| Niels Bohr
Introduction
The first third of the 20th century witnessed a revolutionary development in the field of physics. The atom, which had since antiquity been regarded as the smallest particle of mat-ter in the universe, turned out itself to be a universe. This made it necessary to reject old concepts and ideas and a whole new world with tremendous possibilities and abysmal risks opened up. This took place in an intense interaction between scientists from many countries, partly in the form of direct co-operation, partly through mutual inspiration. During certain periods the new discoveries and the progress came from all parts of the world at such an overwhelming pace that the world picture was enlarged and amplified almost from day to day. For a number of years professor Niels Bohr at the University of Copenhagen was the central figure in the international work on the development of nuclear physics and his modest Institute for Theoretical Physics became the most important meeting place and place of learning for the world's young nuclear physicists. It has left permanent marks in the scientif-ic nomenclature. For instance, the element which has number 72 and which was long unknown was called Hafnium because it was discovered at Niels Bohr's institute and named after the Latin form of Copenhagen, Hafnia. Since then, after Niels Bohr's death, the Soviet scientist who in 1970 produced ele-ment number 105 - one of the radioactive substances that does not exist in nature - suggested that it should be named after Bohr and be called Nielsborium in the same way as other newly discovered elements have been named after other great physicists, for instance Einstein and Rutherford. In this way the names of Copenhagen and Niels Bohr have been incorporated in the periodic system of the elements, the basis for all physical existence. The importance of Niels Bohr was not due solely to the fact that he was a highly talented scientist. Perhaps it came to be even more important that he was at the same time a great educationalist and a great human being who personified the ideal of the responsible citizen and cosmopolite. His scientif-ic efforts were in no way strictly limited to his profession, but were combined with his enthusiastic engagement in the problems of the time and of society, not least the problems that the scientific advances gave rise to. Niels Bohr was in every respect a man of his time in kinship with his fellowmen and therefore exercised a far greater influence than the very considerable influence that was the result of his scientific work. Niels Bohr was born on October 7, 1885 in Copenhagen of parents with widely different, but equally prominent back-grounds. His father was a professor in physiology at the University of Copenhagen while his grandfather and great-grandfather had been headmasters of secondary schools. The educational by way of education of young people who aspired to or received a university education has so far fol-lowed the family for five generations. On his mother's side Niels Bohr descended from a well-to-do Danish-Jewish fami-ly as his maternal grandfather, D.B. Adler, was one of the leading bankers of his time who took part in the establish-ment of two of the banks which today are among the largest banks in Denmark. D.B. Adler also played an active role in politics and was a liberal member of both the Danish Parlia-ment and the municipal government of Copenhagen. A maternal aunt, Hanna Adler, was one of the most prominent educationalists of her time. She created her own school which was influenced by many new thoughts and she introduced among other things co-education for boys and girls. The environment in which Niels Bohr and his younger brother and sister grew up was warm and stimulating. His father often had his colleagues from the university and the Royal Danish Academy of Sciences and Letters as visitors and they were not primarily from his own profession. On the contrary, they included the philosopher Harald Høffding, the philologist Vilhelm Thomsen and the physicist Christian Christiansen who were his closest acquaintances, and the four men's conversations that were often about basic, epistemological problems became very important to the listening boy who has since told about the inspiration to an understanding of the unity of all human search for knowledge that fell to his lot in this way already while he was a boy. His parents wanted to avoid a one-sided, intellectual
up-bringing of their children. They attached much importance to their children's
independent development and prompted
Already as a boy Niels Bohr was vigorously preoccupied with using his hands and his common sense for practical tasks. Thereby he got an opportunity to develop his precision and his powers of observation so that things were made exact-ly and correctly because he took the time that was necessary and was never over-hasty. It is also said that when several chil-dren were together he was able to give the others the impres-sion that they had contributed to the result even though in reality they had perhaps only been spectators. All this was something that also came to be a characteristic of his working methods later in his life. At school he was gifted without making a great show, but when it came to writing essays his early developed sense of precision and logic came into conflict with the school's for-malistic wish for a smooth and superficial treatment of sub-jects that either seemed to him to be unimportant or which were, as far as he was concerned, really too extensive for an ordinary essay. His best subjects were the natural sciences, but also in other fields he held his own so that in 1903 he matriculated with honors. Niels Bohr kept contact with several of his schoolmates throughout his life, among others with Ole Chievitz, who was also the son of a professor and who became a member of the medical profession and profes-sor at the university, but who is not least remembered for his great efforts in the resistance movement when Denmark was occupied by Germany. The years of study at the rather small university in Copen-hagen, which was then still situated in the old part of town and which was for this reason less isolated from the rest of society than many other universities, became valuable in many ways. Most important, perhaps, was the contact with students studying other subjects. This contact was a result of the open classes in fundamental philosophical concepts which were at that time compulsory for all students. The professor was Harald Høffding who has already been mentioned as a frequent guest in Bohr's home, and a number of his students formed a conversation group in order to dis-cuss philosophical and scientific subjects. These were so to speak all the young people who later reached prominent positions within their respective fields of research, but from the very first it was clear that the most promising of them were Niels Bohr and his brother Harald who had matriculat-ed the year after Niels. The two brothers' way of thinking seemed to be co-ordinated, one of their friends said later. They continued and improved each other's argumentation at such a pace that nobody else could take part in it. The only drawback in this connection was that often the chairman had to ask Niels to speak louder, though his request had no effect at all. This continued to be a problem throughout his life. This form of dialogue in which Niels Bohr expatiated together with others became characteristic of all his activi-ties. He could only work in harmony and understanding with those that were closest to him. The constant dialogue with his brother continued as long as they both lived. His colleagues at the Institute for Theoretical Physics have related how Niels Bohr could suddenly stop in the middle of a train of thoughts and say: "I must talk with my brother about this!" - and then he would immediately walk over to the mathematical insti-tute that was situated right next to his own institute. For Harald Bohr had become professor of mathematics after having been awarded a doctor's degree already at the age of 23. In his own field he was perhaps just as brilliantly gifted as Niels Bohr, but in the very nature of things his reputation was limited to the world of his scientific colleagues. Niels Bohr was somewhat behind his younger brother. He was not awarded his doctor's degree until 1911 when he was 26, but even that was early according to the Danish university standard and by then it had already for a long time been clear that he was developing into an exceptional scientist. A fellow student wrote the following already in 1904 in a letter: "It is very interesting to know a genius and I do, I am even together with him every day. I am talking about Niels Bohr . . . and be-sides he is the best, most modest human being that can be imagined". In 1907 Niels Bohr received the gold medal from the Royal Danish Academy of Sciences and Letters for a prize paper on the surface tension of liquids which was to be determined ex-perimentally by the use of a theory that had been advanced by the British physicist Lord Rayleigh. The paper did not ac-tually fulfil the demands made, for Bohr had only deter-mined the surface tension of water whereas the task had been to determine the surface tension of several liquids. However, he had worked so thoroughly on the development of the equipment that was needed for the experiments and which he had to manufacture himself and he had been so meticulously careful in every respect that he could not accomplish more in the time that was available. On the other hand, he had made decisive contributions to Lord Rayleigh's theory, and there was no doubt that he had deserved his gold medal. Two years later the paper was printed in "Philosophi-cal Transactions of the Royal Society of London" as the first paper written by Niels Bohr that reached the world at large. It was not within this specific field of physics that Niels Bohr came to make his main contribution, but his knowledge of liquids later became useful when it turned out that analo-gies could be drawn between drops of liquids and certain conditions of the nuclei of atoms. Bohr was awarded his M.Sc. degree in 1909 and on this oc-casion his main paper concerned the physical properties of metals seen in the light of the electron theory that had in the preceding years been developed by scientists in Europe. He continued his work on this subject in the paper whereby he acquired his doctor's degree two years later. To the Danish physicists of the time it was a completely new field and the opponents at the public defence of the thesis obviously had difficulties in evaluating the difficult thesis with its large mathematical apparatus, even though they were quite aware that they must show it great respect. What was moreover characteristic of the thesis was that it was not limited to an account of the results obtained until then by scientists in this field, even though this alone would have made it epoch making within Danish science, but it contained first and fore-most a demonstration of the many problems that the electron theory was at that time not able to solve. Therefore it pointed to the future, both for science in general and for Niels Bohr himself. It would be reasonable to give a brief account of the level of science at the time when Niels Bohr wrote his doctoral thesis. The decade to come would come to mean a so decisive revolu-tion in the physical world picture that even now - three quart-ers of a century later - it is difficult to imagine. Modern physics is still not very accessible to most people because it is concerned with phenomena that do not directly appear to our senses and because it deals with regularities and facts that seem quite different from the experience gained from practical everyday life. When modern physics first made its appearance it constituted in fact a scientific revolution because it had to break with many customary con-ceptions, not least classical mechanics which was to a great extent based on Isaac Newton's work on gravitation. This very fact meant that the new theories met with strong opposi-tion - like any revolution - and there may be good reason to use the expression about the breakthrough of modern physics phrased by the British nuclear physicist Ernest Rutherford: "the heroic age of physics". The very concept of the atom was old in so far as it origi-nated in the world of antiquity. According to the Greek philosopher Democritus (approx. 400 B.C.) all substances had to be composed of indivisible particles that were identi-cal. However, in the 18th century chemists had identified a number of elements that formed compounds and they understood that the atoms of these elements had to differ. After some time scientists succeeded in determining the compara-tive weight of the atoms with a fair approximation and on the basis of this the Russian chemist Mendelejev in 1869 put for-ward his periodic system in which the elements were arranged according to atomic weight. The epoch-making of this system was that it showed that elements with related chemical properties were not close to each other when it came to weight, but appeared on the contrary at certain intervals in a periodic rhythm, at least when certain spaces were left open for supposed elements that had not yet been discovered. This was based on observations of the individual elements, but so far an explanatory hypothesis that could uncover the reason for the periodicity was not available. The system made it pos-sible, however, to predict the properties of the elements that had not yet been discovered, but which could fill the empty spaces in the system. Empirically the fundamental correct-ness of the system was indeed confirmed as new elements were discovered. The atoms were still regarded, however, as being indivisible and unchangeable. This view was not upset until the last part of the 19th century. In 1896 the French physicist Becquerel discovered that the heaviest of the elements known in nature, uranium, emitted radiation which affected a photographic plate. This radioactivity was investigated further by the French/Polish couple Curie who two years later found the special radioactive element which they named Radium. The radiation from the naturally active substances appeared to consist of several types and gradually it became clear that an atomic transformation took place in connection with the radiation so that one element was transformed into another. Hereby the old picture of the indivisible and unchangeable atom was shattered and completely new fields of research opened up. If one was to be able to explain the radiation and the transformation of elements, a completely new picture of the atom had to be advanced and perhaps it would also be possible in this way to reach an understanding of the circum-stances on which the periodic system was based. At least it was clear that something was going on within the atom which had until then been regarded as the smallest particle of matter. The first task consisted in analyzing the natural radiation from radioactive substances. The so-called alpha radiation appeared to consist of positively charged particles of the same weight as helium atoms while the gamma radiation was electromagnetic radiation with wavelengths that were even smaller than the radiation which the German physicist Rönt-gen had discovered in 1895. The third type, the beta radiation, consisted, however, of negatively charged particles that were smaller than any atom. The same particles had in 1897 been found in cathode rays by the British physicist J.J. Thomson, who was the head of the already then famous Cavendish laboratory in Cambridge, and these newly discovered part-icles were named electrons. It was a decisive breakthrough that the existence of particles smaller than the hydrogen atom could be established. The hydrogen atom had so far been the smallest particle that was known. Thomson thought that the electrons came from the inside of an atom which he imagined as a positively charged sphere that became electrically neutral owing to the negatively charged electrons inside it. This description appeared in 1904. In the German-speaking part of the world two things hap-pened which were of considerable importance to the further development of physics. The German physicist Max Planck presented his quantum theory in 1900 according to which energy that radiates from a hot body is emitted for each wavelength in certain minimum quantities which are in a constant, inverse proportion to the wavelength. Five years later Albert Einstein presented three of his basic theories, among them the relativity theory and a theory to the effect that light is not, as was assumed until then, a continuous wave propagation but consists of small light units, photons. Finally, a number of scientists, among them the Swedish physicist Rydberg, had worked on analyzing the spectra of the elements and the lines that appeared in these. Hereby it was demonstrated that the wavelengths of these lines could be determined by means of rather simple mathematical for-mulae. On the other hand an explanation of how the lines appeared was still not known. A number of the scientific breakthroughs in the years before Bohr's thesis have now been described. On the face of it, it was first and foremost Thomson's electron theory that had been of importance to him and when he went to England in the autumn of 1911 he first went to Cambridge to visit Thomson. However, it came to be another British physicist who had the greatest influence on Bohr's development - Ernest Rutherford, who was a professor in Manchester. Rutherford had worked with alpha particles and by doing so he had found a completely new atomic model, which was incidentally inspired among others by a Japanese physicist. It described the atom as a diminutive solar system which con-sisted of a heavy, positively charged nucleus around which the very small negatively charged electrons circled, fixed by electric forces. It was a revolutionary idea which could be met - and was met - with many objections. But it was the latest new development in the scientific world in 1911 when Niels Bohr came to England. An important characteristic of the development outlined here is that the discoveries were made in many countries and that the scientists of the various countries were inspired by and learnt from each other, perhaps to a greater extent than ever before in the history of science. This international co-operation in the field of natural science came to be particu-larly intense because of Rutherford and Niels Bohr. The Carlsberg Foundation, whose income comes from one of Denmark's largest breweries and which supports first and foremost Danish science, gave Niels Bohr a research scholar-ship for one year's studies in Cambridge. He arrived at this stronghold of physics in September 1911 bringing with him an English translation of his thesis. It was first and foremost J.J. Thomson whom Bohr wanted to meet, learn something from and discuss with. Thomson was friendly towards the keen young Dane whose English was at the beginning rather halt-ing; however, the discussions about the electron theory which Bohr had looked forward to never came. Thomson did not even get to read Bohr's thesis. Perhaps he was no longer interested in electrons. Nor did Bohr succeed in having his thesis published in English as had been planned and this was actually a serious lack in international scientific work for a number of years. His studies in Cambridge gave Bohr many useful impulses and acquaintances, however, even though it came to be a professor from Manchester that became most important to him. The professor was Ernest Rutherford who had in four years placed the laboratory at the Manchester university at the frontline of physics. After some initial contacts in the autumn of 1911 Bohr went to Manchester in March 1912 where he stayed until the end of July. In Manchester a center had been created for the research into radioactivity where a number of the best physicists of the time co-operated under the inspiring leadership of Ruther-ford. Bohr became attached among others to George Hevesy, a Hungarian whose considerable knowledge about chemistry became very useful for Bohr's further work. Later Hevesy also came to Copenhagen when Bohr's institute started its activities. Rutherford's atomic model that had been set up the year before had evoked interest but also skeptical objections. However, Bohr adopted it wholeheartedly as he realized that it constituted a basis for an overall theory concerning the properties of atoms. Such a theory would, however, involve a decisive break with classical physics. According to classical physics electrons that moved around the nucleus of an atom would inevitably emit light while at the same time the orbits became gradually narrower and at last the electrons would be absorbed by the nucleus of the atom. This was contrary both to the fact that the atoms were stable and to the nature of the light that was emitted and which was characterized by the line spectra. The preparatory work for what came to be known as Bohr's atomic theory was made already while Bohr stayed in Manchester, and the three papers in which the theory was published all appeared in England during 1913. During all of the work Bohr was in close contact with Rutherford who was full of enthusiasm for the genius of the young Dane while at the same time he admired Bohr's cheerful and at the same time very modest bearing. In 1914 Rutherford succeeded in having Bohr attached to his university as lecturer and he stayed in Manchester for two years. His friendship with Rutherford continued to develop and deepen and it was of the greatest personal and scientific importance to both of them. Bohr's atomic theory was that the electrons circle around the nucleus in certain so-called stationary orbits in which they can continue without losing energy. On the other hand, light is emitted when an electron jumps from a stationary orbit with high energy to another with low energy and the frequency, and thus the wavelength of this light can be expressed by means of a formula in which Planck's quantum concept forms part, and by the same means accordance with the empirically demonstrated formulae for the wavelengths of the spectrum lines can be shown. In the following years the theory was developed further to explain the relationship between the elements that had already appeared from the periodic system and Bohr continued to refine his atomic model. Bohr was first and foremost a theorist, and a very intuitive theorist at that, but he based his theoretical assumptions on experimental data that were available from scientific research from all over the world and the problems that constantly turned up gave him an opportunity to take an initiative to undertake or propose further experimental work. Contrary to many scientists who accept new and epoch-making results, Bohr never failed to emphasize both the limitation of the scope of the methods employed and the incompleteness of the basis on which the theory had come into existence. Any new result gave rise for him to point to new questions that arose. These were related not least to the paradoxes that had to arise when concepts and ideas from classical physics were used together with the new concepts about quanta. In this connection Bohr set up the so-called correspondence prin-ciple which briefly says that the new theory must be in accord-ance with classical physics when it is used on experience which classical physics has been able to account for in a satis-factory way. Or to put it in another way: any old and tested theory must be implied in the new and more extensive theory. This was of considerable importance for the thinking that was to advance modern physics. In 1911 Niels Bohr had become engaged to Margrethe Nørlund who was the sister of one of his fellow students and in 1912, shortly after his return from the first visit to England, they got married. They had an unusually happy and harmo-nious marriage and for more than fifty years Margrethe Bohr was her husband's indispensable and invaluable support in every respect. There was no post for Bohr when he returned home in 1912, but he continued on his own his work on the epoch--making papers on the atomic theory that were published in 1913 and besides he made use of the right bestowed on him by his doctoral thesis to give private lectures at the University of Copenhagen. The next year a lectureship was established for him, but this job involved many duties that were not in any way related to his actual field of research and in 1914 Bohr therefore accepted the offer from Rutherford, which has al-ready been mentioned, to become a lecturer at the university in Manchester. The university in Copenhagen granted him leave for a year and later extended his leave by yet another year, which meant that his new stay in England extended over two years. Already in 1911 there had been discussions about establish-ing a professorship for Niels Bohr, and scientists in Denmark were beginning to fear that he would stay abroad if reason-able working conditions could not be arranged for him in Denmark. In 1916 a professorship was then granted in theo-retical physics, expressly intended for Bohr, and this post he took up in the autumn of 1916. Even though also the years immediately after this gave evidence of great scientific productivity and Bohr published very important papers which continued the work on his atomic model, in particular the quantum theory for line spectra, he strongly missed an actual laboratory. In 1917 he applied to the State for funds for this purpose and at the same time private funds were collect-ed for the purchase of a site on Blegdamsvej. The building started in 1918 and with considerable support from the Carls-berg Foundation, the University's Institute for Theoretical Physics was ready for inauguration in March 1921. The estab-lishment of the institute had been recommended to the Carls-berg Foundation by Rutherford and the German nuclear physicist Arthur Sommerfeld. The latter emphasized in his statement the internationalism of science with the following: "It is my hope that in future scientists from all countries will get together for studies in Copenhagen and that they will pursue common cultural ideals in Bohr's Institute for Nuclear Physics". This wish came true. The Institute on Blegdamsvej formed part of the University of Copenhagen, but it would not have developed into such an important institution if considerable funds from Danish and foreign foundations had not been granted to it both for scien-tific equipment and for scientific scholarships to foreign students and scientists. Those who held the purse strings had confidence in Bohr's work and they were not disappointed. When Bohr celebrated his 50th birthday in 1935, money was even collected for a national gift which made it possible for the Institute to acquire half a gram of radium - which was at that time the only possible radioactive source of radiation - to be used at the Institute. The chairman of the committee was a prominent person, viz. the former Prime Minister Niels Neergaard who had besides been head of government at the time when the Institute started its activities. When one looks back at this time it seems remarkable that there was so wide support behind scientific work in a field that had a few years earlier been absolutely unknown and the immediate useful effect of which could at that time be difficult to understand. But it was a result of the attention that Niels Bohr's scien-tific efforts had aroused in the world outside Denmark and which could not but make an impression also in Denmark. National pride of being able to join the front row of the scien-tific advances was quite natural. A first culmination was experienced when Niels Bohr was awarded the Nobel Prize in physics for the year 1922 - the year after Einstein. It was the first physics prize that was awarded to a Danish scientist. Since then a large number of scientists from various countries who had spent a short or a long period of time at the Institute in Copenhagen came to follow in Niels Bohr's footsteps as Nobel Prize winners. This was no coincidence. The award of the Nobel Prize was an honor, but not only that. It would also mean something for the future working conditions, Bohr stated in a letter of thanks to Rutherford who had sent him his warmest congratulations. In connection with the celebration in Stockholm when the prize was presented to Bohr he gave the obligatory lecture - naturally on the subject: the Structure of the Atom. This became a kind of stocktaking which in a way was the end of the first pioneer period that had started with Rutherford's discovery of the nucleus of the atom and Bohr's theories concerning the orbits of the electrons. Bohr made no secret of the fact that there was still much left on which to carry on research - he never came to belong to the group of scientists who think that their own contribution means the final coping stone of the development - and he pointed in particular to the unsolved questions concerning the inner structure of the nucleus even though Rutherford's most recent experiments were paving the way for an investigation of this. The lecture could be concluded with an interesting piece of news. At home in Copenhagen Hevesy had started looking for element number 72 that had so far been unknown. Quite apart from the fact that there was the task of filling the gaps in the periodic system, it would serve to confirm Bohr's the-ories if a newly discovered element appeared to correspond to what he had by means of theory been able to predict. On the evening before the lecture Bohr received a telegram stating that the element that had been missed for so long had now been demonstrated and therefore he could inform his au-dience that element number 72 with the predicted chemical properties had been discovered. The element was named after the city in which it was discovered: Hafnium. Because of this the as yet young Institute had made a name for itself in the world of physical science. But in a way this was the last result that was more or less easy to understand for the public. The further work inevit-ably became more and more unintelligible to people other than specialists as the results appeared to be impossible to present in simple, explanatory pictures. The Rutherford/Bohr model of the atom as a diminutive solar system could be rendered intelligible even though it was probably clear that the concepts employed in this connection were bound to cover a form of reality different from the one that was known from the everyday world. But later, abstract concepts had to be used that could not even have any direct meaning in the reality experienced by the senses. Already in his lecture in Stockholm, Bohr in fact warned his audience against believing that the knowledge acquired could be regarded as an explanation of the atom if the word was used in its usual meaning. It had just been possible to classify extensive fields of observation and by means of predictions it had been possible to pave the way for a comple-tion of this classification. The task of the Institute was the further development of the theory and a large number of talented young scientists from different parts of the world contributed to this task. Bohr had taken Cambridge and Manchester as a model for the open scientific environment with an animated exchange of ideas and it was only natural that there would be the most lively interaction with institutes from other places in the world. Perhaps the connection with the Cavendish laboratory in Cambridge, which Rutherford headed from 1919, came to be the most important and owing to the close friendship between Bohr and Rutherford from the time when Bohr was in Manchester it was also the warmest relationship. But as a citizen of Denmark, which had been neutral during the First World War, Bohr had a special opportunity for establishing relations to scientists in defeated Germany who could not as quickly re-establish the connections of earlier times with British colleagues. The scientists in Göttingen, Munich and Berlin quickly joined the circle at the Institute in Copen-hagen. Lise Meitner's early visit became the beginning of a co-operation that in 1939 was to become of importance in world history. The list of foreign scholars and guests is very long. Most of the nuclear physicists who made an effort in the period be-tween the two world wars spent some time in Copenhagen. And Bohr met the rest of them on his lecture tours abroad or at the famous Solvay conferences in Belgium that were a meeting place for a true constellation of physicists of genius. Atomic science was international and no one did more than Bohr to emphasize and promote this fact. But as already mentioned it became increasingly difficult to make the results accessible to the broad public as it was no longer possible to transform the results into simple pictures; instead they had to be expressed in abstract, mathematical formulae. "It's mind-boggling", a young physicist com-plained. "If it doesn't boggle your mind, you understand nothing" replied Bohr. Of course, valuable experiments were carried out at the Institute in Copenhagen, but it was in particular the discus-sions and the sharing of thoughts that created the Copen-hagen contribution to the theoretical development, and Niels Bohr was the soul of the unusually fruitful intellectual en-vironment. All the memories published about the years of study in Copenhagen, and they are extensive in number, give evidence of what Niels Bohr's personality meant to both Danish and foreign participants in the work. "Meeting Niels Bohr was the greatest experience in a lifetime for a young physicist" it has been said and unanimously one account after the other tells of Bohr's openness towards talents, of his human warmth and of his responsiveness to new thoughts. When he encountered valuable scientific contributions he was all enthusiasm and constantly interrupted and objected - "not in order to criticize, only to teach" he repeated again and again. If his reaction was a "how interesting, how very interesting indeed" there was, on the other hand, reason to be worried. It meant that he regarded the contribution in ques-tion as being rather unimportant - but he did not have the heart to express it in that way. One gets a strong impression of a warm and cheerful en-vironment where the co-operation concerning work and studies resulted in friendships and comradeship. It is charac-teristic that there are a lot of photographs of the circle related to the Institute showing the members in their spare time - bathing, playing tennis, skiing, motorbiking, at Bohr's sum-mer house - and Bohr was always at the front. He was an excellent and keen skier, an enthusiastic yachtsman - and his desire to try everything that was new, for instance a motor-cycle, was enough to give grey hairs to those with a greater sense of responsibility. As a further indication of the spontaneous atmosphere at Blegdamsvej it should be mentioned that at the annual con-ference of physicists there was also time for cheerful, cabaret like features - in 1932, for instance, a Faust parody in which a number of the great physicists of the time were presented as Goethe's characters and the current scientific subjects for discussion were parodied. Der Herr - that is God - spoke like Niels Bohr in a mixture of German and English and some characteristic Danicisms that developed at the cosmopolitan institute. Actually this is still great fun - and at that time nobody enjoyed himself more than Niels Bohr. It is this combination of cheerfulness and scientific endeavors that is called the "spirit of Copenhagen" in the memories and with which everybody credits first of all Niels Bohr himself, if Margrethe Bohr is not mentioned in the same breath. She was an invaluable support to her husband in a long and unusually happy marriage that made an impres-sion on everybody who met the couple. During the 1920s it became clear that many phenomena in modern physics could be viewed from two mutually conflict-ing viewpoints. For instance, it turned out that light that had been acknowledged as a wave propagation could also be regarded as particles, photons, and this acknowledgment, which originated in Einstein's discoveries as early as 1905, was supplemented by the acknowledgment that atomic part-icles could, on the other hand, possess wave characteristics. Generally, the conclusion was that waves can possess particle characteristics and particles can possess wave characteristics. This meant that a complete description required the use of two mutually contradictory images and this Niels Bohr named complementarity - the characteristics related to the two images are complementary. He did this for the first time in a speech he held in 1927 which he referred to later saying among other things the following: "It is of the greatest im-portance to realize that an account of all experience - ir-respective of how far the phenomena are from the scope of classical physics - must be expressed by means of classical concepts". This was bound up with the fact that experiments and the experimental devices employed as well as the results had to be described in the usual language with appropriate use of the terminology of classical physics. "This implied", con-tinued Bohr, "the impossibility of a sharp distinction be-tween the behavior of atomic objects and their interplay with the measuring instruments that serve to define the con-ditions under which the phenomena occur". And: "Ex-perience gained under different experimental conditions can-not, therefore, be combined in a single image, but must be regarded as complementary in the sense that only together do the phenomena exhaust the possibility of obtaining informa-tion on the objects". At the beginning Bohr's concept of complementarity met with some resistance, first and foremost from Einstein who, contrary to Bohr, was of the opinion that it must be possible to find an unambiguous explanation of causality of the atomic phenomena and who had in particular difficulties in accepting that it would not be possible to employ the general principle of causality, but that one had to stick to statistical probabilities. It has been related that Einstein said: "Do you really think God plays with dice?" to which Bohr is reported to have answered that one should perhaps be wary of at-tributing qualities to Providence that could be expressed in ordinary language. Since then complementarity has become a permanent component of the conceptual repertoire of modern physics, and Bohr also extended its use to other branches of science, in particular psychology and biology, and he saw this concept as an aid to obtaining greater mutual understanding between cultures and between nations. Also the position of the in-dividual human being in society was in his opinion in-fluenced by complementarity, for instance when the two ideals of justice and mercy were set up in opposition to each other. The Greek tragedians had known and shown this. Again and again Bohr repeated: "Here we are faced with complementary phenomena related to the human situation which, in an unforgettable way, is expressed in ancient Chinese philosophy which reminds us that in the great drama of life we are both actors and audience". During all the rest of his life Niels Bohr continued, concur-rently with all his other extensive activities, to elaborate and refine his considerations concerning the concept of com-plementarity. This resulted in a large number of important papers that were characterized by his constant efforts to reach greater clarity and explicitness with regard to the philosophi-cal system that is most central for all knowledge. From the very first it had been understood that Rutherford/Bohr's atomic model implied the presence of very large quan-tities of energy in the individual atom and authors of science fiction began quickly to talk wildly about the release of nuclear energy. The scientists were very careful even though Rutherford had at an early point in time said that "some fool or other in a laboratory could unintentionally blow up the universe". Scientists were concerned with investigating the atom, not with releasing its energy. To put it roughly one can say that the great breakthroughs in nuclear physics until around 1930 took place in "outer space" - the world of the electrons - whereas most of the secrets about the nuclei of atoms have been uncovered since that time. It was true that Rutherford had earlier predicted that the nucleus must consist of both positively charged and neutral particles, but it was not until 1932 that his associate James Chadwick found the neutral particles that were named neutrons. A number of inventions made in the following years made experiments possible in which the nuclei of the atoms were, so to speak, bombarded with various particles, which caused the transformation of elements. Some of the elements that were created in this way were radioactive isotopes (i.e. elements with the same positive charge, but a different weight) of the elements occurring in nature which were quickly transformed into other, stable elements, but not all of the elements that were produced could be identified immediately. Tentatively it was suggested to speak of "transuranic" elements, i.e. ele-ments with a higher atomic number than uranium, which is number 92 in the periodic system, and since then a number of transuranic elements have actually been identified. The Italian physicist Enrico Fermi had begun using bom-bardments with neutrons whose speed was reduced by means of paraffin or water and it turned out that these neutrons could much more easily be captured by the nucleus of an atom than could particles at higher speeds. This was quite unexpected, but it caused Bohr to develop an ingenious theory about the composition of the nucleus, which also gave an extensive explanation of why different elements behaved in different ways when they were bombarded; something for which there had until then been no explanation. In some cases the atom of an element that absorbed a neutron was transformed into an atom having the atomic number just below, in other cases this did not happen. In one of Fermi's experiments, an element that had been produced through bombardment of uranium remained unidentified for a long time until Otto Hahn in Berlin repeat-ed the experiment in 1938 and found that the element had a strong resemblance to barium - but it could not be barium for had it been that element the atoms of the uranium would have been split into much lighter atoms, which everybody thought was impossible. Disappointed Otto Hahn told of his unsuccessful work in a letter to his earlier colleague Lise Meitner who had to take refuge in Sweden. The letter arrived just around Christmas when Lise Meitner was to be with her nephew Otto Frisch, who worked with Bohr. It end-ed up as an unusual Christmas holiday. Meitner and Frisch began making some calculations to solve the problem and they became convinced that it was really a case of uranium fission - into barium and crypton - and that the fission must have been accompanied by radiation of energy even though this had not been observed during the experiment. Frisch returned to Copenhagen with the calculations in order to present them to Bohr. Niels Bohr was just on the point of leaving for a long stay in the USA, but Frisch succeeded in talking with him for a few minutes - and Bohr was all enthusiasm. He was con-vinced immediately. Frisch and Meitner must write a thesis on the epoch-making discovery right away. It was important that it became known as their contribution. Frisch and Meitner called the splitting of the nucleus "fis-sion". This came to be one of the most important new words in the coming decades and the knowledge of this word spread as quick as lightning. The scientific world was still interna-tional - Fermi's experiment in Rome had inspired the couple Joliot-Curie in France, Otto Hahn had carried out the chemi-cal analyses of the results in Berlin, Lise Meitner and Otto Frisch found the explanation in Sweden, Niels Bohr gave it his approval in Copenhagen, the thesis was to be published in England and even before this had happened American scien-tists had started research within the same field. It was due to a misunderstanding that this piece of news came out so quickly. One of Bohr's associates had been told about it by Bohr himself and had passed it on to American colleagues in the belief that it had already been published and the research laboratories went to work right away. In the course of a few days atoms were split at several places in the USA and at the same time release of energy had been ascertained. Quite remarkably, warnings were voiced immediately against over-hasty hopes. It was thought that the process of releasing the energy of the atom would require more energy than would be produced. Among the future perspectives that were thought then to be unreasonable were "that in the near future it will be possible to have large ships sail by means of energy from atoms... "atom crushers" as energy sources instead of steam or electric plant . . . or superexplosive mat-ter or war weapons". This was in January 1939. When the American press got wind of the matter it became in spite of everything front-page news . . . as an American discovery because it had not been allowed to leak out in Copenhagen. Fermi, who was now in the USA, also forgot to give Frisch and Meitner their due at first. These were perhaps signs that the time of open science internationally was com-ing to a close. There was still some way to go to the release of nuclear energy and to an explanation of how the fission of uranium actually occurred. While staying in the USA Bohr took an important step forward as on the basis of the image of the nucleus of an atom that he had earlier created he was able to demonstrate that it was only a small part of the nuclei of ura-nium that was split. The most frequently occurring uranium isotope, 238U, catches the slow neutrons during the bombard-ment without any fission occurring, while the rare isotope 235U is split. The fission that had been observed during the experiments occurred only in the one per cent of 235U that was to be found in ordinary uranium. Experimentally this was not proved until one year later. In connection with the splitting of 235U, two neutrons were released which could, after their speed had been reduced on account of their collision with suitable, light nuclei of atoms, cause fission of other 235U atoms, but when 99 per cent of the atoms were the more stable 238U atoms the likelihood that it would happen was extremely slight. Statistically it was certain that the process would stop by itself when the outer bombardment stopped. It would be quite different if the 235U isotope could be isolated. In this isotope a chain reac-tion would occur in that the fission of one atom would bring about fission of two more atoms and these could cause the fission of four atoms and so on. If, therefore, pure 235U could be produced, a basis was created for making the stron-gest bomb that had ever existed. In the middle of March 1939 the Germans occupied the last parts of Czechoslovakia and the danger of a world war clearly became more imminent. This made the discovery of fission energy fateful. German scientists knew just as much as scientists in all other countries, and any public announce-ment concerning the future of the research would benefit all countries equally. Therefore, an attempt was made in the USA to stop any further publishing, something that was com-pletely contrary to the practice that had characterized the development of nuclear physics until then, and for some time it was also in vain. Bohr did not feel any sympathy for scien-tific censorship, either. He did not believe it could be of any use for the laws of nature could not be hidden - and futile attempts at keeping them secret would corrupt both the polit-ical and the scientific climate. He came to revert to this issue under quite different circumstances. Besides, it was thought at this time that the necessary precondition for manufacturing an atom bomb - isolation of the uranium isotope 235U - could not for technical reasons be brought about within a foreseeable period of time. Bohr was of the same opinion even though he expected that the problem would be solved as time went on, and from a politi-cal point of view it was more important that Winston Chur-chill (and his scientific adviser Dr. Lindemann) claimed al-ready in August 1939 that the threat of the atom bomb could be ignored. Apparently - but this was not known in other countries - German experts had in advance rejected the possi-bility that Germany could on its own cope with the task of isolating 235U. Nonetheless the new discoveries led to research in
the field of nuclear physics in all warfaring and belligerent countries
becoming subject to military and political interest, and this resulted
on the one hand in heavily increased grants, on the other hand it made
the results of the research military secrets. The basic research was directed
at a specific goal and the free international exchange of new knowledge
stopped.
After the occupation of Denmark on April 9, 1940, the Institute stopped being a place of learning for physicists from all over the world; however, work at the Institute continued though it was now isolated and without contact with Bohr's friends and students from all over the world. But what hap-pened now in England and the USA was initiated by Bohr's theories. The transuranic elements neptunium (No. 93) and plutonium (No. 94) that Bohr had predicted were discovered, and Otto Frisch, who had come to England, prepared together with Peierls a memorandum about the possibility of manufacturing a bomb on the basis of 235U - based on Bohr's fission theory. In reality this memorandum contained so to speak the whole basis for the atom bomb. When at the same time it became clear that there was a race for getting hold of the Norwegian stock of heavy water that could be used for reducing the speed of neutrons in order to achieve a chain reaction in uranium, the authorities immedi-ately took up a different attitude to the research into nuclear energy. Both England and the USA went full speed ahead with the work. Without Bohr's earlier scientific efforts it would not have happened so quickly. Also in Germany some of Bohr's old associates worked on the problems relating to fission, first and foremost the Nobel Prize winner Werner Heisenberg. He knew more than any-body else in Germany about what Bohr had worked on and he had been close to Bohr during a three-year stay in Copen-hagen. In the autumn of 1941 Heisenberg visited Bohr, but under the new circumstances the old intimacy could not be revived. None of the parties dared to talk freely about what they were engaged in. Heisenberg has stated later that he tried to indicate that Germany would not be able to produce atom-ic weapons and that therefore other governments ought to discontinue their experiments. Bohr, on the other hand, got the impression that the Germans were working intensely on the problems. Probably they did, but perhaps it was rather with a view to using nuclear energy for heat production. The conversation seems to have left both Heisenberg and Bohr very unhappy. Bohr was in a very exposed situation in occupied Denmark and at the beginning of 1943 he received the first letter from England via illegal channels. The Nobel Prize winner, James Chadwick, who had discovered the neutron, gave Bohr to understand that he would be very welcome if he were to leave Denmark. Bohr realized that he was in the Germans' search-light, but he was not yet ready to leave his Institute and his associates there, among whom were Jewish refugees. Not until the end of September was the situation so critical that Bohr and his wife felt compelled to take refuge in Sweden. From here Niels Bohr was flown to England in an English "Mosquito". It was a dangerous expedition - the small un-armed plane was to fly through airspace that was controlled by the Germans - and it was also difficult because the bomb--bay was the only place where there was room for a passenger. As, moreover, the oxygen supply failed Niels Bohr had a narrow escape in getting to England alive. In England Bohr was presented with all the research achievements and the technical progress that had been made during the three years when Copenhagen had been isolated from scientific work in the rest of the world. The atom bomb had moved within reach, not least after scientists had in December 1942 succeeded in producing a controlled chain reaction in a reactor in Chicago. Research had been under-taken both in the USA and in England, but the construction of the bomb was to take place in the USA where the technical and economic conditions were better and where there was no risk of damage during air bombardments. After a few months' stay in England, Niels Bohr and his son Aage went to the USA to take part in the final work. At home in Copenhagen the Germans had occupied the Institute, but obviously without any definite plan. Heisen-berg had a trip to Denmark arranged and after having visited the Institute he declared that the work of the Institute was of no military interest, and after some time this made the Ger-mans give up the occupation of the buildings. In the mean-time the Institute had been threatened from other quarters as the German occupation had made certain sections of the resistance movement believe that the Institute was of so great importance to the Germans that it ought to be blown up. Fortunately, an inquiry was sent to Bohr as to whether it was necessary to blow up the Institute and his urgent request for leaving the Institute in peace reached Denmark in time. The hush-hush world that Bohr entered already in England and which continued to surround him in the USA was not his natural element. It formed a glaring contrast to his warm openness. However, the demands of the war made it inevita-ble to attempt keeping Bohr's presence in England and the USA a secret. If the Germans learnt that Bohr stayed in the USA this would be a too obvious indication of what the Allies were doing. It could only mean that they were working intensely on some atom project of importance to the war. It remains a question, however, for how long Bohr's stay was actually kept a secret, for he was not unknown and his characteristic figure could be recognized by many - not only by anyone related to the circle of physicists. When he arrived at the large atomic research plant his anonymity became quite illusory for most of the people he was to work with were old friends and acquaintances, several of them his students from Copenhagen. Admittedly, the plant itself was hermeti-cally sealed off from the surrounding world. Bohr is reported to have said: "They did not need me in order to make the atom bomb". He was the one who had before the war provided the theoretical basis, but what was left was by and large merely the technical side of the question. His importance came to be of quite a different sort. It showed itself in the atmosphere among the scientists and as at the Institute on Blegdamsvej it was due to his warm sense of humanity that influenced anyone he came into contact with. Quite significantly he quickly became everybody's "Uncle Nick" - his cover name was Nicholas Baker. But the subject that he was first and foremost concerned with and which came through him to occupy many, perhaps most of the scientists who worked on the atom bomb, was characterized by anything but cosiness: it was the responsibil-ity of science and the political implications of the bomb. The work on the atom bomb had, like so many military research programmes, been initiated in fear - fear that the enemy was doing the same. But on the way this fear was replaced by new fear - the fear of the terrifying effects of the new weapon. Was it in reality the destruction of civilization the scientists were working on? The scientists who worked on the bomb could not shake off these thoughts and poured out their worries to Bohr. He understood the problem and shared the responsibility. Without him the imminent completion of the atom bomb would hardly have been brought about. But now there was no return. The fight against nazism had to be brought to a close and if Hitler came in possession of an atom bomb before the victory was won there would no longer be any hope. If this happened, worldwide slavery could be expected. But at the same time Bohr saw a different, lighter perspec-tive. The outcome of the research that had been accelerated for military reasons could also be that the coming peace could be based on something better than before. If the new forces were made available to humanity in a spirit of friendli-ness, objectivity and co-operation, a better civilization and a better peace could be envisaged. On the other hand, developments involved a danger of an excessive arms race unless real security was created through a general agreement based on confidence. It was the Soviet Union which had not been initiated in the Anglo-American atom co-operation that was the problem. Endeavors were made to keep the Soviet leaders in ignorance, but nobody knew whether their scientists had started investigating fission energy on their own. Even though the enormous hardship that the war imposed on Soviet society made it for a time impossible for the Soviet Union to produce an atom bomb, couldn't it be expected to happen soon after the war was over? Bohr tried to awaken an understanding for these problems
both in the USA and in England and in both countries some of the high-ranking
officials and politicians accepted his thoughts: England and the USA should
show openness towards the Soviet Union with regard to the atom bomb for
only by this means could mutual confidence be created, which would render
international control possible.
A conversation with Roosevelt some months later took a more favorable course and Bohr got the impression that the President would attempt to make Churchill change his mind. However, nothing came of it, on the contrary. The British Prime Minister regarded Bohr as a security risk who should be carefully watched over, and perhaps the best would be to arrest him. Fortunately, there were trusted scientists both in England and in the USA who could with sufficient firmness refute this to the two heads of state. Even Churchill probably understood at last that Bohr's loyalty was above suspicion and that he had not "by accident come to know too much" - he had known it all before anybody else. However, the central issue - the desire for greater openness that could render inter-national confidence and international control possible - remained a lost case. Even though many others followed up Bohr's initiative, the incredibly shortsighted policy of secrecy continued. Perhaps Roosevelt was about to change his mind shortly before his death, but this did not change anything. One of the reasons why two bombs were dropped was that they differed in type. The Hiroshima bomb was based on 235U, the Nagasaki bomb on the transuranic, artificially produced element plutonium. In both cases the preliminary theoretical work was largely due to Niels Bohr. Therefore, the mushroom clouds over the two Japanese cities were of special importance to Bohr, who on his way home from the USA had reached London. A period had come to a close and even though it had happened in a horrible way the accomplished fact prepared the way for new hope. Until then only a narrow circle of scientists had known that a new force existed as a latent threat against all known forms of society. After Hiroshima it must be possible to make it clear to everybody. Niels Bohr presented the problem soon after - in an article that was published in London on August 11 and already the next day a translation somewhat characterized by haste ap-peared in the feature article of the Danish newspaper Politiken. Some quotes can illustrate his thoughts: "Civilization is faced with a challenge, more serious than any challenge we have ever been faced with before and the fate of humanity will depend on its ability to unite with a view to avoiding the common dangers and jointly reap the fruits of the immense possibilities that scientific progress has to offer". "For a long time scientists have regarded themselves as a brotherhood that works in the service of common human ideals. In none of the fields of science has this doctrine been emphasized more strongly than within the field of atomic research which at this very time brings about consequences of such an overwhelming practical importance". "The world society of physicists was so to speak welded together into a team, which made it more difficult than ever before to separate the individual contributions from each other". "The security that is offered to citizens in a nation by col-lective defence measures is absolutely insufficient. Perhaps no defence is possible against the new destructive forces and the prevention of any use of the new energy sources which does not serve humanity as a whole depends on worldwide co-operation". "Such measures will of course demand removal of barriers that have so far been regarded as necessary to protect nation-al interests but which now stand in the way of common secu-rity against unprecedented dangers". "All of the scientific community will no doubt unite in a strong effort to bring about in broader circles a sufficient understanding of what is at stake and to appeal to humanity as a whole to listen to the warning that has been voiced". In this article Niels Bohr both looked back at the happy international brotherhood of scientists that had created the progress - the progress that had now turned out also to involve the most serious dangers - and forward, further than he could know. He used the strong expression "the present crisis of humanity" and he did so deliberately at the very moment when the Second World War had finally been brought to a close. It is not known whether Churchill read the English version of the article. He had been defeated in the Parliamentary election of that summer and Clement Attlee had taken over as Prime Minister. Both Attlee and Truman, Roosevelt's suc-cessor, sent out vague assurances to the effect that nuclear energy would be used for the benefit of all humanity, but they still wished to preserve the secrecy. Bohr continued for the rest of his life being concerned with this problem. Now and then there seemed to be hope of an international understanding according to the lines he had indicated, but this hope was subsequently extinguished. After the Soviet Union had itself solved the practical problems relating to the atom bomb, there was not much to hope for. The atomic arms race had already started. The last major initiative taken by Bohr was his open letter to the United Nations dated June 9, 1950. It is a document of such clarity and of such historic value that it should not be quoted here but red in its entirety (see later). Unfortunately its value is not due to its having an effect. Outside the Scandinavian coun-tries it was hardly noticed by anybody other than the scien-tists who were influenced by the same worry. The statesmen who apparently decide the fate of the world paid no attention to it. The little effect of the letter did not silence Niels Bohr. He continued emphasizing the importance of these questions and he took them as a basis for his endeavors that Denmark should continue to play a part in the exploration of nuclear physics. If the large countries could not be prevented from carrying out research motivated by military reasons, the small countries had to stake even more on the peaceful utilization of nuclear energy and on pure research. At the end of August 1945, Niels Bohr returned home to Denmark and on August 25 he came back to his Institute after two years' absence - on bicycle, as always. During the first ten years of the Institute's existence his family had lived at the institute, but since 1931 it had been Carlsberg's stately honorary residence that had been its home. Already at that time his position as Denmark's most distinguished scientist had been established and now at his return he was indisput-ably Denmark's most prominent citizen. In spite of this he arrived on bicycle for his first workday after his stay abroad. He was still the modest human being that he had been in his youth. The number of honors he received was large, already long before the war: scientific medals, the award of honorary doctorates, membership of foreign scientific academies and associations, the meritorious service medal in gold on the 25th anniversary of his thesis on atomic theory. This continued abundantly for the rest of his life. Resumption of his work at the Institute must in one respect have felt slow and less satisfactory as the conditions after the long war made it difficult to receive very many foreign scho-lars in Denmark right away. In another respect something was quickly set in motion, however, as the development of nuclear energy made it urgently necessary to carry on research as regards its peaceful utilization and to educate young scientists for this work for Danish society. Plans were made for an extension of the Institute and the necessary grants were provided, as before both from the Danish State and from Danish and foreign foundations. Copenhagen be-came once again an international center of atomic research. As described in the previous section, Niels Bohr was during these years mainly preoccupied with the question of the con-sequences of the atom bomb for the development of world politics and he never tired of demanding an open world with international scientific co-operation. Politically the results were meager, but on both a Scandinavian and a European basis an extended scientific co-operation was initiated with Niels Bohr as a primary driving force. When the Danish Atomic Energy Commission was estab-lished in 1955 it was the most natural thing to appoint Niels Bohr chairman and in his capacity as chairman of this com-mission he was in charge of the establishment of the ex-perimental plant at Risø to which was added a department of the University's Institute for Theoretical Physics. As Prime Minister Viggo Kampmann has written in an account of his co-operation with Niels Bohr, the latter spent quite a con-siderable part of the last 10 years of his life on getting into shape the work relating to peaceful utilization of nuclear energy in Denmark. His good connections to physicists and nuclear energy administrators throughout the world, not least in the USA, was of great help in this connection. It was not all politicians, however, who could appreciate the necessi-ty of a costly Danish research programme in this field and Niels Bohr had to spend a lot of time and energy on convinc-ing the public of the need for staking so much on scientific research. While the work in connection with construction of the Risø plant was being carried out, Niels Bohr received, as the first, the Ford Foundation's prize for the peaceful utilization of nuclear power - in addition to an amount of money also the medal Atoms for Peace. Also on this occasion - in 1957 - he spoke of the responsibility that went with scientific progress. Another medal had been instituted by the Danish Society of Chemical, Civil, Electrical, and Mechanical Engineers in 1955 for Niels Bohr's 70th birthday - the Niels Bohr medal. He was himself the first to receive it and since then it has been awarded every third year to "a person who has made an ex-traordinary effort for the use of nuclear physics in the service of humanity" - often to scientists who had belonged to the circle around Niels Bohr. On his 70th birthday Niels Bohr also received the Grand Cross of the Order of the Dannebrog with diamonds. Earlier, in 1947, he had received the Order of the Elephant, which is only at many years' interval awarded to Danes who are not members of the Royal family and thus he had received the highest honors that could be obtained in Denmark. For the coat-of-arms which was to be placed among coats-of-arms of other Knights of the Order of the Elephant in the chapel at Frederiksborg Castle, Niels Bohr chose the Chinese symbol of Yin and Yang, the two opposite elements that supplement each other, and which together describe the whole world. As his device he chose Contraria sunt complementa, the oppo-sites are complementary. Niels Bohr's unique position in Danish society was
due not only to his scientific contributions, but also to a great extent
to his wide range of commitments and his humbleness of heart when it came
to tasks in society that had to be solved. For 27 years he was chairman
of the Cancer Committee, for 23 years he was president of the Royal Danish
Academy of Sciences and Letters and he had time for a wide range of committees.
In all of these situations he contributed as much as he was capable of.
He did not engage in anything without feeling deeply committed to his various
obligations.
"Even though, as emphasized at the very beginning, the question of what we mean by Danish culture offers many aspects that are inextricably bound up with each other, the view of the community among peoples that has developed throughout the whole of our history may be said to be the most typical characteristic of our culture. Our very demand for and responsibility towards world citizenship as we see it show us our tasks internally and externally and, without making any comparison that would be far removed from this view, we may feel proud of the way in which we have utilized our conditions for our own development and for participa-tion in the co-operation for the advancement of human cul-ture. What fate has in store for us and for others is hidden from our view, but however far-reaching the consequences to all aspects of human life of the crisis that the world is experiencing will be, we have a right to hope that our people will, if only we preserve the freedom to develop our deeply rooted attitude, also in the future be able to serve the cause of hu-manity in an honorable way". Niels Bohr died in 1962, 77 years old, active to the very last. Commemorative words were uttered by the Danish Prime Minister, Jens Otto Krag, in the Danish Parliament and in the United Nations in the following days. What was most in the spirit of Niels Bohr was probably a letter from the Danish Minister for Foreign Affairs, Per Hækkerup, to the Secretary -General of the United Nations, U Thant, in which not only thanks were expressed for the many expressions of sympathy that had been shown but which contained also a reminder of Niels Bohr's open letter to the United Nations in 1950. Perhaps there would today, the letter said, be more fertile soil for many of these thoughts than there had been in 1950. The issue that was very near to Niels Bohr's heart was not allowed to die with him. Open Letter
to the United Nations
I address myself to the organization, founded for the purpose to further co-operation between nations on all problems of common concern, with some considerations regarding the adjustment of international relations required by modern development of science and technology. At the same time as this development holds out such great promises for the im-provement of human welfare it has, in placing formidable means of destruction in the hands of man, presented our whole civilization with a most serious challenge. My association with the American-British atomic energy project during the war gave me the opportunity of submitting to the governments concerned views regarding the hopes and the dangers which the accomplishment of the project might imply as to the mutual relations between nations. While pos-sibilities still existed of immediate results of the negotiations within the United Nations on an arrangement of the use of atomic energy guaranteeing common security, I have been reluctant in taking part in the public debate on this question. In the present critical situation, however, I have felt that an account of my views and experiences may perhaps contribute to renewed discussion about these matters so deeply influenc-ing international relationship. In presenting here views which on an early stage impressed themselves on a scientist who had the opportunity to follow developments at close hand I am acting entirely on my own responsibility and without consultation with the government of any country. The aim of the present account and consider-ations is to point to the unique opportunities for furthering understanding and co-operation between nations which have been created by the revolution of human resources brought about by the advance of science, and to stress that despite previous disappointments these opportunities still remain and that all hopes and all efforts must be centered on their realization. For the modern rapid development of science and in particu-lar for the adventurous exploration of the properties and structure of the atom, international co-operation of an un-precedented extension and intensity has been of decisive importance. The fruitfulness of the exchange of experiences and ideas between scientists from all parts of the world was a great source of encouragement to every participant and strengthened the hope that an ever closer contact between nations would enable them to work together on the progress of civilization in all its aspects. Yet, no one confronted with the divergent cultural tradi-tions and social organization of the various countries could fail to be deeply impressed by the difficulties in finding a common approach to many human problems. The growing tension preceding the second world war accentuated these difficulties and created many barriers to free intercourse be-tween nations. Nevertheless international scientific co-operation continued as a decisive factor in the development which, shortly before the outbreak of the war, raised the prospect of releasing atomic energy on a vast scale. The fear of being left behind was a strong incentive in vari-ous countries to explore, in secrecy, the possibilities of using such energy sources for military purposes. The joint American-British project remained unknown to me until, after my escape from occupied Denmark in the autumn of 1943, I came to England at the invitation of the British government. At that time I was taken into confidence about the great enterprise which had already then reached an ad-vanced stage. Everyone associated with the atomic energy project was, of course, conscious of the serious problems which would con-front humanity once the enterprise was accomplished. Quite apart from the role atomic weapons might come to play in the war, it was clear that permanent grave dangers to world secu-rity would ensue unless measures to prevent abuse of the new formidable means of destruction could be universally agreed upon and carried out. As regards this crucial problem, it appeared to me that the very necessity of a concerted effort to forestall such ominous threats to civilization would offer quite unique opportunities to bridge international divergences. Above all, early consul-tations between the nations allied in the war about the best ways jointly to obtain future security might contribute deci-sively to that atmosphere of mutual confidence which would be essential for co-operation on the many other matters of common concern. In the beginning of 1944, I was given the opportunity to bring such views to the attention of the American and British governments. It may be in the interest of international under-standing to record some of the ideas which at that time were the object of serious deliberation. For this purpose, I may quote from a memorandum which I submitted to President Roosevelt as a basis for a long conversation which he granted me in August 1944. Besides a survey of the scientific back-ground for the atomic energy project, which is now public knowledge, this memorandum, dated July 3rd, 1944, con-tained the following passages regarding the political conse-quences which the accomplishment of the project might imply: It certainly surpasses the imagination of anyone to survey the consequences of the project in years to come, where in the long run the enormous energy sources which will be available may be expected to revolutionize industry and transport. The fact of immediate preponderance is, however, that a weapon of an unparalleled power is being created which will completely change all future conditions of warfare. Quite apart from the question of how soon the weapon
will be ready for use and what role it may play in the present war, this
situation raises a number of problems which call for most urgent attention.
Unless, indeed, some agreement about the control of the use of the new
active materials can be obtained in due time, any temporary advantage,
however great, may be outweighed by a perpetual menace to human security.
In this connection it is above all significant that the enterprise, immense as it is, has still proved for smaller than might have been anticipated and that the progress of the work has continually revealed new possibilities for facilitating the production of the active materials and of intensifying their effects. The prevention of a competition prepared in secrecy will therefore de-mand such concessions regarding exchange of information and openness about industrial efforts including military preparations as would hardly be conceivable unless at the same time all partners were assured of a compen-sating guarantee of common security against dangers of unprecedented acuteness. The establishment of effective control measures will of course involve intricate technical and administrative problems, but the main point of the argument is that the accomplishment of the project would not only seem to necessitate but should also, due to the urgency of mutual confidence, facili-tate a new approach to the problems of international relationship. The present moment where almost all nations are entangled in a deadly struggle for freedom and humanity might at first sight seem most unsuited for any committing arrangement concerning the project. Not only have the aggressive powers still great military strength, although their original plans of world domination have been frustrated and it seems certain that they must ultimately surrender, but even when this happens, the nations united against aggression may face grave causes of disagreement due to conflicting attitudes towards social and economic problems. By a closer consideration, however, it would appear that the potentialities of the project as a means of inspiring confidence just under these circum-stances acquire most actual importance. Moreover the momentary situation would in various respects seem to afford quite unique possibilities which might be forfeited by a postponement awaiting the further development of the war situation and the final completion of the new weapon. In view of these eventualities the present situation would seem to offer a most favorable opportunity for an early initiative from the side which by good fortune has achieved a lead in the efforts of mastering mighty forces of nature hitherto beyond human reach. Without impeding the importance of the project for immediate military objectives, an initiative, aiming at forestalling a fateful competition about the formidable weapon, should serve to uproot any cause of distrust be-tween the powers on whose harmonious collaboration the fate of coming generations will depend. Indeed, it would appear that only when the question is taken up among the United Nations of what concessions the various powers are prepared to make as their contribution to an adequate control arrangement, it will be possible for anyone of the partners to assure themselves of the sincerity of the intentions of the others. Of course, the responsible statesmen alone can have the insight in the actual political possibilities. It would, however, seem most fortunate that the expectations for a future harmonious international co-operation which have found unanimous expression from all sides within the United Nations, so remarkably correspond to the unique opportunities which, unknown to the public, have been created by the advancement of science. Many reasons, indeed, would seem to justify the conviction that an approach with the object of establishing common security from ominous menaces without excluding any nation from participating in the promising industrial development which the accomplishment of the project entails will be welcomed, and be responded with a loyal co-operation on the enforcement of the necessary far reaching control measures. Just in such respects helpful support may perhaps be afforded by the world-wide scientific collaboration which for years has embodied such bright promises for common human striving. On this background personal connections between scientists of different nations might even offer means of establishing preliminary and noncommittal contact. It need hardly be added that any such remark or suggestion implies no underrating of the difficulty and delicacy of the steps to be taken by the statesmen in order to obtain an arrangement satisfactory to all concerned, but aim only at pointing to some aspects of the situation which might facili-tate endeavors to turn the project to lasting benefit for the common cause. The secrecy regarding the project which prevented public knowledge and open discussion of a matter so profoundly affecting international affairs added, of course, to the com-plexity of the task of the statesmen. With full appreciation of the extraordinary character of the decisions which the pro-posed initiative involved, it still appeared to me that great opportunities would be lost unless the problems raised by the atomic development were incorporated into the plans of the allied nations for the post-war world. This viewpoint was elaborated in a supplementary memorandum in which also the technical problem of control measures was further discussed. In particular, I attempted to stress that just the mutual openness, which now was obvious-ly necessary for common security, would in itself promote international understanding and pave the way for enduring co-operation. This memorandum, dated March 24th 1945, contains, besides remarks which have no interest to-day, the following passages: Above all, it should be appreciated that we are faced only with the begin-ning of a development and that, probably within the very near future, means will be found to simplify the methods of production of the active substances and intensify their effects to an extent which may permit any nation possessing great industrial resources to command powers of destruc-tion surpassing all previous imagination. Humanity will, therefore, 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 internation-al control of the manufacture and use of the powerful materials. Any arrangement which can offer safety against secret preparations for the mastery of the new means of destruction would, as stressed in the memorandum, demand extraordinary measures. In fact, not only would universal access to full information about scientific discoveries be neces-sary, but every major technical enterprise, industrial as well as military would have to be open to international control. In this connection it is significant that the special character of the efforts which, irrespective of technical refinements, are required for the production of the active materials, and the peculiar conditions which govern their use as dangerous explosives, will greatly facilitate such control and should ensure its efficiency provided only that the right of supervision is guaranteed. Detailed proposals for the establishment of an effective control would have to be worked out with the assistance of scientists and technologists appointed by the governments concerned, and a standing expert committee, related to an international security organization, might be charged with keeping account of new scientific and technical developments and with recommending appropriate adjustments of the control measures. On recommendations from the technical committee the organization would be able to judge the conditions under which industrial exploitation of atomic energy sources could be permitted with adequate safeguards to prevent any assembly of active material in an explosive state. As argued in the memorandum, it would seem most fortunate that the measures demanded for coping with the new situation, brought about by the advance of science and confronting mankind at a crucial moment of world affairs, fit in so well with the expectations for a future intimate inter-national co-operation which have found unanimous expression from all sides within the nations united against aggression. Moreover, the very novelty of the situation should offer a unique oppor-tunity of appealing to an unprejudiced attitude, and it would even appear that an understanding about this vital matter might contribute most favorably towards the settlement of other problems where history and traditions have fostered divergent viewpoints. With regard to such wider prospects, it would in particular seem that the free access to information, necessary for common security should have far-reaching effects in removing obstacles barring mutual knowledge about spiritual and material aspects of life in the various countries, without which respect and goodwill between nations can hardly endure. Participation in a development, largely initiated by international scientif-ic collaboration and involving immense potentialities as regards human welfare, would also reinforce the intimate bonds which were created in the years before the war between scientists of different nations. In the present situation these bonds may prove especially helpful in connection with the deliberations of the respective governments and the establishment of the control. In preliminary consultations between the governments with the primary purpose of inspiring confidence and relieving disquietude, it should be necessary only to bring up the problem of what the attitude of each partner would be if the prospects opened up by the progress of physical science, which in outline are common knowledge, should be realized to an extent which would necessitate exceptional action. In all the circumstances it would seem that an understanding could hardly fail to result, when the partners have had a respite for considering the conse-quences of a refusal to accept the invitation to co-operate, and convincing themselves of the advantages of an arrangement guaranteeing common security without excluding anyone from participation in the promising utilization of the new sources of material prosperity. All such opportunities may, however, be forfeited if an initiative is not taken while the matter can be raised in a spirit of friendly advice. In fact, a postponement to await further developments might, especially if prepara-tions for competitive efforts in the meantime have reached an advanced stage, give the approach the appearance of an attempt at coercion in which no great nation can be expected to acquiesce. Indeed, it need hardly be stressed how fortunate in every respect it would be if, at the same time as the world will know of the formidable destructive power which has come into human hands, it could be told that the great scientific and technical advance has been helpful in creating a solid founda-tion for a future peaceful co-operation between nations. Looking back on those days, I find it difficult to convey with sufficient vividness the fervent hopes that the progress of science might initiate a new era of harmonious co-operation between nations, and the anxieties lest any opportunity to promote such a development be forfeited. Until the end of the war I endeavored by every way open to a scientist to stress the importance of appreciating the full political implications of the project and to advocate that, before there could be any question of use of atomic weapons, international co-operation be initiated on the elimination of the new menaces to world security. I left America in June 1945, before the final test of the atomic bomb, and remained in England, until the official announcement in August 1945 that the weapon had been used. Soon thereafter I returned to Denmark and have since had no connection with any secret, military or industrial, project in the field of atomic energy. When the war ended and the great menaces of oppression to so many peoples had disappeared, an immense relief was felt all over the world. Nevertheless, the political situation was fraught with ominous forebodings. Divergences in outlook between the victorious nations inevitably aggravated con-troversial matters arising in connection with peace settle-ments. Contrary to the hopes for future fruitful co-oper-ation, expressed from all sides and embodied in the Charter of the United Nations, the lack of mutual confidence soon became evident. The creation of new barriers, restricting the free flow of information between countries, further increased distrust and anxiety. In the field of science, especially in the domain of atomic physics, the continued secrecy and restrictions deemed necessary for security reasons hampered internation-al co-operation to an extent which split the world community of scientists into separate camps. Despite all attempts, the negotiations within the United Nations have so far failed in securing agreement regarding measures to eliminate the dangers of atomic armament. The sterility of these negotiations, perhaps more than anything else, made it evident that a constructive approach to such vital matters of common concern would require an atmos-phere of greater confidence. Without free access to all information of importance for the interrelations between nations, a real improvement of world affairs seemed hardly imaginable. It is true that some degree of mutual openness was envisaged as an integral part of any international arrangement regarding atomic energy, but it grew ever more apparent that, in order to pave the way for agreement about such arrangements, a decisive initial step towards openness had to be made. The ideal of an open world, with common knowledge about social conditions and technical enterprises, including military preparations, in every country, might seem a far remote possibility in the prevailing world situation. Still, not only will such relationship between nations obviously be required for genuine co-operation on progress of civilization, but even a common declaration of adherence to such a course would create a most favorable background for concerted efforts to promote universal security. Moreover, it appeared to me that the countries which had pioneered in the new tech-nical development might, due to their possibilities of offering valuable information, be in a special position to take the in-itiative by a direct proposal of full mutual openness. I thought it appropriate to bring these views to the atten-tion of the American government without raising the delicate matter publicly. On visits to the United States in 1946 and in 1948 to take part in scientific conferences, I therefore availed myself of the opportunity to suggest such an initiative to American statesmen. Even if it involves repetition of argu-ments already presented, it may serve to give a clearer impres-sion of the ideas under discussion on these occasions to quote a memorandum, dated May 17th, 1948, submitted to the Secretary of State as a basis for conversations in Washington in June 1948. The deep-rooted divergences in attitudes to many aspects of human rela-tionship which have grown out of social and political developments in the last decades, were bound to present a serious strain on international rela-tions at the conclusion of the second world war. While, during the war, the efforts in common defense largely distracted attention from such divergences, it was clear that the realization of the hopes acclaimed from all the na-tions united against aggression of a whole-hearted co-operation in true con-fidence would demand a radically new approach to international relations. The necessity of a readjustment of such relations was even further accen-tuated by the great scientific and technical developments which hold out bright prospects for the promotion of human welfare, but at the same time have placed formidable means of destruction in the hands of man. Indeed, just as previous technical progress has led to the recognition of need for adjustments within civilized societies, many barriers between nations which hitherto were thought necessary for the defense of national interests would now obviously stand in the way of common security. The fact that this challenge to civilization presents the nations with a matter of the deepest common concern should offer a unique opportunity for seeking continued co-operation on vital problems. Already during the war, it was, therefore, felt that a favorable foundation for later develop-ments might be created by an early initiative aimed at inviting confidence by making all partners aware of the actual situation which would have to be faced, and by assuring them of willingness to share in the far-reaching concessions as to accustomed national prerogatives which would be demanded from every side. In the years which have passed since the war, the divergences in outlook have manifested themselves ever more clearly and a most desperate feature of the present situation is the extent to which the barring of intercourse has led to distortion of facts and motives, resulting in increasing distrust and suspicion between nations and even between groups within many nations. Under these circumstances the hopes embodied in the establishment of the United Nations Organization have met with repeated great disappoint-ments and, in particular, it has not been possible to obtain consent as regards control of atomic energy armaments. In this situation with deepening cleavage between nations and with spreading anxiety for the future, it would seem that the turning of the trend of events requires that a great issue be raised, suited to invoke the highest aspirations of mankind. Here it appears that the stand for an open world, with unhampered opportunities for common enlightenment and mutual understanding, must form the background for such an issue. Surely, respect and goodwill between nations cannot endure without free access to infor-mation about all aspects of life in every country. Moreover, the promises and dangers involved in the technical advances have now most forcibly stressed the need for decisive steps toward openness as a primary condition for the progress and protection of civilization. The appreciation of this point, it is true, underlies the proposals to regulate co-operation on the development of the new resources, brought before the United Nations Atomic Energy Commission, but just the difficulty ex-perienced in obtaining agreement under present world conditions would suggest the necessity of centering the issue more directly on the problem of openness. Under the circumstances it would appear that most careful consideration should be given to the consequences which might ensue from an offer, extended at a well-timed occasion, of immediate measures towards open-ness on a mutual basis. Such measures should in some suitable manner grant access to information, of any kind desired, about conditions and developments in the various countries and would thereby allow the partners to form proper judgment of the actual situation confronting them. An initiative along such lines might seem beyond the scope of conven-tional diplomatic caution; yet it must be viewed against the background that, if the proposals should meet with consent, a radical improvement of world affairs would have been brought about, with entirely new opportuni-ties for co-operation in confidence and for reaching agreement on effective measures to eliminate common dangers. Nor should the difficulties in obtaining consent be an argument against taking the initiative since, irrespective of the immediate response, the very existence of an offer of the kind in question should deeply affect the situa-tion in a most promising direction. In fact, a demonstration would have been given to the world of preparedness to live together with all others under conditions where mutual relationships and common destiny would be shaped only by honest conviction and good example. Such a stand would, more than anything else, appeal
to people all over the world, fighting for fundamental human rights, and
would greatly strengthen the moral position of all supporters of genuine
international co-operation. At the same time, those reluctant to enter
on the course proposed would have been brought into a position difficult
to maintain since such opposition would amount to a confession of lack
of confidence in the strength of their own cause when laid open to the
world.
The consideration in this memorandum may appear utopian, and the difficulties of surveying complications of non--conventional procedures may explain the hesitations of governments in demonstrating adherence to the course of full mutual openness. Nevertheless, such a course should be in the deepest interest of all nations, irrespective of differences in social and economic organization, and the hopes and aspi-rations for which it was attempted to give expression in the memorandum are no doubt shared by people all over the world. While the present account may perhaps add to the general recognition of the difficulties with which every nation was confronted by the coincidence of a great upheaval in world affairs with a veritable revolution as regards technical resources, it is in no way meant to imply that the situation does not still offer unique opportunities. On the contrary, the aim is to point to the necessity of reconsidering, from every side, the ways and means of co-operation for avoiding mortal menaces to civilization and for turning the progress of science to lasting benefit of all humanity. Within the last years, world-wide political developments have increased the tension between nations and at the same time the perspectives that great countries may compete about the possession of means of annihilating populations of large areas and even making parts of the earth temporarily unin-habitable have caused widespread confusion and alarm. As there can hardly be question for humanity of renounc-ing the prospects of improving the material conditions for civilization by atomic energy sources, a radical adjustment of international relationship is evidently indispensable if civili-zation shall survive. Here, the crucial point is that any guaran-tee that the progress of science is used only to the benefit of mankind presupposes the same attitude as is required for co-operation between nations in all domains of culture. Also in other fields of science recent progress has confront-ed us with a situation similar to that created by the develop-ment of atomic physics. Even medical science, which holds out such bright promises for the health of people all over the world, has created means of extinguishing life on a terrifying scale which imply grave menaces to civilization, unless universal confidence and responsibility can be firmly estab-lished. The situation calls for the most unprejudiced attitude towards all questions of international relations. Indeed, proper appreciation of the duties and responsibilities implied in world citizenship is in our time more necessary than ever before. On the one hand, the progress of science and technol-ogy has tied the fate of all nations inseparably together, on the other hand, it is on a most different cultural background that vigorous endeavors for national self-assertion and so-cial development are being made in the various parts of our globe. An open world where each nation can assert itself solely by the extent to which it can contribute to the common culture and is able to help others with experience and resources must be the goal to be put above everything else. Still, example in such respects can be effective only if isolation is abandoned and free discussion of cultural and social developments per-mitted across all boundaries. Within any community it is only possible for the
citizens to strive together for common welfare on a basis of public knowledge
of the general conditions in the country. Likewise, real co-operation between
nations on problems of common concern presupposes free access to all information
of impor-tance for their relations. Any argument for upholding barri-ers
for information and intercourse, based on concern for national ideals or
interests, must be weighed against the beneficial effects of common enlightenment
and the relieved tension resulting from openness.
In this connection it has to be recognized that abolition of barriers would imply greater modifications in administrative practices in countries where new social structures are being built up in temporary seclusion than in countries with long traditions in governmental organization and international contacts. Common readiness to assist all peoples in overcom-ing difficulties of such kind is, therefore, most urgently re-quired. The development of technology has now reached a stage where the facilities for communication have provided the means for making all mankind a co-operating unit, and where at the same time fatal consequences to civilization may ensue unless international divergences are considered as issues to be settled by consultation based on free access to all relevant information. The very fact that knowledge is in itself the basis for civili-zation points directly to openness as the way to overcome the present crisis. Whatever judicial and administrative interna-tional authorities may eventually have to be created in order to stabilize world affairs, it must be realized that full mutual openness, only, can effectively promote confidence and guarantee common security. Any widening of the borders of our knowledge imposes an increased responsibility on individuals and nations through the possibilities it gives for shaping the conditions of human life. The forceful admonition in this respect which we have received in our time cannot be left unheeded and should hardly fail in resulting in common understanding of the seri-ousness of the challenge with which our whole civilization is faced. It is just on this background that quite unique oppor-tunities exist to-day for furthering co-operation between nations on the progress of human culture in all its aspects. I turn to the United Nations with these considerations
in the hope that they may contribute to the search for a realistic approach
to the grave and urgent problems confronting humanity. The arguments presented
suggest that every initia-tive from any side towards the removal of obstacles
for free mutual information and intercourse would be of the greatest importance
in breaking the present deadlock and encourag-ing others to take steps
in the same direction. The efforts of all supporters of international co-operation,
individuals as well as nations, will be needed to create in all countries
an opinion to voice, with ever increasing clarity and strength, the demand
for an open world.
1885 Born October 7th in Copenhagen.
About Niels Bohr Casimir, Hendrik: Haphazard reality. Half a century of science. New York. Harper & Row. 1983. 356 p. Frisch, Otto Robert: What little I remember. Cambridge. Cambridge University Press. 1979. 227 p. Ill. Gamow, George: My world line: An informal autobiography. New York. The Viking Press. 1970. 178 p. Ill. Hellssen, Henry: Niels Bohr. 1885 - 7. okt. -1955. En billedbiografi (Niels Bohr. 1885 - October 7 - 1955. A biography in pictures). Copenhagen. 1955. 61 p. Ill. Moore, Ruth: Niels Bohr: The man, his science and the world they changed. New York. Knopf 1966. 436 p. Ill. Niels Bohr. His life and work as seen by his friends and colleagues. Ed.: S. Rozental. Amsterdam. North-Holland Publishing Company. 1967. 355 p. Ill. The Niels Bohr Institute. October 7, 1965. Copenhagen. 1965. 16p. Ill. Robertson, Peter: The early years. The Niels Bohr Institute 1921-1930. Copenhagen. Akademisk Forlag. 1979. 175 p. Ill. Rosenfeld, L.: Niels Bohr. An essay dedicated to him on the occasion of his sixtieth birthday. October 7, 1945. 2nd ed. Amsterdam. North-Holland Publishing Company. 1961, 20 p. Rosenfeld, L. and S. Rozental: Niels Bohr. 7 October 1885 - 18 No-vember 1962. Amsterdam. North-Holland Publishing Company. 1963. 16p. The presentation of the first Atoms for Peace Award to Niels Henrik David Bohr, October 24, 1957. Washington, D.C. The National Academy of Sciences. 1957. 33 p. Ill. Silverberg, Robert: Niels Bohr. The man who mapped the atom. Philadelphia, Pa. Macrae Smith Company. 1965. 189 p. Weisskopf Victor Frederick: Physics in the twentieth century. Selected essays. Cambridge, Mass. Massachusetts Institute of Tech-nology Press. 1972. 368 p. Ill. By Niels Bohr Bohr, Niels: Atomic physics and human knowledge. New York. Science Editions. 1961. 101 p. Bohr, Niels: Atomic theory and the description of nature. 1: Four essays (Reprint). Cambridge. The University Press. 1961. 119 p. Bohr, Niels: Collected works. Vol. 1-5. Amsterdam. North-Holland Publishing Company. 1972-84. 5 vols. Ill. Bohr, Niels: Essays 1958-1962 on atomic physics and human knowledge. New York. Interscience Publishers. 1963. 100 p. Bohr, Niels: On the application of the quantum theory to atomic structure. Part 1: The fundamental postulates. Cambridge. 1924. 42 p. Bohr, Niels: On the constitution of atoms and molecules. Papers of 1913 reprinted from the Philosophical Magazine with an introduc-tion by L. Rosenfeld. Copenhagen. Munksgaard. 1963. 77 p. Bohr, Niels: On the quantum theory of line-spectra. parts 1-3. Copenhagen. 1918-22. 3 vols. Bohr, Niels: Open letter to the United Nations June 9th, 1950. Copenhagen. 1950. 12 p. Bohr, Niels: The theory of spectra and atomic constitution. Three essays. Cambridge. 1922. 126 p.  |
