SPRING BOOKS PHYSICS Relativity revealed Peter Coles extols three books on the 1919 eclipse expeditions that proved Einstein right. n 1916, Albert Einstein published No Shadow of a Doubt: The 1919 Eclipse was a critical test, because Einstein’s theory his general theory of relativity in full That Confirmed Einstein’s Theory of predicted a deflection precisely twice the mathematical detail. That opened the Relativity value obtained using Isaac Newton’s law of Iwindow on a radically new framework for DANIEL KENNEFICK universal gravitation. The needed eclipse physics, abolishing established notions of Princeton University Press (2019) came 100 years ago, in 1919. Eddington is space and time and replacing Newton’s Gravity’s Century: From Einstein’s Eclipse to now forever associated with two expeditions Images of Black Holes formulation of the laws of gravity. Ein- to view it: from Sobral in northern Brazil, ANA KOVA BY ILLUSTRATIONS stein’s revolution was to change the course RON COWEN and the island of Príncipe off the coast of Harvard University Press (2019) of science; but in the years immediately West Africa. Those momentous ventures after publication, there was no definitive Einstein’s War: How Relativity Triumphed form the kernel of three books commemo- observational evidence that his theory was Amid the Vicious Nationalism of World War I rating the centenary: No Shadow of a Doubt correct. MATTHEW STANLEY by physicist Daniel Kennefick, Gravity’s Dutton (2019) Enter Arthur Stanley Eddington. An Century by science journalist Ron Cowen, astronomer interested in Einstein’s theory and science historian Matthew Stanley’s because of its wide-ranging implications harnessing a total solar eclipse, he argued Einstein’s War. for astrophysics and cosmology, Edding- that the deflection, or bending, of light by Einstein’s theory, eight years in the ton took on the task of proving it. By the Sun’s gravity could be measured. This making, sprang from insights he had 306 | NATURE | VOL 568 | 18 APRIL 2019 | CORRECTED©2019 Spri nger N17atu rAPRILe Li mited . 2019All ri ghts reserved. ©2019 Spri nger Nature Li mited. All ri ghts reserved. SPRING BOOKS COMMENT developed after he published his theory of the expeditions, remained in England. sophisticated technology. Eddington’s special relativity in 1905. One of the effects Eddington travelled to Príncipe; Andrew plates were lost after he died in 1944 — predicted by the new theory was that light Crommelin, who worked at the Royal his sister might have thrown them away rays passing close to a massive body, such Greenwich Observatory, London, went to when she was forced to move out of the as a star, should be bent by its gravitational Sobral. Cambridge home they had shared. Crom- field. This effect had been predicted qualita- The details of the dual expeditions are melin’s plates seem to have disappeared in tively using Newton’s theory of gravity. Tan- well served by No Shadow of a Doubt. Metic- the course of successive reorganizations at talizingly, Newton himself had written in his ulously researched and vividly written, the the Royal Observatory. 1704 opus Opticks: “Do not Bodies act upon account is sure to become the standard ref- Gravity’s Century concentrates more Light at a distance, and by their action bend erence work on this fascinating example of on the broader ramifications of Einstein’s its Rays…?” But there is no evidence that ‘Big Science’. Eddington, Kennefick reveals, theory in cosmology and astrophysics, he calculated the magnitude of the effect had terrible luck. Met including black holes (the first full calculation was published by by poor weather in and gravitational German mathematician Johann Georg von Príncipe, he man- waves. At fewer than Soldner, in 1804). aged to make fewer THE 200 pages, Cowen’s Newton’s theory of gravity did not, of measurements than book is a breezy and course, formulate gravity as a consequence he had hoped. Then, ARMISTICE enjoyable read, a of curved space. That was Einstein’s innova- a proposed strike by IN NOVEMBER 1918 welcome addition to a tion. And when he calculated the effect, he a steamship company crowded shelf of books confirmed that light is deflected (as in the meant that he could LEFT JUST on these topics. Newtonian theory), but through curved not stay in Príncipe Cowen also goes space. It is this curvature that doubles the long enough to meas- ENOUGH TIME into Einstein’s incar- deflection. ure the star positions TO PUT nation as a cultural on his plates on site, icon. The ‘miracle TEST CONDITIONS and had instead to THE PLAN year’ of 1905, when Einstein first publicly aired the general the- do the analysis after he published papers ory of relativity to the Prussian Academy he had returned to INTO ACTION. on Brownian motion of Sciences in 1915. The First World War England. and the photoelectric was by then well under way, in all its horror. Crommelin had effect as well as on The next year, despite wartime severance of much better conditions in Brazil. Despite special relativity, made Einstein a star of communication channels, Eddington and technical issues with equipment that left physics. The eclipse expeditions of 1919 fellow astronomer Frank Watson Dyson many plates badly blurred, his measure- did much more, cementing his reputation — then director of the Cambridge Obser- ments were decisive, and were noticeably among physicists and transforming him vatory and Astronomer Royal, respectively closer to the Einstein prediction than to the into an international superstar. Yet, in my — managed to obtain Einstein’s published Newtonian. The results were announced col- view, at least part of the reason for that sud- papers. Dyson immediately realized that lectively in November that year, at a special den celebrity is that the expedition came the total solar eclipse in 1919 would prove joint meeting of the Royal Society and Royal just after the war’s end. Moreover, it was an ideal test. Astronomical Society in London. It made a British experiment testing the ideas of a During this eclipse, the Sun would sit front-page news around the globe. German theorist. After four terrible years in front of the Hyades, a cluster of bright of death and destruction, perhaps people stars in the constellation of Taurus. Thus, QUESTIONS AND CONFIRMATION found in Einstein’s triumph a symbol of at totality, many stars would be visible near That initial conclusion by Dyson, some sort of reconciliation. the eclipsed disk. (This was key because the Eddington, Crommelin and their teams was Stanley shares that view in Einstein’s light-bending effect predicted by Einstein subsequently confirmed by many further War. Detailed and readable, the book is greatest for stars observed close to the eclipse experiments. Yet Eddington has complements No Shadow of a Doubt as an Sun.) The stars’ positions relative to the been accused by some of mishandling the account of the eclipse expeditions and their Sun could be recorded and measured on eclipse measurements. Kennefick’s title, No political backdrop. It is especially revealing photographic plates, and then compared Shadow of a Doubt, is thus both a pun and about Einstein’s scientific work and private with reference plates showing the stars a declaration of intent to dispel these sus- life leading up to the momentous events of when the Sun was nowhere near the field picions. Kennefick discusses the criticisms 1919 — particularly in showing how these of view. Any apparent shifts, caused by in some detail. I can add a couple of brief were affected by the First World War. the Sun’s gravitational field, could then points. One of the interesting facts from Stanley’s be calculated. The more stars measured, One is that Eddington had to adopt a account is that Einstein had made a stab at the better the chance the observers would Plan B when analysing the Príncipe data, calculating the bending of light back in 1911, have of correcting for systematic errors and after misfortune had forced his hand; but, before he had formulated the full general reducing random ones. in my opinion, he did nothing unreason- theory of relativity. His result was precisely That was the idea. But there were many able. All the 1919 eclipse measurements the same as the Newtonian value. I was left practical obstacles to surmount, both in are tabulated (in F. W. Dyson et al. Philos. wondering what would have happened to his the technicalities of making the observa- Trans. R. Soc. Lond. A 220, 291–333). It is reputation if measurements had been taken tions, and in the expeditionary logistics. straightforward, and also quite instruc- then. Would they have been a setback? Or The eclipse’s path of totality passed from tive, to analyse them using modern statis- would they just have driven him harder to northern Brazil across the Atlantic to West tical techniques. I’ve done this, and found produce the full theory, with its crucial factor Africa, making it impossible to mount an no evidence that Eddington ‘cooked the of two? ■ expedition from Britain until hostilities books’. It’s a huge misfortune that none of had ceased. The Armistice in November the original plates from either expedition Peter Coles is Professor of Theoretical 1918 left just enough time to put the plan survives: otherwise, it might have been Physics at Maynooth University in Ireland. into action. Dyson, in overall charge of possible to measure them using more e-mail: [email protected] ©2019 Spri nger Nature Li mited. All ri ghts reserved. ©2019 Spri nger Nature Li mited. All ri ghts reserved. 18 APRIL 2019 | VOL 568 | NATURE | 307 COMMENT BOOKS & ARTS CORRECTION In the Books & Arts article ‘Relativity revealed’ (Nature 568, 306-307; 2019), the subtitle of Einstein’s War by Matthew Stanley should have been ‘How Relativity Triumphed Amid the Vicious Nationalism of World War I’.