Best Ears Ever Built

Ready to scan the cosmos for gravity waves.

Illustration by Stephen Rountree
WAVE CATCHER
Two identical LIGO labs have been built 2,500 miles apart so scientists can compare readings and be sure they aren't picking up local noise. 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.
Illustration by Stephen Rountree

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.

Funded in 1990 by the National Science Foundation, LIGO consists of two facilities, in Hanford, Washington, and Livingston, Louisiana. Last year, the Louisiana facility got off to a rocky start. Noise from lumber operations and nearby traffic proved too strong for LIGO's shock absorbers, forcing experiments to a halt. But now LIGO scientists report that these problems have been overcome. Although still in the preparatory stage, LIGO is already the most precise instrument of its kind. "Even if we detect nothing," says LIGO scientist David Shoemaker, "we'll detect nothing at a more sensitive level than it's been detected before."

Here's how it works: Each facility houses two huge L-shaped instruments (see diagram). When a gravity wave arrives, the paths of light beams within the instruments change ever so slightly. If both the Washington and Louisiana labs detect the same minute changes, LIGO's operators know it's a gravity wave and not the sound of a falling pine tree or a rumbling semi-truck.

By studying the precise nature of what's detected, scientists can deduce the origin of the gravity wave--whether from merging black holes, for instance, or spinning neutron stars. Ultimately, the lab should be capable of sensing ripples from upheavals nearly 1 billion years old.