Talk about the middle of nowhere. After landing at the tiny five-gate airport in Pasco, Wash., a visitor has to travel another 20 km by car past brown-gray desert scrub to get to LIGO’s northern outpost. This also happens to be part of the now-desolate Hanford Reservation, famous for its pivotal role in the Manhattan Project, which built the first atomic bomb [see “The Atomic Fortress That Time Forgot,” IEEE Spectrum, April 2005]. It’s a challenge following roads whose intersections are unmarked (a remnant of World War II secrecy), especially in a winter fog so thick that only three dashed lines are visible ahead on the wet asphalt. Finally, at a lone access road, a modest steel sign proclaims in black letters, “LIGO: Hanford Observatory.”

LIGO-Hanford, like its L‑shaped twin in Louisiana, is based on a century-old and fundamentally simple instrument for precisely measuring length. Day and night, 16 000 times a second, LIGO repeatedly compares its two 4-km-long perpendicular arms by measuring the time it takes laser light to travel their lengths. If a gravitational wave passed through Earth, it would momentarily distort space, and scientists are betting it would also distort the lengths of LIGO’s arms in a characteristic pattern.

Even to hope to capture such a subtle phenomenon, though, the scientists and engineers who built LIGO had to tackle two key problems: how to measure that minute distortion and how to reject any other noise coming from terrestrial sources that might mimic or mask the distortion.

The idea of space “distorting,” or having any kind of structure at all, may be foreign to nonphysicists, but physicists talk about the fabric of space and time—and they mean “fabric” almost in the sense of cloth. Just as a smooth piece of cloth is actually made of vertical and horizontal threads woven in a grid, the three-dimensional vacuum of outer space can be envisioned as a grid that extends in all three dimensions. Any points in space, such as the two ends and the apex of the L-shaped LIGO instrument, can be thought of as fixed to three specific locations on that grid.

So what happens when a giant star explodes? Any mass curves the fabric of space and time, stretching the grid—the more massive the object, the greater the curvature. If a mass accelerates, such as when the supernova blasts away most of the star’s matter, the abrupt change in the gravitational field distorts that fabric, and the distortion radiates outward through the universe as a gravitational wave. It’s comparable to the wave you see quickly rippling outward from your hands when you snap a bedsheet.

As the wave passes through it, any pattern on that sheet—say, the capital letter L—is momentarily distorted as well. The two ends and the apex of the L haven’t moved their positions with respect to the threads of the fabric. But the fabric itself has been momentarily warped. In an analogous way, a passing gravitational wave stretches and compresses the fabric of space—stretching and compressing the grid of space itself. In effect, a gravitational wave should momentarily change the lengths of LIGO’s two arms without altering the positions of the arms on Earth.

A gravitational wave coming from directly above LIGO would have a characteristic pattern: first lengthening the X arm while compressing the Y arm, then compressing the X arm while stretching the Y arm [see diagram, “Seeing the Light”]. LIGO is looking for length displacements that oscillate back and forth at frequencies between about 50 and 2000 hertz, within the audio range of a piano. “If your ears were sensitive to gravitational waves, you could literally hear them,” notes LIGO beam tube designer Rainer Weiss, professor emeritus of physics at the Massachusetts Institute of Technology, in Cambridge. The wave’s duration—from seconds to minutes to months—would depend on the type of astronomical event causing it.

Physicists and astronomers hope LIGO is sensitive enough not only to detect the presence of a gravitational wave but also to measure its intensity, duration, and frequency, along with any changes in those characteristics over time. Because as surely as DNA can identify a culprit, gravitational waves emitted by a supernova are expected to look very different from those given off by a black hole swallowing a nearby star, by two galaxies ripping each other apart, or by other acts of astronomical carnage.

Having two LIGO sites instead of just one allows scientists to corroborate observations and reject any spurious measurements that may just mimic gravitational waves. Moreover, if a real gravitational wave should be detected at both sites, the minuscule difference in when the observations occurred and in the patterns of oscillation could give a rough idea of where the waves came from, much as two ears can home in on a cricket’s chirping in the night.

Throughout LIGO-Hanford and LIGO-Livingston, sensors monitor thousands of signals indicating the position, temperature, motion, noise, and other characteristics of every optical, mechanical, and electronic component in the observatory. All that information is funneled into the control room at each site, where it is displayed as brightly colored blinking images on dozens of computer monitors. Encompassing the Hanford control room with a sweep of his arm, LIGO-Hanford director Frederick J. Raab summarizes: “Each LIGO interferometer has only one channel you care about”—namely, the photodetector, where a glimmer of light might signal a gravitational wave—“and several thousand other channels to determine whether you should believe that single channel.”