One of the most influential physicists of the 20th century, Denmark’s Niels Bohr (1885–1962) made significant contributions to understanding atomic structure and quantum mechanics, the work for which he received the Nobel Prize in Physics in 1922.
Helge Kragh is Professor of Physics and Astronomy at Aarhaus University in Denmark. He is the author of several books on physics and cosmology, including “Niels Bohr and the Quantum Atom: The Bohr Model of Atomic Structure 1913-1925.”
Simply Charly: Niels Bohr was one of the 20th century’s greatest scientific minds. What specifically drew you to Bohr and inspired you to write a book about him and the development of quantum mechanics?
Helge Kragh: I started studying physics in 1962 at Copenhagen University’s Institute for Theoretical Physics, which a few years later was officially renamed the Niels Bohr Institute. Even at that time, I took an interest in Bohr’s old atomic theory, which I found fascinating from both a scientific and pedagogical point of view. When I later turned to the history of science, it was natural for me to work on his atomic and quantum theory, which I have done on and off since then. From the historian’s perspective, Bohr is an interesting and rewarding figure to deal with because of the many letters and other unpublished sources that still exist. A study of these sources brings to life not only Bohr’s physics but also his personality and the time of a bygone age. Since many of the sources are in Danish, it is an advantage to be a Dane.
SC: Bohr is best known for his contribution to our understanding of the atom, for which he won the Nobel Prize in 1922. What insights did he reveal about the atom, and what were some of his other, lesser-known findings?
HK: Before Bohr’s breakthrough in 1913, very little was known about the structure of atoms except that they contain electrons. Chemist Ernest Rutherford had proposed the existence of tiny atomic nucleus two years earlier, but the proposal did not win scientific approval. It was only with Bohr, who in 1912 stayed with Rutherford as a postdoc candidate, that the nuclear atom was transformed into a full-fledged atomic theory with great explanatory and predictive power. Moreover, and that was a crucial element in his theory, Bohr realized that the atom was a non-classical system governed by the new and still poorly understood quantum theory. Bohr’s new theory for the simple hydrogen atom (with just a single electron) agreed perfectly with experiments. Even for complex atoms with many electrons, he succeeded in building up provisional models that could explain many of the physical and chemical properties of the elements. Although it may not be generally known, Bohr provided the first correct explanation of the periodic table of the elements, thereby extending atomic theory to the domain of chemistry.
SC: What sort of man was he outside the lab?
HK: First of all, as a scientist, Bohr was extremely ambitious and hardworking. His work capacity was legendary. As a young man, he focused very much on his scientific career, which left him little time for other activities. On the other hand, from the 1920s on, he was also busy organizing his institute and many other practical matters, such as raising money for the research at the Institute. Moreover, he engaged wholeheartedly in Danish and international organizations related to science and culture, and after World War II he spent much time as a member of several Danish government commissions. In addition, he served for three decades as President of the Royal Danish Academy of Sciences founded in 1743. Bohr was very much a family man and loved to spend time with his wife, children, and grandchildren. Sometimes he shared his time with friends and colleagues, mixing social gatherings and scientific discussions. His summer cottage north of Copenhagen served as a place for relaxation and informal discussions with young physicists.
SC: One of Bohr’s other prominent accomplishments was his contribution to the emerging field of quantum physics, specifically the Copenhagen interpretation. What exactly did this entail?
HK: When quantum mechanics emerged in 1925, Bohr was among the first to contemplate the new theory’s physical meaning and philosophical consequences. In close cooperation with Werner Heisenberg, he argued that quantum mechanics necessitated a radical break with the worldview of classical physics. According to the so-called Copenhagen interpretation, quantum physics is inherently acausal and indeterministic: processes occur without a cause, and it is not possible, not even in principle, to determine the future course of a microphysical system. More generally, according to Bohr physics does not reveal truths about the world; it tells us what we can say about the world and confirm experimentally. His complementarity principle of 1927 states that to describe the quantum world we need concepts that are contradictory with a classical point of view. Thus, to understand light we have to use both a particle and a wave picture. There is no way of telling whether light “really” is particle-like or wave-like since the answer depends on the experimental situation. Measurements affect the object of observation, and it is only by measuring an object that we can know something about it; somehow the observer seems to be part of what is observed. Bohr broadened his complementarity principle into a general philosophical principle claimed to be valid also outside physics, but this extension had not been generally accepted by either physicists or philosophers.
SC: Bohr, like Einstein, faced extreme prejudice and even danger from the Nazis. How did this persecution affect his work and life?
HK: Bohr’s mother was Jewish, but he never considered himself a Jew. In this respect, as in many others, he differed from Einstein. During the 1930s, Bohr engaged in a Danish committee trying to help Jewish refugee scientists, several of whom obtained short-time positions at his institute. When German forces occupied Denmark in April 1940, work at the Institute continued, and neither Bohr nor his collaborators faced major problems. However, in the fall of 1943, the Germans declared martial law and planned to round up the Jewish population for deportation to Germany. Bohr was to be arrested, but, like most Danish Jews, he and his family managed to escape by boat to Sweden. From there he flew to England and subsequently to the US, where he participated in the Manhattan Project, which produced the first atomic bomb in July 1945. When the Nazis wanted to arrest Bohr and bring him to Germany, it was not so much because of his Jewish ancestry as his expert knowledge of nuclear physics. For a brief while, the Institute in Copenhagen was closed down and occupied by German soldiers.
SC: Bohr once said that “Anyone who is not shocked by quantum theory has not understood it.” How has our understanding of quantum mechanics advanced since Bohr’s death?
HK: At the time of Bohr’s death in 1962, quantum mechanics had long been the standard theory of atomic and nuclear physics. Most physicists considered the theory just a useful instrument for doing calculations, caring little about its strange philosophical implications. To the extent that these implications were discussed, the Bohr-Heisenberg view, or Copenhagen interpretation, was generally accepted. This consensus was soon challenged by several other interpretations, some of which sought to reinstate classical virtues such as causality in the microworld; other alternatives emerged in the 1960s, including the so-called many-worlds interpretation first proposed by US physicists Hugh Everett and Bryce DeWitt. Bohr maintained that the Copenhagen interpretation based on the complementarity principle was the only correct understanding of quantum mechanics. For the last half-century, there has been a lively discussion about foundational and philosophical problems in quantum physics, one in which philosophers have taken more interest than physicists. In general, Bohr’s ideas still enjoy support, but the issue is not an important one for the average physicist.
SC: Bohr was a contributor to the Manhattan Project, but also a fierce advocate for the peaceful use of nuclear energy. What were Bohr’s feelings about the use of the atom bomb against Japan? Did he ever regret his work on the Manhattan Project?
HK: Bohr contributed to the Manhattan Project, although not in a very direct way. Because of his high status in the physics community, his presence was of some moral importance to the young physicists. His attempt to avoid a post-war nuclear arms race started already in 1944 and culminated with his “Open Letter” to the United Nations in 1950. Contrary to some other physicists more crucially involved in the construction of the bomb, he did not express any regret or bad conscience concerning the bombs over Hiroshima and Nagasaki, at least not in public. It seems that he wanted to look forward rather than backward. He tried to use his status and influence to establish an atmosphere of confidence between West and East, but with limited success. In Denmark, in particular, he was a staunch supporter of peaceful nuclear energy, which at the time was seen as a hope for a better future. Some 15 years after his death, Denmark decided never to use nuclear power
SC: Bohr and Einstein famously had many spirited debates about quantum physics and its impact on the scientific community. What sort of relationship did the two men have? Did they ever influence each other’s work?
HK: Einstein and Bohr first met in Berlin in 1920 and again in Copenhagen the same year. Bohr admired Einstein’s work in both relativity and quantum theory, and as early as 1914 he realized that to explain some details in atomic physics he had to take into account Einstein’s relativity theory. On his side, Einstein was among the first to acknowledge Bohr’s genius. Their letters clearly show the great mutual respect. Although they met several times between 1920 and 1950, they only exchanged a few letters. Focusing on very different fields of physics—general relativity theory (Einstein) and quantum and atomic physics (Bohr)—their scientific interaction was limited. They strongly disagreed concerning the meaning and interpretation of quantum mechanics, which they discussed at the famous Solvay meetings. It is probably fair to say that this discussion was rather unfruitful since neither of them was willing to abandon his own positions. The young Bohr was to some extent influenced by Einstein’s way of doing physics. Although Einstein admired Bohr for what he called his intuition and musicality, I don’t think he was much influenced by him.
SC: In your book, Niels Bohr and the Quantum Atom, you obviously had to research Bohr’s life, as well as that of the other scientists covered in your book. What was the atmosphere like for a scientist in the early 20th century?
HK: There are continuities as well as discontinuities in the history of science. A century ago, physicists worked essentially as they do today, but the social and economic conditions were quite different. First of all, physics was a small field with very limited resources for experimental work. Around 1910, there was a general feeling that physics was at a watershed: relativity theory had already challenged Newtonian physics, and what about the new quantum theory? Bohr was in some respects lucky since he grew up at the right time to make a change in the physical world picture. What strikes me most about the early period is perhaps how similar the game of physics was to what we experience today: a constant interplay between theory and experiment. Although a theorist, Bohr was very interested in experiments and almost obsessed with getting his atomic theory to agree with even minute experimental details. His institute in Copenhagen was devoted to “theoretical physics,” but by that, he meant fundamental physics and not something separate from or contrary to experimental work. From its beginnings in 1921, Bohr’s institute included laboratories for experiments.
SC: When Bohr died in 1962, the world lost one of its greatest scientific minds. Out of those alive today, who would you say is best carrying on the spirit of Bohr’s work?
HK: Niels Bohr, but also Einstein and Erwin Schrödinger, belonged to the last generation of physicists who were also natural philosophers in the tradition of, for example, Newton and Helmholtz. With the increased specialization in physics, and with the increased role of teams rather than individuals, the days of natural philosophers seem to have gone. I can see no obvious candidate to carry on, in some sense, the spirit of Bohr (or, for that matter, the spirit of Einstein). Perhaps the closest is Steven Weinberg, who combines scientific genius with insight and interest in issues outside the domain of physics in the narrow sense.
SC: Lesser known than Niels but no less important was his son Aage Bohr, who died in 2009. What was the relationship like between father and son? Did you ever have a chance to speak with Aage before his passing?
HK: Niels Bohr and his son Aage were very close, and from an early age Aage followed his father’s example and learned from him. For instance, he was with him during the war in England and the US. He later specialized in the theory of the atomic nucleus, improving on the model that his father had pioneered in the 1930s. It was for this work, called the collective theory of the nucleus, which Aage Bohr, together with his collaborator, now 87-year-old Ben Mottelson, received the Nobel Prize in 1976. I had some conversations with Aage and Ben when I wrote about their Nobel Prize, but I was not closely acquainted with either of them. I also knew some of Niels Bohr’s older associates, including the Belgian Léon Rosenfeld, who in the 1960s and 1970s was the chief advocate of the Copenhagen interpretation and, in this regard, Bohr’s successor.