Gravitational Waves 77L 2L 2MirrorMirrors
LaserPhotodiodeBeam
splitterR MirrorFigure 4.3 A schematic view of a LIGO-type interferometer. Reprinted with permission of
A. Abramoviciet al.[3]. Copyright 1992 American Association for the Advancement of Science.
eigenmodes of the incoming wave, and one then expects to observe a change in the
state of oscillation. Today several coordinated and aligned cryogenic bar detectors are
in coordinated operation with sensitivities of approximately 10−^21 Hz−^1 ∕^2. The detec-
tors are tuned to see approximately 1ms bursts occurring within a bandwidth of the
order of 1Hz. In order to eliminate random noise, the data from several detectors are
analyzed for coincidences.
To improve the signal to noise ratio in the high-frequency range one turns
to Michelson interferometers with very long arms. The principle is illustrated in
Figure 4.3. A laser beam is split, travels in two orthogonal directions to mirrors, and
returns to be recombined and detected. A gravitational wave with either theℎ+or
ℎ×component coinciding with the interferometer axes would lengthen the round-trip
distance to one mirror and shorten it to the other. This would be observable as a mis-
match of waves upon recombination, and hence as a decrease in the observed com-
bined intensity of the laser. For isolation against mechanical disturbances the optical
components are carefully suspended in vacuum. The arm lengths in active terrestrial
detectors range from 300m (TAMA in Japan) and 600m (GEO600 in Germany) to 3km
(VIRGO in Italy) and 4km (LIGO at two locations in the United States). Sensitivities
of 10−^21 –10−^22 Hz−^1 ∕^2 can be reached in the high-frequency range. The range is limited
to less than approximately 10^4 Hz by photo-electron shot noise in the components of
the interferometer.
To study sources in the low-frequency range one has to put the interferometer into
space orbiting Earth. This is necessary in order to avoid low-frequency seismic noise
on the ground and thermally induced medium-frequency motion in the atmosphere.
The spectacular solution is the detector LISA (Laser Interferometer Space Antenna)
consisting of three identical spacecraft, forming an equilateral triangle in space, with
sidelength 5 million km, trailing Earth by 20∘in a heliocentric orbit (see Figure 4.4).
From each spacecraft a 1W beam is sent to the two other remote spacecrafts via a
telescope, is reflected by a platinum–gold cubic test mass, and the same telescopes
are then used to focus the very weak returning beams. The interference signals