The Eclipse That Proved Einstein’s Theory Correct

Editor’s note: In May 1919, astronomers flocked to Africa and South America to photograph the total eclipse of the Sun. Their objective was to see whether the stars around the blackened disc of the Sun appeared displaced—an indication that massive objects would indeed bend the course of light, as Einstein had predicted in his general theory of relativity. Just a few months later, the preliminary results were out.

The following is an excerpt of Gravity’s Century: From Einstein’s Eclipse to Images of Black Holes by Ron Cowen. Listen to a radio interview with Cowen about this historic moment.

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Einstein learned about the preliminary results in the summer. On September 27, he wrote to his mother, who was dying of cancer: “Today some happy news. H. A. Lorentz telegraphed me that the English expeditions have locally verified the deflection of light by the Sun.”

The team announced their findings in London on November 6, at a joint meeting of the Royal Society and the Royal Astronomical Society that had been convened expressly to learn of the eclipse results. Most of the astronomers in the audience did not know what was to be announced. J. J. Thomson, president of the Royal Society and a Nobel laureate in physics for his discovery of the electron, presided over the meeting, held in the Great Hall of the colonnaded Burlington House.

“The whole atmosphere of tense interest was exactly like that of the Greek drama,” wrote mathematician and philosopher Alfred Whitehead, who was present in the packed room. “There was a dramatic quality in the very staging:—the traditional ceremonial, and in the background the picture of Isaac Newton to remind us that the greatest of scientific generalizations was now, after more than two centuries, to receive its first modification. Nor was the personal interest wanting: a great adventure in thought had at length come safe to shore.”

Dyson spoke first, outlining the history of the expeditions. In his summary of the results, he focused on the Sobral findings—the problem with the 16 inch coelostat and the high quality of the photographic plates made with the smaller telescope. He concluded: “After a careful study of the plates I am prepared to say there can be no doubt that they confirm Einstein’s prediction. A very definite result has been obtained that light is deflected in accordance with Einstein’s law of gravitation.”

Crommelin was up next, filling in details about the Sobral observations, including the comparison of the positions of stars recorded during the eclipse and their position two months later from the Sobral site, when the Sun was not in the same part of the sky.

“The whole atmosphere of tense interest was exactly like that of the Greek drama.”

Then Eddington took the floor. He described the quality of the two images taken at Príncipe that contained enough stars to be analyzed. Combining the results of the two expeditions pointed to Einstein’s value for the deflection, 1.87, rather than the Newtonian prediction of half that value, Eddington said. “For the half effect we have to assume that gravity obeys Newton’s law; for the full effect which has been obtained we must assume that gravity obeys the law proposed by Einstein. This is one of the most crucial tests between Newton’s law and the proposed new law.”

Eddington added a caveat: “This effect may be taken as proving Einstein’s law rather than his theory.” Eddington meant that although he believed the observations confirmed the light bending predicted by Einstein, the study did not prove Einstein’s claimed source for the bending—the curvature of space-time.

The measurements had large uncertainties, and some scientists in the audience were skeptical. The physicist Ludwik Silberstein proclaimed that although he was convinced that the observations proved the deflection of starlight, they did not convincingly demonstrate that the culprit had to be the curvature of space-time. “We owe it to that great man,” he said, pointing to Newton’s portrait, “to proceed very carefully in modifying or retouching his Law of Gravitation.”

But Thomson spoke for those in the assembly who believed Einstein was right. “This is the most important result obtained in connection with the theory of gravitation since Newton’s day and it is fitting that it should be announced at a meeting of the Society so closely connected with him,” he said.

The next morning, November 7, the front page of the Times of London was full of stories about war and remembrance. It was only a few days before the first anniversary of the armistice, and King George V had just issued an invitation for all workers to take two minutes of silence out of their day to remember and honor “the glorious dead.” But to the right of these stories appeared an article about rebirth and renaissance. In a triple-decker headline, the normally staid Times wrote: “Revolution in Science / New Theory of the Universe / Newtonian Ideas Overthrown.”

The news set off a chain reaction around the globe. The New York Times followed suit with a front-page story on November 10: “Lights All Askew in the Heavens… Einstein Theory Triumphs.”

In Berlin, the originator of the new theory, the forty-year-old Einstein, awoke as usual in the apartment he shared with his second wife and two stepdaughters. Berlin was still consumed with the privations of war and its aftermath, including scarcities of food and fuel for heat, but overnight Einstein had become the first science superstar.

He wrote to a colleague that he felt sure his sudden fame would soon die down. He was wrong. Einstein’s celebrity would endure not just for days or weeks but throughout his lifetime and beyond, just as his theory of gravitation, a century later, continues to open new and unexpected windows on the birth and life of the cosmos.

Excerpted from Gravity’s Century: From Einstein’s Eclipse to Images of Black Holes by Ron Cowen, published by Harvard University Press. Copyright © 2019 by Ron Cowen. Used by permission. All rights reserved.