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How did LIGO use laser interferometers to detect gravitational waves?

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gravitational lens — gravity A galactic cluster, about five billion light-years away, produces a tremendous gravitational field that “bends” light around it. This lens produces multiple copies of a blue galaxy about twice as distant. Four images are visible in a circle surrounding the lens; a fifth is visible near the center of the picture, which was taken by the Hubble Space Telescope.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) detects gravitational waves by measuring the minute distortions they cause in space-time. Each LIGO installation utilizes an L-shaped laser interferometer consisting of two 4-km-long vacuum pipes positioned at right angles to one another.123

A laser beam is split into two parts, sent down these perpendicular arms, reflected back, and then recombined. In the absence of a gravitational wave, the beams are calibrated to cancel each other out through destructive interference. When a gravitational wave passes through the detector, it causes space-time to stretch in one direction and compress in the other, effectively changing the relative lengths of the two arms. This physical displacement prevents the beams from canceling out perfectly, resulting in a detectable shift in the interference fringes.2134

To achieve the extreme precision required to measure changes as small as $10^{- 19}$ meters, the laser beams are bounced back and forth approximately 400 times, creating an effective path length of 1,600 km. Because the instrument is sensitive to various environmental disturbances, two geographically separated installations are used to confirm detections.21