On the use of the proximity force approximation for deriving limits to short-range gravitational-like interactions from sphere-plane Casimir force experiments

We discuss the role of the proximity force approximation in deriving limits to the existence of Yukawian forces - predicted in the submillimeter range by many unification models - from Casimir force experiments using the sphere-plane geometry. Two forms of this approximation are discussed, the first used in most analyses of the residuals from the Casimir force experiments performed so far, and the second recently discussed in this context in R. Decca et al. [Phys. Rev. D 79, 124021 (2009)]. We show that the former form of the proximity force approximation overestimates the expected Yukawa force and that the relative deviation from the exact Yukawa force is of the same order of magnitude, in the realistic experimental settings, as the relative deviation expected between the exact Casimir force and the Casimir force evaluated in the proximity force approximation. This implies both a systematic shift making the actual limits to the Yukawa force weaker than claimed so far, and a degree of uncertainty in the alpha-lambda plane related to the handling of the various approximations used in the theory for both the Casimir and the Yukawa forces. We further argue that the recently discussed form for the proximity force approximation is equivalent, for a geometry made of a generic object interacting with an infinite planar slab, to the usual exact integration of any additive two-body interaction, without any need to invoke approximation schemes. If the planar slab is of finite size, an additional source of systematic error arises due to the breaking of the planar translational invariance of the system, and we finally discuss to what extent this may affect limits obtained on power-law and Yukawa forces.Comment: 11 page, 5 figure

Contribution of drifting carriers to the Casimir-Lifshitz and Casimir-Polder interactions with semiconductor materials

We develop a new theory for Casimir-Lifshitz and Casimir-Polder interactions with semiconductor surfaces that takes into account charge drift in the bulk material. The corresponding frequency-dependent dispersion relations describe a crossover between Lifshitz results for dielectrics and for good conductors. In the quasi-static limit, our calculated reflection amplitudes coincide with those recently computed to account for Debye screening in the thermal Lifshitz force with conducting surfaces with small density of carriers.Comment: 4 pages version 2: improved discussion of perfect conductor and perfect dielectric limits. Version 3; includes discussion of limits of applicability of the analysis. Version $; updated reference

Quantum corrections to the geodesic equation

In this talk we will argue that, when gravitons are taken into account, the solution to the semiclassical Einstein equations (SEE) is not physical. The reason is simple: any classical device used to measure the spacetime geometry will also feel the graviton fluctuations. As the coupling between the classical device and the metric is non linear, the device will not measure the `background geometry' (i.e. the geometry that solves the SEE). As a particular example we will show that a classical particle does not follow a geodesic of the background metric. Instead its motion is determined by a quantum corrected geodesic equation that takes into account its coupling to the gravitons. This analysis will also lead us to find a solution to the so-called gauge fixing problem: the quantum corrected geodesic equation is explicitly independent of any gauge fixing parameter.Comment: Revtex file, 6 pages, no figures. Talk presented at the meeting "Trends in Theoretical Physics II", Buenos Aires, Argentina, December 199

Continuous quantum measurement of a Bose-Einstein condensate: a stochastic Gross-Pitaevskii equation

We analyze the dynamics of a Bose-Einstein condensate undergoing a continuous dispersive imaging by using a Lindblad operator formalism. Continuous strong measurements drive the condensate out of the coherent state description assumed within the Gross-Pitaevskii mean-field approach. Continuous weak measurements allow instead to replace, for timescales short enough, the exact problem with its mean-field approximation through a stochastic analogue of the Gross-Pitaevskii equation. The latter is used to show the unwinding of a dark soliton undergoing a continuous imaging.Comment: 13 pages, 10 figure

Attitudes of isiXhosa-speaking students at the University of Fort Hare towards the use of isiXhosa as a language of learning and teaching (LOLT)

The present work presents and discusses the results of a survey of a sample of isiXhosa-speaking students at the University of Fort Hare (Alice campus) and their attitudes towards the possible introduction of isiXhosa as a medium of instruction at this institution. More specifically, the research focuses on, among other things, the students’ attitudes towards English and isiXhosa and their opinions and beliefs about the introduction of dual-mediumship and its possible consequences. The survey methods used are a questionnaire survey as well as follow-up interviews, supplemented by on-campus observation. The results are first analysed as a whole, and then split into different categories according to a set of background variables (gender, year of study, subject studied etc). This analysis indicates that, while English is recognised as the dominant language in South Africa and, more specifically, in the domain of education, some categories of respondents acknowledge the usefulness of isiXhosa as an additional medium of instruction. This survey clearly shows that it makes little sense to present isiXhosa-speaking students with a rigid choice between the existing English-medium and a dual-medium (English and isiXhosa) policy. If dual-mediumship is ever to be implemented, respondents seem to consider the use of isiXhosa as a medium of instruction more appropriate in the first years of study, for selected subjects and in some domains within the academic context rather than others. This study is part of a growing set of surveys on the attitudes of university students towards the use of African languages in education, and can be fruitfully compared with similar research at other institutions. Moreover, the results of the present research can be used to inform future decisions regarding language policy at the University of Fort Hare

Attitudes of isiXhosa-speaking students at the University of Fort Hare towards the use of isiXhosa as a language of learning and teaching (LOLT)

The present work presents and discusses the results of a survey of a sample of isiXhosa-speaking students at the University of Fort Hare (Alice campus) and their attitudes towards the possible introduction of isiXhosa as a medium of instruction at this institution. More specifically, the research focuses on, among other things, the students’ attitudes towards English and isiXhosa and their opinions and beliefs about the introduction of dual-mediumship and its possible consequences. The survey methods used are a questionnaire survey as well as follow-up interviews, supplemented by on-campus observation. The results are first analysed as a whole, and then split into different categories according to a set of background variables (gender, year of study, subject studied etc). This analysis indicates that, while English is recognised as the dominant language in South Africa and, more specifically, in the domain of education, some categories of respondents acknowledge the usefulness of isiXhosa as an additional medium of instruction. This survey clearly shows that it makes little sense to present isiXhosa-speaking students with a rigid choice between the existing English-medium and a dual-medium (English and isiXhosa) policy. If dual-mediumship is ever to be implemented, respondents seem to consider the use of isiXhosa as a medium of instruction more appropriate in the first years of study, for selected subjects and in some domains within the academic context rather than others. This study is part of a growing set of surveys on the attitudes of university students towards the use of African languages in education, and can be fruitfully compared with similar research at other institutions. Moreover, the results of the present research can be used to inform future decisions regarding language policy at the University of Fort Hare