650 research outputs found
Quantum dynamics of the avian compass
The ability of migratory birds to orient relative to the Earth's magnetic
field is believed to involve a coherent superposition of two spin states of a
radical electron pair. However, the mechanism by which this coherence can be
maintained in the face of strong interactions with the cellular environment has
remained unclear. This Letter addresses the problem of decoherence between two
electron spins due to hyperfine interaction with a bath of spin 1/2 nuclei.
Dynamics of the radical pair density matrix are derived and shown to yield a
simple mechanism for sensing magnetic field orientation. Rates of dephasing and
decoherence are calculated ab initio and found to yield millisecond coherence
times, consistent with behavioral experiments
Impact of stress reduction on negative school behavior in adolescents
BACKGROUND: The purpose of this study was to determine the effect of stress reduction via the Transcendental Meditation program on school rule infractions in adolescents. METHODS: Forty-five African American adolescents (ages 15–18 years) with high normal systolic blood pressure were randomly assigned to either Transcendental Meditation (n = 25) or health education control (n = 20) groups. The meditation group engaged in 15-min sessions at home and at school each day for 4 months. The control group was presented 15-min sessions of health education at school each day for 4 months. Primary outcome measures were changes in absenteeism, school rule infractions and suspension days during the four-month pretest period prior to randomization compared with the four-month intervention period. RESULTS: Comparing the pretest and intervention periods, the meditation group exhibited a mean decrease of 6.4 absentee periods compared to an increase of 4.8 in the control group (p < .05). The meditation group exhibited a mean decrease of 0.1 infractions over the four months compared to an increase of 0.3 in the control group (p < .03). There was a mean reduction of 0.3 suspension days due to behavior-related problems in the meditation group compared to an increase of 1.2 in the control group (p < .04). CONCLUSION: These findings demonstrate that the Transcendental Meditation program conducted in the school setting has a beneficial impact upon absenteeism, rule infractions, and suspension rates in African American adolescents
Towards a Macroscopic Modelling of the Complexity in Traffic Flow
We present a macroscopic traffic flow model that extends existing fluid-like
models by an additional term containing the second derivative of the safe
velocity. Two qualitatively different shapes of the safe velocity are explored:
a conventional Fermi-type function and a function exhibiting a plateau at
intermediate densities. The suggested model shows an extremely rich dynamical
behaviour and shows many features found in real-world traffic data.Comment: submitted to Phys. Rev.
Long-lived states in synchronized traffic flow. Empirical prompt and dynamical trap model
The present paper proposes a novel interpretation of the widely scattered
states (called synchronized traffic) stimulated by Kerner's hypotheses about
the existence of a multitude of metastable states in the fundamental diagram.
Using single vehicle data collected at the German highway A1, temporal velocity
patterns have been analyzed to show a collection of certain fragments with
approximately constant velocities and sharp jumps between them. The particular
velocity values in these fragments vary in a wide range. In contrast, the flow
rate is more or less constant because its fluctuations are mainly due to the
discreteness of traffic flow.
Subsequently, we develop a model for synchronized traffic that can explain
these characteristics. Following previous work (I.A.Lubashevsky, R.Mahnke,
Phys. Rev. E v. 62, p. 6082, 2000) the vehicle flow is specified by car
density, mean velocity, and additional order parameters and that are
due to the many-particle effects of the vehicle interaction. The parameter
describes the multilane correlations in the vehicle motion. Together with the
car density it determines directly the mean velocity. The parameter , in
contrast, controls the evolution of only. The model assumes that
fluctuates randomly around the value corresponding to the car configuration
optimal for lane changing. When it deviates from this value the lane change is
depressed for all cars forming a local cluster. Since exactly the overtaking
manoeuvres of these cars cause the order parameter to vary, the evolution
of the car arrangement becomes frozen for a certain time. In other words, the
evolution equations form certain dynamical traps responsible for the long-time
correlations in the synchronized mode.Comment: 16 pages, 10 figures, RevTeX
Congested Traffic States in Empirical Observations and Microscopic Simulations
We present data from several German freeways showing different kinds of
congested traffic forming near road inhomogeneities, specifically lane
closings, intersections, or uphill gradients. The states are localized or
extended, homogeneous or oscillating. Combined states are observed as well,
like the coexistence of moving localized clusters and clusters pinned at road
inhomogeneities, or regions of oscillating congested traffic upstream of nearly
homogeneous congested traffic. The experimental findings are consistent with a
recently proposed theoretical phase diagram for traffic near on-ramps [D.
Helbing, A. Hennecke, and M. Treiber, Phys. Rev. Lett. {\bf 82}, 4360 (1999)].
We simulate these situations with a novel continuous microscopic single-lane
model, the ``intelligent driver model'' (IDM), using the empirical boundary
conditions. All observations, including the coexistence of states, are
qualitatively reproduced by describing inhomogeneities with local variations of
one model parameter.
We show that the results of the microscopic model can be understood by
formulating the theoretical phase diagram for bottlenecks in a more general
way. In particular, a local drop of the road capacity induced by parameter
variations has practically the same effect as an on-ramp.Comment: Now published in Phys. Rev. E. Minor changes suggested by a referee
are incorporated; full bibliographic info added. For related work see
http://www.mtreiber.de/ and http://www.helbing.org
Autonomous detection and anticipation of jam fronts from messages propagated by inter-vehicle communication
In this paper, a minimalist, completely distributed freeway traffic
information system is introduced. It involves an autonomous, vehicle-based jam
front detection, the information transmission via inter-vehicle communication,
and the forecast of the spatial position of jam fronts by reconstructing the
spatiotemporal traffic situation based on the transmitted information. The
whole system is simulated with an integrated traffic simulator, that is based
on a realistic microscopic traffic model for longitudinal movements and lane
changes. The function of its communication module has been explicitly validated
by comparing the simulation results with analytical calculations. By means of
simulations, we show that the algorithms for a congestion-front recognition,
message transmission, and processing predict reliably the existence and
position of jam fronts for vehicle equipment rates as low as 3%. A reliable
mode of operation already for small market penetrations is crucial for the
successful introduction of inter-vehicle communication. The short-term
prediction of jam fronts is not only useful for the driver, but is essential
for enhancing road safety and road capacity by intelligent adaptive cruise
control systems.Comment: Published in the Proceedings of the Annual Meeting of the
Transportation Research Board 200
Macroscopic traffic models from microscopic car-following models
We present a method to derive macroscopic fluid-dynamic models from
microscopic car-following models via a coarse-graining procedure. The method is
first demonstrated for the optimal velocity model. The derived macroscopic
model consists of a conservation equation and a momentum equation, and the
latter contains a relaxation term, an anticipation term, and a diffusion term.
Properties of the resulting macroscopic model are compared with those of the
optimal velocity model through numerical simulations, and reasonable agreement
is found although there are deviations in the quantitative level. The
derivation is also extended to general car-following models.Comment: 12 pages, 4 figures; to appear in Phys. Rev.
Cardiac index derived from arterial pressure waveform: uncalibrated analysis vs once-only calibrated analysis
Comparison of Insulin Sensitivity of Horses Adapted to Different Exercise Intensities
AbstractDiets high in concentrates and soluble carbohydrates are associated with reduced insulin sensitivity in horses. Exercise training could protect against diet-induced insulin resistance. The objective of this study was to determine the intensity of exercise training required to affect insulin sensitivity in stabled horses fed a diet high in concentrates but moderate in soluble carbohydrates. In all, 31 stabled horses underwent three different exercise regimens: turnout, light exercise, and moderate exercise, while being fed a diet containing 60% concentrate. Blood was sampled monthly and analyzed for insulin. Insulin sensitivity was assessed using basal insulin concentrations and calculated insulin sensitivity (reciprocal of the square root of insulin) and compared across months by analysis of variance with repeated measures. Insulin sensitivity (reciprocal of the square root of insulin) was higher during periods of moderate and light physical activity as compared with turnout. These results indicate that turnout alone may not be adequate to improve insulin sensitivity in horses fed high amounts of concentrate
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