27,518 research outputs found
Blocking and Persistence in the Zero-Temperature Dynamics of Homogeneous and Disordered Ising Models
A ``persistence'' exponent theta has been extensively used to describe the
nonequilibrium dynamics of spin systems following a deep quench: for
zero-temperature homogeneous Ising models on the d-dimensional cubic lattice,
the fraction p(t) of spins not flipped by time t decays to zero like
t^[-theta(d)] for low d; for high d, p(t) may decay to p(infinity)>0, because
of ``blocking'' (but perhaps still like a power). What are the effects of
disorder or changes of lattice? We show that these can quite generally lead to
blocking (and convergence to a metastable configuration) even for low d, and
then present two examples --- one disordered and one homogeneous --- where p(t)
decays exponentially to p(infinity).Comment: 8 pages (LaTeX); to appear in Physical Review Letter
The worldwide air transportation network: Anomalous centrality, community structure, and cities' global roles
We analyze the global structure of the world-wide air transportation network,
a critical infrastructure with an enormous impact on local, national, and
international economies. We find that the world-wide air transportation network
is a scale-free small-world network. In contrast to the prediction of
scale-free network models, however, we find that the most connected cities are
not necessarily the most central, resulting in anomalous values of the
centrality. We demonstrate that these anomalies arise because of the
multi-community structure of the network. We identify the communities in the
air transportation network and show that the community structure cannot be
explained solely based on geographical constraints, and that geo-political
considerations have to be taken into account. We identify each city's global
role based on its pattern of inter- and intra-community connections, which
enables us to obtain scale-specific representations of the network.Comment: Revised versio
Clustering and preferential attachment in growing networks
We study empirically the time evolution of scientific collaboration networks
in physics and biology. In these networks, two scientists are considered
connected if they have coauthored one or more papers together. We show that the
probability of scientists collaborating increases with the number of other
collaborators they have in common, and that the probability of a particular
scientist acquiring new collaborators increases with the number of his or her
past collaborators. These results provide experimental evidence in favor of
previously conjectured mechanisms for clustering and power-law degree
distributions in networks.Comment: 13 pages, 2 figure
Three-dimensional elastic-plastic finite-element analyses of constraint variations in cracked bodies
Three-dimensional elastic-plastic (small-strain) finite-element analyses were used to study the stresses, deformations, and constraint variations around a straight-through crack in finite-thickness plates for an elastic-perfectly plastic material under monotonic and cyclic loading. Middle-crack tension specimens were analyzed for thicknesses ranging from 1.25 to 20 mm with various crack lengths. Three local constraint parameters, related to the normal, tangential, and hydrostatic stresses, showed similar variations along the crack front for a given thickness and applied stress level. Numerical analyses indicated that cyclic stress history and crack growth reduced the local constraint parameters in the interior of a plate, especially at high applied stress levels. A global constraint factor alpha(sub g) was defined to simulate three-dimensional effects in two-dimensional crack analyses. The global constraint factor was calculated as an average through-the-thickness value over the crack-front plastic region. Values of alpha(sub g) were found to be nearly independent of crack length and were related to the stress-intensity factor for a given thickness
A Method for Extracting Light Echo Fluxes Using the NN2 Difference Imaging Technique
Light echoes are interesting because of the wealth of information they offer
about their progenitors and the reflecting dust. Due to their faint surface
brightnesses, difference imaging is necessary to separate most light echoes
from the sky background. However, difference images reveal only the relative
fluxes between two epochs. Obtaining absolute fluxes for individual epochs has
traditionally relied on a single template image that is free of light echoes.
Since such an observation is normally unavailable, a light echo-free template
must be constructed by a complicated and usually subjective process. Here we
present an application of the NN2 method of Barris et al. to extract the
relative fluxes of light echoes across a range of epochs directly from a series
of difference images. This method requires no privileged image and makes
maximal use of the observational data. Statistical methods to estimate the
zero-flux level and thus the absolute flux are also presented. The efficacy of
the technique is demonstrated by an application to the light echoes around SN
1987A. The resulting images reveal new detail and faint light echo structures.
This method can be adapted and applied to other extended variable light
sources, such as stellar outflows and supernova remnants.Comment: To be published in PASP 118 (Oct. 2006
Vacuum Fluctuations of a massless spin-1/2 field around multiple cosmic strings
We study the interaction of a massless quantized spinor field with the
gravitational filed of N parallel static cosmic strings by using a perturbative
approach. We show that the presence of more than one cosmic string gives rise
to an additional contribution to the energy density of vacuum fluctuations,
thereby leading to a vacuum force attraction between two parallel cosmic
strings.Comment: Class. Quantum Grav. 14(1997) 321
THE ACCUSED IS ENTERING THE COURTROOM: THE LIVE-TWEETING OF A MURDER TRIAL.
© 2017 Informa UK Limited, trading as Taylor & Francis GroupThe use of social media is now widely accepted within journalism as an outlet for news information. Live tweeting of unfolding events is standard practice. In March 2014, Oscar Pistorius went on trial in the Gauteng High Court for murder. Hundreds of journalists present began live-tweeting coverage, an unprecedented combination of international interest, permission to use technology and access which resulted in massive streams of consciousness reports of events as they unfolded. Based on a corpus of Twitter feeds of twenty-four journalists covering the trial, this study analyses the content and strategies of these feeds in order to present an understanding of how microblogging is used as a live reporting tool. This study shows the development of standardised language and strategies in reporting on Twitter, concluding that journalists adopt a narrow range of approaches, with no significant variation in terms of gender, location, or medium. This is in contrast to earlier studies in the field (Awad, 2006, Hedman, 2015; Kothari, 2010; Lariscy, Avery, Sweetser, & Howes, 2009 Lasorsa, 2012; Lasorsa, Lewis, & Holton, 2011; Sigal, 1999, Vis, 2013).Peer reviewe
Canonical General Relativity on a Null Surface with Coordinate and Gauge Fixing
We use the canonical formalism developed together with David Robinson to st=
udy the Einstein equations on a null surface. Coordinate and gauge conditions =
are introduced to fix the triad and the coordinates on the null surface. Toget=
her with the previously found constraints, these form a sufficient number of
second class constraints so that the phase space is reduced to one pair of
canonically conjugate variables: \Ac_2\and\Sc^2. The formalism is related to
both the Bondi-Sachs and the Newman-Penrose methods of studying the
gravitational field at null infinity. Asymptotic solutions in the vicinity of
null infinity which exclude logarithmic behavior require the connection to fall
off like after the Minkowski limit. This, of course, gives the previous
results of Bondi-Sachs and Newman-Penrose. Introducing terms which fall off
more slowly leads to logarithmic behavior which leaves null infinity intact,
allows for meaningful gravitational radiation, but the peeling theorem does not
extend to in the terminology of Newman-Penrose. The conclusions are in
agreement with those of Chrusciel, MacCallum, and Singleton. This work was
begun as a preliminary study of a reduced phase space for quantization of
general relativity.Comment: magnification set; pagination improved; 20 pages, plain te
A title-gap flow model for use in aerodynamic loads assessment of space shuttle thermal protection system: Parallel gap faces
The problem of predicting aerodynamic loads on the insulating tiles of the space shuttle thermal protection system (TPS) is discussed and seen to require a method for predicting pressure and mass flux in the gaps between tiles. A mathematical model of the tile-gap flow is developed, based upon a slow viscous (Stokes) flow analysis, and is verified against experimental data. The tile-gap pressure field is derived from a solution of the two-dimensional Laplace equation; the mass-flux vector is then calculated from the pressure gradient. The means for incorporating this model into a lumped-parameter network analogy for porous-media flow is given. The means for incorporating this model into a lumped-parameter network analogy for porous-media flow is given. The flow model shows tile-gap mass flux to be very sensitive to the gap width indicating a need for coupling the TPS flow and tile displacement calculation. Analytical and experimental work to improve TPS flow predictions and a possible shuttle TPS hardware modification are recommended
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