LIGO gravitational wave detector. Aerial photograph of the Hanford detector site for the Laser Interferometer Gravitational-Wave Observatory (LIGO). LIGO compares measurements between two detector sites 3000 kilometres apart, one near Hanford, Washington, USA, and the other near Livingston, Louisiana, USA. Each site is an L-shaped ultra-high vacuum system, four kilometres long on each side. Laser interferometers are used to look for small changes caused by gravitational waves. LIGO has been operating since 2002, with an advanced upgrade (aLIGO) operating since 2015. Photographed on 2 May 2008.