by Abraham Pais*
The year 1917 marked the turbulent birth of a new era. In March the Czar of all Russians abdicated, in October the communist revolution broke out. Germany and Russia signed an armistice at Brest-Litovsk. The first United States division arrived in France and the battle of Passchendaele raged. The Balfour declaration on Palestine was made public, and the trans-Siberian railroad was completed. Degas, Rodin, and Count Zeppelin died, and John Fitzgerald Kennedy was born. Freud published his introduction to psychoanalysis, the Salzburg festival was initiated, and the 100-inch reflecting telescope was installed on Mount Wilson, California.
As to the Danish scene in 1917, the government sold the Virgin Islands to the United States. On April 27, the thirty-one year old Niels Bohr, since April 1, 1916 professor of theoretical physics in the University of Copenhagen, was elected member of Det Konqelige Danske Videnskabernes Selskab (the Royal Danish Academy of Science and Letters). Ten days earlier he had sent a letter to the Science Faculty of his university which begins as follows.1
“I hereby request the Faculty to work for the establishment of an institute for theoretical physics, where the necessary conditions can be created for the growth of this subject here in Denmark. Such an institute would have the dual task of being the center for education in theoretical physics and of giving the opportunity for carrying out numerical computations and experimental investigations in connection with the scientific work in this subject.”
Thus, compelled by the fact that the university had never yet had a physics laboratory, did Bohr begin his campaign for founding his Institut for Teoretisk Fysik. From the start and to this day, it has also been a center for first-rate experimental investigations.
Bohr later explained2 the choice of name for his institute: “There was in Göttingen an institute … called the institute for theoretical physics … they called the new things theoretical physics and we kept the name. It may not be practical, you see; we could perhaps much better have called it an institute for atomic physics.”
In 1965, the year in which Bohr would have been 80, it was renamed Niels Bohr Institutet.
On January 2, 1946, I arrived in Copenhagen to start post-doctoral research. I was the first of the post-World War II crop of youngsters from abroad to settle at Niels Bohr’s Institute.
Shortly thereafter, on March 3, the 25th anniversary of the Institute was celebrated. True to Bohr’s style it was an intimate occasion, the high point of which came as he reminisced about the people and the events of that heroic period. There was no pomp, only a few brief speeches. It was my pleasant task to express the gratitude of the first installment of post-war visitors from abroad. And, of course, that evening there was a feast held by Parentesen, the graduate students’ club. It was the time I learned to sing Videnskabens Fædre (the Fathers of Science), of which the last verse is to be rendered while the participants stand on their chairs, beer in hand: “Nobelmanden Niels Bohr ved vei blandt alle vildspor…” (… knows the way amidst all false tracks). It gave us all a sense of pride to have Bohr in our midst at that moment, also standing on his chair.
I was also in Copenhagen in late April 1971 to attend one of a series of symposia celebrating the 50th anniversary of the Institute. The talks were on philosophical themes and included a dialogue between physicists and psychologists. The affair turned out to be a friendly sort of disaster, but it was pleasant to meet old friends. Margrethe Bohr was still marvelously handsome at 81. Of new acquaintances I remember only some discussions with Jean Piaget, who had spoken on “causality and probability.” I found him to be a rather arrogant old man.
There can only be few among my present audience who were there in 1971, and only fewer who were there on that festive day in March of 1946. In any event, you will appreciate how great a pleasure and privilege it is that today, half a century plus three days later, I may speak once more of an Institute that has so much enriched my life. Your country has indeed been good to me especially because here I have found the lady who has honored me by becoming my wife.
Beginning of science in Denmark
March 3, 1921 is a date of great historical significance, not just for Danish physics, but for science in the world at large. It is of course not the date which marks the beginning of physics being taught in Denmark. That activity started in 1539. Which brings me to the first topic of my discourse. I thought it might be good to hear about the beginning of our beloved science here in Denmark, since many of you are barbaric souls.
I shall comment on the early years, up till the late 1940s, of the Bohr Institute.
Pope Sixtus IV, patron of letters and the arts, who died in 1481, will be remembered for the construction under his aegis of the Sistine chapel, the initiation of the Sistine choir, and the founding of the University of Copenhagen. It is believed3 that this last issue was settled in the course of the pilgrimage to Rome of His Catholic Majesty Christian I, king of Denmark and Norway. At any rate, the necessary authorization for establishing this school was formalized by the papal bull of June 19, 1475. On Tuesday, June 1, 1479, the University was inaugurated as a Catholic institution with a solemn mass de spiritu sancto in Copenhagen’s Vor Frue Kirke (Church of Our Lady), in the king’s presence. Western European exact science was in its infancy then; Copernicus was only a boy of six.
The post-reformation period
In that period universities were essentially religious institutions, meant to prepare future men of the clergy, other church officials, and teachers in Latin schools for their coming tasks. Times were no longer favorable for Catholic institutions when Copenhagen University was founded, however. The struggle against papal power, culminating in the Reformation, had begun. The University languished; by 1531 all its academic activities, limited to start with, had come to an end.4
A new beginning was made after the Reformation had transformed Denmark into a Lutheran state (1536). A new charter, the Fundatio et Ordinatio universalis Scholae Hafniensis, signed in 1539 by King Christian III, would remain in force for the next two hundred years.
Also from 1539 date the first two faculty appointments in the philosophical sciences. One, in mathematics, for teaching theoretical and practical arithmetic, the main topic, and also astronomy, cosmography, Euclid’s geometry, and theoretical and practical music. The other, in physics, for teaching four hours a week Aristotle’s writings on physics and ethics, using a text “in Greek or in Latin translation if (the professor) were not sufficiently familiar with the Greek language.”5 In the words of the charter: “We maintain all sciences at this university regardless of the fact that they teach us many things that cannot be grasped by the common people (but rather) in order to spread God’s works so that others may participate in God’s glorious gifts.”6 The professor’s oath of office may serve as a last example of the continued religious basis of the university: “I swear that with God’s grace I shall faithfully and diligently perform my duties so as to strengthen Christ’s church and honor this University.”7
These few backward glimpses are meant to illustrate that, in Copenhagen as elsewhere, universities then and now had only their name in common. Teaching focused largely on the Scriptures and major texts from antiquity. Learning took precedence over explanation and critical insight, form dominated content. The syllabus was still much like that handed down from the schools of the Roman Empire: the trivium, the lighter sciences, comprising grammar, rhetoric and logic (whence our “trivial”); and the quadrivium, arithmetic, music, geometry, and astronomy. What in post-Reformation and post-Renaissance passed for science was dominated by Aristotelian doctrine, reconciled with theology by men like Thomas Aquinas. At Oxford, in 1586 questions “disagreeing with the ancient and true philosophy” were not even allowed to be discussed.8
Denmark’s eminent scientists
By the end of the seventeenth century Denmark had produced a number of eminent scientists, men like the astronomers Tycho Brahe (1546-1601) and Ole Rømer (1644-1710), the mathematician Rasmus Bartholin (1625-1698), and the polymath Niels Steensen (Steno) (1638-1686). In spite of the appearance of such renowned figures, science at the University had remained weak, however. Its emended charter of 1732, written shortly after the complete destruction of the University by the great fire of 1728, no longer provided for a chair in physics. It states: “Philosophia Naturalis shall be taught by one of the professors in medicine or in mathematics such that one day a week he shall teach physics but the other days, at his preference, mathematics or medicine.” Perhaps that was just as well since whatever physics was then taught continued to be Aristotelian.9
The great change began in the middle of the eighteenth century. In 1742 Det Konlige Danske Videnskabernes Selskab was founded. The trend of its publications was soon directed toward the natural sciences.10 In 1753 Christian Kratzenstein (1723-1795) was appointed professor physices experimentalis designatus medicinae, that is, he became a professor in the faculty of medicine but was charged with teaching experimental physics.11 In 1796 his successor was appointed professor of physics, still in the medical faculty. In 1800 the next professor of physics was assigned to the philosophical faculty.
So it remained until Hans Christian Ørsted (1777-1851) changed physics in Denmark from an appendage to other subjects into a fully fledged independent field of study.
Ørsted’s major scientific contribution was his discovery in 1820 of electromagnetism, which at once created a great sensation. His original paper of 1820 was written in Latin, but in that same year translations appeared in Danish, Dutch, English, French, German, and Italian. Faraday and Ampère wrote in high praise of him. In 1821, volume 31 of the prestigious Journal für Chemie und Physik opened with an editorial announcing a change in format “in part because a new epoch in chemistry and physics appears to have begun with Ørsted’s important discoveries on the connection between magnetism and electricity.” A contributor wrote: Ørsted’s experiments regarding magnetism are the most interesting ones performed in more than a thousand years.”12
As a result of his growing international prestige, Ørsted became an increasingly influential figure on the national scene. This he put to use in fulfilling his long-standing ambition of broadening Danish science at the base. A visit to London during which he attended lectures at the Royal Institution gave him the inspiration for founding, in 1824, Selskabet for Naturlærens Udbredelse, the society for the dissemination of science. He was its president from its beginnings until his death, and himself gave twenty-six of the popular lectures which the Society offered to the general public, both in Copenhagen and in the provinces. The Society still exists and is now housed in the H.C. Ørsted Institute on the Nørre Allé in Copenhagen. Among its later Presidents we find Niels Bohr.
Ørsted was also the driving force behind the founding (1829) of the Polytekniske Læreanstalt (now called the Technical University of Denmark), an institution for education on a scientific basis in engineering and other technical subjects, modeled after the École Polytechnique in Paris, which Ørsted had also visited. He assumed its directorship, which he held for the rest of his life. As concurrent professor at the University, he prompted close ties between the institutions, including joint courses on various subjects.
Finally, by the Royal Decree of September 1, 1850, a separate faculty of mathematics and natural sciences was established at the University. Ørsted, who had suggested this move nearly forty years earlier, became its first professor of physics.13 By and large, physics was housed at Læreanstalt, however. In the first thirty years of the new faculty’s existence, the average number of its students has been estimated at about 20. This year it is about 1,500.
The influence of Niels Bohr
Two men have done more than anyone else to raise physics in Denmark to its current world-class stature: Ørsted in the nineteenth, and Niels Bohr (1885-1962) in the twentieth century.
When in 1903 Bohr began his student years at the University of Copenhagen, that institution had only one professor, not in experimental or in theoretical physics, but in physics tout court – and, as mentioned, had no physics laboratory yet. In 1906 a professor from the Læreanstalt wrote:
“The position of the physical sciences … in this country … is marked by neglect to a high degree … For a hundred years there has existed a physical instrument collection … at the Læreanstalt for joint use with the university. There is, however, a lack of space and equipment for the execution of scientific work. One cannot, in this country, perform modern experiments, one cannot undertake precision measurements of weights or lengths… extremely important recent research elsewhere cannot be taken up here … It will presumably be clear from the foregoing that physics occupies a position unworthy of our country … there is a lack of necessary collaboration between science and technology.“14
In 1916, ten years later, all that had changed when Bohr was appointed the first professor in theoretical physics in Copenhagen, and when he had started to lay plans for the University’s first physics institute.
Bohr’s rise to world fame had meanwhile been meteoric. After having received his Ph.D. in 1911, he had used quantum theory to decode in 1913 the structure of the spectrum of hydrogen. Of his other major contributions in that early period, I only mention two. First, the correspondence principle, which aims to establish links between classical and quantum physics, of which Kramers has written: “In the beginning the correspondence principle appeared to the world of physicists as a rather mystic wand that did not work outside Copenhagen.15 Sommerfeld has called the principle a magic wand.16 Secondly, Bohr’s analysis of the periodic table of the elements, which laid the foundation of quantum chemistry and led to the discovery in Copenhagen of a new chemical element, hafnium, by Hevesy and Coster.17
After many negotiations the construction of his Universitetets Institut for Teoretisk Fysik was completed. At its inauguration on 3 March 1921, Bohr delivered an address in which he stressed what would become the Institute’s main theme: “The task of having to introduce a constantly renewed number of young people into the results and methods of science . . . Through the contributions of the young people themselves new blood and new ideas are constantly introduced into the work.”18
On the evening of December 10, 1922, the day on which Bohr received the Nobel Prize, he proposed a toast “to the vigorous growth of the international work on the advancement of science which is one of the high points of human existence.”19
Bohr’s aspiration, to attract young physicists from all over the world, has been richly fulfilled. Between 1916 and 1961, the year before Bohr’s death, 444 visitors from 35 countries spent at least a month in Copenhagen. During Bohr’s lifetime about 1,200 physics papers were published from Copenhagen (among them about 200 by Bohr himself).
Financial assistance to scientific research
Bohr’s fame helped him greatly in obtaining financial support for his enterprises from outside sources. He needed foundations to provide money. Foundations needed him as evidence for funds well disbursed.
Bohr needed financial assistance for support of collaborators, acquisition of laboratory equipment, and extensions of his institute space. The main Danish source was the Carlsberg Foundation, which has forked out well over a hundred grants to him in his lifetime. The main benefactor from abroad has, since 1924, been the Rockefeller philanthropic empire.
The importance of Bohr’s role in providing guidance to others’ research had soon become widely recognized, as can be seen, for example, by the following lines found20 in the New York Times in 1924: “Working with Dr. Bohr is regarded by scientists as working with the foremost of the exponents of the new atomic physics, which is revolutionizing science.” To that role we must now add a different though related one, his raising of funds for the purpose of providing facilities so that others could be close to him not only in mind but also in body. Those efforts did not just benefit the evolution of the Copenhagen Institute to a world center of theoretical physics. Rather, it is essential to realize that Bohr must be seen above all as a trailblazer who led the way towards new modes of support for physics worldwide, as can be seen by reading once again the New York Times:
“The appropriation [of $40000] was regarded by scientists … as a striking example of the growing recognition accorded to scientific research … It is the hope of many American men of Science that the recognition of the importance of research, shown in the Rockefeller grant to Dr. Bohr, will spur the movement to develop more research laboratories in this country and more American colleges and universities specializing in research.”21
When the new Institute opened its doors, we were still in the era of the so-called old quantum theory, which had produced a rich harvest: quantum numbers, selection rules, the exclusion principle, the first steps toward quantum chemistry, and quantum statistics. While all that went on it had become increasingly obvious that all was far from well with quantum physics, however: the Bohr-Sommerfeld quantum rules appeared quite often to be highly successful, yet, in a deep sense, they were paradoxical, as Bohr well knew. The spectrum of helium – not to mention heavier elements – was impenetrable, and remained so through 1925. The duality of the particle and wave pictures, necessary for light, as experiment had shown, conjectured for matter by de Broglie, was a mystery that did not fit into anything known before, including the correspondence principle.
In retrospect the numerous successes of the old quantum theory of the atom are all the more fabulous and astounding because they are based on analogies – orbits similar to the motions of planets around the sun, spin similar to planets that rotate while they are orbiting – which the new quantum mechanics was to show are in fact false.
The changing world scene
When in 1925 that new mechanics appeared on the scene, many major contributions to this new discipline emanating from Copenhagen were not long in forthcoming. I only mention such gems as young Heisenberg‘s papers on the uncertainty relations and on the quantum mechanics of the helium atom, and young Dirac‘s, on the transformation theory as well as his first paper on quantum electrodynamics. In 1927, Bohr himself contributed importantly to the interpretation of quantum mechanics with his concept of complementarity which says, briefly stated, that particle and wave description of natural phenomena are mutually exclusive, yet that both are necessary for their description.
So it came to pass that during the 1920s and 30s the Copenhagen Institute was the world’s most renowned center for theoretical physics. The international theory conferences annually held in Copenhagen from 1929 through 1938 are remembered as the world’s most outstanding gatherings of their kind during that period.
As already noted, the term “theoretical” in the Institute’s initial name was perhaps somewhat of a misnomer, however, since from the start Bohr laid great emphasis on the need for having theory and experiment pursued under the same roof. Accordingly, in the early 1920s several instruments were installed for doing various kinds of atomic spectroscopic experiments.
In the 1920s, Georg von Hevesy (1885-1966), a physical chemist, had spent some years at Bohr’s institute, during which he had made the first applications of isotope tracer techniques to the life sciences. The radioactive materials then available for such purposes were tiny in quantity and, even worse, highly toxic. The availability of the artificially radioactive substances, discovered in 1934, eliminated both drawbacks. The work of Hevesy in the 1930s at Bohr’s institute made isotope tracer methods flourish. And so Bohr became the godfather, and Hevesy the father, of nuclear medicine.22
Beginning in the 1930s, Bohr became ever more deeply involved, scientifically and otherwise, in the changing world scene.
After the Nazis came to power in 1933, he was quite active in aiding refugees, mostly Jewish, who came to Denmark, and was able to offer temporary hospitality at his institute to physicists in trouble.
As to the changes in science, in the early 1930s Bohr commenced the new task of directing his Institute toward the young field of nuclear physics. Be it recalled that the first book23 ever written anywhere on theoretical nuclear physics is the one by George Gamow, completed in Copenhagen in May 1931. In 1936 Bohr himself came forth with his “liquid drop model” of the atomic nucleus,24 of which Hans Bethe has written: “(This model) dominated the theory of nuclear reactions at least from 1936 to 1954 . . . At Los Alamos, when we tried to get (probabilities), we used (this) model and usually our predictions were quite reasonable. The . . . model could explain many phenomena . . .25 He should know, since in the World War II years he was the head of the theory division at Los Alamos.
In a paper26 from 1939, written at Bohr’s institute, Robert Frisch (1904-1979) and Lise Meitner (1878-1968) interpreted the fission of uranium (the term fission is another Copenhagen contribution), in terms of the “liquid drop model.” Bohr realized, also in 1939, that slow fission of uranium must be attributed to its rare isotope with atomic weight 235 – an insight27 which led to the construction of the bomb dropped on Hiroshima.
Experimental nuclear physics
Now to experimental nuclear physics. When in the 1930s the Institute’s focus shifted to the study of the nucleus, several types of high-energy accelerators were installed, all these activities being personally directed by Bohr. Recall, however, that experimental nuclear studies were performed in Copenhagen even before these new machines became operative. I think of work on hyperfine structure of spectral lines directed by Ebbe Rasmussen; on interactions of a– particles with matter, led by Jacob Jacobsen; of discoveries of new isotopes led by Otto Frisch, using at first old radon tubes from Copenhagen’s Radium Institute and the Finsen Hospital; and later the one-half gram of radium presented to Bohr on his 50th birthday.
The new accelerators, obtained with the help of grants applied for by Bohr, demanded the creation of new laboratory space which was inaugurated on April 5, 1938, in the presence of distinguished guests, including King Christian X. Later in 1938 the first new machine went into operation, a cyclotron, one of the first of its kind in Europe. In 1939 the next machine, a variant of the Cockcroft-Walton design, started working. Both machines were ready just in time for early experiments on nuclear fission. Other accelerators have followed, notably of the Van de Graaff type.
In 1943, circumstances of war caused Bohr to flee from Copenhagen to London, where he became a consultant of the British government for atomic weapons development, in which, however, he only played a minor role. Indeed, from late 1943 onwards his major concern was less with the war effort than with the radical changes in the post-war political climate that could be anticipated because of the new weapons. These, he became convinced, might actually hold out a promise for improved international relations precisely because of their unmanageable threats to the security of nations. He was the pioneer of the idea of an open world but would not live to see his concepts realized.
Staying with physics, Bohr’s post-war activities focused on consolidation and extension of that discipline. He supported the founding (in 1952) of CERN, the European Organization of Nuclear Research and offered hospitality at Copenhagen, from 1952-1957, to its theory group, of which he became its first director. He was the driving force behind the founding of Nordita (the Nordisk Institut for Teoretisk Atomfysik), which started activities in 1957, was the first chairman of its governing board, and arranged for its housing (in 1964) in a building next door to his own Institute. In 1955 he became the first chairman of the Danish Atomic Energy Commission and led the efforts at establishing its research center at Risø, inaugurated in 1958. It is now a multipurpose institution. His final initiative was to establish a branch of his Institute contiguous to the Risø center. That was in 1961, the year before his death.
As, today, we celebrate the Institute’s seventy-fifth anniversary, it is fitting that we pay tribute first and foremost to the memory of Niels Bohr who, more than anyone else, has brought physics in Denmark to high prominence in our century. His contributions are astoundingly varied. I think not only of his researches, his teachings, and his endeavors in the political sphere. There is also Bohr the philosopher, the administrator, the fundraiser, the catalyst in promoting physical applications to biology, the helper of political refugees, the co-founder of international physics institutes as well as the nuclear power projects in Denmark, and, last but not least, the devoted family man.
On October 24, 1957 I attended the presentation in Washington of the first Atoms for Peace Award to Bohr. The eloquent Award citation demonstrates the esteem in which Bohr was held by the world at large. I quote from it:
“Niels Henrik David Bohr, in your chosen field of physics you have explored the structure of the atom and unlocked many of Nature’s other secrets. You have given men the basis for greater understanding of matter and energy. You have made contributions to the practical uses of this knowledge. At your Institute in Copenhagen, which has served as an intellectual and spiritual center for scientists, you have given scholars from all parts of the world an opportunity to extend man’s knowledge of nuclear phenomena. These scholars have taken from your Institute not only enlarged scientific understanding but also a humane spirit of active concern for the proper utilization of scientific knowledge.
In your public pronouncements and through your world contacts, you have exerted great moral force in behalf of the utilization of atomic energy for peaceful purposes.
In your profession, in your teaching, in your public life, you have shown that the domain of science and the domain of the humanities are in reality of single realm.”28
I regret that the brevity of this address does not permit the mention of the many others who have made valuable contributions to physics in Denmark. I shall only mention some further institutional developments.
There are now three big universities in Denmark where physics is practised: Copenhagen, the Technical University of Denmark and the more recently founded Aarhus University, where physics started in 1933. Smaller centers active in physics include Odense University (founded in 1966), the
University Centers at Roskilde and at Aalborg, the Danish Institute for fundamental meteorology and for space research, and the Royal Veterinary and Agricultural University.29
On January 1, 1993, four physics institutes in the Copenhagen region have finally joined30 to constitute the NBIfAFG, Niels Bohr Institutet for Astronomi, Fysik og Geofysik, under the able direction of Ole Hansen.
I conclude with a general comment about our present times.
Our century has not ended well. Today we live in the midst of upheaval and crisis. We do not know where we are going, nor even where we ought to be going. Awareness is spreading that our future cannot be a straight extension of the past or the present, that in the last 1980s an era in world history has ended and a new one has begun.
The century now approaching its end has been one of indiscriminate violence, it has been perhaps the most murderous one in Western history of which we have record. Yet I would think that what will strike people most when, hundreds of years from now, they will look back on our days is that this was the age when the exploration of space began, the microchip was invented, revolutions in transport and communication virtually annihilated time and distance, transforming the world into a “global village,” and relativity theory, quantum mechanics, and the structure of the atom were discovered, in brief that this has been the century of science and technology.
To us, it should be a source of gratification, indeed of pride, to recall how important the contributions of our little Institute to these advances have been. Even today, physics in Denmark is clearly alive and flourishing. Let us keep that up into the next millennium.
1. Aarbog, Københavns Universitet 1915-1920, part IV, p. 316, Schultz, Copenhagen 1922.
2. N. Bohr, interview with T.S. Kuhn.
3. C. Paludan-Müller, Historisk Tidsskr. 2, 241, 1880.
4. H.F. Rørdam, Københavns Universitets Historie, Vol. 1, Bianco, Lunos, Copenhagen 1869.
5. Ref. 4, pp. 59, 84, 314.
6. Ref. 4, p. 110.
7. Ref. 4, p. 111.
8. V.H.H. Green, The Universities, p. 193, Penguin Books, London 1969.
9. Bostrup, Fysisk Tidsskr. 69, 11, 1971.
10. Det Kongelige danske Videnskabernes Selskab 1742-1942, Vol. 2, p. 12, Munksgaard, Copenhagen 1950.
11. For a history of that period see Københavns Universitet 1479-1979, Vol. 12, Gads, Copenhagen 1983. Vol. 2, p. 12, Munksgaard, Copenhagen 1950.
12. I. Schweigger, J. für Chem. und Phys. 31, 1, 1821.
13. O. Bang, Store Hans Christian, Rhodos, Copenhagen 1986.
14. P.K. Prytz, Aarbog for Københavns Universitetet, den polvtekniske Laereanstalt og Kommunitetet, 1906-7, p. 1003, Schultz, Copenhagen 1911.
15. H. A. Kramers, Fysisk Tidsskr. 33, 82, 1935.
16. A. Sommerfeld, Atombau und Spektrallinien, 3rd Ed. , p. 338, Vieweg, Braunschweig 1922.
17. D. Coster and G. von Hevesy, Nature 111, 79, 1923; see also ibid. pp. 182, 252, 322, 462. For more on this discovery see A. Pais, Niels Bohr’s Times, pp. 209-210, Oxford University Press 1991.
18. For the full text see Niels Bohr, Collected Works, Vol. 3, p. 293, North Holland Publ. Cy. New York 1976.
19. Niels Bohr, Collected Works. Vol. 4, p- 26.
20. The New York Times, January 27, 1924.
21. The New York Times, January 28, 1924.
22. For more on the history of tracer methods see Pais, ref. 17, Chapter 17, Section (h).
23. G. Gamow, Constitution of nuclei and radioactivity, Oxford University Press 1931.
24. N. Bohr, Nature 137, 344, 1936.
25. H.A. Bethe, in Nuclear Physics in retrospect, p. 11, R. Stuewer Ed., University of Minnesota Press 1979.
26. L. Meitner and O. Frisch, Nature 143, 239, 1939.
27. N. Bohr, Phys. Rev. 55, 418, 1939.
28. The full speech has been reproduced in a pamphlet printed for the Awards Committee, copy in the Niels Bohr Archive, Copenhagen.
29. More details are found in “Review of Physics in Denmark,” Ministry of Education and Research, Copenhagen 1992.
30. Årbog, Københavns Universitet, 1993, p. 513.
* Address given in Copenhagen on March 6, 1996, on the occasion of the 75th Anniversary of the Niels Bohr Institute.
(The author, a renowned theoretical physicist and scientific historian, died in 2000. His address was published posthumously at Nobelprize.org.)
First published 21 February 2002