Gravitational waves, a feature of General Relativity, are known to be emitted from two different binary pulsar systems. One expects that merging neutron star systems and merging black hole system, supernovae, and spinning neutron stars are emitting gravitational waves. Can these waves be detected? Using ground-based Michelson interferometers, a number of groups are currently trying to detect gravitational waves for the first time. Ground based gravitational wave detectors have a frequency window between about 10Hz and 1000Hz, which make them sensitive to the gravitational waves produced by merging binaries, spinning neutron stars, and supernovae. All of these projects are massive in size, expense, and hype, all are exceedingly difficult engineering feats, and all are high-risk, because after all of the work and expense, they may detect nothing. On the other hand, if they are successful, they will provide the only means of studying objects interacting through strong gravitational fields.
The ground based experiments work largely the same way, with the primary differences in the size of the instrument and whether Fabry-Perot cavity are used. Four of the five are in operation. These systems operate very powerful and stable lasers, and the laser light travels in an ultra-high vacuum through the highest-quality optics. The components in these instruments are suspended on complex pendulum systems to help limit the degradation of the sensitivity from the thermal noise in the suspension system. The instruments are isolated from Earth to limit the impact of earthquake.
The sensitivity is the critical issue, because sensitivity determines the volume of space that is observed. The instrument's sensitivity is a measure of the minimum amplitude of a detectable gravitational wave; a wave's amplitude is proportional to the distance traveled (the energy in the wave, which is proportional to the square of the amplitude, falls as the inverse of the distance squared, as it does for electromagnetic waves). As a consequence, for sources of a set gravitational wave luminosity, the volume sampled by the instrument increases inversely with the cube of the sensitivity.
The LIGO (Laser Interferometer Gravitational Wave Observatory) experiment is a set of three gravitational wave detectors funded by the National Science Foundation (NSF) in the United States. Two detectors are located in Hanford, Washington, and have arm lengths of 2km and 4km. The third detector is located in Livingston, Louisiana, and has an arm length of 4km.
LIGO is the largest of current gravitational wave detectors. It is designed with a Fabre-Pinot cavity, and it is expected to see changes in length as small as 4×10-16 cm, which is on the subatomic length scale.
The LIGO experiment began collecting data in August of 2001 (the data run is referred to as S1), operating for 2 week at a sensitivity of 100 times its design goal sensitivity of 3×10-23. A second data run (S2) covered February 14 to April 14 of 2003, and achieved a sensitivity that was 10 times its design goal. The last completed data run (S3) started on October 31, 2003, and ended January 8, 2004, achieving a sensitivity that is 3.5 time its design goal. Since that last data run, further improvements to LIGO 4km in Hanford have improved the sensitivity to close to the design goal.
According to the President's Requested Budget for fiscal year 2005, the cost of the LIGO experiment for 2003, 2004, and 2005 are $33 million a year.
There is now a proposal to improve the LIGO detectors to lower the sensitivity by a factor of 10. This improved instrument is referred to as Advanced LIGO, and the cost in a FY2003 estimate is $213 million.
The VIRGO gravitational wave detector is operated through a collaboration between French and Italian scientists. The instrument is located in Cascina, near Pisa, Italy.
The Virgo instrument has arms that are 3 km in length. It uses a Fabry-Perot cavity in its design This instrument was completed in June of 2003, and it is currently collecting data as part of its commissioning.
The GEO 600 experiment is a collaboration between German and United Kingdom scientists. The experiment is located in Hannover, Germany.
The instrument has arms that are 600m long. It does not use a Fabry-Perot cavity in its design. The instrument is active, and has had two data runs, one coinciding with LIGO's S1 run, and the other coinciding with LIGO's S3 run.
The TAMA 300 is a gravitational wave detector located in Tokyo, Japan The instrument is a Fabery-Perot Michelson Interferometer with arms that are 300m in length. The purpose of the instrument is to develop technology for a future kilometer-scale instrument.
There have been 9 data runs with the instrument. The latest run, called Data Taking 9, started on November 28, 2003, and ended January 10, 2004, and overlapped the last half of LIGO's S3 run.
Design of the LCGT (Large-scale Cryogenic Gravitational Wave Telescope) is now underway. This Japanese experiment, which is a follow-on to the TAMA 300 instrument, is being constructed in the Kamioka mine. The instrument will be a Fabery-Perot Michelson Interferometer with arms of 3km in length.
AIGO (Australian International Gravitational Observatory). AIGO instrument is located at GinGin, 80km north of Perth, Australia. This instrument is currently being fitted out as a 80m device.