How can gravitational waves be detected?

Knowing that gravitation is a spin 2 interaction—that is, a wave with two directions—is represented by a tensor of rank two, the metrics. This means that the wave will pinch particles along one direction while spreading them along the other. Putting particles in a ring and sending light back and forth between, say, 1 2 and 3 4, will cause the light bouncing back and forth to be out of phase with that between 3 and 4. This type of apparatus is known as an interferometer.

Laser beams split along perpendicular paths and then recombined are used by instruments called interferometers, such as LIGO (Laser Interferometer Gravitational-Wave Observatory), to detect gravitational waves. The minute changes in path length caused by passing gravitational waves are measured as changes in the interference pattern of the recombined beams.