13,442 research outputs found
From Now to Timelike Infinity on a Finite Grid
We use the conformal approach to numerical relativity to evolve hyperboloidal
gravitational wave data without any symmetry assumptions. Although our grid is
finite in space and time, we cover the whole future of the initial data in our
calculation, including future null and future timelike infinity.Comment: 15 pages, 14 figures, revtex
On Killing vectors in initial value problems for asymptotically flat space-times
The existence of symmetries in asymptotically flat space-times are studied
from the point of view of initial value problems. General necessary and
sufficient (implicit) conditions are given for the existence of Killing vector
fields in the asymptotic characteristic and in the hyperboloidal initial value
problem (both of them are formulated on the conformally compactified space-time
manifold)
Three Remarks on “Reflective Equilibrium“
John Rawls’ “reflective equilibrium” ranges amongst the most popular conceptions in contemporary ethics when it comes to the basic methodological question of how to justify and trade off different normative positions and attitudes. Even where Rawls’ specific contractualist account is not adhered to, “reflective equilibrium” is readily adopted as the guiding idea of coherentist approaches, seeking moral justification not in a purely deductive or inductive manner, but in some balancing procedure that will eventually procure a stable adjustment of relevant doctrines and standpoints. However, it appears that the widespread use of this idea has led to some considerable deviations from its meaning within Rawls’ original framework and to a critical loss of conceptual cogency as an ethico-hermeneutical tool. This contribution identifies three kinds of “balancing” constellations that are frequently, but inadequately brought forth under the heading of Rawlsian “reflective equilibrium”: balancing theoretical accounts against intuitive convictions; balancing general principles against particular judgements; balancing opposite ethical conceptions or divergent moral statements, respectively. It is argued that each of these applications departs from Rawls’ original construction of “reflective equilibrium” and also deprives the idea of its reliability in clarifying and weighing moral stances
On the Effect of Constraint Enforcement on the Quality of Numerical Solutions in General Relativity
In Brodbeck et al 1999 it has been shown that the linearised time evolution
equations of general relativity can be extended to a system whose solutions
asymptotically approach solutions of the constraints. In this paper we extend
the non-linear equations in similar ways and investigate the effect of various
possibilities by numerical means. Although we were not able to make the
constraint submanifold an attractor for all solutions of the extended system,
we were able to significantly reduce the growth of the numerical violation of
the constraints. Contrary to our expectations this improvement did not imply a
numerical solution closer to the exact solution, and therefore did not improve
the quality of the numerical solution.Comment: 14 pages, 9 figures, accepted for publication in Phys. Rev.
Group-Theoretical Analysis of Second Harmonic Generation at (110) and (111) Surfaces of Antiferromagnets
Extending our previous work we classify the nonlinear magneto-optical
response at low index surfaces of fcc antiferromagnets, such as NiO.
Antiferromagnetic bilayers are discussed here as models for the termination of
bulk antiferromagnets.Comment: 7 pages, 4 figures (figures no. 1 and 3 updated
Neurosurgery for Psychopaths? An Ethical Analysis
Recent developments in neuroscience have inspired proposals to perform deep brain stimulation on psychopathic detainees. We contend that these proposals cannot meet important ethical requirements that hold for both medical research and therapy. After providing a rough overview of key aspects of psychopathy and the prospects of tackling this condition via deep brain stimulation, we proceed to an ethical assessment of such measures, referring closely to the distinctive features of psychopathic personality, particularly the absence of subjective suffering and a lack of moral motivation. Scrutiny of these factors reveals that two essential bioethical criteria, individual medical benefit and voluntary informed consent, cannot be met in performing neurosurgical experiments or treatments on psychopathic inmates
Femtosecond electron and spin dynamics probed by nonlinear optics
A theoretical calculation is performed for the ultrafast spin dynamics in
nickel using an exact diagonalization method. The present theory mainly focuses
on a situation where the intrinsic charge and spin dynamics is probed by the
nonlinear (magneto-)optical responses on the femtosecond time scale, i.e.
optical second harmonic generation (SHG) and the nonlinear magneto-optical Kerr
effect (NOLIMOKE). It is found that the ultrafast charge and spin dynamics are
observable on the time scale of 10 fs. The charge dynamics proceeds ahead of
the spin dynamics, which indicates the existence of a spin memory time. The
fast decay results from the loss of coherence in the initial excited state.
Both the material specific and experimental parameters affect the dynamics. We
find that the increase of exchange interaction mainly accelerates the spin
dynamics rather than the charge dynamics. A reduction of the hopping integrals,
such as present at interfaces, slows down the spin dynamics significantly.
Besides, it is found that a spectrally broad excitation yields the intrinsic
speed limit of the charge (SHG) and spin dynamics (NOLIMOKE) while a narrower
width prolongs the dynamics. This magnetic interface dynamics then should
become accessible to state of art time resolved nonlinear-optical experiments.Comment: 5 pages with 3 figures, to appear in Applied. Phys. B 68, (1999
Nonlinear Magneto-Optics of freestanding Fe monolayers from first principles
The nonlinear magneto-optical Kerr-effect (NOLIMOKE) is determined for
freestanding Fe monolayers with several in-plane structures from first
principles. Based on the theory of nonlinear magneto-optics by H\"ubner and
Bennemann [Phys. Rev. B, {\bf 40}, 5973 (1989)] we calculate the nonlinear
susceptibilities of the monolayers using the ab initio FLAPW-method WIEN95 with
the additional implementation of spin-orbit coupling and the calculation of the
dipole transition matrix elements appropriate for freestanding monolayers. We
present results for the spectral dependence of the nonlinear susceptibility
tensor elements and the resulting intensities and Kerr angles. Special
emphasize is put on the effects of structural changes such as the variation of
the lattice constant and different surface orientations. The influence of
spin-orbit coupling on the tensor elements for different magnetization
directions is presented as well as the azimuthal dependence of the intensities
generated by several low index surfaces, showing the pronounced sensitivity of
second harmonic generation to lateral structural changes as well as magnetic
properties even in the monolayer range
A Scheme to Numerically Evolve Data for the Conformal Einstein Equation
This is the second paper in a series describing a numerical implementation of
the conformal Einstein equation. This paper deals with the technical details of
the numerical code used to perform numerical time evolutions from a "minimal"
set of data.
We outline the numerical construction of a complete set of data for our
equations from a minimal set of data. The second and the fourth order
discretisations, which are used for the construction of the complete data set
and for the numerical integration of the time evolution equations, are
described and their efficiencies are compared. By using the fourth order scheme
we reduce our computer resource requirements --- with respect to memory as well
as computation time --- by at least two orders of magnitude as compared to the
second order scheme.Comment: 20 pages, 12 figure
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