153,321 research outputs found
The distribution of species range size: a stochastic process
The major role played by environmental factors in determining the geographical range sizes of species raises the possibility of describing their long-term dynamics in relatively simple terms, a goal which has hitherto proved elusive. Here we develop a stochastic differential equation to describe the dynamics of the range size of an individual species based on the relationship between abundance and range size, derive a limiting stationary probability model to quantify the stochastic nature of the range size for that species at steady state, and then generalize this model to the species-range size distribution for an assemblage. The model fits well to several empirical datasets of the geographical range sizes of species in taxonomic assemblages, and provides the simplest explanation of species-range size distributions to date
Online Gaming Can Make a Better World: Jane McGonigal
With personal feelings put aside and sociological theoretical depictions brought to the forefront, it is interesting to compare some of Jane\u27s ideas with that of both Emile Durkheim and Max Weber. The theorist who stood out right away, being exemplified through Jane\u27s positive attitude claims on a much larger, macro-level scale, was Emile Durkheim. Jane\u27s ideas about transcending human\u27s as a resource through the social fabrics of gaming into something that might solve world hunger, poverty, and global warming was nothing short of functionalism at it\u27s best. Jane\u27s platform for social structure and maintaining positive social order is the online world, and online gaming is the vehicle for change
Cardiovascular ephrinB2 function is essential for embryonic angiogenesis
EphrinB2, a transmembrane ligand of EphB receptor tyrosine kinases, is specifically expressed in arteries. In ephrinB2 mutant embryos, there is a complete arrest of angiogenesis. However, ephrinB2 expression is not restricted to vascular endothelial cells, and it has been proposed that its essential function may be exerted in adjacent mesenchymal cells. We have generated mice in which ephrinB2 is specifically deleted in the endothelium and endocardium of the developing vasculature and heart. We find that such a vascular-specific deletion of ephrinB2 results in angiogenic remodeling defects identical to those seen in the conventional ephrinB2 mutants. These data indicate that ephrinB2 is required specifically in endothelial and endocardial cells for angiogenesis, and that ephrinB2 expression in perivascular mesenchyme is not sufficient to compensate for the loss of ephrinB2 in these vascular cells
Efficiency analysis of reaction rate calculation methods using analytical models I: The 2D sharp barrier
We analyze the efficiency of different methods for the calculation of
reaction rates in the case of two simple analytical benchmark systems. Two
classes of methods are considered: the first are based on the free energy
calculation along a reaction coordinate and the calculation of the transmission
coefficient, the second on the sampling of dynamical pathways. We give scaling
rules for how this efficiency depends on barrier height and width, and we hand
out simple optimization rules for the method-specific parameters. We show that
the path sampling methods, using the transition interface sampling technique,
become exceedingly more efficient than the others when the reaction coordinate
is not the optimal one.Comment: 22 pages, 5 figure
Approaching the Problem of Time with a Combined Semiclassical-Records-Histories Scheme
I approach the Problem of Time and other foundations of Quantum Cosmology
using a combined histories, timeless and semiclassical approach. This approach
is along the lines pursued by Halliwell. It involves the timeless probabilities
for dynamical trajectories entering regions of configuration space, which are
computed within the semiclassical regime. Moreover, the objects that Halliwell
uses in this approach commute with the Hamiltonian constraint, H. This approach
has not hitherto been considered for models that also possess nontrivial linear
constraints, Lin. This paper carries this out for some concrete relational
particle models (RPM's). If there is also commutation with Lin - the Kuchar
observables condition - the constructed objects are Dirac observables.
Moreover, this paper shows that the problem of Kuchar observables is explicitly
resolved for 1- and 2-d RPM's. Then as a first route to Halliwell's approach
for nontrivial linear constraints that is also a construction of Dirac
observables, I consider theories for which Kuchar observables are formally
known, giving the relational triangle as an example. As a second route, I apply
an indirect method that generalizes both group-averaging and Barbour's best
matching. For conceptual clarity, my study involves the simpler case of
Halliwell 2003 sharp-edged window function. I leave the elsewise-improved
softened case of Halliwell 2009 for a subsequent Paper II. Finally, I provide
comments on Halliwell's approach and how well it fares as regards the various
facets of the Problem of Time and as an implementation of QM propositions.Comment: An improved version of the text, and with various further references.
25 pages, 4 figure
Rapid Thermal Processing (RTP) of semiconductors in space
The progress achieved on the project entitled 'Rapid Thermal Processing of Semiconductors in Space' for a 12 month period of activity ending March 31, 1993 is summarized. The activity of this group is being performed under the direct auspices of the ROMPS program. The main objective of this program is to develop and demonstrate the use of advanced robotics in space with rapid thermal process (RTP) of semiconductors providing the test technology. Rapid thermal processing is an ideal processing step for demonstration purposes since it encompasses many of the characteristics of other processes used in solid state device manufacturing. Furthermore, a low thermal budget is becoming more important in existing manufacturing practice, while a low thermal budget is critical to successful processing in space. A secondary objective of this project is to determine the influence of microgravity on the rapid thermal process for a variety of operating modes. In many instances, this involves one or more fluid phases. The advancement of microgravity processing science is an important ancillary objective
V/STOL maneuverability and control
Maneuverability and control of V/STOL aircraft in powered-lift flight is studied with specific considerations of maneuvering in forward flight. A review of maneuverability for representative operational mission tasks is presented and covers takeoff, transition, hover, and landing flight phases. Maneuverability is described in terms of the ability to rotate and translate the aircraft and is specified in terms of angular and translational accelerations imposed on the aircraft. Characteristics of representative configurations are reviewed, including experience from past programs and expectations for future designs. The review of control covers the characteristics inherent in the basic airframe and propulsion system and the behavior associated with ontrol augmentation systems. Demands for augmented stability and control response to meet certain mission operational requirements are discussed. Experience from ground-based simulation and flight experiments that illustrates the impact of augmented stability and control on aircraft design is related by example
The curatorial consequences of being moved, moveable or portable: the case of carved stones
It matters whether a carved stone is moved, moveable or portable. This influences perceptions of significance and of form and nature – is it a monument or an artefact? This duality may in turn affect understanding and appreciation of the resource. It has implications for how and if carved stones can be legally protected, who owns them, where and how they are administered, and by whom. The complexities of the legislation mean that all too often this is also a grey area. This paper explores these curatorial issues and their impact
The selection, appraisal and retention of digital scientific data: dighlights of an ERPANET/CODATA workshop
CODATA and ERPANET collaborated to convene an international archiving workshop on the selection, appraisal, and retention of digital scientific data, which was held on 15-17 December 2003 at the Biblioteca Nacional in Lisbon, Portugal. The workshop brought together more than 65 researchers, data and information managers, archivists, and librarians from 13 countries to discuss the issues involved in making critical decisions regarding the long-term preservation of the scientific record. One of the major aims for this workshop was to provide an international forum to exchange information about data archiving policies and practices across different scientific, institutional, and national contexts. Highlights from the workshop discussions are presented
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