2,024 research outputs found
The Mechanics and Statistics of Active Matter
Active particles contain internal degrees of freedom with the ability to take
in and dissipate energy and, in the process, execute systematic movement.
Examples include all living organisms and their motile constituents such as
molecular motors. This article reviews recent progress in applying the
principles of nonequilibrium statistical mechanics and hydrodynamics to form a
systematic theory of the behaviour of collections of active particles -- active
matter -- with only minimal regard to microscopic details. A unified view of
the many kinds of active matter is presented, encompassing not only living
systems but inanimate analogues. Theory and experiment are discussed side by
side.Comment: This review is to appear in volume 1 of the Annual Review of
Condensed Matter Physics in July 2010 and is posted here with permission from
that journa
Entropy production in phase field theories
Allen-Cahn (Ginzburg-Landau) dynamics for scalar fields with heat conduction
is treated in rigid bodies using a non-equilibrium thermodynamic framework with
weakly nonlocal internal variables. The entropy production and entropy flux is
calculated with the classical method of irreversible thermodynamics by
separating full divergences.Comment: 5 pages, no figure
Quantum-Classical Correspondence of Dynamical Observables, Quantization and the Time of Arrival Correspondence Problem
We raise the problem of constructing quantum observables that have classical
counterparts without quantization. Specifically we seek to define and motivate
a solution to the quantum-classical correspondence problem independent from
quantization and discuss the general insufficiency of prescriptive
quantization, particularly the Weyl quantization. We demonstrate our points by
constructing time of arrival operators without quantization and from these
recover their classical counterparts
Structure formation in the presence of relativistic heat conduction: corrections to the Jeans wave number with a stable first order in the gradients formalism
The problem of structure formation in relativistic dissipative fluids was
analyzed in a previous work within Eckart's framework, in which the heat flux
is coupled to the hydrodynamic acceleration, additional to the usual
temperature gradient term. It was shown that in such case, the pathological
behavior of fluctuations leads to the disapperance of the gravitational
instability responsible for structure formation. In the present work the
problem is revisited now using a constitutive equation derived from
relativistic kinetic theory. The new relation, in which the heat flux is not
coupled to the hydrodynamic acceleration, leads to a consistent first order in
the gradients formalism. In this case the gravitational instability remains,
and only relativistic corrections to the Jeans wave number are obtained. In the
calculation here shown the non-relativistc limit is recovered, opposite to what
happens in Eckart's case.Comment: 10 pages, no figure
An inhomogeneous toy-model of the quantum gravity with explicitly evolvable observables
An inhomogeneous (1+1)-dimensional model of the quantum gravity is
considered. It is found, that this model corresponds to a string propagating
against some curved background space. The quantization scheme including the
Wheeler-DeWitt equation and the "particle on a sphere" type of the gauge
condition is suggested. In the quantization scheme considered, the "problem of
time" is solved by building of the quasi-Heisenberg operators acting in a space
of solutions of the Wheeler-DeWitt equation and the normalization of the wave
function corresponds to the Klein-Gordon type. To analyze the physical
consequences of the scheme, a (1+1)-dimensional background space is considered
for which a classical solution is found and quantized. The obtained estimations
show the way to solution of the cosmological constant problem, which consists
in compensation of the zero-point oscillations of the matter fields by the
quantum oscillations of the scale factor. Along with such a compensation, a
slow global evolution of a background corresponding to an universe expansion
exists.Comment: 18 page
On the nonequilibrium entropy of large and small systems
Thermodynamics makes definite predictions about the thermal behavior of
macroscopic systems in and out of equilibrium. Statistical mechanics aims to
derive this behavior from the dynamics and statistics of the atoms and
molecules making up these systems. A key element in this derivation is the
large number of microscopic degrees of freedom of macroscopic systems.
Therefore, the extension of thermodynamic concepts, such as entropy, to small
(nano) systems raises many questions. Here we shall reexamine various
definitions of entropy for nonequilibrium systems, large and small. These
include thermodynamic (hydrodynamic), Boltzmann, and Gibbs-Shannon entropies.
We shall argue that, despite its common use, the last is not an appropriate
physical entropy for such systems, either isolated or in contact with thermal
reservoirs: physical entropies should depend on the microstate of the system,
not on a subjective probability distribution. To square this point of view with
experimental results of Bechhoefer we shall argue that the Gibbs-Shannon
entropy of a nano particle in a thermal fluid should be interpreted as the
Boltzmann entropy of a dilute gas of Brownian particles in the fluid
Analysis of the Reaction Rate Coefficients for Slow Bimolecular Chemical Reactions
Simple bimolecular reactions are analyzed
within the framework of the Boltzmann equation in the initial stage of a
chemical reaction with the system far from chemical equilibrium. The
Chapman-Enskog methodology is applied to determine the coefficients of the
expansion of the distribution functions in terms of Sonine polynomials for
peculiar molecular velocities. The results are applied to the reaction
, and the influence of the non-Maxwellian
distribution and of the activation-energy dependent reactive cross sections
upon the forward and reverse reaction rate coefficients are discussed.Comment: 11 pages, 5 figures, to appear in vol.42 of the Brazilian Journal of
Physic
Gender perspectives on views and preferences of older people on exercise to prevent falls: a systematic mixed studies review
Background: To offer fall prevention exercise programs that attract older people of both sexes there is a need to understand both womens and mens views and preferences regarding these programs. This paper aims to systematically review the literature to explore any underlying gender perspectives or gender interpretations on older peoples views or preferences regarding uptake and adherence to exercise to prevent falls. Methods: A review of the literature was carried out using a convergent qualitative design based on systematic searches of seven electronic databases (PubMed, CINAHL, Amed, PsycINFO, Scopus, PEDro, and OTseeker). Two investigators identified eligible studies. Each included article was read by at least two authors independently to extract data into tables. Views and preferences reported were coded and summarized in themes of facilitators and barriers using a thematic analysis approach. Results: Nine hundred and nine unique studies were identified. Twenty five studies met the criteria for inclusion. Only five of these contained a gender analysis of mens and womens views on fall prevention exercises. The results suggests that both women and men see women as more receptive to and in more need of fall prevention messages. The synthesis from all 25 studies identified six themes illustrating facilitators and six themes describing barriers for older people either starting or adhering to fall prevention exercise. The facilitators were: support from professionals or family; social interaction; perceived benefits; a supportive exercise context; feelings of commitment; and having fun. Barriers were: practical issues; concerns about exercise; unawareness; reduced health status; lack of support; and lack of interest. Considerably more women than men were included in the studies. Conclusion: Although there is plenty of information on the facilitators and barriers to falls prevention exercise in older people, there is a distinct lack of studies investigating differences or similarities in older womens and mens views regarding fall prevention exercise. In order to ensure that fall prevention exercise is appealing to both sexes and that the inclusion of both men and women are encouraged, more research is needed to find out whether gender differences exists and whether practitioners need to offer a range of opportunities and support strategies to attract both women and men to falls prevention exercise.Funding Agencies|Swedish Research Council [2015-03481]; Strategic Research Programme in Care Sciences, Umea University; Karolinska Institute, Sweden; Umea University</p
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