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

by Manjit Kumar

  Einstein was desperately seeking to change the physics as well; for he was not the conservative relic many thought. He was convinced that the concepts of classical physics would have to be replaced by new ones. Since the macroscopic world is described by classical physics and its concepts, Bohr agued that even to seek to go beyond them was a waste of time. He had developed his framework of complementarity in order to save classical concepts. For Bohr there was no underlying physical reality that exists independently of measuring equipment, and that meant, as Heisenberg pointed out, ‘we cannot escape the paradox of quantum theory, namely, the necessity of using the classical concepts’.73 It is the Bohr-Heisenberg call to retain classical concepts that Einstein called a ‘tranquilizing philosophy ’.74

  Einstein never abandoned the ontology of classical physics, an observer-independent reality, but he was prepared to make a decisive break with classical physics. The view of reality endorsed by the Copenhagen interpretation was all the evidence he needed of the necessity to do so. He wanted a revolution more radical than the one offered by quantum mechanics. It was hardly surprising that Einstein and Bohr left so much unsaid.

  In January 1939, Bohr returned to Princeton and stayed for four months as a visiting professor at the institute. Although the two men still enjoyed a warm, friendly relationship, their ongoing dispute over quantum reality had inevitably led to a cooling. ‘Einstein was only a shadow of himself’, recalled Rosenfeld, who had accompanied Bohr to America.75 They did meet, usually at formal receptions, but they no longer talked about the physics that mattered so much to them. During Bohr’s stay Einstein gave only one lecture, on his search for a unified field theory. With Bohr in the audience, he expressed the hope that quantum physics would be derivable from such a theory. But Einstein had already made it known that he would rather not discuss the issue further. ‘Bohr was profoundly unhappy about this’, said Rosenfeld.76 With Einstein unwilling to talk about quantum physics, Bohr found that there were plenty of others in Princeton eager to discuss the latest developments in nuclear physics, given the ominous events in Europe that would lead once again to a world at war.

  ‘No matter how deeply one immerses oneself in work,’ Einstein wrote to Queen Elizabeth of Belgium, ‘a haunting feeling of inescapable tragedy persists.’77 The letter was dated 9 January 1939, two days before Bohr sailed for America and brought with him the news of a discovery that others had made: the splitting apart of a large nucleus into smaller nuclei, with an accompanying release of energy – nuclear fission. It was during the voyage that Bohr realised it was the uranium-235 isotope that undergoes nuclear fission when it is bombarded by slow-moving neutrons, and not uranium-238. At the age of 53, it was Bohr’s last major contribution to physics. With Einstein unwilling to debate the nature of quantum reality, Bohr concentrated on working out the details of nuclear fission with the American John Wheeler from Princeton University.

  After Bohr returned to Europe, Einstein sent a letter, dated 2 August, to President Roosevelt urging him to examine the feasibility of developing an atomic bomb, given that Germany had stopped the sale of uranium ore from mines it now controlled in Czechoslovakia. Roosevelt replied in October, thanking Einstein for his letter and informing him that he had set up a committee to investigate the issues raised. In the meantime, on 1 September 1939, Germany attacked Poland.

  Still a pacifist, Einstein was prepared to compromise until Hitler and the Nazis were defeated. In a second letter, dated 7 March 1940, he urged Roosevelt that more needed to be done: ‘Since the outbreak of the war, interest in uranium has intensified in Germany. I have now learned that research there is carried out in great secrecy.’78 Unknown to Einstein, the man in charge of the German atomic bomb programme was Werner Heisenberg. Once again, the letter failed to solicit much of a response. Bohr’s discovery that it was uranium-235 that underwent fission was far more important to the creation of the atom bomb than anything achieved by Einstein’s two letters to Roosevelt. The American government did not seriously begin thinking about developing an atomic bomb, codenamed the Manhattan Project, until October 1941.

  Even though Einstein had become an American citizen in 1940, the authorities considered him a security risk because of his political views. He was never asked to work on the atomic bomb. Bohr was. On 22 December 1943 he stopped off at Princeton on his way to Los Alamos in New Mexico, where the bomb was being built. He had dinner with Einstein and Wolfgang Pauli, who had joined the Institute for Advanced Study in 1940. Much had happened since the last time Bohr met Einstein.

  In April 1940, German forces had occupied Denmark. Bohr chose to remain in Copenhagen, hoping that his international reputation would provide some semblance of protection to others at his institute. And it did until August 1943, when the illusion of Danish self-rule was finally shattered as the Nazis declared martial law after the government rejected a demand that a state of emergency be declared and acts of sabotage be punishable by death. Then on 28 September, Hitler ordered the deportation of Denmark’s 8,000 Jews. A sympathetic German official informed two Danish politicians that the round-up was to begin at 9pm on 1 October. As word quickly spread of the Nazi plan, almost every Jew disappeared, hidden in the homes of fellow Danes or finding sanctuary in churches, or disguised as patients in hospitals. The Nazis managed to round up fewer than 300 Jews. Bohr, whose mother had been Jewish, managed to escape to Sweden with his family. From there he flew to Scotland in a British bomber, almost dying from a lack of oxygen because he was travelling in the bomb-bay and had an ill-fitting oxygen mask. After meeting British politicians he soon travelled to America, where after his fleeting visit to Princeton he worked on the atomic bomb under the alias ‘Nicholas Baker’.

  After the war, Bohr returned to his institute in Copenhagen, and Einstein said he felt ‘no friendship for any real German’.79 Yet he had abiding sympathy for Planck, who outlived all four children from his first marriage. The death of his youngest son was the bitterest of all the blows Planck endured in his long life. Erwin, an undersecretary of state in the Reich Chancellery before the Nazis came to power, was a suspect in an attempt to assassinate Hitler in July 1944. He was arrested and tortured by the Gestapo and found guilty of complicity in the assassination plot. At one point there was a glimmer of hope as Planck set, in his words, ‘Heaven and Hell in motion’ to have the death penalty commuted to a prison sentence.80 Then, without warning, Erwin was hanged in Berlin in February 1945. Planck had been denied the opportunity to see his son one last time: ‘He was a precious part of my being. He was my sunshine, my pride, my hope. No words can describe what I have lost with him.’81

  When he heard the news that Planck had died, aged 89, following a stroke on 4 October 1947, Einstein wrote to his widow of the ‘beautiful and fruitful time’ he had been privileged to spend with him. As he offered his condolence, Einstein recalled that the ‘hours which I was permitted to spend at your house, and the many conversations which I conducted face to face with that wonderful man, will remain among my most beautiful recollections for the rest of my life’.82 It was something, he reassured her, which could not ‘be altered by the fact that a tragic fate tore us apart’.

  After the war, Bohr was made a permanent non-resident member of the Institute for Advanced Study and could come and stay whenever he wanted to. His first trip in September 1946 was brief, as he came to take part in the bicentennial celebrations of the founding of Princeton University. Then in 1948 he arrived in February and stayed until June. This time Einstein was willing to talk physics. Abraham Pais, a young Dutch physicist who helped Bohr during his visit, later described the occasion when the Dane came bursting into his office ‘in a state of angry despair’, saying, ‘I am sick of myself’.83 When Pais asked what was wrong, Bohr replied that he had been to see Einstein and they had got into an argument about the meaning of quantum mechanics.

  The renewal of their friendship was signalled by the fact that Einstein let Bohr use his office. One day Bohr was dictating a draft
of a paper in honour of Einstein’s 70th birthday to Pais. Stuck on what to say next, Bohr stood looking out of the window, every now and then muttering Einstein’s name aloud. At that moment Einstein tiptoed into the office. His doctor had banned him from buying any tobacco, but had said nothing about stealing it. Pais later recounted what happened next: ‘Always on tiptoes, he made a beeline for Bohr’s tobacco pot, which stood on the table at which I was sitting. Bohr, unaware, was standing at the window, muttering, “Einstein…Einstein…” I was at a loss what to do, especially because I had at that moment not the faintest idea of what Einstein was up to. Then Bohr, with a firm “Einstein”, turned around. There they were, face to face, as if Bohr had summoned him forth. It is an understatement to say that for a moment Bohr was speechless. I myself, who had seen it coming, had distinctly felt uncanny for a moment, so I could well understand Bohr’s own reaction. A moment later the spell was broken when Einstein explained his mission. Soon we were all bursting with laughter.’84

  There were other visits to Princeton, but Bohr never managed to get Einstein to change his mind on quantum mechanics. Nor did Heisenberg, who saw him only once after the war during a lecture tour of the United States that overlapped with Bohr’s last visit in 1954. Einstein invited Heisenberg to his home and, over coffee and cakes, they chatted for most of the afternoon. ‘Of politics we said nothing’, recalled Heisenberg.85 ‘Einstein’s whole interest focused on the interpretation of quantum theory, which continued to disturb him, just as it had done in Brussels twenty-five years before.’ Einstein remained resolute. ‘“I don’t like your kind of physics”, he said.’86

  ‘The necessity of conceiving of nature as an objective reality is said to be superannuated prejudice while the quantum theoreticians are vaunted’, Einstein had once written to his old friend Maurice Solovine.87 ‘Men are even more susceptible to suggestion than horses, and each period is dominated by a mood, with the result that most men fail to see the tyrant who rules over them.’

  When Chaim Weizmann, the first president of Israel, died in November 1952, the prime minister David Ben-Gurion felt compelled to offer Einstein the presidency. ‘I am deeply moved by the offer from our state of Israel, and at once saddened and ashamed because I cannot accept it’, said Einstein.88 He highlighted the fact that he lacked ‘both a natural aptitude and the experience to deal properly with people and to exercise official functions’. ‘For these reasons alone,’ he explained, ‘I should be unsuited to fulfil the duties of high office, even if advancing age was not making increasing inroads on my strength.’

  Ever since the summer of 1950 when doctors discovered that his aortic aneurysm, a bulge in the aorta, was getting larger, Einstein knew he was living on borrowed time. He wrote his will and made it clear that he wanted to be cremated after a private funeral. He lived to celebrate his 76th birthday, and one of his last acts was to sign a declaration written by the philosopher Bertrand Russell calling for nuclear disarmament. Einstein wrote to Bohr asking him to sign it. ‘Don’t frown like that! This has nothing to do with our old controversy on physics, but rather concerns a matter on which we are in complete agreement.’89 On 13 April 1955, Einstein experienced severe chest pains, and two days later he was taken to hospital. ‘I want to go when I want’, he said, refusing surgery. ‘It is tasteless to prolong life artificially; I have done my share, it is time to go.’90

  As fate would have it, his step-daughter Margot was staying in the same hospital. She saw Einstein twice and they chatted for a few hours. Hans Albert, who had arrived in America with his family in 1937, rushed from Berkeley in California to his father’s bedside. For a while Einstein seemed better and asked for his notes, unable to abandon his search for a unified field theory even at the end. Shortly after 1am on 18 April, the aneurysm burst. After saying a few words in German that the night nurse could not understand, Einstein died. Later that day he was cremated, but not before his brain was removed and his ashes scattered at an undisclosed location. ‘If everyone lived a life like mine there would be no need for novels’, Einstein once wrote to his sister. The year was 1899 and he was twenty.91

  ‘Except for the fact that he was the greatest physicist since Newton,’ said Banesh Hoffmann, one of Einstein’s Princeton assistants, ‘one might almost say that he was not so much a scientist as an artist of science.’92 Bohr paid his own heartfelt tribute. He recognised Einstein’s achievements to be ‘as rich and fruitful as any in the whole history of our culture’, and said that ‘mankind will always be indebted to Einstein for the removal of the obstacles to our outlook which were involved in the primitive notions of absolute space and time. He gave us a world picture with a unity and harmony surpassing the boldest dreams of the past.’93

  The Einstein-Bohr debate did not end with Einstein’s death. Bohr would argue as if his old quantum foe were still alive: ‘I can still see Einstein’s smile, both knowing, humane and friendly.’94 Often his first thought when thinking about some fundamental issue in physics was to wonder what Einstein would have said about it. On Saturday, 17 November 1962, Bohr gave the last of five interviews concerning his role in the development of quantum physics. After lunch on Sunday, Bohr went to take his usual nap. When he called out, his wife Margrethe rushed to the bedroom and found him unconscious. Bohr, aged 77, had suffered a fatal heart attack. The last drawing on the blackboard in his study, made the night before as he replayed the argument over once more, was of Einstein’s light box.

  PART IV

  DOES GOD PLAY DICE?

  ‘I want to know how God created this world. I am not interested in this or that phenomenon, in the spectrum of this or that element. I want to know His thoughts, the rest are details.’

  —ALBERT EINSTEIN

  Chapter 14

  FOR WHOM BELL’S THEOREM TOLLS

  ‘You believe in the God who plays dice, and I in complete law and order in a world which objectively exists, and which I, in a wildly speculative way, am trying to capture’, Einstein wrote to Born in 1944.1 ‘I firmly believe, but I hope that someone will discover a more realistic way, or rather a more tangible basis than it has been my lot to find. Even the great initial success of quantum theory does not make me believe in the fundamental dice game, although I am well aware that our younger colleagues interpret this as a consequence of senility. No doubt the day will come when we shall see whose instinctive attitude was the correct one.’ Twenty years passed before a discovery brought that day of judgement closer.

  In 1964 the radio astronomers Arno Penzias and Robert Woodrow detected the echo of the big bang; the evolutionary biologist Bill Hamilton published his theory of the genetic evolution of social behaviour; and the theoretical physicist Murray Gell-Mann predicted the existence of a new family of fundamental particles called quarks. These were just three of the landmark scientific breakthroughs that year. Yet according to the physicist and historian of science Henry Stapp, none rivalled Bell’s theorem, ‘the most profound discovery of science’.2 It was ignored.

  Most physicists were too busy using quantum mechanics as it continued to notch up one success after another to be bothered about the subtleties of the arguments between Einstein and Bohr over its meaning and interpretation. It was little wonder they failed to recognise that a 34-year-old Irish physicist, John Stewart Bell, had discovered what Einstein and Bohr could not: a mathematical theorem that could decide between their two opposing philosophical worldviews. For Bohr there was ‘no quantum world’, only ‘an abstract quantum mechanical description’.3 Einstein believed in a reality independent of perception. The debate between Einstein and Bohr was as much about the kind of physics that was acceptable as a meaningful theoretical description of reality as it was about the nature of reality itself.

  Einstein was convinced that Bohr and the supporters of the Copenhagen interpretation were playing a ‘risky game’ with reality.4 John Bell was sympathetic to Einstein’s position, but part of the inspiration behind his ground-breaking theorem lay in the work done in the
early 1950s by an American physicist forced into exile.

  David Bohm was a talented PhD student of Robert Oppenheimer’s at the University of California at Berkeley. Born in Wilkes-Barre, Pennsylvania in December 1917, Bohm was prevented from joining the top-secret research facility in Los Alamos, New Mexico to work on the development of the atomic bomb in 1943 after Oppenheimer was appointed its director. The authorities cited Bohm’s many relatives in Europe, nineteen of whom were to die in Nazi concentration camps, as the reason they considered him to be a security risk. In truth, having been questioned by US army intelligence, and attempting to secure his position as the scientific leader of the Manhattan Project, Oppenheimer had named Bohm as a possible member of the American Communist party.

  Four years later, in 1947, the self-confessed ‘shatterer of worlds’ took charge of the ‘madhouse’, as Oppenheimer once called the Institute for Advanced Study in Princeton.5 Maybe in an attempt to atone for his earlier naming of Bohm, of which his protégé was unaware, Oppenheimer helped him obtain an assistant professorship at Princeton University. Amid the anti-Communist paranoia sweeping the United States after the Second World War, Oppenheimer was soon under suspicion because of his earlier left-wing political views. Having watched him closely for some years, the FBI had compiled a large dossier on the man who knew America’s atomic secrets.

  In an attempt to smear Oppenheimer, some of his friends and colleagues were investigated by the House Un-American Activities Committee and forced to appear before it. In 1948 Bohm, who had joined the American Communist party in 1942 but left after only nine months, invoked the Fifth Amendment that protected him against self-incrimination. Within a year he was subpoenaed to appear before a grand jury, and once again pleaded the Fifth. In November 1949 Bohm was arrested, charged with contempt of court and briefly imprisoned before being released on bail. Princeton University, worried about losing wealthy donors, suspended him. Although he was acquitted when his case came to trial in June 1950, the university chose to pay off the remaining year of Bohm’s contract, provided he did not set foot on campus. Bohm was blacklisted and unable to find another academic post in the United States, and Einstein seriously considered appointing him as his research assistant. Oppenheimer opposed the idea and was among those who advised his former student to leave the country. In October 1951, Bohm left for Brazil and the University of São Paulo.