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

by Manjit Kumar

  ‘His must be a first-rate mind, extremely critical and far-seeing, which never loses track of the grand design’, was Einstein’s assessment of the young Dane, six years his junior.56 It was October 1919 and such an appraisal was a spur for Planck to get Bohr to Berlin. Einstein had long been an admirer. In the summer of 1905 as the creative storm that had broken loose in his mind began to subside, Einstein found nothing that was ‘really exciting’ to tackle next.57 ‘There would of course be the topic of spectral lines,’ he told his friend Conrad Habicht, ‘but I believe that a simple relationship between these phenomena and those already investigated does not exist at all, so that for the moment, the thing looks rather unpromising to me.’58

  Einstein’s nose for a physics problem ripe for attack was second to none. Having passed on the mystery of spectral lines, he came up with E=mc2, which said that mass and energy were interconvertible. But for all he knew, God Almighty was having a laugh at his expense by leading him ‘around by the nose’.59 So when in 1913 Bohr showed how his quantised atom solved the mystery of atomic spectra, it appeared to Einstein ‘like a miracle’.60

  The uneasy mixture of excitement and apprehension that had taken hold of his stomach as Bohr made his way from the station to the university vanished as soon as he met Planck and Einstein. They put him at his ease by moving quickly from pleasantries to talk of physics. The two men could not have been more dissimilar. Planck was the epitome of Prussian formality and rectitude, while Einstein with his big eyes, unruly hair and trousers that were just a little too short gave the impression of a man at ease with himself, if not the troubled world in which he lived. Bohr accepted Planck’s invitation to stay at his home during the visit.

  His days in Berlin, Bohr said later, were spent ‘discussing theoretical physics from morning to night’.61 It was the perfect break for the man who just loved to talk physics. He particularly enjoyed the lunch that the younger university physicists had thrown for him, from which they excluded all the ‘bigwigs’. It was a chance for them to quiz Bohr after his lecture had left them ‘somewhat depressed because we had the feeling that we had understood very little’.62 Einstein, however, understood perfectly well what Bohr was arguing and he did not like it.

  Like virtually everyone else, Bohr did not believe in the existence of Einstein’s light-quanta. He accepted, like Planck, that radiation was emitted and absorbed in quanta, but not that radiation itself was quantised. For him there was just too much evidence in favour of the wave theory of light, but with Einstein in the audience, Bohr told the assembled physicists: ‘I shall not consider the problem of the nature of radiation.’63 However, he had been deeply impressed by Einstein’s work of 1916 on spontaneous and stimulated emission of radiation and electron transitions between energy levels. Einstein had succeeded where he had failed by showing that it was all a matter of chance and probability.

  Einstein continued to be troubled by the fact that his theory could not predict either the time or the direction in which the light-quantum emitted as an electron jumps from one energy level to a lower one. ‘Nevertheless,’ he had written in 1916, ‘I fully trust in the reliability of the road taken.’64 He believed it was a road that would eventually lead to a restoration of causality. In his lecture, Bohr argued that no exact determination of time and direction was ever possible. The two men found themselves on opposite sides. In the days that followed, each tried to convert the other to his point of view as they walked the streets of Berlin together or dined at Einstein’s home.

  ‘Seldom in my life has a person given me such pleasure by his mere presence as you have’, Einstein wrote to Bohr soon after he returned to Copenhagen. ‘I am now studying your great publications and – unless I happen to get stuck somewhere – have the pleasure of seeing before me your cheerful boyish face, smiling and explaining.’65 The Dane had made a deep and lasting impression. ‘Bohr was here, and I am just as enamoured of him as you are’, Einstein told Paul Ehrenfest a few days later. ‘He is like a sensitive child and walks about this world in a kind of hypnosis.’66 Bohr was equally intent in trying to convey, in his less than polished German, what it meant to him to have met Einstein: ‘It was to me one of my greatest experiences to have met you and to talk to you. You cannot imagine what a great inspiration it was for me to hear your views from you in person.’67 Bohr did so again quite soon, as Einstein made a fleeting visit as he stopped off in Copenhagen in August on his way back from a trip to Norway.

  ‘He is a highly gifted and excellent man’, Einstein wrote to Lorentz after seeing Bohr.68 ‘It is a good omen for physics that prominent physicists are mostly also splendid people.’ Einstein had become the target of two men who were not. Philipp Lenard, whose experimental work on the photoelectric effect Einstein had used in 1905 in support of his light-quanta, and Johannes Stark, the discoverer of the splitting of spectral lines by an electric field, had become rabid anti-Semites. The two Nobel laureates were behind an organisation calling itself the Working Group of German Scientists for the Preservation of Pure Science, whose prime aim was to denounce Einstein and relativity.69 On 24 August 1920 the group held a meeting at Berlin’s Philharmonic Hall and attacked relativity as ‘Jewish physics’ and its creator as both a plagiarist and a charlatan. Not to be intimidated, Einstein went along with Walther Nernst and watched the proceedings from a private box as he was vilified. Refusing to rise to the bait, he said nothing.

  Nernst, Heinrich Rubens and Max von Laue wrote to the newspapers defending Einstein against the outrageous charges levelled at him. Many of his friends and colleagues were therefore dismayed when Einstein wrote an article for the Berliner Tageblatt entitled ‘My Reply’. He pointed out that had he not been Jewish and an internationalist he would not have been denounced, nor his work attacked. Einstein immediately regretted having been riled into writing the article. ‘Everyone has to sacrifice at the altar of stupidity from time to time, to please the Deity and the human race’, he wrote to the physicist Max Born and his wife.70 He was well aware that his celebrity status meant that ‘like a man in the fairy tale who turned everything into gold – so with me everything turns into a fuss in the newspapers’.71 Soon there were rumours that Einstein might leave the country, but he chose to stay in Berlin, ‘the place to which I am most closely tied by human and scientific connections’.72

  In the two years after their meetings in Berlin and Copenhagen, Einstein and Bohr continued their individual struggles with the quantum. Both were beginning to feel the strain. ‘I suppose it’s a good thing that I have so much to distract me,’ Einstein wrote to Ehrenfest in March 1922, ‘else the quantum problem would have got me into a lunatic asylum.’73 A month later, Bohr confessed to Sommerfeld: ‘In the last few years, I have often felt myself scientifically very lonesome, under the impression that my effort to develop the principles of the quantum theory systematically to the best of my ability has been received with very little understanding.’74 His feelings of isolation were about to end. In June 1922, he travelled to Germany and gave a celebrated series of seven lectures spread over eleven days at Göttingen University that became known as the ‘Bohr Festspiele’.

  More than a hundred physicists, old and young, came from all over the country to hear Bohr explain his electron shell model of the atom. It was his new theory about the arrangement of electrons inside atoms that explained the placing and grouping of elements within the periodic table. He proposed that orbital shells, like layers of an onion, surrounded an atomic nucleus. Each such shell was actually made up of a set or subset of electron orbits and was able to accommodate only a certain maximum number of electrons.75 Elements that shared the same chemical properties, Bohr argued, did so because they had the same numbers of electrons in their outermost shell.

  According to Bohr’s model, sodium’s eleven electrons are arranged 2, 8 and 1. Caesium’s 55 electrons are arranged in a 2, 8, 18, 18, 8, 1 configuration. It is because the outer shell of each element has a single electron that sodium and caesium share s
imilar chemical properties. During the lectures Bohr used his theory to make a prediction. The unknown element with atomic number 72 would be chemically similar to zirconium, atomic number 40, and titanium, atomic number 22, the two elements in the same column of the periodic table. It would not, Bohr said, belong to the ‘rare earth’ group of elements that were on either side of it in the table, as predicted by others.

  Einstein did not attend Bohr’s Göttingen lectures, as he feared for his life following the murder of Germany’s Jewish foreign minister. Walther Rathenau, a leading industrialist, had been in office only a few short months when he was gunned down in broad daylight on 24 June 1922 to become the 354th political assassination by the right since the end of the war. Einstein was one of those who had urged Rathenau not to take such a high-profile post within government. When he did, it was deemed ‘an absolutely unheard of provocation of the people!’ by the right-wing press.76

  ‘Here our daily lives have been nerve-racking since the shameful assassination of Rathenau’, Einstein wrote to Maurice Solovine.77 ‘I am always on the alert; I have stopped my lectures and am officially absent, though I am actually here all the time.’ Warned by reliable sources that he was a prime target for assassination, Einstein confided to Marie Curie that he was thinking about giving up his post at the Prussian Academy to find a quiet place to settle down as a private citizen.78 For the man who in his youth had hated authority had now become a figure of authority. He was no longer simply a physicist, but was a symbol of German science and of Jewish identity.

  Despite the turmoil, Einstein read Bohr’s published papers, including ‘The Structure of the Atoms and the Physical and Chemical Properties of the Elements’, which appeared in the Zeitschrift für Physik in March 1922. He recalled nearly half a century later how Bohr’s ‘electron-shells of the atoms together with their significance for chemistry appeared to me like a miracle – and appears to me as a miracle even today’.79 It was, Einstein said, ‘the highest form of musicality in the sphere of thought’. What Bohr had done was indeed as much art as science. Using evidence gathered from a variety of different sources such as atomic spectra and chemistry, Bohr had built up a particular atom, one electron shell at a time, layer by onion layer, until he had reconstructed every element in the entire periodic table.

  At the heart of his approach lay Bohr’s belief that quantum rules apply on the atomic scale, but any conclusion drawn from them must not conflict with observations made on the macroscopic scale where classical physics rules. Calling it the ‘correspondence principle’ allowed him to eliminate ideas on the atomic scale that when extrapolated did not correspond to results that were known to be correct in classical physics. Since 1913 the correspondence principle had helped Bohr bridge the divide between quantum and classical. Some viewed it as a ‘magic wand, which did not act outside Copenhagen’, recalled Bohr’s assistant Hendrik Kramers.80 Others might have struggled to wave it, but Einstein recognised a fellow sorcerer at work.

  Whatever reservations there might have been at the lack of hard mathematics to underpin Bohr’s theory of the periodic table, everyone had been impressed by the Dane’s latest ideas and gained a greater appreciation of the problems that remained. ‘My entire stay in Göttingen was a wonderful and instructive experience for me,’ Bohr wrote on his return to Copenhagen, ‘and I cannot say how happy I was for all the friendship shown me by everybody.’81 He was no longer feeling under-appreciated and isolated. Later that year there was further confirmation, if he needed it.

  As the telegrams of congratulation landed on Bohr’s desk in Copenhagen, none meant more to him than the one from Cambridge. ‘We are delighted that you have been awarded the Nobel Prize’, Rutherford wrote. ‘I knew it was merely a question of time, but there is nothing like the accomplished fact. It is well merited recognition of your great work and everybody here is delighted in the news.’82 In the days that followed the announcement, Rutherford had never been far from Bohr’s thoughts. ‘I have felt so strongly how much I owe you,’ he told his old mentor, ‘not only for your direct influence on my work and your inspiration, but also for your friendship in these twelve years since I had the great fortune of meeting you for the first time in Manchester.’83

  The other person Bohr could not help thinking about was Einstein. He was delighted and relieved that the day he received the 1922 prize, Einstein had been awarded the Nobel Prize for 1921 that had been deferred for a year. ‘I know how little I have deserved it,’ he wrote to Einstein, ‘but I should like to say that I consider it a good fortune that your fundamental contribution in the special area in which I work as well as contributions by Rutherford and Planck should be recognized before I was considered for such an honour.’84

  Einstein was on a ship bound for the other side of the world when the Nobel Prize winners were announced. On 8 October, still fearing for his safety, Einstein and Elsa had left for a lecture tour of Japan. He ‘welcomed the opportunity of a long absence from Germany, which took me away from temporarily increased danger’.85 He did not return to Berlin until February 1923. The original six-week itinerary turned into a grand tour lasting five months, during which he had received Bohr’s letter. He replied during the voyage home: ‘I can say without exaggeration that [your letter] pleased me as much as the Nobel Prize. I find especially charming your fear that you might have received the award before me – that is typically Bohr-like.’86

  A blanket of snow covered the Swedish capital on 10 December 1922 as the invited guests assembled in the Great Hall of the Academy of Music in Stockholm to watch the presentation of the Nobel Prizes. The ceremony began at five o’clock in the presence of King Gustav V. The German ambassador to Sweden received the prize on behalf of the absent Einstein, but only after winning a diplomatic argument with the Swiss over the physicist’s nationality. The Swiss were claiming Einstein as one of their own, until the Germans discovered that by accepting the appointment at the Prussian Academy in 1914 Einstein had automatically become a German citizen, even though he had not given up his Swiss nationality.

  Having renounced his German citizenship in 1896 and taken Swiss citizenship five years later, Einstein was surprised to learn that he was a German after all. Whether he liked it or not, the needs of the Weimar Republic meant that Einstein officially had dual nationality. ‘By an application of the theory of relativity to the taste of readers,’ Einstein had written in November 1919 in an article for the London Times, ‘today in Germany I am called a German man of science and in England I am represented as a Swiss Jew. If I come to be regarded as a bête noire, the descriptions will be reversed and I shall become a Swiss Jew for the Germans and a German man of science for the English!’87 Einstein might have recalled these words had he been at the Nobel banquet and heard the German ambassador propose a toast that expressed the ‘joy of my people that once again one of them has been able to achieve something for all of mankind’.88

  Bohr rose after the German ambassador and gave a short speech as tradition demanded. After paying tribute to J.J. Thomson, Rutherford, Planck and Einstein, Bohr proposed a toast to the international cooperation for the advancement of science, ‘which is, I may say, in these so manifoldly depressing times, one of the bright spots visible in human existence’.89 Given the occasion, it is understandable that he chose to forget the continuing exclusion of German scientists from international conferences. The next day Bohr was on firmer ground as he gave his Nobel lecture on ‘The structure of the atom’. ‘The present state of atomic theory is characterized by the fact that we not only believe the existence of atoms to be proved beyond a doubt,’ he began, ‘but we even believe that we have an intimate knowledge of the constituents of the individual atoms.’90 Having given a survey of the developments in atomic physics of which he had been such a central figure in the past decade, Bohr conclude his lecture with a dramatic announcement.

  In his Göttingen lectures, Bohr had predicted the properties that the missing element with an atomic number of 72 should
possess, based upon his theory of the arrangement of electrons in atoms. At exactly that time a paper was published outlining an experiment performed in Paris that confirmed a long-standing rival French claim that element 72 was a member of the ‘rare earth’ family of elements that occupied slots 57 to 71 in the periodic table. After the initial shock, Bohr began having serious doubts about the validity of the French results. Fortunately his old friend Georg von Hevesy, who was now in Copenhagen, and Dirk Coster devised an experiment to settle the dispute about element 72.

  Bohr had already left for Stockholm by the time Hevesy and Coster completed their investigation. Coster telephoned Bohr shortly before his lecture and he was able to announce that ‘appreciable quantities’ of element 72 had been isolated, ‘the chemical properties of which show a great similarity to those of zirconium and a decided difference from those of the rare earths’.91 Later called hafnium after the ancient name for Copenhagen, it was a fitting conclusion to Bohr’s work on the configuration of electrons within atoms that he had begun in Manchester a decade earlier.92

  In July 1923, Einstein gave his Nobel lecture on the theory of relativity as part of the 300th anniversary celebrations of the founding of the Swedish city of Göteborg. He broke with tradition by choosing relativity, when he had been awarded the prize ‘for his attainments in mathematical physics and especially for his discovery of the law of the photoelectric effect’.93 By limiting the award of the prize for the ‘law’, the mathematical formula that accounted for the photoelectric effect, the committee deftly sidestepped endorsing Einstein’s controversial underlying physical explanation - the light-quantum. ‘In spite of its heuristic value, however, the hypothesis of light-quanta, which is quite irreconcilable with so-called interference phenomena, is not able to throw light on the nature of radiation’, Bohr had said during his own Nobel lecture.94 It was a familiar refrain echoed by every self-respecting physicist. But as Einstein went to meet Bohr for the first time in nearly three years, he knew that an experiment performed by a young American meant that he no longer stood alone in defence of the quantum of light. Bohr had heard the dreaded news before Einstein.