Arthur Stanley Eddington

28 December 1882 22 November 1944

Sir Arthur Stanley Eddington was an English astrophysicist of the early 20th century. The Eddington limit, the natural limit to the luminosity of stars, or the radiation generated by accretion onto a compact object, is named in his honour.

He is famous for his work regarding the Theory of Relativity. Eddington wrote a number of articles which announced and explained Einstein's theory of general relativity to the English-speaking world. World War I severed many lines of scientific communication and new developments in German science were not well known in England. He also conducted an eclipse expedition in 1919 that provided one of the earliest confirmations of relativity, and became known for his popular expositions and interpretations of the theory.

Eddington was born in Kendal, England, son of Quaker parents, Arthur Henry Eddington and Sarah Ann Shout. His father taught at a Quaker training college in Lancashire before moving to Kendal to become headmaster of Stramongate School. He died in the typhoid epidemic which swept England in 1884. When his father died, his mother was left to bring up her two children with relatively little income. The family moved to Weston-super-Mare where at first Stanley (as his mother and sister always called him) was educated at home before spending three years at a preparatory school.

In 1893 Stanley entered Brynmelyn School. He proved to be a brilliant scholar and excelled in mathematics and English literature. His records won him a 60 pounds scholarship in 1898, and he was able to attend Owens College in Manchester once he turned 16 later that year. He spent the first year in a general course, but turned to physics for the next three years. Eddington was greatly influenced by his physics and mathematics teachers, Arthur Schuster and Horace Lamb. At Manchester, Eddington lived at Dalton Hall, where he came under the lasting influence of the Quaker mathematician J.W. Graham. His progress continued to be rapid, winning him several additional scholarships and allowing him to graduate with a B.Sc. in physics with First Class Honours in 1902.

Based on his performance at Owens, he was awarded a 75 pound scholarship at Trinity College, Cambridge, which he entered in 1902. He was coached by the famous mathematician R.A. Herman, and in 1904 became the first second-year student to place as Senior Wrangler. After receiving his B.A. in 1905, he began research on thermionic emission in the Cavendish Laboratory. This did not go well, and meanwhile he spent time teaching mathematics to first year engineering students, without much satisfaction. But fortunately this hiatus was brief.

In January 1906, Eddington was nominated to the post of chief assistant to the Astronomer Royal at the Royal Greenwich Observatory. He left Cambridge for Greenwich the following month. He was put to work on a detailed analysis of the parallax of asteroid 433 Eros on photographic plates that had started in 1900. He developed a new statistical method based on the apparent drift of two background stars, winning him the Smith's Prize in 1907.

The prize won him a Trinity College Fellowship. In December 1912 George Darwin, son of Charles Darwin, died suddenly and Eddington was promoted to his chair as the Plumian Professor of Astronomy and Experimental Philosophy in early 1913. Later that year, Robert Ball, holder of the theoretical Lowndean chair also died, and Eddington was named the director of the entire Cambridge Observatory the next year. He was elected a Fellow of the Royal Society shortly thereafter.

Eddington also investigated the interior of stars through theory, and developed the first true understanding of stellar processes. He began this in 1916 with investigations of possible physical explanations for Cepheid variables. He began by extending Karl Schwarzschild's earlier work on radiation pressure in Emden polytropic models. These models treated a star as a sphere of gas held up against gravity by internal thermal pressure, and one of Eddington's chief additions was to show that radiation pressure was necessary to prevent collapse of the sphere. He developed his model despite knowingly lacking firm foundations for understanding opacity and energy generation in the stellar interior. However, his results allowed for calculation of temperature, density and pressure at all points inside a star, and Eddington argued that his theory was so useful for further astrophysical investigation that it should be retained despite not being based on completely accepted physics. James Jeans contributed the important suggestion that stellar matter would certainly be ionized, but that was the end of any collaboration between the pair, who became famous for their lively debates.

Eddington defended his method by pointing to the utility of his results, particularly his important mass-luminosity relation. This had the unexpected result of showing that virtually all stars, including giants and dwarfs, behaved as ideal gases. In the process of developing his stellar models, he sought to overturn current thinking about the sources of stellar energy. Jeans and others defended the Kelvin-Helmholtz mechanism, which was based on classical mechanics, while Eddington speculated broadly about the qualitative and quantitative consequences of possible proton-electron annihilation and nuclear fusion processes.

With these assumptions, he demonstrated that the interior temperature of stars must be millions of degrees. In 1924, he discovered the mass-luminosity relation for stars. Despite some disagreement, Eddington's models were eventually accepted as a powerful tool for further investigation, particularly in issues of stellar evolution. The confirmation of his estimated stellar diameters by Michelson in 1920 proved crucial in convincing astronomers unused to Eddington's intuitive, exploratory style. Eddington's theory appeared in mature form in 1926 as The Internal Constitution of the Stars, which became an important text for training an entire generation of astrophysicists.

During World War I Eddington became embroiled in controversy within the British astronomical and scientific communities. Many astronomers, chief among them H.H. Turner, argued that scientific relations with all of the Central Powers should be permanently ended due to their conduct in the war. Eddington, a Quaker pacifist, struggled to keep wartime bitterness out of astronomy. He repeatedly called for British scientists to preserve their pre-war friendships and collegiality with German scientists. Eddington's pacifism caused severe difficulties during the war, especially when he was called up for conscription in 1918. He claimed conscientious objector status, a position recognized by the law, if somewhat despised by the public. In 1918 the government sought to revoke this deferment, and only the timely intervention of the Astronomer Royal and other high profile figures kept Eddington out of prison.

Eddington's work in astrophysics in the late 1920s and the 1930s continued his work in stellar structure, and precipitated further clashes with Jeans and E.A. Milne. An important topic was the extension of his models to take advantage of developments in quantum physics, including the use of degeneracy physics in describing dwarf stars. This precipitated his famous dispute with Subrahmanyan Chandrasekhar, who was then a student at Cambridge. Chandrasekhar's narrative of this incident, in which his work is harshly rejected, portrays Eddington as rather cruel and dogmatic, and is unfair to Eddington's character as described by other contemporaries. Eddington's criticism seems to have been based on a suspicion that a purely mathematical derivation from quantum theory was not enough to explain the daunting physical paradoxes that were apparently part of degenerate stars.

During World War I Eddington was Secretary of the Royal Astronomical Society, which meant he was the first to receive a series of letters and papers from Willem de Sitter regarding Einsteins theory of general relativity. Eddington was fortunate in being not only one of the few astronomers with the mathematical skills to understand general relativity, but also (due to his international and pacifist views) one of the few who would have been interested in pursuing a theory developed by a German physicist. He quickly became the chief supporter and expositor of relativity in Britain. He and Astronomer Royal Frank Dyson organized two expeditions to observe a solar eclipse in 1919 to make the first empirical test of Einsteins theory: the measurement of the deflection of light by the Sun's gravitational field. In fact, it was Dysons argument for the indispensability of Eddingtons expertise in this test that allowed him to escape prison during the war.

After the war, Eddington travelled to the island of Prncipe near Africa to watch the solar eclipse of May 29, 1919. During the eclipse, he took pictures of the stars in the region around the Sun. According to the theory of general relativity, stars near the Sun would appear to have been slightly shifted because their light had been curved by its gravitational field. This effect is noticeable only during an eclipse, since otherwise the Sun's brightness obscures the stars. Eddington showed that Newtonian gravitation could also be interpreted to predict half that predicted by Einstein. Somewhat confusingly, this same half-shift was predicted by Einstein with an incomplete version of general relativity.

Eddington's observations published next year (Dyson, F.W., Eddington, A.S., & Davidson, C.R. 1920 A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919 Phil. Trans. Roy. Soc. A, 220, 291-333) confirmed Einstein's theory, and were hailed at the time as a conclusive proof of general relativity over the Newtonian model. The news was reported in newspapers all over the world as a major story. Afterward, Eddington embarked on a campaign to popularize relativity and the expedition as landmarks both in scientific development and international scientific relations.

It has been claimed that Eddington's observations were of poor quality and he had unjustly discounted simultaneous observations at Sobral, Brazil which appeared closer to the Newtonian model. The quality of the 1919 results were of poor quality compared to later observations, but were sufficient to persuade contemporary astronomers. The rejection of the results from the Brazil expedition were due to a defect in the telescopes used, which, again, was completely accepted and well-understood by contemporary astronomers. The myth that Eddington's results were fraudulent is a modern invention, and there is little evidence to support it.

Throughout this period Eddington lectured on relativity, and was particularly well known for his ability to explain the concepts in lay terms as well as scientific. He collected many of these into the Mathematical Theory of Relativity in 1923, which Albert Einstein suggested was "the finest presentation of the subject in any language." He was an early apologist of Einstein's General Relativity, and an interesting anecdote well illustrates his personal intellectual investment: Ludwig Silberstein approached Eddington at the Royal Society's (November 6) 1919 meeting wherein he had defended Einstein's Relativity with his Brazil-Principe Solar Eclipse calculations with some degree of skepticality and ruefully charged Arthur as one who claimed to be one of three men who actually understood the theory. When Eddington refrained from replying, he insisted Arthur not be "so shy", whereupon Eddington replied, "Oh, no! I was wondering who the third one might be!"

During the 1920s and 30s Eddington gave innumerable lectures, interviews, and radio broadcasts on relativity (in addition to his textbook Mathematical Theory of Relativity), and later, quantum mechanics. Many of these were gathered into books, including Nature of the Physical World and New Pathways in Science. Albert Einstein called Eddington's books "the finest presentation of the subject in any language." His skillful use of literary allusions and humor helped make these famously difficult subjects quite accessible. His humor is well demonstrated by an anecdote: Ludwig Silberstein, a physicist who thought of himself as an expert on relativity, once approached Eddington saying that it had been said that Eddington was one of only three men who actually understood the theory (Silberstein of course was including himself and Einstein as the other two). Eddington refrained from replying, and Silberstein insisted that he not be "so shy", whereupon Eddington replied, "Oh, no! I was wondering who the third one might be!"

Eddington's books and lectures were immensely popular with the public, not only because of Eddingtons clear and entertaining exposition, but also for his willingness to discuss the philosophical and religious implications of the new physics. He argued for a deeply-rooted philosophical harmony between scientific investigation and religious mysticism, and also that the positivist nature of modern physics (i.e., relativity and quantum physics) provided new room for personal religious experience and free will. Unlike many other spiritual scientists, he rejected the idea that science could provide proof of religious propositions. He promoted the infinite monkey theorem in his 1928 book The Nature of the Physical World, with the phrase "If an army of monkeys were strumming on typewriters, they might write all the books in the British Museum". His popular writings made him, quite literally, a household name in Great Britain between the world wars.

Eddington was also heavily involved with the development of the first generation of general relativistic cosmological models. He had been investigating the instability of the Einstein universe when he learned of both Lemaitre's 1927 paper postulating an expanding or contracting universe and Edwin Hubble's work on the recession on the spiral nebulae. He soon became an enthusiastic supporter of an expanding universe cosmology, pointing to the nebular recession as evidence of a curved space-time. However, he never accepted the argument that an expanding universe required a beginning. He rejected what would later be known as Big Bang cosmologies as 'too unaesthetically abrupt.' He felt the cosmical constant must have played the crucial role in the universe's evolution from an Einsteinian steady state to its current expanding state, and most of his cosmological investigations focused on the constant's significance and characteristics.

During the 1920s until his death, he increasingly concentrated on what he called "fundamental theory" which was intended to be a unification of quantum theory, relativity and gravitation. At first he progressed along "traditional" lines, but turned increasingly to an almost numerological analysis of the dimensionless ratios of fundamental constants.

His basic approach was to combine several fundamental constants in order to produce a dimensionless number. In many cases these would result in numbers close to 1040, its square, or its square root. He was convinced that the mass of the proton and the charge of the electron, were a natural and complete specification for constructing a Universe and that their values were not accidental. One of the discoverers of quantum mechanics, Paul Dirac, also pursued this line of investigation, which has become known as the Dirac large numbers hypothesis, and some scientists even today believe it has something to it.

A particularly damaging statement in his defence of these concepts involved the fine structure constant a. At the time it was measured to be very close to 1/136, and he argued that the value should in fact be exactly 1/136 for various reasons. Later measurements placed the value much closer to 1/137, at which point he switched his line of reasoning (arguing that the number of degrees of freedom had been miscounted) and claimed that the value should in fact be exactly 1/137, the Eddington number. Wags at the time started calling him "Arthur Adding-one". At this point most other researchers stopped taking his concepts very seriously. The current measured value is estimated at 1/137.035999679(94).

Eddington believed he had discovered an algebraic basis for fundamental physics, which he termed "E-frames" (representing a particular group). While his theory has long been abandoned by the general physics community, similar algebraic notions underlie many modern attempts at a grand unified theory.

He did not complete this line of research before his death in 1944, and his book entitled Fundamental Theory was published posthumously in 1948. Eddington died in Cambridge, England.

Eddington is credited with devising a measure of a cyclist's long distance riding achievements. The Eddington Number in this context is defined as E, the number of days a cyclist has cycled more than E miles. For example an Eddington Number of 70 would imply that a cyclist has cycled more than 70 miles in a day on 70 occasions. Achieving a high Eddington number is difficult since moving from, say, 70 to 75 will probably require more than five new long distance rides since any rides between 70 and 74 miles will no longer be included in the reckoning.

The construct of the Eddington Number for cycling is identical to the h-index that quantifies both the actual scientific productivity and the apparent scientific impact of a scientist.


Bruce Medal (1924)
Henry Draper Medal (1924)
Gold Medal of the Royal Astronomical Society (1924)
Royal Medal of the Royal Society (1928)
Knighted (1930)
Order of Merit (1938)

Named after him:

Eddington Crater on the Moon
Asteroid 2761 Eddington
Royal Astronomical Society's Eddington Medal
Eddington mission, now cancelled

Books by Eddington:

1914. Stellar Movements and the Structure of the Universe. London: Macmillan.
1918. Report on the relativity theory of gravitation. London, Fleetway press, Ltd.
1920. Space, Time and Gravitation: An Outline of the General Relativity Theory. Cambridge University Press. ISBN 0-521-33709-7
1923, 1952. The Mathematical Theory of Relativity. Cambridge University Press.
1926. Stars and Atoms. Oxford: British Association.
1926. The Internal Constitution of Stars. Cambridge University Press. ISBN 0-521-33708-9
1928. The Nature of the Physical World. MacMillan. 1935 replica edition: ISBN 0-8414-3885-4, University of Michigan 1981 edition: ISBN 0-472-06015-5 (192627 Gifford lectures)
1929. Science and the Unseen World. Macmillan. ISBN 0-8495-1426-6, 2004 reprint: ISBN 1-4179-1728-8
19nn. The Expanding Universe: Astronomy's 'Great Debate', 1900-1931. Cambridge University Press. ISBN 0-521-34976-1
1930. Why I Believe in God: Science and Religion, as a Scientist Sees It
1935. New Pathways in Science. Cambridge University Press.
1936. Relativity Theory of Protons and Electrons. Cambridge Univ. Press.
1939. Philosophy of Physical Science. Cambridge University Press. ISBN 0-7581-2054-0 (1938 Tarner lectures at Cambridge))
1925. The Domain of Physical Science. 2005 reprint: ISBN 1-4253-5842-X
1948. Fundamental Theory. Cambridge University Press. LINK: