185,479 research outputs found
Interferometric Constraints on Quantum Geometrical Shear Noise Correlations
Final measurements and analysis are reported from the first-generation
Holometer, the first instrument capable of measuring correlated variations in
space-time position at strain noise power spectral densities smaller than a
Planck time. The apparatus consists of two co-located, but independent and
isolated, 40 m power-recycled Michelson interferometers, whose outputs are
cross-correlated to 25 MHz. The data are sensitive to correlations of
differential position across the apparatus over a broad band of frequencies up
to and exceeding the inverse light crossing time, 7.6 MHz. By measuring with
Planck precision the correlation of position variations at spacelike
separations, the Holometer searches for faint, irreducible correlated position
noise backgrounds predicted by some models of quantum space-time geometry. The
first-generation optical layout is sensitive to quantum geometrical noise
correlations with shear symmetry---those that can be interpreted as a
fundamental noncommutativity of space-time position in orthogonal directions.
General experimental constraints are placed on parameters of a set of models of
spatial shear noise correlations, with a sensitivity that exceeds the
Planck-scale holographic information bound on position states by a large
factor. This result significantly extends the upper limits placed on models of
directional noncommutativity by currently operating gravitational wave
observatories.Comment: Matches the journal accepted versio
Measuring attendance: issues and implications for estimating the impact of free-to-view sports events
A feature of many non-elite sports events, especially those conducted in public places is that they are free-to-view. The article focuses on the methodological issue of estimating spectator attendance at free-to-view events and the consequences of this for impact evaluation. Using empirical data from three case studies, the article outlines various approaches to measuring attendance and discusses the key issues and implications for evaluating free-to-view sports events in the future
Generalized parton distributions: Status and perspectives
We summarize recent developments in understanding the concept of generalized
parton distributions (GPDs), its relation to nucleon structure, and its
application to high-Q2 electroproduction processes. Following a brief review of
QCD factorization and transverse nucleon structure, we discuss (a) new
theoretical methods for the analysis of deeply-virtual Compton scattering
(t-channel-based GPD parametrizations, dispersion relations); (b) the
phenomenology of hard exclusive meson production (experimental tests of
dominance of small-size configurations, model-independent comparative studies);
(c) the role of GPDs in small-x physics and pp scattering (QCD dipole model,
central exclusive diffraction). We emphasize the usefulness of the transverse
spatial (or impact parameter) representation for both understanding the
reaction mechanism in hard exclusive processes and visualizing the physical
content of the GPDs.Comment: 10 pages, 6 figures. Proceedings of SPIN2008, University of Virginia,
October 6-11, 200
Principal Component Analysis of Molecular Clouds: Can CO reveal the dynamics?
We use Principal Component Analysis (PCA) to study the gas dynamics in
numerical simulations of typical MCs. Our simulations account for the
non-isothermal nature of the gas and include a simplified treatment of the
time-dependent gas chemistry. We model the CO line emission in a
post-processing step using a 3D radiative transfer code. We consider mean
number densities n_0 = 30, 100, 300 cm^{-3} that span the range of values
typical for MCs in the solar neighbourhood and investigate the slope
\alpha_{PCA} of the pseudo structure function computed by PCA for several
components: the total density, H2 density, 12CO density, 12CO J = 1 -> 0
intensity and 13CO J = 1 -> 0 intensity. We estimate power-law indices
\alpha_{PCA} for different chemical species that range from 0.5 to 0.9, in good
agreement with observations, and demonstrate that optical depth effects can
influence the PCA. We show that when the PCA succeeds, the combination of
chemical inhomogeneity and radiative transfer effects can influence the
observed PCA slopes by as much as ~ +/- 0.1. The method can fail if the CO
distribution is very intermittent, e.g. in low-density clouds where CO is
confined to small fragments.Comment: 12 pages, 8 figures, accepted for publication in MNRA
Entropy, complexity, and spatial information
We pose the central problem of defining a measure of complexity, specifically for spatial systems in general, city systems in particular. The measures we adopt are based on Shannon's (in Bell Syst Tech J 27:379-423, 623-656, 1948) definition of information. We introduce this measure and argue that increasing information is equivalent to increasing complexity, and we show that for spatial distributions, this involves a trade-off between the density of the distribution and the number of events that characterize it; as cities get bigger and are characterized by more events-more places or locations, information increases, all other things being equal. But sometimes the distribution changes at a faster rate than the number of events and thus information can decrease even if a city grows. We develop these ideas using various information measures. We first demonstrate their applicability to various distributions of population in London over the last 100 years, then to a wider region of London which is divided into bands of zones at increasing distances from the core, and finally to the evolution of the street system that characterizes the built-up area of London from 1786 to the present day. We conclude by arguing that we need to relate these measures to other measures of complexity, to choose a wider array of examples, and to extend the analysis to two-dimensional spatial systems
Time-resolved imaging of non-diffusive carrier transport in long-lifetime halide perovskite thin films
Owing to their exceptional semiconducting properties, hybrid
inorganic-organic perovskites show great promise as photovoltaic absorbers. In
these materials, long-range diffusion of charge carriers allows for most of the
photogenerated carriers to contribute to the photovoltaic efficiency. Here,
time-resolved photoluminescence (PL) microscopy is used to directly probe
ambipolar carrier diffusion and recombination kinetics in hybrid perovskites.
This technique is applied to thin films of methylammonium lead tri-iodide
MAPbI obtained with two different fabrication routes, methylammonium lead
tribromide (MAPbBr), and an alloy of formamidinium lead tri-iodide
(FAPbI) and methylammonium lead bromide
FAMAPb(IBr_). Average diffusion
coefficients in the films leading to the highest device efficiencies and
longest lifetimes, i.e., in FAMAPb(IBr)
and acetonitrile-processed MAPbI, are found to be several orders of
magnitude lower than in the other films. Further examination of the
time-dependence shows strong evidence for non-diffusive transport. In
particular, acetonitrile-processed MAPbI shows distinct diffusion regimes
on short and long timescales with an effective diffusion constant varying over
2 orders of magnitude. Our results also highlight the fact that increases in
carrier lifetime in this class of materials are not necessarily concomitant
with increased diffusion lengths and that the PL quantum efficiency under solar
cell operating conditions is a greater indication of material, and ultimately
device, quality
Representing multifunctional cities: density and diversity in space and time
In this paper, we define measures of urban diversity, density and segregation using newdata and software systems based on GIS. These allow us to visualise the meaning of themultifunctional city. We begin with a discussion of how cities have become moresegregated in their land uses and activities during the last 200 years and how the currentfocus is on reversing this trend through limiting urban sprawl and bringing new lifeback to the inner and central city. We define various indices which show how diversityand density manifest themselves spatially. We argue that multifunctionalism is a relativeconcept, dependent upon the spatial and temporal scale that we use to think about themixing and concentration of urban land uses. We present three examples using spatiallysmoothed indicators of diversity: for a world city ? London, for a highly controlledpolycentric urban region ? Randstad Holland, and for a much more diffusely populatedsemi-urban region ? Venice-Padua-Teviso. We conclude by illustrating that urbandiversity varies as people engage in different activities associated with different landuses throughout the day, as well as through the vertical, third dimension of the city. Thisimpresses the point that we need to understand multifunctional cities in all theirdimensions of space and time
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