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

Chapter 3Gravitational-wave detectorsGravitational radiation is a central prediction of general relativity and its detectionis a key test of the integrity of the theoretical structure of Einstein’s work.However, in the long run, its importance as a tool for observational astronomy islikely to be even more important. We have excellent observational evidence fromthe Hulse–Taylor binary pulsar system (described in chapter 4) that the predictionsof general relativity concerning gravitational radiation are quantitatively correct.However, we have incomplete information from astronomy today about the likelysources of detectable radiation.The gravitational wave spectrum is completely unexplored, and whenever anew electromagnetic waveband has been opened to astronomy, astronomers havediscovered completely unexpected phenomena. This seems to me just as likelyto happen again with gravitational waves, especially because gravitational wavescarry some kinds of information that electromagnetic radiation cannot convey.Gravitational waves are generated by bulk motions of masses, and they encodethe mass distributions and speeds. They are coherent and their low frequenciesreflect the dynamical timescales of their sources.In contrast, electromagnetic waves come from individual electrons executingcomplex and partly random motions inside their sources. They are incoherent, andindividual photons must be interpreted as samples of the large statistical ensembleof photons being emitted. Their frequencies are determined by microphysics onlength scales much smaller than the structure of the astronomical system emittingthem. From electromagnetic observations we can make inferences about thisstructure only through careful modelling of the source. Gravitational waves, bycontrast, carry information whose connection to the source structure and motionis fairly direct.A good example is that of massive black holes in galactic nuclei. Fromobservations that span the electromagnetic spectrum from radio waves to x-rays, astrophysicists have inferred that black holes of masses up to 10 9 M ⊙are responsible for quasar emissions and control the jets that power the giantradio emission regions. The evidence for the black hole is very strong but24