LIGO is a gravitational wave detector that measures spacetime vibrations from cosmic events like black hole mergers.LIGO is a giant ruler that measures how gravitational waves stretch and squeeze space. Two long arms of a laser beam are compared: if a wave passes, one arm becomes slightly longer than the other.
Attempts to detect gravitational waves began in the 1960s with Joseph Weber's solid-state resonators. In the 1970s, the idea of a laser interferometer was proposed by Rainer Weiss and further developed with the participation of Kip Thorne and Ronald Drever. Key funding for the project was obtained in the 1990s, construction was completed in the early 2000s. The first direct detection — event GW150914 — occurred on September 14, 2015.
In each arm, the laser beam is reflected many times (about 280 times) between mirrors, effectively increasing the length to 1120 km and enhancing sensitivity. The combined beam is sent to a photodetector. The gravitational wave, by changing the proper length of the arms in opposite directions according to the metric h = ΔL/L, modulates the brightness of the interference spot. The noise reduction system (seismic, thermal, shot) allows measuring relative strains on the order of 10⁻²¹.
LIGO's sensitivity is such that it detects length changes ten thousand times smaller than the size of a proton — like measuring the distance to the nearest star with the accuracy of a human hair's thickness.















