[Hendrik C. Van de Hulst]
Hendrik C. Van de Hulst (Photo courtesy Harm Habing/Leiden Observatory)



NATIONAL RADIO ASTRONOMY OBSERVATORY ARCHIVES

Papers of Woodruff T. Sullivan III: Tapes Series

Interview with Hendrik Christoffel Van de Hulst
At Leiden
26 November 1973
Interview time: 61 Minutes
Originally transcribed by Bonnie Jacobs (1978), retyped to digitize by Candice Waller (2016)

Note: Note: The interview listed below was originally transcribed as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009). The original transcription was read and edited for clarity by Sullivan, retyped to digitize in 2016, then reviewed, edited/corrected, and posted to the Web in 2016 by Ellen N. Bouton. Sullivan's brief notes about the interview are available on the interviewee Web page. Sullivan sent the transcript to Van de Hulst for review in 1984. Van de Hulst added extensive comments, which have been inserted into the transcript as bracketed notes at the points indicated by Van de Hulst. Places where we are uncertain about what was said are indicated with parentheses and question mark (?). The interview has been split into two parts reflecting the sides of the original audio cassette tapes.

Because of problems with the tape, Sullivan states in his notes that the end of tape 25A (part 1 of this 1973 interview) and beginning of tape 25B (part 2 of this 1973 interview) are garbled. He later deleted the garbled portion at the end of 25A and recorded a summary of what was said; the summary is included in this transcript.

We are grateful for the 2011 Herbert C. Pollock Award from Dudley Observatory which funded digitization of the original cassette tapes, and for a 2012 grant from American Institute of Physics, Center for the History of Physics, which funded the work of posting these interviews to the Web. Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event.

Part 2

Click start to listen to the audio for part 1 of the 1973 interview.

Begin Tape 25A

Sullivan

This is talking with Professor Henk van de Hulst in Leiden on 26 November 1973.

Van de Hulst

Where’s your microphone?

Sullivan

Built in.

Van de Hulst

Built in, okay.

Sullivan

I guess we should go back to the colloquium of course. What was the situation? How did you come to do colloquium in radio astronomy during the war?

Van de Hulst

In the years previous to the War I’d first started to study mathematics as a main subject and then physics. But it became soon clear to me that lots of interesting things were going on in astronomy and so I had not quite decided whether astronomy would be my main subject. Well, those were grim years and Professor Minnaert who was my main professor in Utrecht was in hostage camp.

[Note added by Van de Hulst, January 1984: The full history of the hostage camp (gijzelaarskamp) at St. Michielsgestel, Holland, undoubtedly is in one of the many volumes of De Jong’s History of Holland in War Time (Rijks Instituut voor Oorlogsdocumentatie). When sabotage actions, organized and spontaneous, were increasing, the “authorities” had picked up a well-chosen collection of persons which were held in high esteem by different groups of the total population. This included politicians, religious leaders, scientists, people from the labour movement, etc. Minnaert was among them. Their afflictions were, apart from minor pesterings, like being limited in the number of lines they could write home per month, of the constant threat of death if something serious would happen. At least once three were shot. And at another time several were released to go demonstrate the magnanimity of the fuehrer’. How far vice can go!]

There was for us still some possibility to get around and so an arrangement was made that I could spend a number of months at Leiden Observatory which was, however, closed. – The entire University was closed. Oort was not there, as it would have been too dangerous for him to actually be there, but he came occasionally. And even occasionally gave some lectures in a darkroom.

Sullivan

In a darkroom?

Van de Hulst

Yes, in a dark room. That’s so Hertzsprung wouldn’t know because Hertzsprung was a foreigner and not a very politically-versed person. – He was somewhat of an unreliable link in the total chain.

Sullivan

When did this start?

Sullivan

When was the University closed when all this trouble started?

Van de Hulst

I should know the exact year, but I bet it was already 1940 that the actual University was closed.

Sullivan

Oh, it was that early?

Van de Hulst

Yes.

Sullivan

And that’s when the journals from the outside stopped and so forth also?

Van de Hulst

No, that went on a little longer via different routes. Some came via Portugal and some via different routes, via Sweden, I think. But after a few years that also stopped. Anyhow, I was at Utrecht, but Minnaert was in a hostage camp and finally it boiled down to self-service, and we had a library available there so I read many different things. And when I came here I came especially to get some acquaintance of work going on here and get some further contact with Oort.

[Note added by Van de Hulst, January 1984: The arrangement for me to come to Leiden for those extended visits was made possible by the “Kapteyn Fonds” granting some subsistence and travel money. This is a private fund, still in existence. I have no record of the exact dates.]

And he then suggested that this would be a brand new and very important topic to be discussed at one of our “Astronomenclub” meetings and he suggested that I might be the good person to read up on the subject and to give a review of that.

Sullivan

And he was basing this only on Reber’s work or did he know about Jansky - ?

Van de Hulst

Reber quoted Jansky, but he knew only Reber’s work.

Sullivan

He hadn’t actually seen the Jansky papers?

Van de Hulst

No. It takes some people with a really fine nose to see when something is important, Oort is one of those persons who has that property. [Note added by Van de Hulst, January 1984: My statement that Oort did not know the Jansky papers is a conjecture. Certainly I did not know them. It seemed sufficient to rely on Reber’s judgment in putting the Jansky points into the graph.]

Sullivan

So he just decided to organize a colloquium on this new subject?

Van de Hulst

Yes. And so I spent, I think, those months mostly doing that, doing that reading and reading up everything about the Gaunt factor and all kinds of things. I had done work on astrophysics before and on interstellar matter. It was somewhat familiar to me but this particular thing was quite new to me.

Sullivan

Free-free transitions?

Van de Hulst

Yes.

Sullivan

Were you given a completely free hand as to what to do?

Van de Hulst

Yes. Of course in principle it was to tell what was in the paper of Reber first of all, and then Henyey and Keenan, I think.

Sullivan

Right, but then you expanded a good bit?

Van de Hulst

And of course you try whenever you do something, you try to expand, and so I tried to do the – Well, as I remember I, of course tried to do the continuum work a bit more precisely. I went into detail into the Menzel & Pekeris paper on the Gaunt factor and so on. I also did something which I felt at the time was rather important and which I could have followed up later, but I didn’t. But at the time there were a number of people who made remarks that the population of the different levels could be different from a Boltzmann distribution and that only a small difference in that population would be sufficient to completely overthrow the normal factor hν/kT when you reduce from actual absorption to effective absorption. That was quite well known and I convinced myself and that is in my paper. [Note added by Van de Hulst, January 1984: The theoretical dilemma I refer to here and the context it had in my thinking has been explained more fully in my contribution to your Symposium Volume (Sullivan (ed), The Early Years of Radio Astronomy, C.U.P., 1984) (section ‘The SE of Maser’ in ‘Nanohertz Astronomy’)]

Sullivan

Yes, you have a little note to that effect.

Van de Hulst

In the particular case of the continuum spectrum you can actually prove that there is no worry at all because you can differently rearrange so that you compensate not a transition by the exactly opposite transition but by up and down from the same level. And in fact I found later out that if you do that same thing for the harmonic oscillator, that gives a very nice illustration of the transition from the quantum mechanical approach to the classical approach. I think that was entirely convincing and I think the best answer to that question which was given at the time. But, of course, what I should have added there, that if at some future time a line is found in the radio spectrum, then the warning sign is fully up again because then this trick does not work and then you have all the possibility to have something like a maser. But that particular step I did not take.

Sullivan

That would have been a very nice prediction also.

Van de Hulst

(laughter) But it’s absolutely clear that it’s there.

Sullivan

But your results did agree, even though you did things a bit more carefully, they agreed very exactly the same as Henyey and Keenan?

Van de Hulst

Yes. They agreed quite well, yes. Well, I also at that time, of course, looked whether the individual alpha lines would be observable and I applied the interstellar broadening formula and probably I made an error or they made an error in that earlier paper. Anyhow, I came out with the conclusion that the lines were there, but they would be too strongly broadened to make them observable. And that of course was proven wrong nearly 20 years later.

Sullivan

1964, yeah. I think they made the error. I haven’t checked that paper, but I think they vastly over-estimated the Stark broadening. But I wanted to talk to you a little bit later about that. - I’ve done some calculations. But anyway, that was too bad that that came out that way. That would have been another nice prediction.

Van de Hulst

Well, I’m not sure about it now, but from the note I wrote to Moffet in 1965, I had the impression that at that time I had gone over my old notes and found that I had made a substitution error. The notes are still there and you can see if there isn’t a substitution error there. It’s a lot of work and I never got around to it – it’s not even pleasant one way or the other to find it.

Sullivan

I haven’t actually checked if Inglis and Teller over-estimate that much or not.

Van de Hulst

So, and then of course I systematically went from the continuum – from free-free to free-bound to the bound-bound normal lines and then to the fine structure line and then to the hyperfine structure line. That last one I came to a tentative conclusion that there was a fair chance, although I didn’t believe it myself. From the wording, I was really over-cautious perhaps.

Sullivan

But this had not been measured at all, this line. It was only theoretical prediction.

Van de Hulst

That I remember very well because I was here. Actually it was linking two things together; calculating the frequency, which was nowhere in the literature as far as I know, was very simple. – you had to find some paper of Fermi where generally it gave this splitting, and then I looked in the book of Kopferman, Kernmonente, where they had measured the magnetic moment of the proton. And those two things together did it.

Sullivan

That gave you the dipole moment, essentially.

Van de Hulst

Yes. So to get the frequency was just a matter of one substitution of a number of one book into a formula of another book. In order to get the transition probability of course I went to Condon and Shortly and tried to find what kind of magnetic strength would be necessary to do it and found that it seemed not an unreasonable number. I also found the transition probability expressed in inverse time would correspond to an enormously long time scale. But this is all so far outside the normal experience of that; I felt well, something else might be wrong, nobody can tell. I talked to a number of good theoretical physicists who were all somewhat cautious on their predictions and that’s why I worded my text rather cautiously.

Sullivan

Yes, like you say there had been a little bit of microwave spectroscopy done by that time, but very little.

Van de Hulst

Certainly not that I knew of.

Sullivan

Yes, but I don’t think any for that line. Only, ammonia, I think was the first that was done. So you worked for six months preparing this and then this colloquium…?

Van de Hulst

Well I don’t know it’s six months. Of course at that time we could still travel, over ’43, I think I travelled back and forth from Leiden three or four times for a certain stretch here.

Sullivan

So most of the work was done here?

Van de Hulst

Almost exclusively here, in Leiden. And I know at one time also Oort stopping off at my house in Leiden because he was on his bicycle on his way to- I mean my house in Utrecht which was about half way to the place where he was living in the country and Leiden, about a 100km tour so he stopped off half way in order to fix a flat tire and we discussed some more about interstellar matter. Well, this is typical for that time. [Note added by Van de Hulst, January 1984: For the flavor of that visit (or those visits) of Oort see my contribution “Style of Research” in the book Oort and the Universe, Reidel, Dordrecht, 1980.]

Sullivan

How often did he cycle this back and forth?

Van de Hulst

Oh, I don’t know, he never publicized what he actually did because certainly the official persons should not know where he was.

Sullivan

But he frequently came up here and gave lectures?

Van de Hulst

Yes.

Sullivan

Well, what about this colloquium itself? It was given here in Leiden?

Van de Hulst

Yes.

Sullivan

That was in June ’44?

Van de Hulst

I would have to look up the date.

Sullivan

I think it says on there. [Note added after interview by Sullivan: actually, April, 1944 is correct.]

Van de Hulst

Oh, yes, right.

Sullivan

Can you tell me was this part of a series of colloquia or was this some special thing?

Van de Hulst

Yes. It’s nice that you ask that because it really gives the background of the years before the war. You must realize that certainly at that time there was already quite a boost in astronomy and astrophysics in the Netherlands. Of course, Kapteyn I’d never known – that was long before my time – but the school in Groningen was still quite going strong. There was the Utrecht school of Minnaert who had more or less just started, but after building forth on longer tradition in solar physics. And then there was the school of Pannekoek in Amsterdam who – Pannekoek was completely a self-made man and that he had all the astrophysics on his fingertips and had written a magnificent standard paper in Handbüch der Astrophysik on “Stellar Atmospheres and Ionization Equation” and so with this – there as at one time the axis from Utrecht to Amsterdam with many discussions about stellar atmospheres. And from Gronigan to Leiden was many discussions about say, stellar statistics, the structure of the galaxy and so on. And of course Leiden with Hertzsprung and deSitter has had the biggest name abroad. But all these others counted strongly. And so there was a rather lively group of people wishing to get together to discuss the newest literature and to have these what they call an inter-university symposium, which, in fact…

Sullivan

Especially during the War…

Van de Hulst

The initial format was that the Astronomenclub, which has as its members many people who have studied astronomy a long time ago and now are teaching it at an intermediate level. So that must be a bit popular… to have that afternoon session with a somewhat more general lecture and to fill the morning with a much heavier advanced seminar on certain topics. And I remember Pannekoek could just stand at the blackboard and go on for 2-1/2 hours writing formulae and explaining them. So in that context we had also this inter-university colloquium on radio astronomy.

Sullivan

But did these begin before the War actually?

Van de Hulst

No. We started in the earlier War years. You see there was, like always, when the literature falls down, you get a tendency to do more things among yourselves. And that’s what happened.

Sullivan

You basically pool your knowledge?

Van de Hulst

Yes.

Sullivan

These others, are they published also in the Nederlands Tijdschrift voor Natuurkunde?

Van de Hulst

Yes, there are a number of them published. I think the first one was about the particle and non-thermal emission from the sun which of course in 1940 was a very new subject. And then there’s one about planetary and gaseous nebulae where I have also taken a certain part in the preparation. [Note added by Van de Hulst, January 1984: The "Sterrenkundige Colloquia van de Nederlands Astronomenclub" was an initiative of Minnaert. He very much was the pusher of the first one, of which I do not find the date, but the date of publication is Nederlands Tijdschrift voor Natuurkunde 9, 209-248, May 1942. In our library the full collection, numbered 1-17, is under the Journals and (irregular) serial publications. The authors and title of this first one are: C.J. Bakker, M. Minnaert, A. Pannekoek en J. Veldkamp, “De ver-ultraviolette en de corpusculaire stralingen van de zon.”]

Sullivan

These are in the Nederlands Tijdschrift voor Natuurkunde?

Van de Hulst

They are all in that. There are some which are perhaps less successful. And there are still later ones.

Sullivan

Yes, I know the later ones.

Van de Hulst

Would you like to see those? I must have a collection of the early ones.

Sullivan

Yes, I’d like to see those. After we’re finished I’d like to see those.

Van de Hulst

Let’s not forget it. I’ll try to find them.

Sullivan

Another questions, who is this Bakker that gave the other half?

Van de Hulst

Well, of course you want to bring in real experts and Bakker at that time was a man at the Phillips Research Laboratory who knew all about electronics and antennas. But he later became Physics Professor in Amsterdam, Director of the Zeeman Laboratory. And still later he was, during about four or five years, the Director of CERN, the nuclear research Institute in Geneva. And then he had an accident and was killed, in a helicopter accident.

Sullivan

And did Oort ask him also to give a talk?

Van de Hulst

Well, whether Oort personally invited him, I wouldn’t be surprised if Oort would call up – I can almost guess what happened that Oort via his friends of the Academy of Sciences tried to find out who would be the best man in the country to do just that – then found him.

Sullivan

And what about the actual giving of the colloquium? I’ve seen this picture that was made in the late '50s.

Van de Hulst

Yes, that was, of course an attempt by Kleibrink, a photographer who was an extremely experienced movie maker, you couldn’t call him an amateur because usually he’s a member of the jury rather than somebody making the films. He at that time afterwards found it nice to -

[Note by Sullivan, summarizing the garbled portion of tape: Van de Hulst told me about how Kleibrink assembled together all the people that had been at the original symposium and tried to put them in old clothes so they looked like war-time conditions. But he joked that they couldn’t recreate their waistlines. They were all somewhat more than during the War when they didn’t have much to eat, and were younger. I’m afraid the first five minutes of the other side are garbled, too, because the tape was sticking as it went through. But stick with it and you can get the gist of it.]

End Tape 25A

Part 2


Modified on Wednesday, 13-Jul-2016 08:41:03 EDT by Ellen Bouton, Archivist (Questions or feedback)