Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar (2009)

Part III. TITANS CLASH OVER REALITY

Chapter 12. EINSTEIN FORGETS RELATIVITY

Bohr was stunned. Einstein smiled.

Over the past three years, Bohr had re-examined the imaginary experiments Einstein had proposed at the Solvay conference in October 1927. Each was designed to show that quantum mechanics was inconsistent, but he had found the flaw in Einstein's analysis in every case. Not content to rest on his laurels, Bohr devised some thought experiments of his own involving an assortment of slits, shutters, clocks and the like as he probed his interpretation for any weaknesses. He found none. But Bohr never conjured up anything as simple and ingenious as the thought experiment that Einstein had just finished describing to him in Brussels at the sixth Solvay conference.

The theme of the six-day meeting that began on 20 October 1930 was the magnetic properties of matter. The format remained the same: a series of commissioned reports on various topics related to magnetism, each followed by a discussion. Bohr had joined Einstein as a member of the nine-strong scientific committee and both were therefore automatically invited to the conference. After the death of Lorentz, the Frenchman Paul Langevin had agreed to take on the demanding dual responsibilities of presiding over the committee and the conference. Dirac, Heisenberg, Kramers, Pauli and Sommerfeld were among the 34 participants.

As a meeting of minds it was a close second to Solvay 1927, with twelve current and future Nobel laureates present. It was the backdrop to the 'second round' of the ongoing struggle between Einstein and Bohr over the meaning of quantum mechanics and the nature of reality. Einstein had travelled to Brussels armed with a new thought experiment designed to deliver a fatal blow to the uncertainty principle and the Copenhagen interpretation. An unsuspecting Bohr was ambushed after one of the formal sessions.

Imagine a box full of light, Einstein asked Bohr. In one of its walls is a hole with a shutter that can be opened and closed by a mechanism connected to a clock inside the box. This clock is synchronised with another in the laboratory. Weigh the box. Set the clock to open the shutter at a certain time for the briefest of moments, but long enough for a single photon to escape. We now know, explained Einstein, precisely the time at which the photon left the box. Bohr listened unconcerned; everything Einstein had proposed appeared straightforward and beyond contention. The uncertainty principle applied only to pairs of complementary variables – position and momentum or energy and time. It did not impose any limit on the degree of accuracy with which any one of the pair could be measured. Just then, with a hint of smile, Einstein uttered the deadly words: weigh the box again. In a flash, Bohr realised that he and the Copenhagen interpretation were in deep trouble.

To work out how much light had escaped locked up in a single photon, Einstein used a remarkable discovery he had made while still a clerk at the Patent Office in Bern: energy is mass and mass is energy. This astonishing spin-off from his work on relativity was captured by Einstein in his simplest and most famous equation: E=mc2, where E is energy, m is mass, and c is the speed of light.

By weighing the box of light before and after the photon escapes, it is easy to work out the difference in mass. Although such a staggeringly small change was impossible to measure using equipment available in 1930, in the realm of the thought experiment it was child's play. Using E=mc2 to convert the quantity of missing mass into an equivalent amount of energy, it was possible to calculate precisely the energy of the escaped photon. The time of the photon's escape was known via the laboratory clock being synchronised with the one inside the light box controlling the shutter. It appeared that Einstein had conceived an experiment capable of measuring simultaneously the energy of the photon and the time of its escape with a degree of accuracy proscribed by Heisenberg's uncertainty principle.

'It was quite a shock for Bohr', recalled the Belgian physicist Léon Rosenfeld, who had recently begun what turned into a long-term collaboration with the Dane.1 'He did not see the solution at once.' While Bohr was desperately worried by Einstein's latest challenge, Pauli and Heisenberg were dismissive. 'Ah, well, it will be all right, it will be all right', they told him.2 'During the whole evening he was extremely unhappy, going from one to the other and trying to persuade them that it couldn't be true, that it would be the end of physics if Einstein were right,' recalled Rosenfeld, 'but he couldn't produce any refutation.'3

Rosenfeld was not invited to Solvay 1930, but had travelled to Brussels to meet Bohr. He never forgot the sight of the two quantum adversaries heading back to the Hotel Metropole that evening: 'Einstein, a tall majestic figure, walking quietly, with a somewhat ironical smile on his face, and Bohr trotting near him, very excited, ineffectually pleading that if Einstein's device would work, it would mean the end of physics.'4 For Einstein it was neither an end nor a beginning. It was nothing more than a demonstration that quantum mechanics was inconsistent and therefore not the closed and complete theory that Bohr claimed. His latest thought experiment was simply an attempt to rescue the kind of physics that aimed to understand an observer-independent reality.

A photograph shows Einstein and Bohr walking together, but slightly out of step. Einstein is just ahead as if trying to flee. Bohr, mouth open, is hurrying to keep pace. He leans towards Einstein, desperate to make himself heard. Despite having his coat draped over his left arm, Bohr gestures with his left forefinger to emphasise whatever point he is trying to make. Einstein's hands are by his side, one clutching a briefcase and the other a possible victory cigar. As he listens, Einstein's moustache fails to hide the half-knowing smile of a man who thinks he has just gained the upper hand. That evening, said Rosenfeld, Bohr looked 'like a dog who has received a thrashing'.5

Bohr spent a sleepless night examining every facet of Einstein's thought experiment. He took the imaginary box of light apart to find the flaw that he hoped existed. Einstein did not picture, even in his mind's eye, either the details of the inner workings of the light box or how to weigh it. Bohr, desperate to get to grips with the device and the measurements that would have to be made, drew what he called a 'pseudorealistic' diagram of the experimental set-up to help him.

Figure 18: Bohr's later rendition of Einstein's 1930s light box (Niels Bohr Archive, Copenhagen)

Given the need to weigh the light box before the shutter is opened at a pre-set time and after the photon has escaped, Bohr decided to focus on the weighing process. With mounting anxiety and little time, he chose the simplest possible method. He suspended the light box from a spring fixed to a supporting frame. To turn it into a weighing scale, Bohr attached a pointer to the light box so its position could be read on a scale attached to the vertical arm of what resembled a hangman's gallows. To ensure that the pointer was positioned at zero on the scale, Bohr attached a small weight to the bottom of the box. There was nothing whimsical in the construction, as Bohr included even the nuts and bolts used to fix the frame to a base, and drew the clockwork mechanism controlling the opening and closing of the hole through which the photon was to escape.

The initial weighing of the light box is simply the configuration with the attached weight chosen to ensure that the pointer is at zero. After the photon escapes, the light box is lighter and is pulled upwards by the spring. To reposition the pointer at zero, the attached weight has to be replaced by a slightly heavier one. There is no time limit on how long the experimenter can take to change the weights. The difference in the weights is the mass lost due to the escaped photon, and from E=mc2 the energy of the photon can be calculated precisely.

From the arguments he deployed at Solvay 1927, Bohr held that any measurement of the position of the light box would lead to an inherent uncertainty in its momentum, because to read the scale would require it to be illuminated. The very act of measuring its weight would cause an uncontrollable transfer of momentum to the light box because of the exchange of photons between the pointer and the observer causing it to move. The only way to improve the accuracy of the position measurement was to carry out the balancing of the light box, the positioning of the pointer at zero, over a comparatively long time. However, Bohr argued that this would lead to a corresponding uncertainty in the momentum of the box. The more accurately the position of the box was measured, the greater the uncertainty attached to any measurement of its momentum.

Unlike at Solvay 1927, Einstein was attacking the energy–time uncertainty relation, not the position–momentum incarnation. It was now, in the early hours of the morning, that a tired Bohr suddenly saw the flaw in Einstein's gedankenexperiment. He reconstructed the analysis bit by bit until he was satisfied that Einstein had indeed made an almost unbelievable mistake. Relieved, Bohr went to sleep for a few hours, knowing that when he awoke it would be to savour his triumph over breakfast.

In his desperation to destroy the Copenhagen view of quantum reality, Einstein had forgotten to take into account his own theory of general relativity. He had ignored the effects of gravity on the measurement of time by the clock inside the light box. General relativity was Einstein's greatest achievement. 'The theory appeared to me then, and it still does, the greatest feat of human thinking about Nature, the most amazing combination of philosophic penetration, physical intuition, and mathematical skill', said Max Born.6 He called it 'a great work of art, to be enjoyed and admired from a distance'. When the bending of light predicted by general relativity was confirmed in 1919, it made headlines around the world. J.J. Thomson told one British newspaper that Einstein's theory was 'a whole new continent of new scientific ideas'.7

One of these new ideas was gravitational time dilation. Two identical and synchronised clocks in a room with one fixed to the ceiling and the other on the floor would be out of step by 300 parts in a billion billion, because time flows more slowly at the floor than at the ceiling.8 The reason was gravity. According to general relativity, Einstein's theory of gravity, the rate at which a clock ticks depends upon its position in a gravitational field. Also, a clock moving in a gravitational field ticks slower than one that is stationary. Bohr realised that this implied that weighing the light box affected the time-keeping of the clock inside.

The position of the light box in the earth's gravitational field is altered by the act of measuring the pointer against the scale. This change in position would alter the rate of the clock and it would no longer be synchronised with the clock in the laboratory, making it impossible to measure as accurately as Einstein presumed the precise time the shutter opened and the photon escaped from the box. The greater the accuracy in measuring the energy of the photon, via E=mc2, the greater the uncertainty in the position of the light box within the gravitational field. This uncertainty of position prevents, due to gravity's ability to affect the flow of time, the determination of the exact time the shutter opens and the photon escapes. Through this chain of uncertainties Bohr showed that Einstein's light box experiment could not simultaneously measure exactly both the energy of the photon and the time of its escape.9Heisenberg's uncertainty principle remained intact, and with it the Copenhagen interpretation of quantum mechanics.

When Bohr came down to breakfast he was no longer looking 'like a dog who has received a thrashing' the night before. Now it was Einstein who was stunned into silence as he listened to Bohr explain why his latest challenge, like those of three years earlier, had failed. Later there would be those who questioned Bohr's refutation because he had treated macroscopic elements such as the pointer, the scale, and the light box as if they were quantum objects and therefore subject to limitations imposed by the uncertainty principle. To handle macroscopic objects in this way ran counter to his insistence that laboratory equipment be treated classically. But Bohr had never been particularly clear about where to draw the line between the micro and macro, since in the end every classical object is nothing but a collection of atoms.

Whatever reservations some had later, Einstein accepted Bohr's counter-arguments, as did the physics community at the time. As a result he ceased his attempts to circumvent the uncertainty principle to demonstrate that quantum mechanics was logically inconsistent. Instead Einstein would henceforth focus on exposing the theory as incomplete.

In November 1930 Einstein lectured in Leiden on the light box. Afterwards a member of the audience argued that there was no conflict within quantum mechanics. 'I know, this business is free of contradictions,' replied Einstein, 'yet in my view it contains a certain unreasonableness.'10 In spite of this, in September 1931, he once again nominated Heisenberg and Schrödinger for a Nobel Prize. But after going two rounds with Bohr and his seconds at the Solvay conferences, one sentence in Einstein's letter of nomination was telling: 'In my opinion, this theory contains without doubt a piece of the ultimate truth.'11 His 'inner voice' continued to whisper that quantum mechanics was incomplete, that it was not the 'whole' truth as Bohr would have everyone believe.

At the end of the 1930 Solvay conference, Einstein travelled to London for a few days. He was the guest of honour at a fundraising dinner on 28 October for the benefit of impoverished eastern European Jews. Held at the Savoy Hotel, and hosted by Baron Rothschild, the fundraiser drew almost a thousand people. With the great and the good elegantly dressed, Einstein willingly donned white tie and tails to play his part in what he called the 'monkey comedy' if it helped open wallets.12 George Bernard Shaw was the master of ceremonies.

Although he occasionally departed from his prepared script, the 74-year-old Shaw gave a virtuoso performance that began with him complaining that he had to talk about 'Ptolemy and Aristotle, Kepler and Copernicus, Galileo and Newton, gravitation and relativity and modern astrophysics and Heavens knows what …'13 Then, with his usual wit, Shaw summarised everything in three sentences: 'Ptolemy made a universe, which lasted 1,400 years. Newton, also, made a universe, which lasted for 300 years. Einstein has made a universe, and I can't tell you how long that will last.'14 The guests laughed, none louder than Einstein. After comparing the achievements of Newton and Einstein, Shaw ended with a toast: 'I drink to the greatest of our contemporaries, Einstein!'15

It was a difficult act to follow, but Einstein was every bit as much the showman when the occasion demanded. He expressed his gratitude to Shaw for 'the unforgettable words which you have addressed to my mythical namesake who makes life so difficult for me'.16 He offered words of praise to Jews and Gentiles alike 'of noble spirit and with a strong sense of justice, who had devoted their lives to uplifting human society and liberating the individual from degrading oppression'. 'To you all I say,' knowing that he was addressing a sympathetic audience, 'that the existence and destiny of our people depends less on external factors than on us remaining faithful to the moral traditions which have enabled us to survive for thousands of years despite the fierce storms that have broken over our heads.' 'In the service of life,' Einstein added, 'sacrifice becomes grace.'17 Words said in hope would, for millions, soon be put to the test as the dark clouds of the coming Nazi storm gathered.

Six weeks earlier, on 14 September, the Nazis had gained 6.4 million votes in the Reichstag elections. The size of the Nazi vote stunned many. In May 1924 the party had won 32 seats, and in the December elections that same year, just fourteen. In May 1928 they did even worse, winning a mere twelve seats and 812,000 votes. The result seemed to confirm the Nazis as just another far-right fringe group. Now, little more than two years later, they had increased their share of the vote eight-fold and were the second-largest party in the Reichstag with 107 deputies.18

Einstein was not alone in believing that 'the Hitler vote is only a symptom, not necessarily of anti-Jewish hatred but of momentary resentment caused by economic misery and unemployment within the ranks of misguided German youths'.19 However, only about one quarter of those who voted Nazi were young first-time voters. It was among the older generation of white-collar workers, shopkeepers, small businessmen, Protestant farmers in the north, craftsmen, and unskilled workers outside the industrial centres that Nazi support was strongest. What contributed decisively to the changed German political landscape between the elections of 1928 and 1930 was the Wall Street Crash in October 1929.

Germany was hardest hit by the financial shockwaves emanating from New York. The lifeblood of its fragile economic revival of the past five years had been short-term loans from the United States. With mounting losses, and in disarray, American financial institutions demanded immediate repayment of existing loans. The result was a rapid rise in unemployment from 1.3 million in September 1929 to over 3 million in October 1930. Einstein for the moment saw the Nazis as nothing more than a 'childish disease of the Republic' that would soon pass.20 The disease, however, would kill off an already ailing Weimar Republic that had in all but name abandoned parliamentary democracy in favour of rule by decree.

'We are moving toward bad times', wrote a pessimistic Sigmund Freud on 7 December 1930.21 'I ought to ignore it with the apathy of old age, but I can't help feeling sorry for my seven grandchildren.' Five days earlier, Einstein had left Germany to spend two months at Caltech, the California Institute of Technology in Pasadena. Boltzmann, Schrödinger and Lorentz had all lectured at what had fast become one America's leading centres of scientific excellence. When his ship docked in New York, Einstein was persuaded to give a fifteen-minute press conference to the horde of waiting reporters. 'What do you think of Adolf Hitler?' shouted one. 'He is living on the empty stomach of Germany', replied Einstein. 'As soon as the economic conditions improve, he will no longer be important.'22

A year later, in December 1931, when he set off for a second stint at Caltech, Germany was in an even deeper economic depression and greater political turmoil. 'I decided today that I shall essentially give up my Berlin position and shall be a bird of passage for the rest of my life', Einstein wrote in his diary as he crossed the Atlantic.23 While in California, Einstein happened to meet Abraham Flexner, who was in the process of establishing a unique research centre, the Institute for Advanced Study, in Princeton, New Jersey. Armed with a $5 million donation, Flexner wanted to create a 'society of scholars' devoted entirely to research, freed from the demands of teaching students. Serendipitously meeting Einstein, Flexner wasted little time in taking the first steps that eventually led to the recruitment of the world's most celebrated scientist.

Einstein agreed to spend five months a year at the institute and the remainder in Berlin. 'I am not abandoning Germany', he told the New York Times.24 'My permanent home will still be in Berlin.' The five-year arrangement would begin in the autumn of 1933 because Einstein had already committed himself to another spell at Caltech. He was fortunate that he had, for it was during this third visit to Pasadena that Hitler was appointed Chancellor on 30 January 1933. For Germany's half-million Jews, the exodus began slowly, with only 25,000 leaving by June. Einstein, safely in California, did not speak out, but acted as if he would return when the time came. He wrote to the Prussian Academy asking about his salary, but had already made his decision. 'In view of Hitler,' he wrote to a friend on 27 February, 'I don't dare step on German soil.'25 That very day the Reichstag was set alight. It signalled the beginning of the first wave of state-sponsored Nazi terror.

In the midst of the violence unleashed by the Nazis, 17 million voted for them in the Reichstag election on 5 March. Five days later, on the eve of his planned departure from Pasadena, Einstein gave an interview and made public what he thought about events in Germany. 'As long as I have any choice in the matter,' he said, 'I shall live only in a country where civil liberty, tolerance and equality of all citizens before the law prevail. Civil liberty implies freedom to express one's political convictions, in speech and in writing; tolerance implies respect for the convictions of others whatever they may be. These conditions do not exist in Germany at the present time.'26 As his words were reported around the world, he was condemned in the German press as newspapers vied to demonstrate their allegiance to the Nazi regime. 'Good News of Einstein – He Is Not Coming Back!' read the headline in the Berliner Lokalanzeiger. The article seethed at how 'this puffed up bit of vanity dared to sit in judgement on Germany without knowing what is going on here – matters that forever must remain incomprehensible to a man who was never German in our eyes and who declares himself to be a Jew and nothing but a Jew'.27

Einstein's comments left Planck in a quandary. On 19 March he wrote to Einstein of his 'profound distress' over 'all kinds of rumours which have emerged in this unquiet and difficult time about your public and private statements of a political nature'.28 Planck complained that 'these reports make it exceedingly difficult for all those who esteem and revere you to stand up for you'. He blamed Einstein for making the difficult situation of his 'tribal companions and co-religionists' worse. When his ship docked at Antwerp in Belgium on 28 March, Einstein asked to be driven to the German embassy in Brussels. There he surrendered his passport, renounced his German citizenship for a second time, and handed over a letter of resignation from the Prussian Academy.

While he pondered what to do and where to go, Einstein and Elsa moved into a villa in the small resort of Le Coq-sur-Mer on the Belgian coast. As rumours circulated that Einstein's life might be at risk, the Belgian government assigned two guards to protect him. In Berlin, Planck was relieved when he learnt of Einstein's resignation. It was the only honourable way to sever ties with the Academy and 'at the same time save your friends from an immeasurable amount of grief and pain', he wrote to Einstein.29 There were few prepared to stand up for him in the new Germany.

On 10 May 1933, swastika-clad students and academics carrying torches marched down Unter den Linden to the Opernplatz just across from Berlin University's main entrance and set fire to some 20,000 books plundered from the shelves of the city's libraries and bookstores. A crowd of 40,000 watched as the flames consumed the 'un-German' and 'Jewish-Bolshevik' works by the likes of Marx, Brecht, Freud, Zola, Proust, Kafka, and Einstein. It was a scene repeated in every major university town in the country, and men like Planck read the smoke signals and did little, if anything, to resist. The book-burning was just the beginning of the Nazi assault on 'degenerate' art and culture, but a far more significant event had already occurred for German Jews when anti-Semitism was effectively legalised.

The 'Law for the Restoration of the Career Civil Service', passed on 7April, applied to some 2 million state employees. The law was designed to target the Nazis' political opponents, socialists, communists, and the Jews. Paragraph 3 contained the infamous 'Aryan clause': 'Civil servants not of Aryan origin are to retire.'30 The law defined a non-Aryan as a person who had one parent or grandparent who was not Aryan. Sixty-two years after their emancipation in 1871, German Jews were once again the subject of legalised state discrimination. It was the springboard for the Nazi persecution of the Jews that followed.

Universities were state institutions, and soon more than a thousand academics, including 313 professors, were dismissed or resigned. Almost a quarter of the pre-1933 physics community was forced into exile, including half of all theorists. By 1936 more than 1,600 scholars had been ousted; a third of these were scientists, including twenty who had been or would be awarded the Nobel Prize: eleven in physics, four in chemistry, five in medicine.31 Formally, the new law did not apply to those employed before the First World War, or who were veterans of that war, or anyone who had lost a father or son during the war. But as the Nazi purge of the civil service continued unabated and claimed an increasing number who were exempt, on 16 May 1933 Planck, as president of the Kaiser Wilhelm Society, went to see Hitler. He thought he could limit the damage being done to German science.

Incredibly, Planck told Hitler that 'there are different sorts of Jews, some valuable for mankind and others worthless', and that 'distinctions must be made'.32 'That's not right', said Hitler.33 'A Jew is a Jew; all Jews stick together like leeches. Wherever there is one Jew, other Jews of all sorts immediately gather.' His opening gambit having failed, Planck changed tack. The wholesale expulsion of Jewish scientists would be harmful to Germany's interests, argued Planck. Hitler flew into a rage at the very suggestion: 'Our national policies will not be revoked or modified, even for scientists.' 'If the dismissal of Jewish scientists means the annihilation of contemporary German science, then we shall do without science for a few years!'34

In November 1918, in the immediate aftermath of defeat, Planck had rallied the dispirited members of the Prussian Academy of Sciences: 'If the enemy has taken from our fatherland all defence and power, if severe domestic crises have broken in upon us and perhaps still more severe crises stand before us, there is one thing which no foreign or domestic enemy has yet taken from us: that is the position which German science occupies in the world.'35 For Planck, who had lost his eldest son on the battlefield, all the sacrifices had to be worth something. As his disastrous meeting with Hitler came to an abrupt end, Planck knew that the Nazis were on the verge of achieving what no one else had: the destruction of German science.

Two weeks earlier, the Nazi physicist and Nobel laureate Johannes Stark had been appointed director of the Physikalisch-Technische Reichsanstalt, the Imperial Institute of Physics and Technology. Soon Stark wielded even greater power in the service of 'Aryan physics', as he was placed in charge of disbursing government research funds. From these positions of power he was determined to exact revenge. In 1922 he had stepped down from his professorship at the University of Würzburg to try his hand at business. Anti-Semitic, dogmatic and quarrelsome, Stark had alienated virtually everyone bar the like-minded fellow Nobel laureate and Nazi Philipp Lenard, the leading and long-time proponent of so-called 'Deutsch Physik'. When Stark wanted to return to academia after the failure of his business venture, no one who was in a position to do so was prepared to offer him a job. Already bitterly opposed to the 'Jewish physics' of Einstein and dismissive of modern theoretical physics, Stark was determined to have a say in all appointments to professorial chairs of physics and lobbied to have them occupied by supporters of 'German physics'.

Heisenberg had long wanted to be Sommerfeld's successor at Munich. In 1935 Stark called Heisenberg the 'spirit of Einstein's spirit' and launched a concerted campaign against him and theoretical physics. It culminated on 15 July 1937 with the publication of an article in the SS journal, Das Schwarze Korps, in which Heisenberg was branded a 'white Jew'. He spent the next year trying to remove the slur that, if it stuck, would place him in real danger of being isolated and dismissed. He turned to Heinrich Himmler, head of the SS, who happened to be a family acquaintance. Himmler exonerated Heisenberg, but blocked his appointment as Sommerfeld's successor. There was also a proviso that in future he should 'clearly separate for your audiences, in the acknowledgement of scientific research results, the personal and political characteristics of the researcher'.36 Heisenberg duly obliged in separating the scientist from the science. There would be no more mention by him of Einstein's name in public.

The Göttingen physicists James Franck and Max Born were exempt as war veterans from the 'Aryan clause'. But neither man chose to exercise his right, believing that to do so was tantamount to collusion with the Nazis. Franck was condemned by no fewer than 42 of his colleagues when he submitted his letter of resignation, for fuelling anti-German propaganda by stating that 'we Germans of Jewish descent are being treated as aliens and enemies of the Fatherland'.37 Born, who had no intention of resigning, discovered his name on a list of suspended civil servants published in the local newspaper. 'All I had built up in Göttingen, during twelve years hard work, was shattered', he wrote later.38 'It seemed to me like the end of the world.' He shuddered at the thought of 'standing in front of students who, for whatever reason, have thrown me out, or living among colleagues who were able to live with this so easily'.39

Suspended but not yet sacked, Born had never felt particularly Jewish, he confessed to Einstein. But now he was 'extremely conscious of it, not only because we are considered to be so, but because oppression and injustice provoke me to anger and resistance'.40 Born hoped to settle in England, 'for the English seem to be accepting the refugees most nobly and generously'.41 His wish was granted when he was offered a three-year lectureship at Cambridge University. Believing that he might be depriving a deserving English physicist, Born accepted only after being reassured that the post had been created especially for him. He was one of the lucky few whose contributions to physics were internationally recognised, unlike the 'young ones' for whom Einstein said his 'heart aches'.42 But even scientists of Born's calibre had to endure periods of deep uncertainty about their future. After his time in Cambridge was up, Born spent six months in Bangalore, India and was seriously considering a post in Moscow, when in 1936 he was offered the chair of natural philosophy at the University of Edinburgh.

Heisenberg had tried to convince Born that he was safe, since 'only the very least are affected by the law – you and Franck certainly not'. He hoped, like others, that things would eventually settle down and 'the political revolution could take place without any damage to Göttingen physics'.43 But the damage was already done. It had taken the Nazis a matter of weeks to transform Göttingen, the cradle of quantum mechanics, from a great university to a second-rate institution. The Nazi minister of education asked David Hilbert, the most fêted mathematician in Göttingen, whether it was true 'that your Institute suffered so much from the departure of the Jews and their friends?' 'Suffered? No, it didn't suffer, Herr Minister', replied Hilbert. 'It just doesn't exist any more.'44

As news spread of what was happening in Germany, scientists and their professional bodies quickly swung into action to help colleagues fleeing Nazi oppression with money and jobs. Aid organisations supported by gifts and donations from individuals and private foundations were set up. In England, the Academic Assistance Council, with Rutherford as its president, was established in May 1933 as a 'clearing house' that found temporary posts and offered help for refugee scientists, artists, and writers. Many initially escaped to Switzerland, Holland or France and stayed only a short while before travelling on to Britain and the United States.

In Copenhagen, Bohr's institute became a staging post for many physicists. In December 1931, the Danish Academy of Sciences and Letters had chosen Bohr as the next occupant of the Aeresbolig, 'The House of Honour', a mansion built by the founder of the Carlsberg breweries. His new status as Denmark's leading citizen meant he enjoyed even more influence at home and abroad, which he exercised to help others. In 1933 he and his brother Harald helped set up 'The Danish Committee for Support of Intellectual Workers in Exile'. Through colleagues and former students, Bohr was able to get new posts established or have vacancies filled by refugees. It was Bohr who got James Franck to Copenhagen on a three-year visiting professorship in April 1934. After a year or so, Franck moved on to a tenured position in the United States, which, along with Sweden, was the final destination of many who arrived in Denmark. One man who did not have to worry about a job was Einstein.

In early September, as fears for his safety in Belgium grew, Einstein left for England. For the next month he kept a low profile, staying in a cottage on the Norfolk coast. Soon the tranquillity by the seaside was shattered when he learnt that Paul Ehrenfest, in a fit of despair while estranged from his wife, had committed suicide. It happened during a visit to an Amsterdam hospital to see his sixteen-year-old son Vassily, who suffered from Down's syndrome. Einstein was shocked at the news that Ehrenfest had also shot Vassily. Remarkably, the boy survived but was blinded in one eye.

Although deeply upset at Ehrenfest's suicide, Einstein's thoughts soon turned to the speech he had agreed to give at a fundraising rally highlighting the plight of refugees. The meeting, chaired by Rutherford, took place on 3 October at the Royal Albert Hall. A public eager to get a glimpse of the great man meant that there was not even standing room on the night. Einstein succeeded in addressing the audience of 10,000 in his heavily accented English without once mentioning Germany by name, at the request of the organisers. For the Refugee Assistance Council believed that 'the issue raised at the moment is not a Jewish one alone; many who have suffered or are threatened had no Jewish connection'.45 Four days later, on the evening 7 October, Einstein left for America. Due to spend the next five months at the Institute for Advanced Study, he never returned to Europe.

As he was being driven from New York to Princeton, Einstein was handed a letter from Abraham Flexner. The institute's director was asking him not to attend any public events and to exercise discretion for own his safety. The reason Flexner gave was the danger posed to Einstein by the 'bands of irresponsible Nazis' to be found in America.46 Yet his real concern was the damage that Einstein's public statements might inflict on the reputation of his fledgling institute, and therefore on the donations it relied on. Within a matter of weeks, Einstein found Flexner's restrictions and increasing interference suffocating. Once he even gave his new address as 'Concentration camp, Princeton'.47

Einstein wrote to the trustees of the institute to complain of Flexner's behaviour, and asked them to guarantee him 'security for undisturbed and dignified work, in such a way that there is no interference at every step of a kind that no self-respecting person can tolerate'.48 If they could not, then he would have to 'discuss with you ways and means of severing my relations with your Institute in a dignified manner'.49 Einstein gained the right to do as he pleased, but at a price. He would never have any real influence in the running of the institute. When he backed Schrödinger for a post at the institute, it effectively ruled the Austrian out of the running.

Schrödinger did not have to leave Berlin, but did so as a matter of principle. He had been in exile at Magdalen College, Oxford University less than a week when, on 9 November 1933, he received some unexpected news. The president of the college, George Gordon, informed Schrödinger that The Times had called to say that he would be among the winners of the Nobel Prize that year. 'I think you may believe it. The Times do not say a thing unless they really know', said Gordon proudly.50 'As for me, I was truly astonished, for I thought you had the prize.'

Schrödinger and Dirac were each awarded a half share of the 1933 Nobel Prize, with the deferred prize of 1932 going to Heisenberg alone. Dirac's first reaction was to refuse it because he did not want the publicity. He accepted after Rutherford convinced him that refusing it would generate even greater publicity. While Dirac toyed with the idea of rejecting the prize, Born was deeply hurt at being ignored by the Swedish Academy.

'I have a bad conscience regarding Schrödinger, Dirac, and Born', Heisenberg wrote to Bohr.51 'Schrödinger and Dirac both deserved an entire prize at least as much as I do, and I would have gladly shared with Born, since we have worked together.' Earlier he replied to a letter of congratulations from Born: 'The fact that I am to receive the Nobel Prize alone, for work done in Göttingen in collaboration – you, Jordan and I – this fact depresses me and I hardly know what to write to you.'52 'That Heisenberg's matrices bear his name is not altogether justified, as in those days he actually had no idea what a matrix was', Born complained to Einstein two decades later.53 'It was he who reaped all the rewards of our work together, such as the Nobel Prize and that sort of thing.' He admitted that 'for the last twenty years I have not been able to rid myself of a certain sense of injustice'. Born was finally awarded the Nobel in 1954 for 'his fundamental work in quantum mechanics and especially for his statistical interpretation of the wave function'.

After the difficult start, by the end of November 1933 Princeton was beginning to appeal to Einstein. 'Princeton is a wonderful little spot, a quaint and ceremonious village of puny demigods on stilts', he wrote to Queen Elizabeth of Belgium. 'Yet, by ignoring certain special conventions, I have been able to create for myself an atmosphere conducive to study and free from distractions.'54 In April 1934 Einstein made public that he would be staying in Princeton indefinitely. The 'bird of passage' had found a place to nest for the rest of his life.

Einstein had always been an outsider, even in physics, beginning with his days in the Patent Office. Yet he had led the way for so long and so often. He hoped to do so again as he came up with a new challenge for Bohr and the Copenhagen interpretation.