Complex Wave Numbers in the Vicinity of the Schwarzschild Event Horizon

This paper is devoted to investigate the cold plasma wave properties outside the event horizon of the Schwarzschild planar analogue. The dispersion relations are obtained from the corresponding Fourier analyzed equations for non-rotating and rotating, non-magnetized and magnetized backgrounds. These dispersion relations provide complex wave numbers. The wave numbers are shown in graphs to discuss the nature and behavior of waves and the properties of plasma lying in the vicinity of the Schwarzschild event horizon.Comment: 21 pages, 9 figures, accepted for publication Int. J. Mod. Phys.

Interaction of laser generated ultrasonic waves with wedge-shaped samples

Wedge-shaped samples can be used as a model of acoustic interactions with samples ranging from ocean wedges, to angled defects such as rolling contact fatigue, to thickness measurements of samples with non-parallel faces. We present work on laser generated ultrasonic waves on metal samples; one can measure the dominant Rayleigh-wave mode, but longitudinal and shear waves are also generated. We present calculations, models, and measurements giving the dependence of the arrival times and amplitudes of these modes on the wedge apex angle and the separation of generation and detection points, and hence give a measure of the wedge characteristics

Application of Geometrical Diffraction Theory to QNDE Analysis

The direct problem of the diffraction of time-harmonic·waves by cracks in elastic solids is analyzed for high-frequencies, when the wavelengths are of the same order of magnitude as a characteristic length dimension, a, of the crack. It is shown that good approximations at high frequencies can be obtained on the basis of elastodynamic ray theory. An elastodynamic version of geometrical diffraction theory is briefly reviewed. We also present a hybrid theory, wherein the crack opening displacement is computed on the basis of geometrical diffraction theory, and the scattered field is subsequently obtained by the use of a representation theorem. This hybrid approach avoids the difficulties at shadow boundaries and caustic surfaces that plague a direct application of geometrical diffraction theory. Explicit results are computed for slits and penny-shaped cracks, and these results are compared with numerical results obtained on the basis of exact integral equation formulations. The relatively simply structure of the expressions for the scattered fields displays some characteristic features, whose possible role in the inverse problem is discussed

Application of Geometrical Diffraction Theory to Scattering by Cracks

At high frequencies, the geometrical theory of diffraction provides useful and relatively simple approximations to diffracted fields. In this paper the theory is applied to the diffraction of time-harmonic longitudinal waves by a penny-shaped crack in an elastic solid

Which forms of child/adolescent externalizing behaviors account for late adolescent risky sexual behavior and substance use?

Background: Health risk behaviors like substance use (alcohol, tobacco, soft/hard drugs) and risky sexual behavior become more prevalent in adolescence. Children with behavior problems are thought to be prone to engage in health risk behaviors later in life. It is, however, unclear which problems within the externalizing spectrum account for these outcomes. Methods: Three hundred and nine children were followed from age 4/5 years to 18 years (14-year follow-up). Level and course of parent-rated opposition, physical aggression, status violations and property violations were used to predict adolescent-reported substance use and risky sexual behavior at age 18 years. Results: Both level and change in physical aggression were unique predictors of all forms of adolescent health risk behavior. Levels of status violations predicted smoking and soft drug use only, while change in property violations predicted each of the health risk behaviors. The links between opposition and health risk behaviors were accounted for by co-occurring problem behaviors. Conclusions: Of externalizing problems, physical aggression is the best predictor of adolescent substance use and risky sexual behavior from childhood onwards. Possible explanations and implications of these findings, and future research directions are discussed. © 2007 The Authors