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Why does a passing gravitational wave make one LIGO arm longer while the other becomes shorter?

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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.

Gravitational waves are ripples in the fabric of space-time caused by the acceleration of massive objects, as described by general relativity.12

When a gravitational wave passes through the Laser Interferometer Gravitational-Wave Observatory (LIGO), it causes a physical distortion in the geometry of space itself. Because the wave is a transverse disturbance, it stretches space in one direction while simultaneously compressing it in the perpendicular direction.2

LIGO is designed to exploit this specific effect using an L-shaped interferometer. As the wave passes, the arm oriented along the axis of the stretch increases in length, while the arm oriented along the axis of the compression decreases in length. This differential change in distance alters the travel time of the laser beams sent down each arm. When the beams are recombined, this discrepancy prevents them from canceling each other out, creating a detectable shift in the interference fringes that reveals the presence of the gravitational wave.12