(1885-1962) Danish Theoretical Physicist, Quantum Theorist, Philosopher of Science
Among the revolutionary geniuses of 20th-century physics, the name of Niels Bohr, whose model of the atom laid the basis for quantum mechanics, is second only to that of albert einstein. By wedding max planck’s notion of discrete quanta of energy to ernest rutherford’s nuclear model of the atom, Bohr opened the door to a description of material processes at odds not only with the determinism of classical physics, but with what many, Einstein included, considered to be a coherent, existentially palatable vision of nature.
Niels Henrik David Bohr was born on October 7, 1885, in Copenhagen, Denmark, into a family remarkable for its intellectual attainments: his mother, Ellen Adler Bohr, was a member of a wealthy Jewish family, prominent in Danish banking and parliamentary circles, and his father, Christian Bohr, was a professor of physiology at the University of Copenhagen, known for his work on the physical and chemical aspects of respiration. Niels was considered less brilliant than his younger brother, Harold, who became a renowned mathematician. But he wasted no time in distinguishing himself: three years after entering the University of Copenhagen 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 remained in Copenhagen until 1911, when he received a doctorate for a theory explaining electron behavior in metals.
In 1912, he traveled to England, to continue his research in Cambridge with joseph john (j. j.) thomson, the discoverer of the electron. When Thomson proved indifferent to his ideas, Bohr moved to Manchester to work with Ernest Rutherford, who was making important contributions to the theory of the atom. Rutherford had proposed that atoms consist of electrons orbiting a positively charged nucleus. But it was not understood how electrons could continually orbit the nucleus without radiating energy, as classical physics demanded. According to james clerk maxwell’s equations, orbiting electrons would be accelerating and continuously emitting electromagnetic radiation; this process would cause them to spiral into the nucleus in about a trillionth of a second. In contradistinction to the classical prediction, however, the hydrogen atom was extremely stable.
Niels Henrik David Bohr laid the foundation for the theory of quantum mechanics, an enormously successful description of physical processes that is at odds with classical determinism.
With Rutherford as his inspiration and mentor, Bohr set about explaining this discrepancy. He began with max ernest ludwig planck’s 1900 theory that energy is emitted in discrete packets or quanta and applied it to Rutherford’s nuclear atom. Bohr postulated that electrons are confined to a certain number of stable orbits, in which they neither emit nor absorb energy. Only when it jumps from one discrete orbit to a lower one does the electron lose energy: it sends off an individual photon (particle of light). Since an electron in the innermost orbit has no orbit with less energy to jump to, the atom remains stable. Bohr’s theory explained many of the spectral lines for hydrogen and helium, but he hesitated to publish his results, fearing that no one would take him seriously unless he explained the spectra of all the elements. It was Rutherford who persuaded him that the ability to explain hydrogen and helium would be quite enough to make his model credible. Indeed, when Bohr’s three papers on the structure of the hydrogen atom and on heavier atoms appeared in 1913, they had a profound, unsettling effect. Many of Bohr’s contemporaries balked at accepting so bizarre a picture of the atomic world. But new spectroscopic measurements and other experiments confirmed Bohr’s theory, and in 1914 direct evidence for the existence of such discrete states was found.
In 1916, the University of Copenhagen appointed him to the chair of theoretical physics. When he made known his plans to return to “more ideal” research conditions in England, the Danish university created the Institute of Theoretical Physics for him (now the Niels Bohr Institute for Astronomy, Physics and Geophysics). In 1921, a year before he received the Nobel Prize in physics, he was appointed its director, a post he would retain for the rest of his life. The institute became a mecca for theoretical physicists, who traveled from all over the world to debate the meaning of the new physics, and the birthplace of what came to be called the Copenhagen school. While still in his 20s, Bohr found himself at the very center of the quantum mechanical revolution. Bohr and his colleagues, including wolfgang pauli and werner heisenberg, brainstormed tirelessly in search of a physical interpretation of the new mathematical description of nature. The result was the Copenhagen interpretation of quantum mechanics, which introduced a radical assumption into physical thinking: because the quantum interaction between the “observer” and the “objects to be observed” can never be ignored at the microscopic level, microphysical processes are fundamentally random and probabilistic. Bohr enunciated one of the startling implications of this hypothesis in his complementarity principle, which states that an electron can be regarded as a particle or wave phenomenon, and both characterizations are equally valid, depending on the experimental circumstances:
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 phenomenon.
In the early 1920s, Bohr sought to develop a consistent quantum theory that would supersede classical mechanics and electrodynamics at the atomic level. During this period of intense and wide-ranging exploration, he formulated his principle of correspondence, a philosophical guideline for selection of new physical theories, requiring that they explain all the phenomena for which a preceding theory is valid. Since classical mechanics had met all challenges until physicists began to examine the atom itself, Bohr insisted that quantum mechanics, to be valid, must do what the old physics did—and more: it must describe atomic phenomena correctly and be applicable to conventional phenomena, as well. In 1923, he announced that the new physics could do just that:
Notwithstanding the fundamental departure from the ideas of the classical theory 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. Indeed, in a suitable limit the frequencies calculated by the two very different methods would agree exactly.
If Bohr’s view of quantum theory gradually won almost universal acceptance, one convert he never succeeded in winning, though not for lack of trying, was Einstein. The Bohr-Einstein debates of the 1920s and 1930s are legendary. Einstein, who could never accept the probabilistic nature of quantum mechanics, produced a series of gedanken (thought) experiments designed to disprove the theory. Bohr would then attempt to expose the flaws in Einstein’s reasoning. To Einstein’s insistence that “God does not play dice with the universe,” Bohr would counter, “Einstein, stop telling God what to do!” But if Bohr accepted the strangeness of the theory he had helped bring into being, his attitude toward it was anything but complacent. “Anyone who is not dizzy after his first acquaintance with the quantum of action,” he said, “has not understood a word.”
In the 1930s, Bohr’s interests turned to nuclear physics and in 1939 he proposed the liquid-droplet model for the nucleus, which proved a key to understanding many nuclear processes: Nucleons (neutrons and protons) behave as molecules do in a drop of liquid. If given enough extra energy (by absorbing a neutron), the spherical nucleus may be distorted into a dumbbell shape and then split at the neck into two nearly equal fragments, releasing energy. In this way Bohr, working with john a. wheeler, was able to explain why a heavy nucleus could undergo fission after the capture of a neutron. Bohr validated his theory when he correctly predicted that the nuclei of uranium-235 and ura-nium-238 would behave differently, since the number of neutrons in each nucleus is odd and even, respectively.
During World War II, Bohr was active in the Danish resistance movement; in 1943, he escaped with his family to Sweden and then to England, where he and his son, Aage, took part in the project for making a nuclear fission bomb. He accompanied the British research team to Los Alamos and made significant contributions to physical research on the U.S. atomic bomb. After the war, Bohr became a prominent advocate for control of nuclear weapons, pleading, in a famous 1950 open letter to the United Nations, for an “open world” and exhorting Roosevelt and Churchill to strive for international cooperation. His passionate advocacy won him the first U.S. Atoms for Peace Award in 1957. He was instrumental in creating the European Center for Nuclear Research (CERN), Geneva, in 1952.
In his last years Bohr remained a staunch defender of the Copenhagen interpretation of quantum mechanics and published articles in which he related the quantum mechanical idea of complementarity to aspects of human life and thought. Bohr’s unique approach to science and philosophy, his openness to new ideas, and willingness to learn from even the most junior of his colleagues left a lasting imprint on the generation of physicists who followed him. Until his death in Copenhagen on November 18, 1962, he remained a spirited participant in the great physics debates he had played so central a role in initiating.
