426 research outputs found
Signaling, Entanglement, and Quantum Evolution Beyond Cauchy Horizons
Consider a bipartite entangled system half of which falls through the event
horizon of an evaporating black hole, while the other half remains coherently
accessible to experiments in the exterior region. Beyond complete evaporation,
the evolution of the quantum state past the Cauchy horizon cannot remain
unitary, raising the questions: How can this evolution be described as a
quantum map, and how is causality preserved? What are the possible effects of
such nonstandard quantum evolution maps on the behavior of the entangled
laboratory partner? More generally, the laws of quantum evolution under extreme
conditions in remote regions (not just in evaporating black-hole interiors, but
possibly near other naked singularities and regions of extreme spacetime
structure) remain untested by observation, and might conceivably be non-unitary
or even nonlinear, raising the same questions about the evolution of entangled
states. The answers to these questions are subtle, and are linked in unexpected
ways to the fundamental laws of quantum mechanics. We show that terrestrial
experiments can be designed to probe and constrain exactly how the laws of
quantum evolution might be altered, either by black-hole evaporation, or by
other extreme processes in remote regions possibly governed by unknown physics.Comment: Combined, revised, and expanded version of quant-ph/0312160 and
hep-th/0402060; 13 pages, RevTeX, 2 eps figure
Large-Scale Structure in Brane-Induced Gravity II. Numerical Simulations
We use N-body simulations to study the nonlinear structure formation in
brane-induced gravity, developing a new method that requires alternate use of
Fast Fourier Transforms and relaxation. This enables us to compute the
nonlinear matter power spectrum and bispectrum, the halo mass function, and the
halo bias. From the simulation results, we confirm the expectations based on
analytic arguments that the Vainshtein mechanism does operate as anticipated,
with the density power spectrum approaching that of standard gravity within a
modified background evolution in the nonlinear regime. The transition is very
broad and there is no well defined Vainshtein scale, but roughly this
corresponds to k_*~ 2 at redshift z=1 and k_*~ 1 at z=0. We checked that while
extrinsic curvature fluctuations go nonlinear, and the dynamics of the
brane-bending mode C receives important nonlinear corrections, this mode does
get suppressed compared to density perturbations, effectively decoupling from
the standard gravity sector. At the same time, there is no violation of the
weak field limit for metric perturbations associated with C. We find good
agreement between our measurements and the predictions for the nonlinear power
spectrum presented in paper I, that rely on a renormalization of the linear
spectrum due to nonlinearities in the modified gravity sector. A similar
prediction for the mass function shows the right trends. Our simulations also
confirm the induced change in the bispectrum configuration dependence predicted
in paper I.Comment: 19 pages, 13 figures. v2: corrected typos, added more simulations,
better test of predictions in large mass regime. v3: minor changes, published
versio
Resolving Exceptional Configurations
In lattice QCD with Wilson fermions, exceptional configurations arise in the
quenched approximation at small quark mass. The origin of these large
previously uncontrolled lattice artifacts is identified. A simple well-defined
procedure (MQA) is presented which removes the artifacts while preserving the
correct continuum limit.Comment: Talk presented by E. Eichten at Lattice 97, Edinburgh(UK), July97. 6
pages, LaTeX, 1 table, 5 figure
Chaos and the continuum limit in nonneutral plasmas and charged particle beams
This paper examines discreteness effects in nearly collisionless N-body
systems of charged particles interacting via an unscreened r^-2 force, allowing
for bulk potentials admitting both regular and chaotic orbits. Both for
ensembles and individual orbits, as N increases there is a smooth convergence
towards a continuum limit. Discreteness effects are well modeled by Gaussian
white noise with relaxation time t_R = const * (N/log L)t_D, with L the Coulomb
logarithm and t_D the dynamical time scale. Discreteness effects accelerate
emittance growth for initially localised clumps. However, even allowing for
discreteness effects one can distinguish between orbits which, in the continuum
limit, feel a regular potential, so that emittance grows as a power law in
time, and chaotic orbits, where emittance grows exponentially. For sufficiently
large N, one can distinguish two different `kinds' of chaos. Short range
microchaos, associated with close encounters between charges, is a generic
feature, yielding large positive Lyapunov exponents X_N which do not decrease
with increasing N even if the bulk potential is integrable. Alternatively,
there is the possibility of larger scale macrochaos, characterised by smaller
Lyapunov exponents X_S, which is present only if the bulk potential is chaotic.
Conventional computations of Lyapunov exponents probe X_N, leading to the
oxymoronic conclusion that N-body orbits which look nearly regular and have
sharply peaked Fourier spectra are `very chaotic.' However, the `range' of the
microchaos, set by the typical interparticle spacing, decreases as N increases,
so that, for large N, this microchaos, albeit very strong, is largely
irrelevant macroscopically. A more careful numerical analysis allows one to
estimate both X_N and X_S.Comment: 13 pages plus 17 figure
Chaotic Orbits in Thermal-Equilibrium Beams: Existence and Dynamical Implications
Phase mixing of chaotic orbits exponentially distributes these orbits through
their accessible phase space. This phenomenon, commonly called ``chaotic
mixing'', stands in marked contrast to phase mixing of regular orbits which
proceeds as a power law in time. It is operationally irreversible; hence, its
associated e-folding time scale sets a condition on any process envisioned for
emittance compensation. A key question is whether beams can support chaotic
orbits, and if so, under what conditions? We numerically investigate the
parameter space of three-dimensional thermal-equilibrium beams with space
charge, confined by linear external focusing forces, to determine whether the
associated potentials support chaotic orbits. We find that a large subset of
the parameter space does support chaos and, in turn, chaotic mixing. Details
and implications are enumerated.Comment: 39 pages, including 14 figure
An Arbitrary Curvilinear Coordinate Method for Particle-In-Cell Modeling
A new approach to the kinetic simulation of plasmas in complex geometries,
based on the Particle-in- Cell (PIC) simulation method, is explored. In the two
dimensional (2d) electrostatic version of our method, called the Arbitrary
Curvilinear Coordinate PIC (ACC-PIC) method, all essential PIC operations are
carried out in 2d on a uniform grid on the unit square logical domain, and
mapped to a nonuniform boundary-fitted grid on the physical domain. As the
resulting logical grid equations of motion are not separable, we have developed
an extension of the semi-implicit Modified Leapfrog (ML) integration technique
to preserve the symplectic nature of the logical grid particle mover. A
generalized, curvilinear coordinate formulation of Poisson's equations to solve
for the electrostatic fields on the uniform logical grid is also developed. By
our formulation, we compute the plasma charge density on the logical grid based
on the particles' positions on the logical domain. That is, the plasma
particles are weighted to the uniform logical grid and the self-consistent mean
electrostatic fields obtained from the solution of the logical grid Poisson
equation are interpolated to the particle positions on the logical grid. This
process eliminates the complexity associated with the weighting and
interpolation processes on the nonuniform physical grid and allows us to run
the PIC method on arbitrary boundary-fitted meshes.Comment: Submitted to Computational Science & Discovery December 201
Effects of structure formation on the expansion rate of the Universe: An estimate from numerical simulations
General relativistic corrections to the expansion rate of the Universe arise
when the Einstein equations are averaged over a spatial volume in a locally
inhomogeneous cosmology. It has been suggested that they may contribute to the
observed cosmic acceleration. In this paper, we propose a new scheme that
utilizes numerical simulations to make a realistic estimate of the magnitude of
these corrections for general inhomogeneities in (3+1) spacetime. We then
quantitatively calculate the volume averaged expansion rate using N-body
large-scale structure simulations and compare it with the expansion rate in a
standard FRW cosmology. We find that in the weak gravitational field limit, the
converged corrections are slightly larger than the previous claimed 10^{-5}
level, but not large enough nor even of the correct sign to drive the current
cosmic acceleration. Nevertheless, the question of whether the cumulative
effect can significantly change the expansion history of the Universe needs to
be further investigated with strong-field relativity.Comment: 13 pages, 6 figures, improved version published in Phys. Rev.
Kapitza Resistance between Few-Layer Graphene and Water: Liquid Layering Effects
The Kapitza resistance (RK) between few-layer graphene (FLG) and water was studied using molecular dynamics simulations. The R_K was found to depend on the number of the layers in the FLG though, surprisingly, not on the water block thickness. This distinct size dependence is attributed to the large difference in the phonon mean free path between the FLG and water. Remarkably, R_K is strongly dependent on the layering of water adjacent to the FLG, exhibiting an inverse proportionality relationship to the peak density of the first water layer, which is consistent with better acoustic phonon matching between FLG and water. These findings suggest novel ways to engineer the thermal transport properties of solid–liquid interfaces by controlling and regulating the liquid layering at the interface
Finite-temperature simulations of the scissors mode in Bose-Einstein condensed gases
The dynamics of a trapped Bose-condensed gas at finite temperatures is
described by a generalized Gross-Pitaevskii equation for the condensate order
parameter and a semi-classical kinetic equation for the thermal cloud, solved
using -body simulations. The two components are coupled by mean fields as
well as collisional processes that transfer atoms between the two. We use this
scheme to investigate scissors modes in anisotropic traps as a function of
temperature. Frequency shifts and damping rates of the condensate mode are
extracted, and are found to be in good agreement with recent experiments.Comment: 4 pages, 3 figure
- …