A hundred years ago, Albert Einstein wasn’t a household name. He was a professor in Berlin, known to scientists, intellectuals, his divorced wife and the first cousin who would soon become his second wife — but not to the world.
His rise to superstardom began on May 29, 1919, when the moon and sun lined up just right for a solar eclipse. Photos of the astronomical event showed something strange: A few of the stars visible during the blackout were in the wrong place.
Einstein had foreseen this. Using his theory of general relativity, he made the seemingly crazy bet that the stars’ positions in the sky would shift during an eclipse, and even calculated by how much.
“Stars moved by exactly the amount general relativity predicted,” says Mark Hurn, a departmental librarian at the University of Cambridge’s Institute of Astronomy. “It was the first experimental evidence for general relativity being on the right track.”
Not since Edmond Halley prophesied the appearance of his namesake comet had a scientific prediction come true so spectacularly. But whereas Halley vindicated Isaac Newton’s view of the universe, Einstein sought to topple it. And in a historical twist, the astronomers who made the German-born physicist a superstar came from England: Newton’s birthplace and Germany’s enemy during World War I, which had ended just before the eclipse. Their scientific quest to test Einstein’s theory would be celebrated for moving beyond the horrors of war.
“The romantic nature of this business of postwar reconciliation caught the public’s imagination,” says Daniel Kennefick, a historian of physics at the University of Arkansas. It all added up to sudden fame for the physicist behind the prediction. “The public became fixated on Einstein because of this eclipse.”
Staring at the Sun
General relativity abandoned Newton’s idea that gravity is a force pulling objects together. It reimagined gravity as a warping of time and space — a distortion in the fabric of the universe. According to the mathematics of relativity, light traveling through this distortion will change its path, accommodating the universe’s warps and wefts. The more massive an object, the bigger the distortion, and the more its gravity can bend light.
During the decade Einstein spent developing his theory, he realized that the sun was massive enough to make this effect noticeable. As the sun moves in the sky toward a background star, he said, it should bend the star’s light. The star will appear to move.
Of course, testing this prediction wasn’t easy because stars aren’t visible during the day; they’re washed out by the sun’s light. And at night, when stars do appear, the sun isn’t around to bend their light. Only when the sun is out but its light blocked could Einstein’s work be checked. That’s why, while working out the kinks in his theory in 1911, he asked astronomers to start looking to the heavens during eclipses.
The first to answer that call was the young German astronomer Erwin Finlay-Freundlich, a tragic figure who dedicated much of his life to proving Einstein’s light-bending right and never succeeded. He started by analyzing photographs of historical eclipses, but the stars weren’t clear enough to test Einstein’s idea. So Freundlich raised money — nearly borrowing a sum from Einstein himself — for a voyage to present-day Ukraine, where an eclipse was expected in 1914. His team arrived, armed with telescopes and cameras and glass photographic plates, just as war broke out. Russian soldiers captured Freundlich and confiscated his instruments.
Freundlich wasn’t the only one watching in 1914. Astronomers from California’s Lick Observatory also tried to photograph the eclipse from near Kiev, but they fared little better. Though spared imprisonment because they were Americans, the scientists were thwarted by nature: Clouds obscured their view.
These failures actually may have been a stroke of luck for Einstein. While reviewing his calculations, he found errors. His predictions about how much the stars should move were off. Einstein always was a better physicist than mathematician (though the popular story that he failed math in high school is not true).
By the time Einstein had corrected his mistakes and published the completed theory of general relativity, the Great War was in full swing. And after the war, Germany was in shambles, too wrecked to mount expeditions to the distant parts of the world where an eclipse in 1919 would be visible.
Fortunately, a copy of Einstein’s theory wound up in the hands of Sir Arthur Eddington, director of the Cambridge Observatory, a math prodigy and devout Quaker. Ready to be imprisoned as a conscientious objector, Eddington, like Einstein, believed in pacifism.
He also believed in Einstein and teamed up with Astronomer Royal Sir Frank Watson Dyson to persuade his nation to put relativity to the test. “Eddington was one of the few English-speaking scientists who had a thorough understanding of general relativity,” says Hurn. “It was seen as being very German, very incomprehensible — if not completely wacky — by a lot of people.”
Well aware of how difficult photographing an eclipse could be, Eddington and Dyson planned two expeditions. Eddington headed for Príncipe, an island off the west coast of Africa, while a second team headed to Sobral in Brazil.
The plan was to compare images of the stars taken during the eclipse with images of the same stars taken months later when they appeared in their normal places at night. According to Einstein’s now-corrected math, stars at the edge of the blotted-out sun would be displaced from their usual positions by a mere 1.75 arcseconds — about the thickness of a dime seen from a couple of miles away.
Newton’s theory of gravity made a competing prediction, worked out in detail by a German astronomer in 1801. His math suggested a shift only half as large, based on the notion that the force of the sun’s gravity would pull on the distant stars’ light particles.
When the fateful day arrived in May, Eddington’s Príncipe team faced bad weather. Thunderstorms rolled in, clouds obscured the sun, and all but two of their photographic plates proved worthless. But for Eddington, ever the true believer, what he saw in that pair of plates was enough to write his mother: “The one good plate that I measured gave a result agreeing with Einstein, and I think I have got a little confirmation from a second plate.”
Would confirmation come from the other side of the Atlantic, where the weather in Sobral was more cooperative? Dyson had his own issues analyzing these plates. Photos taken by his primary 16-inch telescope were out of focus and provided values more consistent with Newton.
A 4-inch backup telescope lent by an English Jesuit priest saved the day; it corroborated Eddington’s observations. (In 1979, modern analysis of the blurry images from the main telescope also sided with Einstein.)
After word spread to Germany, it was Einstein’s turn to write to his mother. “Good news today . . . the British expeditions have actually proved the light deflection near the sun.”
On Nov. 6, Eddington and Dyson showed off their results at a joint meeting of the Royal Society and the Royal Astronomical Society in England. The press had a field day, and the public went nuts.
“Revolution in Science,” the front page of The Times of London proclaimed. “New Theory of the Universe: Newtonian Ideas Overthrown.” The New York Times followed suit with “Men of Science More or Less Agog.”
While few people really understood Einstein’s new theory, his views about everything from extraterrestrial life on Mars (nonexistent) to Prohibition (no opinion) wound up in the papers. Crowds greeted him on a visit to Japan. Girls lined up to meet him in the United States, chanting “Einstein, Einstein, rah rah rah.” As a handsome young man, Einstein had been popular with women, but that was nothing compared with the effect he had on them after the eclipse; at least one woman is said to have fainted in his presence. As his stature grew, he found his signature could reach even presidents.
Einstein, Einstein, Einstein
Of course, relativity did not spring, fully formed, from Einstein’s mind alone. He had built on the previous work of scientists, enlisted the help of mathematicians and even faced competition from others hot on his heels. But it was Einstein who would become the public face of science; Einstein, whose name would become synonymous with genius; Einstein, who regularly appeared on lists of the most famous people in history.
He wasn’t entirely comfortable with this international celebrity. Enemies emerged at home, like the right-wing anti-Semite Paul Weyland, who held an anti-relativity rally at which Einstein was accused of plagiarism.
And general relativity itself still wasn’t immediately accepted. Some scientists had trouble understanding it. “The complications of the theory of relativity are altogether too much for my comprehension,” confessed American astronomer George Ellery Hale, in a letter that nonetheless celebrated the results from the 1919 eclipse. Others looked for alternative explanations for the moving stars, clinging to Newton’s vision of the universe.
But further eclipse observations continued to build the case for Einstein. Lick astronomers confirmed relativity again during a 1922 eclipse in Australia and a 1923 eclipse in Mexico. Meanwhile, observations of the star Sirius B seemed to support another prediction, that the gravity of stars stretches the light waves they emit.
By the end of the 1920s, relativity was on solid footing. Eclipses continued to corroborate Einstein’s predictions for decades, eventually eclipsed (so to speak) by the advent of quasar observations. These incredibly bright objects, at first mistaken for stars, can send out powerful radio waves. Measurements of how the sun bends those radio waves have confirmed relativity’s light-bending weirdness with a high degree of certainty.
Even so, Eddington’s original experiment continues to hold fascination. During the 2017 eclipse visible in the United States, an amateur astronomer used a $4,000 telescope to repeat the observation with far greater precision, witnessing the same shift of the stars that Eddington saw, albeit with different stars. Other fans have hunted for artifacts from the 1919 expedition, with mixed results.
“The problem is that when you’re working at an observatory, you tend to cannibalize equipment,” says Robin Catchpole at the University of Cambridge’s Institute of Astronomy. A lens that traveled to Brazil is now part of a telescope in a British castle compound. A mirrored device used to aim Eddington’s telescopes wound up in Dublin. And as for the photographic plates that actually captured the eclipse images, many seem to have been destroyed or lost, says Catchpole. Some may be packed up among thousands of uncataloged glass plates in a town in West England, like the crates at the end of Raiders of the Lost Ark.
The legacy of Eddington’s plates, however, endures. Relativity is now accepted by the scientific community, and its many predictions continue to be tested in wacky ways, from experiments that use atomic clocks to observations of colliding neutron stars.
The media buzz around the 1919 eclipse may have launched Einstein into superstardom. But the staying power of his theory is what kept him there.
A Poem by Sir Arthur Eddington
Oh leave the Wise our measures to collate One thing at least is certain, LIGHT has WEIGHT, One thing is certain, and the rest debate — Light-rays, when near the Sun, DO NOT GO STRAIGHT.
Devin Powell is a science writer living in San Francisco. This story originally appeared in print as "Einstein's Eclipse."