Einstein predicted gravitational waves in 1916

Person: Albert Einstein
Lived: 1879–1955
Field: Physics
From: German-American

The story

Albert Einstein completed his general theory of relativity in late 1915. It described gravity as the curving of space and time by mass and energy. In 1916 he worked out one of its consequences: when massive objects accelerate, such as two stars orbiting each other, they should send out ripples in the fabric of spacetime. These gravitational waves would travel outward at the speed of light, carrying energy away from their source. Newton's theory, in which gravity acts instantly at any distance, had no room for such waves.

The prediction was hard to pin down. The approximations Einstein used led many physicists, at times including Einstein himself, to doubt whether the waves were real. In 1922 Arthur Eddington showed that some of the wave types in Einstein's equations were artefacts of the mathematics, joking that they travelled at the speed of thought. Even if the waves were real, their effect on Earth would be almost unimaginably small.

Indirect evidence came in 1974, when Russell Hulse and Joseph Taylor discovered a binary pulsar 21,000 light-years away using the Arecibo radio telescope in Puerto Rico. Over the following years its two stars were seen to be spiralling closer together at exactly the rate general relativity predicted if they were losing energy as gravitational waves. Hulse and Taylor received the 1993 Nobel Prize in Physics.

Direct detection required instruments of extraordinary sensitivity. On 14 September 2015 the two detectors of the Laser Interferometer Gravitational-wave Observatory, LIGO, in Hanford, Washington, and Livingston, Louisiana, sensed waves from two black holes, of about 29 and 36 times the mass of the Sun, merging about 1.3 billion light-years away. By the time they reached Earth the waves stretched space by an amount 10,000 times smaller than an atomic nucleus. The discovery was announced on 11 February 2016, nearly a century after Einstein's prediction, and in 2017 Rainer Weiss, Kip Thorne and Barry Barish received the Nobel Prize in Physics for it.

Gravitational-wave astronomy has grown rapidly since. In August 2017 detectors caught waves from two colliding neutron stars while telescopes saw the same event in light, and astronomers now study black holes and neutron stars through the vibrations of spacetime itself.