LIGO (Laser Interferometer Gravitational-Wave Observatory)

LIGO uses laser interferometers to measure minute changes in the relative lengths of two perpendicular arms. A passing gravitational wave changes the spacetime distance along the arms, altering the interference pattern of laser light. Such measurements are exceptionally sensitive to environmental disturbances, so the detectors require extensive isolation, monitoring, and analysis to distinguish candidate signals from noise.

The two LIGO interferometers are at Hanford Observatory in Washington and Livingston Observatory in Louisiana, about 3,000 kilometres apart. Comparing data from distant sites helps scientists test whether a signal is consistent with a gravitational wave passing Earth and helps constrain its direction. Having separated detectors also reduces the chance that local disturbances at one site will be mistaken for an astrophysical signal.

On 14 September 2015, the two detectors recorded GW150914, a signal attributed to the merger of two black holes. The LIGO Scientific Collaboration and Virgo Collaboration reported the observation in 2016. It was the first direct observation of gravitational waves and the first detection of a binary black-hole merger. The observation provided a new way to test gravity and investigate events that may be difficult to study through light alone.

LIGO is a collaboration involving scientists and institutions, with the observatories funded by the U.S. National Science Foundation and operated by Caltech and MIT. The 2017 Nobel Prize in Physics recognized Rainer Weiss, Barry C. Barish, and Kip S. Thorne for decisive contributions to the LIGO detector and the observation of gravitational waves. LIGO participates in a broader international network of gravitational-wave observatories and collaborations.