Should active recruitment of health workers from sub-Saharan Africa be viewed as a crime?

[No abstract available]Articl

Graviton n-point functions for UV-complete theories in Anti-de Sitter space

We calculate graviton n-point functions in an anti-de Sitter black brane background for effective gravity theories whose linearized equations of motion have at most two time derivatives. We compare the n-point functions in Einstein gravity to those in theories whose leading correction is quadratic in the Riemann tensor. The comparison is made for any number of gravitons and for all physical graviton modes in a kinematic region for which the leading correction can significantly modify the Einstein result. We find that the n-point functions of Einstein gravity depend on at most a single angle, whereas those of the corrected theories may depend on two angles. For the four-point functions, Einstein gravity exhibits linear dependence on the Mandelstam variable s versus a quadratic dependence on s for the corrected theory.Comment: 29 page

Unitarity constraints on the ratio of shear viscosity to entropy density in higher derivative gravity

We discuss corrections to the ratio of shear viscosity to entropy density $\eta/s$ in higher-derivative gravity theories. Generically, these theories contain ghost modes with Planck-scale masses. Motivated by general considerations about unitarity, we propose new boundary conditions for the equations of motion of the graviton perturbations that force the amplitude of the ghosts modes to vanish. We analyze explicitly four-derivative perturbative corrections to Einstein gravity which generically lead to four-derivative equations of motion, compare our choice of boundary conditions to previous proposals and show that, with our new prescription, the ratio $\eta/s$ remains at the Einstein-gravity value of $1/4\pi$ to leading order in the corrections. It is argued that, when the new boundary conditions are imposed on six and higher-derivative equations of motion, $\eta/s$ can only increase from the Einstein-gravity value. We also recall some general arguments that support the validity of our results to all orders in the strength of the corrections to Einstein gravity. We then discuss the particular case of Gauss-Bonnet gravity, for which the equations of motion are only of two-derivative order and the value of $\eta/s$ can decrease below $1/4\pi$ when treated in a nonperturbative way. Our findings provide further evidence for the validity of the KSS bound for theories that can be viewed as perturbative corrections to Einstein Gravity.Comment: Sign error in the equations of motion corrected, leading to several numerical changes. Clarifications added, references added. Main results and cnclusions essentially unchanged. V3 published version. Clarifications added, discussion of Gauss-Bonnet moved to main tex

Present day challenges in understanding the geomagnetic hazard to national power grids

Power grids and pipeline networks at all latitudes are known to be at risk from the natural hazard of geomagnetically induced currents. At a recent workshop in South Africa, UK and South African scientists and engineers discussed the current understanding of this hazard, as it affects major power systems in Europe and Africa. They also summarised, to better inform the public and industry, what can be said with some certainty about the hazard and what research is yet required to develop useful tools for geomagnetic hazard mitigation

Above ground woody community attributes, biomass and carbon stocks along a rainfall gradient in the savannas of the central lowveld, South Africa

Enumeration of carbon stocks at benchmark sites is a necessary activity in assessing the potential carbon sequestration and possible generation of credits through restoration of intensively impacted sites. However, there is a lack of empirical studies throughout much of the savannas of sub-Saharan Africa, including South Africa. We report an estimation of species specific and site biomass and carbon stocks, and general vegetation structural attributes from three protected areas along a rainfall gradient in the central lowveld, South Africa. Estimates of biomass and carbon stocks were effected through destructive sampling to establish locally derived allometric equations. There was a gradient of increasing woody density, height of the canopy, number of species, density of regenerative stems and a greater proportion of stems in small size classes from the arid locality to the mesic locality, with the semi-arid locality being intermediate. The proportion of spinescent species decreased with increasing rainfall. The mesic locality was significantly more woody than either the arid or semi-arid sites, having double the biomass, four times the density and 40% higher basal area. Above ground carbon pools were also higher; carbon stocks were approximately 9 t/ha for the arid and semi-arid sites and 18 t/ha for the mesic site.

Evaluating the Wald Entropy from two-derivative terms in quadratic actions

We evaluate the Wald Noether charge entropy for a black hole in generalized theories of gravity. Expanding the Lagrangian to second order in gravitational perturbations, we show that contributions to the entropy density originate only from the coefficients of two-derivative terms. The same considerations are extended to include matter fields and to show that arbitrary powers of matter fields and their symmetrized covariant derivatives cannot contribute to the entropy density. We also explain how to use the linearized gravitational field equation rather than quadratic actions to obtain the same results. Several explicit examples are presented that allow us to clarify subtle points in the derivation and application of our method

Welcome Address of the Vice‐Chancellor of Rhodes University, Dr Saleem Badat, to First‐Year students

From introduction: Our first purpose is to produce knowledge, so that we can better understand our natural and social worlds and also enrich our scientific and cultural heritage. The second purpose of a university is to disseminate knowledge and to develop critical and creative minds. Our goals, yours and ours, must be for you to think imaginatively, ‘effectively and critically;’ to ‘achieve depth in some field of knowledge;’ to appreciate how we ‘gain knowledge and understanding of the universe, of society, and of ourselves;’ to have ‘a broad knowledge of other cultures and other times;’ to critique ideas and views and construct alternatives, and to communicate cogently, orally and in writing. Our final purpose as a university is to undertake community engagement, whether this is as part of academic courses or your voluntary participation in community projects organized by our Community Engagement Office

Nathaniel Merriman's lecture: "On the study of Shakspeare".

Nathaniel Merriman’s lectures on Shakespeare were published in 1857 and 1858. The first, “On the Study of Shakspeare,” was delivered in the Court House, Grahamstown on the 2nd September 1857 to an audience of more than four hundred and fifty people. The second, “Shakspeare, as Bearing on English History,” was given in the same venue two months later, on Friday, 6 November 1857, and was also well attended. The lectures were published under the auspices of the Committee of “The General Institute,” which sponsored the lectures, and printed at the Anglo-African Office in the High Street. The two lectures and their context are little known in Shakespeare studies because the original pamphlets are rare. The first lecture appears in Mendelssohn’s South African Bibliography (1910), while the second is picked up only in the 1979 revision of that work. Copies of “On the Study of Shakspeare” are held by the Mendelssohn Library in the Library of Parliament, Cape Town; by The South African Library, Cape Town; and in the Oppenheimer Collection, Johannesburg. Copies of “Shakspeare, as Bearing on English History” are held by the Mendelssohn Library; by the University of the Witwatersrand Library, Johannesburg; and by the Kimberley Public Library. The purpose of preparing annotated editions of these lectures is to make them more accessible to scholars and draw them further into the mainstream of international discussion on colonial Shakespeare

Fluorescence studies of quantum dots and zinc tetraamino phthalocyanine conjugates

CdTe Qds capped with mercapto propionic acid (MPA) were covalently linked to zinc tetraamino phthalocyanine (ZnTAPc) using N-ethyl-N(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxy succinimide (NHS) as the coupling agents. The results presented give evidence in favour of the formation of an amide bond between ZnTAPc and CdTe QDs. Both the linked ZnTAPc–QD complexes and a mixture of QDs and ZnTAPc (i.e. without chemical linking) showed Förster resonance energy transfer (FRET). ZnTAPc quenched the QDs emission, giving quenching constants in the order of 103 M−1