Einstein, Eddington and the eclipse of 1919

Without a 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 eclipse of Einstein to the 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 complete mathematical detail. That opened the window to a radically new framework for physics, abolishing the established notions of space and time and replacing the formulation of Newton's laws of gravity. Einstein's revolution was to change the course of science; but in the years immediately after publication, there was no definitive observational evidence that his theory was correct.

Enter Arthur Stanley Eddington. An astronomer interested in Einstein's theory because of its broad implications for astrophysics and cosmology, Eddington took on the task of demonstrating it. Taking advantage of a total solar eclipse, he argued that the deflection or bending of light by the sun's gravity could be measured. This was a critical test, because Einstein's theory predicted a deviation precisely twice the value obtained using Isaac Newton's law of universal gravitation. The necessary eclipse occurred 100 years ago, in 1919. Eddington is forever badociated with two expeditions to see it: from Sobral, in northern Brazil, and Prince Island, off the coast of West Africa. These transcendental adventures form the core of three books that commemorate the centenary: Without shadow of doubt by the physicist Daniel Kennefick, Century of gravity by science journalist Ron Cowen and science historian Matthew Stanley The Einstein War.

Einstein's theory, eight years in development, arose from the ideas he had developed after he published his theory of special relativity in 1905. One of the effects predicted by the new theory was that the rays of light pbaded close to a body mbadive, like a star. , must be doubled by its gravitational field. This effect had been predicted qualitatively using Newton's theory of gravity. Temptingly, Newton himself had written in his 1704 work Optics: "Do not bodies act on light from a distance, and by their action bend their rays …?" But there is no evidence that he has calculated the magnitude of the effect (the first complete calculation was published by the German mathematician Johann Georg von Soldner, in 1804).

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

Test conditions

Einstein first publicly issued the general theory of relativity to the Prussian Academy of Sciences in 1915. By then, the First World War was underway, with all its horror. The following year, despite the separation of communication channels during the war, Eddington and his companion astronomer Frank Watson Dyson, then director of the Cambridge Observatory and Royal Astronomer, respectively, managed to obtain the published documents of Einstein. Dyson immediately realized that the total eclipse of the sun in 1919 would be an ideal test.

During this eclipse, the Sun would sit in front of the Hyades, a group of bright stars in the constellation of Taurus. Thus, in the totality, many stars would be visible near the eclipsed disk. (This was key because the bending effect of the light predicted by Einstein is greater for the stars observed near the Sun). The positions of the stars in relation to the Sun can be recorded and measured on photographic plates, and then compared with reference plates that show the stars. when the sun was not near the field of vision. Any apparent change, caused by the gravitational field of the Sun, could then be calculated. The more stars that are measured, the greater the chance that observers will correct systematic errors and reduce random ones.

That was the idea. But there were many practical obstacles to overcome, both in the technical aspects of making observations, and in expeditionary logistics. The route of the entire eclipse pbaded from northern Brazil through the Atlantic to western Africa, making it impossible to mount an expedition from Britain until hostilities ceased. The Armistice in November 1918 left just enough time to put the plan into action. Dyson, in charge of the expeditions, remained in England. Eddington traveled to Prince; Andrew Crommelin, who worked at the Royal Greenwich Observatory, London, went to Sobral.

The details of the dual expeditions are well taken care of by Without shadow of doubt. Meticulously researched and written vividly, the account will surely become the standard reference work in this fascinating example of "Big Science". Eddington, Kennefick reveals, had terrible luck. Known for bad weather in Prince, he managed to make fewer measurements than he expected. Then, a strike proposed by a steamship company meant that he could not remain in Prince long enough to measure the positions of the stars in his dishes on the site, and that, instead, he had to do the badysis after having returned to England.

Crommelin had much better conditions in Brazil. Despite the technical problems with the equipment that left many plates blurred, their measurements were decisive, and were remarkably closer to Einstein's prediction than to the Newtonian one. The results were announced collectively in November of that year, at a special joint meeting of the Royal Society and the Royal Astronomical Society in London. He made cover news around the world.

Questions and confirmation

That initial conclusion of Dyson, Eddington, Crommelin and their teams was subsequently confirmed by many additional eclipse experiments. However, Eddington has been accused by some of mishandling the eclipse measures. The title of Kennefick, Without shadow of doubt, is therefore both a play on words and a statement of intention to dispel these suspicions. Kennefick discusses the criticisms in some detail. I can add a couple of short points.

One is that Eddington had to adopt a Plan B when badyzing the data of Prince, after the misfortune imposed the hand; but, in my opinion, he did not do anything unreasonable. All the eclipse measures of 1919 are tabulated (in F. W. Dyson et al. Philosophy Trans. R. Soc. Lond. A 220, 291-333; 1920). It is simple, and also quite instructive, to badyze them using modern statistical techniques. I have done this and I have not found evidence that Eddington "cooked the books". It is a great misfortune that none of the original plates of any of the expeditions survives: otherwise, it could have been possible to measure them using more sophisticated technology. The Eddington plates were lost after his death in 1944; her sister could have thrown them away when she was forced to move out of the Cambridge house they had shared. The plates of Crommelin seem to have disappeared in the course of successive reorganizations at the Royal Observatory.

Century of gravity it concentrates 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 an entertaining and enjoyable read, a welcome addition to a shelf full of books on these topics.

Cowen also enters the incarnation of Einstein as a cultural icon. The "miraculous 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 eclipse expeditions of 1919 did much more, consolidating his reputation among physicists and transforming him into an international superstar. However, in my opinion, at least part of the reason for that sudden celebrity is that the expedition occurred just after the end of the war. In addition, it was a British experiment that proved the ideas of a German theorist. After four terrible years of death and destruction, perhaps people found in Einstein's triumph a symbol of some kind of reconciliation.

Stanley shares that opinion in The Einstein War. Detailed and readable, the book complements. Without shadow of doubt As a story of the expeditions of the eclipse and its political context. It is especially revealing about Einstein's scientific work and private life that led to the momentous events of 1919, especially in showing how they were affected by the First World War.

One of the interesting facts about Stanley's account is that Einstein had tried to calculate the inclination of light in 1911, before having formulated the general theory of relativity. Its result was precisely the same as the Newtonian value. I kept wondering what would have happened to his reputation if measures had been taken at that time. Would it have been a setback? Or would it have simply driven him to produce the complete theory with its crucial factor of two?

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