7,158 research outputs found
Gravitational lensing in modified Newtonian dynamics
Modified Newtonian dynamics (MOND) is an alternative theory of gravity that
aims to explain large-scale dynamics without recourse to any form of dark
matter. However the theory is incomplete, lacking a relativistic counterpart,
and so makes no definite predictions about gravitational lensing. The most
obvious form that MONDian lensing might take is that photons experience twice
the deflection of massive particles moving at the speed of light, as in general
relativity (GR). In such a theory there is no general thin-lens approximation
(although one can be made for spherically-symmetric deflectors), but the
three-dimensional acceleration of photons is in the same direction as the
relativistic acceleration would be. In regimes where the deflector can
reasonably be approximated as a single point-mass (specifically low-optical
depth microlensing and weak galaxy-galaxy lensing), this naive formulation is
consistent with observations. Forthcoming galaxy-galaxy lensing data and the
possibility of cosmological microlensing have the potential to distinguish
unambiguously between GR and MOND. Some tests can also be performed with
extended deflectors, for example by using surface brightness measurements of
lens galaxies to model quasar lenses, although the breakdown of the thin-lens
approximation allows an extra degree of freedom. Nonetheless, it seems unlikely
that simple ellipsoidal galaxies can explain both constraints. Further, the
low-density universe implied by MOND must be completely dominated by the
cosmological constant (to fit microwave background observations), and such
models are at odds with the low frequency of quasar lenses. These conflicts
might be resolved by a fully consistent relativistic extension to MOND; the
alternative is that MOND is not an accurate description of the universe.Comment: MNRAS, in press; 11 pages, 10 figure
Constraint interface preconditioning for topology optimization problems
The discretization of constrained nonlinear optimization problems arising in
the field of topology optimization yields algebraic systems which are
challenging to solve in practice, due to pathological ill-conditioning, strong
nonlinearity and size. In this work we propose a methodology which brings
together existing fast algorithms, namely, interior-point for the optimization
problem and a novel substructuring domain decomposition method for the ensuing
large-scale linear systems. The main contribution is the choice of interface
preconditioner which allows for the acceleration of the domain decomposition
method, leading to performance independent of problem size.Comment: To be published in SIAM J. Sci. Com
Nashbots: How Political Scientists have Underestimated Human Rationality, and How to Fix It
Political scientists use experiments to test the predictions of game-theoretic models. In a typical experiment, each subject makes choices that determine her own earnings and the earnings of other subjects, with payments corresponding to the utility payoffs of a theoretical game. But social preferences distort the correspondence between a subject’s cash earnings and her subjective utility, and since social preferences vary, anonymously matched subjects cannot know their opponents’ preferences between outcomes, turning many laboratory tasks into games of incomplete information. We reduce the distortion of social preferences by pitting subjects against algorithmic agents (“Nashbots”). Across 11 experimental tasks, subjects facing human opponents played rationally only 36% of the time, but those facing algorithmic agents did so 60% of the time. We conclude that experimentalists have underestimated the economic rationality of laboratory subjects by designing tasks that are poor analogies to the games they purport to test
Labor in the New Urban Battlegrounds: Local Solidarity in a Global Economy
[Excerpt] Labor in the New Urban Battlegrounds is an energizing, optimistic book. By using the contemporary metropolis as a comparative laboratory to see what contexts and strategies contribute best to labor revitalization, Lowell Turner, Daniel Cornfield, and their collaborators generate a fresh sense of positive possibilities for labor and new insights as to how creative actors can best take advantage of those possibilities.
Energizing optimism should not be confused with seeing things through rose-colored glasses. The book fully acknowledges the odds against labor revitalization and the structural obstacles to a more equitable society. Optimism is generated by pairing obstacles with possibilities, often brought to light by another city in which similar obstacles have been overcome with innovative strategies. This book builds on a new tradition of recent analyses of U.S. labor that compellingly contests previous premature obituaries of the labor movement while making a distinctive contribution. Its power is rooted in the comparative metropolis analytical theme and the editors\u27 skill in bringing a diverse baker\u27s dozen of substantive studies to bear on it.
The individual chapters are empirically diverse, complementing a gamut of metropolitan areas in the United States with comparative cases from Europe. They employ varied methodological approaches to look at the social infrastructure and strategic choices that underlie urban successes and failures. Many chapters are in-depth case studies of individual cities, while others (e.g., Greer, Byrd, and Fleron; Hauptmeier and Turner) are paired comparisons. Still others (Applegate; Luce; Reynolds) draw their evidence from larger numbers of cities. One (Sellers) employs an ingenious analysis of cross-national data to draw inferences about differences in urban strategic possibilities. The result is much more powerful analytically than it would have been had the editors collected thirteen metropolitan case studies and then tried to figure out their comparative implications.
Empirical range and methodological diversity augment the power of the volume, but the overarching focus on comparative metropolitan analysis is what gives the book its distinctive analytical punch. Even though a variety of organizations and social actors populate the stage—campaigns, nongovernmental organizations, individual unions, and ethnic communities—defining the urban area as the stage on which the dramas occur was a critical decision. From this decision flows the book\u27s special contribution to refocusing contemporary labor debates
Density regulation in strictly metric-free swarms
There is now experimental evidence that nearest-neighbour interactions in
flocks of birds are metric free, i.e. they have no characteristic interaction
length scale. However, models that involve interactions between neighbours that
are assigned topologically are naturally invariant under spatial expansion,
supporting a continuous reduction in density towards zero, unless additional
cohesive interactions are introduced or the density is artificially controlled,
e.g. via a finite system size. We propose a solution that involves a
metric-free motional bias on those individuals that are topologically
identified to be on an edge of the swarm. This model has only two primary
control parameters, one controlling the relative strength of stochastic noise
to the degree of co-alignment and another controlling the degree of the
motional bias for those on the edge, relative to the tendency to co-align. We
find a novel power-law scaling of the real-space density with the number of
individuals N as well as a familiar order-to-disorder transition
Gravitational lensing and modified Newtonian dynamics
Gravitational lensing is most often used as a tool to investigate the
distribution of (dark) matter in the universe, but, if the mass distribution is
known a priori, it becomes, at least in principle, a powerful probe of gravity
itself. Lensing observations are a more powerful tool than dynamical
measurements because they allow measurements of the gravitational field far
away from visible matter. For example, modified Newtonian dynamics (MOND) has
no relativistic extension, and so makes no firm lensing predictions, but
galaxy-galaxy lensing data can be used to empirically the deflection law of a
point-mass. MONDian lensing is consistent with general relativity, in so far as
the deflection experienced by a photon is twice that experienced by a massive
particle moving at the speed of light. With the deflection law in place and no
invisible matter, MOND can be tested wherever lensing is observed. The
implications are that either MONDian lensing is completely non-linear or that
MOND is not an accurate description of the universe.Comment: PASA (OzLens edition), in press; 5 pages, 1 figur
Gravitational lensing and modified Newtonian dynamics
Gravitational lensing is most often used as a tool to investigate the
distribution of (dark) matter in the universe, but, if the mass distribution is
known a priori, it becomes, at least in principle, a powerful probe of gravity
itself. Lensing observations are a more powerful tool than dynamical
measurements because they allow measurements of the gravitational field far
away from visible matter. For example, modified Newtonian dynamics (MOND) has
no relativistic extension, and so makes no firm lensing predictions, but
galaxy-galaxy lensing data can be used to empirically the deflection law of a
point-mass. MONDian lensing is consistent with general relativity, in so far as
the deflection experienced by a photon is twice that experienced by a massive
particle moving at the speed of light. With the deflection law in place and no
invisible matter, MOND can be tested wherever lensing is observed. The
implications are that either MONDian lensing is completely non-linear or that
MOND is not an accurate description of the universe.Comment: PASA (OzLens edition), in press; 5 pages, 1 figur
The distance to the young open cluster Westerlund 2
Evidence is presented indicating that the young star cluster Westerlund~2
lies kpc in the direction of Carina. The distance is tied
partly to new photometry and revised spectral classifications for
cluster stars, which imply that dust in the direction of Carina is
characterized by an anomalous extinction law (). That result was
determined from a multi-faceted approach relying on the variable-extinction and
color excess methods.Comment: 4 pages, 4 eps figure, proceedings of IAU Symposium 289, Beijing,
August 201
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