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Interview of Hendrik van de Hulst by David DeVorkin on 1978 July 20, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/25894
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Brief interview concentrating on early life in Utrecht; training at the University of Utrecht and physics from Orenstein; Minnaert’s influence and conditions during World War II; Leiden University and prize essay leading to study of growth of interstellar grains; transition to work on interstellar hydrogen.
Well in the time we have available could we identify the early influences upon you that brought you into astronomy and specifically into the astronomy that you are quite well known for?
That’s always a very difficult question.
Let’s start with your home life.
Yes. I think I knew from an early age that I wanted to do something involving mathematics. It was a thing I was good at and I liked arithmetic and all kinds of things. Whether that would finally become an interest in pure mathematics, or of technical study, or of physics or astronomy, I had no inkling. And I think that remained until, well, really until I started university studies.
Let’s identify your home town, the schools and that sort of influence, and your father’s background and your mother’s background.
My home town was Utrecht in the Netherlands. And until my Ph.D. I never had a longer bicycle ride than 10 minutes to get all my education. That was an advantage. I’ve always been grateful for this. So I had my grade school teaching at my father’s school. My father was a head of a grade school of which very few people ever got any education beyond the age of twelve at that time.
And why was that?
Simply, it was not customary. That luxury of advanced education beyond the age of twelve or thirteen has come up only later.
At the time were there exams to determine whether you’d go on?
Yes. In principle there were exams. In practice of course it was determined by economic factors in many cases.
What years were these that we’re talking about?
Let’s see. I was born in 1918 and so I finished the sixth grade I think in 1928 at the age of ten and from there I had to decide what to do next.
And what were the factors in your decision?
Well it was obvious that I had to go on. I could do things quite easily. In fact when I got my first set of books for the Gymnasium, which was the high school, at the age of twelve, among them was a book containing geometry exercises through I think, all the years including the final exam, so I did that in summer vacation — a lot of those, I think I did most of my geometry, already during that year.
So it was very interesting to you?
Very interesting to have a new challenge but I always am happy that from my father I learned that it’s better to know a few things well then to know a lot of things, partially.
Was he a generalist in his education?
No. Actually he had two lives, the one life was as a school teacher and the other life was as a writer of children’s’ books, children’s’ novels and when I say two lives, I mean two. He worked about as many hours on the one as on the other into the late hours of the night. And he was very successful, yes.
What kind of home life did that make for you?
Oh fine. I mean he was very well aware of that. We were a family of six, and just kind of a happy family.
Were you oldest or youngest?
I was, well, somewhere in the middle.
You made your career decision which must have been pretty obvious that you were going to go on then.
Well actually the only key decision point came at the end of the gymnasium when I was seventeen. And then of course many people talk to you and say, “you ought to do so and so and ought to do so and so.” I don’t believe in very specialized intelligence. People might do one thing well but should usually do well in other things too. I also had a hobby for languages so somebody tried to make me take Chinese languages. Finally we had somebody come in to give us the real low down on what was involved in going to, say, the technical university; doing some technical or industrial work, or to the other university to do some real physics and mathematics. And he was very clear. After talking for half an hour he said: “No, your interest clearly is more theoretical — you ought to go — actually to the more theoretical side in the university.” I’m very grateful for that type of advice because many people don’t get it.
What was this person’s background?
Oh he had himself done the technical university, he had just finished in there. So he knew the differences.
At that time had you had any interest or awareness of an interest in astronomy?
Yes, indeed, during the teenage period, time is unlimited in a sense. So we did all kinds of hobbies by the side including astronomy I did a fair amount of riding around on bicycle collecting stones to do some geology and I did a fair amount of playing chess, I did a fair amount of botany, a fair amount of astronomy also on the side.
But astronomy was one of many interests.
Yes. And I got my first formal astronomy training from somebody who was at that same gymnasium, somebody who was taking the linguistic side, and who was going to be a theological minister, you see. So my first astronomy training I got from somebody who was not at all a professional astronomer but an ardent amateur. The first explanation about meteors, about stars, came from this man. And so again, it’s a nice thing that these amateur societies do this and it’s very important.
Was there any question which university you would go to?
Not really because we had a university in our home town; we couldn’t tell the difference.
So you went to Utrecht.
Was your course mainly in mathematics, was that your direction?
Yes when I started out I had the feeling mathematics was my main subject and later physics and astronomy came along on the side.
Who were your instructors?
Well the names hardly count. But I got a splendid, but at that time already old fashioned, introduction into projective and analytic geometry from a man called Barrau, who had a long beard. And while it was really a splendid form of teaching it was in a sense historical. At the time I studied I did not at all know what was going on but I was determined to find my way. It’s like going somewhere without a map in your pocket. I did a lot of what now would be called analysis, but also in a quite old fashioned way with all the “epsilon-tics” and I found my way through there knowing that that was not the real thing. By chance, if I would have had somebody who at that time would do modern mathematics like topology or so, the chances would have been that that would have been my major occupation later. And of course numerical work hardly did exist yet. So I don’t claim that I have made a deliberate choice for myself. Things just happened. But what I do have is a vivid memory of the physics I got from Professor Ornstein who had started as a theoretical physicist. Of course he’s known, worked with Zernicke on different problems. He clearly had a wide theoretical interest in physics but also was aware of its various applications and at that time was very deep in a number of applications. I think his course on physical optics, on Diffraction Theory and all kinds of things, which he gave in the undergraduate course in the second year, has perhaps been my most inspiring individual course in setting the pattern to my further work. And it was completely disorganized but personally extremely inspiring.
In what direction? How did you become inspired?
No. I was inspired by the man. It was clearly a subject of which he knew all the different corners, but he had hardly taken the trouble to organize it as a student course. So it took a fair amount of initiative to glue the different pieces of the course together into an organized notebook and that was the inspiring part of it for me. And the funny thing is that about the time of this course, which was in the depths of the Depression, it was 1936 when I got to the university, the formal director of the Observatory and professor of astronomy at Utrecht University had died and the University was debating if it should get rid of astronomy altogether. So during the first half year I was there I had no astronomy lectures whatever and during the second half year they had found somebody to substitute who was also a well-known man of his own right, van der Billt; a good popularize in astronomy but originally from the Merchant Marine. So he used to explain certain things finishing with a smile: “this is mostly a problem for the seaman.”
That gives you an interesting view of astronomy.
Yes. Right. So in the first year I didn’t really get any astronomy at all. And then they discovered Minnaert. Supposedly Minnaert had had an offer to come to this country as a professor and that finally triggered the decision to offer him this chair in Utrecht, which was vacant already for a year. And Minnaert was a very vigorous man and started from absolutely nothing to build up that astronomy department.
Well he’s known not only for solar research but for his education.
He emphasized his laboratory work.
Oh yes, I mean, it was very clear after he was there, we got for the first time some real practical training.
How did your interests develop under Minnaert?
That’s a hard question to answer in a general sense, of course, because I could drink in the inspiration he gave. But at the same time he obviously did not have the type of theoretical interest or background that I had. So I needed something else at the side, but his inspiration was extremely strong and I still, in my own teaching for instance, can trace the influences of Minnaert, really.
But you stayed at Utrecht through to your PhD?
Yes, that was normal. It was not customary to change universities and still isn’t in Holland. So that was nothing exceptional. What was exceptional, of course, was that then the war started. In 1939 I was drafted into the Army, and after I came back the university was half closed and Minnaert was taken into a hostage camp. So for part of my exams I had to send notes there, but the number of lines he could write back were limited, indeed by the line. That was an awful time.
You had been in the Army yet he had been sent. I know many civilians had been sent to hostage camps, why did he go particularly? I know that not everyone went.
No, no, I mean the Germans just as a sheer deterrent took on the order of hundred or two hundred hostages which, as you say, were people held in good esteem by different population groups in the country. And they were very very well aware of what people were in good esteem. So that included Church people, that included people like Minnaert, that included many labor union people, all kinds.
I know that many people went into hiding in Leiden, was this the same in Utrecht too?
Well yes. There was a fair amount of hiding but all these things take detailed explanations because as long as you don’t know from what you hide exactly, the hiding is never complete. So at a later time I got a chance to spend a number of months at Leiden Observatory simply to expand my knowledge and contacts. Oort was there, but the university was already closed and Oort was already in fairly complete hiding because he was one of the twenty or thirty people who had been most vocal against sending the Jewish professors away. So it was distinctly unsafe for him to be at a known place. But that didn’t prevent him from coming occasionally on his bicycle to Leiden and actually giving lectures there. But again the fact that lectures were given there should be known to the students who wanted to attend, including me, but should not be known to the director of the observatory because that was Hertzsprung who had a Danish nationality and was known to be not a typically courageous man, so it was better to leave him out.
I see he was not actually a sympathizer?
He remained at the observatory while everyone else went on strike.
In any such situations you can clearly see that 70 or 80% of the people are those people who in principle are courageous and in practice are not so.
But your first contacts with Oort, are these the famous near legendary contacts where you began work on the 21-centimeter line?
No, the first contact was rather different. The Leiden University had this long standing tradition of having prize essays. So prize questions were published and I think that may have been in 1942 or so, that Oort and Kramers together had thought up, as a prize question the question of the growth of interstellar grains. I had seen that announcement but then Minnaert advised me to try and work on that, you see. That must have been a couple of months before he was taken into this hostage camp. Because I remember I then took his advice, it was clear. There were one or two references there including that famous Linblad paper of 1933 where Linblad made a suggestion for growth of grains and so there were four or five references that then existed, finishing about 1938.
That’s pretty much when all correspondence was cut off with the allied countries?
Yes, right. And so what I did then during that time in my home town I did completely by myself without any advice even from Minnaert who was away or from Oort whom I didn’t know. I had to go around and go to libraries and find out about accommodation and growth coefficients in the chemistry library and things like that. And so I wrote an essay. But they did not award a first prize. Per Haar and I, who had both given an independent answer to this thing, both got an honorable mention. So it was really at the occasion when I came to the University in Leiden and when Kramers and Oort were there that I got to know Oort. It was the first face to face contact after the award was made.
That clarifies it in my mind.
From there on we had many further discussions of what should be done next or what could be done next. Kramers was quite critical in this. I remember the critical remarks. The reason why they did not think that any of the answers were complete is that Per Haar had concentrated completely on the first new formation of a multi-atomic molecule whereas I, impressed by the chemical literature on the new formation of condensation nuclei, had decided that is a completely impossible subject. I started from a nucleus and then estimated temperatures and growth rates. I had done all of that. I feel some satisfaction, from all we know now, that my part has stuck better than the other thing which was too speculative, I would say.
Can I assume that Woody Sullivan and others have already asked you these questions in this area?
No. I don’t think so because most people think they should start with the 21-centimeter line but that comes later.
But this is really where you began?
So then later on your continued contact with Oort then helped to develop that?
Yes and indeed what happened is that we held an inter-university colloquium on the interstellar dust or smoke, as I felt it should be called to remain in line with the chemical terminology. We had these inter-university meetings still going on, organized by the Netherlands Astronomical Society.
During the occupation?
Yes, right. So that was the first publication of this work the growth rate and then Oort and I started to write a paper so I got to know Oort’s method of work.
You got your Ph.D. at Utrecht, but when were you officially at Leiden?
I was never officially at Leiden at that time. I just had a kind of an informal fellowship to visit Leiden occasionally. In fact it amounted to a travel grant to take the train for the thirty miles. But at that time it took a fair time to take the train because of war conditions. Once, part of a bridge was damaged and couldn’t be repaired.
What was the transition from the growth of interstellar molecules and grains to the neutral hydrogen atom?
Of course I was writing a thesis, this was all pre Ph.D. In fact, since the university had stopped the exams had stopped and I took my Master’s Degree exam only in 1945. The Doctor’s degree I got in ‘46, so that was kind of delayed. But during that time I was working on the thesis and had of course a far too ambitious program to write something which would include everything about interstellar matter. So I included a lot about interstellar gas and a lot about interstellar grains and about the distribution, and about reddening, everything. And since I had done that at a certain time Oort again was the man who suggested that one of these national colloquia should be dedicated to this new possibility of receiving radio waves from space. He then personally asked me to do the theoretical side of it because he knew of my background. And so as part of that work I also started to look at the lines. It was the obvious thing to do but it fitted also into that grandiose scheme of writing a thesis containing everything.
That certainly fits. Well that concludes this half hour and I really appreciate it.
Sullivan has been interviewing many astronomers associated with the growth of radio astronomy, and did interview van de Hulst.