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Interview of Carl Weizsäcker by Karl Hufbauer on 1978 April 18, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4948
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Background information, early interest in stars; meets Werner Heisenberg in Copenhagen and Berlin, 1927; studies at Universitat Berlin, 1929; Heisenberg to Universitat Leipzig, fall 1929. Fellow students Edward Teller, Guido Beck, Felix Bloch; Universitat Göttingen , 1931; emphasis on astronomy (Robert Atkinson, origin of the elements); interior of the stars (Arthur S. Eddington) and problems of energy sources; origin of the planetary system of keen interest. Ph.D. from Leipzig (Heisenberg), 1933; Niels Bohr's Institute, Copenhagen (George Gamow, Lev Landau, Bengt Strömgren ), 1933-1934; to Leipzig as assistant, writes Die Atomkerne; sets out to solve the problem of the energy sources, 1936; Dozent Universitat Berlin. Lise Meitner, Peter Debye, 1936; work on nuclear origin of energy in the stars. Work on carbon cycle (Hans Bethe's identical findings, George Gamow's role); returns to physics. Also prominently mentioned are: Peter Josef William Debye, Himmel swelt , Aloys Mueller, Walther Nernst, and Elis Strömgren.
I will be interviewing Professor Carl Friedrich von Weizsäcker, Director of the Max-Planck-Institut zur Erforschung der Lebensbedingungen der wissenschaftlich - technischen Welt. On account of the shortness of time for the interview, I will focus my questions on Professor Weizsäcker's early contacts with astrophysics,especially his work during the 1930's on the problems of the source of stellar energy and the origin of the elements. My first question, Professor Weizsäcker, is about your early plans to become an astronomer. Under what circumstances did you first consider going into astronomy, and how did your relatives react to this? Especially I ask this question because it's not often that people from the nobility went into science. It’s somewhat unusual.
Yes. Now, first about nobility — my father's family is what you would call a bourgeois family. And his father was nobilitated because he was a jurist and he became prime minister of the little kingdom of Wurttemberg in southern Germany. This was when I was already a boy of four years. I am born under the name Carl Friedrich Weizsäcker without any "von.” And my father’s family had been rather a family of university people. My father, however, actually went into the German navy before the First World War, and afterwards into diplomacy. But he had a brother who was a well-known medical professor, and his grandfather had been a professor of Protestant theology, so it was quite natural. That was a family tradition. I just don't remember when I first discovered that I was so much interested in the stars. I may have been at the age of eight or so. I don't know. And when I was of the age of 12 I think or 11, I even wrote a little poem in which I said — I will quote it in German, because it's the kind of poetry a child writes. "Wenn ich haft genugend Gelt um zu leben sicher, Wurd am liebsten auf der Welt ich ein Sternkuddiger" So, it was my elementary interest. And my family was of a very liberal nature in this respect, and they said, "Well, the boy wants to be an astronomer, let him be an astronomer." There was no objection to it. It was a natural thing. They were not sure that I would stick to it, when I would grow up, but at least that was my interest. Then, the decisive experience in this connection was that my father then became a member of the German embassy in Copenhagen.
In what year?
It was in '25. We went to Copenhagen in '25, we stayed there until '27, and there my mother one day came home and told me, "Yesterday evening I was invited by people, so and so, and there I met some very interesting people, especially a young man by the name of Werner Heisenberg, who seems to be working at the institute of that famous Danish physicist, I think Bohr is his name, and he plays the piano very well and he belongs to the German youth movement, and I had a long talk with him about the youth movement, because I am rather skeptical of it." I had already read his name. At that time, I was at the age of 14, I had a popular periodical on astronomy which I had read, and there I had just read an article that the problems of atomic physics had now first been solved by a young man by the name of Heisenberg.
Which journal was this, was it Himmelsweit?
It was Die Sterne, Henseling's DIE STERNE — Robert Henseling was the man who edited it. This thing happened in December, '26. Then of course I told my mother, we must invite Heisenberg, and she invited him, and he immediately convinced me that what I was out after was better done by studying physics instead of astronomy — because he realized, and correctly realized, that my interest was not so much an interest in the stars in special, but that it was an interest in fundamental laws of nature. And also, that astronomy itself depended on physics for its understanding, he could easily make clear to me. And so from that moment on, it was my idea that I would be a physicist. But I always kept the interest in astronomy. And the real temptation then was philosophy and not astronomy.
When you were in Copenhagen, you mentioned in your letter to me that you were at the observatory once?
And that you had contact with Strömgren?
Yes. Elis Strömgren, who was director of the observatory, was a man who had close connection with Germany. I'm not sure, whether he had been a professor in Germany for a while also. At least he was one of those people in Denmark who were glad to have good connections with their German embassy — which was not the common thing. And my father met him, and my father told him that he had a son who was interested in astronomy, and then Strömgren said, "Well, why doesn't your son visit the observatory once?" So I was invited to go there, and I saw it, and Strömgren also had a son who was interested in astronomy. That was Bengt Strömgren who was I think about, I was 14 at the time and he was 16 or so. And then Bengt Strömgren showed me around, and that was the way I came into contact with the Strömgren family.
Did you at that time see anything more of Strömgren?
I think it was just this one visit. I don't remember at least that it was more than that. And later on, I met Bengt Strömgren again and again, but never in any very close connection. Only we had known each other for so long and so it was as easy contact.
In the spring of '27, your family moved back to Berlin.
I guess then you went to the gymnasium.
Yes, I did.
What I would like to know is something about your science training in the gymnasium, and contact possibly with astronomy during this period? Did you visit for instance the Einsteinturm?
No, I didn't. There was no definite contact with astronomy. My contact at that time was mainly physics, and with one person. Let me just tell you that in April, '27 Heisenberg once traveled from Copenhagen to Munich, which was his home, and he wrote me a postcard that I might meet him at the one station, the Stettiner Bahnhoff, in Berlin, go with him by taxi to the other station, Anhalter Bahnhoff, where he would have to leave. So I did, and there he told me the Uncertainty Principle, which was not yet printed! So that was not with astronomy. it was not exalted. ledge of physics was my real contact with science at that time — but not with astronomy. And the science training in the gymnasium — well, it was not exalted During the last two years of the gymnasium, my knowledge of physics was probably a little bit better than that of my teacher.
That wouldn’t have been a strong influence.
Then in the spring of 1929, according to what I've seen, you began your university studies in Berlin.
And that fall, when Heisenberg returned from a trip around the world, you went to Leipzig, where you spent the next three semesters.
What I’d like to know now is, what contacts if any did you have with astronomy during these first two years of your education as a physicist?
Well, my memory is not quite precise. I decided very early that I would take the three fields — physics, mathematics and astronomy for my doctor's examination, and at that time no other examination than the doctor was needed, and I didn’t want, to have any other one and I didn't have any other one. That meant that I would have to study astronomy besides physics. In Leipzig, the professor of astronomy was Hopman, and I am not sure whether he was there already in '29 when I came there, but in any case, he was the only one with whom I studied astronomy, but not very intensively. I did more of my astronomy by just reading books and so on, and I never had any training in practical astronomy at all.
I take it you did not attend the astronomy seminar at Leipzig?
Certainly not in the earlier time. I may have done it later. But I have no very strong remembrance of the Leipzig observatory and the Leipzig astronomers.
What about astronomers, did they come to the physics seminars?
I don't remember any astronomer whom I met in Leipzig as a student. In Heisenberg's seminar, that was fairly exclusive, and in any case, I don’t remember an astronomer. I mean, in Heisenberg's seminar, when I entered it for the first time — that was in winter, '29, '30 — I think there were five Germans, including Heisenberg and Hund the two professors and six Japanese students, and perhaps the rest amounted to four or five, Teller was one of them. And there was the assistant, Beck, who was Austrian, and Bloch I think was there at that time, who was Swiss. Teller was Hungarian. And one or two Americans and that was it.
You became rather close friends with Teller in this early period?
Yes. My main friendship of course was with Heisenberg. That became very soon a close personal friendship. And of the others, I would say that probably the contact with Teller was closest. I mean, the beginning of it was not at a very high emotional level. We just loved talking to each other, and he was so very eager to have someone with whom he could discuss whatever was interesting for hours and hours.
Then going to Göttingen in the spring of 1931 you spent a semester there — I have the impression that Teller went to Göttingen about the same time.
Yes, he was in Göttingen at the same time.
His decision to go there, I guess he went as an assistant?
He went as an assistant I think with Eucken, with the physical chemist Eucken, if I remember rightly.
Did his going there at that time influence the timing of your going there?
No, not at all. Not at all. I had the tendency once to study at another university — as you know, in the German tradition this was quite natural — and Heisenberg said, "Well, now if you'd like to go somewhere else, Göttingen probably is the best place, Göttingen or Munich, and I don't remember why I preferred Göttingen to Munich. Probably Heisenberg had the idea that in that particular time, the mathematical school of Göttingen was more important than Sommerfeld. And so I went there, and Teller had finished his studies in Leipzig and went there independently, and I saw him in Göttingen for every lunch because we had a common, what we called Mittagstisch, with several other people. My contact with Teller in Göttingen was close, but not extremely close, and I don’t remember precisely what we did together in Göttingen. And in Göttingen I had my main contacts with astronomy.
That's what I would like to get to next. There were several astronomers there — Kienle, Biermann, Heckmann, and others. Why don't you tell us a little bit about your contacts in astronomy there and then something of your memories of the experience?
Yes. Well, my studies took part in Kienle's seminar. But I forget what was the topic that we treated at that time.
He was very interested in this period in cosmogony, and that may have come up in the seminar —
It may well be, but in fact I don't remember. I don't remember. I also participated in physics in the seminar of Born, and there again, I don't remember what was actually the subject matter. I remember — I don't remember very much of scientific content, but more of astronomy than anything else, because there suddenly, I was in a school of astronomers who were at the level which was the level I knew from physics, from Heisenberg in physics, and Kienle was the professor, the director of the observatory, and Heckmann was what we called "Privatdozent" and Biermann I think was a student like myself.
Possibly by that time he'd finished his dissertation?
Maybe. It may well be. I'm not sure. And as I wrote you, there at some time I met Atkinson in Göttingen, but it may have been at another opportunity.
He came through Göttingen?
Yes, he came through Göttingen .
He came in June of '31, and at that time he had just finished his —
This is exactly the time that I was there.
He had just finished his paper on stellar energy and the origin of elements.
And he was very eager to talk about it. He gave a lecture on it, I know, at the Einsteinturm.
— yes, exactly, exactly. I mean, that makes it clear. I had the vague remembrance. And that means that certainly I heard what Atkinson told about the description of the origin of the elements, the processes, within stars, and my knowledge of the theory of the interior stars was not sufficient, and my knowledge of nuclear physics was not sufficient, and it was somehow above my level. I would not remember if it had not been interesting to me.
What about Flügge? Was he there then? As a physicist, he's one of the others in Germany who was interested in nuclear processes in stars.
Yes. I don’t remember exactly. I met Flügge in Leipzig, and I think I also met him in Göttingen , but I couldn't tell you now where we first met. And in any case I had long talks with Flügge too. At that time, later times too. He was my successor for instance at a later period in the Institute of Otto Hahn, where I had been for a while and later on he was what we called "Haustheoretiker." It may well be that Flügge was present in Göttingen at the same time and was a member of that group of young people who were interested in astronomy.
Do you recall whether any other physics students might have been interested in astronomy? I have the impression that there wasn't very close contact between astronomy and physics.
No. But for instance, take a man like Teller. I didn't discover any topic in which Teller was not interested. And of course we said, astronomy there is, one great problem — I would say, every physicist who was working in fields like ours, like, for instance nuclear physics, knew that the problem of the interior of the stars was probably solved by Eddington, with the exception of the problem of the energy and that this was a problem of physics was clear, too. It was not clear how it was to be solved. Atkinson offered a solution. Atkinson and Houtermans, by the way — and so we liked discussing this, of course. There was always some interest in the question whether the nuclear energy might be technically used. And later on, I learned that Rutherford had said, "Who is talking about that is talking moonshine." But it was an interesting subject. And in this sense, I would say that people like say Nordheim, who at that time was also in Göttingen, or — Placzek, Weisskopf, Bethe, the whole group, Bloch — they all would have taken some academic interest. I mean, not an active interest but some general interest in astronomical questions. But none of them, I think, had the idea that he would be working in astronomy.
Shortly after your stay in Göttingen —
I’ll say one thing more. I had been interested in the problem of the origin of the planetary system. And this was not quite a common interest in the group of physicists. But I had read a little bit about that, and had the impression that the problem was absolutely not solved, and on the other: hand, it probably would be possible to solve it but probably only after having better understood the problem of the history of a star, which on its side depended on the question of energy production. And so I tried to know what was known in the astronomic circles, on the problem of the origin of planets.
Do you recall any books you read in the early period, popular books maybe, that would be of interest?
Well, one book I read, I don't remember exactly when, was Eddington, INTERNAL CONSTITUTION OF THE STARS, which I did not read very carefully, but I read it the way you read a novel. I didn't do the calculations and so on, And then, there was a popular book on the origin of the planetary system in German by a man named Nölke, I think Friedrich Nölke, but I don’t remember the first name. He was not a university man. I think he, was a gymnasium teacher in Bremen, and he thought that he had solved the problem, that he knew the solution, and I was never convinced that he knew the solution, but he also told about the earlier attempts, and from him, I think, from his book I think I learned that there had been a theory by Kant and Laplace and so on, and that Kant and Laplace were not identical and so on. I mean, not the persons of course but also the theories.
You didn't meet Nölke?
No, I never met him. I exchanged letters with him, because after I had published my own theory on the origin of planets he wrote me a letter that I was mistaken because he had already solved the problem.
Shortly after your Göttingen period, you were only there one semester, you had an article in the journal HIMMELSWELT.
On the relation between philosophy and modern physics.
I was wondering if contacts that you made in Göttingen led to that publication? How did you happen to publish —?
Now, why did I publish in HIMMELSWELT? Who was the editor of HIMMELSWELT, do you know?
Sorry, I don't know.
I think the point was that HIMMELSWELT, it may have come through my astronomica1 friends, for instance, Heckmann or — I don't know now. In any case, the point was that HTMMELSWELT had published a paper by a philosopher, Aloys Müller, if I remember rightly, on philosophy and modern physics. And he said that modern physics was no good because it didn't correspond to the classical views of philosophy. And then, they felt that this, was not a good article for them to publish, at least not the final one. They looked for somebody to answer the philosopher. And they found me because they knew I was interested in philosophy.
In the fall of 1931 you returned to Leipzig.
And a year and a half later there you received your Ph.D.
I'm wondering why you returned to Leipzig so soon, after just one semester.
Well, my idea had always been to finish my studies with Heisenberg. And the idea was, just that I had wanted to see some other place, and I was not quite sure when I left, whether I would leave for one semester or two, but in principle it was clear that I would return to Heisenberg.
Did your astronomical studies in Göttingen incline you to seek out astronomers in Leipzig more than before?
Just for the exams?
It was just for the exams, and I didn't prepare very much for the examination, and I don't remember how it went but it went well. Hopman was a nice man, but I was not very deeply impressed by the astronomy I could learn with him.
Heisenberg was impressed with your performance on the exam. At least in a letter to Bohr he mentioned it.
I don't know.
After taking your Ph.D. at Leipzig in 1933, you spent several months, September 1933 to February 1934, in Copenhagen, at Bohr's Institute.
Physics surely was your first interest there, physics and maybe philosophy.
The institute was a broad ranging place, and I'm sure that discussions of astrophysical problems came up from time to time?
Yes, I would think so, but I don’t remember it.
During the time you were there Gamow and Landau were there and both — Gamow for instance had collaborated with Houtermans and Atkinson on that first paper. Weizsäcker ; He did? This I don’t remember. I think Gamow is one of the people with whom again you could talk about anything. He was always interested. He had always new ideas about things, and left it to others to find out whether they were correct or not, and I'm sure I talked about astrophysical matters — with Gamow. But in fact I cannot remember it.
What about Landau? He was also there at the time.
Yes, with Landau, partly we had contact of a completely different nature, because we had very interesting and complicated talks on political matters, and he told us many interesting things. But my main contact with Landau I think was that he participated in discussion of the problems of radiation of what we called "Bremstrahlung," of radiation produced in electrons by being deflected by atomic nuclei. That was a long discussion in which I took part. And I finally wrote a paper about it, in that winter, and that was one of the main contacts with Landau. I don’t remember contacts with Landau on astronomy. It may well have been but I don’t remember.
And Strömgren? You mentioned earlier that your contacts continued at a low level.
Contacts may have continued. But I don't even remember whether Bengt Strömgren was in Copenhagen at that time.
He was still, I think.
You know better.
Yes, I know he was there, but I don't know for instance whether he would have been a regular at Bohr's Institute?
Yes, I would say that, if I remember rightly, that when Bengt Strömgren was in Copenhagen, different from his father Elis, he would in general also have had fairly close contact with the physicists, and would have come to Bohr's Institute whenever there was any good reason for going there.
What about Bohr? Do you recall any interest that he might have had in astronomical or astrophysical questions?
I can't remember definite astronomical or astrophysical interests on the side of Bohr. No, I don't think so. He was open minded, but I can't remember any specific interests.
Then in February 1934, you returned to Leipzig as an assistant.
And during the next two or three years you concentrated on nuclear physics, culminating in your monograph ATOMKERNE.
As you were finishing this monograph, you seem to have given increasingly serious consideration to the possibility of working on nuclear processes in stellar interiors.
And by the fall of 1936, when you were again at the Bohr Institute for an extended period, you were hard at work on the stellar problem — that's clear from this letter to Bethe.
I'm interested in your decision to go in this direction. I want to raise various questions.
Flügge came to Leipzig, I think in '35, and he had had this earlier interest, had in fact written a paper on neutrons in stellar interiors.
Yes, that's true. That's true, I remember it.
He didn't write any subsequent papers in this period, I'm wondering if the two of you discussed such things?
Yes, I think so. And I think that I now remember that I once gave a talk in the physics colloquium which was directed by Debye in Leipzig, upon problems of the stellar interior, and energy production, and since I had not yet finished my monograph, it must have been rather, a report on the stellar interior for the physicists. I don't remember exactly what it was, but I think I remember that during the last minute of the talk, I said — I was asked in the discussion, perhaps, by somebody, on the idea that neutrons were in the stars. And I answered, "Assume there are a lot of neutrons in the star in this very moment — and wait a minute, then no neutron would be left, because they would all be absorbed by the atomic nuclei." And I think, for this reason, I didn't believe Flügge's paper. I remember precisely. When I wrote the final chapters of my monograph, in which I briefly mentioned the question of stellar reactions in the stars, I suddenly — it was in summer, '36 — I suddenly had the idea "Now the problem can be solved." I had always been aware of the problem, and I felt, now we know so much about nuclear reactions, the only thing to do is to draw a chart of all nuclei and possible nuclear reactions, and to find out what is the right one. And then I had the idea that thereby, you can solve the problem of the abundances, of the elements, of the isotopes, which seemed difficult to understand, under the idea that it would be thermodynamic equilibrium at any temperature, and this I started immediately, when I had finished my monograph, and I started it I think in Bern in my father's house, who then was the German minister or ambassador to Bern, to Switzerland, and went on with it in Copenhagen.
In Copenhagen you may have had some discussions with Strömgren. He refers, in a letter written in early '37, to seeing a manuscript by you. Do you recall that? It's rather an oblique reference.
I don't recall any detail in this, but certainly I discussed the matter with Strömgren, I discussed it with Placzek, whom I mention in the letter here, and I don't remember who was in that particular year in Copenhagen, but in any case I discussed it with the people.
Why would you have referred, in your letter to Bethe of September '36, to your work in this area as "astrophysical fantasy?"
Well, of course, it wasn't clear — I mean, it was fantasy. One didn't know. I felt that I could ask Bethe on a matter on which he and I had been working before — that was the semi-empirical formula for nuclei, for the energies, and radii and so on — and then I felt that this was a good way of introducing the theme in which I was more interested at the moment, which was, the astrophysical ideas. And it's fantasy because the way I put it, it was, wrong.
But you referred to the other things as "normal physics." I'm wondering if there was perhaps some skepticism on the part of physicists about indulging in something that was so speculative?
Oh no. Well, speculative, of course — but I mean, physicists would have said, "Well, if you want to do good work in astronomy" — astrophysics was not yet a term very much used at that time — "If you want to do some work in astronomy, you must learn astronomy." That's a lot to learn. So I would have referred to that as something outside our common normal field. And besides, questions like production of elements of course were speculative. And they still are speculative.
In November '36 you went to Berlin, and I guess you were there first with Meitner and then with Debye, and you soon became a dozent in Berlin University.
It's during these first two years in Berlin that you showed the greatest interest in stellar problems.
Well, later on again I was interested in the origin of the planetary system, and after the war even I did some work in cosmogony. But in the nuclear origin of the energy of the stars, it was the Berlin years.
Could you say something about who you might have discussed the paper with in Berlin? You already told me in the letter, why you submitted to the PHYSIKALISCHE ZEITSCHRIFT?
That answer I gave you, I think — that Debye was the editor. In the institute there was a certain tendency that we should publish in the PHYSIKALISCHE ZEITSCHRIFT. And I think my main contact in Berlin at that time in matters of astrophysics was Biermann, who then was in Babelsberg, if I remember correctly, and we even had I think some common days in the week when we met, so we had long talks about these matters.
In the fall of '37 there was a colloquium series to which you must be referring?
And Biermann said in a letter to me that he, that Flügge was in some way instrumental in putting this together too.
And that Biermann, Flügge and yourself — do you recall any other participants? Maybe Freundlich had been interested earlier?
Freundlich may have been, yes.
Or ten Bruggencate?
Ten Bruggencate, yes. But the main intensity I remember was with Biermann and me, and perhaps Flüiggle. And the others, ten Bruggencate and Grotrian, did their own work and took some interest in these things.
Then there was this conference in January, '38, in Göttingen, on stellar interiors, yourself and Biermann and others —
Looking over the list of attendees, you were the only physicist in the group. How did the astronomers feel about that? Do you recall any impressions they might have had?
No, I have no definite — I even didn’t remember the conference. But when I saw your letter, I thought, yes, I think there was a conference at that time in Göttingen. When did you say, January '38?
In January of '38.
Yes, Yes. Well, I think probably Biermann was instrumental in bringing 'me there, But since I knew, the astronomers, there was no great difficulty, and no great surprise, at least not on my side.
Then in May or June of ’38, Gamow visited you in Berlin.
Can you recall any details from this visit? He would have Have been able at that time to have reported some of the developments in America — the conference that there had been in America in March — also, some of the details maybe of Bethe’s work, though I’m not clear about that.
The things I remember are quite limited. At that time I had the idea of the carbon cycle, and I felt that this was the solution, and Gamow came and told me that Bethe had probably found the solution to the problem and that it was the carbon cycle. Then I said, "Well, I think he's right, I found it too," and my paper at that time was perhaps already in print, I don't remember. In any case, I felt that I had already done it and submitted it for print. But on the other hand, it was very good if Bethe had done it too, and it was not the first time I had done something parallel with Bethe. It had been the same thing with the mass-defect formula for the atomic nuclei, of which I spoke before.
Then I felt that at least he should tell Bethe that I had done it. And I might even have written a letter to Bethe about that, But since Bethe didn’t have it, perhaps I didn’t. Perhaps I asked Gamow to tell him.
Then I was a little bit disturbed by the fact that Bethe’s paper didn't appear earlier — because it was delayed, because it was submitted for some Festschrift — because that gave the impression that I had true priority, while I would say that we were just independent.
Simultaneous. I may have been a bit earlier, but I don't remember.
It's really hard to tell, when so much is going on —
Also, you acknowledge in your paper at this time discussions with Strömgren , after his return from the United States, which may have been spring of '38. Do you recall whether he came to Berlin? I don't believe you were in Copenhagen at that time.
I went to Copenhagen every year in autumn. And the most probable thing is that I met him in Copenhagen. Hufbaer: Just two more questions — I know you have an engagement. Can you say, something about the reactions on the part of colleagues in Germany to your two papers the physical colleagues on the one hand, the astronomical colleagues on the other?
In fact, I don't remember quite precisely what it was. I think in general it was accepted as interesting. I gave a talk about that in the physics colloquium in Berlin, at the university, and there, I said that now this problem of the history of stars could be treated, because the problem. of the energy' sources was solved, and Nernst was there, and Nernst didn't like the idea at all because he had some ideas of his own…And I said, "It's so very good that we now have an age available for the sun which is similar to the age of the universe." And Nernst said the idea of the age of the universe was metaphysics, Was an impossible idea, was no science, because of course the universe was eternal. Then we had a long discussion about that, and we went to Nernst’s room, and finally Debye, who was also there and who was my institute director, gave a very good solution to the problem. He said to Nernst, Well, now, look — you have quite different interests. You, Herr Geheimrat, want to know the great problem of the history of the universe, and Weizsäcker wants to understand the energy sources of the sun, and of course if you are both right, there can be no discrepancy between the two and wait, Herr Geheimrat, he will come to your view. Then he took me away, and I was very happy to get out of the room.
Then one final question. I guess that was your last work before the war in the area — there's one little article summarizing results of these nuclear investigations, stellar interiors — in FORSCHUNGEN UND FORTSCHRITTE.
— yes —
But I'm wondering, why your interest at this point turned away from stellar interiors?
Well, I mean, the question was just that, since I felt the problem of the energy sources was probably solved, I had the choice either or going on, really entering the theory of the internal structure of the stars, as my main field; or, to leave that to astronomers, and to do the kind of physics again which I had done before. And I felt that it was quite hard to learn all these things in astrophysics, numerical integrations of Eddington’s equations and all that — I wasn't sure whether I would be successful there, and I preferred to stay a nuclear physicist. And then the war came, and we were engaged in the problem of uranium and energy from uranium, and that drove me away from active work on the stellar interiors. Then I became enthusiastic again, when I suddenly had the idea quite independently of all this that the problem of the origin of the planetary-system could be solved. But that's quite another story.
Speaking of the difficulty with this astrophysical mathematics, Landau one time referred to it as "astrophysical pathology."
Well, that it’s like Landau. I mean, that's the way Landau expressed himself. But I didn’t feel it was pathology. I felt it was a field of study which would certainly be a possible field, but then I would have to do nothing else. I would not be able to do it just with my left hand. Because what I had done before was physics, applied to astrophysics but it was nuclear physics. And then suddenly I would have had to determine myself what was the temperature, under the influence of all these many cooperating factors, and I was just a bit afraid of that.
Thank you very much.
 Note added: After the interview with Dr. Hufbauer I asked my mother (she is still alive), and she looked up the date in her diary. He was in our home February 5th, 1927
 Weizsäcker to Hufbauer, 10 March 1978
Teller went to Gottingen in 1930
Biermann took his Ph.D. in 1933
Weizsäcker to Hufbauer, 10 March 1978
Weizsäcker to Bethe, 30 Sept. 1936 (original in Bethe Papers, Cornell, University Archives)
Strömgren to O. Struve, early 1937 (Struve Papers, Yerkes Observatory Archives)
Weizsäcker to Bohr, early 1938(Bohr Scientific Correspondence)
Weizsäcker to Hufbauer, 10 March 1978
Physikalische Zeitschrift (1937, 1938)
Landau to Bohr, 1937/38 (Bohr Scientific Correspondence)