By Emily Levesque, University of Washington
Albert Einstein published the primary results on his general theory of relativity in 1915. He had already conceived of one elegant and straightforward means of proving something as seemingly cerebral as the bending of spacetime by mass and the test involved astronomy. Now, Einstein was eager to find a champion and collaborator who could put his theory to the test. Enter Arthur Eddington.
Triumph of German Science
Around this time, Albert Einstein became a member of the Prussian Academy of Sciences and moved to a prestigious directorship and research position in Berlin. It marked a new step in a difficult relationship between Einstein, his science, and Germany.
In the early days of World War I, Einstein’s theory of relativity was seen politically as a triumph of German science over the laws of motion and gravity made famous by Englishman Isaac Newton. In Britain, Einstein’s work was framed as politically unpalatable, and engaging with it was met with wary skepticism.
Shifting Position of Stars Near the Sun
According to general relativity, our own Sun should influence the light of background stars as it passes in front of them. The subtle warping of spacetime from our Sun’s mass should deflect the path of the light as it travels from the background stars to Earth, subtly shifting the apparent position of stars near the Sun. Einstein’s general relativity equations allowed him to calculate exactly what this expected shift would be, and while it was small, the observational astronomy experts of the era—like George Ellery Hale—confirmed to Einstein that it would be detectable with cutting-edge telescope technology.
The difficulty, of course, is that when the Sun is visible it outshines the light of any stars in the sky! For the best test, astronomers would need to observe stars incredibly close to the Sun, right at the most blinding point in the daytime sky. The only way to accomplish this would be during a total solar eclipse.
During a total solar eclipse, at the moment of totality—a few short minutes when the Moon perfectly blocks the Sun—the Sun would be in position to bend the light of background stars and the stars themselves would actually be visible! Astronomical photographs of those stars near the Sun taken during an eclipse could then show whether there was the shift in apparent position predicted by Einstein.
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Astronomical Experts to Test the Theory
The challenge became testing this theory during an actual eclipse. Einstein was not an observational astronomer himself and needed collaborators who were astronomical experts to test his theory. Eclipses are also rare and difficult to observe; they happen less than once a year, on average, and can only be seen from a narrow swath of the Earth’s surface that happens to lie beneath the path of the Moon’s shadow.
An expedition to Brazil for a 1912 eclipse was foiled by weather. A second attempt in August of 1914 took some of Einstein’s German colleagues to Crimea just weeks after Germany declared war on Russia, and political strife cut the expedition short. General relativity may have offered a grand solution to physicists’ models of space, time, and gravity, but even ground-breaking science seemed to be bending under the influence of global politics. This was when Arthur Eddington entered the story.
Arthur Eddington
Arthur Eddington was a British astronomer, three years younger than Einstein, born and raised in northern England in the Quaker faith. By the mid 1910s, he was an expert observational and theoretical astronomer, studying everything from asteroids to detailed theories that could explain the strange Cepheid variable stars discovered by Henrietta Leavitt at Harvard. His research ultimately led to ground-breaking discoveries on the interior physics of stars, including discovering that stars are powered by internal fusion and describing the physical limits on how luminous stars could get.
Eddington held the prestigious position as secretary of the Royal Astronomical Society during World War I. His scientific prowess made him uniquely suited to test the implications of general relativity.
Two Expeditions to the Atlantic Equator
Encouraging his British colleagues to pursue confirmation of a German theory in the midst of World War I was no easy feat, but Eddington ultimately worked with colleagues at the Royal Astronomical Society to organize two expeditions to the Atlantic equator in 1919, sailing with huge telescopes and heaps of equipment to set up portable observations beneath the path of the eclipse in Sobral, Brazil, and Principe Island off the coast of West Africa.
The two stations were meant to offer redundancy. The astronomers would have only minutes to capture the photographs that would prove Einstein right or wrong, and a poorly timed cloud or rainstorm at an inopportune moment could ruin years of planning and months of expedition travel.
Eddington’s Discovery
Fortunately, despite clouds in Sobral and rain in Principe, the weather cleared at both stations and Eddington and his colleagues captured crucial observations of the stars near the Sun during the eclipse.
Eddington’s early analysis was enough to confirm that the stars’ apparent position had indeed moved. Thus, Einstein had been proven correct in his prediction of there being a shift in apparent position of the stars near the Sun.
When the final analysis of the data was released, it was heralded as a major worldwide news story. Eddington’s discovery made the front page of newspapers all over the world and catapulted Einstein and his newly proven general theory of relativity to worldwide fame.
Later on, many more eclipse observations repeatedly backed up what Eddington and his colleagues had found, cementing in place this first confirmation of Einstein’s model for space, time, and gravity.
Common Questions about Arthur Eddington’s Discovery
In the early days of World War I, Einstein’s theory of relativity was seen politically as a triumph of German science over the laws of motion and gravity made famous by Englishman Isaac Newton.
According to general relativity, the subtle warping of spacetime from our Sun’s mass should deflect the path of the light as it travels from the background stars to Earth, subtly shifting the apparent position of stars near the Sun.
Arthur Eddington‘s research ultimately led to ground-breaking discoveries on the interior physics of stars, including discovering that stars are powered by internal fusion and describing the physical limits on how luminous stars could get.
