GEO 600 is a gravitational wave detector located near Sarstedt, Germany. 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 one part in 10−21, about the size of a single atom compared to the distance from the Sun to the Earth. GEO 600 is capable of detecting gravitational waves in the frequency range 50 Hz to 1.5 kHz.[1] Construction on the project began in 1995.[2]
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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 GEO 600. 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.
On January 15, 2009, it was reported in New Scientist that some yet unidentified noise that was present in the GEO 600 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.[3] 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.[4]
The New Scientist story states that Hogan sent his prediction of "holographic noise" to the GEO 600 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 GEO 600 motivates further study."[5] Hogan cites a 2007 talk from the GEO 600 collaboration which already mentions "mid-band 'mystery' noise", and where the noise spectra are plotted.[6] 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 GEO 600 paper submitted in October 2007 and published in May 2008.[7]
It is also a very common occurrence for gravitational wave detectors to find excess noise that is subsequently eliminated. According to Karsten Danzmann, the GEO 600 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."[3] 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.[8]
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