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

Modelling discontinuities and Kelvin-Helmholtz instabilities in SPH

In this paper we discuss the treatment of discontinuities in Smoothed Particle Hydrodynamics (SPH) simulations. In particular we discuss the difference between integral and differential representations of the fluid equations in an SPH context and how this relates to the formulation of dissip ative terms for the capture of shocks and other discontinuities. This has important implications for many problems, in particular related to recently highlighted problems in treating Kelvin-Helmholtz instabilities across entropy gradients in SPH. The specific problems pointed out by Agertz et al. (2007) are shown to be related in particular to the (lack of) treatment of contact discontinuities in standard SPH formulations which can be cured by the simple application of an artificial thermal conductivity term. We propose a new formulation of artificial thermal conductivity in SPH which minimises dissipation away from discontinuities and can therefore be applied quite generally in SPH calculations.Comment: 31 pages, 10 figures, submitted to J. Comp. Phys. Movies + hires version available at http://www.astro.ex.ac.uk/people/dprice/pubs/kh/ . v3: modified as per referee's comments - comparison with Ritchie & Thomas formulation added, quite a few typos fixed. No major change in metho

Group selection theories are now more sophisticated, but are they more predictive?

Human beings are unique among species in their ability to cooperate in large groups of genetically unrelated individuals, and in this book, Samuel Bowles and Herbert Gintis attempt to account for the origins of this ability. The authors specialize in the use of formal models and agent-based simulations in order to precisely specify their theories of cooperation, and they often draw on studies conducted in hunter gatherer societies and in experimental economic laboratories for evidence that they find relevant to evaluating these theories. The book is a valuable review of these anthropological and economic literatures, and a thorough showcase of the authors’ expert formal theorizing about how cooperation may have evolved. However, I often found myself disagreeing with the authors’ focus on group selection as an explanation for human cooperation, and with their views on how well the empirical findings provide support for group selectionist theories

Fun, fear, and frustration: Experiences and opportunities in the classroom

In lieu of an abstract, here is the article\u27s first paragraph: As the outreach and special collections librarian at a small liberal arts college, I also serve as the liaison to the Wegmans School of Nursing, the Wegmans School of Pharmacy, and the Departments of Biology, Chemistry, and Physics. My formal training is in secondary education, with an undergraduate degree in Spanish. So when my liaison work brought me into a cellular biology course, I struggled with the content. Although I could think of great ways to engage the students in active learning, I was lacking enough basic science knowledge to create activities that tied closely to the curriculum. To remedy that situation I was offered the chance to join a student working group in the cellular biology lab. For an entire summer I spent an hour each day learning lab basics, attending lectures, and doing research with C. elegans, a small transparent worm. With a pipette in my hand and worms on my plate, I became a true novice again

The popular and the avant-garde: performance, incorporation and resistance

Many modernists in the late nineteenth century harboured a strong suspicion of the emerging mass culture produced for the working classes. Adopting a position similar to Matthew Arnold’s (2009) that culture was the speciality of a cultured few, and not the uneducated working classes, some sought to protect ‘high art’ from ‘low’ mass forms. Modernists associated with the avant-garde, however, do not appear to have held such rigid views; and for Peter Bürger (1984) this is what distinguishes it from modernism. Historical avant-garde artists tended to be more forthcoming about their connections to other cultural forms, not only acknowledging mass and popular culture, but openly co-opting and integrated it into their art. Bürger sees this not only as part of the avant-garde’s resistance to modernism, but its ‘attack on the status of art in bourgeois society’ (1984, 49). In this paper I will interrogate the tensions between these two modernist positions, and will consider the ways that mass and popular culture forms became an important tool for aesthetic experimentation amongst avant-garde artists. I will focus on three distinct areas in which such experimentation occurred: early Parisian cabaret, Futurism and Dada. In doing so, the paper aims to do two things: (1) to demonstrate the fundamental role that popular and mass cultural forms played in the development of avant-garde aesthetics; and (2) to show how, through such appropriation, avant-garde practices intended to critique mass culture, the belief in high and low art categories, and the capitalist system responsible for their production