Einstein, Eddington and the 1919 eclipse

Observations of solar eclipse confirmed accuracy of theory of relativity

This week, one of the most famous experiments of 20th century science will be celebrated at an international conference in Paris. On May 29th, 1919, two British astronomical expeditions took advantage of a solar eclipse to measure a bending of distant starlight by our sun. The results, announced a few months later, offered dramatic support for Einstein’s general theory of relativity, transforming him from a moderately well-known German theorist into the world’s most famous scientist.

The bending of light by a massive body such as our sun has been observed many times since, but the observations of 1919 have a special place in the annals of science. One reason was the timing; after the terrible ravages of the first World War, journalists, politicians and scientists were happy to discuss an experiment carried out by English astronomers to test a German’s theory. Another reason was the nature of the experiment; the concept of a bending of light by celestial bodies was more than a little intriguing.

Indeed, the New York Times led with the memorable headline “Light All Askew in the Heavens . . . Einstein Theory Triumphs . . . Stars Not Where They Seemed To Be . . . But Nobody Need Worry”.

For physicists, the result was no less dramatic. In 1905, a young Albert Einstein had suggested that space and time are not absolute, but entities that are experienced differently by observers in relative motion. By 1915, he had produced the general theory of relativity, an astounding theory that suggested that the force of gravity is in reality a warping of space by a massive body.


Deflected light

In his 1915 paper, Einstein suggested an intriguing method to put general relativity to the test. According to the theory, a warping of space by our sun would cause the light from a distant star to be deflected whenever the sun lay between the star and the earth.

Thus, an earthbound astronomer might observe a slight change in the “normal” position of the star at certain times of the year. This displacement would not normally be observable due to the brightness of the sun, but could be detected by measuring the position of the star during a solar eclipse.

Einstein's prediction reached the ears of the prominent British astronomer Sir Frank Dyson. Dyson noted that, during the solar eclipse of 1919, the sun would sit in front of the Hyades, a cluster of bright stars in the constellation of Taurus. Thus, during totality, a number of stars would be visible in the vicinity of the eclipsed sun, offering an ideal opportunity to test the theory.

As the eclipse's path was predicted to pass from northern Brazil to West Africa, Dyson organised two separate expeditions to carry out the experiment. One expedition, led by the noted astronomer Arthur Stanley Eddington was to observe the eclipse from Sobral in northern Brazil; another, led by Arthur Crommelin, was to make observations from the island of Príncipe off the coast of West Africa.

Technical problems

As so often in science, not everything went to plan. The Sobral expedition managed very few observations due to poor weather conditions, while the Crommelin expedition suffered from technical problems such as a warping of optical lenses in the heat. Nevertheless, after months of analysis, Eddington was able to present a definitive result to a joint meeting of the Royal Society and the Royal Astronomical Society in London in November 1919. Despite some uncertainties, the data supported Einstein's prediction.

Of course, such experiments are never cut and dried and Eddington’s results were questioned by some American groups. However, most British and European astronomers were convinced by his data. As for the media, the reception was extraordinary. Einstein’s theory was fêted on the front pages of major national newspapers, transforming him overnight into an iconic figure of science.

Since 1919, there have been many observations of a bending of light by the sun and by other celestial bodies. Other predictions of general relativity have also been observed, from the detection of gravitational waves in 2015 to last month’s exciting observation of the event horizon of a black hole. So far, the bizarre forecasts of Einstein’s theory have been supported time and again by ever more sophisticated observations, to the amazement of physicists and the public alike.

Dr Cormac O'Raifeartaigh lectures in physics at Waterford Institute of Technology and is a fellow of the Royal Astronomical Society. He will present the talk 'Eddington, Relativity and the Expanding Universe' at the Eddington Conference in Paris – https://www.eddingtonstudies.org/