Get the feel of a gravitational wave detector

AEI at the “Highlights of Physics” from 15 to 18 September 2008 in Halle

Albert Einstein already predicted the existence of gravitational waves as early as 1916 as a consequence of his general theory of relativity. He was firmly convinced that these minute changes in space-time would never be able to be measured. He was probably wrong in this regard, as the international scientific community has developed highly sensitive gravitational wave detectors over the last decades that can be used to measure the miniscule changes in length that occur during the passing through of a gravitational wave. One of these revolutionary projects, the German-British gravitational wave detector GEO600, is located near Hannover and is operated by the scientists of AEI as well as their colleagues from Cardiff and Glasgow.

The direct measurement of gravitational waves would allow us to gain completely new insights into the universe, because for the first time we would be able to “see” parts thereof that have previously been inaccessible. Because we always look back into the past with astronomical methods, we would then be able to look back into the first moments of the universe and to better understand how it came about. Using the observation methods now available, astronomers can see up to around 380,000 years after the Big Bang occurred. This is the point at which the universe became transparent for electromagnetic radiation, e.g. X-ray, gamma or infrared radiation. Gravitational wave astronomy is therefore the perfect complement to existing astronomical observation methods. Research into this area is currently being undertaken on a worldwide basis.

Background information

Measurement of gravitational waves

The best method for detecting the miniscule compressions and expansions of space-time as gravitational waves pass through has, over the past decades, proven to be laser interferometry. The L-shaped Michelson interferometer uses a laser, the light of which is divided into two beams. These laser beams travel back and forth between two mirrors in an evacuated tube. The distance between the precisely positioned mirrors is measured using the laser beams. If a gravitational wave crosses the detector, it changes the length of the measuring path and thus the signal at the output of the detector.

If a violent cosmic event takes place in the Milky Way or a neighbouring galaxy, then the effect on the Earth in terms of gravitational waves only produces relative changes with the length of at most 10^-18 (one trillionth), typically even 10^-21 (one sextillionth), i.e. a distance of one kilometre only changes by one thousandth of the diameter of a proton. This illustrates the challenge that direct gravitational-wave detection is confronted with.

GEO600 and the most advanced lasers in the world

Together with British colleagues from Glasgow and Cardiff, the scientists of the AEI operate the gravitational wave detector GEO600 in Ruthe near Hannover. Thanks to its innovative and reliable technologies, the GEO project enjoys an excellent reputation worldwide and is regarded as a think tank for international gravitational wave research. This is where laser technology is perfected: the GEO600 system features extremely stable lasers with an output power that is three times stronger than previously available. The GEO lasers are now used in the US LIGO detectors, as well as in the French-Italian Virgo project. If the scientists of the Japanese LCGT project (Large Cryogenic Gravitational Wave Telescope) also opt for this system, then all gravitational wave detectors worldwide would be using GEO lasers. With the GEO600 measurement accuracy, neutron stars can be observed up to a distance of 40 million light years from the Earth. The accessible part of our universe contains several dozens of galaxies in the neighbourhood of our Milky Way.

LISA, the gravitational-wave observatory in outer space

LISA (Laser Interferometer Space Antenna) is the pioneering project of gravitational wave research, a detector in outer space. LISA will consist of three identical satellites that fly in a permanent formation along with Earth's orbit. The distance from the earth will be about 50 million kilometres. The LISA satellites will form the vertices of an isosceles triangle, the sides of which are formed by the 5 million-kilometre-long laser arms of the interferometer. The launch of the LISA mission is planned for 2018. LISA should be able to detect gravitational waves of supermassive black holes throughout the entire universe, and perhaps even "hear" the gravitational waves produced by the Big Bang – certainly the most spectacular source of gravitational waves. Its measurements could provide information about the birth of the universe. First, the ESA mission LISA Pathfinder will test the key technologies of the measurement and control systems in space - the launch of this mission is scheduled for early 2010. At the AEI in Hannover the new optical measurement technology is being developed and tested in the very heart of the satellite.