Superkicks in ultrarelativistic encounters of spinning black holes

We study ultrarelativistic encounters of two spinning, equal-mass black holes through simulations in full numerical relativity. Two initial data sequences are studied in detail: one that leads to scattering and one that leads to a grazing collision and merger. In all cases, the initial black hole spins lie in the orbital plane, a configuration that leads to the so-called "superkicks". In astrophysical, quasicircular inspirals, such kicks can be as large as ~3,000 km/s; here, we find configurations that exceed ~15,000 km/s. We find that the maximum recoil is to a good approximation proportional to the total amount of energy radiated in gravitational waves, but largely independent of whether a merger occurs or not. This shows that the mechanism predominantly responsible for the superkick is not related to merger dynamics. Rather, a consistent explanation is that the "bobbing" motion of the orbit causes an asymmetric beaming of the radiation produced by the in-plane orbital motion of the binary, and the net asymmetry is balanced by a recoil. We use our results to formulate some conjectures on the ultimate kick achievable in any black hole encounter.Comment: 10 pages, 6 figures, 2 table

Stability and the Gauge Problem in Non-Perturbative Cosmology

In this paper, we describe the first steps towards fully non-perturbative cosmology. We explain why the conventional methods used by cosmologists based on the ADM formulation are generally inadequate for this purpose and why it is advantageous instead to adapt the harmonic formulation pioneered and utilized in mathematical and numerical relativity. Here we focus on using this approach to evaluating the linear mode stability in homogeneous and nearly homogeneous backgrounds and devising a valid scheme and diagnostics for numerical computation. We also briefly touch on the relevance of these methods for extracting cosmological observables from non-perturbative simulations.Comment: 43 page

Can Black Holes Decay to Naked Singularities?

We investigate thermodynamic properties of two types of asymptotically anti-de Sitter spacetimes: black holes and singular scalar field spacetimes. We describe the possibility that thermodynamic phase transitions can transform one spacetime into another, suggesting that black holes can radiate to naked singularities.Comment: 5 pages, Essay for 2001 Gravity Research Foundation competition, to appear in IJMP

Learning and knowing from the arts : an interdisciplinary model of aesthetic knowing

Published ArticleIn this article the author analyzes the types of knowledge and learning involved when educationalists study works of art to further their professional development. A theoretical background of experiential learning is presented. An Interdisciplinary Model of Aesthetic Knowing (IMAK) is developed and presented, utilizing the Model of Aesthetic Understanding as Informed Experience of Richard Lachapelle, Deborah Murray and Sandy Neim. Additional kinds of knowledge and learning processes included in the IMAK are discussed. Attending a concert in South Africa by Neil Diamond, the popular singer/songwriter stimulated the author to reflect upon his own metacognitive thinking and learning, both during and after the concert, and on the application of what he learned to the IMAK and in his lecturing room

Nonperturbative analysis of the evolution of cosmological perturbations through a nonsingular bounce

In bouncing cosmology, the primordial fluctuations are generated in a cosmic contraction phase before the bounce into the current expansion phase. For a nonsingular bounce, curvature and anisotropy grow rapidly during the bouncing phase, raising questions about the reliability of perturbative analysis. In this paper, we study the evolution of adiabatic perturbations in a nonsingular bounce by nonperturbative methods including numerical simulations of the nonsingular bounce and the covariant formalism for calculating nonlinear perturbations. We show that the bounce is disrupted in regions of the universe with significant inhomogeneity and anisotropy over the background energy density, but is achieved in regions that are relatively homogeneous and isotropic. Sufficiently small perturbations, consistent with observational constraints, can pass through the nonsingular bounce with negligible alteration from nonlinearity. We follow scale invariant perturbations generated in a matter-like contraction phase through the bounce. Their amplitude in the expansion phase is determined by the growing mode in the contraction phase, and the scale invariance is well preserved across the bounce.Comment: 38 pages + appendices, 22 figure

Die menslike liggaam as fisiologiese eenheid*

Die menslike liggaam is ’n fisiologiese eenheid wat uit ongeveer 12 miljard sub-eenhede, die lewende selle saamgestel is. Elke individuele sel kan beskou word as ’n lewende eenheid, om dat elkeen m eeste of al die fisiologiese lewenskenm erke van die geheel soos prikkelbaarheid, geleidingsvermoë, energietransformering en voortplanting vertoon. Die eenheid van die lewende sel kom tot stand om dat talryke biochemiese en biofisiese prosesse van molekules en selorganelle en boueenhede van die sel noukeurig beheer word. Die funksies van die selle word in die weefsels en organe só gekoordineer dat ’n volgende vlak van eenheid m et wyer omvang tot stand kom. Die afsonderlike funksies van die weefsels en organe word op hul beurt w eer gekottrdineer en geïntegreer in die groter geheel van die eenheid van die liggaam, waarbinne die voortbestaan en optimale funksionering van elkeen van die lewende onderdele verseker word. Terwyl elke sel dus die voordeel van die instandhouding van die geheel as eenheid geniet, dra d it op sy beurt weer aktief by om die voortbestaan en doeltreffende funksionering van die geheel moontlik te maak

Flood modelling using data available on the Internet

Includes bibliographical references (p. 95-102).The aim of this study was to determine if sufficient data at no charge is available on the Internet to use as input to a free and open source hydrological model for use in a flood monitoring system. As such, the monitoring system would be SensorWeb enabled. The study area is the C83A quaternary catchment (746 km2) in the Northern Free State, part of the Vaal primary catchment in South Africa

Eccles se mistisisme en kuhlendeck se boeddhisme as moontlike grondslag vir fisiologie

The discussion ranging around the issues of human worth, uniqueness, the typically human, spirituality and human freedom also includes the subject of Physiology. The sense and the meaning of physiological knowledge, foundations for ethical norms and religious functions of man are of cardinal importance for the subject philosophy of Physiology

The unresolved search for the proper standard of review of affirmative action: Solidarity obo Barnard v SAPS

From text: In Solidarity obo Barnard v SAPS, the Supreme Court of Appeal (SCA) set aside the order of the Labour Appeal Court (LAC), which upheld the decision of the National Commissioner of Police not to appoint a white female (Barnard) to the position of superintendent in the National Evaluation Services Division of the South African Police Service (SAPS). The most important issue raised by the judgments is the proper standard of review of affirmative action measures in terms of section 9(2) of the Constitution and section 6(2)(a) of the Employment Equity Act (EEA). The Labour Court (LC), the LAC and the SCA all subscribed to different versions of what the standard of review ought to be. In so doing, they also applied different interpretations of the leading judgment of the Constitutional Court on affirmative action, Minister of Finance v Van Heerden. In this note, the different interpretations will be identified and analysed