Conceptual illustration of gravitational waves over the Louvre, Paris, France. Gravitational waves are a prediction of Einstein's theory of general relativity. Gravity is the distortion of space-time by mass, and changes in this distortion travel in waves at the speed of light. The effect is most pronounced where extremely massive objects are subject to high acceleration. This is seen, for instance, where black holes or neutron stars are in a close orbit. In February 2016, gravitational waves were finally detected, 100 years after Einstein's prediction. The waves, detected by the two LIGO detectors in the USA, emanated from the collision of two black holes some 1.3 billion light years away.