A 1919 eclipse confirmed Einstein's general relativity

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

The story

Albert Einstein's special theory of relativity of 1905 left out gravity. In 1907 he took a first step toward including it with his equivalence principle: an observer falling freely in a closed box could find no evidence of a gravitational field. Over the next eight years he worked toward a new theory, and by late 1915 he had completed the general theory of relativity, which describes gravity not as a force pulling across empty space but as the curving of space and time by mass and energy.

The theory made testable predictions. It explained a long-standing puzzle, the slow drift of Mercury's orbit around the Sun, and it predicted that light from distant stars should bend as it passes close to the Sun. In 1911 Einstein had calculated a deflection of 0.83 seconds of arc, the same as a Newtonian estimate; the finished theory doubled it to 1.75 seconds of arc. Attempts to measure it during eclipses in 1912 and 1914 were defeated by rain, clouds and the outbreak of war.

The total solar eclipse of 29 May 1919 offered a fresh chance, with the Sun passing in front of a bright cluster of stars, the Hyades. The British Astronomer Royal, Frank Dyson, and the Cambridge astronomer Arthur Eddington organised two expeditions, one to the island of Príncipe off West Africa and one to Sobral in Brazil. By photographing the stars around the darkened Sun and comparing their positions with photographs of the same field taken at night, they could measure how far the starlight had been bent. Despite clouds on Príncipe, the results agreed with Einstein's prediction rather than the Newtonian value.

The findings were announced at a joint meeting of the Royal Society and the Royal Astronomical Society in November 1919. On 7 November The Times of London ran the headline "Revolution in Science", adding that Newtonian ideas had been overthrown. The story was reported around the world and turned Einstein into perhaps the world's first celebrity scientist. Later eclipse expeditions, including measurements of more than 200 stars during the 1922 eclipse in Australia, confirmed the result.

General relativity became the framework for modern cosmology, from black holes to the expansion of the universe, and the bending of light by gravity, known as gravitational lensing, has itself become an important tool of astronomy.

Why it matters

General relativity is essential to modern astronomy and cosmology, and satellite navigation systems must correct for its effects to give accurate positions.