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GEO600 is a laser-interferometric gravitational wave detector located near Sarstedt in the South of Hanover, Germany. This instrument, and its sister interferometric detectors, 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&minus;21, about the size of a single atom compared to the distance from the Sun to the Earth. GEO600 is capable of detecting gravitational waves in the frequency range 50 Hz to 1.5 kHz. Construction on the project began in 1995.

Hardware
GEO600 is a Michelson interferometer. It consists of two 600 meter long arms, which the laser beam passes twice, so that the effective optical arm length is 1200 m. The major optical components are located in an ultra-high vacuum system. The residual pressure is in the range of 10&minus;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 cm and a height of 10 cm. The beam splitter (with dimensions of 26 cm diameter and 8 cm 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/cm.

Advanced features
GEO600 uses many advanced techniques and hardware that are planned to be used in the next generation of ground based gravitational wave detectors:


 * Monolithic suspensions: The mirrors are suspended as pendulums. While steel wires are used for secondary mirrors, GEO’s main mirrors are hanging from so called ‘monolithic’ suspensions. This means that the wires are made from the same material as the mirror: fused silica. The reason is that fused silica has less mechanical losses, and losses lead to noise.


 * Electrostatic drives: Actuators are needed to keep the mirrors in their position and to align them. Secondary mirrors of GEO600 have magnets glued to them for this purpose. They can then be moved by coils. Since gluing magnets to mirrors will increase mechanical losses, the main mirrors of GEO600 use electrostatic drives (ESDs). The ESDs are a comb-like structure of electrodes at the back side of the mirror. If a voltage is applied to the electrodes, they produce an inhomogeneous electric field. The mirror will feel a force in this field.


 * Thermal mirror actuation system: A system of heaters is sitting at the far east mirror. When heated, a thermal gradient will appear in the mirror, and its radius of curvature of the mirror changes due to thermal expansion. The heaters allow thermal tuning of the mirror’s radius of curvature.


 * Signal recycling: An additional mirror at the output of the interferometer forms a resonant cavity together with the end mirrors and thus increases a potential signal.


 * Homodyne detection (also called ‘DC readout’)


 * Output Mode Cleaner (OMC): An additional cavity at the output of the interferometer in front of the photodiode. Its purpose is to filter out light that does not potentially carry a gravitational wave signal.


 * Squeezing: Squeezed vacuum is injected into the dark port of the beam splitter. The use of squeezing can improve the sensitivity of GEO600 above 700 Hz by a factor of 1.5.

A further difference to other projects is that GEO600 has no arm cavities.

Sensitivity and measurements
The sensitivity for gravitational wave strain is usually measured in amplitude spectral density (ASD). The peak sensitivity of GEO600 in this unit is 2×10&minus;22 1/√Hz at 600 Hz. At high frequencies the sensitivity is limited by the available laser power. At the low frequency end, the sensitivity of GEO600 is limited by seismic ground motion.

Data/ Einstein@home
Not only the output of the main photodiode is registered, but also the output of a number of secondary sensors, for example photodiodes that measure auxiliary laser beams, microphones, seismometers, accelerometers, magnetometers and the performance of all the control circuits. These secondary sensors are important for diagnosis and to detect environmental influences on the interferometer output. The data stream is partly analyzed by the distributed computing project ‘Einstein@home’, software that volunteers can run on their computer.

From September 2011, both VIRGO and the LIGO detectors will be shut down for upgrades, leaving GEO600 as the only operating large scale laser interferometer searching for gravitational waves.

History and current operations
The original proposal for the GEO gravitational wave detector included 3 kilometer long arms. The construction of a down-scaled 600 meter long version started with the groundbreaking in 1995. Subsequently, the vacuum system and the optical components were installed.

GEO600 was operational for the first time in .... in the configuration of a power-recycled Michelson interferometer. Since ...., the signal-recycling cavity was added, making GEO600 a dual-recycled Interferometer.

In the following years, GEO600 took part in several extended data-taking ('science') runs, often joint with other laser-interferometric gravitational wave detectors. The time between those science runs would be used for upgrades and maintenance of the detector.

In November 2005, it was announced that the LIGO and GEO instruments have begun an extended joint science run. The three instruments (LIGO's instruments are located near Livingston, Louisiana and on the Hanford Site, Washington in the U.S.) will collect data for more than a year, with breaks for tuning and updates. This will be the fifth science run of GEO600. No signals were detected on previous runs, but the sensitivity of the instruments (and the quality of the data analysis) is continually improving, and once the data from the current run are analyzed, it is hoped that they will perhaps reveal the arrival at Earth of two unambiguous bursts of gravitational waves. This would constitute the first direct detection of gravitational radiation.

Claimed link between GEO600 detector noise and holographic properties of spacetime
On January 15, 2009, it was reported in New Scientist that some yet unidentified noise that was present in the GEO600 detector measurements might be because the instrument is sensitive to extremely small quantum fluctuations of space-time affecting the positions of parts of the detector. This claim was made by Craig Hogan, a scientist from Fermilab, on the basis of his own theory of how such fluctuations should occur motivated by the holographic principle.

The New Scientist story states that Hogan sent his prediction of "holographic noise" to the GEO600 collaboration in June 2008, and subsequently received a plot of the excess noise which "looked exactly the same as my prediction". However, Hogan knew before that time that the experiment was finding excess noise. Hogan's article published in Physical Review D in May 2008 states: "The approximate agreement of predicted holographic noise with otherwise unexplained noise in GEO600 motivates further study." Hogan cites a 2007 talk from the GEO600 collaboration which already mentions "mid-band 'mystery' noise", and where the noise spectra are plotted. A similar remark was made ("In the region between 100 Hz and 500 Hz a discrepancy between the uncorrelated sum of all noise projections and the actual observed sensitivity is found.") in a GEO600 paper submitted in October 2007 and published in May 2008.

It is also a very common occurrence for gravitational wave detectors to find excess noise that is subsequently eliminated. According to Karsten Danzmann, the GEO600 principal investigator, "The daily business of improving the sensitivity of these experiments always throws up some excess noise (...). We work to identify its cause, get rid of it and tackle the next source of excess noise." Additionally, some new estimates of the level of holographic noise in interferometry show that it must be much smaller in magnitude than was claimed by Hogan.