Arthur Eddington

Arthur Stanley Eddington was born in Kendal, in the English Lake District, in 1882, and raised a Quaker — a faith whose pacifism would shape both his career and the most famous experiment he ever performed. He was a prodigy at mathematics, took his degree at Cambridge as Senior Wrangler (top of his year) in 1904, and by 1913 held the Plumian Professorship of Astronomy there, with the directorship of the Cambridge Observatory following the next year. For three decades he was the dominant figure in British astrophysics and one of its greatest expositors, writing books that taught a generation what stars are made of and what relativity means.

Eddington's scientific reputation rests above all on the internal constitution of stars. In the 1910s and 1920s he worked out that a star is a ball of gas held up against its own gravity by the pressure of radiation and hot matter, and that this balance fixes a relationship between a star's mass and its luminosity. He understood, before the source was known, that the energy must come from the subatomic transmutation of the elements — that the Sun shines by turning hydrogen into helium. When colleagues objected that the interior was not hot enough, he famously replied that they should 'go and find a hotter place.' His 1926 treatise *The Internal Constitution of the Stars* founded modern stellar astrophysics.

But it is the eclipse of May 29, 1919, that made Eddington a household name. As war raged, Eddington — a conscientious objector who was spared internment partly so he could lead a scientifically valuable expedition — organized two parties to photograph a total solar eclipse and test Einstein's prediction that the Sun would deflect starlight by 1.75 arcseconds, twice the Newtonian value. Eddington himself sailed to the island of Príncipe; a second team went to Sobral in Brazil. Despite clouds, the plates yielded a deflection consistent with general relativity and inconsistent with Newton. The November 1919 announcement turned Einstein into the most famous scientist in the world, and Eddington into relativity's foremost champion in the English-speaking world.

Eddington spent his later years on two grand and increasingly idiosyncratic projects: popularizing relativity and quantum theory in best-selling books such as *Space, Time and Gravitation* (1920) and *The Nature of the Physical World* (1928), and an unfinished 'fundamental theory' that sought to derive the constants of nature from pure number. He clashed bitterly with the young Subrahmanyan Chandrasekhar in 1935 over whether massive stars could collapse without limit — Eddington was wrong, and his prestige delayed acceptance of what became black-hole physics. He died in Cambridge in 1944. He is remembered as the man who weighed the stars and who, on a cloudy morning off the coast of Africa, photographed the bending of light.