LIGO

LIGO is a pair of gravitational-wave observatories in the United States that use laser interferometers to measure tiny changes in the distance between suspended mirrors.

Michelson interferometer: An optical device that measures differences in light’s travel along two paths by recombining the beams. LIGO’s perpendicular arms form a Michelson interferometer whose output changes when their lengths shift differently.
GW150914: The first directly detected gravitational-wave signal, observed by LIGO in September 2015. The signal established that LIGO could detect the merger of two black holes.
Rainer Weiss: An American physicist whose interferometer designs helped establish the experimental basis for LIGO. Weiss developed key early ideas for using laser interferometry to detect gravitational waves.
KAGRA: A Japanese underground gravitational-wave detector with cryogenically cooled mirrors. KAGRA complements LIGO with an underground, cryogenic design rather than LIGO’s surface facilities.
Multi-messenger astronomy: The study of an astronomical event using multiple kinds of signals, such as light and gravitational waves. LIGO’s neutron-star detections made coordinated gravitational-wave and electromagnetic observations possible.
Fabry–Pérot interferometer: An optical cavity formed by two mirrors that makes light pass repeatedly between them. Light bounces many times in LIGO’s arms, increasing the effective distance over which phase shifts accumulate.
GW170817: A gravitational-wave signal from two merging neutron stars, detected in August 2017. LIGO’s observation triggered coordinated electromagnetic follow-up that identified the merger’s host galaxy.
Kip Thorne: An American theoretical physicist known for work on gravitation and black holes. Thorne helped develop the scientific case and organize support for building LIGO.
LISA: A planned space-based gravitational-wave observatory with three spacecraft in solar orbit. LISA will target lower-frequency waves inaccessible to LIGO’s ground-based detectors.
Nobel Prize in Physics: An annual international award recognizing major contributions to physics. The 2017 prize recognized foundational contributions to LIGO and gravitational-wave detection.

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