by Professors Ben Mottelson and Chris Pethick
First published in the Nordita Annual Report 1996
At a round birthday it is natural to look back and survey what has happened over the years. Our aim here is to give a number of examples that illustrate how NORDITA has responded to the changing challenges in theoretical physics, as well as to the changing needs within the Nordic countries. We shall not attempt to give a detailed historical account of NORDITA, and will place most emphasis on what has happened over the second half of the institute's life. For the early history of NORDITA we refer to the report "NORDITA 1957-82" issued on the occasion of NORDITA's 25th anniversary, as well as the personal accounts by Torsten Gustafsson (1) and Pekka A. O. Jauho (2).
The background for the foundation of NORDITA was the long traditions for collaboration between Nordic physicists that stretch back to the middle of the last century. These were further strengthened by the key role that Niels Bohr and his institute played in the creation of modern physics during the 1920's and 1930's. The idea of establishing a Nordic institute for theoretical physics arose in the years immediately following the Second World War, and after lengthy deliberations at various levels, the establishment of NORDITA was authorized at a meeting of the Nordic Council in Helsinki on 21st February 1957. On 1st October 1957 the institute started operations, with Christian Møller as director, and Ben Mottelson, Gunnar Källèn, and Leon Rosenfeld as professors. Källèn resigned to take up a chair at the University of Lund in 1958, and later Jim Hamilton and Gerry Brown became professors at NORDITA. For the first decade of its existence, NORDITA's activity was primarily in nuclear and particle physics, the areas in which staff members were then working.
During the 1960's condensed matter physics in the Nordic countries was growing, and it was recognized that the scientific environment at NORDITA and its relevance to developments in the Nordic countries would be strengthened in an important way if a significant activity in this area could be developed. An initial step was the appointment of a number of prominent condensed matter physicists, among them Gordon Baym, Vic Emery, Stig Lundqvist, David Pines, and Alf Sjölander as visiting professors. An especially important role was played by John Wilkins, who was a visiting professor for a total of more than three years (including the academic years 1975-77 and 1979-1980). He built up a very active group and trained a number of NORDITA fellows. The first permanent appointment was that of Chris Pethick (1975), who at the time was working on quantum liquids as well as on topics in astrophysics. Subsequently Alan Luther, whose research at the time focussed on low-dimensional systems, was appointed in 1976, and John Hertz, whose research was on spin glasses and later on neural networks, in 1980. The latter position represented an expansion of the NORDITA staff which had been argued for by pointing to the desirability of increasing the breadth of the scientific environment in the condensed matter field. In making these appointments the Board laid special emphasis on the role of NORDITA in developing an activity in frontier areas of physics which were otherwise less strongly represented in the Nordic area. It is significant that many of the NORDITA fellows from the early years of NORDITA's expansion into condensed matter physics, among them Risto Nieminen, Matti Manninen, Petter Minnhagen, Martti Salomaa, and Jussi Timonen are now professors in condensed matter physics at Nordic universities.
In the development of the condensed matter activity, an important role was played by the special connection with the Landau Institute that NORDITA was able to establish, building on the strong traditions of the Niels Bohr Institute for collaboration with physicists in the Soviet Union. During the difficult period of the 1980's Copenhagen was an almost unique centre where scientists from both East and West could meet and collaborate on scientific matters.
Another notable feature of NORDITA's activity in condensed matter physics was a strong collaboration with the Low Temperature Laboratory of the Helsinki University of Technology, where forefront experiments on the properties of the superfluid phases of liquid 3He were being carried out. NORDITA supported visiting scientists in Helsinki to strengthen the theoretical environment, and scientists and fellows at NORDITA were engaged in theoretical analysis of data from Helsinki.
Over the years NORDITA has been a centre for studies of aspects of condensed matter outside the main focus of the permanent staff. Among these we may mention the work on electronic properties of metals which was a major activity at the institute while John Wilkins was visiting professor. This tradition was continued by Jens Nørskov, who was assistant professor at NORDITA in 1982 - 1985. He was subsequently appointed as an adjunct professor at NORDITA and spent much of his time at the institute while employed by Haldor Topsøe, a major producer of industrial catalysts, and later he became a professor at the Danish Technical University. Also during the years 1986-1988 when Allan Mackintosh was director, NORDITA had a significant group working on electronic properties of metals. More recently, the study of electronic phenomena in small systems, an area of growing importance for physics reasons as well as because of applications to devices, was a major thrust of NORDITA's activity, during and following the period 1992-1993 when Antti-Pekka Jauho was a Nordic assistant professor.
Another area in which the activity at NORDITA has been expanding in response to new opportunities in contemporary physics is the study of complex systems. The discovery of universalities that link such diverse systems as planetary and galactic orbits, populations of predators and prey, turbulent flows, and the avoided crossings of the levels of the hydrogen atom in a magnetic field created a new field within statistical physics in which NORDITA together with the Niels Bohr Institute has been able to create what is one the leading centers for such studies. This was possible due to the fortunate confluence of a number of different personalities. At the time of his appointment, John Hertz was carrying out research on spin glasses. The activity in dynamical systems was initiated in 1981 by Predrag Cvitanovic, who at the time was assistant professor in particle physics (1980 -1984). At about the same time, across Fælledparken at the H.C. Ørsted Institute of the University of Copenhagen, Per Bak, a former NORDITA fellow, and Tomas Bohr, and Mogens Høgh Jensen, who were then graduate students, had independently taken up the study of complex systems. These people formed the core around which the activity in complex systems in Copenhagen grew. After postdoctoral work in the United States, Mogens Høgh Jensen was appointed assistant professor at NORDITA (1986-1992), and he, like Predrag Cvitanovic and Per Bak, has now gone on to a permanent position in the Danish system.
The study of complex systems has been one of the major themes at NORDITA over the past 15 years, and NORDITA has played an important role in developing expertise in this area within the Nordic countries by training fellows, and arranging numerous summer schools, workshops and conferences on the subject. Examples within recent years are the summer school on "Complex systems: Turbulence, Chaos, and Neural Networks" in 1993 and the "Nordic Non-linear Days" in 1996. The fact that many prominent workers in the field, including Mitchell Feigenbaum, Leo Kadanoff, and Itamar Procaccia (who is an adjunct professor at NORDITA) frequently visit contributes to the excellent environment at NORDITA for this work. Over time many different aspects have been taken up, including quantum chaos, turbulence, growth phenomena, neural networks, and, more recently, biological problems, a topic on which the current Nordic assistant professor in the area, Kim Sneppen, works. Another aspect of the work on complex systems is that it has been a source of inspiration also to scientists working in other areas of physics, including condensed matter physics, nuclear physics and astrophysics. Considering how young the field is, it is noteworthy that a number of former NORDITA fellows, including Per Dahlqvist, Kristian Lindgren and Mats Nordahl already have permanent positions at Nordic universities.
Another major development over the past two decades has been NORDITA's expansion into the area of theoretical astrophysics, which played a significant role in the development of this field in the Nordic area. The revolutionary observational discoveries concerning the basic cosmological issues and phenomena involving high energies pose profound questions that theorists have to struggle with. The germ for growth of astrophysics at NORDITA can be traced back to Bengt Strömgren's becoming affiliated with NORDITA on his return to Denmark in 1967. The following year Bengt Gustafsson came as a NORDITA fellow to work with Bengt Strömgren. Subsequently series of seminars on astrophysical topics were held on the Niels Bohr Institute and NORDITA, and a number of astrophysicists and physicists with interests in astrophysical phenomena, including Charles Barnes, Jesse Greenstein, Thomas Gold, Vittorio Canuto were visiting professors at NORDITA. In 1975, Chris Pethick was appointed as professor, and in that fall he and Bengt Strömgren arranged an intensive two-week school, the Astrophysics Novemberfest, on a range of topics in theoretical astrophysics. This attracted 56 students from the Nordic countries, among them Claes Fransson, Einar Gudmundsson, and Roland Svensson, all of whom are now full professors in astrophysics but who at that time had yet to begin their graduate studies.
In 1976 Richard Epstein was appointed as assistant professor in astrophysics, greatly strengthening the breadth of activity at NORDITA. In addition during this period Gerry Brown, and Hans Bethe, who was a regular visitor, were engaged in an active collaboration on supernova explosions. NORDITA became one of the few centres in the Nordic countries for work on some of the modern themes in contemporary astrophysics, especially what is generally termed `relativistic astrophysics', and fostered the development of young Nordic scientists working in this area. The more recent assistant professors in astrophysics each brought with them expertise in some new facet of astrophysics: Bernard Jones (1983-1989) working on galaxy formation and cosmology, Roland Svensson (1985-1990) on active galactic nuclei and processes in relativistic plasmas, Marek Abramowicz (1991-1993) on general relativity and accretion discs, and the current one, Sasha Kashlinsky (appointed 1994) on microwave background fluctuations and cosmology. Following the retirement of Gerry Brown, Bernard Pagel was appointed as the first professor of astrophysics at NORDITA in 1990. His broad interests spanning a wide range of topics in both theoretical and observational astronomy, especially centering on issues of nuclear abundances and their interpretation in terms of galactic and cosmological history, greatly expanded the interface between NORDITA and the astronomical community.
The impact of NORDITA's efforts in astrophysics may judged from the fact that many people appointed in recent years to permanent positions in the field at Nordic universities were NORDITA fellows or had been closely associated with NORDITA. These include Claes-Ingvar Björnsson, Jørgen Christensen-Dalsgaard, Claes Fransson, Einar Gudmundsson, Bengt Gustafsson, Per Lilje, Jes Madsen, Åke Nordlund, Roland Svensson, and Esko Valtaoja.
The "classical" fields at NORDITA (nuclear and particle physics) have also changed profoundly in response to the evolving scientific challenges. The boundaries between the two areas are not well defined and are constantly changing. Thus many topics which fell within the domain of particle physics two decades ago are today regarded as being part of nuclear physics. The overwhelming success of the Standard Model of the weak, electromagnetic and strong interactions has attracted a large part of the particle physics community to the issues involved in formulating a theory that can provide a consistent unification of all known interactions including gravity. This has required the introduction of new symmetries such as, for instance, supersymmetry, which is a symmetry between fermions and bosons, and the extension of field theories based on point-like objects to string theories based on one-dimensional extended objects (strings). Since string theories provide a quantum mechanically consistent extension of Einstein's theory of general relativity unified with the gauge theories that are at the basis of the Standard Model, they have been proposed as unified theories of all interactions at vastly greater energies than those presently available. Thus the presence at NORDITA of Paolo Di Vecchia, who does research in these areas, represented an important new chapter in the particle physics activity at the institute. He was first an assistant professor at NORDITA (1974-1978), and was later appointed as Jim Hamilton's successor as professor (1986). The activity in particle physics has also broadened to encompass cosmic phenomena, the area now generally referred to as astroparticle physics, and Kari Enqvist, who works in this area, was an assistant professor during the period 1990-1994.
The nuclear physics activity, besides maintaining a strong interest in the current issues of nuclear structure (effects of large angular momentum and extreme isospin), has actively cultivated the connections to other fields: astrophysics (Pethick, Brown), atomic clusters (Mottelson), non-linear systems and especially particle physics. At the latter interface one may mention the work on models of nucleons and nuclear forces of Brown and of Andreas Wirzba (assistant professor, 1986-1992), on the use of chiral perturbation theory to understand particle interactions by Johan Bijnens (assistant professor, 1992-1997), on properties of mesons in nuclei and sub-nuclear degrees of freedom by Wolfram Weise (adjunct professor, 1992-present) and relativistic heavy-ion interactions by Henning Heiselberg (Nordic assistant professor, 1995-present). Another significant area is the study of Quantum Chromodynamics, where Paul Hoyer (Senior fellow, 1977-1981, director, 1994-present) and Vladimir Braun (assistant professor, 1995-present) have active research programmes. In providing this breadth of scientific environment NORDITA has relied heavily on the possibility of appointing assistant and adjunct professors as well as Nordic assistant professors, as in clear from the names of the leaders in the interdisciplinary connections sketched above.
As in other areas of physics, many NORDITA fellows in particle physics, among them Hans Hansson, Paul Hoyer, Jon Magne Leinaas, Carsten Lütken, Per Osland, Finn Ravndal, and Bo-Sture Skagerstam, have became professors at Nordic universities. In nuclear physics there are likewise many faculty at Nordic universities who are closely associated with NORDITA. These include Ikuko Hamamoto, Eivind Osnes, Dan Olof Riska, Jan Vaagen and Sven Åberg.
The form of NORDITA's activities and its administrative structures have undergone major developments reflecting the changing structure and needs of the Nordic scientific community. The contact with this community was greatly strengthened by the appointment of Nils Robert Nilsson (coming from Uppsala) as the chief of administration in 1972. Nils Robert Nilsson's enthusiasm, his creativity in assisting physicists in realizing their projects, and his broad network of connections in the Nordic countries and elsewhere were of immense importance to NORDITA and Nordic collaboration more generally for more than two decades. Similarly it was of great value to NORDITA that Paul Hoyer (Helsinki) was able to accept appointment to the directorship (1994), thus for the first time bringing a scientist from outside the Copenhagen area to this position.
Another initiative to strengthen contact between the Nordic scientific community and NORDITA was the setting up of sub-field committees in the various areas of physics. At the suggestion of John Wilkins, this was initiated in 1975 in condensed matter physics, an area of physics in which the contacts within the Nordic countries were less well developed than in nuclear and particle physics, in which there already existed strong Nordic links stemming from the central role that the institutes in Copenhagen had played in the scientific life in these areas. Subsequently sub-field committees were also established in other areas of physics, and these have played important roles as a channel for communication between NORDITA and the Nordic community, and as part of NORDITA's decision-making process.
NORDITA has in the second half of its 40 years initiated activities that aim at making contact with younger scientists at an earlier stage in their career than represented by the usual fellowship program. These new activities have taken the form of summer schools focused on current research in the different subfields of physics, and contact weekends to which PhD students from the Nordic area are invited to spend a few days at NORDITA in order to hear lectures from the staff and to get to know each other. Typically in any year NORDITA is strongly involved in the organizing of about two summer schools, and in 1996 there were ones on magnetic fields in astrophysics, on high-energy physics phenomenology, and on non-linear phenomena in physics and biology. In 1997, new initiatives include a master class aimed to introduce undergraduates to exciting current topics, and a two-week cross-disciplinary programme to bring forefront areas of research to the attention of beginning graduate students.
An important step to increase NORDITA's scientific breadth and to strengthen ties to the international scientific community has been the appointment of prominent scientists as adjunct professors. These come from a number of different countries, and are broadly distributed in terms of fields of interest.
In the structure of activities at NORDITA another innovation has been the attempt to have focussed programs addressing current problems in some well defined area for a period typically of order a few months, inspired to some extent by the programs at the Institute for Theoretical Physics at Santa Barbara. Among programmes arranged by NORDITA were the ones on "Nuclear Structure in the Era of New Spectroscopy", 1989, to explore perspectives opened up by the new generation of nuclear detectors, "Hot Spots in Astrophysics", a three-week workshop in 1990 on frontier problems in relativistic astrophysics organized by Igor Novikov, and that on "Perturbative QCD" held in Lund 1991. The latter brought together experts on analytical solutions with the outstanding group in Lund working more phenomenologically. Also in 1991 there was a programme on "Physics of Quantum Chaos and Measurement" that brought together the leading workers in the subjects. A more extended programme on the physics of nuclei that can be investigated using facilities with beams of radioactive nuclei took place in 1993-1995. This program was proposed by physicists at Nordic universities (T. England, J. Vaagen and S. Åberg), and was run by them in collaboration with members of the staff at NORDITA. This mode has now evolved into what are termed "Nordic Projects", of which there are at the present time ones on accretion disks in astrophysics, relativistic heavy-ion collisions, supersymmetric theories, computational materials and surface physics, fundamental constituents of matter, and the nature of cosmic gamma-ray bursts.
NORDITA has made considerable efforts to find additional financial support from a variety of funding agencies. Among these are the research councils in the Nordic countries, which support graduate students at the institute for periods from months up to about a year, the EU, which supports postdoctoral positions and networks, NorFA, which supports summer schools and workshops, and the special funds from the Nordic Council of Ministers for the Baltic countries and NW Russia, which provide fellowships for young scientists. The total support from these sources is in the region of 10% of the total budget and it adds significantly to the activities of the institute. It is also important to bear in mind that NORDITA receives a considerable amount of indirect support because, by virtue of its attractiveness as a scientific centre, many visitors come supported by funds from their home institutions.
The political developments in the Baltic countries have opened the possibility for NORDITA to greatly strengthen its interaction with physicists in this area and partipate in the renaissance of scientific life that is being experienced there. Initially this took the form of visits of senior Baltic scientists to NORDITA, and visits by NORDITA staff and other Nordic physicists to centres in the Baltic countries. More recently, Nordic summer schools have been held with participation by both Baltic and Nordic students, and two of these schools have taken place in the Baltic countries. A further step has been the establishing of a fellowship programme for promising young scientists to work at NORDITA, and a number of the fellows have begun collaborations with NORDITA staff.