4,897 research outputs found
Bringing Together Gravity and the Quanta
Due to its underlying gauge structure, teleparallel gravity achieves a
separation between inertial and gravitational effects. It can, in consequence,
describe the isolated gravitational interaction without resorting to the
equivalence principle, and is able to provide a tensorial definition for the
energy-momentum density of the gravitational field. Considering the conceptual
conflict between the local equivalence principle and the nonlocal uncertainty
principle, the replacement of general relativity by its teleparallel equivalent
can be considered an important step towards a prospective reconciliation
between gravitation and quantum mechanics.Comment: 9 pages. Contribution to the proceedings of the Albert Einstein
Century International Conference, Paris, 18-22 July, 200
Simple two-layer dispersive models in the Hamiltonian reduction formalism
A Hamiltonian reduction approach is defined, studied, and finally used to
derive asymptotic models of internal wave propagation in density stratified
fluids in two-dimensional domains. Beginning with the general Hamiltonian
formalism of Benjamin [1] for an ideal, stably stratified Euler fluid, the
corresponding structure is systematically reduced to the setup of two
homogeneous fluids under gravity, separated by an interface and confined
between two infinite horizontal plates. A long-wave, small-amplitude
asymptotics is then used to obtain a simplified model that encapsulates most of
the known properties of the dynamics of such systems, such as bidirectional
wave propagation and maximal amplitude travelling waves in the form of fronts.
Further reductions, and in particular devising an asymptotic extension of
Dirac's theory of Hamiltonian constraints, lead to the completely integrable
evolution equations previously considered in the literature for limiting forms
of the dynamics of stratified fluids. To assess the performance of the
asymptotic models, special solutions are studied and compared with those of the
parent equations.Comment: 29 pages, 4 figure
Life cycle assessment of biogas production in small-scale household digesters in Vietnam
Small-scale household digesters have been promoted across Asia as a sustainable way of handling manure. The major advantages are that they produce biogas and reduce odor. However their disadvantages include the low recycling of nutrients, because digestate is dilute and therefore difficult to transport, and the loss of biogas as a result of cracks and the intentional release of excess biogas. In this study, life cycle assessment (LCA) methodology was used to assess the environmental impacts associated with biogas digesters in Vietnam. Handling 1,000 kg of liquid manure and 100 kg of solid manure in a system with a biogas digester reduced the impact potential from 4.4 kg carbon dioxide (CO2) equivalents to 3.2 kg CO2 equivalents compared with traditional manure management. However, this advantage could easily be compromised if digester construction is considered in the LCA or in situations where there is an excess of biogas which is intentionally released. A sensitivity analysis showed that biogas digesters could be a means of reducing global warming if methane emissions can be kept low. In terms of eutrophication, farms with biogas digesters had 3 to 4 times greater impacts. In order to make biogas digesters sustainable, methods for recycling digestates are urgently required
On the Locality of Nash-Williams Forest Decomposition and Star-Forest Decomposition
Given a graph with arboricity , we study the problem of
decomposing the edges of into disjoint forests in the
distributed LOCAL model. Barenboim and Elkin [PODC `08] gave a LOCAL algorithm
that computes a -forest decomposition using rounds. Ghaffari and Su [SODA `17] made further progress by
computing a -forest decomposition in rounds when ,
i.e. the limit of their algorithm is an -forest decomposition. This algorithm, based on a combinatorial
construction of Alon, McDiarmid \& Reed [Combinatorica `92], in fact provides a
decomposition of the graph into \emph{star-forests}, i.e. each forest is a
collection of stars.
Our main result in this paper is to reduce the threshold of
in -forest decomposition and star-forest decomposition.
This further answers the open question from Barenboim and
Elkin's "Distributed Graph Algorithms" book. Moreover, it gives the first
-orientation algorithms with {\it linear dependencies} on
.
At a high level, our results for forest-decomposition are based on a
combination of network decomposition, load balancing, and a new structural
result on local augmenting sequences. Our result for star-forest decomposition
uses a more careful probabilistic analysis for the construction of Alon,
McDiarmid, \& Reed; the bounds on star-arboricity here were not previously
known, even non-constructively
Compact Brillouin devices through hybrid integration on Silicon
A range of unique capabilities in optical and microwave signal processing
have been demonstrated using stimulated Brillouin scattering. The desire to
harness Brillouin scattering in mass manufacturable integrated circuits has led
to a focus on silicon-based material platforms. Remarkable progress in
silicon-based Brillouin waveguides has been made, but results have been
hindered by nonlinear losses present at telecommunications wavelengths. Here,
we report a new approach to surpass this issue through the integration of a
high Brillouin gain material, As2S3, onto a silicon chip. We fabricated a
compact spiral device, within a silicon circuit, achieving an order of
magnitude improvement in Brillouin amplification. To establish the flexibility
of this approach, we fabricated a ring resonator with free spectral range
precisely matched to the Brillouin shift, enabling the first demonstration of
Brillouin lasing in a silicon integrated circuit. Combining active photonic
components with the SBS devices shown here will enable the creation of compact,
mass manufacturable optical circuits with enhanced functionality
Multiple-point statistical simulation for hydrogeological models: 3D training image development and conditioning strategies
Most studies about the application of geostatistical simulations based on multiple-point statistics (MPS) to hydrogeological modelling focus on relatively fine-scale models and concentrate on the estimation of facies-level, structural uncertainty. Much less attention is paid to the use of input data and optimal construction of training images. For instance, even though the training image should capture a set of spatial geological characteristics to guide the simulations, the majority of the research still relies on 2D or quasi-3D training images. In the present study, we demonstrate a novel strategy for 3D MPS modelling characterized by: (i) realistic 3D training images, and (ii) an effective workflow for incorporating a diverse group of geological and geophysical data sets. The study covers an area of 2810 km2 in the southern part of Denmark. MPS simulations are performed on a subset of the geological succession (the lower to middle Miocene sediments) which is characterized by relatively uniform structures and dominated by sand and clay. The simulated domain is large and each of the geostatistical realizations contains approximately 45 million voxels with size 100 m × 100 m × 5 m. Data used for the modelling include water well logs, high-resolution seismic data, and a previously published 3D geological model. We apply a series of different strategies for the simulations based on data quality, and develop a novel method to effectively create observed sand/clay spatial trends. The training image is constructed as a small 3D voxel model covering an area of 90 km2. We use an iterative training image development strategy and find that even slight modifications in the training image create significant changes in simulations. Thus, the study underlines that it is important to consider both the geological environment, and the type and quality of input information in order to achieve optimal results from MPS modelling. In this study we present a possible workflow to build the training image and effectively handle different types of input information to perform large-scale geostatistical modellin
Zooming in on local level statistics by supersymmetric extension of free probability
We consider unitary ensembles of Hermitian NxN matrices H with a confining
potential NV where V is analytic and uniformly convex. From work by
Zinn-Justin, Collins, and Guionnet and Maida it is known that the large-N limit
of the characteristic function for a finite-rank Fourier variable K is
determined by the Voiculescu R-transform, a key object in free probability
theory. Going beyond these results, we argue that the same holds true when the
finite-rank operator K has the form that is required by the Wegner-Efetov
supersymmetry method of integration over commuting and anti-commuting
variables. This insight leads to a potent new technique for the study of local
statistics, e.g., level correlations. We illustrate the new technique by
demonstrating universality in a random matrix model of stochastic scattering.Comment: 38 pages, 3 figures, published version, minor changes in Section
Stable Propagation of a Burst Through a One-Dimensional Homogeneous Excitatory Chain Model of Songbird Nucleus HVC
We demonstrate numerically that a brief burst consisting of two to six spikes
can propagate in a stable manner through a one-dimensional homogeneous
feedforward chain of non-bursting neurons with excitatory synaptic connections.
Our results are obtained for two kinds of neuronal models, leaky
integrate-and-fire (LIF) neurons and Hodgkin-Huxley (HH) neurons with five
conductances. Over a range of parameters such as the maximum synaptic
conductance, both kinds of chains are found to have multiple attractors of
propagating bursts, with each attractor being distinguished by the number of
spikes and total duration of the propagating burst. These results make
plausible the hypothesis that sparse precisely-timed sequential bursts observed
in projection neurons of nucleus HVC of a singing zebra finch are intrinsic and
causally related.Comment: 13 pages, 6 figure
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