Einstein on the Run Read online

  Far from regarding [Einstein’s] theory as a threat to Cambridge views on geometry and electrodynamics, Eddington saw it as a new, physical explanation of gravitation that attributed gravitational effects with a finite rate of propagation, explained a puzzling anomaly in Mercury’s motion, opened exciting avenues of research concerning the large-scale structure of the universe, and made further predictions that were testable by astronomers.

  In addition to its scientific appeal, general relativity also attracted Eddington for what might be called political reasons, connected with the disaster of the First World War. Both he and Einstein were pacifists: Eddington because of his religious upbringing since birth as a Quaker, Einstein because of his lifelong dislike of German authoritarianism.

  In October 1914, two months after the outbreak of the war, ninety-three leading Germans from the world of the arts, humanities and sciences had enthusiastically signed what would become a notorious document during and after the war, entitled ‘Manifesto to the Cultured World’. In November 1919, after the announcement of Eddington’s eclipse observations, The Times noted approvingly that Einstein had not been one of the document’s signatories.

  Published in the leading German dailies, the manifesto was also translated into ten languages throughout the world. It protested ‘the lies and defamations with which our enemies are trying to besmirch Germany’s pure cause in the hard life-and-death struggle forced upon it’. It denied that Germany had started the war, defended Germany’s breach of Belgium’s neutrality, dismissed the stories of atrocities committed by Germany’s troops as fabrications, and proclaimed that Germany’s cultural legacy – Goethe, Beethoven and Kant were mentioned by name – and its current militarism, were one. Among the scientists who signed it, in addition to two Nobel laureates, Wien and Lenard, were three colleagues and friends of Einstein: Planck, Nernst and Haber.

  As a Swiss citizen, Einstein had not been asked for his signature (and was of course exempt from German military service). Yet he now decided to make his first-ever public political statement by signing a counter-manifesto drafted by a well-known German physician and physiologist, Georg Friedrich Nicolai, whose patients included the imperial family. This ‘Manifesto to the Europeans’, though it openly rejected the ‘Manifesto of the 93’, did not analyse the causes of the war and attribute guilt. Instead, in deliberately restrained language, it urged educated people everywhere to try to ‘create an organic unity out of Europe. . . . Should Europe, too, as Greece did earlier, succumb through fratricidal war to exhaustion and destruction? For the struggle that is raging today will hardly leave a victor but only vanquished behind.’ Nicolai circulated the counter-manifesto among the staff at the University of Berlin in late 1914. Many indicated agreement (some, including Planck, were having regrets about signing the nationalist manifesto) – but only two others apart from Einstein, one of whom was an unknown, were willing to sign the appeal. A distressed and isolated Nicolai gave up, and the counter-manifesto did not appear in print until 1917 – and then in Zurich, not Berlin. Nicolai’s career was ruined by it; German ultra-nationalists regarded him as a traitor and succeeded in having him banned from teaching in 1920.

  Meanwhile, among the patriots, Wien sent round his own circular. It called upon his German academic colleagues to avoid quoting scholars from the enemy camp in Britain, even in footnotes, unless such quotations were indispensable. Einstein’s influential physicist colleague, Arnold Sommerfeld, declared himself pleased to sign the appeal in a letter to Wien written on Christmas Day 1914 – of all inappropriate days – while German and Allied troops were fraternising in a truce in the trenches. Two decades later, under Adolf Hitler, German academic references to the work of Jewish physicists, especially Einstein’s, were verboten, unless they were derogatory. But in the Nazi case this was by government fiat, with serious penalties for transgression. What is particularly dispiriting about German scientific self-censorship in the First World War is that it was entirely voluntary.

  The national self-delusion was perfectly encapsulated in a vignette from Einstein the following year. After every meeting of the Berlin University Senate, he remarked in 1915 to a Swiss colleague, laughing aloud, all the professors would meet in a restaurant and ‘invariably’ the conversation would begin with the question: ‘Why are we hated in the world?’ Then there would be a discussion in which everyone would supply his own answer while ‘most carefully steering clear of the truth’.

  In Britain, Eddington, after the introduction of military conscription in 1916, was in a more challenging situation than the technically Swiss Einstein in Germany, since he was a British citizen. Some British Quaker conscientious objectors in Cambridge (such as Cunningham) were sent to work in agriculture or minesweeping, others to prison. Russell, an active pacifist (though not a Quaker), who eventually went to prison, remembered the Cambridge colleges during this stressful period as melancholy places: ‘dead, except for a few Indians and a few pale pacifists and bloodthirsty old men hobbling along victorious in the absence of youth. Soldiers are billeted in the courts and drill on the grass; bellicose parsons preach to them in stentorian tones from the steps of the hall.’

  Eddington made no secret of his conscientious objection and expected to suffer the official consequences. He informed a tribunal:

  To assert that it is our religious duty to cast off the moral progress of centuries and take part in the passions and barbarity of war is to contradict my whole conception of what the Christian religion means. Even if the abstention of conscientious objectors were to make the difference between victory and defeat, we cannot truly benefit the nation by wilful disobedience to the divine will.

  However, he was rescued from agricultural labour or even prison by Dyson, the Astronomer Royal – and by the happy chance of the forthcoming total solar eclipse in May 1919, which both Dyson and Eddington realised could be used to test general relativity. In March 1917, despite the exigencies of war, Dyson persuaded the British government to give £1,000 to investigate the eclipse (assuming that by May 1919 the war was over), under the auspices of a Joint Permanent Eclipse Committee of the Royal Society and the Royal Astronomical Society, of which Dyson was chairman. He was determined that Eddington should be the expedition’s leader. In 1918, Dyson wrote in support of Eddington:

  I should like to bring to the notice of the tribunal the great value of Prof. Eddington’s researches in astronomy, which are, in my opinion, to be ranked as highly as the work of his predecessors at Cambridge – Darwin, Ball, and Adams. They maintain the high position and traditions of British science at a time when it is very desirable that they be upheld, particularly in view of a widely spread but erroneous notion that the most important scientific researches are carried out in Germany.

  He added: ‘Under present conditions the eclipse will be observed by very few people. Prof. Eddington is peculiarly qualified to make these observations, and I hope the tribunal will give him permission to undertake this task.’

  The tribunal agreed. In effect, Eddington was exempted from military conscription in order to undertake scientific research judged to be of national importance. Although his religious objections to the war were officially ignored, the members of the tribunal did declare that they were convinced he was a genuine conscientious objector. For Eddington, who presciently foresaw his proving of Einstein’s scientific theory as an important, ‘pacifist’ opportunity to restore post-war Anglo-German relations, the tribunal’s statement was sufficient recognition of his religious objections. He accepted its decision.

  SOLAR ECLIPSE EXPEDITIONS AND RELATIVITY

  In early 1919, following the armistice of November 1918, planning of the British eclipse expeditions began in earnest. The solar eclipse was predicted to occur on 29 May in front of the Hyades, a cluster of bright stars in the constellation Taurus. A series of test photographs of the Hyades, against a reference frame of other stars, was taken by Eddington through telescopes in Britain in January and February 1919. Comparison
of these baseline measurements of each star’s position with future corresponding measurements made during the darkness of the eclipse, when these stars were just visible at the limb of the Sun, should enable astronomers to determine whether the deflection of the stars’ light by the Sun conformed to the prediction of Newton’s theory of gravitation or to the prediction of Einstein’s general relativity (that is, a deflection of starlight twice that of Newton’s theory).

  In March, the four astronomers assigned to make the observations – Eddington and E. T. Cottingham in Principe, an island off the coast of West Africa, and A. C. D. Crommelin and C. R. Davidson in Sobral, a city in northeastern Brazil – met for a final briefing in Dyson’s study at Flamsteed House in Greenwich. At one point Cottingham asked: ‘What will it mean if we get double the Einstein deflection?’ ‘Then,’ said Dyson, ‘Eddington will go mad and you will have to come home alone.’ The psychological validity of this story is supported by the fact that in later years Eddington told the young astrophysicist Subrahmanyan Chandrasekhar that he was so utterly convinced by Einstein’s theory in 1919, ‘had he been left to himself, he would not have planned the expeditions’!

  The following morning, both parties set off from England by ship. Eddington and Cottingham, after two weeks’ delay in Portugal waiting for a steamer, arrived in Principe on 29 April, exactly a month before the predicted date of the solar eclipse on 29 May. After a lot of hard work under mosquito netting, building waterproof huts for their equipment, setting up the telescopes and instruments, taking test photographs from 16 May, and even hunting monkeys interfering with the equipment, they were ready for the moment of truth.

  But cloudy weather, obscuring the solar disc, was against them – not to speak of high temperatures, affecting photographic development. The great day, 29 May, began with heavy rain, which ceased only around noon. The eclipse – scheduled to reach totality at 2.15 p.m. – had already started by the time the two astronomers got their first glimpse of the Sun. According to Eddington’s diary:

  About 1:30 when the partial phase was well advanced, we began to get glimpses of the Sun, at 1:55 we could see the crescent (through cloud) almost continuously, and there were large patches of clear sky appearing. We had to carry out our programme of photographs in faith. I did not see the eclipse, being too busy changing plates, except for one glance to make sure it had begun, and another half-way through to see how much cloud there was. We took 16 photographs. . . . They are all good pictures of the Sun, showing a very remarkable prominence; but the cloud has interfered with the star-images. The first ten photographs show practically no stars. The last six show a few images which I hope will give us what we need; but it is very disappointing.

  Only on 3 June did they get their first scientific results:

  We developed the photographs two each night for six nights after the eclipse, and I spent the whole day measuring. The cloudy weather upset my plans, and I had to treat the measures in a different way from what I intended; consequently I have not been able to make any preliminary announcement of the result. But one good plate that I measured gave a result agreeing with Einstein and I think I have got a little confirmation from a second plate.

  This, writes Eddington’s biographer, ‘was a moment which Eddington never forgot. On one occasion in later years he referred to it as the greatest moment in his life.’ Turning to Cottingham in Principe, he said – recalling their conversation in Dyson’s Greenwich study – ‘Cottingham, you won’t have to go home alone.’ To Dyson Eddington sent a noncommittal telegram: ‘Through cloud. Hopeful.’

  Soon after his return from West Africa, Eddington attended a dinner of the Royal Astronomical Society. In an affectionate parody of Edward FitzGerald’s Rubáiyát of Omar Khayyám, the ‘Astronomer-Poet of Persia’, he confidently announced:

  Oh leave the Wise our measures to collate.

  One thing at least is certain, LIGHT has WEIGHT

  One thing is certain, and the rest debate –

  Light-rays, when near the Sun, DO NOT GO STRAIGHT.

  More seriously, Eddington developed four more photographic plates from Principe. He detected in them Einstein’s value for the deflection of light, though within a rather large margin of error. As for the photographs taken in Sobral, these were all brought back to England only in late August 1919 for development and measurement. The first set to be developed showed a deflection half that of the Principe photographs, that is, Newton’s value – to the surprise and dismay of Eddington in September. But fortunately, the second set of Sobral photographs, which had been taken through a technically better telescope and could be developed only in October, supported Einstein’s value with total unambiguity and excellent accuracy. ‘They gave a final verdict,’ wrote Eddington, ‘definitely confirming Einstein’s value of the deflection, in agreement with the results obtained in Principe.’ Although Eddington’s results would long remain controversial, amidst unsubstantiated allegations by some scientists that he had fudged his measurements in favour of Einstein’s theory, subsequent evidence for general relativity has shown that Eddington and the other expedition astronomers were undoubtedly honest victims of adverse weather in Principe and some inadequate technology in Sobral.

  During this five-month-long scientific assessment, Einstein, sitting in Berlin, was naturally anxious for news from the British expeditions. He was still not in direct contact with Eddington, so he asked his close physicist friend at Leiden (de Sitter’s university), Paul Ehrenfest, for up-to-date information. On 22 September, another physicist friend at Leiden, Lorentz, replied by telegram, to the effect that Eddington had confirmed a deflection lying somewhere between Newton’s and Einstein’s values: a consequence of Eddington’s uncertainty prior to the development of the Sobral plates in October (of which Einstein was unaware).

  Despite this doubtful experimental result, Einstein – like Eddington – had all along been convinced that general relativity was true. He showed Lorentz’s telegram to a doctoral student, and told her: ‘I knew all the time that the theory was correct.’ But supposing the result had been equivocal or contradicted his theory, the student asked. ‘In that case I’d have felt sorry for God, because the theory is correct,’ Einstein famously replied. Almost thirty years later, when Planck died, Einstein told a friend, after warmly praising Planck, ‘but, you know, he didn’t really understand physics. During the eclipse of 1919, Planck stayed up all night to see if it would confirm the bending of light by the gravitational field of the Sun. If he had really understood the way the general theory of relativity explains the equivalence of inertial and gravitational mass, he would have gone to bed the way I did.’

  Not until 23 October did Einstein receive private but explicit confirmation of his theory by the eclipse observations. It came on a visit to Leiden, when a colleague showed Einstein a letter written by Eddington after he had measured the photographic plates from Sobral. Immediately, Einstein informed Planck of the experimental evidence, and also reported happily to his mother in Switzerland: ‘The result is now definite and means an exact confirmation of my theory.’

  GENERAL RELATIVITY CONFIRMED

  Public confirmation occurred in London on 6 November 1919, when the observations were presented as the sole item on the agenda of the joint meeting of the Royal Society and the Royal Astronomical Society. So important was the occasion that the audience was virtually a roll-call of the greatest names in British physics, astronomy and mathematics. Alfred North Whitehead, mathematician and philosopher, who had come specially from Cambridge, described the scene:

  The whole atmosphere of tense interest was exactly that of the Greek drama. We were the chorus, commenting on the decree of destiny in the unfolding development of a supreme incident. There was a dramatic quality in the very staging – the traditional ceremonial, and in the background the picture of Newton to remind us that the greatest of scientific generalisations was now, after more than two centuries, to receive its first modification. Nor was the personal in
terest wanting; a great adventure in thought had at length come safe to shore.

  First the Astronomer Royal, Dyson, who had launched the whole enterprise in March 1917, outlined the course of the two expeditions to West Africa and Brazil and the essentials of the photographic plates taken on 29 May 1919. Dyson declared: ‘A very definite result has been obtained that light is deflected in accordance with Einstein’s law of gravitation.’ Then Eddington, speaking for the Principe expedition, and Crommelin, speaking for the Sobral expedition, presented their observations in detail. Finally, the presidents of the two scientific societies supported Dyson and Eddington. Thomson, the president of the Royal Society – and thus the successor to Newton – said of general relativity:

  this result is not an isolated one; it is part of a whole continent of scientific ideas. . . . This is the most important result obtained in connection with the theory of gravitation since Newton’s day, and it is fitting that it should be announced at a meeting of the Society so closely connected with him. . . . If it is sustained that Einstein’s reasoning holds good . . . then it is the result of one of the highest achievements of human thought.

  A leader article published in The Times on 7 November, the day after the meeting, summed up: ‘It is confidently believed by the greatest experts that enough has been done to overthrow the certainty of ages and to require a new philosophy of the universe, a philosophy that will sweep away nearly all that has hitherto been accepted as the axiomatic basis of physical thought.’ Three headlines in the newspaper – ‘Revolution in science’, ‘New theory of the universe’ and ‘Newtonian ideas overthrown’ – were followed by six headlines in the New York Times on 10 November, including ‘Lights all askew in the heavens’, ‘A book for 12 wise men’ and ‘No more in all the world could comprehend it, said Einstein when his daring publishers accepted it.’