LIGO (or Laser Interferometer Gravitational-wave Observatory) is a pair of gravitational wave observatories that work in unison, in Livingston, Louisiana (LIGO L or LLO for LIGO Livingston Observatory) Hanford, Washington (LIGO H or LHO for LIGO Hanford Observatory), 3002 kilometers apart. They are interferometers, looking for unexplained changes in distance between two lengths at right angles, i.e., Michelson interferometers, with arms 4km in length. The delta distance they aim to detect is on the order of 10-21, which less than the width of an atomic nucleus. Other sources of distance change such as ground movements like earthquakes, are filtered out by detecting them independently, and by the use of two separated detectors which should both register something affecting the whole Earth at once, i.e., an actual gravitational wave. The use of two detectors also allows some determination of the direction from which the wave came. The two operated from 2002 to 2010, shutting down for a planned four year upgrade as Advanced LIGO (AdLIGO) which aims at a sensitivity that covers a thousand times the volume for a given wave-strength, thus can detect a thousand times the number of events, or see something a thousand times more often. Further plans: a 2020 upgrade called A+ to double sensitivity, and a 2027 upgrade named Voyager to double the sensitivity again and also extend the frequency range.
As of 2018, LIGO has detected several gravitational wave events, which analysis has shown are generally from black hole mergers, taking place with a redshift in the .1-.2 general range. It had been previously thought that neutron star merger detections would be far more common. The first detection (GW150914) was during Advanced LIGO's initial testing, and the second was in the Spring of 2016.
LIGO's time alternates between periods of maintenance/upgrades versus periods of observation , the latter periods generally lasting last several months each. The first six of these were termed Science Run 1 through 6 and had no detections. With the upgrade to Advanced LIGO, they have been termed Observation Run 1 (or Observing Run 1, O1, 9/12/2015-1/19/2016, with two detections) and Observation Run 2 (O2, 11/30/2016 to 8/25/2017, with four detections), with Observation Run 3 (O3) planned to commence after additional upgrades, beginning early 2019. With to Virgo's upgrades, runs between LIGO and Virgo will be coordinated because of the increased science possible if all three detectors detect an event.