Figure 3: Gravitational waves on the ground
The LIGO Hanford Observatory in Washington state, US, is one of several ground-based interferometers that may soon make the first direct detection of gravitational waves. The instrument is a Michelson interferometer, which measures tiny changes to the lengths of its two perpendicular 4 km-long arms by bouncing a powerful laser off two large mirrors at the end of each arm. This design was chosen because the displacement of masses caused by a passing gravitational wave has a transverse and quadrupolar character: the wave alternatively stretches and compresses in perpendicular directions transverse to the direction of propagation. The longer the interferometer arms, the more sensitive the instrument is to such displacements. Source: LIGO Laboratory.