6.1 Gravitational wave observatories
Some time in the next decade, a new opportunity for testing relativistic gravity will be realized, when a
worldwide network of kilometer-scale, laser interferometric gravitational wave observatories
in the U.S. (LIGO project), Europe (VIRGO and GEO600 projects), and Japan (TAMA300
project) begins regular detection and analysis of gravitational wave signals from astrophysical
sources. These broad-band antennas will have the capability of detecting and measuring the
gravitational waveforms from astronomical sources in a frequency band between about 10 Hz
(the seismic noise cutoff) and 500 Hz (the photon counting noise cutoff), with a maximum
sensitivity to strain at around 100 Hz of (rms), for the kilometer-scale
LIGO/VIRGO projects. The most promising source for detection and study of the gravitational
wave signal is the “inspiralling compact binary” – a binary system of neutron stars or black
holes (or one of each) in the final minutes of a death spiral leading to a violent merger. Such is
the fate, for example, of the Hulse–Taylor binary pulsar B1913+16 in about 300 Myr, or the
“double pulsar” J0737-3039 in about 85 Myr. Given the expected sensitivity of the “advanced
LIGO” (around 2010), which could see such sources out to many hundreds of megaparsecs, it has
been estimated that from 40 to several hundred annual inspiral events could be detectable.
Other sources, such as supernova core collapse events, instabilities in rapidly rotating newborn
neutron stars, signals from non-axisymmetric pulsars, and a stochastic background of waves,
may be detectable (for reviews, see [1, 256]; for updates on the status of various projects,
see [114, 45]).
A similar network of cryogenic resonant-mass gravitational antennas have been in operation for many
years, albeit at lower levels of sensitivity (h 10–19). While modest improvements in sensitivity may be
expected in the future, these resonant detectors are not expected to be competitive with the large
interferometers, unless new designs involving masses of spherical, or nearly spherical shape come to fruition.
These systems are primarily sensitive to waves in relatively narrow bands about frequencies in the hundreds
to thousands of Hz range [206, 123, 32, 217], although future improvements in sensitivity and increases in
bandwidth may be possible .
In addition, plans are being developed for an orbiting laser interferometer space antenna
(LISA for short). Such a system, consisting of three spacecraft orbiting the sun in a triangular
formation separated from each other by five million kilometers, would be sensitive primarily in
the very low frequency band between 10–4 and 10–1 Hz, with peak strain sensitivity of order
h 10–23 .
In addition to opening a new astronomical window, the detailed observation of gravitational waves
by such observatories may provide the means to test general relativistic predictions for the
polarization and speed of the waves, for gravitational radiation damping and for strong-field