The Laser Interferometry Gravitational Wave Observatory (LIGO) detects gravitational waves from the surface of Earth. LIGO consists of two observatories in the USA, one in Louisiana and one in Washington.

LIGO detector in Washington, USA
Credit: U Andersen

Each detector consists of a L-shaped structure, with each arm 4000 metres long. Inside the detector is a vacuum, and laser beams which travel to the ends of each arm, bounce off a mirror, and travel back again. Scientific instruments detect a change in the length of the arms, by measuring the time taken for the laser beam to travel to the mirror and back.

If the length of the arms changes, a gravitational wave may have passed through the detector. However, a gravitational wave causes the length of the arms to change by about 10-18 metres, less than one thousandth of the diameter of a proton. It is very difficult to detect such a small change, and so gravitational wave detectors must be very sensitive!

On 11th February 2016, the LIGO and Virgo Collaborations announced the first detection of a gravitational wave. The 'squeezing and stretching' signal was found in the data on 14th September 2016. The signal was matched in both detectors. Scientists believe the gravitational wave was caused by a pair of black holes merging together.

Two further detections have been recorded, and another detection which is not yet confirmed. In the future, there are plans to upgrade the existing LIGO detectors, and build several new detectors across the world. The LISA detector will be the first gravitational wave observatory in space.