Einstein, Eddington and the 1919 eclipse

The shadow of a doubt: the 1919 eclipse that confirmed Einstein's theory of relativity Daniel Kennefick Princeton University Press (2019)

The century of gravity: from the Einstein eclipse to images of black holes Ron Cowen Harvard University Press (2019)

Einstein's war: how relativity conquered the world Matthew Stanley Dutton (2019)

In 1916, Albert Einstein published his general theory of relativity in all its mathematical details. This opened the window to a radically new framework for physics, abolishing established notions of space and time and replacing Newton's formulation of the laws of gravity. Einstein's revolution was to change the course of science; but in the years immediately following publication, there was no definitive evidence that his theory was correct.

Enter Arthur Stanley Eddington. An astronomer interested in Einstein's theory because of its many implications for astrophysics and cosmology, Eddington set out to prove it. In exploiting a total solar eclipse, he argued that the deviation, or curvature, of the light caused by the gravity of the Sun could be measured. It was a critical test, because Einstein's theory predicts a deviation exactly twice the value obtained with Isaac Newton's law of universal gravitation. The necessary eclipse happened 100 years ago, in 1919. Eddington is now associated with two expeditions to see it: from Sobral to the north of Brazil and the island of Príncipe off the coast of the island. 39, West Africa. These major adventures form the core of three books commemorating the centenary: No shadow of a doubt by the physicist Daniel Kennefick, The century of gravity by science journalist Ron Cowen and science historian Matthew Stanley's Einstein's war.

Einstein's theory, which began eight years ago, stems from the ideas he had developed after publishing his theory of special relativity in 1905. One of the effects predicted by the New theory was that light rays passing close to a massive body, such as a star, should be bent by its gravitational field. This effect was predicted qualitatively using Newton's theory of gravity. Too bad, Newton himself had written in his opus of 1704 Opticks: "Do not the bodies act on the light at a distance, and by their action curl its rays …?" But nothing proves that he calculated the magnitude of the effect (the first complete calculation was published by the German mathematician Johann Georg von Soldner, in 1804).

Of course, Newton's theory of gravity did not formulate gravity as a consequence of curved space. It was Einstein's innovation. And when he calculated the effect, he confirmed that the light is deflected (as in Newtonian theory), but through a curved space. It is this curvature that doubles the deviation.

Test conditions

Einstein published for the first time in 1915 the general theory of relativity at the Prussian Academy of Sciences. The First World War was already well underway, in all its horror. The following year, despite the interruption of communication channels in war, Eddington and his astronomer colleague Frank Watson Dyson – respectively director of the Cambridge Observatory and astronomer Royal – managed to obtain the documents published by Einstein . Dyson immediately realized that the 1919 total eclipse was an ideal test.

During this eclipse, the sun would sit in front of the Hyades, a cluster of bright stars in the constellation Taurus. Thus, in total, many stars would be visible near the eclipsed disc. (This was essential because the light tilt effect predicted by Einstein was more important for the stars observed near the sun.) The position of the stars with respect to the sun could be recorded and measured on plates photographic and then compared to reference plates showing the stars. when the sun was far from the field of vision. All apparent displacements, caused by the gravitational field of the Sun, could then be calculated. The more measured stars are, the better the chance for observers to correct systematic errors and reduce random errors.

That was the idea. But there were many practical obstacles to overcome, both in the technical details of observation and in the expeditionary logistics. The path of the entire eclipse moved from North Brazil across the Atlantic to West Africa, making it impossible for an expedition from Britain before the end of hostilities. The November 1918 armistice left just enough time to execute the plan. Dyson, in charge of the expeditions, stays in England. Eddington went to Príncipe; Andrew Crommelin, who worked at the Royal Greenwich Observatory in London, went to Sobral.

The details of the double expedition are well served by No shadow of a doubt. Meticulously researched and written with brilliance, the account will surely become the reference book on this fascinating example of "Big Science". Eddington, reveals Kennefick, had a very bad luck. Encountered by the bad weather in Príncipe, he managed to do fewer steps than he had hoped. Then, a strike proposed by a shipping company meant that he could not stay in Príncipe long enough to measure the position of the stars on his plates on the spot, but had to carry out the analysis after his return to England.

Crommelin had much better conditions in Brazil. Despite technical problems with equipment that left many plates very fuzzy, his measurements were decisive and were significantly closer to Einstein's prediction than that of the Newtonian. The results were announced collectively in November of the same year, at a special joint meeting of the Royal Society and the Royal Astronomical Society in London. This made the headlines around the world.

Questions and confirmation

This first conclusion of Dyson, Eddington, Crommelin and their teams was then confirmed by many other eclipse experiences. Eddington was however accused by some of having mismanaged the measures of the eclipse. Kennefick title, No shadow of a doubtis therefore both a play on words and a statement of intent to dispel these suspicions. Kennefick discusses the criticisms in detail. I can add a few brief points.

The first is that Eddington had to adopt a Plan B to analyze Príncipe's data, after the misfortune forced his hand; but, in my opinion, he did not do anything unreasonable. All the eclipse measurements of 1919 are presented in tabular form (in F. W. Dyson et al. Philos. Trans. R. Soc. Lond. A 220291 to 333; 1920). It is simple, and also quite instructive, to analyze them using modern statistical techniques. I did it and found no evidence that Eddington had "cooked the books". It is a great misfortune that none of the original plates of either expedition survives: otherwise, it would have been possible to measure them using more sophisticated technologies. The Eddington plates were lost after his death in 1944 – his sister could have thrown them in the trash while she was forced to leave Cambridge home that they had shared. The plates of Crommelin seem to have disappeared during the successive reorganizations of the Royal Observatory.

The century of gravity focuses more on the broader ramifications of Einstein's theory in cosmology and astrophysics, including black holes and gravitational waves. With less than 200 pages, Cowen's book is a pleasant and enjoyable read, a welcome addition to a book-cluttered bookshelf on these topics.

Cowen also embodies the embodiment of Einstein as a cultural icon. The "miracle" year of 1905, when he published articles on Brownian motion and the photoelectric effect as well as on special relativity, made Einstein a star of physics. The expeditions of the 1919 eclipse did much more, consolidating his reputation among physicists and making him an international superstar. However, in my opinion, at least part of the reason for this sudden celebrity is that the expedition took place just after the end of the war. Moreover, it was a British experiment testing the ideas of a German theorist. After four terrible years of death and destruction, people may have found in Einstein's triumph a symbol of a kind of reconciliation.

Stanley shares this view in Einstein's war. Detailed and readable, the complete book No shadow of a doubt record of eclipse expeditions and their political context. It is particularly revealing of Einstein's scientific work and private life that preceded the memorable events of 1919 – showing in particular how they were affected by the First World War.

One of the interesting facts in Stanley's account is that Einstein had tried to calculate the bending of light in 1911, even before he had formulated the theory of general relativity. Its result was exactly the same as the Newtonian value. I wondered what would have happened to his reputation if action had been taken at that time. Would they have been a setback? Or would they have just pushed harder to produce the complete theory, with its crucial factor of two?