NBI History


The Niels Bohr Institute was founded on March 3, 1921.

For more information on the history of the Niels Bohr Institute, contact the Niels Bohr Archive.


The Institute Today

If you are interested to know what the Niels Bohr Institute is today, just have a look at all the other pages in this WWW server, they have been written exactly for this purpose! In any case, here is a short summary.

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History of the Niels Bohr Institute from 1921 to 1965

These notes have been written in 1965 in the following occasion.

On Niels Bohr's 80 birthday - October 7, 1965 - the Institute for Theoretical Physics of the University of Copenhagen will be given the name it has had unofficially for many years:

The Niels Bohr Institute
It is the purpose of these lines to present a short survey of the development of the Institute, so intimately connected with Bohr's life-work as a physicist.

The Founding of the Institute

Niels Bohr's first epoch-making papers on the quantum theory of atomic structure were published in 1913. Building on Ernest Rutherford's discovery of the atomic nucleus and the existence of the universal quantum action first recognized by Max Planck, Bohr formulated his "quantum postulates". On the basis of these postulates he succeeded in explaining in a convincing manner the structure of the hydrogen spectrum and thereby took a decisive step in the founding of modern atomic theory.
Niels Bohr was at that time assistant professor at the University of Copenhagen. In 1914, Rutherford invited him to take over the readership in mathematical physics at the University of Manchester, where the stimulating atmosphere had been such an inspiration to Bohr during a previous stay. Two years later, in 1916, he was appointed professor to the newly created chair of theoretical physics in the University of Copenhagen.
In the beginning, Bohr had only a small office on Sølvgade at his disposal and he was assisted by a secretary and a mechanic. His first scientific assistant was the young Dutch physicist Kramers. Soon Bohr took the initiative in establishing an institute to further the research in atomic physics, the future of which seemed so promising. A site on Blegdamsvej was purchased with funds collected by a group of far-sighted persons and the building was erected by the State. The Carlsberg Foundation contributed annual grants in support of the scientific investigations. The Institute was completed in 1921 and Niels Bohr continued as director until his death in November 1962.
Although the Institute was connected with a chair in theoretical physics and was denoted accordingly, its working domain included experimental investigations in the field of atomic physics. Indeed, Bohr laid great emphasis on the close contact between theoretical and experimental physicists. The experimental work at the Institute concentrated mainly on spectroscopy investigations, which played a prominent part in testing and stimulating the development of atomic theory.

The Development of Atomic Physics

Already during the first years after the establishment of the Institute, many young physicists from abroad came to Copenhagen in order to join the investigations in atomic theory and to participate in experimental research. Important progress was achieved in these years through Niels Bohr's explanation of the periodic system of the elements. This interpretation was soon given striking confirmation in Coster and Hevesy's discovery of a new element, which was found to possess just the properties predicted by the theory. The existence of the element was demonstrated in X-ray spectra taken at the Institute, and it was named Hafnium.
In order to provide increased possibilities for the experimental work and to make room for the growing number of collaborators from Denmark and abroad, an extension of the Institute was soon needed. The means for this extension, which took place in 1924-26, were made available by the International Education Board (later Rockefeller Foundation) in the U.S.A., and the additional land was put at the disposal of the Institute by the municipality of Copenaghen.
The development initiated by Niels Bohr's work in 1913 reached its climax in the middle of the twenties. Though an intense collaboration between physicists from many different countries, unique in its kind in the history of science, there was created a new physics - quantum mechanics - which provided the basis for a comprehensive description of atomic phenomena. The Institute became a meeting place for atomic physicists coming to Copenhagen to learn Bohr's views on this development and to work under his inspiring guidance. From the group associated with the Institute came many of the fundamental contributions to the new atomic theory.
The great progress in understanding the constitution of atoms made possible a deeper insight into the properties of matter, and new prospects were opened within wide domains of science. Many of the physicists at the Institute took up the problem of applying the quantum description to numerous branches of physic, chemistry, astronomy and biology.
It became increasingly clear to the group in Copenhagen that the departure from ordinary "classical" physical concepts implied by the new mechanics was even more radical than had been envisaged earlier. Following Heisenberg's formulation of the indeterminacy relations, Bohr took a decisive step in the recognition that our position as observers of atomic phenomena is different in principle from that in which we find ourselves when studying the events of every-day life. It is i the nature of the quantum phenomena that the interaction with the measuring instruments cannot be made arbitrarily small. Atomic properties observed under different experimental conditions cannot, therefore, be combined in the usual manner. Bohr used the word "complementarity" to characterise the relationship between apparently contradictory phenomena which only together give a complete description. The epistemological aspects of these questions became a recurring theme in the discussions at the Institute, and Bohr used examples from other domains of experience to exhibit the great scope of the idea of complementarity.

Nuclear Physics

In the years of break-through in atomic physics, interest had been focussed on the outer structure of atoms. Gradually, however, the problem of the properties of atomic nuclei and elementary particles came into the foreground. The construction of accelerators which could produce energetic beams of ions was of great importance. If the energy is sufficiently high, the ion can penetrate into the nucleus and in this way produce a multitude of reactions. At an early stage of this development, it became possible for the Institute to participate in the promising field of research due to the installation of new laboratories with a high voltage generator and a cyclotron. Funds for these instruments were made available by the Carlsberg Foundation and the Thrige Foundation as well as the Rockefeller Foundation.
Nuclear reactions can also be initiated by neutron bombardment, and in these years radium-beryllium sources were used to produce neutrons. On the occasion of Niels Bohr's fiftieth birthday, a group of Danish enterprises and foundations presented him with funds for the purchase of a strong radium source.
In the latter part of the thirties, Niels Bohr and his co-workers contributed important ideas to the understanding of nuclear structure. Essential features of nuclear reactions could be explained on the basis of a comparison of the nucleus with a liquid drop.
When uranium fission was discovered, Frisch and Meitner were able to give the first interpretation of this new process on the basis of Bohr's liquid drop model. Working with the Institute's high voltage generator and the radium-beryllium sources, they also succeeded in demonstrating the large energy released in the fission process. In the same year, 1939, Niels Bohr recognized the different roles of the two uranium isotopes in the fission of naturally occurring uranium. This fact was to play a very important role in the technical development which soon led to the release of atomic energy.
The cyclotron was used not only for the study of nuclear reactions, but also for the production of radioactive isotopes for biological research. Hevesy was the first to recognise the vast perspectives opened by the use of induced radioactivity in biological studies. From 1934 to 1943 he directed the biological tracer work at the Institute, and he and his coworkers established close collaboration with biologists and doctors in a number of research centres in Copenhagen. Deriving a lifelong interest in biological problems from his father, Niels Bohr gave enthusiastic support to the endeavours of the Institute in this field.

The War Years

From the early thirties, the political development in Europe affected international scientific cooperation. The group of visitors at the Institute contained an increasing number of scientists who on their escape from Nazism had found a refuge in Copenhagen. Some of them remained in Denmark, while the majority proceeded with Bohr's help to England and the United States.
After the occupation in 1940, the Institute became isolated from the rest of the world, but work continued. In 1943, Niels Bohr had to escape to Sweden in order to avoid arrest. Subsequently, the Institute was occupied by the German authorities for a few months.
During the last two years of the war, Niels Bohr participated in the Anglo-American atomic energy project. From the outset he recognized the far-reaching consequences of this revolutionary enterprise. To the leading statesmen he emphasised the decisive significance of openness between nations in the new situation which was both dangerous and at the same time held great promises for mankind. During the years following the war, and until his death, Niels Bohr strove untiringly to promote the acceptance of these ideas, and to foster the development of open international cooperation.

The Years after the War

Niels Bohr returned to Denmark in August 1945 and immediately began to plan an extension of the Institute to permit active participation in nuclear research which was rapidly developing. This expansion, which was financed by the State and took place during 1946--48, was also aimed at training additional numbers of students in recognition of the increasing role of atomic physics in postwar society. Again the Carlsberg Foundation and the Thrige Foundation provided large grants to scientific research.
Investigation carried out in many different countries disclosed regularities in the properties of atomic nuclei that could not be understood on the basis of the liquid drop model and which had to be ascribed to the motion of individual protons and neutrons in the nucleus. In the following years, these problems became a main topic of the work at the Institute. A close interplay between theoretical and experimental investigations has led to a more comprehensive description of the nucleus, comprising the motion of the individual particles as well as the collective types of motion which were first suggested by the liquid drop model.
The construction of still more powerful accelerators has made it possible to explore the structure of the "elementary particles". The study of atomic reactions at very high energies has resulted in the discovery of a richness of new phenomena which have increased our knowledge of the fundamental laws of nature. This latest branch of atomic physics---high energy physics---is another focus of interest for the work of the Institute. When the European Organisation for Nuclear Research (CERN) was established in 1952, the Institute was host to the CERN theoretical study group for a number of years.
Immediately after the war, the Institute resumed its international contacts and again a large number of physicists from many countries came to Copenhagen, contributing to a fruitful scientific environment. For the last ten years (1955--1965), the Ford Foundation has provided funds for the support of international cooperation at the Institute.
To promote cooperation in the field of theoretical atomic physics among the Scandinavian countries, the Nordic Institute for Theoretical Atomic Physics (Nordita) was established in 1956 through an agreement between the five governments. This new organisation works in intimate association with the Niels Bohr Institute and is located in the buildings at Blegdamsvej. Nordita offers fellowships to junior Scandinavian physicists for participation in the scientific work in Copenhagen and has succeeded in attracting a number of prominent scientists from abroad.

Present (1965!) activities of the Institute

In recent years, the field of activity of the Institute has been considerably broadened, in keeping with the expansion of physics all over the world. A large part of the advanced teaching in physics at Copenhagen University takes place at the Institute.
The experimental equipment is especially oriented towards problems of nuclear structure and high energy physics. The cyclotron, now more than 25 years old, has been reconstructed many times and has continued to be a valuable instrument for nuclear investigations. The old high voltage machine has been replaced by electrostatic accelerators producing ion beams of higher energy.
A large electrostatic generator of the so-called tandem type was acquired in 1959 and has been of great value to the whole nuclear research work at the Institute. Because of inadequate space at Blegdamsvej, this accelerator was placed on a site close to that of the Atomic Energy Establishment at Risø, about 20 miles west of Copenhagen.
Spectroscopic studies of the radiation from radioactive nuclei have likewise played an important part in the study of nuclear structure, and the Institute is also participating in this line of research.
The high energy experimental program at the Institute is based on the analysis of bubble chamber exposures made at CERN in Geneva. Studies are also being made of the production of elementary particles in cosmic radiation.
The work of the theoretical group comprises a broad section of modern physics, even though the main emphasis is on nuclear physics and high energy physics.
After Niels Bohr's death, his scientific papers were deposited at the Institute, as an archive in which source material for the study of the history of quantum physics is being collected.
At the present time (1965!), the staff of the Institute includes about forty scientists, and a similar number of physicists from abroad participate each year in the work at the Institute, in addition to the Nordita physicists (about 20). In the course of the years, more than 600 physicists from c. 40 different countries have been associated with the Institute.
To Niels Bohr international cooperation was a decisive element in the development of science. He understood how fruitful personal contacts can be and attached a great value to the friendships thus established, recognising also their importance as a means of furthering understanding between nations. From the outset, the Institute thus became a meeting place for the new generation of atomic physicists. Through the years, the contacts of the Institute with the community of physicists in all parts of the world has been steadily expanded, and this participation in international cooperation will continue to be an important aspect of the activities of the Institute.

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Quantum Theory in 1929

Recollections from the first Copenhagen conference

L. Rosenfeld

These notes were written in the occasion of the Fiftieth Anniversary of the Niels Bohr Institute -- March 3, 1971.

Although in the historical perspective the first Copenhagen conference in 1929 rather marks the completion of a heroic period in the life of the Institute than the initiation of its activity fifty years ago, the celebration of the jubilee may be a fitting occasion for calling up recollections of an elating week when the Copenhagen spirit hovered over the troubled waters from which quantum theory had just emerged. The circumstances of this gathering loom large in my own memory: I was then a mere tiro, and the pictures I have kept of the events has all he vividness of the first glimpse into a wonderful world; I have therefore no scruple to revive it, as a testimony of the mood in which a new generation was then taking the cue from the pioneers to carry on the unending quest. In fact, I have once before written down some of my impressions of this famous week under the title "My initiation", which conveys this mood. It was a lighthearted, but truthful tale, which had the honour of publication in the 1945 issue of the Journal of Jocular Physics; I do not want the present account to be much more solemn, and I shall take the liberty of quoting occasionally from the previous one.

As most good things in the world, the idea of this conference was due to chance. In Bohr's own words: Der Plan zu dieser Konferenz ist dadurch entstanden, dass mehrere Physiker, die früher hier gearbeitet haben, besuche in Kopenhagen in den Osterferien angemeldet haben. Da wir unter anderen auf die Anwesenheit von Kramers und Pauli rechnen, wird es wohl zu lebhaften und lehrreichen Diskussionen Anlass geben könen ... [1] It was quite in line with Bohr's paternal attitude that this fortuitous conjunction of visits should have prompted him to summon a full-scale family reunion. Not all could manage to be there, but a good twenty of them from all over Europe responded to the call: from Cambridge, Bohr's old friend from the Manchester time, Darwin; from Leiden another near friend, one of the great masters from the classical era, Ehrenfest, accompanied by one of his youngest students, Casimir; from Utrecht, Bohr's first collaborator, Kramers, who had left Copenhagen three years before to take up the Utrecht chair; from Zürich came Pauli, who with his acutely critical mind, was already then, as Bohr used to put it, "the conscience of the physicists"; further from Holland, Germany and Scandinavia a number of those who, each in his domain of predilection, had shared in the edification of the atomic theory: Goudsmit and Kronig, Hückel, Fues, Jordan, Heitler and Nordheim, Rosseland, Holtsmark and Waller. My own good luck was due to the circumstance that I had some time before written to Bohr to enquire whether I could come to work under his guidance; he generously counted me as a disciple in spe. At the time, four foreign visitors were staying in Copenhagen; they added their disctintive features to a variegated assembly: Mott's Cantabrigian elegance, Trumpy's Norse cheerfulness, the quiet composure of the Chinese Chou were overshadowed by the whimsical fantasy that Gamow had brought to the West from the lively group of young Soviet physicists. As to the Institute's staff, it consisted of two experimenters, S. Werner and J.C. Jacobsen, and of Bohr's closest collaborator, Oskar Klein, who had succeeded Kramers and Heisemberg in this position of trust. Among the handful of Danish students who attended the proceedings, we find the familiar names of Christian Møller, Bengt Strömgren, Mogens Pihl and the regretted Ebbe Rasmussen.

The conference was due to start on Monday the 8th of April, and a number of the pilgrims travelled on the preceding Sunday. They all met inevitably on the deck of the ferry from Warnemünde to Gedser. There was much handshaking, exchange of news and shop talk. This was my maiden trip, and a propitious day for a first view of the Danish scenery, in the timid budding of spring, with flags daily flying in front of every thatch roofed farmhouse (not, as I learned from my more experienced companions, in honour of the conference, but just as a weekly manifestation of homely content). The old-fashioned look of the ferries and railway carriages, the queer funnels of the locomotives, the easy-going demeanour of the railway people and of the local passengers at the cosy red-brick stations where the train unhurriedly lingered, all concurred to build up the impression of a simple-minded, un demanding peasant community, happily confined to its own well protected little world. Tourist agents had not yet discovered how idyllic a country it was, how wonderful its capital.
The pleasant feeling of old-time hospitality conveyed by the Danish countryside came to a climax on our arrival at Copenhagen. Niels Bohr himself was awaiting us on the platform, together with his brother Harald, his lieutenant Klein and a few boys of various sizes, obviously his sons. I recognized Harald, whose lectures on almost periodic functions I ha attended in Göttingen, and I shook hands with Niels for the first time. He received me with a broad, benevolent smile; I was struck by the cordial simplicity with which he greeted old friends and newcomers alike. Fro the station I strolled to the boarding house I had been assigned, on the Vester Boulevard, then a spacious promenade lined with stately elms, which formed a worthy setting for the little trumpeter's eternally unavailing last call. At dinner, I had occasion to experience how the Danes could sometimes overdo their hospitable attentions. They subjected me to the ordeal of pronouncing rødgrød med fløde, and adding insult to injury, expected me to find the stuff delicious.

On the next morning, we all flocked to the Institute lecture room; old acquaintances were reniewed, new ones formed. I happened to be nearby when a beaming Ehrenfest came in and went straight to greet Bohr, followed by a tall, fair-haired, rosy-cheecked youth of rather indolent gait, who did not quite know what to do with his arms. Ich bringe Dir diesen Knaben, he said to Bohr, while he affectionately put his hand on the boy's shoulder. Er kann shon was, aber er braucht noch Prügel. Well, in the course of the week, Casimir was going to show us, unobtrusively, that he indeed could do something. Goudsmit was then applying his extraordinary skill to the classification of hyperfine spectra and was naturally anxious to know what the contribution of the electron spin to the hyperfine structure would look like. As soon as he heard Goudsmit mention the problem, Casimir keenly responded: he asked Goudsmit some brief questions and, lost in meditation, withdrew to a quiet corner; but let us hear the story from himself [2]: At the time of the conference Dirac's theory of the spinning electron was still fairly new ... I had studied the paper and also Weyl's Gruppentheorie und Quantenmechanik and had been much impressed by the beautiful simplicity of the current distribution in the fundamental state of the hydrogen atom. So when Goudsmit asked me whether I could calculate the hyperfine interaction in a S state I realized at once that one had only to calculate the magnetic field of the current. I arrived at the formula

DELTA E = ...
where the nuclear moment is given by Nuclear moment. Goudsmit supplied an estimate for Magnitude of PSI for "Tauchbahnen" and we obtained pretty nice results for Na if I remember well. After the conference I wrote a manuscript --a fairly clumsy one I fear-- which I sent to Goudsmit. I got it back, much later but soon afterwards Fermi's manuscript "Ueber die magnetischen Momente der Atomkerne" was received (18. Dec. 1929) by Zs. für Physik and appeared (Z. für Physik 60, 320, 1930). It was quite edifying to watch Casimir's eagerness in working out this problem. One evening we, some of us went to see the pictures, and persuaded him to join us. In those days, they showed one reel of film at the time, and lit the room every time they changed reels; at each recurrence of the light, we could see our friend bending over odd scraps of paper and hastily filling them with formulae.
Commenting on the decisive part of the unravelling of the spectral regularities had played in the development of atomic theory, and mentioning especially Goudsmit's virtuosity in this art, Bohr told us that the latter's talent for tracking hidden order in apparent capriciousness was by no means limited to physics; he also exercised it on classifying the types of representation of sacred scarabs by the ancient Egyptians. On Goudsmit's first visit to Copenhagen in 1926, Bohr had been with him to see the collection of Egyptian sculpture at the Glyptotek; as he started translating the Danish labels for Goudsmit's benefit, the latter quietly told him it was not necessary, as he could read the inscribed hieroglyphs. Bohr had a large stock of such pointed anecdotes, reflecting his warm interest in people, that he was never tired of telling in those conversation with the older and younger physicists clustered around him, during which he freely expressed his thoughts on the prospects of current research as well as the widest implications of science, and displayed the best of his wisdom and deep humanity.

Bohr as a lecturer is a different matter. It is much glossed, but very little written about. Perhaps the only one who has put his view of it in print so far is Larmor; ina speech (later published [3]) at the Maxwell celebration in Cambridge in 1931, he commented upon Maxwell's reputation of being a poor lecturer and roundly added: So perhaps with our friend Bohr: he might want to instruct us about the correlations of too many things at once ... I was sitting near Bohr when the speech was delivered; as this judgement was expressed, Bohr whispered to me: Imagine, he thinks I am a poor lecturer! Bohr's lectures, composed with tremendous labour, were indeed masterpieces of allusive evocation of a subtle dialectic; the trouble was that the audience was usually unprepared to catch subtle allusions to conceptions and arguments which were anyhow unfamiliar and hard to grasp.
I am not sure whether Bohr's introductory talk at the conference was really worse than the average; perhaps he had not prepared it so thoroughly, since the idea was to have quite informal discussions: no programme had been set up in advance - Bohr took in turn each of the participants aside and asked him what topic he wished to bring up. At any rate, here is the impression this talk has left in my memory, as I described it (with some hindsight) in 1945: He had begun with a few general considerations calculated, no doubt, to convey to the audience the peculiar sensation of having the ground suddenly removed from under their feet, which is so effective in promoting receptiveness for complementary thinking. This preliminary result being readily achieved, he had eagerly hastened to his main subject and stunned us all (except Pauli) with the non-observability of the electron spin. I spent the afternoon with Heitler pondering on the scanty fragments of the hidden wisdom which we had been able to jot down in our note books.
It was comforting to hear from Klein, when I told him some time ago of our failure to understand what Bohr meant by the impossibility of measuring the spin of the electron, that he had the same difficulty when Bohr first discussed the matter with him in the autumn of 1928. Guided by the general correspondence idea, Bohr argued that such a purely quantal concept as the electron spin, vanishing from the theory in the classical limit, could not possibly be brought in direct relation with classical quantities like angular momentum or magnetic moment. It was not immediately clear to Klein, however, how this correspondence argument could be reconciled with the Stern-Gerlach effect, which clearly exhibited a contribution to the magnetic moment of an atom from an electron bound in a 2S state; but what Bohr demonstrated was precisely that with a free electron a Stern-Gerlach experiment could not succeed, because the effect of the Lorentz force would inevitably blur any Stern-Gerlach pattern. This is the point he ineffectually tried to make in his talk. Fortunately, Mott, during his stay at the Institute, had been engaged in the problem of electron polarisation, and in the paper [4] in which he brilliantly showed how this property could in principle be ascertained by a double scattering experiment, he gave a very clear account of the whole situation. He finished writing this paper shortly after the conference (it was sent off by Bohr on the 25th of April) and we were thus soon able to appreciate at leisure the full force of Bohr's famous argument.

The incorporation of the spin into the relativistic quantum theory of the electron had not removed from this theory the riddle of the negative energy states. Klein had just given a striking illustration of the acuteness of the difficulty by showing that electrons impinging upon a sufficiently high and steep potential wall would not only suffer reflexion, but that a sizable fraction of them would penetrate through the wall and undergo a transition to states of negative kinetic energy [5]. At the conference, he submitted, tentatively, a way to escape from this "paradox": it amounted to treating the electrostatic potential as an operator whose eigenvalues would have a finite higher limit. Jordan pointed out that in order to eliminate the paradox, this highest potential value should be of an order of magnitude quite within the range of observation; probably it ought to be something like 2 m c**2 (m denoting the electron mass). Potential differences of this magnitude are indeed observed between thunderstorm clouds and the earth.
Jordan's remark caught Bohr's imagination: here was a possible test of Klein's assumptions. The limiting potential would presumably entail a limit to the stability of matter: if it did occur in a thunderstorm, a bird flying in the region of highest potential would then be killed. The discussion went on, to and fro, as various consequences of Klein's formalism were either put forth in support of it or found to raise difficulties. Bohr kept musing about the fate of the bird, to Ehrenfest's boundless amusement. Whenever the discussion ebbed out for a moment: Now, Bohr, he would ask with boyish mischief, is the bird still alive?
When we gathered again after lunch, Klein declared that he was now convinced that his proposal would not work and that he withdrew it. This was probably the shortest lifespan ever meted out to any theory. For all his apparent playfulness, Ehrenfest took very much to heart the difficulties in which relativistic quantum theory seemed to be bogged. I got an inkling of this the next day, when, much to my surprise, I was told, shortly after the end of the session, that Bohr wanted me to come to his house (which was adjacent to the Institute and now part of it). I was ushered into his study, where I found him and Ehrenfest installed in comfortable armchairs and, judging by their smiles, engaged in pleasant conversation. Could you tell us, Ehrenfest asked me with his wonted directness, why the relativistic Klein-Gordon theory is unacceptable? - Because, I replied pedantically, the charge-current density is not of definite sign. - Yes, said Ehrenfest, but could you just give us an example in which this property leads to unphysical consequences? I never felt so sheepish in my life. Ehrenfest turned to Bohr: You see, he said, just as I told you! From this remark I surmised with some relief, - and my guess was soon confirmed by the ensuing conversation, - that they did not impute the fiasco to my personal stupidity, but took it as an illustration of a widespread failing of the young generation, prompt to uncritical acceptance of dogmatism. Bohr intervened with a soothing pronouncement about how hard it was to imagine how the behaviour of the electron could be described beyond the limit where correspondence with the classical point-charge picture afforded legitimate guidance; we must here be prepared for further renouncement in the application of classical concepts. How often were we to hear this warning in the following years, until the discoveries of the neutron and the positive electron broke the spell!
In retrospect, one cannot help admiring the penetration and independence of judgment revealed by Ehrenfest's iconoclastic questioning. And yet (as I later learned from Bohr), he was at the same time turning his searching criticism against himself: he realized that progress in such untried regions of thought and imagination as quantum theory had opened, demanded adventurous minds, unhampered by the scruples and doubts that assailed him; he imagined he was losing his grip on physics and - worse still for one always ready to pour out a wealth of affection on the youth - getting out of touch with the coming generation. Of this inner tension there was no outward sign, however: to the last we saw him as cheerful, witty and warmhearted as ever.

Pauli, so far as I remember, was rather subdued, except on one spectacular occasion. Heitler, by lecturing on the theory of homopolar bond, unexpectedly excited his wrath: for, as it turned out, he had a strong dislike to this theory. Hardly had Heitler finished, that Pauli moved to the blackboard in a state of great agitation; pacing to and fro he angrily started to voice his grievance, while Heitler sat down on a chair at the edge of the Podium. At long distances, Pauli explained, the theory is certainly wrong, since we have there the Van der Waals attraction; at short distances, obviously, it is also entirely wrong. At this point he had reached the end of the podium opposite to that where Heitler was sitting. He turned round and was now walking towards him, threateningly pointing in his direction the piece of chalk he was holding in his hand: Un nun, he exclaimed, gibt es eine an den guten Glauben der Physiker appellierende Aussage, die behauptet, dass diese Näherung, die falsch ist in grossen Abständen und falsch in kleinen Abständen, trotzdem in einem Zwischengebiet qualitativ richtig sein soll! He was now quite near to Heitler. The latter leaned back suddenly, the back of the chair gave way with a great crash, and poor Heitler tumbled backward (luckily without hurting himself too much). Casimir, who also remembers the incident, notes that Gamow was the first to shout: Pauli-effect! And as an afterthought he adds: Sometimes I wonder whether Gamow had not done something to the chair beforehand.

I found Gamow as jolly as I had always known him in Göttingen since the first day I saw him appear at Born's institute and heard of his surprising intention of applying quantum mechanics to alpha-radioactivity. His direct approach to the problem was, of course, most congenial to Bohr, who had supported him wholeheartedly when he had to counter Laue's doubts about the lawfulness of the new type of solution he had introduced [6]. I was then too much indoctrinated with Göttingen lore not to side with Laue in this debate; the more so as Born had just shown us how he wanted to treat the problem by a somewhat hazardous, but very ingenious, extension of hortodox perturbation technique [7]. When remonstrating with Gamow in the library, I went so far as to express doubt whether Gamow wavefunctions, with their infinite norm, could at all be regarded as solutions of a Schrödinger equation. Pauli happened to overhear this rather wild statement: They are certainly solutions, he interjected, but whether they are allowed in quantum mechanics, this is questionable. Gamow looked at us with an expression of wonder in his face: But my solution, he said, just represents a damped resonance process; the exponential increase at large distances has a simple interpretation, and after all I get the decay constant and an improved Geiger-Nuttall law. What else do you want? -- Anyhow, Pauli concluded as he went away, it was great fun to see the flood of papers your theory let loose.
I don't understand what all this fuzz is about, Gamow continued, It is just a solution of a partial differential equation, of the kind we use in mechanics and elasticity. When I showed it to Bohr, he was at once enthusiastic. I was so pleased and proud that I wanted to show him something more. I had noticed that the Schrödinger equation could be written as a classical diffusion equation, - only with a purely imaginary diffusion coefficient. Now, I had better kept that to myself: Bohr's reaction to it was anything but enthusiastic ...

On Gamow's work, which ushered in nuclear physics, there was no debate. Mott, however, presented his analysis of the scattering of alpha-particles by helium [8], which was received with considerable interest. It was a beautifully simple case of an interference effect arising from the identity of the interacting particles, the first concrete example of the intervention of Bose statistics in an individual process. It was also a good lesson in the correct interpretation of a wave-function - an art then so new that Mott's argument was not accepted without some resistance on the part of one or two participants.
Darwin was deeply interested in these problems of interpretation; he did extremely helpful work discussing in great detail, with consummate mathematical skill in the best Cambridge tradition, a variety of idealized experimental devices illustrating typical features of quantum mechanics. The one he submitted at the conference [9] was a rather complicated case of constructive interference of wave-packets producing spontaneously the results which simple intuition would suggest could only be due to particles. The analysis was as brilliant as ever, but unfortunately carried him away, at the end, onto splippery ground. He argued that the seat of an observation could be shifted from the physical receptor to the retina, and thence to somewhere in the brain, where we are absolutely compelled to stop. Right up to the brain the process is non-committal: it is only after our consciousness has animated the proceedings that it is possible to infer back and describe what actually happened in the familiar language of particles. Thus, we have a sub-world described by a wave-function, a dead world, not involving definite events, but instead the potentiality for all possible events. It becomes animated by our consciousness, which so to speak cuts sections of it when it makes observations.
This remarkable anticipation of von Neumann's Schnitt conception struck me as being quite beside the point. It cannot make any difference by how many eyes, or brains, a scintillation on a screen is observed, and there is no reference in the formalism (then my only anchor!) than to the particle producing scintillation: the structure of the eye or the brain, or even that of the screen, is quite irrelevant. These were (as near as I can figure out after so many years) the confused thoughts that crossed my mind as I listened to Darwin. What troubled me mightly, however, was Bohr's failure to react to propositions which seemed to me so alien to the ideas he had expressed in the Como lecture. I was so disturbed by what I mistook for silent acquiescence that, mustering all my courage, I approached Bohr and cautiously started to voice my doubts. Oh! he said, interrupting me, this was all nonsense!
Then he motioned me to a neighbouring room, which had been Hevesy's laboratory: it was lined with cupboards, behind whose glass doors one could see rows of flasks containing brilliantly coloured solutions of rare-earth compounds. In the middle of the room there was a long table, and I stood at one extremity of it. I cannot find better words to tell what happened next than those I wrote in my previous article: Bohr described around the table, "at a rather lively pace, a Keplerian ellipse of large excentricity, of which the place where I was standing was a focus. All the time, he was talking in a soft, low voice, explaining to me the broad outlines of his philosophy. He walked with bent head and knit brows; from time to time he looked up at me and underlined some important point by a sober gesture. As he spoke, the words and the sentences which I read before in his papers suddendly took life and became loaded with meaning. It was one of the few solemn moments that count in an existence, the revelation of a world of dazzling thought, truly an initiation." Bohr was particularly well prepared to comment on the problems Darwin had raised: he was just writing up (in a short essay [10] in honour of the 50th anniversary of Planck's doctorate) his reflexions on the complementary aspects of psychical phenomena. As an example of the shift that can occur in the separation between observing subject and object of observation, he mentioned the case of a cane held in the hand: if it is held firmly, it is an instrument extending the range of tactile perception of the subject, who feels it as a part of himself; if it is held loosely, it ceases to serve as an instrument and becomes an object of observation. He was quite excited about this effect. He insisted that I should try with a pencil that was lying on the table, and he watched me intently during the operation, eager to catch on my face a sign of the joyous wonder I could not fail to experience.

I have not too much to tell about the obligatory three-castle tour, with a stop at Bohr's country house at Tisvilde, a long walk through the wood to Brantebjerg and along the strand to the village. The more enterprising displayed their sportive talents; Bohr was outstanding at ricochet throws of pebbies on the sea, but most spectacular was Jordan's acrobatics (it was rumoured he had lately taken to jiu-jitsu). I would rather recall an afternoon outing of a small group of us, among whom Ehrenfest, to Christianshavn, where Bohr wanted to show us Our Saviour's church. We took the boat at Nyhavn and landed near the famous church, with its queer outer staircase winding up along the steeple. I was thrilled of the thought of seeing this monument, about which I had read as a boy in Jules Verne's Voyage au centre de la terre. From the landing place the steeple looked curiously flat; Ehrenfest had the same impression: It looks as if it had an elliptic cross-section - if this were possible, he said. One ascends to the steeple from inside the church; one first arrives at a kind of platform, on which lies, idle and rusty, a huge clockwork dating, as we ascertained, from the time of Huygens, and Newton. Here was the real goal of the trip. Bohr exulted: This shows us what classical mechanics is about, he exclaimed. Nobody that sees this can doubt that our measuring instruments must be big bodies!
Next time Jauch comes to Copenhagen, I must take him to Our Saviour's church.

References

[1] Letter from N. Borh to P. Jordan (5 March 1929)
[2] Letter from H. Casimir to L. Rosenfeld (7 December 1970)
[3] James Clark Maxwell: A Commemoration Volume 1831-1931 (Cambridge University Press 1931) p. 78
[4] N. Mott, Proc. Roy. Soc. A124 (1929) 425
[5] O. Klein, Z. Physik 53 (1929) 157
[6] M. v. Laue, Z. Physik 52 (1928) 726; G. Gamov, Z. Physik 53 (1929) 601
[7] M. Born, Z. Physik 58 (1929) 306
[8] N. Mott, Proc. Roy. Soc. A125 (1929) 222; A126 (1930) 259
[9] C. G. Darwin, Proc. Roy. Soc. A124 (1929) 375
[10] N. Bohr, Naturwiss. 17 (1929) 483

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