What is the purpose of the LIGO project?

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large observatories were built in the United States, one located in Hanford, Washington, and the other in Livingston, Louisiana.

Gravitational waves are variations in the gravitational field that are transmitted as waves. Variations of the gravitational field should be transmitted from place to place as waves, just as variations of an electromagnetic field travel as waves. LIGO is designed to detect the gravitational waves released when two neutron stars or black holes spiral into each other.

Each installation of LIGO is an underground L-shaped laser interferometer with arms 4 km (2.5 miles) long. Each arm is inside an evacuated pipe 1.3 metres (4 feet) in diameter. When a gravitational wave passes through the interferometer, it will make one arm of the interferometer shorter and the other longer, and these changes in distance will appear as a change in the interference fringes between the two beams.

On September 14 the two detectors made the first observation of gravitational waves. Two black holes about 1.3 billion light-years away spiralled into each other. The black holes were 36 and 29 times the mass of the Sun and formed a new black hole 62 times the mass of the Sun.