3,750 research outputs found
A Method to Find Community Structures Based on Information Centrality
Community structures are an important feature of many social, biological and
technological networks. Here we study a variation on the method for detecting
such communities proposed by Girvan and Newman and based on the idea of using
centrality measures to define the community boundaries (M. Girvan and M. E. J.
Newman, Community structure in social and biological networks Proc. Natl. Acad.
Sci. USA 99, 7821-7826 (2002)). We develop an algorithm of hierarchical
clustering that consists in finding and removing iteratively the edge with the
highest information centrality. We test the algorithm on computer generated and
real-world networks whose community structure is already known or has been
studied by means of other methods. We show that our algorithm, although it runs
to completion in a time O(n^4), is very effective especially when the
communities are very mixed and hardly detectable by the other methods.Comment: 13 pages, 13 figures. Final version accepted for publication in
Physical Review
The Universe is not a Computer
When we want to predict the future, we compute it from what we know about the
present. Specifically, we take a mathematical representation of observed
reality, plug it into some dynamical equations, and then map the time-evolved
result back to real-world predictions. But while this computational process can
tell us what we want to know, we have taken this procedure too literally,
implicitly assuming that the universe must compute itself in the same manner.
Physical theories that do not follow this computational framework are deemed
illogical, right from the start. But this anthropocentric assumption has
steered our physical models into an impossible corner, primarily because of
quantum phenomena. Meanwhile, we have not been exploring other models in which
the universe is not so limited. In fact, some of these alternate models already
have a well-established importance, but are thought to be mathematical tricks
without physical significance. This essay argues that only by dropping our
assumption that the universe is a computer can we fully develop such models,
explain quantum phenomena, and understand the workings of our universe. (This
essay was awarded third prize in the 2012 FQXi essay contest; a new afterword
compares and contrasts this essay with Robert Spekkens' first prize entry.)Comment: 10 pages with new afterword; matches published versio
The Ithaca Interpretation of Quantum Mechanics
I list several strong requirements for what I would consider a sensible
interpretation of quantum mechanics and I discuss two simple theorems. One, as
far as I know, is new; the other was only noted a few years ago. Both have
important implications for such a sensible interpretation. My talk will not
clear everything up; indeed, you may conclude that it has not cleared anything
up. But I hope it will provide a different perspective from which to view some
old and vexing puzzles (or, if you believe nothing needs to be cleared up, some
ancient verities.)Comment: 21 pages, plain TEX. Notes for a lecture given at the Golden Jubilee
Workshop on Foundations of Quantum Theory, Tata Institute, Bombay, September
9-12, 199
The Transit Light Curve Project. VI. Three Transits of the Exoplanet TrES-2
Of the nearby transiting exoplanets that are amenable to detailed study,
TrES-2 is both the most massive and has the largest impact parameter. We
present z-band photometry of three transits of TrES-2. We improve upon the
estimates of the planetary, stellar, and orbital parameters, in conjunction
with the spectroscopic analysis of the host star by Sozzetti and co-workers. We
find the planetary radius to be 1.222 +/- 0.038 R_Jup and the stellar radius to
be 1.003 +/- 0.027 R_Sun. The quoted uncertainties include the systematic error
due to the uncertainty in the stellar mass (0.980 +/- 0.062 M_Sun). The timings
of the transits have an accuracy of 25s and are consistent with a uniform
period, thus providing a baseline for future observations with the NASA Kepler
satellite, whose field of view will include TrES-2.Comment: 15 pages, including 2 figures, accepted Ap
The Transit Light Curve Project. VIII. Six Occultations of the Exoplanet TrES-3
We present photometry of the exoplanet host star TrES-3 spanning six
occultations (secondary eclipses) of its giant planet. No flux decrements were
detected, leading to 99%-confidence upper limits on the planet-to-star flux
ratio of 0.00024, 0.0005, and 0.00086 in the i, z, and R bands respectively.
The corresponding upper limits on the planet's geometric albedo are 0.30, 0.62,
and 1.07. The upper limit in the i band rules out the presence of highly
reflective clouds, and is only a factor of 2-3 above the predicted level of
thermal radiation from the planet.Comment: To appear in AJ [14 pages
The Transit Light Curve Project. IX. Evidence for a Smaller Radius of the Exoplanet XO-3b
We present photometry of 13 transits of XO-3b, a massive transiting planet on
an eccentric orbit. Previous data led to two inconsistent estimates of the
planetary radius. Our data strongly favor the smaller radius, with increased
precision: R_p = 1.217 +/- 0.073 R_Jup. A conflict remains between the mean
stellar density determined from the light curve, and the stellar surface
gravity determined from the shapes of spectral lines. We argue the light curve
should take precedence, and revise the system parameters accordingly. The
planetary radius is about 1 sigma larger than the theoretical radius for a
hydrogen-helium planet of the given mass and insolation. To help in planning
future observations, we provide refined transit and occultation ephemerides.Comment: To appear in ApJ [22 pages
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