Sir Arthur Eddington (1882–1944) was a British astronomer, mathematician, physicist, science writer, and science "popularizer", somewhat in the mold of Carl Sagan and Neil deGrasse Tyson in modern times.
In the early decades on the 20th century, two new theories of "modern physics", Relativity and Quantum Mechanics, had come into existence. Both were known for being very abstruse. He popularized them, both for experts and for the general public. He particularly worked on getting relativity better known, both by the scientific community and by the public. General Relativity was something he worked especially hard at.
He is best known for his observations of the bending of light from stars in the vicinity of the sun during a total eclipse. This had been predicted by General Relativity, but the theory was so esoteric that even the scientific community had a difficult time with it. The problem was compounded by the fact that Einstein was German, and the controversy was around the time of World War I, when America and Western Europe were not positively disposed toward Germans. His claimed success (not fully justified—see below) immediately propelled Einstein (and, to a lesser extent, Eddington), to worldwide fame, even though the public still understood virtually nothing about relativity.
Eddington was a conscientious objector during World War I. This wasn't a popular position at his time. When general conscription was introduced in 1916, Eddington entered his plea to claim exemption on grounds of conscience, but his university had requested the authorities not to withdraw him from his scientific researches. Eddington had declared:
"My objection to war is based on religious grounds. I cannot believe that God is calling me to go out to slaughter men, many of whom are animated by the same motives of patriotism and supposed religious duty that have sent my countrymen into the field. To assert that it is our religious duty to cast off the moral progress of centuries and take part in the passions and barbarity of war is to contradict my whole conception of what the Christian religion means. Even if the abstention of conscientious objectors were to make the difference between victory and defeat, we cannot truly benefit the nation by wilful disobedience to the divine will."
He was exempted until Aug 1, 1918, and the Astronomer Royal Sir Frank Dyson supported the idea of an expedition - led by Eddington - to observe the solar eclipse of 1919.
The Eclipse of 1919
The theory of general relativity of Albert Einstein predicts that light from stars grazing the sun will be deflected by 1.75 arc-seconds. Newton's "corpuscular" theory predicts a deflection of 0.875 arc seconds. Scientists had mostly discarded Newton's theory in terms of the "wave" theory, which didn't address the issue at all. The predicted deflections were near the limits of the technology of the time. Furthermore, an eclipse was required to darken the sun so that the starlight may be seen in proximity to the sun.
Eddington mounted two expeditions, with the best equipment available, to observe the solar eclipse of May 29, 1919. One team went to Sobral, Brazil, and the other went to the island of Principe, off the African coast. Upon his return, Eddington presented his observations, that he had successfully measured a deflection in the amount of Einstein's 1.75 arc-seconds, to the Royal Society, as a validation of the theory of relativity.
Recent analysis of Eddington’s work revealed that he was biased in selecting his data, and that overall his data were inconclusive about the theory of relativity. Modern techniques for analyzing data from experiments involves extremely careful statistical analysis of the data, and generally demands a "5 sigma" level of statistical certainty, which means that there is only a one-in-three-million chance that the measurements came out that way by chance rather than by the phenomenon being measured.
Eddington's observation protocol was somewhat muddled by an incorrect initial calculation, by several people including Einstein himself, of what the effect would be, and some "cherry picking" of the data to be used. The data selection could be considered "manipulation" or "fudging", by Eddington, who had a personal stake in the outcome. His analysis techniques would not pass muster today. Nevertheless, the deflection has been verified with ever-increasing precision in subsequent eclipses, and, with exquisite precision, in the observations of quasar 3C273.
Promotion of General Relativity, and the "Who is the Third?" Anecdote
Eddington spent much of his career promoting relativity, and particularly general relativity, to the English-speaking scientific community. He was a very popular and respected writer and lecturer. General relativity was so abstruse at the time (it still is, though much less so) that even the leading scientists didn't understand it. His 1923 book The Mathematical Theory of Relativity was a major undertaking in this regard.
He also participated in many scientific lectures and colloquia. There is an anecdote (sources differ on whether it is apocryphal) about an exchange during a lecture that he gave in 1919 at the Royal Society on the topic of general relativity. An audience member, Ludvig Silberstein, suggested to him "Professor Eddington, you must be one of three persons in the world who understands general relativity." Silberstein was presumably thinking of himself, Eddington, and Einstein. Eddington paused for a moment. Silberstein then said "Don't be modest, Eddington!" Eddington replied "I'm trying to think who is the third."
The Black Hole Controversy with Chandrasekhar
In the 1930s Eddington got into a dispute with Subrahmanyan Chandrasekhar, who had proposed a model of stellar evolution for large stars—white dwarves with a mass greater than 1.44 times the mass of the sun, which is known as the Chandrasekhar limit. This work presaged the notion of black holes. Eddington rejected the notion that a purely mathematical analysis could be accepted if it led to such a seemingly absurd physical result. Chandrasekhar turned out to be right.
The Fine Structure Constant
Physical quantities that are dimensionless always fascinate physicists. One is the "fine structure constant", represented as . It is named because it is the ratio of certain line spacings in the fine structure lines in spectra. It is defined as . It had been measured at the time to be very close to 1/136. Eddington proposed, apparently on grounds of pure mathematical elegance, that it was exactly 1/136. Later measurements showed that it was instead very close to 1/137, and Eddington proposed that it was exactly 1/137. He was wrong both times; the modern value of is 137.036.
- The Nature of the Physical World, Arthur Eddington, MacMillan, 1929, ISBN 0-8414-3885-4
- A. Vibert Douglas "The Life of Arthur Stanley Eddington", Thomas Nelson and Sons Ltd, 1956, p. 93
- From the New York Times: "Eddington, a Quaker and conscientious objector, was concerned that wartime resentments were damaging the international scientific community. He leapt on Einstein’s relativity as a means of restoring harmony. As the historian Matthew Stanley has documented, Eddington’s superiors in London and Cambridge lobbied British government officials to let him devote his mandatory wartime service to preparing an astronomical expedition to test one of Einstein’s major predictions, that gravity could bend the path of starlight. By leading a British team to test the work of a German physicist, Eddington hoped to 'heal the wounds of war.'" David Kaiser, "How Politics Shaped General Relativity," NY Times (Nov. 6, 2015).
- Johann von Soldner calculated this deflection, and showed that would be independent of the (presumably very fast) speed of the "corpuscles", in 1801.
- Einstein's Luck, John Waller, Oxford University Press, ISBN 0-19-860719-9
- Peter Coles (University of Nottingham, UK): "Einstein, Eddington and the 1919 Eclipse"
- The Mathematical Theory of Relativity, Arthur Eddington, Cambridge University Press, ISBN 0-19-860719-9