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

The strategic and the stratigraphic: a working paper on the dynamics of organisational evolution.

Despite large debates over fundamental issues a broadly evolutionary paradigm of organisations is growing in legitimacy. It may though be preferable to replace the metaphor of the organisation as an organism with the literal assertion that both social organisations are ecologies (Weeks and Galunic, 2003). They are still classes of complex systems maintained, and specified by, replicators (or schemata Gell Mann 1994) but the interactor is not necessarily the individual organisation, or population of organisations. Conceptual evolution has been argued as a post-Kuhnian analysis of the scientific process (Hull 1988), a rival economic paradigm (references in Hodgson 1993), a view of strategy (e.g. Lloyd 1990) and an explanation of organisational transformation and learning (Price and Evans 1993, Price 1994, 1995).My concern in this paper is to compare strategic extinction and speciation events in both systems. The stratigraphic record shows a dominant pattern of extinctions and radiative speciations which then settle to stabilised ecosystems. The historical and commercial (or strategigraphic?) record illustrates a similar pattern (Rothschild 1990, Tylecote 1993, Arthur 1994). The causes of extinction events may be genuinely external to the system affected (e.g. asteroid impacts interrupting a reptilian dominated system cannot plausibly be traced to feedback processes in any coupled eco/ lithosphere) or they may be internal when the success of a particular replicator system disturbs a wider systemic balance (e.g. ice-house glaciations terminating plant dominated episodes of earth history). Strategic scale parallels of both forms of extinction event can be seen in commercial and technological history. Keywords Organisational evolution, Punctuated equilibrium, Narrative ecology, memetics

Decision-based Probabilities in the Everett Interpretation: Comments on Wallace and Greaves

It is often objected that the Everett interpretation of QM cannot make adequate sense of quantum probabilities, in one or both of two senses: either it cannot make sense of probability at all, or cannot explain why probability should be governed by the Born rule. David Deutsch has attempted to meet these objections. He argues not only that rational decision under uncertainty makes sense in the Everett interpretation, and that under reasonable assumptions, the credences of a rational agent in an Everett world should be constrained by the Born rule. David Wallace has recently developed and defended Deutsch's proposal, and greatly clarified its conceptual basis. In this note I outline some concerns about the Deutsch argument, as presented by Wallace, and about related proposals by Hilary Greaves. In particular, I argue that the argument is circular, at a crucial point

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