The Livingston Observatory, located in Livingston, Louisiana facility houses a laser interferometer, consisting of mirrors suspended at each of the corners of a gigantic L-shaped vacuum system, measuring 4 kilometers (2.5 miles) on each side. Precision laser beams in the interferometer will sense small motions of the mirrors, which are caused by gravitational waves.
The Handord Observatory, located near Richland, Washington on the Hanford Nuclear Reservations, houses a nominally identical laser interferometer as the Livingston Observatory. Also, there is a smaller, though no less complicated, interferometer housed in parallel. This second detector is half the length (2 kilometers), and thus half as sensitive.
The theoretical gravitational waves that originate hundreds of millions of light years from Earth are expected to distort the 4 kilometer mirror spacing by about 10-18 m (a hydrogen atom is about 5×10-11 m). These waves were first predicted by Einstein's Theory of General Relativity in 1916, when the technology necessary for their detection did not yet exist. Now, at the turn of the 21st century, we believe technology has reached the point where detection of gravitational waves is possible.