450 research outputs found
Fundamental Bounds on First Passage Time Fluctuations for Currents
Current is a characteristic feature of nonequilibrium systems. In stochastic
systems, these currents exhibit fluctuations constrained by the rate of
dissipation in accordance with the recently discovered thermodynamic
uncertainty relation. Here, we derive a conjugate uncertainty relationship for
the first passage time to accumulate a fixed net current. More generally, we
use the tools of large-deviation theory to simply connect current fluctuations
and first passage time fluctuations in the limit of long times and large
currents. With this connection, previously discovered symmetries and bounds on
the large-deviation function for currents are readily transferred to first
passage times.Comment: 7 pages including S
Proof of the Finite-Time Thermodynamic Uncertainty Relation for Steady-State Currents
The thermodynamic uncertainty relation offers a universal energetic
constraint on the relative magnitude of current fluctuations in nonequilibrium
steady states. However, it has only been derived for long observation times.
Here, we prove a recently conjectured finite-time thermodynamic uncertainty
relation for steady-state current fluctuations. Our proof is based on a
quadratic bound to the large deviation rate function for currents in the limit
of a large ensemble of many copies.Comment: 3 page
Parity Violation, the Neutron Radius of Lead, and Neutron Stars
The neutron radius of a heavy nucleus is a fundamental nuclear-structure
observable that remains elusive. Progress in this arena has been limited by the
exclusive use of hadronic probes that are hindered by large and controversial
uncertainties in the reaction mechanism. The Parity Radius Experiment at the
Jefferson Laboratory offers an attractive electro-weak alternative to the
hadronic program and promises to measure the neutron radius of 208Pb accurately
and model independently via parity-violating electron scattering. In this
contribution we examine the far-reaching implications that such a determination
will have in areas as diverse as nuclear structure, atomic parity violation,
and astrophysics.Comment: 5 pages, 5 figures, proceedings to the PAVI06 conferenc
Universal thermodynamic bounds on nonequilibrium response with biochemical applications
Diverse physical systems are characterized by their response to small
perturbations. Near thermodynamic equilibrium, the fluctuation-dissipation
theorem provides a powerful theoretical and experimental tool to determine the
nature of response by observing spontaneous equilibrium fluctuations. In this
spirit, we derive here a collection of equalities and inequalities valid
arbitrarily far from equilibrium that constrain the response of nonequilibrium
steady states in terms of the strength of nonequilibrium driving. Our work
opens new avenues for characterizing nonequilibrium response. As illustrations,
we show how our results rationalize the energetic requirements of two common
biochemical motifs.Comment: 21 pages, 15 figure
Insensitivity of the elastic proton-nucleus reaction to the neutron radius of 208Pb
The sensitivity--or rather insensitivity--of the elastic proton-nucleus
reaction to the neutron radius of 208Pb is investigated using a
non-relativistic impulse-approximation approach. The energy region (Tlab=500
MeV and Tlab=800 MeV) is selected so that the impulse approximation may be
safely assumed. Therefore, only free nucleon-nucleon scattering data are used
as input for the optical potential. Further, the optical potential includes
proton and neutron ground-state densities that are generated from
accurately-calibrated models. Even so, these models yield a wide range of
values (from 0.13 fm to 0.28 fm) for the poorly known neutron skin thickness in
208Pb. An excellent description of the experimental cross section is obtained
with all neutron densities. We have invoked analytic insights developed within
the eikonal approximation to understand the insensitivity of the differential
cross section to the various neutron densities. As the diffractive oscillations
of the cross sections are controlled by the matter radius of the nucleus, the
large spread in the neutron skin among the various models gets diluted into a
mere 1.5% difference in the matter radius. This renders ineffective the elastic
reaction as a precision tool for the measurement of neutron radii.Comment: 17 pages with 5 figure
Does action disrupt Multiple Object Tracking (MOT)?
While the relationship between action and focused attention has been well-studied, less is known about the ability to divide attention while acting. In the current paper we explore this issue using the multiple object tracking (MOT) paradigm (Pylyshyn & Storm, 1988). We asked whether planning and executing a display-relevant action during tracking would substantially affect the ability track and later identify targets. In all trials the primary task was to track 4 targets among a set of 8 identical objects. Several times during each trial, one object, selected at random, briefly changed colour. In the baseline MOT trials, these changes were ignored. During active trials, each changed object had to be quickly touched. On a given trial, changed objects were either from the tracking set or were selected at random from all 8 objects. Although there was a small dual-task cost, the need to act did not substantially impair tracking under either touch condition.peer-reviewe
What are the shapes of response time distributions in visual search?
Many visual search experiments measure response time (RT) as their primary dependent variable. Analyses typically focus on mean (or median) RT. However, given enough data, the RT distribution can be a rich source of information. For this paper, we collected about 500 trials per cell per observer for both target-present and target-absent displays in each of three classic search tasks: feature search, with the target defined by color; conjunction search, with the target defined by both color and orientation; and spatial configuration search for a 2 among distractor 5s. This large data set allows us to characterize the RT distributions in detail. We present the raw RT distributions and fit several psychologically motivated functions (ex-Gaussian, ex-Wald, Gamma, and Weibull) to the data. We analyze and interpret parameter trends from these four functions within the context of theories of visual search
Dissipation Bounds All Steady-State Current Fluctuations
Near equilibrium, small current fluctuations are described by a Gaussian distribution with a linear-response variance regulated by the dissipation. Here, we demonstrate that dissipation still plays a dominant role in structuring large fluctuations arbitrarily far from equilibrium. In particular, we prove a linear-response-like bound on the large deviation function for currents in Markov jump processes. We find that nonequilibrium current fluctuations are always more likely than what is expected from a linear-response analysis. As a small-fluctuations corollary, we derive a recently conjectured uncertainty bound on the variance of current fluctuations.Gordon and Betty Moore Foundation (Grant GBMF4513)Gordon and Betty Moore Foundation (Grant GBMF4343
Visual Attention Measures Predict Pedestrian Detection in Central Field Loss: A Pilot Study
Purpose The ability of visually impaired people to deploy attention effectively to maximize use of their residual vision in dynamic situations is fundamental to safe mobility. We conducted a pilot study to evaluate whether tests of dynamic attention (multiple object tracking; MOT) and static attention (Useful Field of View; UFOV) were predictive of the ability of people with central field loss (CFL) to detect pedestrian hazards in simulated driving. Methods: 11 people with bilateral CFL (visual acuity 20/30-20/200) and 11 age-similar normally-sighted drivers participated. Dynamic and static attention were evaluated with brief, computer-based MOT and UFOV tasks, respectively. Dependent variables were the log speed threshold for 60% correct identification of targets (MOT) and the increase in the presentation duration for 75% correct identification of a central target when a concurrent peripheral task was added (UFOV divided and selective attention subtests). Participants drove in a simulator and pressed the horn whenever they detected pedestrians that walked or ran toward the road. The dependent variable was the proportion of timely reactions (could have stopped in time to avoid a collision). Results: UFOV and MOT performance of CFL participants was poorer than that of controls, and the proportion of timely reactions was also lower (worse) (84% and 97%, respectively; p = 0.001). For CFL participants, higher proportions of timely reactions correlated significantly with higher (better) MOT speed thresholds (r = 0.73, p = 0.01), with better performance on the UFOV divided and selective attention subtests (r = −0.66 and −0.62, respectively, p<0.04), with better contrast sensitivity scores (r = 0.54, p = 0.08) and smaller scotomas (r = −0.60, p = 0.05). Conclusions: Our results suggest that brief laboratory-based tests of visual attention may provide useful measures of functional visual ability of individuals with CFL relevant to more complex mobility tasks
Relativistic mean-field study of neutron-rich nuclei
A relativistic mean-field model is used to study the ground-state properties
of neutron-rich nuclei. Nonlinear isoscalar-isovector terms, unconstrained by
present day phenomenology, are added to the model Lagrangian in order to modify
the poorly known density dependence of the symmetry energy. These new terms
soften the symmetry energy and reshape the theoretical neutron drip line
without compromising the agreement with existing ground-state information. A
strong correlation between the neutron radius of 208Pb and the binding energy
of valence orbitals is found: the smaller the neutron radius of 208Pb, the
weaker the binding energy of the last occupied neutron orbital. Thus, models
with the softest symmetry energy are the first ones to drip neutrons. Further,
in anticipation of the upcoming one-percent measurement of the neutron radius
of 208Pb at the Thomas Jefferson Laboratory, a close relationship between the
neutron radius of 208Pb and neutron radii of elements of relevance to atomic
parity-violating experiments is established.Comment: 14 pages, 5 figure
- …