Niels Henrik David Bohr (born October 7, 1885; died November 18, 1962) was a Danish scientist who studied how atoms and energy work. He won the Nobel Prize in Physics in 1922 for his important discoveries. He also worked as a philosopher and supported scientific research.
Bohr created the Bohr model of the atom. In this model, he explained that electrons move in specific paths around the nucleus and can jump between energy levels. Even though newer models have replaced this idea, the basic principles are still correct. Bohr also introduced the principle of complementarity, which means that things can be studied in different ways, such as acting like a wave or a group of particles. This idea influenced his work in science and philosophy.
In 1920, Bohr started the Institute of Theoretical Physics at the University of Copenhagen, now called the Niels Bohr Institute. He worked with scientists like Hans Kramers, Oskar Klein, George de Hevesy, and Werner Heisenberg. He predicted the properties of a new element similar to zirconium, which was later named hafnium after Copenhagen. A synthetic element, bohrium, was named after him because of his work on atomic structure.
During the 1930s, Bohr helped people fleeing from Nazi Germany. After Germany occupied Denmark, he met with Heisenberg, who led Germany’s nuclear weapons project. In 1943, Bohr learned he was about to be arrested by the Germans and fled to Sweden. From there, he traveled to Britain and joined the British Tube Alloys nuclear project. He also worked on the Manhattan Project. After World War II, Bohr promoted international cooperation on nuclear energy. He helped create CERN and the Research Establishment Risø in Denmark. He became the first chairman of the Nordic Institute for Theoretical Physics in 1957. In 1999, he was named the fourth greatest physicist of all time.
Early life and education
Niels Henrik David Bohr was born on October 7, 1885, in Copenhagen, Denmark. He was the second of three children in a family with Christian Bohr, a professor of physiology at the University of Copenhagen, and Ellen Adler, the daughter of a Danish Jewish banker named David Baruch Adler. Niels had an older sister named Jenny and a younger brother named Harald. Jenny became a teacher, and Harald became a mathematician and a football player who played for Denmark’s national team at the 1908 Summer Olympics in London. Niels also loved playing football and played with his brother for the Copenhagen-based Akademisk Boldklub (Academic Football Club), where Niels was the goalkeeper.
Bohr attended Gammelholm Latin School when he was seven years old. In 1903, he began studying physics at the University of Copenhagen. His physics professor was Christian Christiansen, the only physics professor at the university at that time. Bohr also studied astronomy and mathematics with Thorvald Thiele and philosophy with Harald Høffding, a friend of his father.
In 1905, the Royal Danish Academy of Sciences and Letters organized a competition to find a way to measure the surface tension of liquids, a method proposed by Lord Rayleigh in 1879. This involved measuring how the radius of a water jet moved. Bohr used his father’s laboratory at the university to conduct experiments, as the university had no physics lab. To complete his work, he made his own glassware, including test tubes with oval-shaped cross-sections. Bohr improved on Rayleigh’s method by considering the thickness of water and using measurements for larger movements instead of only tiny ones. His essay, written at the last minute, won the competition. Later, he sent an improved version of his paper to the Royal Society in London for publication in the Philosophical Transactions of the Royal Society.
Harald was the first of the Bohr brothers to earn a master’s degree, which he received in mathematics in April 1909. Niels earned his master’s degree nine months later for his work on the electron theory of metals, a topic assigned by his professor, Christiansen. Bohr later expanded his master’s thesis into a longer Ph.D. thesis. He reviewed existing research and focused on a model developed by Paul Drude and expanded by Hendrik Lorentz, which compared electrons in metals to gas particles. Bohr improved Lorentz’s model but could not explain certain effects, like the Hall effect, and concluded that electron theory could not fully explain the magnetic properties of metals. His thesis was accepted in April 1911, and he defended it formally on May 13. Harald had received his doctorate the previous year. Bohr’s work was important but received little attention outside Scandinavia because it was written in Danish, as required by the university at the time. In 1921, the Dutch physicist Hendrika Johanna van Leeuwen independently discovered a rule in Bohr’s thesis, now known as the Bohr–Van Leeuwen theorem.
Physics
In September 1911, Bohr, with financial support from the Carlsberg Foundation, traveled to England, where scientists were studying the structure of atoms and molecules. He met J. J. Thomson of the Cavendish Laboratory and Trinity College, Cambridge. He attended lectures on electromagnetism by James Jeans and Joseph Larmor, and did research on cathode rays, but did not impress Thomson. He had better success with younger scientists like William Lawrence Bragg from Australia and Ernest Rutherford from New Zealand. Rutherford had proposed a model of the atom with a small central nucleus, which challenged Thomson’s earlier "plum pudding" model. Rutherford invited Bohr to work at Victoria University of Manchester, where Bohr met George de Hevesy and Charles Galton Darwin, whom Bohr called "the grandson of the real Darwin."
Bohr returned to Denmark in July 1912 for his wedding and traveled through England and Scotland on his honeymoon. After returning, he became a Privatdocent at the University of Copenhagen, teaching about thermodynamics. Martin Knudsen recommended Bohr for a docent position, which was approved in July 1913. Bohr then taught medical students. In 1913, he published three famous papers, later called "the trilogy," in Philosophical Magazine. In these papers, Bohr combined Rutherford’s nuclear model with Max Planck’s quantum theory to create his Bohr model of the atom.
Planetary models of atoms were not new, but Bohr’s approach was different. He used a 1912 paper by Darwin about electrons and nuclei as a starting point. Bohr proposed that electrons move in specific, quantized "stationary states" around the nucleus to keep atoms stable. He later showed that the chemical properties of elements depend on the number of electrons in their outer orbits. He also introduced the idea that electrons can move between energy levels, emitting light in the process. This became a key part of the old quantum theory.
In 1885, Johann Balmer discovered a formula, now called the Balmer series, to describe the visible light emitted by hydrogen atoms. The formula worked well, but no one could explain why for 30 years. In his first trilogy paper, Bohr used his model to explain the Balmer series mathematically. The formula involved the electron’s mass, charge, Planck’s constant, and the atom’s atomic number.
One challenge for Bohr’s model was the Pickering series, which did not fit Balmer’s formula. When asked about this by Alfred Fowler, Bohr explained that the lines were caused by ionized helium atoms. His model worked for such ions. Many older scientists, like Thomson, Rayleigh, and Hendrik Lorentz, disliked the trilogy, but younger scientists, including Rutherford, Einstein, and Arnold Sommerfeld, saw it as a breakthrough. Einstein praised Bohr’s model as "the highest form of musicality in the sphere of thought." The trilogy gained acceptance because it explained problems other models could not and predicted results later confirmed by experiments. Today, the Bohr model is no longer used, but it remains widely known in school textbooks.
Bohr did not enjoy teaching medical students. He later admitted he was not a good lecturer, as he struggled to balance clarity and truth. He returned to Manchester, where Rutherford offered him a job as a reader. He took a leave of absence from the University of Copenhagen, visiting Tyrol with his brother and aunt. There, he met Sommerfeld and gave lectures on the trilogy. World War I began during their trip, making it harder to return to Denmark. Bohr and his wife moved to England in 1914, where he worked until 1916. He was then appointed to the Chair of Theoretical Physics at the University of Copenhagen, a position created for him. He still had to teach medical students, and his docentship was abolished. New professors were introduced to King Christian X, who was pleased to meet Bohr, a famous football player.
In 1917, Bohr began working to create an Institute of Theoretical Physics. He gained support from the Danish government, the Carlsberg Foundation, and private donors, including many Jewish contributors. The institute was officially established in 1918 and opened in 1921 as the Niels Bohr Institute, with Bohr as its director. His family moved into an apartment on the first floor. The institute became a center for quantum mechanics research in the 1920s and 1930s, attracting scientists like Hans Kramers and George de Hevesy. Klein and Rosseland published the institute’s first paper before it opened.
Bohr’s model worked well for hydrogen and ionized helium but could not explain more complex elements. By 1919, he moved away from the idea of electrons orbiting the nucleus. Chemists struggled to classify rare-earth elements because they had similar chemical properties. In 1924, Wolfgang Pauli discovered the Pauli exclusion principle, which helped solidify Bohr’s models. Bohr predicted that element 72 was not a rare-earth element but had properties similar to zirconium. This claim was challenged by French chemist Georges Urbain, who claimed to have discovered a rare-earth element called "celtium." Dirk Coster and George de Hevesy tested Bohr’s prediction by searching for a zirconium-like element in mineral samples. They found it and named it hafnium, after Copenhagen’s Latin name, "hafnia." Hafnium was more common than gold.
The Bohr Festival, known as the "Bohrfestival," was a series of seven lectures given by Bohr in 1922 at the Institute of Theoretical Physics in Göttingen. These lectures, funded by the Wolfskehl Foundation, were held before the Göttingen International Handel Festival and became known as the Bohr Festival. In 1991, Friedrich Hund suggested that James Franck was responsible for naming the event. The lectures were part of the Wolfskehl Lectures.
Philosophy
Niels Bohr once said that Niels Henrik David Bohr was "primarily a philosopher, not a physicist." Bohr studied the writings of Søren Kierkegaard, a 19th-century Danish philosopher who focused on personal choices and the meaning of life. Richard Rhodes wrote in The Making of the Atomic Bomb that Bohr was influenced by Kierkegaard through another philosopher named Høffding. In 1909, Bohr gave his brother a book by Kierkegaard titled Stages on Life's Way as a birthday gift. In a letter that came with the book, Bohr 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." Bohr liked Kierkegaard's writing style but said he disagreed with some of Kierkegaard's ideas. Some of Bohr's biographers suggested that this disagreement might have been because Kierkegaard supported Christianity, while Bohr did not believe in religion.
There has been debate about how much Kierkegaard influenced Bohr's ideas. David Favrholdt argued that Kierkegaard had little influence on Bohr's work, taking Bohr's statement about disagreeing with Kierkegaard as true. Jan Faye, however, believed that people can disagree with the details of a theory while still agreeing with its overall structure.
Bohr served on the Editorial Board of a book series called World Perspectives, which published works on philosophy.
There has been much discussion about Bohr's views on quantum mechanics. Some people have described Bohr as an anti-realist, an instrumentalist, a phenomenological realist, or another type of realist. Others thought Bohr might have been a subjectivist or a positivist, but most philosophers say this is incorrect because Bohr never supported the idea that personal experience directly affects scientific results.
Bohr is often quoted as saying, "There is no quantum world, only an abstract quantum physical description." However, this was not a public statement by Bohr but a private comment attributed to him by Aage Petersen after Bohr's death. N. David Mermin later said that Victor Weisskopf believed Bohr would not have made such a statement and criticized Petersen for misrepresenting Bohr.
Many scholars believe that the philosophy of Immanuel Kant influenced Bohr. Like Kant, Bohr thought that understanding the difference between a person's experience and the object being studied is important for gaining knowledge. This requires using ideas like cause and effect, and how things change over time and space. According to Jan Faye, Bohr believed that classical ideas such as "space," "time," and "causation" are necessary to describe objects and their existence. Bohr also thought that these classical ideas are part of everyday language and that the ideas used in classical physics are more detailed versions of them. For Bohr, classical concepts must be used to describe experiments involving the quantum world.
Faye explains that there are several reasons why Bohr believed classical concepts were important for describing quantum phenomena. These reasons include:
1. Empiricism (a belief that knowledge comes from experience),
2. Kantianism (a focus on how knowledge is structured),
3. Pragmatism (a focus on how people interact with the physical world),
4. Darwinianism (the idea that humans evolved to use classical concepts), and
5. Experimentalism (a focus on how experiments must be described classically).
These ideas are not always separate, and Bohr sometimes emphasized one over others.
Faye notes that Bohr believed atoms are real, not just tools or logical ideas. However, Bohr did not think that the mathematical framework of quantum mechanics was a direct picture of reality. Instead, Bohr's theory of complementarity is a way of understanding quantum mechanics that focuses on how knowledge is gained, with some implications about the nature of reality.
Faye also explains that Bohr's idea of "indefinability" means that certain quantum concepts cannot be clearly defined in the same way as classical ones.
Faye points out that Bohr never mentioned the idea of a "wave function collapse" during measurements. Instead, Bohr supported the Born statistical interpretation, which says the wave function is a symbolic tool, not a direct representation of reality. Because Bohr did not believe the wave function was a literal picture of the world, he did not think the wave function could "collapse" into a real state.
A major topic in recent discussions is whether Bohr believed atoms have a reality beyond what we observe. Some, like Henry Folse, argue that Bohr saw a difference between what we observe and a deeper, hidden reality. Jan Faye disagrees, saying Bohr only described the quantum world using the quantum formalism and complementarity. Faye adds that Bohr did not write about atoms having properties that exist independently of measurements, beyond what classical ideas describe.
World War II
The rise of Nazism in Germany caused many scholars to leave their countries, either because they were Jewish or because they opposed the Nazi government. In 1933, the Rockefeller Foundation started a program to help refugee scientists. In May 1933, Niels Bohr talked about this program with Max Mason, the president of the Rockefeller Foundation, during a visit to the United States. Bohr helped refugees by giving them temporary jobs at his institute, offering financial support, arranging fellowships from the Rockefeller Foundation, and finding them positions at institutions worldwide. Some of the people he helped included Guido Beck, Felix Bloch, James Franck, George de Hevesy, Otto Frisch, Hilde Levi, Lise Meitner, George Placzek, Eugene Rabinowitch, Stefan Rozental, Erich Ernst Schneider, Edward Teller, Arthur von Hippel, and Victor Weisskopf.
In April 1940, during World War II, Nazi Germany invaded Denmark. To protect Nobel Prize medals belonging to Max von Laue and James Franck from being taken by the Nazis, Bohr asked George de Hevesy to dissolve the medals in a strong chemical called aqua regia. The melted gold was stored on a shelf at the Institute until after the war, when it was used to make new medals. Bohr’s own medal had been sold at an auction to help a relief fund in Finland. Later, the two medals were donated to the Danish Historical Museum in Frederiksborg Castle, where they remain. In 2023–2024, Bohr’s medal briefly traveled to space with Andreas Mogensen on the International Space Station.
Bohr kept his institute running, but all the foreign scientists left.
Bohr knew that uranium-235 could be used to build an atomic bomb. He mentioned this in lectures in Britain and Denmark before and after the war began, but he did not think it was possible to get enough uranium-235 to make a bomb. In September 1941, Heisenberg, who led Germany’s nuclear energy project, visited Bohr in Copenhagen. During their meeting, they had a private conversation outside the institute. The details of their discussion are unclear because Heisenberg and Bohr gave different accounts. Heisenberg said he talked about nuclear energy, morality, and the war, but Bohr ended the conversation quickly without sharing his views. A student of Heisenberg, Ivan Supek, claimed the main topic was Carl Friedrich von Weizsäcker, who wanted Bohr to help negotiate peace between Britain and Germany.
In 1957, Heisenberg wrote to Robert Jungk, who was writing a book about atomic scientists. Heisenberg said he visited Bohr to share German scientists’ concerns about the possibility of making nuclear weapons and the responsibilities this created for scientists. When Bohr saw Jungk’s book in Danish, he wrote a letter to Heisenberg (but never sent it) saying he disagreed with Heisenberg’s account. Bohr claimed Heisenberg’s visit was to encourage cooperation with Nazi Germany and was shocked that Heisenberg led Germany’s nuclear weapons program.
In 1998, Michael Frayn wrote a play called Copenhagen, which imagined what happened during Heisenberg and Bohr’s 1941 meeting. A BBC television version of the play aired in 2002. Historians later criticized the play for being too biased toward Heisenberg. The meeting was also dramatized in a 1992 BBC science documentary and in a Norwegian/Danish/British miniseries called The Heavy Water War.
In September 1943, Bohr and his brother Harald learned the Nazis considered their family Jewish because their mother was Jewish. This made them a target for arrest. Danish resistance fighters helped Bohr and his wife escape to Sweden by boat on September 29. The next day, Bohr convinced Sweden’s King Gustaf V to publicly offer asylum to Jewish refugees. On October 2, 1943, Sweden announced it would help Jewish refugees, which led to the rescue of thousands of Danish Jews. Some historians say Bohr’s actions directly caused this rescue, while others believe his efforts were not the main reason for the success. Eventually, over 7,000 Danish Jews escaped to Sweden.
When news of Bohr’s escape reached Britain, Lord Cherwell sent a telegram asking Bohr to join Britain. Bohr arrived in Scotland on October 6, 1943, in a fast, unarmed aircraft called a de Havilland Mosquito. The plane flew at high speed and altitude to avoid German fighters. During the flight, Bohr did not wear his oxygen mask because the helmet was too small. He passed out from lack of oxygen and only recovered when the plane descended over the North Sea. A week later, Bohr’s son, Aage, joined him in Britain and became his assistant.
In Britain, Bohr was welcomed by scientists like James Chadwick and Sir John Anderson, but for safety, he stayed out of public view. He lived in an apartment at St. James’s Palace and worked with the British Tube Alloys team, which developed nuclear weapons. Chadwick arranged for Bohr to visit the United States as a consultant, with Aage as his assistant. In December 1943, Bohr arrived in Washington, D.C., where he met the director of the Manhattan Project, General Leslie Groves. He visited scientists like Albert Einstein and Wolfgang Pauli in Princeton, New Jersey, and toured Los Alamos, New Mexico, where nuclear weapons were being designed. In the United States, Bohr used the name “Nicholas Baker,” and Aage became “James Baker.” In May 1944, a Danish newspaper reported that “Professor Niels Bohr” had fled Denmark to London and possibly supported the war effort from Moscow.
Bohr did not stay at Los Alamos but visited the site multiple times over the next two years. Robert Oppenheimer, a leader of the Manhattan Project, said Bohr acted like a “scientific father figure” to younger scientists, including Richard Feynman. Bohr once said, “They didn’t need my help in making the atom bomb.” Oppenheimer credited Bohr with helping solve a difficult problem related to neutron initiators in nuclear weapons. He said, “This device remained a stubborn puzzle, but in early February 1945, Niels Bohr clarified what had to be done.”
Later life
After the war ended, Bohr returned to Copenhagen on August 25, 1945. He was re-elected President of the Royal Danish Academy of Arts and Sciences on September 21. At a memorial meeting of the Academy on October 17, 1947, to honor King Christian X, who had died in April, the new king, Frederik IX, announced that Bohr would be awarded the Order of the Elephant. This award was usually given only to royalty and heads of state, but the king explained it recognized Bohr’s contributions to Danish science. Bohr designed his own coat of arms, which included a taijitu (a symbol of yin and yang) and a Latin motto: contraria sunt complementa, meaning “opposites are complementary.”
The Second World War showed that science, especially physics, needed large amounts of money and resources. To prevent scientists from leaving Europe for the United States, twelve European countries formed CERN, a research organization similar to U.S. national laboratories. CERN aimed to conduct large-scale science projects that no single country could manage alone. Debates arose about where to build CERN’s facilities. Bohr and Kramers believed the Institute in Copenhagen would be the best location. Pierre Auger, who led early discussions, disagreed, saying Bohr and his Institute were no longer as strong, and Bohr’s presence might dominate others. After much discussion, Bohr supported CERN in February 1952, and Geneva was chosen as the site in October. The CERN Theory Group remained in Copenhagen until new buildings in Geneva were ready in 1957. Victor Weisskopf, who later became CERN’s Director General, noted that while others started the idea of CERN, Bohr’s support was essential for its success.
At the same time, Scandinavian countries created the Nordic Institute for Theoretical Physics in 1957, with Bohr as its chairman. He also helped establish the Research Establishment Risø of the Danish Atomic Energy Commission and served as its first chairman from February 1956.
Bohr died of heart failure on November 18, 1962, at his home in Carlsberg, Copenhagen. He was cremated, and his ashes were buried in the family plot at Assistens Cemetery in Nørrebro, Copenhagen, alongside his parents, his brother Harald, and his son Christian. Later, his wife’s ashes were also placed there. On October 7, 1965, the Institute for Theoretical Physics at the University of Copenhagen was officially renamed the Niels Bohr Institute, a name it had been called informally for many years.
Family
In 1910, Bohr met Margrethe Nørlund, the sister of mathematician Niels Erik Nørlund. On April 16, 1912, Bohr gave up his membership in the Church of Denmark. He and Margrethe had a non-religious ceremony at the town hall in Slagelse on August 1. Later, Bohr’s brother, Harald, also left the church before getting married. Bohr and Margrethe had six sons. Their oldest son, Christian, died in a boat accident in 1934. Another son, Harald, had serious mental challenges and was sent to a facility away from his family’s home when he was 4 years old. He later died from childhood meningitis six years later. Aage Bohr became a successful physicist and won the Nobel Prize in Physics in 1975, just like his father. Aage’s son, Vilhelm A. Bohr, is a scientist who works with the University of Copenhagen and the National Institute on Aging in the United States. Hans became a doctor; Erik became a chemical engineer; and Ernest became a lawyer. Like his uncle Harald, Ernest Bohr played field hockey for Denmark at the 1948 Summer Olympics in London.
Commemoration
The 50th anniversary of the Bohr model was celebrated in Denmark on 21 November 1963 with a postage stamp showing Bohr, a hydrogen atom, and the formula for the difference between two hydrogen energy levels: h ν = ϵ 2 − ϵ 1 {displaystyle hnu =epsilon _{2}-epsilon _{1}} . Many other countries have also issued stamps featuring Bohr. In 1997, the Danish National Bank introduced a 500-krone banknote with a portrait of Bohr holding a pipe. On 7 October 2012, a Google Doodle honored Bohr's birthday by showing the Bohr model of the hydrogen atom. An asteroid named 3948 Bohr, the Bohr lunar crater, and the chemical element bohrium (atomic number 107) were named in recognition of Bohr's work on atomic structure.