6,596 research outputs found
Limits on violations of Lorentz Symmetry from Gravity Probe B
Generic violations of Lorentz symmetry can be described by an effective field
theory framework that contains both general relativity and the standard model
of particle physics called the Standard-Model Extension (SME). We obtain new
constraints on the gravitational sector of the SME using recently published
final results from Gravity Probe B. These include for the first time an upper
limit at the 10^(-3) level on the time-time component of the new tensor field
responsible for inducing local Lorentz violation in the theory, and an
independent limit at the 10^(-7) level on a combination of components of this
tensor field.Comment: 8 pages, 1 figur
Core-periphery organization of complex networks
Networks may, or may not, be wired to have a core that is both itself densely
connected and central in terms of graph distance. In this study we propose a
coefficient to measure if the network has such a clear-cut core-periphery
dichotomy. We measure this coefficient for a number of real-world and model
networks and find that different classes of networks have their characteristic
values. For example do geographical networks have a strong core-periphery
structure, while the core-periphery structure of social networks (despite their
positive degree-degree correlations) is rather weak. We proceed to study radial
statistics of the core, i.e. properties of the n-neighborhoods of the core
vertices for increasing n. We find that almost all networks have unexpectedly
many edges within n-neighborhoods at a certain distance from the core
suggesting an effective radius for non-trivial network processes
The Lifecycles of Apps in a Social Ecosystem
Apps are emerging as an important form of on-line content, and they combine
aspects of Web usage in interesting ways --- they exhibit a rich temporal
structure of user adoption and long-term engagement, and they exist in a
broader social ecosystem that helps drive these patterns of adoption and
engagement. It has been difficult, however, to study apps in their natural
setting since this requires a simultaneous analysis of a large set of popular
apps and the underlying social network they inhabit.
In this work we address this challenge through an analysis of the collection
of apps on Facebook Login, developing a novel framework for analyzing both
temporal and social properties. At the temporal level, we develop a retention
model that represents a user's tendency to return to an app using a very small
parameter set. At the social level, we organize the space of apps along two
fundamental axes --- popularity and sociality --- and we show how a user's
probability of adopting an app depends both on properties of the local network
structure and on the match between the user's attributes, his or her friends'
attributes, and the dominant attributes within the app's user population. We
also develop models that show the importance of different feature sets with
strong performance in predicting app success.Comment: 11 pages, 10 figures, 3 tables, International World Wide Web
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Time travel paradoxes, path integrals, and the many worlds interpretation of quantum mechanics
We consider two approaches to evading paradoxes in quantum mechanics with
closed timelike curves (CTCs). In a model similar to Politzer's, assuming pure
states and using path integrals, we show that the problems of paradoxes and of
unitarity violation are related; preserving unitarity avoids paradoxes by
modifying the time evolution so that improbable events bewcome certain. Deutsch
has argued, using the density matrix, that paradoxes do not occur in the "many
worlds interpretation". We find that in this approach account must be taken of
the resolution time of the device that detects objects emerging from a wormhole
or other time machine. When this is done one finds that this approach is viable
only if macroscopic objects traversing a wormhole interact with it so strongly
that they are broken into microscopic fragments.Comment: no figure
Percolation, depinning, and avalanches in capillary condensation of gases in disordered porous solids
We propose a comprehensive theoretical description of hysteresis in capillary
condensation of gases in mesoporous disordered materials. Applying mean-field
density functional theory to a coarse-grained lattice-gas model, we show that
the morphology of the hysteresis loops is influenced by out-of-equilibrium
transitions that are different on filling and on draining. In particular,
desorption may be associated to a depinning process and be percolation-like
without explicit pore-blocking effects.Comment: 4 pages, 5 figure
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