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Niels Henrik David Bohr

Niels Bohr studied at the University of Copenhagen which he entered in 1903. He won a gold medal from the Royal Danish Academy of Sciences for his theoretical analysis of vibrations of water jets as a means of determining surface tension. He received his Master’s degree from the University of Copenhagen in 1909 and his doctorate in 1911 with a thesis Studies on the electron theory of metals.

Bohr went to England to study with Sir J.J. Thomson at Cambridge. He had intended to spend his entire study period in Cambridge but he did not get on well with Thomson so, after a meeting with Ernest Rutherford in Cambridge in December 1911, Bohr moved to Manchester in 1912. There he worked with Rutherford’s group on the structure of the atom. Rutherford became Bohr’s role model both for his personal and scientific qualities. Using quantum ideas due to Planck and Einstein, Bohr conjectured that an atom could exist only in a discrete set of stable energy states.

Bohr returned to Copenhagen during 1912 and continued to develop his new theory of the atom completing the work in 1913. The same year he published three papers of fundamental importance on the theory of the atom that influenced Einstein and other scientists. The first paper was on the hydrogen atom, the next two on the structure of atoms heavier than hydrogen.

After being a lecturer in Copenhagen, then in Manchester, Bohr was appointed to a chair of theoretical physics at the University of Copenhagen in 1916. An Institute of Theoretical Physics was created for him there and, from its opening in 1921, he was its director for the rest of his life.

Bohr is best known for the investigations of atomic structure referred to above and also for work on radiation, which won him the 1922 Nobel Prize for physics. He said in 1923:-

Not with standing the fundamental departure from the ideas of the classical
theories of mechanics and electrodynamics involved in these postulates, it has
been possible to trace a connection between the radiation emitted by the atom
and the motion of the particles which exhibits a far-reaching analogy to that
claimed by the classical ideas of the origin of radiation.

It was Bohr’s view of quantum theory which was eventually to become accepted. Einstein expressed grave doubts about Bohr’s interpretation and Bohr, Einstein and Ehrenfest spent many hours in deep discussion, but Bohr’s view prevailed. Bohr expressed this view saying:-

Evidence obtained under different experimental conditions cannot be
comprehended within a single picture, but must be regarded as complementary in
the sense that only the totality of the phenomena exhausts the possible
information about the objects.

H B G Casimir wrote describing what it was like working with Bohr in his Institute:-

Even Bohr who concentrated more intensely and had more staying power than
any of us, looked for relaxation in crossword puzzles, in sports, and in facetious

Bohr’s other major contributions, in addition to quantum theory, include his theoretical description of the periodic table of elements around 1920, his theory of the atomic nucleus being a compound structure in 1936, and his understanding of uranium fission in terms of the isotope 235 in 1939.

Bohr was of Jewish origins and, when the Nazis occupied Denmark his life became exceeding difficult. He had to escape in 1943 by being taken to Sweden by fishing boat. From there he was flown to England where he began to work on the project to make a nuclear fission bomb. After a few months he went with the British research team to Los Alamos in the USA where they continued work on the project.

However Bohr was deeply concerned about the control of nuclear weapons and from 1944 he tried to persuade Churchill and Roosevelt for the need to have international cooperation. He wrote a public letter to the United Nations in 1950 arguing for rational, peaceful atomic policies. Bohr received the first U.S. Atoms for Peace Award in 1957.

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