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

32 Gravitational-wave detectorsthose carried by waves. The environment always contains changes in theNewtonian fields of nearby objects. Besides obvious ones, like people,there are changes caused by density waves in ground vibrations, atmosphericpressure changes, and many other disturbances. Below about 1 Hz,these gravity gradient changes will be stronger than waves expected fromastronomical objects, and they make it impossible to do observing at lowfrequencies from Earth. This is the reason that scientists have proposed theLISA mission, discussed later. Above 1 Hz, this noise does not affect thesensitivity of present detectors, but in ten years this could become a limitingfactor.Besides these noise sources, which are predictable and therefore can becontrolled by detector design, it is possible that there will be unexpected orunpredicted noise sources. Interferometers will be instrumented with many kindsof environmental monitors, but there may occasionally be noise that is impossibleto identify. For this reason, short bursts of gravitational radiation must beidentified at two or more separated facilities. Even if detector noise is not atall understood, it is relatively easy to estimate from the observed noise profile ofthe individual detectors what the chances are of a coincident noise event betweentwo detectors.3.2.1 New interferometers and their capabilitiesInterferometers work over a broad bandwidth and they do not have any naturalresonance in their observing band. They are ideal for detecting bursts, since onecan perform pattern-matching over the whole bandwidth and detect such signalsoptimally. They are also ideal for searching for unknown continuous signals, suchas surveying the sky for neutron stars. And in observations of stochastic signalsby cross-correlating two detectors, they can give information about the spectrumof the signal.If an interferometer wants to study a signal with a known frequency, suchas known pulsars, then there is another optical technique available to enhance itssensitivity in a narrow bandwidth, at the expense of sensitivity outside that band.This is called signal recycling [3]. In this technique, a further mirror is placed infront of the photodetector, where the signal emerges from the interferometer (seefigure 3.1(d)). If the mirror is chosen correctly, it will build up the signal, but onlyin a certain bandwidth. This modifies the shot noise in the detector, but not othernoise sources. Therefore, it can improve sensitivity only at the higher frequencieswhere shot noise is the limiting factor.Four major interferometer projects are now under construction, and theycould begin acquiring good data in the period between 2000–2003. They will alloperate initially with a sensitivity approaching 10 −21 over a bandwidth between50–1000 Hz. Early detections are by no means certain, but recent work hasmade prospects look better for an early detection than when these detectors werefunded.