The Eclipse That Revealed the Universe
In 1919, British astronomers photographed a solar eclipse and proved that light bends around our Sun—affirming Einstein’s theory of general relativity.
A view of the 1919 solar eclipse, observed in Sobral, Brazil. Arthur Eddington set out to verify Einstein’s prediction that gravity could affect the course of starlight. Credit Arthur Eddington
So, this is what it is like to play cosmic pinball. The worlds move, and sometimes they line up. Then you find yourself staring up the tube of blackness that is the Moon’s shadow, a sudden hole in the sky during a total solar eclipse.
Such moments have left their marks on human consciousness—like the monoliths in the classic movie, “2001: A Space Odyssey”—since before history was recorded.
Few eclipses have had more impact on modern history than the one that occurred on May 29, 1919, more than six minutes of darkness sweeping across South America and across the Atlantic to Africa. It was during that eclipse that the British astronomer Arthur Eddington ascertained that the light rays from distant stars had been wrenched off their paths by the gravitational field of the Sun.
That affirmed the prediction of Einstein’s theory of general relativity, ascribing gravity to a warp in the geometry of space-time, that gravity could bend light beams. “Lights All Askew in the Heavens,” read a headline in this newspaper.
Eddington’s report made Einstein one of the first celebrities of the new 20th century and ushered in a new dynamic universe, a world in which space and time could jiggle, grow, warp, shrink, rip, collapse into black holes and even disappear. The ramifications of his theory are still unfolding; it was only two years ago that a rippling of space-time—gravitational waves produced by colliding black holes—was discovered.
The British astronomer Arthur Eddington, who discovered that light rays from distant stars would bend from the gravitational field of the Sun, an affirmation of one of the more dramatic predictions of Einstein’s theory of general relativity. Credit Oxford Science Archive, via Getty Images
But the first step wasn’t easy. How it happened illustrates that even the most fundamental advances in science can be hostage to luck and sometimes divine inspiration.
The bending of light by gravity was the most stunning and obvious prediction of Einstein’s theory. Astronomers had been trying to detect the effect at solar eclipses since before he had even finished formulating the theory. Nature and politics did not always cooperate.
One of the earliest to try was Erwin Finlay-Freundlich, an astronomer at the Berlin Observatory who was to become a big Einstein booster. Freundlich led an expedition to the Crimea in 1914 to observe an eclipse, but World War I began and he was arrested as a spy before the eclipse occurred. A team from the Lick Observatory in California did make it to the Crimean eclipse—but it rained.
“I must confess that I never before seriously faced the situation of having everything spoiled by clouds,” said William W. Campbell, the team’s frustrated leader. “One wishes that he could come home by the backdoor and not see anybody.”
Worse, Lick’s special eclipse camera was impounded by the Russians and not returned in time for the next eclipse, in Venezuela in 1916.
The next big chance to prove Einstein correct came in 1918, when the Moon’s shadow tracked right up the Columbia River between Washington State and Oregon. Lick sent another team of observers, but their camera was still not back from the Crimea and their improvised optics fell short, leaving the stars looking like fuzzy dumbbells as darkness fell.
So, the universe was still up for grabs in March 1919, when Eddington and his colleagues set sail for Africa to observe the next eclipse. Astronomically, the prospects were as good as they could get. During the eclipse, the Sun would pass before a big cluster of stars known as the Hyades, so there ought to be plenty of bright lights to see yanked askew.
Eddington was the right man for the job. A math prodigy and professor at Cambridge, he had been an early convert to Einstein’s new theory, and an enthusiastic expositor to his colleagues and countrymen.
A story went that he was once complimented on being one of only three people in the world who understood the theory. Admonished for false modesty when he didn’t respond, Eddington replied that, on the contrary, he was trying to think of who the third person was.
Instruments used during the solar eclipse expedition in Sobral, Brazil. Credit SSPL, via Getty Images
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.
“Dear Mother, joyous news today,” Einstein wrote when he got wind of the result.
Astronomers and historians have argued ever since about whether Eddington’s belief that he already knew the answer led him to fudge the eclipse analysis by leaving out the warped astrograph.
In 2007, however, Daniel Kennefick, an astrophysicist and historian at the University of Arkansas, con-cluded after a long study of the records of the eclipse expedition that it was Dyson, the astronomer royal, who had decided to exclude the results from the astrograph. Dyson was well known to be skeptical of Einstein’s new theory.
Eddington and Dyson were right. The experiment was repeated during an eclipse in 1922 and at many other eclipses over the years, always with the same Einsteinian result. With improvements in technology, today even small universities can do the requisite observations.
During the eclipse coming this month, Bobby E. Powell, a physicist at the University of West Georgia, will be redoing the experiment with his students at a site near Lexington, SC. His is only one of half a dozen universities that will try it. He hopes to develop a lab manual for individuals or schools that might want to do it during the next eclipse in American skies, in April 2024.
In modern times, some of the most precise measurements of light-bending have come from radio observations of distant galaxies. In 2009 Edward Fomalont, of the National Radio Astronomy Observatory in Charlottesville, Va., and his colleagues used a set of antennas known as the Very Long Baseline Array to obtain results that supported Einstein’s predictions to within 0.02 percent.
Astronomers in the meantime have learned to use the light-bending and amplifying abilities of immense galaxies as telescopes—Einstein’s own telescope, if you like—to study exploding stars on the other side of the cosmos and to map the mysterious dark matter that pervades the universe.
In November 1919, news of Einstein’s triumph was announced to the world with all due pomp and circumstance at a joint meeting of Royal Society and the Royal Astronomical Society in London.
Presiding over the meeting, the physicist J. J. Thomson called general relativity one of the highest achievements of mankind, describing it “as a whole continent of new scientific facts.” Black holes and the Big Bang were still in the future.
Indeed, what emerged from the Moon’s shadow that cloud-speckled day in May was an entirely new universe.
 Dennis Overbye, “Out There,” New York Times (July 31, 2017). A version of this article appears in print on August 1, 2017, on Page D4 of the New York edition with the headline: “When We Knew Light Could Bend”. Downloaded August 6, 2017