111,838 research outputs found
Self-force on extreme mass ratio inspirals via curved spacetime effective field theory
In this series we construct an effective field theory (EFT) in curved
spacetime to study gravitational radiation and backreaction effects. We begin
in this paper with a derivation of the self-force on a compact object moving in
the background spacetime of a supermassive black hole. The EFT approach
utilizes the disparity between two length scales, which in this problem are the
size of the compact object and the radius of curvature of the background
spacetime, to treat the orbital dynamics of the compact object, described as an
effective point particle, separately from its tidal deformations. Ultraviolet
divergences are regularized using Hadamard's {\it partie finie} to isolate the
non-local finite part from the quasi-local divergent part. The latter is
constructed from a momentum space representation for the graviton retarded
propagator and is evaluated using dimensional regularization in which only
logarithmic divergences are relevant for renormalizing the parameters of the
theory. As a first important application of this framework we explicitly derive
the first order self-force given by Mino, Sasaki, Tanaka, Quinn and Wald. Going
beyond the point particle approximation, to account for the finite size of the
object, we demonstrate that for extreme mass ratio inspirals the motion of a
compact object is affected by tidally induced moments at , in
the form of an Effacement Principle. The relatively large radius-to-mass ratio
of a white dwarf star allows for these effects to be enhanced until the white
dwarf becomes tidally disrupted, a potentially process, or
plunges into the supermassive black hole. This work provides a new foundation
for further exploration of higher order self force corrections, gravitational
radiation and spinning compact objects.Comment: 22 pages, 5 figures; references added, revised Appendices B & C,
corrected typos, revisions throughout for clarification particularly in
Section IV.B; submitted to PR
Kirigami Actuators
Thin elastic sheets bend easily and, if they are patterned with cuts, can
deform in sophisticated ways. Here we show that carefully tuning the location
and arrangement of cuts within thin sheets enables the design of mechanical
actuators that scale down to atomically-thin 2D materials. We first show that
by understanding the mechanics of a single, non-propagating crack in a sheet we
can generate four fundamental forms of linear actuation: roll, pitch, yaw, and
lift. Our analytical model shows that these deformations are only weakly
dependent on thickness, which we confirm with experiments at centimeter scale
objects and molecular dynamics simulations of graphene and MoS nanoscale
sheets. We show how the interactions between non-propagating cracks can enable
either lift or rotation, and we use a combination of experiments, theory,
continuum computational analysis, and molecular dynamics simulations to provide
mechanistic insights into the geometric and topological design of kirigami
actuators.Comment: Soft Matter, 201
Universal bounds on current fluctuations
For current fluctuations in non-equilibrium steady states of Markovian
processes, we derive four different universal bounds valid beyond the Gaussian
regime. Different variants of these bounds apply to either the entropy change
or any individual current, e.g., the rate of substrate consumption in a
chemical reaction or the electron current in an electronic device. The bounds
vary with respect to their degree of universality and tightness. A universal
parabolic bound on the generating function of an arbitrary current depends
solely on the average entropy production. A second, stronger bound requires
knowledge both of the thermodynamic forces that drive the system and of the
topology of the network of states. These two bounds are conjectures based on
extensive numerics. An exponential bound that depends only on the average
entropy production and the average number of transitions per time is rigorously
proved. This bound has no obvious relation to the parabolic bound but it is
typically tighter further away from equilibrium. An asymptotic bound that
depends on the specific transition rates and becomes tight for large
fluctuations is also derived. This bound allows for the prediction of the
asymptotic growth of the generating function. Even though our results are
restricted to networks with a finite number of states, we show that the
parabolic bound is also valid for three paradigmatic examples of driven
diffusive systems for which the generating function can be calculated using the
additivity principle. Our bounds provide a new general class of constraints for
nonequilibrium systems.Comment: 19 pages, 13 figure
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