J.H.D. Jensen

 Personal recollection by Berthold Stech

 Please do not expect a usual after dinner speech with many jokes. No, this is an attempt of an homage to a great man to whom I had the priviledge to be close for many years. Hans Jensen's personality, his views on life, his scientific and human wisdom had a great influence on the lives of his friends and students, and certainly on mine.

In 1948 I was a student of Walter Bothe who was the only professor of physics in Heidelberg. It was the time right after World War II with little to eat, overcrowded lecture rooms, many war-wounded students and only outdated physics instruments. As a member of the student body I learned from Bothe that the local government was reluctant to give the final o.k. for the appointment of Hans Jensen as a professor, who was at Hannover at that time. Together with a fellow student I went to the local newspaper with an aggressive open letter scolding the government. We asked the owner of the newspaper to publish it. Well, he did not, but he went personally to Stuttgart and talked with the government. I am not sure whether this helped, but in any case, little afterwards, Jensen got the contract. At first he sent his assistant Helmut Steinwedel and somewhat later Mike Danos. They told us about Jensen with great admiration, his attitude during the "Third Reich" and how he managed to survive inhuman times and still to do interesting physics. Steinwedel also told us about Jensen 's moving about at the end of the war with a wreck of an old car working by burning wood, and how he managed to stop without brakes. (Picture of Steinwedel and Danos). But as Jensen finally arrived - we, the students, were greatly disappointed at first. In his lectures he jumped frorn one subject to another, apparently incoherently, and only later we realized how interesting it is to see all these connections. Moreover, he spoke with a low voice, mumbled a bit, so that it was not easy to understand him, and he gave the impression to be a somewhat cynical person. But soon we saw everything different and we admired him. We students called him "der große Meister"(the great master ). His presence brought big changes. Thanks to him we came suddenly in contact with modern theoretical ideas and approaches. He gave us interesting tasks and started very lively seminars. The exercises he gave were often not fully formulated. The students should recognize the problem and think about it themselves. It was challenging. Jensen was unconventional. Once during a lecture he was puzzled and did not know how to continue. Then he suddenly went to his room and brought back the whole blackboard on which he had done the calculation before.

We also could see that he was now the motor for the development of physics in Heidelberg. First the theory group increased: Arnold Schoch and Heinz Koppe (now emeritus in Kiel) came to Heidelberg. Then, in a very difficult but always fair combat with the ministerium in Stuttgart, lasting for years, he won the necessary support to make Heidelberg a leading center of nuclear physics in experiment and theory. He managed that Otto Haxel and somewhat later Hans Kopfermann left Göttingen and came to Heidelberg. To bring those outstanding scientists to Heidelberg was a very wise decision. Haxel was a very experienced nuclear physicist, who persuaded Jensen to see in the magic numbers a key for the structure of nuclei. Kopfermann was the authority for the measurements of nuclear magnetic moments and nuclear quadrapol moments which were decisive for learning the properties of nuclei. And Kopfermann had already an excellent and competent group of people who came with hirn (Peter Brix, Krüger and Steudel). It was almost unbelievable how much Jensen could achieve besides working very hard in research, giving lectures and seminars. He also organised the library and managed many administrative tasks. Every week there was a tea meeting where Bothe, Maier-Leibniz, Jensen, and later Haxel and Schmelzer, informally discussed new results, new publications. For us students it was very impressive to see how they argued, how they made rough estimates, suggested solutions etc.. Of course, it was the high time of nuclear physics and the shell model. Each new information on spins, parity, energy levels and cross sections was of importance. We students were witnessing an exciting period with hot and lively discussions. But even more important, we experienced the outstanding scientific and social atmosphere created by Bothe and Jensen which extended to the newcomers and to the students. Besides scientific competence there was also heart. Coming back from years of war the institute became our home where we spent all day and half of the night.

The reputation of physics at Heidelberg was growing fast. Jensen and Bothe, could bring to an end the international isolation after the war. An excellent opportunity was the 60th birthday of Walther Bothe (1951). Wolfgang Pauli, who for some time was reluctant to come to Germany, came to this festival. (Pictures 2+3). Little later Maria Meyer, Bethe, Gamov, Rabi, Weisskopf, Wigner, Fermi and many other outstanding scientists came for visits.

At first the theory group had only two rooms in the physics institute and a few outdated books. It was difficult to get periodicals such as Physical Review. If at all, it arrived with a delay of many months. Around 1953, with much effort, Jensen could persuade the state to buy the house on Philosophenweg 16 -a very fortunate event. And the state got it at little cost. It was the time of the Korean war. It was, and still is, a wonderful old villa in beautiful surroundings. Now the group had some rooms, a library, and a secretary (Herrn Sach). Very fortunate for the institute was that Jensen got two private rooms in this house. Usually, he got up at 6 o'clock in the morning and worked for 2 or 3 hours in the garden. He was proud of the garden and cared for it very much. As the son of the gardener of Planten and Bloomen in Hamburg he knew much about plants. After gardening he went into the house and looked for the mail. He first took out the weather chart which arrived every day. He studied the pressure and temperature distributions and liked to talk about it. In fact he was an expert on weather from his wartime experience. Then he dictated letters and started his research.

Already some time before moving to the new house I had finished my PhD under Bothe and had followed a suggestion by Jensen to work on multipole radiation of isomeric nuclei. There was then ample opportunity to discuss with Jensen and to learn frorn him. He usually came and asked to come to his room, offered a drink, brought along a tremendously long chart of all nuclei, mentioned the problems with them, their known and their not yet established properties. I was always impressed how much he knew, how critical he was to his own suggestions, and how conscientiously and systematically he worked. He was not a formalist, more phenomenologically oriented. But he did not fully trust intuition as long as there was not a good mathematical formula for it or a direct experimental proof.

Mathematics was for him a language developed to a large part by physicists. The mathematicians he considered more as the herolds of grammatic of this language watching that it is not misused. Very often Jensen came to the working rooms in the middle of the night after some concert or some private invitation and was happy if there was still somebody with whom he could discuss and chat. He liked those nightly meetings so much that he could not suppress his disappointment when his collaborators -I for instance -got married and did not appear anyimore at night. All meetings with him were very rewarding because of his comprehensive knowledge, his historical, cultural and philosophical insights. He very much liked Shakespeare, questioned Schiller's pathos and was particularly fond of Heinrich Heine whom he often quoted when making ironical remarks about others and about himself. Although not a member of a church, for any situation in life he knew an appropriate quotation from the bible. One day -much later -as I was unsure whether or not I should accept a good position at another university I found a paper on my desk with a quotation from the St. Luke's Gospel: "Listen to your teachers..." Höret auf Eure Lehrer.. .

Yes, Jensen furthered and helped many students and friends in a critical but always encouraging way. On the morning as it became known that he would receive the Nobel- price he was asked by the minister of the state if he has a particular wish. O yes, was Jensen's reply, there is a student named Jamil Daboul who was driven out of Iraq. You can give him the German citizenship. Daboul got it. He is now professor in Israel and always likes to visit us here in Heidelberg. In many ways Jensen advised his students and collaborators in their scientific and personal lives. Much of the success his students achieved is based on the delightful human atmosphere and the freedom and opportunities he provided. Jensen together with Gentner also initiated the first contact with Israel and made this collaboration a success.

From the outside Jensen was sometimes seen differently. He could be quite sarcastic, and liked it to be, perhaps in order not to show his real feelings. In the physics colloquium on Friday afternoon he was often sitting in the first row with the periodical "The New Yorker" in front of him. If the lecture got boring, he opened it, scaring the lecturer. He also liked good jokes and said: Better to loose a good friend than to suppress a good joke. Once I was in a Heidelberg clinic. The night before a small operation Jensen came to see me. I was really touched. But before leaving he said: "I also visited Michael Schön ( a solid state physicist I had also known) on the eve of his operation. He had a large furuncle on his bottom. Next day he was dead." Of course, knowing Jensen, I was amused.

The institute was Jensen's home where he looked after orderly behaviour. There was a student whose wife had to be in the Springer publishing house close to our institute very early in the morning. So he arrived early, but then lay down on a couch with a newspaper over his eyes and slept. Jensen did not like a snoring person in the room, removed the newspaper from the student's face and soon also the couch.

In the fifties, thanks to Jensen's efforts, the research and education conditions for physicists improved even more. Jensen supported the foundation of the Institute of Ap- plied Physics under Christof Schmelzer and Konrad Tamm and opened the way for the foundation of the Max-Planck-Institut for Nuclear Physics on the Königsstuhl under Wolfgang Gentner. For the Institute of Theoretical Physics he organized new chairs, the first one for Walter Wessel in 1956, and in fact built a department with professors having the same rights as he, long before the idea of a department came up in Germany. He was the boss by authority but not because of his position. Jensen was very progressive and conservative at the same time. He hated bureaucratic and organisational "reforms" of established rules and of things which worked well, but initiated drastic changes were it was necessary. The ideas coming up in the late sixties and early seventies about fashionable ways of learning and lecturing were distasteful to him, e.g. the request that everything that could possibly be asked in examinations should be presented in lectures. According to him a lecture should give stimulation and hints. To get more knowledge and abilities was the task of the students themselves. Today he would certainly, and rightly, make very sarcastic remarks about the new reform virus which now infects us again. On the progressive side he was e.g. the initiator of a modern molecular biology in Heidelberg. It would take much time to mention all his initiatives.

I mentioned already how much he furthered his students and collaborators. He did all he could to encourage and to help. I was by far not the only one who owes him very much and for whom he cared almost like a father for his sons. I will mention only Hans-Jörg Mang, Hans Weidenmüller, Wolfgang Wild and Klaus Dietrich. But I can serve as an example. As early as in 1954 -you may remember that I had been an experimentalist before -he recommended me as guest lecturer in Norway. In 1956 he wanted me to represent him in a Summer School in Mexico and to give there lectures on the nuclear shell model, even though I had worked much more on the theory of beta-decay than on nuclear structure. Jensen organized for me a scholarship for a stay in the U.S. which at that time was not easy to get. Thus I could go from Mexico first to the famous 56 conference in Seattle where Yang and Lee postulated parity non-conservation in weak transitions, and then go on to stay as a research fellow at CALTECH in Pasadena. I am mentioning the Seattle conference because Jensen also came to Seattle and I can show a few pictures which I have taken there. (Jensen with G. Chew on a boat trip; Jensen talking to Oppenheimer; on the way from Seattle to Pasadena with Feynman). One year later Jensen visited me at CALTECH. Here I can show another picture. He is sitting in front of the little house I had rented in Altadena. I show this because Hans Weidenmüller will certainly remember it: After I left CALTECH this house became a much appreciated domicile of Volker Soergel, Hans Weidenmüller, Theo Meyer-Kuckuck and Karl-Heinz Althoff.

Let me now go back to Heidelberg. Some of the highlights of the fifties and the early sixties were visits of Maria Goeppert-Mayer, the writing of the book and, of course, the nobel prize in 1963. An appropriate picture to show is this one: Ben Mottelson, Maria G.-M. , Jensen, Aage Bohr. Some years before, in 1957, Rudolf Mößbauer did his exciting experiments in Heidelberg. Jensen was enormously interested. Ten years before, Jensen and Steinwedel had written a paper about the recoil of nuclei in molecules and lattices. But they had missed the important point. Every week Otto Haxel came by to chat with Jensen. Haxel was the first director of the center of nuclear physics in Karlsruhe. The intense and fruitful collaboration between Karlsruhe and Heidelberg was a consequence of the remarkably good and solid friendship between the two great men. (Picture: Hörsaal).

Although Jensen was mainly interested in nuclear physics and statistical mechanics, he was open and very well informed about the developments in particle physics. In a way he even anticipated already the close connection of statistical mechanics and field theory. Anyhow he helped decisively that Heidelberg got a strong stand in high energy physics. On his initiative Joachim Heintze, Volker Soergel and Heinz Filthuth came and made Heidelberg well respected also in the field of experimental high energy physics.

In the last years of his life Jensen still did intellectually pretentious work. He got interested in the problem of justifying d' Alembert's principle from a quantum theory point of view. Also problems connected with spontaneous symmetry breaking fascinated him. He even had a beam of wood in his bathtube and verified his calculation of the symmetry- breaking positions!

Jensen was very social. He had many friends in the whole world. He laughed a lot and had humorous, spirited remarks on all situations of life -always precisely to the point. Often he invited us to sit in his room and to listen to an interesting piece of music. He liked chamber music more than symphonies and was very fond of baroque music, and liked Telemann even more than Bach.

As a last picture I will show Jensen in his garden. He proudly showed it to his visitors. He liked the architecture of the house, and only after an inner struggle he applied for an annex to it. It was necessary to get room for the secretarial staff. But after the monument preservation board gave the permission he looked again at the plan, and went to a neighbour asking him to make a legal complaint against the permission!

The social and human atmosphere among Heidelberg's physicists, the good contact between the faculty members are to a large part due to the influence of Hans Jensen. All who knew him are aware of this. It still determines their ways and doings, and they all want to preserve it for the following generations.