Michele's wiki/blog

Authors: David Alba (Firenze Univ.), Luca Lusanna (INFN, Firenze)

Date: Fri, 28 Jan 2005

Abstract: All existing 4-coordinate systems centered on the world-line of an accelerated observer are only locally defined like it happens for Fermi coordinates both in special and general relativity. As a consequence, it is not known how non-inertial observers can build {\it equal-time surfaces} which a) correspond to a conventional observer-dependent definition of synchronization of distant clocks; b) are good Cauchy surfaces for Maxwell equations. Another type of coordinate singularities are those connected to the relativistic rotating coordinate systems (the rotating disk).We show that the use of Hamiltonian methods based on 3+1 splittings of space-time allows to define as many observer-dependent globally defined radar 4-coordinate systems as nice foliations of space-time with space-like hyper-surfaces admissible according to M$\o$ller (for instance only differentially rotating relativistic coordinate system are allowed). All these conventional notions of an {\it instantaneous 3-space} for the observer have the leaves not orthogonal to the observer world-line and can be empirically defined by introducing generalizations of Einstein ${1\over 2}$ convention for clock synchronization in inertial frames. Each admissible 3+1 splitting corresponds to a non-rigid non-inertial frame centered on the observer. When there is a Lagrangian description of an isolated relativistic system, its reformulation as a parametrized Minkowski theory allows to show that all the admissible synchronization conventions are gauge equivalent, as it also happens in canonical metric and tetrad gravity, where, however, the chrono-geometrical structure of space-time is dynamically determined.

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Authors: Adrian C. Melissinos, William E.Butler (Department of Physics and Astronomy University of Rochester, Rochester NY)

Date: Thu, 27 Jan 2005

Abstract: Laser interferometer gravitational wave detectors can be operated at their free spectral range frequency. We show that in this case and when the interferometer is well understood one could detect a stochastic background using a single detector.

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Authors: Masaki Ishii, Masaru Shibata, Yasushi Mino

Date: Thu, 27 Jan 2005

Abstract: We derive a tidal potential for a self-gravitating fluid star orbiting Kerr black hole along a timelike geodesic extending previous works by Fishbone and Marck. In this paper, the tidal potential is calculated up to the third and fourth-order terms in $R/r$, where $R$ is the stellar radius and $r$ the orbital separation, in the Fermi-normal coordinate system following the framework developed by Manasse and Misner. The new formulation is applied for determining the tidal disruption limit (Roche limit) of corotating Newtonian stars in circular orbits moving on the equatorial plane of Kerr black holes. It is demonstrated that the third and fourth-order terms quantitatively play an important role in the Roche limit for close orbits with $R/r \agt 0.1$. It is also indicated that the Roche limit of neutron stars orbiting a stellar-mass black hole near the innermost stable circular orbit may depend sensitively on the equation of state of the neutron star.

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Authors: The LIGO Scientific Collaboration

Date: Mon, 24 Jan 2005

Abstract: We have performed a search for bursts of gravitational waves associated with the very bright Gamma Ray Burst GRB030329, using the two detectors at the LIGO Hanford Observatory. Our search covered the most sensitive frequency range of the LIGO detectors (approximately 80-2048 Hz), and we specifically targeted signals shorter than 150 ms. Our search algorithm looks for excess correlated power between the two interferometers and thus makes minimal assumptions about the gravitational waveform. We observed no candidates with gravitational wave signal strength larger than a pre-determined threshold. We report frequency dependent upper limits on the strength of the gravitational waves associated with GRB030329. Near the most sensitive frequency region, around 250 Hz, our root-sum-square (RSS) gravitational wave strain sensitivity for optimally polarized bursts was better than h_RSS = 6E-21 Hzˆ{-½}. Our result is comparable to the best published results searching for association between gravitational waves and GRBs.

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Authors: Michael D. Hartl, Alessandra Buonanno

Date: Mon, 17 Jan 2005

Abstract: We investigate the (conservative) dynamics of binary black holes using the Hamiltonian formulation of the post-Newtonian (PN) equations of motion. The Hamiltonian we use includes spin-orbit coupling, spin-spin coupling, and mass monopole/spin-induced quadrupole interaction terms. In the case of both quasi-circular and eccentric orbits, we search for the presence of chaos (using the method of Lyapunov exponents) for a large variety of initial conditions. For quasi-circular orbits, we find no chaotic behavior for black holes with total mass 10 - 40 solar masses when initially at a separation corresponding to a Newtonian gravitational-wave frequency less than 150 Hz. Only for rather small initial radial distances, for which spin-spin induced oscillations in the radial separation are rather important, do we find chaotic solutions, and even then they are rare. Moreover, these chaotic quasi-circular orbits are of questionable astrophysical significance, since they originate from direct parametrization of the equations of motion rather than from widely separated binaries evolving to small separations under gravitational radiation reaction. In the case of highly eccentric orbits, which for ground-based interferometers are not astrophysically favored, we again find chaotic solutions, but only at pericenters so small that higher order PN corrections, especially higher spin PN corrections, should also be taken into account.

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Authors: Ted Jacobson, Donald Marolf, Carlo Rovelli

Date: Fri, 14 Jan 2005

Abstract: A trialogue. Ted, Don, and Carlo consider the nature of black hole entropy. Ted and Carlo support the idea that this entropy measures in some sense ‘the number of black hole microstates that can communicate with the outside world.’ Don is critical of this approach, and discussion ensues, focusing on the question of whether the first law of black hole thermodynamics can be understood from a statistical mechanics point of view.

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Authors: Serge Reynaud, Marc-Thierry Jaekel

Date: Wed, 12 Jan 2005

Abstract: Experimental tests of Newton law put stringent constraints on potential deviations from standard theory with ranges from the millimeter to the size of planetary orbits. Windows however remain open for short range deviations, below the millimeter, as well as long range ones, of the order of or larger than the size of the solar system. We discuss here the relation between long range tests of the Newton law and the anomaly recorded on Pioneer 10/11 probes.

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Authors: Don N. Page

Date: Fri, 31 Dec 2004

Abstract: An inexhaustive review of Hawking radiation and black hole thermodynamics is given, focusing especially upon some of the historical aspects as seen from the biased viewpoint of a minor player in the field on and off for the past thirty years.

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Authors: D. Fargion

Date: Thu, 30 Dec 2004

Abstract: The present gravitational wave detectors are reaching lowest metric deviation fields able to detect galactic and extra-galactic gravitational waves, related to Supernova explosions up to Virgo cluster. The same gravitational wave detector are nevertheless almost able to reveal near field gravitational perturbations due to fast huge mass displacements as the ones occurring during largest Earth-Quake or Tsunami as the last on 26nd December 2004 in Asiatic area. The prompt gravitational near field deformation by the Tsunami may reach the LIGO threshold sensitivity, but not frequency band, and LISA within 3000 km distances. Their eventual discover (in future on-line detector arrays) may offer the most rapid warning alarm system on earth. Nevertheless the later continental mass rearrangement and their gravitational field assessment on Earth must induce, for Richter Magnitude 9 Tsunami, a different terrestrial inertia momentum and a different rotation axis, as well as a detectable shrinking of the Earth radius of nearly R =3.4 micron. and a consequent faster Earth spinning by a ratio Delta w/w_= -1.08 10ˆ{-12} and a year duration shortening of the order of Delta {t_{year}}= - 34 micron s; if all the energy released is absorbed by the terrestrial rotation energy the opposite would occur, Delta w/w = 2.59 10ˆ{-10}, with a longer year lenght: Delta t {year} = 8.17 10ˆ{-3} s; because the terrestrial gravitational energy is nearly 480 times its rotational one, the mutual energy exchange may in general leads to a spin up or a spin down of the day lenght within Delta t= 22.4 10ˆ{-6} s. well within detection.

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Authors: Jim Hough, Sheila Rowan, B.S. Sathyaprakash

Date: Tue, 4 Jan 2005

Abstract: Experiments aimed at searching for gravitational waves from astrophysical sources have been under development for the last 40 years, but only now are sensitivities reaching the level where there is a real possibility of detections being made within the next five years. In this article a history of detector development will be followed by a description of current detectors such as LIGO, VIRGO, GEO 600, TAMA 300, Nautilus and Auriga. Preliminary results from these detectors will be discussed and related to predicted detection rates for some types of sources. Experimental challenges for detector design are introduced and discussed in the context of detector developments for the future.

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Authors: Christian Koenigsdoerffer, Achamveedu Gopakumar

Date: Wed, 5 Jan 2005

Abstract: We derive Keplerian-type parametrization for the solution of post-Newtonian (PN) accurate conservative dynamics of spinning compact binaries moving in eccentric orbits. The PN accurate dynamics that we consider consists of the third post-Newtonian accurate conservative orbital dynamics influenced by the leading order spin effects, namely the leading order spin-orbit interactions. The orbital elements of the representation are explicitly given in terms of the conserved orbital energy, angular momentum and a quantity that characterizes the leading order spin-orbit interactions in Arnowitt, Deser, and Misner-type coordinates. Our parametric solution is applicable in the following two distinct cases: (i) the binary consists of equal mass compact objects, having two arbitrary spins, and (ii) the binary consists of compact objects of arbitrary mass, where only one of them is spinning with an arbitrary spin. As an application of our parametrization, we present gravitational wave polarizations, whose amplitudes are restricted to the leading quadrupolar order, suitable to describe gravitational radiation from spinning compact binaries moving in eccentric orbits. The present parametrization will be required to construct ‘ready to use’ reference templates for gravitational waves from spinning compact binaries in inspiralling eccentric orbits. Our parametric solution for the post-Newtonian accurate conservative dynamics of spinning compact binaries clearly indicates, for the cases considered, the absence of chaos in these systems. Finally, we note that our parametrization provides the first step in deriving a fully second post-Newtonian accurate ‘timing formula’, that may be useful for the radio observations of relativistic binary pulsars like J0737-3039.

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Authors: S. P. Drake

Date: Tue, 11 Jan 2005

Abstract: In this paper we show how the student can be led to an understanding of the connection between special relativity and general relativity by considering the time dilation effect of clocks placed on the surface of the Earth. This paper is written as a Socratic dialog between a lecturer Sam and a student Kim.

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Authors: Richard H. Price

Date: Sat, 8 Jan 2005

Abstract: The kind of flat-earth gravity used in introductory physics appears in an accelerated reference system in special relativity. From this viewpoint, we work out the special relativistic description of a ballistic projectile and a simple pendulum, two examples of simple motion driven by earth-surface gravity. The analysis uses only the basic mathematical tools of special relativity typical of a first-year university course.

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