GEO600 is a gravitational wave detector located near Sarstedt, a town 20 kilometres (12 mi) to the south of Hanover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics, Max Planck Institute of Quantum Optics and the Leibniz Universität Hannover, along with University of Glasgow, University of Birmingham and Cardiff University in the United Kingdom, and is funded by the Max Planck Society and the Science and Technology Facilities Council (STFC). GEO600 is capable of detecting gravitational waves in the frequency range 50 Hz to 1.5 kHz, and is part of a worldwide network of gravitational wave detectors. This instrument, and its sister interferometric detectors, when operational, are some of the most sensitive gravitational wave detectors ever designed. They are designed to detect relative changes in distance of the order of 10−21, about the size of a single atom compared to the distance from the Sun to the Earth. Construction on the project began in 1995. In March 2020 the COVID-19 pandemic forced the suspension of operation of other gravitational wave observatories such as LIGO and Virgo (and in April 2020, KAGRA), but GEO600 continued operations. GEO600 will shut down and permanently cease operations on 31 December 2026.
History In the 1970s, two groups in Europe, one led by Heinz Billing in Germany and one led by Ronald Drever in UK, initiated investigations into laser-interferometric gravitational wave detection. In 1975 the Max Planck Institute for Astrophysics in Munich started with a prototype of 3-metre (9.8 ft) armlength, which led to a prototype with 30-metre (98 ft) armlength at the Max Planck Institute of Quantum Optics (MPQ) in Garching in 1983. In 1977 the Department of Physics and Astronomy of the University of Glasgow began similar investigations, and in 1980 started operation of a 10-metre (33 ft) prototype. In 1985 the Garching group proposed the construction of a large detector with 3-kilometre (2 mi) armlength, the British group an equivalent project in 1986. The two groups combined their efforts in 1989 – the project GEO was born, with the Harz mountains in northern Germany considered an ideal site. The project was, however, not funded, because of financial problems. Thus in 1994 a smaller detector was proposed: GEO600, to be built in the lowlands near Hannover, with arms of 600 metres (2,000 ft) in length. The construction of this British-German gravitational wave detector started in September 1995. In 2001 the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Potsdam took over the Hannover branch of the MPQ, and since 2002 the detector is operated by a joint Center of Gravitational Physics of AEI and Leibniz Universität Hannover, together with the universities of Glasgow and Cardiff. Since 2002 GEO600 participated in several data runs in coincidence with the LIGO detectors. In 2006, GEO600 has reached the design sensitivity, but up to now no signal has been detected. The next aim is to reduce the remaining noise by another factor of about 10, until 2016. In March 2026, it was announced that GEO600 would cease operations on 31 December that year, following the retirement of AEI director Karsten Danzmann and a subsequent termination of funding for the project.
Hardware GEO600 is a Michelson interferometer. It consists of two 600-metre-long (2,000 ft) arms, which the laser beam passes twice, so that the effective optical arm length is 1,200 metres (3,900 ft). The major optical components are located in an ultra-high vacuum system, with a pressure of less than 10−8 mbar.
Suspensions and seismic isolation For precise measurements, the optics must be isolated from ground motion and other influences from the environment. For this reason, all ground-based interferometric gravitational wave detectors suspend their mirrors as multi-stage pendulums. For frequencies above the pendulum resonance frequency, pendulums provide a good isolation against vibrations. All the main optics of GEO600 are suspended as triple pendulums, to isolate the mirrors from vibrations in the horizontal plane. The uppermost and the intermediate mass are hung from cantilever springs, which provide isolation against vertical movement. On the uppermost mass are six coil-magnet actuators that are used to actively dampen the pendulums. Furthermore, the whole suspension cage sits on piezo crystals. The crystals are used for an 'active seismic isolation system'. It moves the whole suspension in the opposite direction of the ground motion, so that ground motion is cancelled.
Optics The main mirrors of GEO600 are cylinders of fused silica with a diameter of 18 centimetres (7.1 in) and a height of 10 centimetres (3.9 in). The beam splitter, with dimensions of 26 centimetres (10 in) diameter and 8 centimetres (3.1 in) thickness, is the only transmissive piece of optics in the high power path, therefore it was made from special grade fused silica. Its absorption has been measured to be smaller than 0.25 ppm per 1 centimetre (0.39 in).
Advanced GEO600 uses many advanced techniques and hardware that are planned to be used in the next generation of ground based gravitational wave detectors:
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