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Niels Bohr

Inventors and scientists

Niels Bohr
October 7, 1885, Copenhagen, Denmark
November 18, 1962, Copenhagen

Danish physicist Niels Bohr is generally regarded as one of the foremost physicists of the 20th century. He was the first to apply the quantum concept, which restricts the energy of a system to certain discrete values, to the problem of atomic and molecular structure.

For this work he received the Nobel Prize for Physics in 1922. His manifold roles in the origins and development of quantum physics may be his most important contribution, but through his long career his involvements were substantially broader, both inside and outside the world of physics.

Niels Bohr

Bohr Atomic Model

Bohr’s first contribution to the emerging new idea of quantum physics started in 1912. Only the year before, Ernest Rutherford and his collaborators at the University of Manchester had established experimentally that the atom consists of a heavy positively charged nucleus with substantially lighter negatively charged electrons circling around it at considerable distance.

According to classical physics, such a system would be unstable, and Bohr felt compelled to postulate, in a substantive trilogy of articles published in The Philosophical Magazinein 1913, that electrons could only occupy particular orbits determined by the quantum of action and that electromagnetic radiation from an atom occurred only when an electron jumped to a lower-energy orbit. Although radical and unacceptable to most physicists at the time, the Bohr atomic model was able to account for an ever-increasing number of experimental data, famously starting with the spectral line series emitted by hydrogen.

Already in his 1913 trilogy, Bohr had sought to apply his theory to the understanding of the periodic table  of  elements. At the University of Copenhagen, where Bohr had established an Institute for Theoretical Physics, he improved upon this aspect of his work, developing an elaborate scheme building up the periodic  table by adding electrons one after another to the atom according to his atomic model.

When Bohr was awarded the Nobel Prize for his work in 1922, the Hungarian physical chemist Georg Hevesy, together with the physicist Dirk Coster from Holland, were working at Bohr’s institute to establish experimentally that the as-yet-undiscovered atomic element 72 would behave as predicted by Bohr’s theory. They succeeded in 1923, thus proving both the strength of Bohr’s theory and the truth in practice of Bohr’s words at the institute’s inauguration about the important role of experiment. The element was named hafnium (Latin for Copenhagen).

Copenhagen Interpretation of Quantum Mechanics

During the academic year 1926–27, Werner Heisenberg served as Bohr’s assistant in Copenhagen, where he formulated the fundamental uncertainty principle  as  a consequence of quantum mechanics. Bohr, Heisenberg, and a few others then went on to develop what came to be known as the Copenhagen interpretation of quantum mechanics, which still provides a conceptual basis for the theory. A central element of the Copenhagen interpretation is Bohr’s complementarity principle, presented for the first time in 1927 at a conference in Como, Italy.

According to complementarity, on the atomic level a physical phenomenon expresses itself differently  depending on the experimental setup used to observe it. Thus, light appears sometimes as waves and sometimes as particles. For a complete explanation, both aspects, which according to classical physics are contradictory, need to be taken into account. The other towering figure of physics in the 20th century, Albert Einstein, never accepted the Copenhagen interpretation, famously declaring against its probabilistic implications that “God does not play dice.” The discussions between Bohr and Einstein, especially at two of the renowned series of Solvay Conferences in physics, in 1927 and 1930, constitute one of the most fundamental and inspired discussions between physicists in the 20th century. For the rest of his life, Bohr worked to generalize complementarity as a guiding idea applying far beyond physics.

Nuclear Physics

In the early 1930s Bohr, together with Hevesy and the Danish physiologist August Krogh, applied for support from the Rockefeller Foundation to build a cyclotron-a kind of particle accelerator recently invented by Ernest O. Lawrence in the United States-as a means to pursue biological studies. Although Bohr intended to use the cyclotron primarily for investigations in nuclear physics, it could also produce isotopes of elements involved in organic processes, making it possible in particular to extend the radioactive indicator method, invented and promoted by Hevesy, to biological purposes.

Splitting the Atom

After the German physicists Otto Hahn and Fritz Strassmann in late 1938 had made the unexpected and unexplained experimental discovery that a uranium atom can be split in two approximately equal halves when bombarded with neutrons, a theoretical explanation based on Bohr’s recently proposed theory of the compound nucleus was suggested by two Austrian physicists close to Bohr-Lise Meitner and her nephew Otto Robert Frisch; the explanation was soon confirmed in experiments by Meitner and Frisch at the institute. By this time, at the beginning of 1939, Bohr was in the United States, where a fierce race to confirm experimentally the so-called fission of the nucleus began after the news of the German experiments and their explanation had become known. In the United States, Bohr did pathbreaking work with his younger American colleague John Archibald Wheeler at Princeton University to explain fission theoretically.

The Atomic Bomb

After the discovery of fission, Bohr was acutely aware of the theoretical possibility of making an atomic bomb. In early 1943 Bohr received a secret message from his British colleague James Chadwick, inviting Bohr to join him in England to do important scientific work. Although Chadwick’s letter was vaguely formulated, Bohr understood immediately that the work had to do with developing an atomic bomb. Convinced of the infeasibility of such a project, Bohr answered that there was greater need for him in occupied Denmark.

In the fall of 1943, the political situation in Denmark changed dramatically after the Danish government’s collaboration with the German occupiers broke down. After being warned about his imminent arrest, Bohr escaped by boat with his family across the narrow sound to Sweden. In Stockholm the invitation to England was repeated, and Bohr was brought by a military airplane to Scotland and then on to London.

Upon being briefed about the state of the Allied atomic bomb project on his arrival in London, Bohr changed his mind immediately about its feasibility. Concerned about a corresponding project being pursued in Germany, Bohr willingly joined the Allied project. Taking part for several weeks at a time in the work in Los Alamos, N.M., to develop the atomic bomb, he made significant technical contributions, notably to the design of the so-called initiator for the plutonium bomb. His most important role, however, was to serve, in J. Robert Oppenheimer’s words, “as a scientific father confessor to the younger men.”

Bohr was allowed to return home only after the atomic bomb had been dropped on Japan in August 1945. He later took part in the establishment of CERN, the European experimental particle physics facility near Geneva, Switz., as well as of the Nordic Institute for Atomic Physics (Nordita) adjacent to his institute.

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