Stability of Gravitational and Electromagnetic Geons

Recent work on gravitational geons is extended to examine the stability properties of gravitational and electromagnetic geon constructs. All types of geons must possess the property of regularity, self-consistency and quasi-stability on a time-scale much longer than the period of the comprising waves. Standard perturbation theory, modified to accommodate time-averaged fields, is used to test the requirement of quasi-stability. It is found that the modified perturbation theory results in an internal inconsistency. The time-scale of evolution is found to be of the same order in magnitude as the period of the comprising waves. This contradicts the requirement of slow evolution. Thus not all of the requirements for the existence of electromagnetic or gravitational geons are met though perturbation theory. From this result it cannot be concluded that an electromagnetic or a gravitational geon is a viable entity. The broader implications of the result are discussed with particular reference to the problem of gravitational energy.Comment: 40 pages, 5 EPS figures, uses overcite.st

The sound motion controller: a distributed system for interactive music performance

We developed an interactive system for music performance, able to control sound parameters in a responsive way with respect to the user’s movements. This system is conceived as a mobile application, provided with beat tracking and an expressive parameter modulation, interacting with motion sensors and effector units, which are connected to a music output, such as synthesizers or sound effects. We describe the various types of usage of our system and our achievements, aimed to increase the expression of music performance and provide an aid to music interaction. The results obtained outline a first level of integration and foresee future cognitive and technological research related to it

Laser Interferometric Detectors of Gravitational Waves

A laser interferometric detector of gravitational waves is studied and a complete solution (to first order in the metric perturbation) of the coupled Einstein-Maxwell equations with appropriate boundary conditions for the light beams is determined. The phase shift, the light deflection and the rotation of the polarization axis induced by gravitational waves are computed. The results are compared with previous literature, and are shown to hold also for detectors which are large in comparison with the gravitational wavelength.Comment: 13 pages, LaTe

Gravitational Geons Revisited

A careful analysis of the gravitational geon solution found by Brill and Hartle is made. The gravitational wave expansion they used is shown to be consistent and to result in a gauge invariant wave equation. It also results in a gauge invariant effective stress-energy tensor for the gravitational waves provided that a generalized definition of a gauge transformation is used. To leading order this gauge transformation is the same as the usual one for gravitational waves. It is shown that the geon solution is a self-consistent solution to Einstein's equations and that, to leading order, the equations describing the geometry of the gravitational geon are identical to those derived by Wheeler for the electromagnetic geon. An appendix provides an existence proof for geon solutions to these equations.Comment: 18 pages, ReVTeX. To appear in Physical Review D. Significant changes include more details in the derivations of certain key equations and the addition of an appendix containing a proof of the existence of a geon solution to the equations derived by Wheeler. Also a reference has been added and various minor changes have been mad

Overcoming Limitations of the Trackpad for 3D Docking Operations

International audienceFrom notebook trackpads to mobile phones to tabletop surface computing, multitouch input surfaces have become one of the most dominant interfaces for human-computer interaction. Although these are clearly e ective for interaction with 2D graphical user interfaces, we suspect that they are not as well suited for interaction requiring greater degrees of freedom (DoF). Here, we consider the possibility of exploiting two such surfaces, one for each hand, as a means of a ording e cient control over higher dimensional tasks. We investigate performance on a 6 DoF task, comparing such a two-surface multitouch input device against the results obtained using a standard 2D mouse, a single multitouch surface, and a 6 DoF free-space device. Our results indicate that two multitouch surfaces signi cantly improve user performance compared to the mouse and to a single surface

Negative Pressure and Naked Singularities in Spherical Gravitational Collapse

Assuming the weak energy condition, we study the nature of the non-central shell-focussing singularity which can form in the gravitational collapse of a spherical compact object in classical general relativity. We show that if the radial pressure is positive, the singularity is covered by a horizon. For negative radial pressures, the singularity will be covered if the ratio of pressure to the density is greater than -1/3 and naked if this ratio is $\leq -1/3$.Comment: 7 pages, LaTeX Fil

A Time-Like Naked Singularity

We construct a class of spherically symmetric collapse models in which a naked singularity may develop as the end state of collapse. The matter distribution considered has negative radial and tangential pressures, but the weak energy condition is obeyed throughout. The singularity forms at the center of the collapsing cloud and continues to be visible for a finite time. The duration of visibility depends on the nature of energy distribution. Hence the causal structure of the resulting singularity depends on the nature of the mass function chosen for the cloud. We present a general model in which the naked singularity formed is timelike, neither pointlike nor null. Our work represents a step toward clarifying the necessary conditions for the validity of the Cosmic Censorship Conjecture.Comment: 4 pages, Revtex4, To appear in Physical Review

Nonlinear Gravitational Waves: Their Form and Effects

A gravitational wave must be nonlinear to be able to transport its own source, that is, energy and momentum. A physical gravitational wave, therefore, cannot be represented by a solution to a linear wave equation. Relying on this property, the second-order solution describing such physical waves is obtained. The effects they produce on free particles are found to consist of nonlinear oscillations along the direction of propagation.Comment: 15 pages, no figures. v2: presentation changes aiming at clarifying the text; matches published versio

The gravitational interaction of light: from weak to strong fields

An explanation is proposed for the fact that pp-waves superpose linearly when they propagate parallely, while they interact nonlinearly, scatter and form singularities or Cauchy horizons if they are antiparallel. Parallel pp-waves do interact, but a generalized gravitoelectric force is exactly cancelled by a gravitomagnetic force. In an analogy, the interaction of light beams in linearized general relativity is also revisited and clarified, a new result is obtained for photon to photon attraction, and a conjecture is proved. Given equal energy density in the beams, the light-to-light attraction is twice the matter-to-light attraction and four times the matter-to-matter attraction.Comment: 17 pages, LaTeX, no figures. To appear in General Relativity and Gravitatio

Energy-Momentum of a regular MMaS-class black hole

We compute the energy and momentum of a regular black hole of type defined by Mars, Martin-Prats, and Senovilla using the Einstein and Papapetrou definitions for energy-momentum density. Some other definitions of energy-momentum density are shown to give mutually contradictory and less reasonable results. Results support the Cooperstock hypothesis.Comment: 16 pages, 3 figures, LaTex2e; made minor corrections (in content and in references) at the behest of two anonymous referees. Paper to appear in IJMP