niels henrik david bohr,...

Niels Henrik David Bohr, 1885-1962

J. D. Cockcroft

, 37-53, published 1 November 196391963 Biogr. Mems Fell. R. Soc.

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NIELS H EN R IK DAVID BOHR

1885-1962

N iels Bohr was born on 7 October 1885, the son of Professor Christian Bohr, the physiologist, and Ellen, daughter of D. B. Alder, a banker. He had one sister, Jenny, and a brother, Harald, who became Professor of Mathematics in the University of Copenhagen. They grew up in a home of culture. From his father, Niels became acquainted with the problems of biology and in later years became interested again in biological problems through his interests in complementarity. He was educated in Copenhagen, first at the Gammelholm School and then at the University. Whilst there, he became interested in philosophical problems through attending the lectures of Hoffding. According to Rosenfeld (Nordita Publication No. 57) ‘he was fascinated by some of the great religious figures of the past, especially the Jewish prophets and the Buddha; he earnestly endeavoured to penetrate the human side of their teachings and arrived at interpretations of striking originality. Into these sacred texts, coming from the innermost recesses of the human soul, he read an effort to account in a peculiar language for that deepest complementarity between rational knowledge and living experience of the cosmos which in his eyes characterized man’s ambiguous position and rules his whole activity. No wonder that such deep seated views were to play a leading part in his thought in later years and even pervade every shade of his sensitive reaction to the various impressions of everyday life’. During his adolescent years Niels Bohr together with his brother, Harald, were good footballers and his passion for sport was transferred in time to skiing and sailing. He was a lover of the arts and when Eric Gill’s carving of Rutherford installed in the Mond Laboratory at Cambridge was being criticized he was delighted to be presented by Dirac and Kapitza with a copy for his Institute at Copenhagen.

He graduated in physics in the University of Copenhagen in 1907 and gained the Gold Medal of the Royal Danish Academy of Sciences for his work as a student on surface tension by the method of jet vibration. This was published in an enlarged form in the Philosophical Transactions of the Royal Society in 1909. He then wrote a dissertation on the electron theory of metals and in the remarkably short time of two years gained his doctor’s degree.

In the autumn of 1911 Bohr went to work with J. J . Thomson in Cambridge bearing with him the manuscript of his doctor’s thesis. This did not appear to arouse much interest in J. J. He became interested in J. J.



g8 Biographical Memoirs

Thomson’s ideas on the electronic constitution of atoms—‘the plum pudding model’—in which the electrons were embedded like currants in a sphere of positive electricity. He heard Rutherford speak at the Cavendish Laboratory dinner and soon after this met Rutherford in Manchester and obtained his agreement to join the Manchester Laboratory in the spring. He arrived in Manchester a year after Rutherford had ‘devised a new atom’ to explain the Geiger-Marsden experiments on the scattering of alpha particles. During his first few weeks in the Laboratory he attended the course organized by Geiger, Makower and Marsden on experimental methods in radioactive research for the benefit of students and visitors. However, he very rapidly became involved in the theoretical problems of the work of the Laboratory and in particular he discussed with Hevesy the possibility of explaining the physical and chemical properties of the elements by what is now known as the atomic number. These discussions led amongst other things to the conclusion that as a result of radioactive decay an element would shift its place in the periodic table by two steps down or one step up accompanying the emission of alpha or beta rays respectively. Bohr wrote to Andrade later: ‘Rutherford showed a kind sympathy for my youthful enthusiasm and became till the end of his life almost a second father to me. Naturally to begin with Rutherford was cautious and I remember that when a few months after my arrival I told him of my views on the origin of isotopes and, in particular, of the expectation that alpha and beta decay had to be accompanied by displacement in the table of elements, he answered that such views appeared to him to contain a somewhat extravagant extrapolation of the conclusions derived from the scattering experiments.’ However, evidence in support of Bohr’s views was soon produced by Hevesy, and Russell published this work in a lecture to the Chemical Society in the late autumn of 1912. Marsden considers (private communication) that Bohr ‘contributed to the ideas underlying isotopes more than any other person’.

Bohr then took an interest in the electronic constitution of the Rutherford atom and became convinced that this was ‘governed throughout by the quantum of action’. He found support for his views partly from Whiddington’s results on the excitation of characteristic X-rays by electron bombardment of different elements. These could be related to the binding energy of electrons rotating in a Planck orbit round a nucleus with charge given by the atomic number. Between January 1913 and 6 March 1913 his letter to Rutherford showed that he had in this period become acquainted with Rydberg’s work on spectral series. Rosenfeld reports that Bohr said to him in 1934 that ‘as soon as I saw Balmer’s formula the whole thing was immediately clear to me’. He was helped in this by discussions with H. M. Hansen, a Danish spectroscopist, who had worked with Voigt in Gottingen.

During the last month of his stay at Manchester he worked on the stopping power of matter for alpha and beta rays. The transfer of energy from the particles to the atomic electrons was calculated by working out the reaction of the electrons to the electric pulse of the passing particle taking account of



Niels Henrik David Bohr 39the electronic frequencies through dispersion theory. The stopping power measurements provided additional evidence for ascribing to hydrogen and helium the atomic numbers 1 and 2.

O f this period, A. B. Wood has written that ‘Bohr was a great help not only to Rutherford directly but also to many of the research staff down to the most junior research student. For example, he suggested a diffusion method of measuring the atomic weight of radioactive emanations in the radium, thorium and actinium series, and Ernest Marsden and I made the first measurements of the atomic weight of actinium by this method. He was always a source of encouragement and inspiration to everyone in the laboratory. His unassuming modesty, willingness to discuss problems and to make helpful suggestions were characteristic of him’.

He returned to Copenhagen on 26 July 1912 and on 1 August married Margrethe, daughter of Alfred Norlund. The marriage was a very happy one throughout their lives. It has been said that Margrethe made as great an impression on Rutherford in her way as Niels did in his.

After their wedding they called at Manchester and delivered to Rutherford the manuscript of the paper on stopping power and thereafter enjoyed a honeymoon in Scotland. On returning to Copenhagen Bohr took up again the stability problem of the Rutherford atom in intervals between lecturing and helping Knudsen with experimental work. He wrote to Rutherford several times reporting his progress and difficulties and on 6 March 1913 he sent to Rutherford the first part of his paper on the constitution of atoms. This gave the interpretation of the law of the spectrum of hydrogen and

calculated the Rydberg constant 27r2m 4~ ¥ ~

. In the theory ‘the electrons were

taken to follow orbits in accordance with Newtonian mechanics but of all possible such orbits only those could occur from which the action integrals are integral multiples of Planck’s constant. While the motion in the orbit is described by classical theory the transition from one orbit to another in the process of emission or absorption of radiation or in atomic collisions occurs in quantum jumps which cannot be understood in classical terms’. Bohr wrote to Rutherford ‘I hope you will find that I have taken a reasonable point of view as to the delicate question of the simultaneous use of the old mechanics and of the new assumptions introduced by Planck’s theory of radiation’. To this Rutherford quickly replied that ‘your ideas as to the mode of origin of the spectra of hydrogen are very ingenious and work out very well; but the mixture of Planck’s ideas with the old mechanics makes it very difficult to form a physical idea as to what is the basis of it. There appears to me to be one grave difficulty in your hypothesis, which I have no doubt you fully realize, namely, how does an electron decide what frequency it is going to vibrate at when it passes from one stationary state to another? It seems to me that you have to assume that the electron knows beforehand where it is going to stop’.



Biographical Memoirs

Rutherford also criticized the length of the paper and suggested that Bohr should authorize him to ‘cut out any matter I may consider unnecessary . After receiving a new and still longer version Rutherford returned to the charge ‘as you know it is the custom in England to put things very shortly and tersely in contrast to the Germanic method, where it appears to be a virtue to be as long-winded as possible’—to which Bohr replied that he had revised the paper and made it still longer to throw some light on Rutherford’s difficulties! He then visited Manchester to discuss the paper with Rutherford and by sheer obstinacy managed to preserve his paper from damaging curtailment.

In June 1913 Bohr visited Manchester again bearing the second part ol his paper which dealt with the origin of the Barkla characteristic radiation and the ground state of atoms containing several electrons. In this he tried to arrange the electrons in closed rings resembling the shell structure introduced by J. J . Thomson in his atomic model.

He returned again to attend the British Association meeting in Birmingham in September 1913. There was a general discussion about the problem of radiation in which Rayleigh, Larmor, Lorentz, Rutherford and Jeans participated. Jeans gave an introductory survey of the applications of quantum theory to the atomic constitution. Bohr remarked in his Rutherford lecture to the Physical Society that ‘his lucid expression was in fact the first public expression of serious interest in considerations which outside the Manchester Group were generally received with much scepticism . He said shoitly after the meeting that the discussion had gone as well as he could possibly have hoped. This slow adoption of radically new ideas had been characteristic of the scientific world for the previous half century. ‘Even Planck remained for long reluctant to accept the drastic break with classical physics to which his own hypothesis seems to lead and made many attempts to reconcile his results with the classical picture.’ On 23 September 1913 Hevesy wrote to Bohr that he had spoken to Einstein and asked his views on Bohr’s theory. He told me that ‘it was very interesting and important if it is right and that he had very similar ideas many years ago but he had no pluck to develop it . I told him that it was established now with certainty that the Pickering- Fowler spectrum belongs to helium. When he learnt this he was extremely astonished and said ‘then the frequency of the light does not depend at all

the frequency of the electron! This is an enormous achievement. Bohr replied to Hevesy on 13 October that ‘it is a great pleasure to me to hear that Einstein thought that there might possibly be something in the theory\

In early 1914 Moseley wrote to Rutherford from Oxford to say ‘here there is no one interested in atom building. I should be glad to do something towards knocking on the head the very prevalent view that Bohr s work is all juggling with numbers until they can be got to fit. I myself feel convinced that what I have called the “A” hypothesis is true; that is to say, one will be able to build atoms out of e, m and h and nothing else’.

In the autumn of 1913 Stark discovered the splitting of spectral lines by



Niels Henrik David Bohr

electric fields and Rutherford at once wrote to Bohr to say that ‘I think it is rather up to you to write something on the Zeeman and electric effects if it is possible to reconcile them with your theory’. By the end of the year Bohr replied to say ‘I think that I have succeeded in accounting, at least partly, for the experiments of Stark on the basis of my theory. The agreement seems inside the limits of experimental error’.

In 1913 Bohr was appointed Assistant Professor in the University of Copenhagen and by March 1914 was asking Rutherford for a testimonial to help to create a post in theoretical physics at the University. In June 1914 Rutherford invited Bohr to succeed to Darwin’s Readership at Manchester for a period of a year and he arrived at Manchester in the early autumn after a stormy voyage round Scotland. By this time the Manchester Group had almost completely dissolved owing to the war and Rutherford was still in America on his way back from Australia.

At the British Association meeting in Melbourne in August 1914 Rutherford had opened a discussion on the structure of atoms and molecules and spoke of the single scattering of alpha particles. He said that ‘Niels Bohr has faced the difficulty of bringing in the idea of the quantum in a novel way. At all events there is something going on in the atom which is inexplicable on the older mechanics’.

Bohr took part in the teaching of students together with Evans and Makower and started experimental work with Makower on the Franck- Hertz experiments, only to have his intricate quartz apparatus built by the German glassblower destroyed by fire, whereupon he abandoned experimental work. During this period up to the summer of 1916 when he returned to Denmark, Bohr’s wider interests were shown by his membership of the monthly discussion group consisting of a number of Rutherford’s friends including Alexander, the philosopher; Tout, the historian; Elliott Smith, the anthropologist and Chaim Weizmann.

Bohr’s next paper, ‘On the quantum theory of line spectra’, was published in 1918. During the intervening period he was much engaged with teaching but worked with Kramers on the theory of the helium spectrum. He wrote to Rutherford that ‘the method of applying the Quantum Theory is in the main line the same as Sommerfeld in his theory of the fine structure of the hydrogen lines but we have succeeded in finding some quite simple and beautiful solutions of the mechanical problem which for a long time looked so complicated’. A year later his paper still unpublished he wrote ‘I have tried also by taking the new developments of quantum theory into account to follow up the analogy between this theory and the ordinary theory of electrodynamics and in this way to develop a general argument which allows to a certain extent to include the questions of intensity and polarization in the considerations which were previously concerned only with the frequency of spectral lines. The theory also allows to explain in detail the Zeeman effect on the hydrogen lines which has hitherto given rise to so much difficulty for the Quantum Theory’.

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42 Biographical Memoirs

In November 1918 Bohr wrote to Rutherford rejoicing on the end of the war and looking forward to hearing more about Rutherford’s transmutation results. He also told him about the decision to found the Institute for Theoretical Physics in the University of Copenhagen thanks to a benefaction amounting to £4500, and this rapidly became a world centre for theoretical atomic physics. By this time Bohr’s work was receiving general recognition. He was awarded the Hughes Medal of the Royal Society in 1921 and the Nobel Prize for Physics in 1922. The work of Bohr’s Institute was enriched by Oscar Klein and Svein Rosseland from Norway and in 1922 by Pauli and two years later by Heisenberg, both pupils of Sommerfeld. The connexion with Cambridge was maintained by visits of Darwin, Dirac, Fowler, Hartree, Mott and others.

Bohr has said that ‘in these years a unique co-operation of a whole generation of theoretical physicists from many countries created step by step, a logically consistent generalization of quantum mechanics and electro magnetics and has sometimes been designated as the heroic age in quantum physics . . . To everyone following this development it has been an unforgettable experience to witness how through the combination of different lines of approach and the introduction of appropriate mathematical methods, a new outlook emerged requiring the comprehension of physical experience. Many obstacles had to be overcome before this goal was reached and time and again decisive progress was achieved by some of the youngest among us’.

About this period Weisskopf has written December 1962)‘In lively discussions in groups of two or more the deepest problems on the structure of matter were brought to light. You can imagine what atmosphere, what life, what intellectual activity reigned in Copenhagen at that time. Here was Bohr’s influence at its best. Here it was that he created his style the “Kopenhagener Geist” , the style which he has imposed on physics—the style of a very special character. We see him, the greatest among his peers, acting, talking, living as an equal in a group of young, optimistic, jocular, enthusiastic people, approaching the deepest riddles of nature with a spirit of attack, a spirit of freedom from conventional bonds and a spirit of joy which can hardly be described’.

A major advance was made by Heisenberg in 1925 ‘in which all use of orbital pictures was avoided. The canonical equations of mechanics were retained in the original form but the conjugate variables were replaced by operators subject to a non-commutative algorith involving Planck’s constant as well as the symbol V — 1. This so called quantum mechanics to which from the outset BoHI and Jordan made important contributions opened the way to a consistent statistical treatment of many atomic problems which hitherto were only answerable to a semi-empirical approach’.

Bohr wrote to Rutherford in January 1926 to say that ‘due to the last work of Heisenberg, prospects have with a stroke been realized which although only vaguely grasped have for a long time been the centre of our wishes. At the same time as we see the possibility of developing a quantitative theory



of atomic structure, another and as I believe very important contribution to the atomic theory has been made by two young Dutchmen, Goudsmit and Uhlenbeck, who have got the idea that the fine structure of spectral lines is to be traced in a spin of the electron round its orbit’.

The development by Schrodinger of wave mechanics was described at the Copenhagen Institute in 1926 and the equivalence of the two different mathematical formulisms was ‘completely elucidated by the transformation theory formulated independently by Dirac in Copenhagen and Jordan in Gottingen’.

A major development of Bohr’s philosophical views was put forward in a lecture to the International Physical Congress held at Como in commemoration of Volta in September 1927. In this he introduced the point of view of ‘complementarity’ ‘suited to embrace the characteristic features of individuality of quantum phenomena and at the same time to clarify the observation problems in this field of experience’ (. physics and humanknowledge p. 39). He pointed out ‘the impossibility of any sharp separation between the behaviour of atomic objects and their interaction with the measuring instruments which serve to define the conditions under which the phenomena appear’. This principle was used to remove the difficulties and paradoxes in discussing the wave and corpuscular aspects of electrons and photons. Then followed a long series of discussions with Einstein and others on the interpretation of the notions of complementarity and indeterminacy. Thus at the 1930 Solvay Conference Einstein proposed a device to measure the time of escape of a photon from a box using a shutter actuated by a clock and simultaneously to measure the energy of the photon by weighing the box, thus contradicting the principle of indeterminacy SE = h. Bohr was able in reply to show that the use of this apparatus as a means of accurately measuring the energy prevented the control of the moment of escape. O f this L. Rosenfeld has said (Speech at the Danish Academy of Sciences on 13 December 1962) ‘In devising such imaginary experiments Einstein’s inventiveness was supreme but Bohr was unsurpassed for penetrating analysis of their implications. As the outcome of this patient and unrelenting effort of elucidation there emerged in all its generality the concept of complementarity as a logical relationship between two physical phenomena both representing aspects of a physical system equally necessary for its complete description but corresponding to mutually exclusive experimental conditions’. In November 1929 Rutherford wrote to Bohr inviting him to give the Scott lectures in 1930. He said that ‘I have heard rumours that you are wanting to upset the conservation of energy both microscopically and macroscopi- cally. I will wait to see before expressing an opinion but I always feel that there are more things in heaven and earth than are dreamt of in our philosophy’. I was given by Rutherford the task of reporting Bohr’s three lectures. The first difficulty was to hear correctly most of what Bohr said and for this it was important to sit as near the front as possible. The second, more important difficulty was to understand what Bohr meant and in this case

Niels Henrik David Bohr 43



44 Biographical Memoirsfortunately understanding was helped by the large number of drawings on the blackboard of electrons going through holes with diffraction effects and of clocks regulating the time of escape of photons through a hole.

In the summer of 1923 Bohr was offered a Royal Society Research Professorship to work in Cambridge. He had great difficulties in deciding whether to accept, being divided by his loyalties to the Danish authorities and individuals who had supported his Institute, and his friendship with Rutherford. He asked Rutherford whether it might be possible for him to retain the Directorship of his Institute and to resign his Professorship in the University of Copenhagen. He would stay in Copenhagen for the larger part of the year but would visit Cambridge at regular short intervals. He suggested that ‘such regular visits might open the possibility of preparation for both sides which might make exchanges of ideas especially fruitful and be helpful in planning of theoretical research on a larger scale and of a proper direction of such important but laborious calculations which are previously unavoidable for progress in this field’. The correspondence with Rutherford and Jeans, then Secretary of the Royal Society, went on throughout the summer but in the end the Royal Society Committee decided that the plan to divide Bohr’s time and energy between two countries was not acceptable.

In 1926 Bohr was elected a Foreign Member of the Royal Society.Bohr took a great interest in the development of nuclear physics in the

1930’s following the application of accelerators to transmutation of atomic nuclei. Of this period Peierls has written (Proc. Soc. 81, 793, 1963):

‘In 1936 one might well have expected Bohr to continue only with his work on epistemology and the clarification of the basic principles, without contributing further to the solution of specific, practical problems of physics. But he still produced two ideas of outstanding importance. The first related to the problem of collisions of neutrons with nuclei. At that time the neutron had been used by Fermi and others for the study of nuclei, and this work had shown many surprising features.

‘It was then usual to think of neutrons and protons in a nucleus as moving largely independently of each other, each being subject to a field of force which was the average result of the motion of all the others. This followed the way in which Bohr had so successfully described the behaviour of electrons in the atom. These ideas suggested that the effectiveness of neutrons in collisions should not depend sensitively on their energy, and that radiative capture of neutrons should be a rather rare event. The experiments showed many extremely sharp resonances in the cross section for neutron collisions as a function of energy, and at reasonably low energies radiative capture was the dominant process. Bohr pointed out that a nucleus must be regarded as a system of many particles all strongly coupled together, like the molecules in a drop of water.

‘The numerous resonances were then at once understood as the large number of quantum levels of such a system at a high excitation energy; their sharpness reflected the long life of such a state. Although there was enough




energy to eject a neutron (since the state had been formed by neutron impact) this energy was shared between many degrees of freedom, and it took a long time before by chance the whole energy was again concentrated on one neutron. During this time some of this energy could easily be lost as radiation.

‘This picture of the “compound nucleus” had a profound influence on the theory of nuclear reactions. It was for a time misinterpreted as ruling out the description in terms of independent particles even in the normal state of a nucleus or in states of low excitation. We learned later that the two pictures are perfectly compatible, and that even at the high energies to which the compound nucleus picture applies one can also recognize single-particle features if one looks at energy averages (which by the uncertainty principle relate to the short-time behaviour of the process) rather than at well-defined energies.

‘Yet even the misunderstanding was fruitful since it led to speculations about a liquid-drop model for a nucleus, of which a direct descendent, the collective model, became in the hands of Aage Bohr and Mottelson, and their collaborators, an extremely fruitful tool in the understanding of nuclei.

‘Bohr’s other contribution of that period was the analysis of the fission process. He had taken a great interest in the discovery of fission, and his ideas about the compound nucleus provided just the appropriate approach for the study of its mechanism. His paper with Wheeler in 1939 showed in quantitative detail how to understand the competition between the decay of the compound nucleus by fission, radiation, and neutron emission, as well as the different behaviour of fast and slow neutrons, and the parts played by the uranium isotopes.’

In particular they showed that it was mainly the isotope 235U which was fissioned by slow neutrons whilst the 238U isotope would usually absorb the neutrons without fission. This led to the realization that isotope separation by 235U would be necessary to produce a fast chain reaction.

Seven months later the Second World War started and before long Denmark was occupied. Until the autumn of 1943 conditions of work for scientists in Denmark were quite tolerable. Hevesy has said that he himself could travel as many times as he liked to Stockholm. However, early in 1943 the British Government had been anxious to get Bohr to England and Chadwick sent a message to say that he would be delighted to see Bohr again and that his assistance would be of great value in a particular unspecified problem. Bohr replied through underground channels that ‘neither such duties nor even the danger of retaliation to my collaborators and relatives might carry sufficient weight to detain me here if I felt that I could be of real help in other ways but I do not think this is probable. Above all I have to the best of my judgement convinced myself that in spite of all future prospects any immediate use of the latest marvellous discoveries of atomic physics is impracticable. However, there may, and perhaps in the near future, come a moment where things look different and where I, if not in other ways, might be able modestly

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to assist in the restoration of international collaboration in human progress. At that moment I shall gladly make an effort to join my friends’.

In September 1943 the regime in Denmark underwent a great change. The King was interned in one of his castles; patriots were imprisoned and Jews were deported to Czechoslovakia. The instructions of the Nazi authorities were not to incommode people who were only partly Jewish or who had 100 per cent Arian spouses. So Bohr was not involved in the anti- Jewish measures. He was however in danger of imprisonment as a patriot and he became cognizant of this intention and escaped together with members of his family in overcrowded fishing boats to Sweden (Hevesy— private communications).

After his arrival in Stockholm arrangements were made by the Director of Tube Alloys and the British Intelligence to transport Bohr and his younger son, Aage, also a physicist, to London and on 6 October 1943 he was flown in an unarmed Mosquito back to England being accommodated in the empty bomb rack. He was unconscious for most of the journey to England owing to his not hearing the instructions to put on his oxygen mask. However, he recovered consciousness by the time he reached Scotland and was thence flown to London, Aage following a week later. He was met by Chadwick and other British scientists and learnt about the atomic energy developments and the state of Anglo-American relations. Shortly before this the Quebec agreement had restored the possibility of Anglo-American collaboration in atomic energy and in September 1943 Chadwick had been the British representative in Washington responsible for making detailed arrangements with the Americans. By the end of November Chadwick was back again in the United States with the first members of the U.K. team who were to work in Los Alamos, New York and Berkeley. It was arranged after some argument between Chadwick and General Groves that the Bohrs should visit the United States—Niels as a Consultant to the Tube Alloys directorate and Aage as a Junior Scientific Officer of the D.S.I.R. General Groves allowed Bohr to visit different parts of the project. He spent a good deal of time in Washington and Los Alamos where he made some technical contributions to the atomic bomb development particularly to the initiator problem {New World, p. 317) and was also useful as an elder statesman in the supercharged atmosphere of Los Alamos full of scientists of great distinction. Bohr’s main interests were however in the implications of atomic bombs for the future of the world and in February 1944 he wrote to Sir John Anderson, Minister in charge of the Tube Alloys project, expressing his concern about the future control of these enormous powers. Sir John Anderson arranged for Bohr to have direct access to the British Ambassador, Lord Halifax, and the Minister, Sir Ronald Campbell, and he discussed the problem with them on several occasions. Bohr felt that after the surrender of Germany the U.S.A. and Britain would be able to use the fact of the possession of the bomb favourably to influence the future state of the world since the necessity of co-operation to avoid a potential catastrophe provided a great opportunity




to establish an improved basis for the whole relationship with the Soviet Union. Bohr was advised by the Ambassador that the initiative in this would have to come from President Roosevelt.

Bohr was then able through the intermediacy of his friend, Mr Justice Frankfurter, to transmit his views about the political implications of the bomb, mutually described as ‘x’, to the President (New , p. 326) thoughwithout disclosing any technical information about the bomb to Mr Frankfurter. The President told Mr Frankfurter that the whole thing ‘worried him to death’ and authorized Frankfurter to tell Bohr that the President was eager to explore with Mr Churchill the proper safeguards in relation to the bomb.

During April and May 1944 Bohr had long talks with Sir John Anderson who together with Lord Cherwell impressed on the Prime Minister the importance of the future international control of atomic energy. Sir Henry Dale wrote to the Prime Minister urging him to see Bohr and as a result Bohr and Lord Cherwell met the Prime Minister, but according to Bohr the meeting was a complete failure. Soon after this, Cherwell and Anderson discussed the problem with Smuts who in turn talked to Bohr. After this, Smuts urged Churchill to discuss the whole question with President Roosevelt when they next met.

On 26 August 1944 Bohr met Roosevelt privately after preparing a memorandum which was transmitted to the President by Frankfurter. This memorandum was later published in Bohr’s ‘Open letter to the United Nations’ in June 1950. The essential point of Bohr’s presentation was that the Russians should be informed about the existence of the bomb before it was used though it would not be necessary to give any technical information. The opportunity should be used to try to establish mutual confidence and co-operation in the industrial applications of atomic energy and to avoid an atomic bomb race.

The outcome of the Hyde Park meeting between the President and the Prime Minister of September 1944 was very different, and the participants decided that the atomic bomb project should continue to have the utmost secrecy ( New World, p. 312).

However, quite independent moves towards international control were being made by Dr Vannevar Bush and Dr Conant, the leading scientists of the U.S. project {New World, p. 326). Dr Bush discussed the problem with the President immediately after the Hyde Park Conference. He and Dr Conant were considering the foundation of an International Control Agency with Russian members. In April 1945 Bohr had further talks with Lord Halifax and it was agreed that he should discuss the question with Bush on a personal basis. Bush then sent Bohr’s latest memorandum to Bundy, adviser to Mr Stimson, Secretary of State for War, and urged that an Advisory Committee should be formed to consider the whole question World, p. 844).Mr Stimson then formed the Interim Committee and chaired this himself. An account of the deliberations of this Committee has been given by Arthur

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Compton ( Atomic Quest) and in the New World, p. 367. The Committee decided early in June 1945 that no information should be revealed to Russia until the first bomb had been dropped on Japan but by the end of June they recommended that President Truman should tell the Russians at the Potsdam Conference that the U.S.A. had a new weapon and they expected to use it against Japan. When Stalin was told at Potsdam he appeared to show little interest.

Bohr’s work, therefore, had little direct fruit. We know now that Stalin must have known about the bomb in 1944 from the disclosures of Fuchs and others.

Nevertheless the views of Bohr and Bush and Conant led to the Acheson- Lilienthal plan for the international control of atomic energy. This prepared the ground for Mr Bernard Baruch’s participation in the work of the United Nations Atomic Energy Commission which was established in 1946. The negotiations, however, came to nothing, foundering on the rock of Russia’s objections to any inspection system to make control effective.

When Bohr returned to Copenhagen after the war these international problems remained a principal source of interest and he continued to discuss them with Sir John Anderson on his visits to England. In 1950 he published in a letter to the United Nations a plea for 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’. This could be effective ‘only if isolation is abandoned and free discussion of cultural and social developments permitted across all boundaries’.

During the 1950’s Bohr took a leading part in the foundation and development of CERN. The programme of building the synchro-cyclotron and 28 BeV proton synchrotron was decided at a Conference in his Institute. For a time the theoretical division of CERN was housed in the Institute and on its move to CERN a joint Scandinavian Research Institute in Theoretical Physics, NORDITA, was founded in association with Bohr’s Institute.

Bohr became Chairman of the Danish Atomic Energy Commission on its foundation and was responsible for the building of the Riso Atomic Research Establishment which with its three reactors was an ambitious project for a country of the size of Denmark. He was President of the Royal Danish Academy of Sciences between 1939 and his death, ‘but he still found time for physics, time to follow the post-war developments of the subject and to add contributions to his favourite problems—the foundation of quantum theory, the problem of measurement and the passage of charged particles through matter’ (Peierls—Physical Society Proceedings).

His last publication was the Rutherford Memorial Lecture in the Proceedings of the Physical Society which gives his reminiscences of the Manchester days and brings out not only a very vivid picture of Rutherford but also of Niels Bohr himself.

Throughout his Directorship of the Institute Bohr’s kind interests in people made the personal relations in his Institute very much like those in a




family. His home in the ‘House of Honour’ in the grounds of the Carlsberg brewery and maintained by the Carlsberg Foundation provided a centre of hospitality for the Institute and his numerous visiting friends. Institute parties were held in the large conservatory, the walls lined with casks of Carlsberg beer for the parties. The large grounds of the Carlsberg house provided the space required for the meditative perambulatory walks of Bohr going round and round with a friend arguing atomic physics or in later years the problems of obtaining an open world. Over this hospitable and beautiful home Margrethe Bohr presided with charm and wisdom and throughout his life was the greatest help to Niels. Their Copenhagen home and their summer home by the sea was enlivened by their family and a large number of grandchildren. Many of us will never to the end of our lives forget the delightful days spent with the Bohrs.

Whilst at the Lindau Conference in June 1962 Bohr had a slight stroke. During the summer he rested at his summer home and appeared to have made a good recovery and after three happy weeks in October at Amalfi with Margrethe his doctors agreed to his resuming work. On Friday, 1^ November 1962, two days before his death he chaired a meeting of the Danish Royal Academy of Sciences and on the Sunday he had planned to have a party of friends at his home. He lay down in the afternoon for a rest and shortly after awakening, became unconscious and died.

In preparing this memoir I have been greatly helped by Professor E. N. da C. Andrade, F.R.S., Professor Aage Bohr, Mrs Margrethe Bohr, Professor G. de Hevesy, For.Mem.R.S., Mrs M. M. Gowing, Sir Ernest Marsden, F.R.S., Professor R. Peierls, F.R.S., Professor L. Rosenfeld and Dr A. B. Wood.

J. D. C ockcroft

49

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