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On the narrative form of simulations.
Understanding complex physical systems through the use of simulations often takes on a narrative character. That is, scientists using simulations seek an understanding of processes occurring in time by generating them from a dynamic model, thereby producing something like a historical narrative. This paper focuses on simulations of the Diels-Alder reaction, which is widely used in organic chemistry. It calls on several well-known works on historical narrative to draw out the ways in which use of these simulations mirrors aspects of narrative understanding: Gallie for "followability" and "contingency"; Mink for "synoptic judgment"; Ricoeur for "temporal dialectic"; and Hawthorn for a related dialectic of the "actual and the possible". Through these reflections on narrative, the paper aims for a better grasp of the role that temporal development sometimes plays in understanding physical processes and of how considerations of possibility enhance that understanding
Calibrating AGN Feedback in Clusters
Whether caused by AGN jets, shocks, or mergers, the most definitive evidence
for heating in cluster cores comes from X-ray spectroscopy. Unfortunately such
spectra are essentially limited to studying the emission spectrum from the
cluster as a whole. However since the same underlying emission measure
distribution produces both the observed CCD and RGS spectra, X-ray imaging can
still provide spatial information on the heating process. Using Chandra
archival data for a sample of 9 clusters, we demonstrate how imaging data can
be used to constrain departures from a canonical, isobaric cooling flow model
as a function of position in a given cluster. The results of this analysis are
also shown for the deep archival exposure of the Perseus cluster. Such heating
maps can provide constraints on both the location and magnitude of the heating
in the cores of clusters. When combined with detections and spectral index maps
from low-frequency radio observations, these maps can be used to distinguish
between different models for heating in these objects.Comment: 4 pages, 2 figures; to appear in the proceedings of "The Monsters'
Fiery Breath", Madison, Wisconsin 1-5 June 2009, Eds. Sebastian Heinz & Eric
Wilcot
Convergence Analysis of the Fast Subspace Descent Methods for Convex Optimization Problems
The full approximation storage (FAS) scheme is a widely used multigrid method
for nonlinear problems. In this paper, a new framework to design and analyze
FAS-like schemes for convex optimization problems is developed. The new method,
the Fast Subspace Descent (FASD) scheme, which generalizes classical FAS, can
be recast as an inexact version of nonlinear multigrid methods based on space
decomposition and subspace correction. The local problem in each subspace can
be simplified to be linear and one gradient descent iteration (with an
appropriate step size) is enough to ensure a global linear (geometric)
convergence of FASD.Comment: 33 page
Bioeconomic meta-modelling of Indonesian agroforests as carbon sinks
In many areas of developing countries, economic and institutional factors often combine to give farmers incentives to clear forests and repeatedly plant food crops without sufficiently replenishing the soils. These activities lead to large-scale land degradation and contribute to global warming through the release of greenhouse gases into the atmosphere. We investigate whether agroforestry systems might alleviate these trends when carbon-credit payments are available under the Clean Development Mechanism of the Kyoto Protocol. A meta-modelling framework is adopted, comprising an econometric-production model of a smallholding in Sumatra. The model is used within a dynamic-programming algorithm to determine optimal combinations of tree/crop area, tree-rotation length, and firewood harvest. Results show the influence of soil-carbon stocks and discount rates on optimal strategies and reveal interesting implications for joint management of agriculture and carbon.bio-economic meta-modelling, Indonesia, agroforestry, carbon credits, dynamic programming, Environmental Economics and Policy, Resource /Energy Economics and Policy,
Mascons as structural relief on a lunar Moho
Mascons as structural relief on lunar Moh
Imprints of a Primordial Preferred Direction on the Microwave Background
Rotational invariance is a well-established feature of low-energy physics.
Violations of this symmetry must be extremely small today, but could have been
larger in earlier epochs. In this paper we examine the consequences of a small
breaking of rotational invariance during the inflationary era when the
primordial density fluctuations were generated. Assuming that a fixed-norm
vector picked out a preferred direction during the inflationary era, we explore
the imprint it would leave on the cosmic microwave background anisotropy, and
provide explicit formulas for the expected amplitudes of
the spherical-harmonic coefficients. We suggest that it is natural to expect
that the imprint on the primordial power spectrum of a preferred spatial
direction is approximately scale-invariant, and examine a simple model in which
this is true.Comment: 7 pages, no figures; v5: Corrections, as well as use of more standard
convention, in section I
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