LIGO uses laser interferometry to detect gravitational waves. A laser beam is split into two beams that travel back and forth along two perpendicular arms of the interferometer, each 4 kilometers long. The beams are then recombined, and any difference in their travel lengths, caused by the stretching and squeezing of spacetime, indicates the presence of a gravitational wave. The changes in distance detected by LIGO are incredibly small, on the order of 10^-18 meters, which is less than one-thousandth the diameter of a proton.
