(ed.). Gravitational waves (IOP, 2001)(422s).

4 Gravitational waves, theory and experiment (an overview)interferometers in space, LISA, to Doppler tracking of interplanetary spacecrafts,there is, aimed at increasing the chances of future detections, a strongly relatedtheoretical and computational work to understand and predict the emission orgravitational waves from astrophysical systems in strong field conditions [3]. Inthis book contributions of leading experts in the field of gravitational waves, boththeoretical and experimental, are presented.The basic contribution by Bernard Schutz and Franco Ricci deals with themain features of gravitational waves, sources and detectors. The contributionis divided into six chapters and some chapters are followed by a few exercises.The first chapter describes the linearized theory and the fundamental propertiesof weak gravitational waves, perturbations of a flat background, analysed inthe so-called transverse-traceless gauge. The second and third chapters dealwith detectors and astrophysical sources; in particular an overview is presentedof the most important detectors under construction (their physics, sensitivityand opportunity for the future) and the main expected sources of gravitationalwaves, such as binary systems, neutron stars, pulsars, γ -ray bursts, etc. Thefourth chapter deals with the mathematical theory of waves in general, stressenergytensor and energy carried by gravitational waves. The subsequent chapterdescribes radiation generation in linearized theory: mass- and current-quadrupoleradiation, i.e. the quadrupole formulae for the outgoing flux of gravitational-waveenergy emitted by a system characterized by slow motion. Finally, the last chapterdescribes some applications of radiation theory to some sources: binary systemsand especially r-modes of neutron stars.The contribution by Guido Pizzella deals with bar detectors of gravitationalwaves. A gravitational-wave resonant detector is usually a cylindrical barof length L. The small change δL in the length of the whole bar at thefundamental resonance angular frequency, ω 0 , can be described by the solution ofthe equation of a harmonic oscillator, with resonance angular frequency ω 0 (witha supplementary 4/π 2 factor obtained by solving the problem of a continuousbar). In a gravitational-wave resonant detector the mechanical oscillations ofthe bar induced by a gravitational wave are converted by an electromechanicaltransducer into electric signals which are amplified with a low noise amplifier,such as a dc SQUID. Then the data analysis is performed. Using a resonantantenna one measures the Fourier component of the metric perturbation nearthe antenna resonance frequency ω 0 . The typical damping time of the resonantdetector is 2Q/ω 0 , where Q is the so-called quality factor of the resonant detector.The ultimate sensitivity of bar antennae to a fractional change in dimension dueto a short burst of gravitational radiation has been estimated to be of the orderof 10 −20 or 10 −21 . Bar detectors, usually 3 m long aluminum bars, work at atypical frequency of about 10 3 Hz. Resonant antennae were first built by J Weber,around 1960, at the University of Maryland. Subsequently, gravitational-waveresonant detectors have been operated by the following universities: Beijing,Guangzhou, Louisiana, Maryland, Moscow, Rome, Padua, Stanford, Tokyo andWestern Australia at Perth. The contribution of Pizzella deals with the bandwidth