynamical evidence for the widely held belief that M31 is more massive -- it
may even be less massive.Comment: In press, The Astrophysical Journal (Letters
Johnson-Kendall-Roberts theory applied to living cells
Johnson-Kendall-Roberts (JKR) theory is an accurate model for strong adhesion
energies of soft slightly deformable material. Little is known about the
validity of this theory on complex systems such as living cells. We have
addressed this problem using a depletion controlled cell adhesion and measured
the force necessary to separate the cells with a micropipette technique. We
show that the cytoskeleton can provide the cells with a 3D structure that is
sufficiently elastic and has a sufficiently low deformability for JKR theory to
be valid. When the cytoskeleton is disrupted, JKR theory is no longer
applicable
Kinetic pathways of multi-phase surfactant systems
The relaxation following a temperature quench of two-phase (lamellar and
sponge phase) and three-phase (lamellar, sponge and micellar phase) samples,
has been studied in an SDS/octanol/brine system. In the three-phase case we
have observed samples that are initially mainly sponge phase with lamellar and
micellar phase on the top and bottom respectively. Upon decreasing temperature
most of the volume of the sponge phase is replaced by lamellar phase. During
the equilibriation we have observed three regimes of behaviour within the
sponge phase: (i) disruption in the sponge texture, then (ii) after the sponge
phase homogenises there is a lamellar nucleation regime and finally (iii) a
bizarre plume connects the lamellar phase with the micellar phase. The
relaxation of the two-phase sample proceeds instead in two stages. First
lamellar drops nucleate in the sponge phase forming a onion `gel' structure.
Over time the lamellar structure compacts while equilibriating into a two phase
lamellar/sponge phase sample. We offer possible explanatioins for some of these
observations in the context of a general theory for phase kinetics in systems
with one fast and one slow variable.Comment: 1 textfile, 20 figures (jpg), to appear in PR
Higgs Mechanism in String Theory
In first-quantized string theory, spacetime symmetries are described by inner
automorphisms of the underlying conformal field theory. In this paper we use
this approach to illustrate the Higgs effect in string theory. We consider
string propagation on M^{24,1} \times S^1, where the circle has radius R, and
study SU(2) symmetry breaking as R moves away from its critical value. We find
a gauge-covariant equation of motion for the broken-symmetry gauge bosons and
the would-be Goldstone bosons. We show that the Goldstone bosons can be
eliminated by an appropriate gauge transformation. In this unitary gauge, the
Goldstone bosons become the longitudinal components of massive gauge bosons.Comment: 12 pages, Te
Assessing non-Markovian dynamics
We investigate what a snapshot of a quantum evolution - a quantum channel
reflecting open system dynamics - reveals about the underlying continuous time
evolution. Remarkably, from such a snapshot, and without imposing additional
assumptions, it can be decided whether or not a channel is consistent with a
time (in)dependent Markovian evolution, for which we provide computable
necessary and sufficient criteria. Based on these, a computable measure of
`Markovianity' is introduced. We discuss how the consistency with Markovian
dynamics can be checked in quantum process tomography. The results also clarify
the geometry of the set of quantum channels with respect to being solutions of
time (in)dependent master equations.Comment: 5 pages, RevTex, 2 figures. (Except from typesetting) version to be
published in the Physical Review Letter
An Infinite Dimensional Symmetry Algebra in String Theory
Symmetry transformations of the space-time fields of string theory are
generated by certain similarity transformations of the stress-tensor of the
associated conformal field theories. This observation is complicated by the
fact that, as we explain, many of the operators we habitually use in string
theory (such as vertices and currents) have ill-defined commutators. However,
we identify an infinite-dimensional subalgebra whose commutators are not
singular, and explicitly calculate its structure constants. This constitutes a
subalgebra of the gauge symmetry of string theory, although it may act on
auxiliary as well as propagating fields. We term this object a {\it weighted
tensor algebra}, and, while it appears to be a distant cousin of the
-algebras, it has not, to our knowledge, appeared in the literature before.Comment: 14 pages, Plain TeX, report RU93-8, CTP-TAMU-2/94, CERN-TH.7022/9
A model of hyphal tip growth involving microtubule-based transport
We propose a simple model for mass transport within a fungal hypha and its
subsequent growth. Inspired by the role of microtubule-transported vesicles, we
embody the internal dynamics of mass inside a hypha with mutually excluding
particles progressing stochastically along a growing one-dimensional lattice.
The connection between long range transport of materials for growth, and the
resulting extension of the hyphal tip has not previously been addressed in the
modelling literature. We derive and analyse mean-field equations for the model
and present a phase diagram of its steady state behaviour, which we compare to
simulations. We discuss our results in the context of the filamentous fungus,
Neurospora crassa.Comment: 5 pages, 5 figure
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