Figure 12-36Laser Interferometer Gravitational Wave Observatory (LIGO) Located in Hanford, Washington, this is one of several gravitational wave (colloquially, gravity wave) detectors around the world. It has two perpendicular arms, each 4 km (2.5 mi) long. Gravity waves that pass the detector cause unequal changes in the lengths of the arms. These changes are detected by lasers inside each arm.