Niels Bohr

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Niels Henrik David Bohr ; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr was also a philosopher and a promoter of scientific research. In 1943, having a Jewish mother, he fled to Sweden to escape nazi-persecution and pleaded with the Swedish king to allow all 8.000 Danish Jews refuge in the neutral kingdom. The escape succeeded.

Bohr developed the Bohr model of the atom with the atomic nucleus at the centre and electrons in orbit around it, which he compared to the planets orbiting the Sun. He helped develop quantum mechanics, in which electrons move from one energy level to another in discrete steps, instead of continuously. He founded the Institute of Theoretical Physics at the University of Copenhagen, now known as the Niels Bohr Institute, which opened in 1920. Bohr mentored and collaborated with physicists including Hans Kramers, Oskar Klein, George de Hevesy and Werner Heisenberg. He predicted the existence of a new zirconium-like element, which was named hafnium, after Copenhagen, when it was discovered. Later, the element bohrium was named after him. He conceived the principle ofcomplementarity: that items could be separately analysed as having contradictory properties, like behaving as a wave or a stream of particles. The notion of complementarity dominated his thinking on both science and philosophy.


Niels Bohr was born in Copenhagen, Denmark, on 7 October 1885, the second of three children of Christian Bohr, a professor of physiology at the University of Copenhagen, and Ellen Adler Bohr, who came from a wealthy Danish Jewish family prominent in banking and parliamentary circles. He had an older sister, Jenny, and a younger brother Harald. Jenny became a teacher, while Harald became a mathematician and Olympic footballer who played on the Danish national team at the 1908 Summer Olympics in London. Niels was a passionate footballer as well, and the two brothers played a number of matches for the Copenhagen-based Akademisk Boldklub, with Niels as goalkeeper.

Bohr was educated at Gammelholm Latin School, which he started when he was seven. In 1903, Bohr enrolled as an undergraduate at Copenhagen University. His major was physics, which he studied under Professor Christian Christiansen, the university’s only professor of physics at that time. He also studied astronomy and mathematics under Professor Thorvald Thiele, and philosophy under Professor Harald Høffding, a friend of his father.

Niels Bohr as a young man. Exact date of photo unknown.

Niels Bohr as a young man. Exact date of photo unknown.

In 1905, there was a gold medal competition, sponsored by the Royal Danish Academy of Sciences and Letters, to investigate a method for measuring the surface tension of liquids that had been proposed by Lord Rayleigh in 1879. Bohr conducted a series of experiments, using his father’s laboratory in the university because the university had no physics laboratory. To accomplish them, he had to become his own glass blower, creating test tubes with the required elliptical cross-sections. He went beyond the original task, incorporating improvements into both the theory and the method. His essay, which he submitted at the last minute, won the prize. He subsequently submitted an improved version of the paper to the Royal Society in London for publication in thePhilosophical Transactions of the Royal Society.

Harald became the first of the two Bohr brothers to earn a master’s degree, for mathematics in April 1909. Although he was older, Niels took another nine months to earn his. Students had to submit a thesis on a subject assigned by their supervisor. Bohr’s supervisor was Christiansen, and the topic he chose was on the electron theory of metals. Bohr subsequently elaborated his master’s thesis into his much larger Doctor of Philosophy (PhD) thesis. He surveyed the literature on the subject, and settled on a model postulated by Paul Drudeand elaborated by Hendrik Lorentz, in which the electrons in a metal are considered to behave like a gas. Bohr extended Lorentz’s model but was still unable to account for phenomena like the Hall effect, and concluded that electron theory could not fully explain the magnetic properties of metals. The thesis was accepted in April 1911, and Bohr conducted his formal defence on 13 May. Once again, Bohr was behind his younger brother Harald, who had received his PhD the previous year.

Earlier in 1910, Bohr met Margrethe Nørlund, the sister of the mathematician Niels Erik Nørlund. Bohr resigned his membership in the Lutheran Church on 16 April 1912, and he and Margrethe were married in a civil ceremony at the town hall in Slagelse on 1 August. Years later, his brother Harald would similarly leave the church before getting married. Niels and Margrethe had six sons. The oldest, Christian, died in a boating accident in 1934, and another, Harald, died from childhood meningitis. The other four went on to lead successful lives. Aage Bohr became a successful physicist and in 1975, like his father, was awarded the Nobel Prize in physics. His other sons were Hans Henrik, a physician; Erik, a chemical engineer; and Ernest, a lawyer, who, like Niel’s brother Harald, became an Olympic athlete, and played field hockey for Denmark at the 1948 Summer Olympics in London.


Bohr model

In 1911, Bohr travelled to England, where he met with J. J. Thomson, of Trinity College, Cambridge, and the Cavendish Laboratory. He attended lectures on electromagnetismgiven by James Jeans and Joseph Larmor, and did some research on cathode rays, but failed to impress Thomson. He had more success with Ernest Rutherford fromVictoria University of Manchester, whose 1911 Rutherford model of the atom had recently challenged Thomson’s plum pudding model, and Bohr received an invitation to conduct post-doctoral work at Manchester. As a post-doctoral student there, Bohr met George de Hevesy, and Charles Galton Darwin (whom Bohr referred to as “the grandson of the real Darwin“), and was intrigued by a paper by Darwin on electrons.

Bohr returned to Denmark in July 1912 for his wedding, and travelled around England and Scotland on his honeymoon. On his return, he became a privatdocent at the University of Copenhagen, and gave lectures on thermodynamicsMartin Knudsen put Bohr’s name forward for a docent, which was approved in July 1913, and Bohr began teaching medical students. His three papers, which later became famous as “the Trilogy”, were published in Philosophical Magazine in July, September and November of that year. He adapted Rutherford’s nuclear structure to Max Planck‘s quantum theory and so created his Bohr model of the atom. Here he introduced the theory of electrons travelling in orbits around the atom’s nucleus, with the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms. He also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a quantum of discrete energy. This became a basis for what is now called the old quantum theory.

The Bohr model of the hydrogen atom. A negatively charged electron confined to an atomic shell orbits a small, positively charged atomic nucleus and a quantum jump between orbits is accompanied by an emitted or absorbed amo

The Bohr model of the hydrogen atom. A negatively charged electron confined to an atomic shell orbits a small, positively charged atomic nucleus and a quantum jump between orbits is accompanied by an emitted or absorbed amo

In 1885, Johann Balmer had come up with his Balmer series for describing the visible spectral lines of a hydrogen atoms:

\frac{1}{\lambda}=R_\mathrm{H}\left(\frac{1}{2^2} - \frac{1}{n^2}\right) \quad \mathrm{for~} n=3,4,5,...

where λ is the wavelength of the absorbed/emitted light and RH is known as the Rydberg constant. Balmer’s formula was corroborated by the discovery of additional spectral lines; but for thirty years, no one could explain why it worked. In the first paper of his Trilogy, Bohr was able to derive it from his model:

 R_Z={ 2\pi^2 m_e Z^2 e^4 \over h^3 }

where me is the electron‘s mass, e is its charge, h is Planck’s constant and Z is the atom’s atomic number (which is 1 for Hydrogen). One problem was the Pickering series, lines which did not fit Balmer’s formula. When challenged on this by Alfred Fowler, Bohr replied that they were caused by ionised helium – helium atoms with only one electron. The model was found to work for them too. Reaction to the Trilogy was mixed. Many older physicists like Thomson and Rayleigh did not like it, althoughEmil Warburg thought highly of it, but the younger generation, including Ernest Rutherford, David HilbertAlbert EinsteinMax Born and Arnold Sommerfeld saw the Trilogy as a breakthrough.

Bohr did not enjoy teaching medical students, and decided to return to Manchester, where Rutherford had offered him a job as a reader in place of Darwin, whose tenure had expired. Bohr accepted. He took a leave of absence from the University of Copenhagen, but started by taking a holiday in Tyrol with his brother Harald and aunt Hanna Adler, visiting the University of Göttingen and the Ludwig Maximilian University of Munich, where he met Sommerfeld, and conducted seminars on the Trilogy. The First World War broke out while they were in Tyrol, greatly complicating this trip back to Denmark, and Bohr’s subsequent voyage with Margrethe to England, where he arrived in October 1914. They stayed until July 1916, by which time he had been appointed to the Chair of Theoretical Physics at the University of Copenhagen, a position created especially for him. His docentship was abolished at the same time, so he still had to teach physics to medical students. New professors were formally introduced to King Christian X, who expressed his delight at meeting such a famous football player

Institute of Physics

In April 1917, Bohr began a campaign to establish an Institute of Theoretical Physics. He gained the support of the Danish government and the Carlsberg Foundation, and sizeable contributions were also made by industry and private donors, many of them Jewish. Legislation establishing the Institute was passed in November 1918. Now known as the Niels Bohr Institute, it opened its doors on 3 March 1921, with Bohr as its director and his family moving into an apartment on the first floor.[30][31] Bohr’s institute served as a focal point for researchers into quantum mechanics and related subjects in the 1920s and 1930s, when most of the world’s best known theoretical physicists spent some time in his company. 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. Bohr became widely appreciated as their congenial host and eminent colleague. Klein and Rosseland produced the Institute’s first paper even before it opened.

The Niels Bohr Institute

The Niels Bohr Institute

While the Bohr model worked well for hydrogen, it could not explain more complex elements, and by 1919, Bohr was moving away from the idea that electrons orbited the nucleus, and he developed heuristics to describe them. The rare earth elements posed a particular classification problem for chemists, on account of their being so chemically similar. An important development came in 1924 with Wolfgang Pauli‘s discovery of the Pauli exclusion principle, which put Bohr’s models on a firm theoretical footing. Bohr was able to then declare that the as-yet-undiscovered element 72 was not a rare earth element at all, but an element with chemical properties similar to thatzirconium. He was immediately challenged by the French chemist, Georges Urbain, who claimed to have discovered a rare earth element 72, which he called “celtium”. At the Institute in Copenhagen, Dirk Coster and George de Hevesy took up the challenge of proving Bohr right and Urbain wrong. Starting with a clear idea of the chemical properties of the unknown element greatly simplified the search process. They went through samples from Copenhagen’s Museum of Mineralogy looking for a zirconium-like element, and soon found it. The element, which they named hafniumHafnia being the Latin name for Copenhagen, turned out to be more common than gold.

In 1922, Bohr was awarded the Nobel Prize in physics “for his services in the investigation of the structure of atoms and of the radiation emanating from them”. The award thus recognised both the Trilogy and his early leading work in the emerging field of quantum mechanics. For his Nobel lecture, Bohr treated his audience to a comprehensive survey of what was then known about the structure of the atom, including his correspondence principle.

The discovery of Compton scattering by Arthur Holly Compton in 1923 convinced most physicists that light was composed of photons, and that energy and momentum were conserved in collisions between electrons and photons. In 1924, Bohr, Kramers and John C. Slater, an American physicist working at the Institute in Copenhagen, proposed the Bohr-Kramers-Slater theory (BKS) in 1924. It was more a program than a full physical theory, as the ideas that were developed were not worked out in a quantitative way. BKS theory became the final attempt at understanding the interaction of matter and electromagnetic radiation on the basis of the old quantum theory, in which quantum phenomena were treated by imposing quantum restrictions on a classical wave description of the electromagnetic field.

One aspect, the idea of modelling atomic behaviour under incident electromagnetic radiation using “virtual oscillators” at the absorption and emission frequencies, rather than the (different) apparent frequencies of the Bohr orbits, led Max Born, Werner Heisenberg and Kramers to explore mathematics that strongly inspired the subsequent development of matrix mechanics, the first form of modern quantum mechanics. The theory also generated great discussion and renewed attention to the difficulties in the foundations of the old quantum theory. However, the most provocative element, that momentum and energy would not necessarily be conserved in each interaction but only overall, statistically, was soon shown to be in conflict with experiments conducted by Walter Bothe and Hans Geiger. In the light of these results, Bohr informed Darwin, “there is nothing else to do than to give our revolutionary efforts as honourable a funeral as possible.”

Quantum mechanics

The introduction of spin by George Uhlenbeck and Samuel Goudsmit in November 1925 was a milestone. The next month, Bohr travelled to Leiden to attend celebrations of the 50th anniversary of Hendrick Lorentz receiving his doctorate. When his train stopped in Hamburg, he was met by Wolfgang Pauli and Otto Stern, who asked for his opinion of the spin theory. Bohr pointed out that he had concerns about the interaction between the electron and the magnetic field. On arrival in Leiden, however, he was met by Paul Ehrenfest and Albert Einstein, who informed him that Einstein had resolved this problem using relativity. Bohr then had Uhlenbeck and Goudsmit incorporate this into their paper. Thus, when he met Werner Heisenberg and Pascual Jordan in Göttingen on the way back, he had become, in his own words, “a prophet of the electron magnet gospel”.

Heisenberg first came to Copenhagen in 1924. He returned to Göttingen in June 1925, but when Kramers left the Institute in 1926 to take up a chair as professor of theoretical physics at the Utrecht University, Bohr arranged for Heisenberg to take Kramers’s place as a lektor at the University of Copenhagen. Heisenberg worked as an assistant to Bohr and university lecturer in Copenhagen from 1926 to 1927, and it was in Copenhagen, in 1925, that Heisenberg developed the mathematical foundations of quantum mechanics. When he showed his results to Max Born in Göttingen, Born realised that they could best be expressed using matrices. This work also attracted the attention of the British physicist Paul Dirac, who came to Copenhagen for six months in September 1926. Austrian physicist Erwin Schrödinger also visited in 1926. His attempt at explaining quantum physics in classical terms using wave mechanics impressed Bohr who believed it contributed “so much to mathematical clarity and simplicity that it represents a gigantic advance over all previous forms of quantum mechanics”.

Niels Bohr and Albert Einstein. Picture taken by Paul Ehrenfest at his home in Leiden (December 1925).

Niels Bohr and Albert Einstein. Picture taken by Paul Ehrenfest at his home in Leiden (December 1925).

Bohr was now convinced that light behaved like both waves and particles, and in 1927, experiments confirmed the de Broglie hypothesis that matter like electrons also behaved like waves. Bohr conceived the principle of complementarity: that items could be separately analysed as having several contradictory properties, such as a wave or a stream of particles depending on the experimental framework – two apparently mutually exclusive properties – on the basis of this principle. “Shortly before his death [Bohr] complained that no professional philosopher had ever understood his doctrine of complementarity.”

It was in Copenhagen in 1927 that Heisenberg developed his uncertainty principle. Bohr embraced the new principle, and in a paper he presented at the Volta Conference atComo in September 1927, he demonstrated that the uncertainty principle could be derived from classical arguments, and without quantum terminology or matrices. Einstein much preferred the determinism of classical physics over the probabilistic new quantum physics to which Einstein himself had contributed. Philosophical issues that arose from the novel aspects of quantum mechanics became widely celebrated subjects of discussion. Einstein and Bohr had good-natured arguments over such issues throughout their lives.

In 1914, Carl Jacobsen, the heir to Carlsberg breweries, bequeathed his mansion to be used for life by the Dane who had made the most prominent contribution to science, literature or the arts, as an honorary residence (DanishÆresbolig). Harald Høffding had become the first occupant, and when he died in July 1931, the Royal Danish Academy of Sciences and Letters decided that Bohr should be the house’s next occupant. Bohr and his family moved in in the summer of 1932. He was elected president of the Academy on 17 March 1939.

By 1929, the phenomenon of beta decay once again had Bohr suggesting that the law of conservation of energy be abandoned, but Enrico Fermi‘s hypothetical neutrinoand the subsequent 1932 discovery of the neutron provided another explanation. This prompted Bohr to create a new theory of the compound nucleus in 1936, which explained how neutrons could be captured by the nucleus. In this model, the nucleus could be deformed like a drop of liquid. He worked on this with a new collaborator, the Danish physicist Fritz Kalckar, who died suddenly in 1938.

Bohr brought the news of the discovery of nuclear fission by Lise Meitner and Otto Hahn to the United States in 1938. When Bohr told George Placzek that this resolved all the mysteries of transuranic elements, Placzek told him that one remained: the neutron capture energies of uranium did not match those of its decay. Bohr thought about it for a few minutes and then announced to Placzek, Leon Rosenfeld and John Wheeler that “I have understood everything.” He explained that only the uranium-235isotope, which makes up just 0.7 per cent of natural uranium, is fissile. Bohr wrote a short paper explaining this, but it was only after it was confirmed by experiment in 1940 that it would be accepted by physicists like Enrico Fermi.





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