Gravitational waves interact with spacetime by compressing it in one direction while stretching it in the perpendicular direction. Detecting gravitational waves requires measuring the length of the laser to precise measurements, equivalent to measuring the distance to the nearest star, around four light years away, down to the width of a human hair.
Measuring Gravitational Waves
Gravitational waves are detected by measuring the change in distance between two points, which is on the order of one part in 1021. The Laser Interferometer Gravitational-Wave Observatory (LIGO) uses two 4-kilometer-long perpendicular arms in an L-shaped configuration to measure these tiny changes. Mirrors are suspended at the ends of the arms, and laser light is split and travels down each arm, bouncing off the mirrors, and then recombined.
Laser Precision
When a gravitational wave passes through the interferometer, it will alternately stretch and squeeze the space between the mirrors, causing the lengths of the two arms to change by a tiny, but measurable, amount. This changes the relative phase of the laser light in each arm, which can be detected when the light is recombined. The laser light must be stabilized to one part in 1022 to detect these minute changes.