General relativity was so obviously true, he said later, that if it had been up to him he wouldn’t have bothered trying to prove it.
But it wasn’t up to him, due to a quirk of history. Eddington was also a Quaker and so had refused to be drafted into the army. His boss, Frank Dyson, the Astronomer Royal of Britain, saved Eddington from jail by promising that he would undertake an important scientific task, namely the expedition to test the Einstein theory.
Eddington also hoped to help reunite European science, which had been badly splintered by the war, Germans having been essentially disinvited from conferences. Now, an Englishman was setting off to prove the theory of a German, Einstein.
According to Einstein’s final version of the theory, completed in 1915, as their light rays curved around the sun during an eclipse, stars just grazing the sun should appear deflected from their normal positions by an angle of about 1.75 second of arc, about a thousandth of the width of a full moon.
According to old-fashioned Newtonian gravity, starlight would be deflected by only half that amount, 0.86 second, as it passed the sun during an eclipse.
A second of arc is about the size of a star as it appears to the eye under the best and calmest of conditions from a mountaintop observatory. But atmospheric turbulence and optical exigencies often smudge the stars into bigger blurs.
So Eddington’s job, as he saw it, was to ascertain whether a bunch of blurs had been nudged off their centers by as much as Einstein had predicted, or half that amount — or none at all. It was Newton versus Einstein.
No pressure there.
And what if Eddington measured twice the Einstein deflection?, Dyson was asked by Edwin Cottingham, one of the astronomers on the expedition. “Then Eddington will go mad and you will come home alone,” Dyson answered.
To improve the chances of success, two teams were sent: Eddington and Cottingham to the island of Principe, off the coast of Africa, and Charles Davidson and Andrew Crommelin to Sobral, a city in Brazil. The fail-safe strategy almost didn’t work.
In Sobral, the weather was unusually cloudy, but a clearing in the clouds opened up only one minute before totality, the moment the moon fully eclipsed the sun. On Principe, it rained for an hour and a half on the morning of the eclipse, and Eddington took pictures through fleeting clouds, hoping that some stars would show up.
A few blurry stars were visible on a couple of his photographic plates, and a preliminary examination convinced Eddington that the positions of the stars had moved during the eclipse. He turned to his colleague and said, “Cottingham, you won’t have to go home alone.”
In the end, there were three sets of plates from which the deflection of starlight could be measured. How Eddington and his colleagues played them off against one another sealed the fate of Einstein’s theory.
The best-looking data had come from an Irish telescope at Sobral. The images indicated a deflection of 1.98 seconds of arc — more than Einstein had predicted.
Another Sobral telescope, known as an astrograph, also produced lots of star images, but they were blurred and out of focus, perhaps because heat from the sun had affected the telescope mirror. The images gave a value of 0.86 for the deflection, about in line with Newton’s formula, but with large uncertainties.
Finally, there was the Principe telescope, which recorded only a handful of stars, from which Eddington heroically derived a reading of 1.61 seconds of arc.
Which result should Eddington use? If he averaged all three, he would wind up in the unhappy middle ground between Newton and Einstein.
If he just depended on the best telescope, as the astronomers and historians John Earman and Clark Glymour pointed out in an influential essay in 1980, the figure of 1.98 would have cast doubt on Einstein’s theory of general relativity.
In the end, Eddington wound up throwing out the Sobral astrograph data on the grounds that it was unreliable. Both of the remaining plates “point to the full deflection 1”.75 of Einstein’s generalized relativity theory,” Dyson and his colleagues wrote in their official report.
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