Bursts and Shocks in a Continuum Shell Model

We study a "burst" event, i. e. the evolution of an initial condition having support only in a finite interval of k-space, in the continuum shell model due to Parisi. We show that the continuum equation without forcing or dissipation can be explicitly written in characteristic form and that the right and left moving parts can be solved exactly. When this is supplemented by the appropriate shock condition it is possible to find the asymptotic form of the burst.Comment: 15 pages, 2 eps figures included, Latex 2e. Contribution to the proceedings of the conference: Disorder and Chaos, in honour of Giovanni Paladin, September 22-24, 1997, in Rom

Exact Periodic Solutions of Shells Models of Turbulence

We derive exact analytical solutions of the GOY shell model of turbulence. In the absence of forcing and viscosity we obtain closed form solutions in terms of Jacobi elliptic functions. With three shells the model is integrable. In the case of many shells, we derive exact recursion relations for the amplitudes of the Jacobi functions relating the different shells and we obtain a Kolmogorov solution in the limit of infinitely many shells. For the special case of six and nine shells, these recursions relations are solved giving specific analytic solutions. Some of these solutions are stable whereas others are unstable. All our predictions are substantiated by numerical simulations of the GOY shell model. From these simulations we also identify cases where the models exhibits transitions to chaotic states lying on strange attractors or ergodic energy surfaces.Comment: 25 pages, 7 figure

Transport coefficients for electrolytes in arbitrarily shaped nano and micro-fluidic channels

We consider laminar flow of incompressible electrolytes in long, straight channels driven by pressure and electro-osmosis. We use a Hilbert space eigenfunction expansion to address the general problem of an arbitrary cross section and obtain general results in linear-response theory for the hydraulic and electrical transport coefficients which satisfy Onsager relations. In the limit of non-overlapping Debye layers the transport coefficients are simply expressed in terms of parameters of the electrolyte as well as the geometrical correction factor for the Hagen-Poiseuille part of the problem. In particular, we consider the limits of thin non-overlapping as well as strongly overlapping Debye layers, respectively, and calculate the corrections to the hydraulic resistance due to electro-hydrodynamic interactions.Comment: 13 pages including 4 figures and 1 table. Typos corrected. Accepted for NJ

Remarks on the entanglement entropy for disconnected regions

Few facts are known about the entanglement entropy for disconnected regions in quantum field theory. We study here the property of extensivity of the mutual information, which holds for free massless fermions in two dimensions. We uncover the structure of the entropy function in the extensive case, and find an interesting connection with the renormalization group irreversibility. The solution is a function on space-time regions which complies with all the known requirements a relativistic entropy function has to satisfy. We show that the holographic ansatz of Ryu and Takayanagi, the free scalar and Dirac fields in dimensions greater than two, and the massive free fields in two dimensions all fail to be exactly extensive, disproving recent conjectures.Comment: 14 pages, 4 figures, some addition

Regional trends in breast cancer incidence and mortality in Denmark prior to mammographic screening.

To provide a basis for the evaluation of mammographic screening programmes in Denmark, a study was undertaken of the regional differences in breast cancer incidence and mortality. All 16 regions were followed for the 20 year period, 1970-89, before the start of the first population-based mammographic screening programme in the Copenhagen municipality in 1991. Multiplicative Poisson models were used for the analysis. In general, the incidence increased during this period from 55 to 70 [per 100,000 standardised world standard population (WSP)], and the analysis shows this to be most pronounced among women below age 60. The mortality was more stable, changing only from 24 to 28 (per 100,000 standardised WSP), but a significant increase occurred in the late 1980s. The study showed regional differences in both incidence and mortality of breast cancer in Denmark. Both the incidence and the mortality varied between the regions, with maximum differences of 22%. The analysis showed no variation in the time trends in the different regions, and thus indicates that the use of a regional comparison group would be a valid basis for evaluation of the Copenhagen programme. Our study, however, underlies the difficulties inherent in the evaluation of screening programmes without internal control groups

AdS/CFT and Strong Subadditivity of Entanglement Entropy

Recently, a holographic computation of the entanglement entropy in conformal field theories has been proposed via the AdS/CFT correspondence. One of the most important properties of the entanglement entropy is known as the strong subadditivity. This requires that the entanglement entropy should be a concave function with respect to geometric parameters. It is a non-trivial check on the proposal to see if this property is indeed satisfied by the entropy computed holographically. In this paper we examine several examples which are defined by annuli or cusps, and confirm the strong subadditivity via direct calculations. Furthermore, we conjecture that Wilson loop correlators in strongly coupled gauge theories satisfy the same relation. We also discuss the relation between the holographic entanglement entropy and the Bousso bound.Comment: 29 pages, harvmac, 7 figures, references adde

The Savvidy ``ferromagnetic vacuum'' in three-dimensional lattice gauge theory

The vacuum effective potential of three-dimensional SU(2) lattice gauge theory in an applied color-magnetic field is computed over a wide range of field strengths. The background field is induced by an external current, as in continuum field theory. Scaling and finite volume effects are analyzed systematically. The first evidence from lattice simulations is obtained of the existence of a nontrivial minimum in the effective potential. This supports a ``ferromagnetic'' picture of gluon condensation, proposed by Savvidy on the basis of a one-loop calculation in (3+1)-dimensional QCD.Comment: 9pp (REVTEX manuscript). Postscript figures appende

Picturing protest: visuality, visibility and the public sphere (special issue introduction).

Aims and Scope: This special issue is concerned with how and why certain visual images picturing protest events and social movements are rendered visible or invisible in the public sphere. ‘Picturing Protest’ responds to the growing interest in a new protest culture and new ways of ‘doing politics’, ranging from Arab revolts to the Occupy Movement, the Indignados and anti-austerity protests in Europe. Since 2011 these new activisms have gained momentum in media and scholarly debates. Contemporary activisms are seen as powerfully tied in to the possibilities that social media platforms and web 2.0 technologies offer to those involved in practices of dissent in physical squares and streets as much as in virtual environments. Of special interest here is how new forms of political participation and the practice of dissent go in tandem with the widespread use of visual images and internet memes facilitated by technological devices with documentation facilities (e.g., smartphones, tablets) and social network technologies (Bennett and Segerberg 2012). Iconic images like the image of dying Neda, a 26-year-old Iranian woman killed by a sniper bullet during a protest event, go viral in social media platforms and have the power to galvanize the attention of global publics. Hence, this new protest culture demands for a different approach in the study of how protest images are constituted, analysed, interpreted and circulated in both old and new media environments. Taken all together, the different contributions ask how and why activists, photojournalists, citizen journalists and journalists use protest images, ranging from maps, posters, to amateur and professional photographs, to communicate with a range of audiences within and beyond nationally-defined public spheres. The contributors do so by employing theoretical tools and methods that originate from within a variety of disciplines, including media and communication, political science, sociology, semiotics and art history. In pursuing their research, the contributors draw on a variety of political contexts, including Spain, Portugal, the West Bank and the Gaza Strip, Greece, Germany, Italy, Austria and the UK. One of the key aims of this special issue is to overcome the overemphasis on the intended symbolic meanings of protest images (Philipps, 2011), by directing the analytical lens to issues of image production and diffusion. It does so to show how certain visual images, and not others, end up circulating in a range of traditional and new media environments