View from VIRGO; August 2003; Scientific American Magazine; by Alexander Hellemans; 2 Page(s)

High on any astrophysicist's wish list is the detection of gravitational waves, ripples of spacetime caused by such violent phenomena as supernova and merging black holes. Researchers are pinning their hopes on kilometer-long detectors. The world's biggest, the $371-million Laser Interferometer Gravitational Observatory (LIGO), began taking data last year. This past July a French-Italian collaboration inaugurated VIRGO, which, though second fiddle in size to LIGO, may be in a better position to register the tiny, elusive wrinkles. And costing about 75 million euros (roughly $87 million), it is substantially cheaper.

Like LIGO, VIRGO is a so-called Michelson interferometer: light from a laser passes a beam splitter and travels down two perpendicular evacuated pipes. The beams are reflected back by mirrors at the end of the pipes and "interfere" with each other. Specifically, they recombine destructively-that is, the waves cancel each other out. Any slight change in arrival time (phase) gives itself away as a faint beam that can be detected by an optical sensor.