LIGO

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. The detectors are interferometers, watching for otherwise-unexplained changes in distance between two lengths at right angles to each other, i.e., Michelson interferometers with arms 4 km in length. Gravitational waves (GWs) are "ripples in space itself" such that distances between two points in space change by tiny amounts far too small for us to perceive. The detectors look for effects on the light travel time over the 4 km arms, looking to sense tiny distance-changes that are 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. Each instrument also catches leftover light that would be lost, reflecting it back to increase the effective power of the laser (a power recycling mirror).

To measure the tiny distance change, other mechanisms that change the distance between mirrors are identified and discounted. These naturally include earthquakes (even distant ones), as well as shaking caused by nearby moving vehicles, etc., which the mirrors' mountings can't completely insulate against. The use of two well-separated GW detectors which should simultaneously register anything 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 to become Advanced LIGO (AdLIGO or aLIGO), increasing the sensitivity to detect events of a given wave-strength throughout a larger portion of the universe, a thousand times larger by volume, multiplying the number of events detectable by a thousand. A project in the early 2020s called A+ (Advanced plus) improved the sensitivity. An additional such project for further upgrades during the latter 2020s has been called Voyager or A# (Advanced sharp), with the intention of doubling the sensitivity again and also extend the frequency range.

In 2015-2017, LIGO detected several gravitational wave events, which analysis showed 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 the more common detections. The first GW detection (GW150914) was during Advanced LIGO's September 2015 initial testing, and the second was in December 2015.

LIGO's time alternates between considerable periods of maintenance/upgrades versus periods of observation, the latter periods generally lasting several months (or even years) each. The earliest six observation periods (which were before 2015), termed Science Run 1 through 6, had no detections. With the upgrade to Advanced LIGO, the periods of observation have been labeled Observing Runs:

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 Virgo's upgrades and the subsequent completion of KAGRA, the observation runs of all three are being coordinated so as to have all the three or four detectors active simultaneously because that provides very useful additional data for the detected events (including additional chance of confirming a particular detection through at least two of the detectors recording analogous signals).

LIGO's most recent run, O4 began on May 24, 2023, with some of the intended A+ upgrades that gave it a sensitivity increase on the order of 30%. After a scheduled maintenance outage in early 2024, it resumed official operation on April 10, 2024 (with Virgo, as O4b, with the earlier part of O4 termed O4a), running until the end of March 2025. After another round of maintenance, they both resumed operation (with KAGRA as well) on June 11, 2025, as O4c, which ended on November 18, 2025. O4 produced roughly 250 candidates.

O5 is planned to begin in 2027 or 2028 after the next round of major upgrades.