Physicists of the LIGO and Virgo projects first observed gravitational waves, the existence of which was predicted by Henri Poincaré in 1905.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves. Prior to LIGO, all data about the universe had come in the form of light and other forms of electromagnetic radiation, from limited direct exploration on relatively nearby Solar System objects such as the Moon, Mars, Venus, Jupiter and their moons, asteroids etc., and from high energy cosmic particles. Initially, two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. Two additional, smaller gravitational wave observatories are now operational: one in Japan (KAGRA), and one in Italy (Virgo). The two LIGO observatories use mirrors spaced 4 kilometres (13,000 ft) apart to measure changes in length—over an effective span of 1,120 kilometres (700 mi)—of less than one ten-thousandth the charge diameter of a proton.