DRIVING through the countryside south of Hanover, it would be easy to miss the GEO600 experiment. From the outside, it doesn't look much: in the corner of a field stands an assortment of boxy temporary buildings, from which two long trenches emerge, at a right angle to each other, covered with corrugated iron. Underneath the metal sheets, however, lies a detector that stretches for 600 metres. For the past seven years, this German set-up has been looking for gravitational waves - ripples in space-time thrown off by super-dense astronomical objects such as neutron stars and black holes. GEO600 has not...