57,985 research outputs found
Elastic Effects in Disordered Nematic Networks
Elastic effects in a model of disordered nematic elastomers are numerically
investigated in two dimensions. Networks crosslinked in the isotropic phase
exhibit unusual soft mechanical response against stretching. It arises from
gradual alignment of orientationally correlated regions that are elongated
along the director. A sharp crossover to a macroscopically aligned state is
obtained on further stretching. The effect of random internal stress is also
discussed.Comment: 5 pages, 5 figure
Timelike Infinity and Asymptotic Symmetry
By extending Ashtekar and Romano's definition of spacelike infinity to the
timelike direction, a new definition of asymptotic flatness at timelike
infinity for an isolated system with a source is proposed. The treatment
provides unit spacelike 3-hyperboloid timelike infinity and avoids the
introduction of the troublesome differentiability conditions which were
necessary in the previous works on asymptotically flat spacetimes at timelike
infinity. Asymptotic flatness is characterized by the fall-off rate of the
energy-momentum tensor at timelike infinity, which makes it easier to
understand physically what spacetimes are investigated. The notion of the order
of the asymptotic flatness is naturally introduced from the rate. The
definition gives a systematized picture of hierarchy in the asymptotic
structure, which was not clear in the previous works. It is found that if the
energy-momentum tensor falls off at a rate faster than , the
spacetime is asymptotically flat and asymptotically stationary in the sense
that the Lie derivative of the metric with respect to \ppp_t falls off at the
rate . It also admits an asymptotic symmetry group similar to the
Poincar\'e group. If the energy-momentum tensor falls off at a rate faster than
, the four-momentum of a spacetime may be defined. On the other
hand, angular momentum is defined only for spacetimes in which the
energy-momentum tensor falls off at a rate faster than .Comment: 19 pages, LaTex, the final version to appear in J. Math. Phy
The evolution of cooperation by social exclusion
The exclusion of freeriders from common privileges or public acceptance is
widely found in the real world. Current models on the evolution of cooperation
with incentives mostly assume peer sanctioning, whereby a punisher imposes
penalties on freeriders at a cost to itself. It is well known that such costly
punishment has two substantial difficulties. First, a rare punishing cooperator
barely subverts the asocial society of freeriders, and second, natural
selection often eliminates punishing cooperators in the presence of
non-punishing cooperators (namely, "second-order" freeriders). We present a
game-theoretical model of social exclusion in which a punishing cooperator can
exclude freeriders from benefit sharing. We show that such social exclusion can
overcome the above-mentioned difficulties even if it is costly and stochastic.
The results do not require a genetic relationship, repeated interaction,
reputation, or group selection. Instead, only a limited number of freeriders
are required to prevent the second-order freeriders from eroding the social
immune system.Comment: 28 pages, 3 figures, supplementary material (materials and methods,
and 6 supplementary figures
Optimal Timber Rotation on Multiple Stands with an Asymmetric Externality
Replaced with revised version of paper 07/28/05.forest economics, multiple stands, non-timber goods, flood risk, spatial externality, additivity properties, Resource /Energy Economics and Policy, Q23, Q57,
Synchronization in A Carpet of Hydrodynamically Coupled Rotors with Random Intrinsic Frequency
We investigate synchronization caused by long-range hydrodynamic interaction
in a two-dimensional, substrated array of rotors with random intrinsic
frequencies. The rotor mimics a flagellated bacterium that is attached to the
substrate ("bacterial carpet") and exerts an active force on the fluid.
Transition from coherent to incoherent regimes is studied numerically, and the
results are compared to a mean-field theory. We show that quite a narrow
distribution of the intrinsic frequency is required to achieve collective
motion in realistic cases. The transition is gradual, and the critical behavior
is qualitatively different from that of the conventional globally coupled
oscillators. The model not only serves as a novel example of non-locally
coupled oscillators, but also provides insights into the role of intrinsic
heterogeneities in living and artificial microfluidic actuators.Comment: 5 pages, 5 figure
Quantum Radion on de Sitter branes
The quantum fluctuation of the relative location of two (n-1)-dimensional de
Sitter branes (i.e., of n spacetime dimensions) embedded in the
(n+1)-dimensional anti-de Sitter bulk, which we shall call the quantum radion,
is investigated at the linear perturbation level. The quantization of the
radion is done by deriving the effective action of the radion. Assuming the
positive tension brane is our universe, the effect of the quantum radion is
evaluated by using the effective Einstein equations on the brane in which the
radion contributes to the effective energy momentum tensor at the linear order
of the radion amplitude. Specifically, the rms effective energy density arising
from the quantum radion is compared with the background energy density. It is
found out that this ratio remains small for reasonable values of the parameters
of the model even without introducing a stabilizing mechanism for radion,
although the radion itself has a negative mass squared and is unstable. The
reason behind this phenomenon is also discussed.Comment: 17 pages, no figure
- …