Several years ago the sensitivities of some of these detectors
reached a level - better than
for millisecond pulses - such that the technology could be
considered sufficiently mature to propose the construction of
detectors of much longer baseline which should be capable of
reaching the performance required to have a real possibility of
detecting gravitational waves. Thus an international network of
gravitational wave detectors is now under construction.
The American LIGO project comprises the building of two detector systems with arms of 4 km length, one in Hanford, Washington State, and one in Livingston, Louisiana. One half length, 2 km, interferometer is also being built inside the same evacuated enclosure at Hanford. Construction at both sites is proceeding well and should allow prelimimary coincident operation to be carried out in 2001. A birds-eye view of the Hanford site showing the central building and the directions of the two arms is shown in Fig. 9 .
The French/Italian VIRGO detector of 3 km arm length at Cascina near Pisa is also well into construction with the interferometry in the central part being ready for testing in 2000 and final operation expected for late 2002. As mentioned earlier it is designed to have better performance, down to 10 Hz than the other detectors as shown in Fig. 10, where the level of contributing noise sources and some possible signal levels are shown.
The TAMA 300 detector, which has arms of length
300 m, is at a relatively advanced stage of construction at
the Tokyo Astronomical Observatory. This detector is being built
mainly underground; initial operation of the interferometer has
been achieved in 1999 and power recycling is now being
implemented. All the systems mentioned above are designed to use
resonant cavities in the arms of the detectors and use standard
wire sling techniques for suspending the test masses. The
German/British detector, GEO 600, is somewhat different. It
makes use of a four pass delay line system with advanced optical
signal enhancement techniques, utilises very low loss fused
silica suspensions for the test masses, and should have a
sensitivity at frequencies above a few hundred Hz comparable to
the first phases of VIRGO and LIGO when they are in initial
operation. Construction is advancing well and initial operation
of the GEO 600 detector is expected to commence in 2001.
During the following years we can expect some very interesting
coincidence searches for gravitational waves, at a sensitivity
level of approximately
for pulses of several milliseconds duration. This level of
sensitivity is expected to be improved on when the longer LIGO
and VIRGO detectors are upgraded. Indeed plans for an upgraded
LIGO, LIGO 2, are already on the drawing board. Plans are to
use 30 kg sapphire test masses for this detector, suspended
by fused silica fibers or ribbons, along with an improved seismic
isolation system, increased laser power, of the order of
100 W, and signal recycling [43
]. The contribution of the different noise source to the expected
interferometer sensitivity as contained in [43], is shown in Fig.
11
.
It should be noted that very recent work by Braginsky and colleagues in Moscow [17] is suggesting that a form of mechanical loss known as thermoelastic damping [73] is important in bulk crystalline materials such as as sapphire, and may be represented by a noise line somewhat higher than the thermal noise in the above figure. This is currently under investigation (at the beginning of 2000).
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Gravitational Wave Detection by Interferometry (Ground
and Space)
Sheila Rowan and Jim Hough http://www.livingreviews.org/lrr-2000-3 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |