An experiment that has been constructed to detect gravitational waves. When gravitational waves interact with solid matter they induce vibrations in the matter causing it to be squeezed and stretched. However, since gravitational waves only interact very weakly with matter, the amount by which the matter is compressed or stretched is very small (about a thousandth of the diameter of a proton). This has made the direct detection of gravitational waves very difficult. The LIGO experiment has attempted to detect this very small effect using laser interferometers. In LIGO there are two interferometers about 4 km long, with laser beams being used to measure any difference in the length of an object due to gravitational waves. These are based at two locations in the USA that are about 3000 km apart: Hanford, Washington and Livingston, Louisiana.

The first version of LIGO, which operated between 2002 and 2010, did not detect any gravitational waves. However, an upgraded version of the experiment successfully detected gravitational waves in 2015, with an account of the discovery being published in 2016. This was the first direct detection of gravitational waves. The event which gave rise to these waves was the merger of two black holes with masses of about 30 times the mass of the sun, about 1.3 billion light years from the earth. The success of the LIGO experiment has initiated the field of gravitational wave astronomy, i.e. the use of gravitational waves to study astronomical processes such as the formation and merger of dense objects such as black holes and neutron stars. It is hoped that the success of LIGO will lead to a network of gravitational wave detectors around the world, which might also be able to detect gravitational waves from the very early universe. See also LISA.