Black hole merger and gravitational waves, conceptual image. Computer illustration representing the merger of two black holes causing gravitational waves (spiralling lines). Gravitational waves are a prediction of Einstein's theory of general relativity. The effect is most pronounced where extremely massive objects (black holes or neutrons stars) are subject to extremely high acceleration, as shown here. In February 2016, gravitational waves were detected for the first time, 100 years after Einstein's prediction. The waves emanated from the collision of two black holes around 1.3 billion light years away. The waves were detected by the LIGO (Laser Interferometer Gravitational-Wave Observatory) detectors in the USA.