Sir Arthur Eddington was a prominent English astrophysicist of the early 20th Century. He is perhaps best known for his observational confirmation of Einstein’s General Theory of Relativity and the bending of light due to gravity, and his early adoption and popular expositions of relativity were instrumental in gaining publicity for the theory and disseminating its ideas to the English-speaking world. However, he also helped develop the first true understanding of stellar processes and the internal structure of stars, and he established the Eddington limit, which dictates the natural limit to the luminosity of stars.
Arthur Stanley Eddington was born on 28 December 1882 in Kendal, northern England. His father, a Quaker schoolteacher, died of typhoid when Eddington was just two, and the family moved to Weston-super-Mare, where he was raised in relative poverty. From 1893 to 1898, he attended Brynmelyn School, where he distiguished himself, particularly in mathematics and English literature.
His school performance earned him a scholarship to Owens College, Manchester in 1898, where he soon turned to physics. He graduated with a First Class BSc in physics in 1902, and was awarded a scholarship to Trinity College, Cambridge, where he was tutored by the distinguished mathematician R. A. Herman. He earned his BA in 1905, and spent some time researching thermionic emission in the Cavendish Laboratory and teaching mathematics to first year engineering students.
In 1906, Eddington made the move to astronomy when he was nominated to the post of chief assistant to the Astronomer Royal (then William Christie) at the Royal Greenwich Observatory. He developed a new statistical method based on the apparent drift of two background stars, winning him the Smith's Prize in 1907 and a Fellowship of Trinity College, Cambridge. In 1913, he was promoted to the position of Plumian Professor of Astronomy and Experimental Philosophy at Cambridge, and the next year he was named the director of the entire Cambridge Observatory, as well as a Fellow of the Royal Society.
During World War I, Eddington struggled to keep wartime bitterness out of astronomy and, as a Quaker pacifist, he repeatedly called for British scientists to preserve their pre-war friendships and collegiality with German scientists. When he was finally conscripted in 1918, he claimed conscientious objector status, and only the timely intervention of the Astronomer Royal, Frank Dyson, and other high profile figures kept Eddington out of prison for his views.
As Secretary of the Royal Astronomical Society during World War I, Eddington was one of the first to receive a series of letters and papers from the Dutch physicist Willem de Sitter regarding Albert Einstein’s new General Theory of Relativity. Eddington was one of the few British astronomers with the mathematical skills to properly understand it (and who was still interested in pursuing a theory developed by a German physicist), and he quickly became the chief supporter and expositor of relativity in Britain. Eddington’s observations and photographs during a solar eclipse on the African island of Príncipe in 1919 effectively confirmed Einstein's predictions of a slight shift in starlight caused by the gravitational field of the Sun. This verification of the bending of light passing close to the Sun (as predicted by relativity theory) was hailed at the time as a conclusive proof of general relativity, even if in retrospect the proof was actually far from conclusive.
In 1916, Eddington began to investigate possible physical explanations for Cepheid variable stars and to develop the first true understanding of stellar processes, extending Karl Schwarzschild's earlier work on radiation pressure and going on to show that the internal thermal pressure of a star was necessary to prevent the collapse of the sphere of gas due to gravity. He defined the so-called Eddington luminosity (or Eddington limit) of a star as the point where the gravitational force inwards equals the continuum radiation force outwards, assuming hydrostatic equilibrium and spherical symmetry. He demonstrated that virtually all stars, including giants and dwarfs, behaved as “ideal gases”, and that the interior temperature of stars must be millions (not just thousands) of degrees. In 1924, he discovered the empirical mass-luminosity relationship for stars, whereby the luminosity of a star is roughly proportional to the total mass to the power of 3.5.
When Eddington learned of Georges Lemaitre's 1927 paper postulating an expanding or contracting universe, and Edwin Hubble's work on the recession of spiral nebulae, he soon became an enthusiastic supporter of an expanding universe cosmology. However, he rejected what would later be known as the Big Bang model of cosmology as “too unaesthetically abrupt”, preferring Einstein’s cosmological constant as an explanation for the universe's evolution from a Newtonian and Einsteinian static universe to its current expanding state.
Eddington's books and lectures were immensely popular with the public, largely because of his clear and entertaining exposition. Einstein himself suggested that Eddington’s 1923 book “Mathematical Theory of Relativity” was “the finest presentation of the subject in any language”. His 1926 “The Internal Constitution of the Stars” became an important text for training an entire generation of astrophysicists. His popular writings on relativity and quantum theory were to make him, quite literally, a household name in Great Britain between the world wars.
In later life, Eddington (like Einstein, Dirac and others) continued to pursue what he called a "fundamental theory" which might unify quantum theory, relativity and gravitation. Although he never completed this research before his death, some of his tentative or abandoned theories underlie many modern attempts at a grand unified theory, and many of his more intuitive, exploratory theories were later borne out by empirical observations.
Eddington was knighted in 1930, and received the Order of Merit in 1938, as well as many other honours from astronomical societies throughout the world. He never married. He died in Cambridge, England on 22 November 1944, aged 61, and is buried at the Parish of the Ascension Burial Ground in Cambridge.