Niels Bohr
The Atom, the Argument With Einstein, and the Escape From Denmark
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Bohr's scientific contributions include the 1913 atomic model with electrons orbiting in fixed energy levels. The audiobook details his participation in the 1927 Solvay Conference where he argued with Einstein over quantum mechanics' interpretation. It also covers his secret meetings with German scientists during World War II, his escape from Denmark in 1943, and his later work on international atomic weapons control.
This audiobook offers a clear account of Bohr's scientific breakthroughs and personal struggles during wartime. Readers interested in the history of physics, the development of quantum theory, or the stories of scientists during World War II will find this biography both accessible and compelling.
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Niels Henrik David Bohr entered the world on 7 October 1885 in Copenhagen, as the second child of Christian Bohr, a professor of physiology at the University of Copenhagen, and Ellen Adler, whose father was David Baruch Adler, a Danish Jewish banker. He had an older sister, Jenny, who became a teacher, and a younger brother, Harald, who would grow up to be both a mathematician and a footballer. Harald played for Denmark’s national team at the 1908 Summer Olympics in London. Niels, too, was deeply involved in football, serving as goalkeeper for Akademisk Boldklub, a club based in Copenhagen.
Bohr began his education at Gammelholm Latin School at the age of seven. In 1903, he entered the University of Copenhagen as an undergraduate, where he studied physics under Christian Christiansen, the university’s only professor of physics at the time. He also pursued astronomy and mathematics with Thorvald Thiele, and philosophy with Harald Høffding, who was a friend of his father's.
In 1905, Bohr entered a competition organized by the Royal Danish Academy of Sciences and Letters to evaluate a technique for measuring surface tension first suggested by Lord Rayleigh in 1879. The challenge required determining the frequency of a water jet's oscillation. Working in his father’s laboratory at the university—where no dedicated physics lab existed—he crafted his own glass apparatus, including test tubes shaped with specific elliptical cross-sections. Going beyond the basic requirements, he enhanced both Rayleigh’s theory and the experimental method by factoring in water’s viscosity and using finite, rather than infinitesimal, movements. His final submission, delivered at the last moment, earned him the prize. He later submitted a revised version of the paper to the Royal Society in London.
Harald Bohr earned his master's degree in mathematics in April 1909, while Niels took nine more months to complete his own, focusing on the electron theory of metals under supervisor Christiansen. Niels expanded his thesis into a full Ph.D. dissertation, reviewing existing work and building on models by Paul Drude and Hendrik Lorentz, which treated electrons in metals like a gas. Though he refined their approach, Niels couldn't explain certain phenomena such as the Hall effect, concluding that electron theory couldn't fully account for metals' magnetic properties. His thesis was accepted in April 1911, and he defended it on May 13. Harald had already received his doctorate the year before. Written in Danish, a requirement at Copenhagen University, the work attracted little attention outside Scandinavia. In 1921, Dutch physicist Hendrika Johanna van Leeuwen would independently arrive at a theorem from Bohr's work now known as the Bohr–Van Leeuwen theorem.
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In April 1917, Bohr began pushing to found an Institute of Theoretical Physics. He won backing from the Danish government and the Carlsberg Foundation, plus major contributions from industry and private donors, many of them Jewish. By November 1918, the legislation was passed, and on March 3, 1921, the institute opened with Bohr as director. His family moved into an apartment on the first floor. The Niels Bohr Institute became a hub for quantum mechanics research during the 1920s and 1930s, drawing top physicists from around the world. Early arrivals included 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 even published the institute’s first work before it officially opened.
By 1919, Bohr had moved beyond the idea that electrons orbited the nucleus, crafting new ways to describe their behavior. The rare-earth elements puzzled chemists due to their chemical similarity. Then in 1924, Wolfgang Pauli’s discovery of the Pauli exclusion principle gave Bohr’s model a firmer theoretical foundation. With it, Bohr predicted that element 72 wasn’t a rare earth but shared properties with zirconium. The French chemist Georges Urbain claimed to have found it and named it “celtium.” In Copenhagen, Dirk Coster and George de Hevesy took up the test, searching through samples from the city’s Museum of Mineralogy for a zirconium-like element. They soon located it and named the new element hafnium, after the Latin name for Copenhagen.
In June 1922, Niels Bohr delivered seven lectures at the Institute of Theoretical Physics in Göttingen. These were the Wolfskehl Lectures, supported by the Wolfskehl Foundation. Held during the fortnight before the Göttingen International Handel Festival, the series came to be called the Bohr Festival. Some years later, in 1991, Friedrich Hund suggested that James Franck was responsible for this comparison. During the talks, Bohr summarized the progress of the Bohr-Sommerfeld theory, observing "how incomplete and uncertain everything still is."
In 1922, Bohr received the Nobel Prize in Physics for his work on the structure of atoms and the radiation they emit. The award honored not only his groundbreaking trilogy but also his early contributions to quantum mechanics. For his Nobel lecture, he presented a broad overview of atomic knowledge at the time, including the correspondence principle he had developed. This principle explains that quantum theory's behavior aligns with classical physics when dealing with large quantum numbers.
In 1923, Arthur Holly Compton discovered Compton scattering, which showed that light behaves like particles called photons and confirmed that energy and momentum are conserved in electron-photon collisions. The next year, Bohr, Kramers, and John C. Slater—working together at the Institute in Copenhagen—put forward what became known as the Bohr–Kramers–Slater theory, or BKS. It wasn’t a full theory but more of a research program, since its ideas weren't fully worked out mathematically. The BKS theory was an effort to explain how matter and light interact using the older quantum theory, where quantum rules were applied to classical descriptions of electromagnetic waves.
Using "virtual oscillators" at absorption and emission frequencies instead of the apparent frequencies of Bohr orbits, Max Born, Werner Heisenberg, and Kramers explored new mathematical models that led to matrix mechanics, the first form of modern quantum mechanics. Their work, known as the BKS theory, brought renewed attention to problems in the old quantum theory. The most controversial idea—that energy and momentum might not be conserved in individual interactions, only statistically—was soon disproven by experiments of Walther Bothe and Hans Geiger. In response, Bohr told Darwin, "there is nothing else to do than to give our revolutionary efforts as honourable a funeral as possible."
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In November 1925, George Uhlenbeck and Samuel Goudsmit introduced the concept of spin, a key development in quantum theory. The following month, Niels Bohr traveled to Leiden for the 50th anniversary celebration of Hendrick Lorentz’s doctorate. During a stop in Hamburg, he was approached by Wolfgang Pauli and Otto Stern, who wanted his thoughts on the new spin theory. Bohr expressed concerns about how electrons interact with magnetic fields. Upon arriving in Leiden, Paul Ehrenfest and Albert Einstein told him that Einstein had already solved this issue using relativity. Bohr then asked Uhlenbeck and Goudsmit to include this solution in their paper. On his way back through Göttingen, he met Werner Heisenberg and Pascual Jordan, and said of himself, “a prophet of the electron magnet gospel.”
In 1924, Heisenberg first traveled to Copenhagen, later returning to Göttingen in June 1925 to develop the mathematical framework of quantum mechanics. When he shared his results with Max Born, Born understood they were best expressed through matrices. The significance of this work drew the interest of the British physicist Paul Dirac, who visited Copenhagen for six months in September 1926. Also in 1926, the Austrian physicist Erwin Schrödinger came to town. His effort to explain quantum physics using classical wave mechanics left Bohr impressed, saying it contributed “so much to mathematical clarity and simplicity that it represents a gigantic advance over all previous forms of quantum mechanics.”
After Kramers departed the institute in 1926 to accept a professorship in theoretical physics at Utrecht University, Bohr made arrangements for Heisenberg to return to Copenhagen. Heisenberg took over Kramers’s role as a lektor at the University of Copenhagen. From 1926 through 1927, he served in this capacity as a university lecturer and assistant to Bohr.
Bohr became convinced that light acted as both waves and particles, and in 1927, experiments validated de Broglie’s idea that matter, like electrons, also behaved like waves. He developed the concept of complementarity, which explained how things could seem to have opposite properties—like being a wave or a stream of particles—depending on how they were observed. Bohr believed this principle wasn’t fully grasped even by professional philosophers.
In February 1927, Heisenberg introduced the uncertainty principle through a thought experiment involving a gamma-ray microscope. Bohr wasn’t satisfied, arguing that Heisenberg’s version only suggested measurement disturbed pre-existing properties, rather than embracing the deeper idea that electron properties don’t exist independently of measurement. At the Como Conference in September 1927, Bohr argued the uncertainty relations could be understood through classical ideas about optical instruments. He believed fully grasping complementarity would need “closer investigation.” Einstein preferred classical physics’ determinism to quantum mechanics’ probability, even though he had helped lay the groundwork for it. Their debates over these new ideas became widely discussed philosophical topics, continuing throughout their lives with good-natured disagreement.
In 1914, Carl Jacobsen gave his mansion—now known as the Carlsberg Academy—to be used as an honorary residence for the most distinguished Dane in science, literature, or the arts. Harald Høffding was the first occupant, and after he died in July 1931, the Royal Danish Academy of Sciences and Letters selected Niels Bohr to live there. Bohr and his family moved into the home in 1932. On March 17, 1939, he was elected president of the Academy.
By 1929, beta decay had led Bohr to propose that the law of conservation of energy should be abandoned, but Wolfgang Pauli's idea of a hypothetical neutrino and the discovery of the neutron in 1932 offered another explanation. That prompted Bohr to develop a new theory of the compound nucleus in 1936, which described how neutrons could be captured by a nucleus. In this model, the nucleus could behave like a droplet of liquid. He worked on this with Fritz Kalckar, a Danish physicist who died suddenly in 1938.
The discovery of nuclear fission by Otto Hahn in December 1938 sent shockwaves through the physics community. Bohr carried the news to America, where he opened the fifth Washington Conference on Theoretical Physics alongside Fermi on 26 January 1939. George Placzek responded to Bohr's claim that this solved all puzzles about transuranic elements by pointing out one unresolved issue: the neutron capture energies of uranium didn't align with its decay patterns. After a few moments, Bohr told Placzek, Léon Rosenfeld, and John Wheeler, "I have understood everything." Drawing on his liquid drop model, he determined that it was the uranium-235 isotope, not uranium-238, driving fission with thermal neutrons. By April 1940, John R. Dunning confirmed Bohr's conclusion. During this time, Bohr and Wheeler also worked out a theoretical framework, publishing their findings in a September 1939 paper titled "The Mechanism of Nuclear Fission."
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Heisenberg once described Bohr as “primarily a philosopher, not a physicist,” and Bohr’s engagement with philosophy ran deep. He read the works of the 19th-century Danish thinker Søren Kierkegaard, whose existentialist ideas left a strong impression. In 1909, Bohr gave his brother Kierkegaard’s Stages on Life’s Way for a birthday present. In the accompanying letter, he wrote, “It is the only thing I have to send home; but I do not believe that it would be very easy to find anything better... I even think it is one of the most delightful things I have ever read.” Though Bohr appreciated Kierkegaard’s writing style, he found some philosophical disagreements. Some biographers believe those tensions stemmed from Kierkegaard’s Christian beliefs, while Bohr himself was an atheist.
There has been some debate about how much Kierkegaard influenced Bohr’s thinking. David Favrholdt believed Kierkegaard had little impact, pointing to Bohr’s own statement that he disagreed with Kierkegaard. But Jan Faye took a different view, suggesting that someone could reject the ideas of a theory while still accepting its overall structure and basic assumptions.
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There has been much debate about Bohr’s views on quantum mechanics, with scholars interpreting his philosophy in different ways—some seeing him as an anti-realist or instrumentalist, others as a phenomenological realist. Though some have called him a subjectivist or positivist, most philosophers agree this misreads him, since Bohr never supported verificationism or the idea that observers directly affect measurement outcomes. One famous quote often attributed to Bohr—"there is no quantum world, but only an abstract quantum physical description"—was actually said privately by Aage Petersen in a memoir after Bohr’s death. N. David Mermin recalled Victor Weisskopf strongly objecting, saying, “Shame on Aage Petersen for putting those ridiculous words in Bohr's mouth!”
Bohr believed that understanding the world requires a clear difference between what we experience and what exists independently of our experience. He thought this distinction is only possible through concepts like space, time, and causation—what he called "classical" ideas. According to Jan Faye, these familiar notions help us talk about objects and their existence. Bohr argued that basic ideas such as time are already part of how we speak, and classical physics simply refines them. So, when describing quantum experiments, we must use classical language. As Bohr wrote: “The account of all evidence must be expressed in classical terms.”
According to Faye, Bohr believed classical concepts were needed to describe quantum phenomena, offering five frameworks for this view: empiricism or logical positivism; Kantianism, also called Neo-Kantian models of epistemology; Pragmatism, which looks at how humans interact with atomic systems based on their needs and interests; Darwinianism, the idea that we evolved to use classical concepts, a view Léon Rosenfeld supported; and Experimentalism, which insists experiments must be described classically because of their function and outcome. These ideas aren't mutually exclusive, and Bohr shifted between them depending on context. Faye notes that Bohr considered atoms real, not just tools or logical constructions. Yet he didn't think the quantum mechanical formalism offered a literal, pictorial picture of reality. Instead, his complementarity theory was first and foremost about meaning and knowledge, carrying some implications about what exists. As Faye explains, Bohr's indefinability thesis is that...
Bohr said that to understand what happens to an atom, you have to consider not just the object itself but also the setup of the experiment and what actually occurred during it. This idea was central to his view of quantum physics. Faye points out that Bohr never talked about a “collapse of the wave function” during measurements, even though others later described it that way. Instead, Bohr accepted the statistical interpretation introduced by Born, but he believed the ψ-function did not represent anything real—it was only symbolic. Because of this, there could be no actual collapse of the wavefunction in Bohr’s view.
There is ongoing disagreement among scholars about Bohr’s views on the nature of atoms and reality. Henry Folse believes Bohr distinguished between what we observe and a deeper, underlying truth. Jan Faye challenges that view, arguing that for Bohr, quantum mechanics and the principle of complementarity were all we could truly say about the quantum world. Faye further points out that there is no evidence in Bohr’s writings suggesting he thought atomic objects possess hidden, measurement-independent properties beyond those revealed through observation.
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When Nazism rose in Germany, many scholars were forced to flee—some for being Jewish, others for opposing the regime. In 1933, the Rockefeller Foundation created a fund to support refugee academics, and Bohr discussed the program with its President, Max Mason, during a visit to the United States. He provided temporary jobs at his institute, helped secure financial support, arranged fellowships through the Foundation, and ultimately found positions for these scholars around the world. Among those he assisted were George Placzek, Arthur von Hippel, Victor Weisskopf, and others.
In April 1940, when Nazi Germany invaded Denmark, Bohr helped hide Max von Laue’s and James Franck’s gold Nobel medals by having them dissolved in aqua regia and stored on a shelf at the Institute until after the war. The gold was then recovered and the medals re-struck by the Nobel Foundation. Bohr’s own medal had been sold at auction in March 1940, along with August Krogh’s, to raise money for the Finnish Relief Fund. The buyer later gave both medals to the Danish Historical Museum in Frederiksborg Castle, where they remain. In 2023–2024, Bohr’s medal traveled to space with Andreas Mogensen on ISS Expedition 70.
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Bohr knew that uranium-235 could potentially be used to build an atomic bomb, and he mentioned this in talks he gave in Britain and Denmark before and after the war began. In September 1941, Werner Heisenberg, who was now leading Germany’s nuclear energy project, came to visit Bohr in Copenhagen. The two men stepped outside for a private conversation, though what they discussed remains unclear because their accounts differ. Heisenberg said he brought up topics like nuclear energy, morality, and the war, but Bohr reportedly ended the talk quickly without giving him any clues about his own views. One of Heisenberg’s students, Ivan Supek, claimed the main topic was Carl Friedrich von Weizsäcker, who had suggested getting Bohr to help negotiate peace between Britain and Germany.
In 1957, Heisenberg wrote to Robert Jungk, who was then working on the book Brighter than a Thousand Suns: A Personal History of the Atomic Scientists. He described visiting Copenhagen to convey the views of several German scientists, noting that creating a nuclear weapon was possible with great effort and that this posed enormous responsibilities for scientists on both sides. When Bohr encountered Jungk’s portrayal in the Danish translation of the book, he prepared — though never sent — a letter to Heisenberg. In it, he said he strongly disagreed with Heisenberg’s version of their meeting. Bohr remembered that Heisenberg had come not to discuss collaboration but to promote cooperation with the surely triumphant Nazis, and he was stunned that Germany would pursue nuclear weapons under Heisenberg's direction.
Michael Frayn's play Copenhagen, first performed in 1998, imagines what might have occurred during the 1941 meeting between Heisenberg and Bohr. A BBC television adaptation aired in 2002, with Stephen Rea portraying Bohr. After the release of Bohr’s letters, historians criticized the play as a “grotesque oversimplification and perversion of the actual moral balance,” noting its pro-Heisenberg stance.
The meeting between Bohr and Heisenberg was adapted for television in 1992 by the BBC’s Horizon science documentary series. In that production, Anthony Bate portrayed Bohr and Philip Anthony played Heisenberg. A second dramatization of the encounter appears in the Norwegian/Danish/British miniseries titled The Heavy Water War.
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In September 1943, Niels Bohr and his brother Harald were told by the Nazis that they were considered Jewish and therefore in danger of arrest. The Danish resistance arranged for Bohr and his wife to flee by sea to Sweden on 29 September. The following day, Bohr persuaded King Gustaf V of Sweden to publicly support offering asylum to Jewish refugees. On 2 October 1943, Swedish radio made that declaration, which led quickly to the rescue of many Danish Jews. Some historians argue that Bohr’s actions directly caused the mass rescue, while others say his efforts, though significant, did not determine the overall outcome. Over 7,000 Danish Jews eventually found safety in Sweden.
When word reached Britain that Bohr had escaped, Lord Cherwell wired him a message asking him to come to Scotland. Bohr arrived on 6 October, traveling on a de Havilland Mosquito operated by BOAC. These aircraft were fast, unarmed bombers converted to carry important passengers or small, valuable cargoes. Flying at high speed and altitude, they could cross German-occupied Norway and avoid fighters. Bohr, equipped with a parachute, flying suit, and oxygen mask, spent three hours lying on a mattress in the bomb bay. He didn't wear his flying helmet because it was too small, so he didn't hear the pilot's intercom instruction to turn on his oxygen supply during the climb over Norway. He passed out from oxygen starvation and only woke when the plane descended over the North Sea. A week later, his son Aage followed him to Britain and became his personal assistant.
Bohr reached Washington, D.C., on 8 December 1943, where he met Brigadier General Leslie R. Groves Jr., who led the Manhattan Project. He was assigned an apartment at St James's Palace and a workspace with the British Tube Alloys team working on nuclear weapons. While in the United States, he traveled to Princeton, New Jersey, to see Einstein and Pauli at the Institute for Advanced Study, then made his way to Los Alamos, New Mexico, where the bombs were being designed. For security reasons, he went by the name “Nicholas Baker,” with Aage serving as “James Baker.” Bohr was amazed by how much progress had been made. In May 1944, a Danish resistance newspaper reported that Professor Niels Bohr had fled Denmark the previous October via Sweden to London and then on to Moscow, from where he could be assumed to support the war effort.
Bohr didn’t stay at Los Alamos, but visited frequently over the next two years. Robert Oppenheimer said Bohr acted "as a scientific father figure to the younger men", especially Richard Feynman. Bohr himself said, "They didn't need my help in making the atom bomb." Still, Oppenheimer credited Bohr with helping solve a tough problem involving modulated neutron initiators. He noted that this device had been a stubborn puzzle until early February 1945, when Bohr clarified what needed to be done.
Bohr understood early on that nuclear weapons would reshape global power. In April 1944, he got a letter from Peter Kapitza while he was in Sweden, inviting him to come to the Soviet Union. That note made Bohr believe the Soviets knew about the Anglo-American effort and planned to keep up. He replied to Kapitza without committing, showing the response to British authorities before sending it. Later that year, on 16 May 1944, Bohr met with Churchill, but found they couldn’t agree. Churchill rejected sharing information with the Russians, writing that “It seems to me Bohr ought to be confined or at any rate made to see that he is very near the edge of mortal crimes.”
After discussing the Manhattan Project with President Franklin D. Roosevelt, Bohr was advised by Oppenheimer to seek support from the Soviets, hoping their involvement might accelerate progress. Through the help of his friend Supreme Court Justice Felix Frankfurter, Bohr met with Roosevelt on 26 August 1944. The president suggested that Bohr travel to the United Kingdom to try to win British approval. When Churchill and Roosevelt convened at Hyde Park on 19 September 1944, they decided against sharing details of the project. Their conversation included a note stating that “enquiries should be made regarding the activities of Professor Bohr and steps taken to ensure that he is responsible for no leakage of information, particularly to the Russians.”
In June 1950, Bohr wrote an “Open Letter” to the United Nations asking for global cooperation on nuclear energy. During the 1950s, after the Soviet Union tested its first nuclear weapon in 1949, the International Atomic Energy Agency was formed based on ideas Bohr had suggested. Then, in 1957, he was given the very first Atoms for Peace Award.
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After the war ended, Bohr went back to Copenhagen on August 25, 1945, and was elected President of the Royal Danish Academy of Arts and Sciences again on September 21. At a memorial meeting for King Christian X, who had died in April, the new king, Frederik IX, said he was giving Bohr the Order of the Elephant. That honor is usually only given to royalty and world leaders, but the king said it recognized not just Bohr, but all of Danish science. Bohr also created his own coat of arms, using a taijitu symbol and a Latin motto: "contraria sunt complementa," which means "opposites are complementary."
The Second World War revealed that science, particularly physics, now demanded substantial funding and materials. In response, twelve European nations formed CERN to pursue large-scale research projects beyond what any one country could manage alone. When selecting a site, debate arose. Bohr and Kramers favored the Copenhagen Institute. Pierre Auger, who guided early discussions, disagreed, feeling both Bohr and his institution were no longer at their peak and that Bohr’s influence might dominate. After prolonged discussion, Bohr backed the effort in February 1952, with Geneva chosen as the location in October. The CERN Theory Group remained in Copenhagen until new facilities in Geneva opened in 1957. Victor Weisskopf, later CERN’s Director General, recalled that “the enthusiasm and ideas of the other people would not have been enough, however, if a man of his stature had not supported it.”
In 1957, Scandinavian countries came together to form the Nordic Institute for Theoretical Physics, with Bohr named as its chairman. At the same time, he was involved in creating the Research Establishment Risø, which was part of the Danish Atomic Energy Commission. Bohr became the first chairman of this new organization in February 1956.
Niels Bohr passed away on 18 November 1962 after heart failure, at his home in Carlsberg, Copenhagen. His remains were cremated and later laid to rest in the family plot at Assistens Cemetery, in the Nørrebro area of the city. There, alongside his parents, brother Harald, and son Christian, he was joined by his wife’s ashes years afterward. In honor of his legacy, the Institute for Theoretical Physics at the University of Copenhagen was officially renamed the Niels Bohr Institute on 7 October 1965 — a day that would have marked his 80th birthday.
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In 1910, Niels Bohr met Margrethe Nørlund, the sister of mathematician Niels Erik Nørlund. On April 16, 1912, he resigned from the Church of Denmark, and the couple married in a civil ceremony in Slagelse on August 1. Later, his brother Harald also left the church before marrying. Bohr and Margrethe had six sons. Their eldest, Christian, died in a boating accident in 1934. Another son, Harald, was severely mentally disabled and placed in an institution at age four, where he died of meningitis six years later. Aage Bohr became a physicist and won the Nobel Prize in Physics in 1975. His son Vilhelm A. Bohr works with the University of Copenhagen and the National Institute on Aging in the U.S. Hans became a physician, Erik a chemical engineer, and Ernest a lawyer. Ernest also played field hockey for Denmark at the 1948 Summer Olympics in London.
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