An Interferometric Gravitational Wave Detector
GEO600 measures the tiny distorsions in space-time by means of a laser interferometer. A semi-transparent mirror divides the incident laser beam. Both partial beams are sent into the 600 m long vacuum tubes which are at right angles, thus forming the interferometer arms of GEO600. The beams are reflected at the other end, run back and then are recombined at a photo detector. The lengths of the arms are chosen in such a way that the returning light waves are oscillating in the opposite sense so that they cancel each other out during normal operation: the output is dark. If a gravitational wave changes the lengths of the arms, the partial beams lose the beat and they no longer cancel out: at the output a signal will be seen.