LIGO Hanford Observatory

Einstein's general theory of relativity, which describes gravitation, predicts the existence of “gravitational waves,” ripples in space-time triggered by moving masses. However, only under extraordinary circumstances, involving enormous masses moving at very high speeds, might gravitational waves be potentially detectable.

This is the first instrument (one of two run in tandem, the other being in Louisiana) to actually detect gravitional waves. LIGO stands for Laser Interferometer Gravitational-Wave Observatory, and that summarizes the technique: laser light is split into two perpendicular beams, which are sent out along a 2.5 mile (4 kilometer) vacuum line, where they are reflected back and then recombined. The passage of a gravitational wave induces very slight changes in the lengths of the paths followed by the two beams. When the beams are recombined, they no longer match up exactly (are "out of phase"), and the degree of mismatch is proportional to the distortion.

The changes are on the order of one-hundred-millionth the diameter of a hydrogen atom. This vanishingly small effect is the reason gravitational waves have eluded detection for so long. Much of the LIGO Exploration Center, the public museum and interpretive center at the site, describes the extraordinary engineering needed to achieve such unprecedented precision.

Detailed graphs on the sensitivity of the detector, related to such things as seismic noise, are also on display. Although the detector is supposed to be decoupled from ground motion, a earthquake of sufficient magnitude can “unlock” the detector, making measurements impossible for minutes to hours. Probably the most astonishing thing, though, is that the decoupling usually works.

The Exploration Center also includes many hands-on exhibits on waves. Wave behavior is high-school physics, and so the principles behind LIGO are simple in conception. What’s extraordinary is the precision of the measurements.

The first detection of gravity waves was on September 14, 2015, at both LIGO observatories. (Detecting a signal at both observatories is a further check that the signal is real.) This event resulted from the merger of two black holes, of 36 and 29 solar masses, to yield a final mass of 62 solar masses. In the merger, three solar masses were converted to energy and radiated as gravitational waves, a staggering amount of energy. But since the merger event was about 1.3 billion light-years away, the energy was dispersed to the point of near-indetectability.

Since that first detection, new events continue to be observed routinely, and the 2017 Nobel Prize in Physics went to researchers associated with the observatory.