LIGO (or Laser Interferometer Gravitational-wave Observatory) is a pair of gravitational-wave detectors that work in unison, located in Livingston, Louisiana (LIGO L or LLO for LIGO Livingston Observatory) and Hanford, Washington (LIGO H or LHO for LIGO Hanford Observatory), 3002 kilometers separated. They are interferometers, watching for unexplained changes in distance between two lengths at right angles to each other, i.e., Michelson interferometers with arms 4 km in length. The tiny fraction of the light-travel distance represented by the change ratio they aim to detect is on the order of 10-21, a distance change less than the width of a proton. By nature, the instrument is most sensitive for a wave whose wave-period is twice the interferometer's light path's time of travel, and LIGO uses a Fabry-Pérot cavity, reflecting the light multiple times over the 4 km to put it in the most sensitive regime. It also catches the leftover light that would be lost, reflecting it back to increase the effective power of the laser (a power recycling mirror).
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 pair of detectors also allows some determination of the direction from which the wave came. The pair operated from 2002 to 2010, shutting down for a planned four year upgrade as Advanced LIGO (AdLIGO or aLIGO) which aims at a sensitivity that detects events of a given wave-strength throughout a larger portion of the universe, a thousand times larger by volume, thus can detect a thousand times the number of events or detect events a thousand times more often. Further plans include 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.
In 2015-2017, LIGO has detected several gravitational wave events, which analysis has shown are generally from black hole mergers, taking place with a redshift in the 0.1-to-0.2 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, the periods of observation have been labeled Observing Runs:
|O3||4/1/2019-active||53 candidates as of 2/2020|
During the O1/O2 time frame, six candidate-detections were accepted as real, but since then, five more from that time period have been accepted as well. With to Virgo's upgrades, LIGO and Virgo runs are being coordinated so as to have the three detectors active simultaneously because that provides very useful additional data for the detected events.