How Einstein’s General Theory of Relativity Was Proven Correct a Century Ago This Week

Some six months after the Great War of 1914-18 ended, Arthur Eddington travelled at the head of a team on a scientific expedition to the island of Principe off the coast of Equatorial Guinea in West Africa. He headed one of the two teams of astronomers assigned by a Joint Permanent Eclipse Committee of the Royal Society and the Royal Astronomical Society of Britain to observe and record photographically the full solar eclipse scheduled to take place on May 29, 1919.

At the time under Portuguese rule, Principe was selected as one of the two sites – the other was Sobral in the Brazilian Nordeste – from where the total solar eclipse and its full effect could be best observed. The expedition was proposed by Eddington, a rising star among British astronomers, to test Einstein’s general theory of relativity published in the middle of the Great War.

Eddington’s expedition to Principe a century ago tested and confirmed Einstein’s general theory of relativity as the single most outstanding scientific achievement in history by one individual.  For more than two centuries Newton’s theory of gravitation, of space and time and motion, had stood as the definitive theory in explaining the mechanics of the universe, and had marked a paradigmatic shift in thinking that characterized the birth of the modern world as Newtonian.

Einstein’s theory when confirmed signified a revolution of even greater magnitude in scientific thinking than what Newton wrought with the publication of his Principia in 1687.

In 1916, Einstein published “The Foundation of the General Theory of Relativity” in the journal Annalen der Physik. Eddington was one of the very few individuals who grasped the implication of Einstein’s theory and was selected by Frank Dyson, the head of the Royal Astronomical Association, to witness and record the full solar eclipse of May 1919 from the island of Principe.

The testable proposition in Einstein’s general relativity was that light, like any material object, was bent when passing through the gravitational field of a massive body such as the sun. For verification of this conjecture, Einstein proposed that photographs taken of stars bordering the sun during a full solar eclipse be compared with those same stars made at another time.

Eddington’s mission was to photograph the cluster of stars, known as the Hyades, on a clear night without any obstruction, and then to photograph the same cluster during the total solar eclipse when the sun was fully covered by the moon. The resulting comparison of the two sets of photographs at two different times would either disprove or confirm Einstein’s proposition. And since Einstein staked his entire theory of general relativity on such a test, the results from Eddington’s mission came to be hugely anticipated by the scientific community worldwide.

At the moment of the total solar eclipse of May 29, 1919, the Hyades would be situated behind the darkened sun providing the right moment for photographs to measure the light deflection, if any, from the stars when compared with photographs of the same stars taken on a clear night.

The weather on the day of the eclipse was poor. It rained during much of the morning, but the sky cleared just ahead of the eclipse. By early afternoon the sky was clear enough and the solar eclipse fully visible for photographs to be taken with the Hyades situated in the background of the sun.

Eddington confided in his diary, “We took sixteen photographs. They are all good of the sun, showing a very remarkable prominence; but the cloud has interfered with the star images. The last six photographs show a few images which I hope will give us what we need.”

The expedition that had headed to Sobral, Brazil, observed the solar eclipse in a perfectly clear sky.  But the equatorial heat on the site where the photographs were taken warped the equipment, and this resulted in rendering useless the measurements on the photographic plates of the eclipse.

It was left to Eddington’s mission to develop the photographs and provide the measurements of the deflection of starlight, if any, from the Hyades passing through the gravitational field of the sun. On November 6, 1919 at the joint meeting of the Royal Society and the Royal Astronomical Society of Britain, Eddington presented the results of his expedition. The photographs confirmed that the deflection of the starlight in the gravitational field of the sun measured in the average 1.64 seconds.

The measurement, according to what the accuracy of the instruments allowed, was almost equal to that predicted by Einstein.

The story goes, as told by Einstein’s biographer, Ronald W. Clark, that Einstein had remained unusually calm and confident through the entire saga of the Eddington mission to Principe and the announcement of the results. Ilse Rosenthal-Schneider, one of Einstein’s students, recalled the moment when he handed her a telegram from Eddington. She wrote:

“It was Eddington’s cable with the results of measurement of the eclipse expedition. When I was giving expression to my joy that the results coincided with his calculations, he said, quite unmoved, ‘But I knew that the theory is correct,’ and when I asked, what if there had been no confirmation of his prediction, he countered: ‘Then I would have been sorry for the dear Lord -- the theory is correct.”

Einstein’s theory is correct, and more. Pedro G. Ferreira, an astrophysicist at the University of Oxford in his book The Perfect Theory (2014), which he has described as the biography of general relativity, writes, “For almost a century, it has been considered by many to be the perfect theory, a source of profound admiration to anyone who has had the privilege of coming across it.”

Einstein during a lecture in Vienna, 1921

Photograph by Ferdinand Schmutzer, restored by Adam Cuerden

In the century since Einstein published his theory, and Eddington tested and confirmed it, the theory of general relativity has provided us with the most detailed and comprehensive accounting of the nearly 14 billion year history of the universe. Only over the past decade, scientists have learned new astounding facts; for instance, that only 4 percent of the universe is made of known forms of matter and energy of which we humans are an insignificantly tiny part, and the rest is the mysterious “dark matter,” and the even more mysterious “dark energy.”

Here we might also note, in the field of epistemology Einstein’s theory of general relativity as the “perfect theory” and Eddington’s verifiability test, have become the gold standard of what Karl Popper termed “conjectures and refutations” as the basis of advancement in our knowledge of the universe and its working. How do we know what is scientifically true, and Popper indicated “by saying that the criterion of the scientific status of a theory is its falsifiability, or refutability, or testability” (italics given).

This is why in the shadow of the perfect theory, all the accompanying hoopla surrounding the theory of man-made global warming or climate change in our time fails to meet the criterion of testability, such as Eddington’s expedition put to test Einstein’s theory of general relativity and, hence, the so-called science of man-made global warming might well be described as voodoo-science.

Salim Mansur is professor emeritus in political science, Western University, London, Ontario, Canada, and author of The Qur’an Problem and Islamism (2017).

If you experience technical problems, please write to