Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality

by Manjit Kumar

  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.9 Heisenberg’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 7 April, 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’.