A solar eclipse observed over a century ago confirmed one of the key predictions of Einstein’s general theory of relativity, helped establish the theory’s credibility, and ignited one of the greatest scientific revolutions in history.
“Revolution in Science – New Theory of the Universe – Newtonian Ideas Overthrown," declared the British newspaper The Times on November 7, 1919. The report was published the day after a special meeting of the British Royal Society, at the conclusion of which the society’s president, the renowned physicist J.J. Thomson proclaimed: "The result is one of the greatest achievements of human thought." The agenda of the special meeting included only one item: a report on an astronomical observation conducted during a solar eclipse that had occurred six months earlier. The results of that observation—as can be understood—changed the course of scientific history.
Light bends in the sky, stars are not where they appear to be | New York Times report on the scientific breakthrough
A Question for Astronomers
The young and unknown scientist Albert Einstein sparked a revolution in physics when he published the theory of relativity in 1905. In this theory, he showed that time and space can be warped in a system moving at speeds close to the speed of light—at least from the perspective of an external observer. This theory later became known as special relativity, because it applied only to systems moving at constant speeds. In the following years, Einstein worked to expand the theory to include systems experiencing acceleration, and he aimed to develop a general relativity that would explain phenomena such as the laws of gravity.
Relatively early in this effort, Einstein concluded that the gravity of a sufficiently massive body would also affect light rays, causing them to bend. In a paper he published in 1911 in the journal Annalen der Physik, he predicted that the gravity of our Sun would cause a light ray passing near it to be deflected by 0.83 arcseconds (this figure was later corrected to 0.85 arcseconds).
Einstein also proposed in the paper a method for testing this prediction.
"Since during a solar eclipse it is possible to observe fixed stars in the sky near the edge of the Sun, it might be possible to compare this consequence of the theory with experimental findings [...] I hope that astronomers will urgently address the question raised here," he wrote.
What Einstein suggested was that astronomers photograph the Sun during a total solar eclipse. Under such conditions, when the Moon completely covers the face of the Sun, it becomes possible to see stars that appear to lie near the Sun's edge—though in reality, they are much farther away. The astronomers would then compare the eclipse-time photographs with standard nighttime images of the same region of the sky, and examine the positions of the distant stars. If, during the eclipse, the stars appear slightly farther from the Sun than their actual positions—by the amount Einstein predicted—it would prove that the Sun’s gravity indeed bends light as he had calculated.
Because of light bending, a star located behind the Sun will appear from Earth to be near its edge, and this can be observed during a solar eclipse | Illustration: Alexandra Avrutine
Going to War
One of the readers of the paper was the German astronomer Erwin Freundlich, from the Berlin University Observatory. Freundlich, who had only recently completed his doctorate at the University of Göttingen, was eager to carry out the measurement and prove Einstein’s prediction. The next solar eclipse suitable for such a measurement was expected on August 21, 1914, and the ideal observation point was the Crimean Peninsula, then under Russian control. Freundlich organized an expedition to photograph the eclipse, and Einstein did everything he could to raise funding for the venture. He even promised to cover part of the expedition's budget out of his own pocket if donors could not be found.
In the end, the funding was secured, and about a month before the date, Freundlich set out from Berlin with two colleagues toward Crimea. Unfortunately for them, as they traveled eastward, World War I broke out. Germany and Russia were now enemies. The Russian army arrested the three astronomers, and since they were carrying telescopes, advanced cameras, and measuring instruments, they had little chance of convincing the soldiers that they were scientists rather than spies.
Einstein was deeply concerned for the fate of his colleague and friend. Fortunately, he didn’t have to worry for long: Freundlich and his team were released a few weeks later in a prisoner exchange for Russian officers detained in Germany, and they returned to Berlin in early September.
In the end, even if Freundlich and his colleagues had reached Crimea safely, they likely would not have succeeded in their mission—the sky was overcast at the peak of the eclipse, and American researchers who were there failed to capture even a single clear photograph of the eclipse. Einstein was disappointed, but in retrospect, it seems the turn of events actually spared him some embarrassment.
Few data points, but enough. One of the photographs Eddington managed to take, showing stars around the Sun | Science Photo Library
A New Prediction
After several years of relatively slow progress in developing general relativity, Einstein decided to refocus his efforts on its mathematical formulation rather than on broad physical ideas. He was scheduled to present the theory in a series of lectures in Germany in November 1915, and in preparation for that, he continued to revise and refine the theory toward its final form. One result of the changes he made to the equations was a revised prediction regarding the deflection of light by the Sun’s gravitational field. Einstein now estimated that the Sun would bend a light ray by 1.7 arcseconds—twice the amount he had previously predicted. This value was also roughly double the deflection calculated based on the laws formulated by Isaac Newton.
When Einstein published general relativity, it caused a stir in the world of physics. Many researchers resisted the idea that space and time were a single interconnected fabric, and that gravity was a warping of this fabric. They preferred to stick with Newton’s familiar laws, which had successfully explained many phenomena. One phenomenon that didn’t quite fit with Newton’s laws was the irregularities in the orbit of the planet Mercury. These deviations had puzzled scientists for decades, but no convincing explanation had been found. Einstein suspected the reason lay in Mercury’s proximity to the Sun, where gravitational forces are extremely strong—strong enough that Newton’s laws no longer applied effectively. When he calculated Mercury’s orbit using general relativity, he found that it produced an accurate result.
However, the scientific community remained unconvinced by Einstein’s calculations. The data on Mercury’s orbit were already known, and skeptics suspected he may have simply tailored his math to fit the known results. To truly prove general relativity, the theory needed to make a prediction about a phenomenon that had not yet been measured—something that could be tested independently.
The theory is correct: gravity bends light just as predicted. Eddington (right) and Einstein in the 1930s | Source: Science Photo Library
An Observation for Peace
During the war, there was almost total disconnection between Germany and Britain or France. Yet, the paper on general relativity managed to cross the English Channel and reached Arthur Eddington, the director of the Cambridge Observatory. Unlike many of his colleagues, Eddington was immediately convinced the theory was correct, and he even published the first English-language article explaining it. As a Quaker and a pacifist, Eddington deeply appreciated the idea that British scientists might photograph a solar eclipse and confirm the theory of a German scientist.
The suitable eclipse was set to occur only three years later, on May 29, 1919, and would be visible only near the equator over the Atlantic Ocean. With the blessing of the Astronomer Royal, Frank Dyson, Eddington began planning an expedition to photograph the eclipse. The planning also solved another issue for Eddington: he had refused military service on conscientious grounds, and his offer to volunteer as an ambulance driver for a Red Cross unit had been rejected. Now, with a scientific mission at hand, Dyson persuaded the authorities that Eddington would contribute more to the country by leading an expedition that could prove the superiority of Newton's British theory over the German one—though both Dyson and Eddington hoped the result would instead favor the new science.
Eddington was granted exemption from military service and began planning the expedition to a region still patrolled by German submarines eager to sink any British vessel. Fortunately, the war ended about six months before the eclipse, allowing the expedition to focus on the other significant challenges it faced. In addition to the harsh conditions of remote locations, the scientists had to work quickly—the total eclipse would last less than seven minutes—and use the heavy, cumbersome equipment of that era, in the early days of astronomical photography. Most crucially, they had to hope for clear skies.
It was therefore decided to split the expedition into two teams: one would head to the coast of Brazil, and the other to the island of Príncipe, off the west coast of Africa—hoping at least one would get a clear shot of the eclipse.
Almost three months before the eclipse, the expedition departed Liverpool for the island of Madeira, where it split into two groups. Eddington sailed to Príncipe, and the second team, led by astronomer Andrew Crommelin, traveled to Sobral in Brazil. The weather was not kind to Eddington’s team—there was heavy rain in the morning, and although the skies gradually cleared by the afternoon eclipse, they remained patchy with clouds. The team managed to take 16 photographs, but it was unclear how much the clouds had obscured the stars. In Brazil, the weather was better, but to truly assess the data, the astronomers had to wait for the photographic plates to return to England. Only there could the images be developed, the visible stars identified, and their positions compared with astronomical photographs taken in Oxford at a different time—when the Sun was not present in that region of the sky.
An instrument built by Eddington to measure star positions and compare photographic plates | Source: Science Photo Library
God’s Sorrow
The process took several months. When Eddington finally had the data in hand, they were not as conclusive as he had hoped. He dismissed the results from one of the telescopes in Brazil, which showed a star deflection close to the Newtonian prediction, arguing that the telescope mirror had likely warped—the images from that device were blurry and had a wide margin of error. Better-quality images, taken with another Brazilian telescope, showed a deflection of 1.98 arcseconds. The photographs from Africa were less successful: only two plates showed stars with sufficient clarity, and complex calculations were needed to determine that their average deviation from the expected positions was 1.6 arcseconds. Ultimately, after weighing the data from both sites, Eddington concluded that the average measured deflection was 1.7 arcseconds—exactly as Einstein had predicted.
On September 22, Einstein received the results via telegram from his friend, the Dutch physicist Hendrik Lorentz. In Einstein: His Life and Universe, biographer Walter Isaacson recounts how Einstein showed the telegram to a graduate student visiting him. When she asked how he would have responded if the experiments had disproved his theory, he replied: "Then I would feel sorry for the good Lord. The theory is correct."
On November 6, Dyson presented the findings at a special meeting of the Royal Society together with representatives of the British Astronomical Association. He explained the calculations in detail and concluded: “The results of the expeditions to Sobral and Príncipe leave little doubt that light is deflected near the Sun, and that the amount of this deflection agrees with the predictions of Einstein’s general theory of relativity.”
A few weeks later, the scientific paper was published in a journal of the Royal Society.
Proof of the validity of Einstein’s theory made headlines, as mentioned, in the British Times, and two days later in the United States. The New York Times reported:
“Lights All Askew in the Heavens – Einstein’s Theory Triumphs,” and added: “Stars Not Where They Seemed, or Were Calculated to Be, but Nobody Need Worry.” The paper also noted Einstein’s own estimate that only a dozen people in the world could fully understand his theory.
The news created a global sensation, and Einstein became the first scientific celebrity—sought after by the media and a welcome guest at any scientific or even commercial event. General relativity became a topic of conversation in living rooms, even among people who were not among the few scientists who truly understood it in depth.
One of relativity’s strange predictions turned out to be real. A simulation (right) and an actual image of a black hole | Science Photo Library
A New Understanding of the Universe
Although general relativity quickly gained popularity with the general public—and perhaps because of that—it still faced strong opposition within the scientific community, alongside its enthusiastic supporters. The Nobel Prize committee, for example, could not reach a decision for years on awarding Einstein the prize. Eventually, a compromise was reached: the 1921 Nobel Prize in Physics was granted to him for a different achievement—the law of the photoelectric effect, presented in a paper he published in 1905.
Einstein continued his work in physics, focusing mainly on various aspects of general relativity and efforts to develop a theory that would unify all the fundamental forces of nature. However, he never achieved another breakthrough of the same magnitude as the theory of relativity or his earlier work. With the rise of the Nazis, he left Germany for the United States, where he lived until his death from an abdominal aortic aneurysm in 1955.
Eddington also continued working in physics and cosmology until his death from cancer in 1944. Despite his many contributions to science, he is remembered today primarily for the once-in-a-lifetime astronomical observation that led to the first proof of general relativity.
Over time, additional eclipse measurements confirmed that the conclusion drawn by Eddington and Dyson was correct: the bending of light by the Sun’s gravity indeed causes an apparent shift of 1.7 arcseconds in the position of stars, just as Einstein had predicted. Later, as technology and observational tools improved, many more confirmations of Einstein’s theory followed.
The German physicist Karl Schwarzschild, before his untimely death in 1916, managed to publish two important papers based on the equations of general relativity. In one of them, he demonstrated the mathematical possibility of objects so dense that their gravity wouldn’t allow even a beam of light to escape. This was the first theoretical prediction of black holes. Scientists later proved that such objects do indeed exist, and only a few weeks ago, the first direct observational evidence of them was obtained.
One problem that arose from general relativity was that, when applying it to the universe as a whole, the equations suggested that the universe is expanding—contrary to Einstein’s belief, shared by most of his contemporaries, that the universe was static. To reconcile this, he introduced a correction to the equation: the "cosmological constant," a kind of repulsive force that counteracts gravitational attraction and keeps the universe unchanging. Einstein admitted that this constant was an inelegant solution, marring the beauty and elegance of the theory—but he had no better option at the time.
Other scientists, however, did find a better option. Russian mathematician Alexander Friedmann passionately argued that the universe is indeed expanding and even debated Einstein on this in articles and letters. But he too died young, at only 37, before he could settle the argument. Belgian physicist and priest Georges Lemaître, unaware of Friedmann and his work, independently arrived at the same conclusion. He also presented his findings to Einstein—who opposed them. Once again, it was the astronomers who resolved the dispute: Edwin Hubble and others showed through observations that the universe is indeed expanding. In 1931, Einstein admitted his mistake and removed the cosmological constant from his equation.
The insight that the universe is expanding—first derived from general relativity—became a central pillar in the development of the Big Bang theory, which today remains the best explanation for the origin of the universe.
An application of general relativity: gravitational lensing allows astronomers to use the bending of light for astronomical observations | Illustration: Science Photo Library
Another prediction derived from general relativity was the existence of gravitational waves—tiny ripples in spacetime caused by the movement of massive bodies in the universe. For decades, scientists attempted to detect such waves to prove their existence, but only succeeded a few years ago, when waves generated by the merger of two black holes were measured. A year later, several leading researchers in the field were awarded the 2017 Nobel Prize in Physics for this achievement.
The understanding that strong gravity bends light rays has allowed astronomers to use distant stars—and even entire galaxies—to observe even farther galaxies using a method called gravitational lensing, which essentially uses gravity as a magnifying lens. General relativity provides the scientific basis for this phenomenon and enables astrophysicists to interpret what they see. The method has led to numerous discoveries in astronomy.
These major breakthroughs have been joined by many other observations and measurements that confirmed various aspects of general relativity. Today, the theory is fully accepted by the global scientific community—although, even after more than a hundred years, the number of people who fully understand it remains very small (full disclosure: the author of these lines is not among them). It's likely that without Eddington’s expedition, the theory would have been confirmed eventually by other observations, but the fact that it happened so soon after the theory's introduction had a profound impact on the development of physics and cosmology in the 20th century.