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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.

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.

==Conscientious objector==

==Conscientious objector==

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:

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:

<blockquote>"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."<ref>A. Vibert Douglas ''"The Life of Arthur Stanley Eddington"'', Thomas Nelson and Sons Ltd, 1956, p. 93</ref></blockquote>

<blockquote>"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."<ref>A. Vibert Douglas ''"The Life of Arthur Stanley Eddington"'', Thomas Nelson and Sons Ltd, 1956, p. 93</ref></blockquote>

==The Eclipse of 1919==

==The Eclipse of 1919==

The [[theory of relativity|theory of general relativity]] of [[Albert Einstein]] predicts that light from stars grazing the sun will be deflected by 1.75 arc-seconds.  [[Isaac Newton|Newton]]'s "corpuscular" theory predicts a deflection of 0.875 arc seconds.<ref>Johann von Soldner calculated this deflection, and showed that would be independent of the (presumably very fast) speed of the "corpuscles", in 1801.</ref>  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.

The [[theory of relativity|theory of general relativity]] of [[Albert Einstein]] predicts that light from stars grazing the sun will be deflected by 1.75 arc-seconds.  [[Isaac Newton|Newton]]'s "corpuscular" theory predicts a deflection of 0.875 arc seconds.<ref>Johann von Soldner calculated this deflection, and showed that would be independent of the (presumably very fast) speed of the "corpuscles", in 1801.</ref>  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.

    

    

==Promotion of General Relativity, and the "Who is the Third?" Anecdote==

==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''<ref>''The Mathematical Theory of Relativity'', Arthur Eddington, Cambridge University Press, ISBN 0-19-860719-9</ref> was a major undertaking in this regard.

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''<ref>''The Mathematical Theory of Relativity'', Arthur Eddington, Cambridge University Press, ISBN 0-19-860719-9</ref> was a major undertaking in this regard.

==The Black Hole Controversy with Chandrasekhar==

==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&mdash;[[white dwarf|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.

In the 1930s Eddington got into a dispute with Subrahmanyan Chandrasekhar, who had proposed a model of stellar evolution for large stars&mdash;[[white dwarf|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==

==The Fine Structure Constant==

Physical quantities that are dimensionless always fascinate physicists.  One is the "fine structure constant", represented as <math>\alpha\,</math>.  It is named because it is the ratio of certain line spacings in the fine structure lines in spectra.  It is defined as <math>\frac{1}{4 \pi \varepsilon_0} \frac{e^2}{\hbar c}</math>.  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 <math>1/\alpha\,</math> is 137.036.

Physical quantities that are dimensionless always fascinate physicists.  One is the "fine structure constant", represented as <math>\alpha\,</math>.  It is named because it is the ratio of certain line spacings in the fine structure lines in spectra.  It is defined as <math>\frac{1}{4 \pi \varepsilon_0} \frac{e^2}{\hbar c}</math>.  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 <math>1/\alpha\,</math> is 137.036.

==References==

==References==

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