That is where two giant facilities in Louisiana and Washington state come in, operating together as the Laser Interferometer Gravitational-Wave Observatory (LIGO). At each of the facilities, a laser shoots a 35-watt infrared beam through a Faraday isolator, which directs and polarizes the light. The beam then enters a vacuum chamber where a partially-silvered mirror splits it in two, sending each half down one of the L-shaped system’s 2.5-mile-long arms. The beams bounce around a system of mirrors about 75 times before returning to the splitter. Normally, the split light waves fall out of sync on their journeys and so cancel each other out when recombined. When a gravitational wave moves through the detector, though, it should stretch one arm of LIGO while shortening the other, changing the path of the beams and causing the rejoined waves to produce a detectable pattern of light.
Since 2005 LIGO has tuned in to a small portion of the universe, to no avail. But last October the experiment was suspended for a major upgrade, including a new laser 20 times as powerful as the current model. When it goes back online in 2014, LIGO will be able to scan a swath of sky 1,000 times bigger. “Instead of having a few percent likelihood of seeing a signal in a year, we should see a signal once a week or once a month,” says Caltech physicist Jay Marx, LIGO’s executive director.