Eclipse that revealed the universe

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 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 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.

Testing Einstein’s theory

Arthur’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. But the first step was not 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, Germany who was to become a big Einstein booster. Erwin 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, USA did make it to the Crimean eclipse — but it rained.

So the universe was still up for grabs in March 1919, when Arthur 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. Arthur 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 did not respond, Arthur replied that, on the contrary, he was trying to think of who the third person was. General relativity was so obviously true, he said later, that if it had been up to him he would not have bothered trying to prove it. But it was not up to him, due to a quirk of history. Arthur was also a Quaker and so had refused to be drafted into the army. His boss, Frank Dyson, the Astronomer Royal of Britain, saved Arthur from jail by promising that he would undertake an important scientific task, namely the expedition to test the Einstein theory.

Arthur 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 Arthur’s job, as he saw it, was to ascertain whether a bunch of blurs had been nudged off their centres by as much as Einstein had predicted, or half that amount — or none at all.

And what if Arthur measured twice the Einstein deflection? Frank was asked by Edwin Cottingham, one of the astronomers on the expedition. “Then Arthur will go mad and you will come home alone,” Frank answered. To improve the chances of success, two teams were sent: Arthur and Edwin 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 Arthur that the positions of the stars had moved during the eclipse. In the end, there were three sets of plates from which the deflection of starlight could be measured. How Arthur and his colleagues played them off against one another sealed the fate of Einstein’s theory.
Varied data

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 Arthur heroically derived a reading of 1.61 seconds of arc.

Which result should Arthur 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 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, Arthur 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 generalised relativity theory,” Frank and his colleagues wrote in their official report.

In modern times, some of the most precise measurements of light-bending have come from radio observations of distant galaxies. Astronomers in the meantime have learned to use the light-bending and -amplifying abilities of immense galaxies as telescopes to study exploding stars on the other side of the cosmos and to map the mysterious dark matter that pervades the universe.