It looked bad for LIGO last year. Critics were grumbling that one of the most sensitive listening devices in history was, at $365 million, too expensive. LIGO (short for Laser Interferometer Gravitational Wave Observatory) is designed to hunt for gravity waves—energetic ripples that distort the shape of space. These waves could inform scientists about violent events that occurred long ago in the distant universe, such as supernovas or the merging of black holes. But gravity waves lose strength as they travel across the cosmos. A passing wave could alter Earth’s waistline by much less than the width of a single atom. To sense such tiny disturbances, scientists need extremely precise listening devices and almost perfectly quiet laboratories. After overcoming a host of challenges, scientists now say that LIGO will provide both.

Each LIGO facility houses two concrete-encased, 4-kilometer-long vacuum tubes aligned in an L-shape, as above. A laser sends a beam of light, first through a mode cleaner, which filters and balances it, then to a splitter that divides it into two identical beams. Each beam then travels down a separate tube and bounces back and forth between mirrors at the ends of the tubes. If a gravity wave arrives, one vacuum tube will stretch and the other shrink, both by less than the width of a proton, and a detector will sense that the beams have traveled different distances. Popular Science