Gravitational waves, one of the more exotic predictions of Einstein’s General Theory of Relativity may, after decades of controversy over their existence, be detected within the next five years.
Sources such as interacting black holes, coalescing compact binary systems, stellar collapses and pulsars are all possible candidates for detection; observing signals from them will significantly boost our understanding of the Universe. New unexpected sources will almost certainly be found and time will tell what new information such discoveries will bring. Gravitational waves are ripples in the curvature of space-time and manifest themselves as fluctuating tidal forces on masses in the path of the wave. The first gravitational-wave detectors were based on the effect of these forces on the fundamental resonant mode of aluminium bars at room temperature. Initial instruments were constructed by Joseph Weber [310, 311] and subsequently developed by others. Reviews of this early work are given in [299, 128]. Following the lack of confirmed detection of signals, aluminium bar systems operated at and below the temperature of liquid helium were developed [253, 261, 76, 170], although work in this area is now subsiding, with only two detectors, Auriga  and Nautilus , continuing to operate. Effort also continues to be pursued into cryogenic spherical bar detectors, which are designed to have a wider bandwidth than the cylindrical bars, with the two prototype detectors the Dutch MiniGRAIL [233, 158] and Brazilian Mário Schenberg [159, 70]. However, the most promising design of gravitational-wave detectors, offering the possibility of very high sensitivities over a wide range of frequency, uses widely-separated test masses freely suspended as pendulums on Earth or in a drag-free craft in space; laser interferometry provides a means of sensing the motion of the masses produced as they interact with a gravitational wave.
Ground-based detectors of this type, based on the pioneering work of Forward and colleagues (Hughes Aircraft) , Weiss and colleagues (MIT) , Drever and colleagues (Glasgow/Caltech) [130, 129] and Billing and colleagues (MPQ Garching) , will be used to observe sources whose radiation is emitted at frequencies above a few Hz, and space-borne detectors, as originally envisaged by Peter Bender and Jim Faller [126, 140] at JILA, will be developed for implementation at lower frequencies.
Gravitational-wave detectors of long baseline have been built in a number of places around the world; in the USA (LIGO project led by a Caltech/MIT consortium) [45, 212], in Italy (Virgo project, a joint Italian/French venture) [61, 304], in Germany (GEO600 project built by a collaboration centred on the University of Glasgow, the University of Hannover, the Max Planck Institute for Quantum Optics, the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Golm and Cardiff University) [321, 151] and in Japan (TAMA300 project) [78, 294]. A space-borne detector, called LISA [125, 217, 216], was until earlier this year (2011) under study as a joint ESA/NASA mission as one L-class candidate within the ESA Cosmic Visions program (a recent meeting detailing these missions can be found here ). Funding constraints within the US now mean that ESA must examine the possibility of flying an L-class mission with European-only funding. The official ESA statement on the next steps for LISA can be found here . When completed, this detector array would have the capability of detecting gravitational wave signals from violent astrophysical events in the Universe, providing unique information on testing aspects of general relativity and opening up a new field of astronomy.
It is also possible to observe the tidal effects of a passing gravitational wave by Doppler tracking of separated objects. For example, Doppler tracking of spacecraft allows the Earth and an interplanetary spacecraft to be used as test masses, where their relative positions can be monitored by comparing the nearly monochromatic microwave signal sent from a ground station with the coherently returned signal sent from the spacecraft . By comparing these signals, a Doppler frequency time series , where is the central frequency from the ground station, can be generated. Peculiar characteristics within the Doppler time series, caused by the passing of gravitational waves, can be studied in the approximate frequency band of 10–5 to 0.1 Hz. Several attempts have been made in recent decades to collect such data (Ulysses, Mars Observer, Galileo, Mars Global Surveyor, Cassini) with broadband frequency sensitivities reaching 10–16 (see  for a thorough review of gravitational-wave searches using Doppler tracking). There are currently no plans for dedicated experiments using this technique; however, incorporating Doppler tracking into another planetary mission would provide a complimentary precursor mission before dedicated experiments such as LISA are launched.
The technique of Doppler tracking to search for gravitational-wave signals can also be performed using pulsar-timing experiments. Millisecond pulsars  are known to be very precise clocks, which allows the effects of a passing gravitational wave to be observed through the modulation in the time of arrival of pulses from the pulsar. Many noise sources exist and, for this reason, it is necessary to monitor a large array of pulsars over a long observation time. Further details on the techniques used and upper limits that have been set with pulsar timing experiments can be found from groups such as the European Pulsar Timing Array , the North American Nanohertz Observatory for Gravitational Waves [190, 191], and the Parkes Pulsar Timing Array .
All the above detection methods cover over 13 orders of magnitude in frequency (see Figure 1) equivalent to covering from radio waves to X-rays in the electromagnetic spectrum. This broadband coverage allows us to probe a wide range of potential sources.
We recommend a number of excellent books for reference. For a popular account of the development of the gravitational-wave field the reader should consult Chapter 10 of Black Holes and Time Warps by Kip S. Thorne , or the more recent books, Einstein’s Unfinished Symphony, by Marcia Bartusiak  and Gravity from the Ground Up, by Bernard Schutz . A comprehensive review of developments toward laser interferometer detectors is found in Fundamentals of Interferometric Gravitational Wave Detectors by Peter Saulson , and discussions relevant to the technology of both bar and interferometric detectors are found in The Detection of Gravitational Waves edited by David Blair .
In addition to the wealth of articles that can be found on the home site of this journal, there are also various informative websites that can easily be found, including the homepages of the various international collaborative projects searching for gravitational waves, such as the LIGO Scientific Collaboration .
Living Rev. Relativity 14, (2011), 5
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