13,806 research outputs found
On the existence of quantum representations for two dichotomic measurements
Under which conditions do outcome probabilities of measurements possess a
quantum-mechanical model? This kind of problem is solved here for the case of
two dichotomic von Neumann measurements which can be applied repeatedly to a
quantum system with trivial dynamics. The solution uses methods from the theory
of operator algebras and the theory of moment problems. The ensuing conditions
reveal surprisingly simple relations between certain quantum-mechanical
probabilities. It also shown that generally, none of these relations holds in
general probabilistic models. This result might facilitate further experimental
discrimination between quantum mechanics and other general probabilistic
theories.Comment: 16+7 pages, presentation improved and minor errors correcte
Spherically symmetric equilibria for self-gravitating kinetic or fluid models in the non-relativistic and relativistic case - A simple proof for finite extension
We consider a self-gravitating collisionless gas as described by the
Vlasov-Poisson or Einstein-Vlasov system or a self-gravitating fluid ball as
described by the Euler-Poisson or Einstein-Euler system. We give a simple proof
for the finite extension of spherically symmetric equilibria, which covers all
these models simultaneously. In the Vlasov case the equilibria are
characterized by a local growth condition on the microscopic equation of state,
i.e., on the dependence of the particle distribution on the particle energy, at
the cut-off energy E_0, and in the Euler case by the corresponding growth
condition on the equation of state p=P(\rho) at \rho=0. These purely local
conditions are slight generalizations to known such conditions.Comment: 20 page
Phenotypic switching of populations of cells in a stochastic environment
In biology phenotypic switching is a common bet-hedging strategy in the face
of uncertain environmental conditions. Existing mathematical models often focus
on periodically changing environments to determine the optimal phenotypic
response. We focus on the case in which the environment switches randomly
between discrete states. Starting from an individual-based model we derive
stochastic differential equations to describe the dynamics, and obtain
analytical expressions for the mean instantaneous growth rates based on the
theory of piecewise deterministic Markov processes. We show that optimal
phenotypic responses are non-trivial for slow and intermediate environmental
processes, and systematically compare the cases of periodic and random
environments. The best response to random switching is more likely to be
heterogeneity than in the case of deterministic periodic environments, net
growth rates tend to be higher under stochastic environmental dynamics. The
combined system of environment and population of cells can be interpreted as
host-pathogen interaction, in which the host tries to choose environmental
switching so as to minimise growth of the pathogen, and in which the pathogen
employs a phenotypic switching optimised to increase its growth rate. We
discuss the existence of Nash-like mutual best-response scenarios for such
host-pathogen games.Comment: 17 pages, 6 figure
Breaking Kelvin: Circulation conservation and vortex breakup in MHD at low Magnetic Prandtl Number
In this paper we examine the role of weak magnetic fields in breaking
Kelvin's circulation theorem and in vortex breakup in two-dimensional
magnetohydrodynamics for the physically important case of a low magnetic
Prandtl number (low ) fluid. We consider three canonical inviscid solutions
for the purely hydrodynamical problem, namely a Gaussian vortex, a circular
vortex patch and an elliptical vortex patch. We examine how magnetic fields
lead to an initial loss of circulation and attempt to derive scaling
laws for the loss of circulation as a function of field strength and diffusion
as measured by two non-dimensional parameters. We show that for all cases the
loss of circulation depends on the integrated effects of the Lorentz force,
with the patch cases leading to significantly greater circulation loss. For the
case of the elliptical vortex the loss of circulation depends on the total area
swept out by the rotating vortex and so this leads to more efficient
circulation loss than for a circular vortex.Comment: 21 pages, 12 figure
A pairwise maximum entropy model describes energy landscape for spiral wave dynamics of cardiac fibrillation
Heart is an electrically-connected network. Spiral wave dynamics of cardiac
fibrillation shows chaotic and disintegrated patterns while sinus rhythm shows
synchronized excitation patterns. To determine functional interactions between
cardiomyocytes during complex fibrillation states, we applied a pairwise
maximum entropy model (MEM) to the sequential electrical activity maps acquired
from the 2D computational simulation of human atrial fibrillation. Then, we
constructed energy landscape and estimated hierarchical structure among the
different local minima (attractors) to explain the dynamic properties of
cardiac fibrillation. Four types of the wave dynamics were considered: sinus
rhythm; single stable rotor; single rotor with wavebreak; and multiple wavelet.
The MEM could describe all types of wave dynamics (both accuracy and
reliability>0.9) except the multiple random wavelet. Both of the sinus rhythm
and the single stable rotor showed relatively high pairwise interaction
coefficients among the cardiomyocytes. Also, the local energy minima had
relatively large basins and high energy barrier, showing stable attractor
properties. However, in the single rotor with wavebreak, there were relatively
low pairwise interaction coefficients and a similar number of the local minima
separated by a relatively low energy barrier compared with the single stable
rotor case. The energy landscape of the multiple wavelet consisted of a large
number of the local minima separated by a relatively low energy barrier,
showing unstable dynamics. These results indicate that the MEM provides
information about local and global coherence among the cardiomyocytes beyond
the simple structural connectivity. Energy landscape analysis can explain
stability and transitional properties of complex dynamics of cardiac
fibrillation, which might be determined by the presence of 'driver' such as
sinus node or rotor.Comment: Presented at the 62nd Biophysical Society Annual Meeting, San
Francisco, California, 201
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