Victor Weisskopf: looking back on a distinguished career

29 May 2002

Wolfgang Kummer pays tribute to former CERN director-general Victor Weisskopf, who died on 22 April at the age of 93.


In the spring of 1960, CERN’s proton synchrotron (PS) was delivering its first beams. In the middle of this critical phase for European particle physics, CERN’s director-general, Cornelis Bakker, was killed in an aeroplane accident. Although CERN’s governing Council acted swiftly by appointing John Adams as acting director-general, this step necessarily prolonged the period that in retrospect may be characterized by the dominance of brilliant accelerator scientists.

At the same meeting in June 1960 which confirmed Adams’ appointment, the “modern” structure of research committees with at least as many members from outside as inside the laboratory was also approved, and the search for Bakker’s successor began. In any case, Adams would have to leave CERN to take up an important position in the UK. The discussion centred around two eminent scientists – Hendrik B G Casimir and Victor F Weisskopf. Weisskopf was already well known at CERN, having worked in the Theory Division from 1957 until 1958. With characteristic modesty he doubted his talents for such a position, but he expressed his willingness to act as a director of research. Casimir made it clear that his position with Philips would make it very difficult to take over the post of CERN director-general.

During the following months, a formal nomination procedure of candidates in the Scientific Policy Committee (where Weisskopf was formally proposed by Greece), extensive deliberations and successful persuasion led to Weisskopf’s election by Council on 8 December 1960. His term was envisaged to run from 1 August 1961 until 31 July 1963, but this was later extended until 31 December 1965. It is no exaggeration that in that period, under Weisskopf’s guidance, the future of CERN was shaped for many years to come.


CERN was fortunate to be led by a personality such as Weisskopf at this time. The difficult situation for the laboratory, whose harmonious development had been interrupted at a critical point in its evolution, needed a director-general with special abilities. Every fast-developing scientific organization must cope with the effects that its very size has on its aims. Scientists with little inclination towards administrative matters must submit to administrative and bureaucratic rules, especially in an international organization.

The selection of collaborators and the future style of work is determined at the stage of most rapid initial growth, because the natural inertia of a structure made up of human beings makes it extremely difficult later on to rectify earlier mistakes. At the end of 1960 the number of CERN staff and visiting scientists was 1166; this rose to 2530 at the time of Weisskopf’s departure in 1965.

Therefore, at this time in the history of CERN even more than at others, the director-general had to be a physicist who set the direction of the laboratory towards an absolute priority of science. To achieve this he had to rely on a high reputation in his field, together with an ability to deal with the administrative needs of a rapidly growing organization. CERN was placed in the delicate position of having to restore European research parity with that of the US, profiting as much as possible from the experience gained already in the US, while retaining the European character of the new organization.

Distinguished career


Born in Vienna in 1908, Weisskopf followed a truly cosmopolitan scientific career as a theoretical nuclear physicist, working with the most important founding fathers of modern quantum theory, and contributing important results himself. He was familiar not only with Germany (his collaboration with Heisenberg), Switzerland (with Pauli) and the Nordic countries (with Niels Bohr at Copenhagen) from extended stays in these countries, but also with Russia (with Landau at Kharkov), and eventually accepted a position at Rochester, US, in 1937.

His qualities as a leader of a technological project in which theoretical physics only played an auxiliary role was exploited in the Manhattan Project (Los Alamos) towards the end of the Second World War. The European background of many of his collaborators there was an excellent preparation for the task of leading a European laboratory. Even when pursuing the same scientific goal, the individual style of scientists varies greatly, especially if they are of different nationalities.

After the war, as professor at the Massachusetts Institute of Technology (MIT), Weisskopf resumed contacts with Europe, which was slowly recovering from the dark years. In addition to his outstanding qualifications as a theoretical physicist and as a leader of scientific enterprises, Weisskopf possessed a special quality that physics in Europe is lacking to a large degree. Possibly because of the general structure of secondary education in Europe, mathematics plays an extremely important role in theoretical physics. Hence theoretical physics frequently becomes almost a mathematical discipline, with the physical ideas being submerged by an overemphasized mathematical formalism. Among experimentalists this can cause uncertainty or even refusal as far as the judgement of theoretical ideas is concerned.

In the US only a handful of gifted physicists knew how to bridge this gap. Weisskopf was a master of this. Before coming to CERN, he had already taught a generation of nuclear physicists how to pick out the essential physical ideas which are always transparent and simple (once they have been understood), but which may be hidden under many layers of mathematical formalism. The true masters of mathematical physics always knew how to isolate the physical content of complicated mathematical arguments, but unfortunately the majority of theoreticians in Europe are to this day sometimes over-fascinated by the mathematical aspects of the physical description of nature.

The understanding of physical phenomena often does not even require the use of precise formulae. Students at MIT had invented the notion of the “Weisskopfian”, which naturally takes care of numerical factors such ±1, i, 2p, etc. Also in the book Theoretical Nuclear Physics by John M Blatt and Weisskopf, which remains a standard textbook to this day, the emphasis on simple, physically transparent arguments by Weisskopf and the more precise, but more formal presentation topics by his co-author are clearly discernible.

From MIT to CERN

To facilitate his transition from MIT to CERN, and to make optimal use of his period as director-general of CERN, Weisskopf became a part-time member of the CERN directorate in September 1960, dividing his time equally between MIT and CERN. Unfortunately in February 1961 he was involved in a traffic accident, and needed complicated hip surgery and a long stay in hospital. At the start of his term as director-general and less so during a large part of his stay in Geneva, Weisskopf was hampered in his movement. I vividly remember his tall figure walking with crutches through the corridors, obviously in pain, but he never lost his friendly disposition.

The first progress report to CERN Council in December 1961 clearly reflects the situation of CERN at the beginning of the Weisskopf era. Two years after the first beam at the PS, breakdowns and construction work on beams had prevented completely satisfactory use of this machine, whereas the smaller synchrocyclotron was working very well. Research director Gilberto Bernardini aptly remarked that European researchers with a nuclear physics background had had little difficulty orienting their work towards the synchrocyclotron. The PS, on the other hand, was a novelty for physicists, so certain mistakes had been made, particularly with insufficient time for preparation of experiments.

Nevertheless, 1961 was the first year with a vigorous research programme at CERN. Not surprisingly, organizational problems and difficulties in the management of relations with universities in the member states became acute. It was recognized that at least track chamber experiments required the collaboration with institutes outside CERN for the scanning, measuring and evaluation of data. For electronic experiments such a need was not yet seen.

The construction of the 2 m bubble chamber was continuing well, but experimental work was still done on the basis of data from the tiny 30 cm chamber and with the 81 cm Saclay chamber. The heavy liquid chamber had looked in vain for fast neutrinos in the neutrino beam. Simon van der Meer’s neutrino horn, intended to improve this situation, had just finished its design stage.

Addressing Council for the first time on the problem of the term future of CERN, the new director-general already strongly emphasized two directions of development which, as subsequent history has shown, were decisive for the laboratory’s future success. One project, based upon design work by Kjell Johnson and collaborators, foresaw the construction of storage rings; the other was aimed at a much larger 300 GeV accelerator.

The financial implications of such proposals and the necessity to formalize budget preparations more than a year in advance led to the creation of a working group headed by the Dutch delegate, Jan Bannier. From this group emerged the remarkable “Bannier procedure”, under which firm and provisional estimates of budget figures for the coming years are fixed annually. It was decided that the cost variation index should not be provided automatically, and that Council should make a decision on this index each year.

First research successes

The discovery that different neutrinos came from electrons and from muons was made in 1962, not at CERN, but at Brookhaven. In retrospect it was clear that CERN’s attempt was bound to fail for technical reasons. However, the disappointment did not overshadow some remarkable successes in the first full year of CERN under Weisskopf’s leadership. The shrinking of the diffraction peak in elastic proton collisions was first seen at CERN – in agreement with the new ideas of Regge pole theory, which had also originated in Europe. The cascade anti-hyperon was found simultaneously with Brookhaven, but the beta decay of the p meson and the anomalous magnetic moment of the muon were “pure” CERN discoveries. For the first time development of a novel type of scanning device for bubble-chamber pictures (the Hough-Powell device) which started at CERN was taken over by US institutions. However, Weisskopf had to complain to Council about the “equipment gap” at the PS, caused by the lack of increase in real value of the budgets in 1960 and 1961.

In some sense, the most important experimental result of 1963 was the determination of the positive relative parity between the L and the S hyperon, obtained at CERN in the evaluation of data from the 80 cm bubble chamber. This result was in disagreement with some much-publicized predictions from Heisenberg, and gave further support to the growing confidence in internal symmetries. Despite a long shutdown of the PS in order to install the fast ejection mechanism giving extracted beam energies up to 25 GeV, it now began its reliable and faithful operation, which to this day is the basis of all accelerator physics at CERN. Thanks to a neutrino beam 50 times more intense than that at Brookhaven, the first bubble-chamber pictures of neutrino events were made.
<textbreak=Parliament>In 1963 a new body of European physicists was created under the chairmanship of Edoardo Amaldi. Taking into account future plans outside Europe, this body strongly recommended the storage ring project, as well as the plans for a 300 GeV accelerator. CERN Council authorized a “supplementary programme” for 1964 to study the technical implications of these two projects. This Amaldi Committee was set up as a working group of CERN’s Scientific Policy Committee, and was the forerunner of the European Committee for Future Accelerators (ECFA), which was founded three years later, again under Amaldi’s chairmanship. ECFA has been the independent “parliament” of European particle physicists ever since.

Weisskopf’s clear vision of the importance of education resulted in his legendary theoretical seminars for experimentalists at CERN. I had the privilege of collaborating with him at that time on some aspects of the preparation of these seminars, and my view of theoretical physics has been decisively influenced by his insistence on stressing the physical basis of new theoretical methods.

From 1964, CERN’s synchrocyclotron started to concentrate on nuclear physics alone, whereas the PS was now the most intensive and most reliable accelerator in the world. Another world premiere was the first radiofrequency separator, allowing K-meson beams of unprecedented energy. At CERN, also for the first time, small online computers were employed in electronic experiments. A flurry of fluctuating excitement was caused by the analysis of muon and muon-electron pairs in the neutrino events seen in the spark chamber. When it turned out that they could not have been produced by the intermediate W-boson (to be discovered at CERN exactly 20 years later at much larger energies), these events were more or less disregarded. Only 10 years later, after the charmed quark was found in the US, was it realized that these events were examples of charm decay – admittedly very difficult to understand on the basis of the knowledge in 1964. The unsuccessful hunt for free quarks also started in 1964, together with the acceptance of the concept of quarks as fundamental building blocks of matter.

Making decisions

Thanks to Weisskopf’s relentless prodding in 1964, CERN member states were convinced that the time was ripe for a decision on the future programme of CERN. Rather than rush into an easier but one-sided decision, Weisskopf was careful to emphasize the need for a comprehensive step involving three elements:

*further improvements of existing CERN facilities, comprising among other things two very large bubble chambers containing respectively 25 m3 of hydrogen and 10 m3 of heavy liquid;

*the construction of intersecting storage rings (ISR) on a new site offered by France adjacent to the existing laboratory;

*the construction of a 300 GeV proton accelerator in Europe.

Although a decision had to be postponed in 1964 – due to the difficult procedure to be set up for the site selection of the new 300 GeV laboratory – optimism prevailed that such a decision would be possible in 1965. After recommending the ISR supplementary programme in June 1965, the formal decision by Council was finally taken in December 1965.


The novel ISR project had no counterpart elsewhere in the world. Although experience had been gained at the CESR test ring for stacking electrons and for high ultravacuum, this decision reflected the increasing self-confidence of European physics. Thus the foundation was laid for the dominating role of European collider physics which eventually led to the antiproton-proton collider, the LEP electron-positron collider, and the LHC proton collider. At the same time as the ISR project was authorized, a supplementary programme for the preparation of the 300 GeV project was also approved.

When Weisskopf’s mandate ended at the end of 1965, particle physics had passed through perhaps its most important stage of development. From being an appendix to nuclear physics and cosmic-ray experiments, it had become a field with genuine new methods and results. The many new particle states disentangled by CERN and other laboratories gradually found a place in a framework determined by a new substructure, the quarks. In addition, many new discoveries in weak interactions, and especially at the unique neutrino beam of CERN, showed close similarities between weak and electromagnetic interactions, and paved the way for their unified field theory.

Human touch


Much of the enthusiasm that enabled CERN experimentalists to participate so successfully was due to Weisskopf. He made a point of regularly talking to the scientists, and more than once he visited experiments during the night. These frequent contacts on the experimental floor with physicists at all levels gave CERN a new atmosphere and created contacts between different groups – something which was lacking before. Weisskopf himself was aware of this. When asked on his departure from CERN what he thought his main contribution had been, he replied that the administration and committees would have functioned perfectly well without him, but that he thought he had given CERN “atmosphere”.

During the Weisskopf era, directions were set for the distant future. Almost 40 years later, the basis of the CERN programme is still determined by those decisions taken in 1965. How could Weisskopf have been so successful in his promotion of CERN in Europe, at a time when there was always at least one member state with special problems regarding the support of particle physics and CERN?

Politicians must trust valued experts. Weisskopf achieved so much for the laboratory because he was deeply trusted by the representatives of the member states. Although enthusiastic in the support of new ideas in scientific projects, he never lost his self-critical attitude, and was quick to try to understand opposing points of view in science and in scientific policy. The enthusiasm, honesty and modesty of Victor Weisskopf have proved to be a rich inheritance, and have determined the future of CERN.

An obituary for Victor Weisskopf appears in the People section of this issue.

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