Constrained Hyperbolic Divergence Cleaning for Smoothed Particle Magnetohydrodynamics

We present a constrained formulation of Dedner et al's hyperbolic/parabolic divergence cleaning scheme for enforcing the \nabla\dot B = 0 constraint in Smoothed Particle Magnetohydrodynamics (SPMHD) simulations. The constraint we impose is that energy removed must either be conserved or dissipated, such that the scheme is guaranteed to decrease the overall magnetic energy. This is shown to require use of conjugate numerical operators for evaluating \nabla\dot B and \nabla{\psi} in the SPMHD cleaning equations. The resulting scheme is shown to be stable at density jumps and free boundaries, in contrast to an earlier implementation by Price & Monaghan (2005). Optimal values of the damping parameter are found to be {\sigma} = 0.2-0.3 in 2D and {\sigma} = 0.8-1.2 in 3D. With these parameters, our constrained Hamiltonian formulation is found to provide an effective means of enforcing the divergence constraint in SPMHD, typically maintaining average values of h |\nabla\dot B| / |B| to 0.1-1%, up to an order of magnitude better than artificial resistivity without the associated dissipation in the physical field. Furthermore, when applied to realistic, 3D simulations we find an improvement of up to two orders of magnitude in momentum conservation with a corresponding improvement in numerical stability at essentially zero additional computational expense.Comment: 28 pages, 25 figures, accepted to J. Comput. Phys. Movies at http://www.youtube.com/playlist?list=PL215D649FD0BDA466 v2: fixed inverted figs 1,4,6, and several color bar

Citizenship, Gender, and Racial Differences in the Publishing Success Of Graduate Students and Young Academics

Although extensive research exists on the publishing success of academics, few studies have examined factors influencing the publishing success of graduate students and young academics. Data from a survey of 12,000 graduate students in the Humanities and related social sciences was used to examine citizenship, gender and racial/ethnic differences in publishing success during graduate school and the first three years after graduation. The results of this analysis indicate that international students have the highest publication rates during graduate school as well as in the first three years following receipt of degree. Results also indicate that female graduate students are less likely than male graduate students to publish, a gap that remains in the years following graduate school. Finally, results indicate that U.S. citizen minority students exhibit lower levels of publishing success compared with non-minority students during graduate school, but that this gap that disappears within the first few years after graduate school

Does Time-Symmetry Imply Retrocausality? How the Quantum World Says "Maybe"

It has often been suggested that retrocausality offers a solution to some of the puzzles of quantum mechanics: e.g., that it allows a Lorentz-invariant explanation of Bell correlations, and other manifestations of quantum nonlocality, without action-at-a-distance. Some writers have argued that time-symmetry counts in favour of such a view, in the sense that retrocausality would be a natural consequence of a truly time-symmetric theory of the quantum world. Critics object that there is complete time-symmetry in classical physics, and yet no apparent retrocausality. Why should the quantum world be any different? This note throws some new light on these matters. I call attention to a respect in which quantum mechanics is different, under some assumptions about quantum ontology. Under these assumptions, the combination of time-symmetry without retrocausality is unavailable in quantum mechanics, for reasons intimately connected with the differences between classical and quantum physics (especially the role of discreteness in the latter). Not all interpretations of quantum mechanics share these assumptions, however, and in those that do not, time-symmetry does not entail retrocausality.Comment: 22 pages, 6 figures; significant revision

Letter to Secretary of Energy Bodman and Report

Letter to Secretary of Energy Bodman and report – December 18, 2005. Regards Bakken crude oil resource of the Williston Basin and calling for a fresh look at resources in place, recovery factors, state-of-the-art best practices, and how to push them forward to the next logical step

Bidding among friends and enemies

We consider an auction setting in which bidders, even if they fail to obtain the good, care about the price paid by the winner. We study the impact of these price externalities on the first-price auction and the second-price auction in a symmetric information framework. We establish a distinction between price externalities that do not depend on the identity of the winner and price externalities that depend on the identity of the winner. We prove that the outcome of the first-price auction is not affected by the first type of price externalities while the outcome of the second-price auction is. In contrast, the second type of price externalities affects the outcome of both auction formats. In any case, in comparison with the first-price auction, the second-price auction exacerbates the effects of price externalities whatever their types are. The two auction formats are generically not equivalent

Price pressures

We study price pressures in stock prices—price deviations from fundamental value due to a risk-averse intermediary supplying liquidity to asynchronously arriving investors. Empirically, twelve years of daily New York Stock Exchange intermediary data reveal economically large price pressures. A $100,000 inventory shock causes an average price pressure of 0.28% with a half-life of 0.92 days. Price pressure causes average transitory volatility in daily stock returns of 0.49%. Price pressure effects are substantially larger with longer durations in smaller stocks. Theoretically, in a simple dynamic inventory model the ‘representative’ intermediary uses price pressure to control risk through inventory mean reversion. She trades off the revenue loss due to price pressure against the price risk associated with remaining in a nonzero inventory state. The model’s closed-form solution identifies the intermediary’s relative risk aversion and the distribution of investors’ private values for trading from the observed time series patterns. These allow us to estimate the social costs—deviations from constrained Pareto efficiency—due to price pressure which average 0.35 basis points of the value traded. JEL Classification: G12, G14, D53, D6