Bohr, Aage Niels

Aage Niels Bohr

Danish physicist Aage Niels Bohr (born 1922) is the son of another famed physicist, Niels Bohr. Both father and son received Nobel prizes, in 1922 and 1975, respectively. In the early part of his career, Aage Bohr was involved in his father's research, including work on the U.S. government-sponsored Manhattan Project. In the late 1940s and 1950s, Bohr conducted his own research on the structure of the atomic nucleus, which led to a share of the 1975 Nobel Prize in Physics with James Rainwater and Benjamin R. Mottelson.

The fourth son of Niels Bohr and Margrethe (Nørlund) Bohr, Aage Bohr was born in Copenhagen, Denmark, on June 19, 1922. His family boasted a strong scientific background. Niels Bohr (1885-1962) was a famed physicist who made significant contributions to nuclear physics and the understanding of atomic structure and won the Nobel Prize in Physics the year Aage was born. Aage Bohr's uncle, Harald Bohr, was an eminent mathematician.

In 1913, Niels Bohr developed a new theory of the atom that built on the "Rutherford model," which viewed the atom as a compact nucleus surrounded by many lighter electrons. Bohr's new theory advanced the idea that electrons travel only in certain orbits and that a particular atom can exist only in a discrete set of stable states. Further, an atom's outer orbits, which can hold more electrons than inner orbits, determine the atom's chemical properties. In his theory, Niels Bohr speculated that atoms emit electromagnetic radiation when an electron jumps from an outer orbit to an inner one. At first, his theory met with skepticism, but by 1922 it had become accepted, earning him the Nobel Prize. Other physicists would later expand Bohr's theory into quantum mechanics.

Influenced by Famous Scientists

Aage Bohr's early education took place at the Sortedam Gymnasium, which he attended for twelve years. While he later expressed a debt to his teachers for their inspiration and encouragement in learning the humanities and sciences, the informal learning he received at his father's Institute for Theoretical Physics was, perhaps, the most influential element of his early education. Niels Bohr founded the Institute at the University of Copenhagen in 1921, and the Bohr family lived at the Institute during Aage's early childhood. As a result, Aage Bohr essentially grew up among the world's leading physicists, among them Wolfgang Pauli, Werner Heisenberg, Yoshio Nishina, and Oskar Klein, the last two who developed the well-known Klein-Nishina formula involving scattered radiation. This environment helped steer him in the direction of becoming a nuclear physicist.

When Aage Bohr was about ten years old, his parents moved to a mansion at Carlsberg, Denmark, where they were hosted many scholars, artists, and public figures. In 1940, he followed in his father's footsteps and entered the University of Copenhagen to study theoretical physics. By that time, he was already assisting his father with correspondence and the writing of some articles, as well as with his work in physics. During that period, Niels Bohr's work involved both the problems of nuclear physics and those relating to the penetration by atomic particles of matter.

Family Fled Denmark

By the time Aage Bohr began his formal higher education, World War II had already started in Europe. His enrollment at the University of Copenhagen occurred a few months after Germany began its occupation of Denmark. By October 1943, because of his Jewish heritage, Niels Bohr found it necessary to flee Denmark to avoid arrest by the Nazis. The entire family escaped to the neutral country of Sweden, first by boat and then by plane. From 1943 to 1945, father and son traveled and worked together in London, Washington, D.C., and Los Alamos, New Mexico. Aage Bohr acted as his father's assistant and secretary on Niels Bohr's ongoing research.

In London, the Bohr team joined a British group working on the development of atomic weapons. Aage Bohr's official title was junior scientific officer in the Department of Scientific and Industrial Research. In the United States, the Bohrs became involved in the Manhattan Project, and worked at Los Alamos, where Aage Bohr served the elder Bohr as secretary and lab assistant.

Niels Bohr played a large part in the development of the Manhattan Project, which was a secret program dedicated to building an atomic bomb during World War II. The project, which began in 1942, was the government's response to a growing fear that Germany was coming very close to developing a useable atomic weapon. Some of the greatest nuclear physicists in the world were involved in the project, among them J. Robert Oppenheimer, Enrico Fermi, James Chadwick, Isidor Rabi, and Richard Tolman. The project resulted in the first atomic bomb test, at Los Alamos, and eventually culminated in the bombing of Hiroshima and Nagasaki and, as is often maintained, the end of the war.

The Bohr family was able to return to Denmark in August of 1945, and Aage Bohr was able to resume his studies at the University of Copenhagen. In 1946, he obtained his master's degree on the strength of a thesis involving some aspects of atomic stopping problems. He went on to earn his Ph.D. from that institution in 1954.

Conducted Research on Atomic Structure

During his graduate work, Bohr accepted a research position at his father's Institute for Theoretical Physics, but he returned to the United States in 1948. In the spring of that year, he became a member of Princeton University's Institute for Advanced Study in Princeton, New Jersey. He also visited Columbia University and met with noted physics professor I. I. Rabi. Discussions with Rabi sparked Bohr's interest in the recently discovered effect in the hyperfine structure in deuterium, or "heavy hydrogen," and he began a short but fruitful association with Columbia University on a research fellowship which lasted from January 1949 to August 1950. He worked with Rabi in the Pupin Laboratory, an environment Bohr later described as "stimulating." Many important discoveries were made in that laboratory, and one of the most important areas of research involved the study of nuclear moments, an area proving to be crucial in the development of the new ideas on nuclear structure.

While at Columbia, Bohr began a collaboration with James Rainwater in which the two scientists conducted research into the structure of the atomic nucleus. As they endeavored to understand the basic composition of the nucleus, they concluded that a new model was needed to more effectively clarify its structural and functional properties. The two existing model theories contradicted each other, and the validity of each model theory was disputed among physicists. Ironically, one of the models, the so-called "liquid-drop model," had been developed by Niels Bohr in 1936. This model proposed that the protons and the neutrons, or nucleons, were held together by nuclear forces in much the same way that molecules attract each other in a drop of water. The second model, the so-called "shell model," developed by Maria Goeppert-Mayer and J. Hans D. Jensen in 1949, proposed that the protons and neutrons traveled in orbits that were concentric, or that had a common center—that is, these nucleons moved in much the same way that electron shells do in an atom. The theory also suggested that the sum of the forces of the nucleons resulted in a spherical "force field." This theory was disputed, too, particularly when later experiments revealed that in instances where the outermost shell was incomplete, the charge distribution around the nucleus was non-spherical. Rainwater suggested that interactions between particles in an incomplete outer shell and particles deep within the nucleus might cause the nucleus to become distorted.

Began Long Collaboration with Mottelson

Bohr continued to perform research in this area when he returned to Copenhagen in 1950, as a fellow of the Institute for Theoretical Physics. His return initiated his long collaboration with Benjamin R. Mottelson. Using Rainwater's theory as the basis of their work, Bohr and Mottelson created a new model to describe nuclear structure, calling it the "collective model." According to this new model, which combines the liquid and shell models, the surface of the nucleus indeed acts like a liquid drop, but the shell structure is subject to centrifugal distortions that account for some nuclei being spherical and some oblong. The new model resolved the contradictions of the previous models and led to a more unified model of the nucleus. It was considered one of the greatest achievements of modern physics, as it led to new knowledge of the nuclei.

A major part of Bohr and Mottelson's collaboration involved developing a monograph that summarized their understanding of nuclear structure. The first volume of the monograph, Single-Particle Motion, was published in 1969. The second volume, Nuclear Deformations, was published in 1975.

In 1975, Bohr and Mottelson received the Nobel Prize in Physics for their work on the monograph and for their collaborative research on nuclear theory and their new model of nuclear structure. They shared the prize with Rainwater, who was recognized for his research on the quantum mechanical description of nucleons orbiting inside an unsteady rotating droplet. In honoring the three scientists with the Nobel Prize, the Royal Swedish Academy of Sciences lauded their "discovery of the connection between collective motion and particle motion in the atomic nucleus and the development of the theory of the structure of the atomic nucleus based on this connection." In his speech accepting his share of the Nobel Prize, Bohr pointed out that their research had influenced a variety of scientific fields "ranging from celestial mechanics to the spectra of elementary particles."

Directed His Father's Institute

Following his return to Denmark, Bohr remained connected with the Institute for Theoretical Physics. In addition, he became professor of physics at the University of Copenhagen in 1956. After Niels Bohr's death in 1962, Aage Bohr succeeded his father as the director of the Institute, which by now had been renamed the Niels Bohr Institute. He remained active with the Institute on an administrative basis until he relinquished his post in 1970, when he sought to resume his own active research.

Bohr returned to administration in 1975, when he became director of the Nordic Institute for Theoretical Atomic Physics, or Nordita. Nordita was founded in 1957 on the basis of an initiative of Niels Bohr and Torsten Gustafsson, and it followed the premises of the Niels Bohr Institute. The goal of Nordita is to encourage research within theoretical physics and to strengthen the Nordic collaboration within the basic areas of theoretical physics. Research areas include astrophysics, condensed matter physics, and subatomic physics. It is located on the premises of the Niels Bohr Institute at the University of Copenhagen. As such, the two institutes operate in close association. Aage Bohr was a member of the board of Nordita from 1957 until 1975, when he assumed the organization's directorship, and he retired in 1981.

In addition to the Nobel Prize, Bohr has been honored with the 1963 Pope Pius XI Medal, the 1969 Atoms for Peace Award, the 1970 Orsted Medal, the 1974 John Price Wetherill Medal, and the 1976 Ole Romer Medal, in addition to several honorary degrees. A member of six academies of science throughout Europe as well as the National Academy of Science in the United States, he also held membership in many professional associations throughout his long career. Bohr was married to Marietta Soffer in March of 1950 in New York City. The couple had two sons, Vilhelm and Tomas, and one daughter, Margrethe. Following Marietta's death in 1978, Bohr remarried, wedding Bente Meyer Scharff in 1981.

Books

Notable Scientists: From 1900 to the Present, Gale Group, 2001.

World of Scientific Discovery, second edition, Gale Group, 1999.

Online

"AageN.Bohr-Autobiography,"NobelPrizeWebsite, http://nobelprize.org/physics/laureates/1975/bohr-autobio.html (January 8, 2005).