Niels Bohr

• 1. Introduction

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  Niels Bohr once described the tension at the heart of all good teaching as the struggle between "Klarheit und Wahrheit" - clarity and truth. It was a conflict he never entirely resolved, and it drove him to build an entirely new picture of the atom, win the Nobel Prize, shelter refugees from fascism, and fly unconscious in the bomb bay of a converted de Havilland Mosquito over Nazi-occupied Norway. Born on the 7th of October 1885 in Copenhagen, he would die in the same city on the 18th of November 1962, having spent nearly eight decades asking questions that no one else had thought to ask. What does it mean for an electron to jump between orbits? Can a single thing be simultaneously a wave and a particle? Should scientists share the secret of the atomic bomb with an enemy's allies? This documentary follows Bohr through those questions - through his cramped university lab, his institute on the first floor of which his family lived, his clandestine escape by sea, and his arguments with Einstein that lasted a lifetime.

• 2. A Family of Athletes and Scholars

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  Christian Bohr, Niels's father, held the chair of Physiology at the University of Copenhagen, and the household he ran was one where science and intellectual argument were simply part of daily life. Niels was the second of three children. His younger brother Harald would go on to become a mathematician - and, less predictably, a footballer who represented Denmark at the 1908 Summer Olympics in London. Niels was equally passionate about football. Both brothers played for Akademisk Boldklub, the Copenhagen-based Academic Football Club, with Niels taking the position of goalkeeper.

  At age seven, Niels enrolled at Gammelholm Latin School, and in 1903 he entered the University of Copenhagen as an undergraduate in physics. The department was small enough that his only professor of physics was Christian Christiansen. He also studied astronomy and mathematics under Thorvald Thiele, and philosophy under Harald Høffding, who was a friend of his father - a connection that would later be cited as the channel through which the Danish existentialist philosopher Søren Kierkegaard entered Bohr's thinking.

  In 1905, the Royal Danish Academy of Sciences and Letters offered a gold medal for an investigation into a method for measuring the surface tension of liquids, originally proposed by Lord Rayleigh in 1879. Bohr entered. Because the university had no physics laboratory of its own, he worked in his father's lab. He even made his own glassware, crafting test tubes with elliptical cross-sections to suit his measurements. He went beyond the brief, incorporating water's viscosity into his calculations and working with finite amplitudes rather than the infinitesimal ones Rayleigh had assumed. He submitted at the last minute and won. He later published a refined version of the paper in the Philosophical Transactions of the Royal Society in London.

  Harald earned his master's degree in mathematics in April 1909. Niels followed nine months later with a thesis on the electron theory of metals. When he expanded that work into a doctoral thesis, drawing on a model developed by Paul Drude and elaborated by Hendrik Lorentz, he found he could push Lorentz's framework further but still could not account for phenomena like the Hall effect. His conclusion - that electron theory could not fully explain the magnetic properties of metals - was honest and groundbreaking. The thesis was accepted in April 1911 and defended on the 13th of May. Almost no one outside Scandinavia read it, because Copenhagen University required it to be written in Danish.

• 3. The Trilogy That Changed Physics

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  In September 1911, a fellowship from the Carlsberg Foundation took Bohr to England, where most of the serious theoretical work on atomic structure was then happening. He met J. J. Thomson at Cambridge's Cavendish Laboratory, attended lectures by James Jeans and Joseph Larmor, and tried to impress Thomson with observations about the deficiencies of Thomson's own plum pudding model of the atom. Thomson was not impressed. Ernest Rutherford, whose 1911 model placed a small, dense nucleus at the centre of the atom, was more receptive. Rutherford invited Bohr to Manchester for post-doctoral work, and there Bohr met George de Hevesy and Charles Galton Darwin - Darwin, as Bohr noted, being the grandson of the real Darwin.

  Bohr returned to Denmark in July 1912 for his wedding to Margrethe Nørlund, and in the months that followed he began writing what would become three papers so significant that they are still referred to simply as "the trilogy." Published in Philosophical Magazine in July, September and November 1913, the papers adapted Rutherford's nuclear structure to Max Planck's quantum theory. Bohr proposed that electrons travel in stable, quantised orbits around the nucleus. An electron could drop from a higher-energy orbit to a lower one, releasing a discrete quantum of energy as it did. This was the foundation of what became known as the old quantum theory.

  Since 1885, Johann Balmer's formula had described the visible spectral lines of hydrogen with mathematical precision, but for three decades no one could explain why it worked. In the first paper of his trilogy, Bohr derived it directly from his model. When the astronomer Alfred Fowler challenged him on lines that did not fit Balmer's formula, Bohr replied that they came from ionised helium - helium with only one electron - and the model held.

  Many senior physicists rejected the trilogy outright. Thomson, Rayleigh, and Lorentz were among them. But the younger generation saw it differently. Albert Einstein described the model as "the highest form of musicality in the sphere of thought." The trilogy survived not on authority but on predictive power: it explained phenomena that other models could not, and its predictions were borne out by subsequent experiments. When new professors at the University of Copenhagen were formally introduced to King Christian X, the king expressed his delight at meeting such a famous football player - he had apparently heard more about Bohr's athletic career than his physics.

• 4. The Institute and Hafnium

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  In April 1917, Bohr began lobbying to establish an Institute of Theoretical Physics in Copenhagen. He secured backing from the Danish government, the Carlsberg Foundation, and private donors, many of them Jewish. Legislation passed in November 1918, and the institute opened on the 3rd of March 1921, with Bohr as director. His family moved into a flat on the first floor.

  Physicists arrived from across Europe: Hans Kramers from the Netherlands, Oskar Klein from Sweden, George de Hevesy from Hungary, Wojciech Rubinowicz from Poland, and Svein Rosseland from Norway. Klein and Rosseland produced the institute's first publication before it had even officially opened. Throughout the 1920s and 1930s, almost every leading theoretical physicist in the world spent time in Copenhagen.

  The institute's most dramatic early success came from a prediction. By 1919, limitations in the Bohr model had become apparent as physicists tried to apply it to elements more complex than hydrogen. Wolfgang Pauli's 1924 discovery of the exclusion principle - the rule that no two electrons in an atom can occupy the same quantum state - gave Bohr's models a firm theoretical footing. Armed with this, Bohr declared that the undiscovered element 72 was not a rare-earth element, as French chemist Georges Urbain claimed, but something chemically similar to zirconium. Dirk Coster and de Hevesy searched samples in Copenhagen's Museum of Mineralogy and found it. They named the new element hafnium, from hafnia, the Latin name for Copenhagen. It turned out to be more common than gold.

  In June 1922, Bohr gave seven lectures at the Institute of Theoretical Physics in Göttingen - the Wolfskehl Lectures, funded by the Wolfskehl Foundation - in the fortnight before the Göttingen International Handel Festival. The event became known as the Bohr Festival, or Bohrfestspiele. In the lectures, Bohr was candid about the limits of his own theory, remarking that "how incomplete and uncertain everything still is." That same year, the Nobel Committee awarded him the Prize in Physics for his work on atomic structure and radiation.

• 5. Complementarity and the Einstein Debates

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  Werner Heisenberg first came to Copenhagen in 1924. He returned to Göttingen in June 1925 and shortly afterwards produced the mathematical foundations of quantum mechanics. When Max Born saw the work, he recognised that matrices could express it best. This led Paul Dirac to Copenhagen for six months from September 1926. Erwin Schrödinger also visited that year, and his attempt to render quantum physics in classical wave-mechanics terms drew from Bohr the assessment that it represented "a gigantic advance over all previous forms of quantum mechanics" in mathematical clarity.

  When Kramers left in 1926 to take a professorship at Utrecht University, Bohr arranged for Heisenberg to return as a lecturer and his own assistant. Working together from 1926 to 1927, they pushed the interpretation of quantum mechanics further than anyone had gone. In February 1927, Heisenberg developed the first version of the uncertainty principle, framing it through a thought experiment involving an electron observed through a gamma-ray microscope. Bohr found the argument too limited: Heisenberg had only shown that measurement disturbs pre-existing properties, whereas Bohr believed the more radical position - that an electron's properties cannot be discussed at all outside the context in which they are measured.

  From these disagreements, Bohr articulated the principle of complementarity: that a single quantum object can display mutually exclusive properties, such as being a wave or a particle, depending entirely on the experimental setup. He introduced this at the Como Conference in September 1927. Einstein rejected the probabilistic implications of quantum mechanics for the rest of his life, preferring the determinism of classical physics. He and Bohr engaged in good-natured arguments on the subject for decades. Bohr had previously made a key contribution to the spin debates: in November 1925, when George Uhlenbeck and Samuel Goudsmit introduced spin, Bohr raised concerns about electron-magnetic field interactions. At a celebration in Leiden for Hendrik Lorentz's doctoral anniversary, Einstein told Bohr he had resolved the problem using relativity. Bohr relayed this to Uhlenbeck and Goudsmit, who incorporated it into their paper.

  In 1931, when the philosopher Harald Høffding died, the Royal Danish Academy gave Bohr occupancy of the Carlsberg Honorary Residence - a mansion that had been bequeathed in 1914 by the heir to the Carlsberg breweries for use by whoever had made the most prominent Danish contribution to science, literature, or the arts. Bohr and his family moved in the following year, and he was elected president of the Academy on the 17th of March 1939.

• 6. War, Escape, and the Bomb

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  When Nazi Germany invaded Denmark in April 1940, Bohr kept the institute running, but the foreign scholars departed. To prevent the Germans from seizing Max von Laue's and James Franck's gold Nobel medals, de Hevesy dissolved them in aqua regia. The resulting solution sat on a shelf at the institute until after the war, when the gold was precipitated back out and the medals were re-struck by the Nobel Foundation. Bohr's own medal had been auctioned for the Finnish Relief Fund in March 1940 and later donated to the Danish Historical Museum at Frederiksborg Castle.

  In September 1941, Heisenberg - now head of the German nuclear energy project - visited Bohr in Copenhagen. The two men stepped outside for a private conversation whose content has never been settled. Heisenberg later wrote that he had come to share scientists' growing concerns about the moral weight of nuclear weapons. Bohr, in a letter he drafted but never sent after reading the Danish translation of Robert Jungk's book Brighter than a Thousand Suns, stated that he remembered the visit very differently - as an attempt to encourage cooperation with what Heisenberg appeared to assume would be a victorious Germany, and that he had been shocked to learn Germany was pursuing nuclear weapons under Heisenberg's direction. Michael Frayn's 1998 play Copenhagen dramatised this unresolved encounter. A BBC television film followed in 2002, with Stephen Rea playing Bohr.

  In September 1943, Bohr learned that the Germans considered him and his brother Jewish - their mother was Jewish - and planned to arrest them. The Danish resistance smuggled Bohr and his wife across the sea to Sweden on the 29th of September. The next day, Bohr persuaded King Gustaf V of Sweden to publicly declare Sweden's willingness to shelter Jewish refugees. On the 2nd of October, Swedish radio broadcast the offer, and more than 7,000 Danish Jews subsequently escaped to Sweden.

  Lord Cherwell sent a telegram to Bohr in Sweden asking him to come to Britain. On the 6th of October, Bohr arrived in Scotland aboard a de Havilland Mosquito operated by the British Overseas Airways Corporation. These converted bomber aircraft carried passengers lying in the former bomb bay. Bohr's flying helmet was too small for him to wear, so he never heard the pilot's instruction to activate his oxygen supply when the plane climbed over Norway. He lost consciousness from oxygen starvation and only revived when the aircraft descended over the North Sea. His son Aage flew over a week later and became his personal assistant.

  Bohr joined the British Tube Alloys project and then, arriving in Washington on the 8th of December 1943 under the name Nicholas Baker, he entered the Manhattan Project. He visited Einstein and Pauli at Princeton, then went to Los Alamos, where Robert Oppenheimer later credited Bohr with acting as a scientific father figure to younger scientists, including Richard Feynman. In early February 1945, Oppenheimer noted, Bohr clarified the solution to a stubborn problem with modulated neutron initiators. Bohr himself quoted as saying, "They didn't need my help in making the atom bomb."

  Bohr recognised that the bomb would transform international relations. He met Churchill on the 16th of May 1944, but found the encounter fruitless - Churchill later wrote that Bohr ought to be confined for being dangerously close to leaking information to the Soviets. A letter in April 1944 from Soviet physicist Peter Kapitza had convinced Bohr that Moscow already knew about the Anglo-American project. Through Supreme Court Justice Felix Frankfurter, Bohr reached President Roosevelt. They met on the 26th of August 1944. Roosevelt suggested Bohr return to Britain to seek British support; when Churchill and Roosevelt conferred at Hyde Park on the 19th of September 1944, they rejected international disclosure and noted that Bohr's activities should be monitored.

• 7. After the War

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  Bohr returned to Copenhagen on the 25th of August 1945. In June 1950, he addressed an open letter to the United Nations calling for international cooperation on atomic energy. That call was partially answered in the following decade when the International Atomic Energy Agency was established, modelled on his proposal, after the Soviet Union's first nuclear weapon test in 1949. In 1957, he received the first ever Atoms for Peace Award.

  When European nations set about creating CERN to pool resources for large-scale physics research, Bohr and Kramers initially argued that the institute in Copenhagen should be its home. Pierre Auger, who organised the preliminary discussions, disagreed, feeling that Bohr and his institute were past their prime. After a long debate, Bohr pledged his support to CERN in February 1952. Geneva was chosen as the site in October of that year. The CERN Theory Group operated out of Copenhagen until their Geneva accommodation was ready in 1957. Victor Weisskopf, who later became CERN's Director General, said that without a person of Bohr's stature behind the project, the enthusiasm of others would not have been sufficient to carry it forward.

  Also in 1957, Bohr became the first chairman of the newly formed Nordic Institute for Theoretical Physics. He had served as first chairman of the Research Establishment Risø of the Danish Atomic Energy Commission from February 1956. King Frederik IX conferred on Bohr the Order of the Elephant - normally reserved for royalty and heads of state - at a memorial meeting on the 17th of October 1947. Bohr designed his own coat of arms for the occasion, incorporating the taijitu, the yin-and-yang symbol, with the Latin motto in contraria sunt complementa: "opposites are complementary."

  Bohr died of heart failure on the 18th of November 1962 at his home in Carlsberg, Copenhagen. His ashes were buried in the family plot at Assistens Cemetery in the Nørrebro section of the city, alongside those of his parents, his brother Harald, and his son Christian. On the 7th of October 1965, what would have been his 80th birthday, the Institute for Theoretical Physics at the University of Copenhagen was officially renamed the Niels Bohr Institute - a name it had already carried informally for years. In 1997, the Danish National Bank issued the 500-krone banknote bearing his portrait; in 1999, a survey named him the fourth greatest physicist of all time.