Gravitational waves were first predicted by Einstein's Theory of General Relativity in 1916, before the technology necessary for their detection existed. Gravitational waves are ripples in the fabric of space-time, which are produced by interstellar collisions, explosions, or the dramatic movement of large and extremely dense objects such as neutron stars. When a gravitational wave passes Earth, space-time in the plane normal to its direction of propagation is altered by its passage. Along one axis the space is stretched, while at an orthogonal axis it is compressed. By probing these small changes in two dimensions, a very sensitive instrument can detect its passage. The change in the linear dimension due to the passage of a gravitational wave is very small, so the sensitivity of the instrumentation must be great. The LIGO instruments are sensitive enough to measure displacements as small as one-thousandth of the diameter of a proton.

LIGO allows the detection of gravitational waves using a device called a laser interferometer, which measures the difference in the distance between two sets of suspended mirrors hanging four kilometers apart at the ends of each tube as shown above.