In 1915, Albert Einstein portrayed a completely new picture of our world in his general theory of relativity: In contrast to Newton, gravitation is not a force, but a consequence of the geometry of space and time: Big masses such as stars and galaxies deform space-time around them. If other objects move through such areas, they are diverted from their original path, apparently attracted by the big mass. According to Einstein, what in fact happens is that the objects just follow the path mapped out for them by the deformation of space-time around the big mass. Moving masses give rise to pertubations in the space-time continuum that propagate in all directions with the velocity of light. These moving space-time disturbances are called gravitational waves. They alternately stretch and compress space - so that the distances between the objects in space are changed.
However, these changes in distance are tiny: even the gravitational wave produced by a powerful event in our vicinity, like a supernova explosion within the Milky Way, changes the total distance between Earth and Sun only by about the diameter of a hydrogen atom - and that merely for several thousandths of a second. For shorter distances the effect is correspondingly smaller: when measuring over a distance of only one kilometer a change of a thousandths of the diameter of a proton has to be detected to determine the passing of a gravitational wave. This is the effect GEO600 will measure. The great challenge is to get rid of the many disturbances, like air pressure and temperature fluctuations as well as seismic vibrations of all sorts, that would conceal a signal.