The solar eclipse on 11 August should reveal stars shining close to the sun. Those stars will not be where they seem to be. We know this because of Albert Einstein's general theory of relativity, and it was the solar eclipse of 1919 that tested the theory and found it to be true. The eclipse also made Einstein, almost overnight, into the world's most famous scientist. His theories were incomprehensible to almost everyone, but that, if anything, increased his popularity, not unlike Stephen Hawking with his Brief History of Time decades later.
Every factor helped to make an inspirational news story for the general public: the timing of the eclipse soon after the armistice ending the first world war; the dramatic corroboration of a revolutionary theory that could "overturn" Newtonian physics; the hopeful precedent of Anglo-German scientific collaboration; and the photogenic, convention-defying man of genius himself.
Einstein's general theory of relativity presented a radically different concept of gravity from the Newtonian model. Gravity, it argued, is not a force but a property of space, and space is not empty but has a four-dimensional structure. This can be distorted by the presence of mass, which can bend light beams as well as the path of material objects. Einstein predicted that during a solar eclipse the gravitational pull of the sun would "bend" light from nearby stars, which would appear in different positions from those expected.
Years before the theory was published, Erwin Finlay Freundlich, a German astronomer, planned to test it by observing an eclipse from southern Russia in August 1914. But that month war was declared between Russia and Germany; Freundlich, arriving in the Crimea, was imprisoned and deported a month later - missing the eclipse. Perhaps it was as well; Einstein's mathematical formulations were then unfinished and the theory might not have stood the test.
The theory was published in 1916. Einstein sent a copy to a Dutch colleague, who forwarded it to Arthur Eddington, secretary of the Royal Astronomical Society, in London. The 1919 solar eclipse presented the opportunity for tests. The best viewing, inconveniently, was from Principe island, in the Gulf of Guinea, and Sobral, in Brazil. On 29 May in Principe it rained heavily until noon, and Eddington was so busy changing photographic plates that he didn't see the eclipse at 1.30pm. He developed the pictures immediately; cloud obscured the star images, but one plate gave results that matched Einstein's prediction.
The Brazilian plates were developed and measured later, and Einstein himself didn't get confirmation until late October. The following month a packed joint meeting of the Royal Society and the Royal Astronomical Society made the news public. The president of the Royal Society, J J Thomson, described the general theory as "the greatest discovery in connection with gravitation since Newton . . . Our conceptions of the fabric of the universe must be fundamentally altered."
The next day, the Times reported the meeting and summarised the theory under the heading "Space warped", launching a torrent of publicity that engulfed Einstein for the rest of his life. Within a year, more than 100 books had been written on the subject; prizes were offered for the best explanation; lecturers and discussion groups even in small villages grappled with it; an impresario tried to book Einstein for a run at the London Palladium. Endless jokes and rhymes were coined, such as the limerick: " There was a young lady called Bright/whose speed was much faster than light./She went out one day/in a relative way/and came back the previous night."
Einstein was portrayed as a scientific magician who showed that light could bend and explained afresh the motion of stars in a universe at once finite and unending. Journalists hung on every word from their crumpled, sockless, always (mis)quotable quarry. Legions of stories circulated about the peripatetic genius who had to ring up his wife to remind him where he was and where he should be.
Einstein was popular everywhere except in his home country. As a pacifist, socialist, Zionist Jew and supporter of the new republic, he stood for everything most hated by German patriots bewildered by their country's defeat, who believed it had been engineered from within by the Jews and the Weimar government. The new physics was denounced by some as a "Jewish fraud". Einstein's science became enmeshed with political issues, and his celebrity later made him a leading target for the Nazis.
Einstein was 40 in 1919; his career had already peaked with the completion of the general relativity theory. Asked what he would study next, the scientist whose great work was vindicated by a solar eclipse replied: "For the rest of my life I want to reflect on what light is."