Graphs showing the gravitational wave signals detected on 14th September 2015 by the LIGO (Laser Interferometer Gravitational-Wave Observatory) gravitational wave detector laboratories in Hanford, Washington, USA, and Livingston, Louisiana, USA. Top and centre are the predicted signals, bottom is the actual signal. The signals were detected on 14th September 2015, and were the result of two black holes colliding 1.3 billion light years away. Gravitational waves are ripples in the curvature of spacetime that travel outward from a central source. In this case the source was two colliding black holes. Gravitational waves were predicted by Einstein in 1916, and remained undetected for a century.