Ligo (Laser Interferometer Gravitational-Wave Observatory，激光干涉仪引力波探测器) observatories operate by beaming a high power laser into a splitter that divides the beam into two parts. Each part is then directed towards two 4km tunnels perpendicular to each other.
A mirror at the end of the tunnels reflects the rays back into a detector where they are recombined.
Since both tunnels are equally long, when the two halves meet in the detector the signal shows no pattern. But this is not the case if a gravitational wave were passing through the Earth.
When [the gravitational waves] reach Earth they distort space and time. In particular, they will change the separation of the mirrors.