1,178 research outputs found
Characterizing Information Propagation in Plants
This paper considers an electro-chemical based communication model for
intercellular communication in plants. Many plants, such as Mimosa pudica (the
"sensitive plant"), employ electrochemical signals known as action potentials
(APs) for communication purposes. In this paper we present a simple model for
action potential generation. We make use of the concepts from molecular
communication to explain the underlying process of information transfer in a
plant. Using the information-theoretic analysis, we compute the mutual
information between the input and output in this work. The key aim is to study
the variations in the information propagation speed for varying number of plant
cells for one simple case. Furthermore we study the impact of the AP signal on
the mutual information and information propagation speed. We aim to explore
further that how the growth rate in plants can impact the information transfer
rate and vice versa.Comment: 6 pages, 5 Figures, Submitted to IEEE Conference, 201
What impact did a Paediatric Early Warning system have on emergency admissions to the paediatric intensive care unit? An observational cohort study
Summary
The ideology underpinning Paediatric Early Warning systems (PEWs) is that earlier recognition of deteriorating in-patients would improve clinical outcomes.
Objective
To explore how the introduction of PEWs at a tertiary children's hospital affects emergency admissions to the Paediatric Intensive Care Unit (PICU) and the impact on service delivery. To compare ‘in-house’ emergency admissions to PICU with ‘external’ admissions transferred from District General Hospitals (without PEWs).
Method
A before-and-after observational study August 2005–July 2006 (pre), August 2006–July 2007 (post) implementation of PEWs at the tertiary children's hospital.
Results
The median Paediatric Index of Mortality (PIM2) reduced; 0.44 vs 0.60 (p < 0.001). Fewer admissions required invasive ventilation 62.7% vs 75.2% (p = 0.015) for a shorter median duration; four to two days. The median length of PICU stay reduced; five to three days (p = 0.002). There was a non-significant reduction in mortality (p = 0.47). There was no comparable improvement in outcome seen in external emergency admissions to PICU. A 39% reduction in emergency admission total beds days reduced cancellation of major elective surgical cases and refusal of external PICU referrals.
Conclusions
Following introduction of PEWs at a tertiary children's hospital PIM2 was reduced, patients required less PICU interventions and had a shorter length of stay. PICU service delivery improved
Could the Pioneer anomaly have a gravitational origin?
If the Pioneer anomaly has a gravitational origin, it would, according to the
equivalence principle, distort the motions of the planets in the Solar System.
Since no anomalous motion of the planets has been detected, it is generally
believed that the Pioneer anomaly can not originate from a gravitational source
in the Solar System. However, this conclusion becomes less obvious when
considering models that either imply modifications to gravity at long range or
gravitational sources localized to the outer Solar System, given the
uncertainty in the orbital parameters of the outer planets. Following the
general assumption that the Pioneer spacecraft move geodesically in a
spherically symmetric spacetime metric, we derive the metric disturbance that
is needed in order to account for the Pioneer anomaly. We then analyze the
residual effects on the astronomical observables of the three outer planets
that would arise from this metric disturbance, given an arbitrary metric theory
of gravity. Providing a method for comparing the computed residuals with actual
residuals, our results imply that the presence of a perturbation to the
gravitational field necessary to induce the Pioneer anomaly is in conflict with
available data for the planets Uranus and Pluto, but not for Neptune. We
therefore conclude that the motion of the Pioneer spacecraft must be
non-geodesic. Since our results are model independent within the class of
metric theories of gravity, they can be applied to rule out any model of the
Pioneer anomaly that implies that the Pioneer spacecraft move geodesically in a
perturbed spacetime metric, regardless of the origin of this metric
disturbance.Comment: 16 pages, 6 figures. Rev. 3: Major revision. Accepted for publication
in Phys. Rev. D. Rev. 4: Added two reference
Zeldovich flow on cosmic vacuum background: new exact nonlinear analytical solution
A new exact nonlinear Newtonian solution for a plane matter flow superimposed
on the isotropic Hubble expansion is reported. The dynamical effect of cosmic
vacuum is taken into account. The solution describes the evolution of nonlinear
perturbations via gravitational instability of matter and the termination of
the perturbation growth by anti-gravity of vacuum at the epoch of transition
from matter domination to vacuum domination. On this basis, an `approximate' 3D
solution is suggested as an analog of the Zeldovich ansatz.Comment: 9 pages, 1 figure
Metric of a tidally perturbed spinning black hole
We explicitly construct the metric of a Kerr black hole that is tidally
perturbed by the external universe in the slow-motion approximation. This
approximation assumes that the external universe changes slowly relative to the
rotation rate of the hole, thus allowing the parameterization of the
Newman-Penrose scalar by time-dependent electric and magnetic tidal
tensors. This approximation, however, does not constrain how big the spin of
the background hole can be and, in principle, the perturbed metric can model
rapidly spinning holes. We first generate a potential by acting with a
differential operator on . From this potential we arrive at the metric
perturbation by use of the Chrzanowski procedure in the ingoing radiation
gauge. We provide explicit analytic formulae for this metric perturbation in
spherical Kerr-Schild coordinates, where the perturbation is finite at the
horizon. This perturbation is parametrized by the mass and Kerr spin parameter
of the background hole together with the electric and magnetic tidal tensors
that describe the time evolution of the perturbation produced by the external
universe. In order to take the metric accurate far away from the hole, these
tidal tensors should be determined by asymptotically matching this metric to
another one valid far from the hole. The tidally perturbed metric constructed
here could be useful in initial data constructions to describe the metric near
the horizons of a binary system of spinning holes. This perturbed metric could
also be used to construct waveforms and study the absorption of mass and
angular momentum by a Kerr black hole when external processes generate
gravitational radiation.Comment: 17 pages, 3 figures. Final PRD version, minor typos, etc corrected.
v3: corrected typo in Eq. (35) and (57
Self Interacting Dark Matter in the Solar System
Weakly coupled, almost massless, spin 0 particles have been predicted by many
extensions of the standard model of particle physics. Recently, the PVLAS group
observed a rotation of polarization of electromagnetic waves in vacuum in the
presence of transverse magnetic field. This phenomenon is best explained by the
existence of a weakly coupled light pseudoscalar particle. However, the
coupling required by this experiment is much larger than the conventional
astrophysical limits. Here we consider a hypothetical self-interacting
pseudoscalar particle which couples weakly with visible matter.
Assuming that these pseudoscalars pervade the galaxy, we show that the solar
limits on the pseudoscalar-photon coupling can be evaded.Comment: 17 pages, 2 figure
Gravitational collapse in asymptotically Anti-de Sitter/de Sitter backgrounds
We study here the gravitational collapse of a matter cloud with a
non-vanishing tangential pressure in the presence of a non-zero cosmological
term. Conditions for bounce and singularity formation are derived for the
model. It is also shown that when the tangential pressures vanish, the bounce
and singularity conditions reduce to that of the dust case studied earlier. The
collapsing interior is matched with an exterior which is asymptotically de
Sitter or anti de Sitter, depending on the sign of cosmological constant. The
junction conditions for matching the cloud to exterior are specified. The
effect of the cosmological term on apparent horizons is studied in some detail,
and the nature of central singularity is analyzed. We also discuss here the
visibility of the singularity and implications for the cosmic censorship
conjecture.Comment: 11 pages, 1 figure, Revtex
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