1919 Eclipse and General Relativity
Times Higher Educational Supplement

Next week, from Cornwall to the Bay of Bengal, millions of people will witness the awe-inspiring sight of a total eclipse. For some, witnessing the obliteration of the sun and being plunged into darkness will be an almost spiritual experience, but for others, the celestial spectacle will provide a an opportunity to conduct valuable astronomical experiments. The eclipse has been the subject of scientific interest ever since the time of the Ancient Babylonian astronomers, but of all the thousands of eclipses studied by scientists, the most important one was the eclipse of 1919, which was able to provide the clinching evidence in favour of one of the most revolutionary ideas in the history of physics, namely Einstein’s theory of general relativity.

Although general relativity was a radically new formulation of gravity, its predictions were largely consistent with Newton’s highly successful theory of gravity. However, Einstein’s theory did make one or two predictions which distinguished it from Newton’s theory, and, if true, these predictions would show that Einstein’s model was closer to reality. For example, Einstein predicted that a gravitational field should bend rays of light much more than was expected by Newton’s theory of gravity. Although the effect was too small to be observed in the laboratory, Einstein calculated that the immense gravity of the massive sun would deflect a ray of light by 1.75 seconds of arc – less that one thousandth of a degree, but twice as large as the deflection according to Newton, and significant enough to be measured.

Einstein pictured a scenario whereby the straight line of sight between a star and an observer on earth would be just blocked by the edge of the sun. Einstein believed that the star would still be visible because gravity would bend the rays of light around the sun and towards the earth. The sighting of a star that should have been blocked by the sun would prove Einstein right, but it is generally to impossible to see starlight that passes close to the sun, because it is swamped by the brilliance of the sun itself. However, during an eclipse, the sun is blacked out by the moon, and under such conditions a gravitationally distorted star should be visible.

General relativity was born in 1915 during the First World War, and as soon as the war ended the Astronomer Royal Sir Frank Dyson began preparing for the next total eclipse, which would occur on 29 May 1919, and which would be an opportunity to test Einstein’s theory. He had already recruited Arthur Eddington, Plumian Professor of Astronomy at Cambridge, to make the observations, a decision that was largely a consequence of Eddington’s pacifist beliefs. The son of devout Quakers, Eddington had nearly spent the war as a conscientious objector peeling potatoes in an army camp, but instead Dyson had arranged for a letter of deferment, which allowed him to carry on his astronomical research. However, in return, Eddington had to promise to make the trek to the island of Principe, off the coast of West Africa, one of the best locations for observing the 1919 eclipse.

The eclipse seemed almost too good to be true. Totality would last for 410 seconds, almost seven minutes, which is extraordinarily long for an eclipse, and which would provide plenty of opportunity for measurements to be taken. Also, the eclipse would occur against the rich background of the Hyades constellation, increasing the likelihood for an appropriately positioned star. However, as the vital day approached a cloud, or rather several, loomed on the horizon. It rained every day for the nineteen days prior to the eclipse, and as the eclipse began on 29 May, the sun was obscured by clouds. For 400 seconds the eclipse was hidden from view, and throughout this period Eddington prayed. Then with only ten seconds of the eclipse remaining, the skies miraculously cleared, and he was able to take just one meaningful photograph.

Eddington compared his eclipse photos with images taken when the sun was not present, and announced that the sun had caused a deflection of roughly 1.61 seconds of arc, a result that was in agreement with Einstein’s prediction, thereby validating the theory of general relativity. In recent years, scientists have questioned Eddington’s margin of error, arguing that his equipment was not sufficiently accurate to discriminate between the predicted effects of the rival gravitational theories.

In other words, Eddington believed in Einstein’s theory and wanted to prove that it was true, and therefore he subconsciously minimised his errors in order to get the right result. Regardless of whether or not this was the case, Eddington’s result was hailed as a wondrous piece of science, experimental validation of the greatest intellectual achievement of the of the youthful twentieth century, a sign of optimism in a world that had been torn apart by war. J.P. McEvoy, author of the “Eclipse”, encapsulated the significance of the announcement: “A new theory of the universe, the brain-child of a German Jew working in Berlin, had been confirmed by an English Quaker on a small African island.”

Next week’s eclipse will resonate with the 1919 eclipse, because it will follow an uncanny path that seems to pay homage to Einstein. The science journalist Marcus Chown noted that the eclipse, the last one of the millennium, will pass over the German city of Ulm, the birthplace of Einstein, arguably the greatest intellect of the millennium. An eclipse occurs somewhere in the world every eighteen months, but the time between eclipses above the same location is typically three centuries. Hence, the fact that this particular eclipse passes over Ulm appears to be a special celestial tribute to the physicist who concocted the general theory of relativity, the theory that dictates the motions of the stars and planets.