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

Gravitational waves, theory and experiment (an overview) 3a mixture of the two, and may hold itself together by its own gravitationalattraction. A collection of radiation held together in this way, is called a geon(gravitational electromagnetic entity) and studied from a distance, such an objectwould present the same kind of gravitational attraction as any other mass. Yet,nowhere inside the geon is there a place where there is ‘mass’ in the conventionalsense of the term. In particular, for a geon made of pure gravitational radiation—a gravitational geon—there is no local measure of energy, yet there is globalenergy. The gravitational geon owes its existence to a dynamical localized—buteverywhere regular—curvature of spacetime, and to nothing more. Thus, a geonis a collection of electromagnetic or gravitational-wave energy, or a mixture ofthe two, held together by its own gravitational attraction, that was described byWheeler as ‘mass without mass’.In the 1960s, Joseph Weber began the experimental work to detectgravitational waves. He was essentially alone in this field of research [10]. Then,the theoretical work of Wheeler, Bondi, Landau and Lifshitz, Isaacson, Thorneand others and the experimental work of Weber, Braginski, Amaldi and othersopened a new era of research in this field. In 1972 Steven Weinberg wrote ‘. . .gravitational radiation would be interesting even if there were no chance of everdetecting any, for the theory of gravitational radiation provides a crucial linkbetween general relativity and the microscopic frontiers of physics’ [11].Today gravitational waves, both theory and experiment, are one of the maintopics of research in general relativity and gravitation [3].In the same way as electromagnetic waves other than visible light, that isradio, millimetre, infrared, ultraviolet, x-ray and gamma-ray astronomy openednew windows and brought radical changes in our knowledge of the universe,gravitational-wave astronomy is expected to bring a revolution in our knowledgeof the universe by observing new exotic phenomena such as formation andcollision of black holes, fall of stars into supermassive black holes, primordialgravitational waves emitted just after the big bang .... Nevertheless, today,about 85 years after the prediction of gravitational waves by Einstein, the onlyevidence for their actual existence is indirect and comes from the observationof the energy loss from the binary pulsar system PSR 1913+16, discovered in1974 by Hulse and Taylor [12]. Quite remarkably, though of no surprise, theobserved energy loss of the binary pulsar is in agreement with the theoreticalprediction by general relativity for the energy loss by gravitational radiationemitted by a binary system, to within less than 0.3% error (in this respect, itmight be interesting to note here that, in regard to the field that in generalrelativity is formally analogous to the magnetic field in electrodynamics, i.e. theso-called gravitomagnetic field, predicted by Lense and Thirring in 1916, thefirst evidence and measurement of the existence of such an effect on Earth’ssatellites, due to the Earth’s rotation, was published only in 1996, that is 80years after the derivation of the effect [13]). Thus, today, together with theenormous experimental efforts to detect gravitational waves, from bar detectorsto laser interferometers on Earth, GEO-600, LIGO, VIRGO, ..., and from laser