Quantum mechanics constitutes the essential framework for twentieth-century physics. Providing a means for understanding the microworld, it has led to a host of major new technologies, including the transistor, the silicon chip, and nuclear energy. It has provided a far more comprehensive explanation of the chemical bond and brought about new ways of understanding biological phenomena and so lies at the root of a multitude of new methods of manipulating nature. Today, even cosmology depends upon quantum ideas, and apart from changing the mechanics of everyday life, quantum theory also lies behind broad shifts in philosophical thought. Of all those who developed quantum theory, the most eminent was the Danish physicist Niels Bohr.
Bohr's importance is doubly manifest, through his own work and through his overarching influence upon theoretical physics in the first quarter of the twentieth century. Beginning in 1913, his proposal of a highly influential model of the atom laid the basis for quantum mechanics as it finally emerged at the end of the 1920s. Bohr also examined the larger implications of the theory, which entails a radical break with determinism and common-sense notions of cause and effect; and his "Copenhagen interpretation" of the quantum world remains dominant today. With Niels Bohr there comes an end to mainstream efforts to discover some "ultimate" reality. "It is wrong to think that the task of physics is to find out how nature is", according to Bohr. "Physics concerns what we can say about nature."
In 1911 when Bohr received his doctorate from the University of Copenhagen, the revolution in understanding the structure of the atom was already under way. Indeed, his doctoral thesis concerned the theory of electrons, which had been discovered about a decade earlier and were known to be the universal constituents of all matter. It had been suggested that the number of electrons in an atom corresponded to its weight, accounting for a variety of different stable atoms. Bohr moved to England to work with the scientist who made the crucial discovery that atoms possessed a compact, massive nucleus. This led physicists to abandon previous theories about atoms in favor of a new model of electrons orbiting a tiny nucleus.
In 1913, Bohr published three papers that concern atomic structure. They effectively changed the course of physics. Although the new model of the atom solved certain important problems, the crucial question remained why the orbiting electrons - evidently attracted to the nucleus - were not destined to be absorbed into it. In brief, the model did not account for the stability of the atom, which is one of its central features.
Bohr perceived that classical Newtonian mechanics could not explain the behavior of matter at atomic scale. He was inspired to turn to quantum physics. During a relatively brief period of intense work in 1912, Bohr examined ho a hydrogen atom radiated light and developed a theory which had an exceptionally close fit with observable facts. He assumed the electron radiated light only when it changed orbits, and so identified the emission of a “quantum” with the “jump” of an electron from one orbit to another. Einstein, upon learning of Bohr's results, commented with characteristic laconic certitude: "This is an enormous achievement"
The new Bohr model of the atom was a fundamental advance, and soon was used to gain a new understanding of the atomic structure of all the known elements. One of Bohr’s accomplishments in 1913 was to identify the quantum jumps of electrons with the X-ray spectra. The next year brought a new, definitive order to the periodic table, subjecting each of the chemical elements to X-ray spectral analysis and assigning to each an atomic number. Over the next several years Bohr managed a number of technical achievements which, a historian wrote, “In retrospect…are all the more fabulous and astounding because they are based on analogies – atomic orbits similar to the motion of the planets around the sun, and spin similar to the rotation of the planets while orbiting – which are in fact false.” Bohr was awarded the Nobel Prize for Physics in 1922.
Indeed Bohr’s model of the atom had several significant flaws. What sometimes is called the “first quantum revolution” did not resolve certain problems with the behavior of more complex atoms. Although the theory was developed in various ways between 1913 and 1925, it accumulated serious problems, which would ultimately lead to what is known as the “second quantum revolution”.
During the 1920s Bohr was a key figure in helping to resolve the crisis in physics generated in the atomic structure he himself had proposed. Returning to the University of Copenhagen in 1916, he became professor of theoretical physics and participating in the opening, five years later, of the Institute of Theoretical Physics. In this way Copenhagen became a magnet for physics, with Bohr the central figure. The “second quantum revolution” gave birth to a purely mathematical model of the atom, which effectively acknowledged the limitations of human perception in subatomic events. Advances in wave mechanics, matrix mechanics, and the famous Uncertainty Principle - which acknowledged restrictions upon direct knowledge of physical systems - epitomized it.
In the late 1920s Bohr evolved two principles to help guide the quantum revolution to a successful rebirth. In a famous lecture in 1927, “The Philosophical Foundations of Quantum Theory”, he first discussed the concept of “complementarity”. This is the idea that, although subatomic systems may be measured by contrasting, exclusive models – as waves or as particles – both are needed for a full description of the phenomena. Intrigued by the philosophical implications of the idea, Bohr eventually argued that the complementarity principle might be applied to the problem of free will and basic life processes. Perhaps the most important yield of this idea was that quantum theory was subsequently taken to offer a basically complete description of nature that would not be altered by future discoveries: There is no “deeper” reality lying beyond quantum measurements. Although this notion has been challenged in various ways, it remains the bedrock of the “Copenhagen spirit” – various thought experiments, the “mind of God,” and theories of multiple universes notwithstanding. This doctrine was never fully accepted by Albert Einstein, or a number of physicists, but it endures, basically unchanged, to the present.
During the 1930s Bohr began investigating the expanding field of nuclear physics and in 1934 suggested the “liquid drop” model for the nucleus of the atom; it proved important for understanding nuclear fission. In 1936 he provided a summary theory of the atomic nucleus that became a general guide for physicists over the next decade. In Bohr’s theory, neutrons and protons were tightly knit together in the nucleus by the “strong force”. Although the energy would clearly be released if this compound nucleus were disturbed, at this time the effects of splitting the atom were by no means clear.
After the onset of WWII, Bohr initially remained in Denmark, which was invaded by the Nazis in 1940. Because of his stature, he was able to help some of his colleagues escape persecution, and he refused to cooperate with the Nazi war aims. But in 1943, after rumors convinced him that he was soon to be imprisoned, he and his family escaped to Sweden, then to England and the United States. He soon joined the Manhattan Project, where, he was known by the assumed name of Nicholas Baker for security reasons (everyone called him “Uncle Nick” while he was there). His role in the project was important as a knowledgeable consultant or "father confessor" on the project. He was concerned about a nuclear arms race, and is quoted as saying, "That is why I went to America. They didn't need my help in making the atom bomb."
Bohr believed that atomic secrets should be shared by the international scientific community. After meeting with Bohr, the laboratory director suggested Bohr visit President Franklin D. Roosevelt to convince him that the Manhattan Project should be shared with the Russians in the hope of speeding up its results. Roosevelt suggested Bohr return to the United Kingdom to try to win British approval. Winston Churchill disagreed with the idea of openness towards the Russians.
Returning to Denmark after the war, Bohr was active until the end of his life, retiring from the University of Copenhagen in 1955. A committed scientist who remained opposed to the production of atomic weapons, Bohr wrote a famous public letter to the United Nations in 1950 arguing for rational, peaceful atomic policies:
“Humanity will be confronted with dangers of unprecedented character unless, in due time, measures can be taken to forestall a disastrous competition in such formidable armaments and to establish an international control of the manufacture and use of powerful materials.”
Bohr received the first U.S. Atoms for Peace Award in 1957. He died from a heart attack in his home in 1962 and following this scientists and leading figures worldwide joined in paying tributes to him. President Kennedy wrote, “American scientists, indeed all American citizens who knew doctor Bohr's name and his great contributions, have respected and venerated him for more than two generations.”
Highly collaborative in his approach to physics – rather unlike Einstein in this respect – Bohr was often the subject of lavish praise by colleagues, just as his family and friends adored him. Bohr created the ‘Copenhagen Style’: “We see him, the greatest among his colleagues, acting, taking, living as an equal in a group of young, optimistic, jocular, enthusiastic people, approaching the deepest riddles of nature with a spirit of attack, a spirit of freedom from conventional bonds, and a spirit of joy that can hardly be described.” This is truly the embodiment of academic freedom.
His happy marriage to Margrethe produced six sons, one of who, Aage Bohr, also became a theoretical physicists, and in 1975, he joined his father as a Nobel laureate.
How important were Bohr’s contribution to science? Historian Richard Rhodes (The Making of the Atom Bomb and Arsenals of Folly: The Making of the Nuclear Arms Race) puts it simply: “Bohr’s contributions to twentieth-century physics would rank second only to Einstein’s.” Author John Simmons (The Scientific 100: A Ranking of the Most Influential Scientists, Past and Present) placed him third, behind only his friendly rival Albert, and Sir Isaac Newton.