Interview with Hendrik Christoffel Van de Hulst on 26 November 1973
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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 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.
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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 h?/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
Begin Tape 25B
Sullivan
And these people were from all over?
Van de Hulst
I would estimate some 30 – 40 or so.
Sullivan
And the article that was finally written – was that written right after you gave the colloquium and then it could not be published.
Van de Hulst
In fact it must have been written almost immediately after the symposium – the normal slack to get it in a neat form. The publishing delay was not too much, five months perhaps.
Sullivan
Yes, the summer of ’45, I think.
Van de Hulst
Ok, but if it appeared summer of ’45 it seems that (?) in the war. Because the end of the war was only in May ’45. [Note added by Van de Hulst, January 1984: I cannot guess what is on the line missing in the transcript here. Clearly the date printed on the journal (Dec.1945) does not necessarily coincide with the date that it actually appeared. I have no idea when that was.]
Sullivan
That’s true. Well, what was your next step as far as radio astronomy – you got off into dust and things like that?
Van de Hulst
We had been in dust well before that time. The first time I worked with Oort was on the formation of dust in the interstellar medium. That preceded the radio work. In 1945 I had to finish my thesis and I wanted to include one chapter on that dust and one chapter on the actual optics and Mie computations. That one chapter expanded so much that it became my entire thesis in 1946. And then I got to Chicago to Yerkes Observatory, Kuiper was there and invited me over as a postdoctoral fellow. Just after my degree I went over there.
Sullivan
What year was that?
Van de Hulst
’46. Just after the war.
Sullivan
So I guess you just missed being there when the discovery of the polarization?
Van de Hulst
I was already long back by then.
Sullivan
Yes, that’s a shame that you missed that.
Van de Hulst
Well, I did not miss much of it because by that time from ’46 to ’48 I was with a fellowship at Yerkes Observatory. And, of course I did a fair number of things there, among other things a big review paper on light scattering in planetary atmospheres, which I’m still interested in now, and on the zodiacal light which is another piece of interest I did at that time. But on radio astronomy during that time I remember only visiting Reber at his original place in Wheaton, Illinois. And also a fair number of discussions with Struve, and with Kuiper and with others on the possible funding of larger instrumentation in radio astronomy by the American astronomers. At that time nobody was exactly certain how much there would be in it, how much of an investment would be warranted to go really full- fledged into radio astronomy. Of course there were a few exceptions like at Cornell University. But the main astronomical community at that time was somewhat holding off. There were so many new things to be done, the entire infrared scene was opening up with the lead sulphide cell and there were a number of other important developments. But they certainly missed out on radio astronomy to some extent.
Sullivan
Right. And America took a long while to catch up.
Van de Hulst
Yes.
Sullivan
But, of course, in the other countries it wasn’t the astronomers that got it going, it was radar labs and radiophysics labs.
Van de Hulst
That is true for England and for Australia, but it is not true for Holland.
Sullivan
Not true for Holland, right. But then Holland didn’t really come in until five years later after the War, also. I’m talking about the late ‘40s now.
Van de Hulst
Yes, right. Nothing much to boast of until the line in ’51.
Sullivan
I’m interested, you say you talked with Struve and Kuiper. Well, I guess this was their attitude that you just described to me that it looked interesting, but they weren’t -
Van de Hulst
Oh, their attitude was this merits close examination, but when you start a close examination you reach a point where you must gamble, and that they did not do. Or at least, I mean they took other gambles.
Sullivan
You say you visited Wheaton, Illinois. Can you give me your impressions of what you saw there, or how Grote Reber impressed you at that time?
Van de Hulst
Well, he was a very jolly fellow living there with his brother and sister-in-law, I think. Anyhow, he was very much the man who had one big hope in life, but is not too serious about it. He wanted to – he was clearly competent. He, for instance, was very proud of a number of small things, he was very proud that he put up the declination axis with a man from a neighboring garage on a Ford rear axle. And he did a number of these things just all by himself. He was also very proud of his electronics and one of his things – of course I had no electronics knowledge at all – but one of the things he was proud about is that he – he had various stages of amplification – he designed a number of these five-stage amplifiers and on each of them he could monitor the output to see if one stage went bad. At that time it was not at all standard practice, but he still… well, he was really an artist in that type of work.
Sullivan
Was this with a meter?
Van de Hulst
With a meter, yes.
Sullivan
And then he had the one final strip chart which he shows many examples of it in his papers.
Van de Hulst
Yes.
Sullivan
So you say this was his whole life, really? He spent almost all his free time?
Van de Hulst
No. that I never asked him, but the impression I got from others is that he really had some kind of consulting job which he could take longer or shorter time, and then took a number of months off and then did this on his own account.
Sullivan
I see. He went up to Chicago and took some courses I understand there, just to learn something about the astrophysics side. And, of course, he was really the first one to propose the free-free emission mechanism – just very briefly in a paper.
Van de Hulst
Yes.
Sullivan
But was he getting input from the astronomers as to what would be the next good thing to do?
Van de Hulst
Of course by the time that these papers had been published, and I don’t know whenever that is, ’38, I mean the fact that they appear in the same journal together – the Reber observation and the Henyey and Keenan interpretations – that already means that at that time a certain close tie had been made. That was long before I ever heard about it.
Sullivan
But what I’m trying to get at is that the optical astronomers, I mean he was listening to them more or less about…?
Van de Hulst
Yes.
Sullivan
It wasn’t just a matter of reading some books and then doing it all by himself?
Van de Hulst
No. Oh, and he actually at one time, just after the war when everything started anew, he was actively making plain propaganda for doing something bigger in radio astronomy.
Sullivan
Yes, and he wanted a 100-ft dish or something like that?
Van de Hulst
Yes. But the first bigger thing that happened in America was this Cornell setup with Seeger and Williamson, that was a very nice start off.
Sullivan
That’s right. So you went to the States and you had some contact with radio astronomy there, but you were not working directly in it.
Van de Hulst
That’s right.
Sullivan
Then you came back to Holland and what was going on when you got back here in ’48?
Van de Hulst
In ’48, well, quite a number of things. I still had to finish part of a paper on the interstellar dust that I did, this ’49 paper. – I think it was published in ’49 – where the first somewhat more detailed calculations of the growth of dust and the color and the expected composition which later was called the “dirty ice.” At that time, I mainly had to learn the job of teaching and of doing things here at the Observatory and reading the literature and keeping up-to-date.
Sullivan
But I was referring to what was going on in radio astronomy.
Van de Hulst
In radio astronomy at that time they had already started a kind of national committee which was not called ‘radio astronomy’, but something called ‘Solar Research’ or so, because it was obvious that the sun as a source was something important.
[Note added by Van de Hulst, January 1984: I remember that at the time of my arrival at Leiden, after I had taught a summer course at Harvard, so it must have been about 1 September 1948, I felt a little excluded because I had not been invited to the meetings where solar radio-astronomy were discussed. This must have been a very brief interval because the record of the first meeting of SRZW on 26 Oct. 1948 shows that I was present there. In the meantime, the “Werkgroep Zon en Ionosfeer” continued, because I find in the same arch.box 41 the record of a meeting at Kootwijk 23 May 1949, referring to an earlier (undated meeting). Solar-terrestrial relations, including the modifications of the ionosphere and the impact on radio communications to distant countries, were the driving factors by the PTT (Ir.A.M.de Voogt was a high officer in the telecommunications branch). In 1952 systematic solar radio patrol was started at the NERA station and Aad Fokker’s Thesis "Studies of enhanced solar radio emission at frequencies near 200 MMz," Leiden, 22 June 1960, with me as promotor, was a kind of late result of these scientific efforts. SRZM followed these developments but did not have much of a grip on them.]
And Minnaert was interested in that and Oort was also interested in it, and they had made it national order to, well, to join forces, so there were several experts from the Post and Telegraph Service and there was a man from the Philips Laboratory, so in order to make sure that you had the right type of advice jointly rather than in competition. And this very first part I have not been, well, I was a youngster, I have not been in there. But I think a year or sometime later this was changed into what is now called the Radio Astronomy Foundation and there I have been a member from the beginning.
Sullivan
So that was the forbearer of the Stichting? [Stichting Radiostraling von Zone n Melweg]
Van de Hulst
No, it’s the Stichting itself.
Sullivan
Right, but -
Van de Hulst
No, this earlier thing was the forbearer. We had then already at that time de Voogt had put this Würzburg telescope in Kootwijk at the place where they had the radio receiver station of PTT, mostly for link with Indonesia. And so we had some free space there and a sufficiently free hand to be able to do that. So from the Kamerlingh Omnes Laboratory we got some electronic equipment and partly dump equipment. And there was a man there who tried to get the receiver working and it worked to some extent, but really not enough to get good signals.
Sullivan
Is this Hoo?
Van de Hulst
Yes. And that of course changed rather – well, it changed absolutely completely from the time that Muller came there. And Muller was, perhaps at that time already before he joined the Stichting, the one man or one of the very few men in Holland who knew anything at all about noise, electronic noise, because that was his subject on which he had done his major research work during his studies at Delft University.
Sullivan
But now before Muller came were there any astronomical observations? Or was it just not quite getting there?
Van de Hulst
I’m not entirely certain. No, I don’t want to bet on that - nothing to speak of. [Note added by Van de Hulst, January 1984: I remember visits with Hoo at Kootwijk, sitting in the co-rotating Cabin of the Würzburg mirror, trying to do things well with the equipment he had, but without really having a grasp of things. I do not remember having seen any result arising from beyond the Sun.]
Sullivan
Yes, nothing was published certainly.
Van de Hulst
Nothing to speak of, no.
Sullivan
And the aim was the sun, or was it?
Van de Hulst
No, the aim at the time was, of course, first of all to do as well as Reber had done. I mean that was a clear objective you could put for yourself. And then successively to do better. But it was absolutely clear from the computations that it should be possible to get the Galaxy and to get the sun.
Sullivan
But working at rather short wavelengths for those times?
Van de Hulst
True, you are right, but in the beginning we were working on a number of wavelengths, you see. I’m pretty certain that we started also out near 3 meter or 1½ meter wavelength, but again I would have to look that up in the old documents.
Sullivan
But then it got shorter?
Van de Hulst
Yes. No, and then at that time, of course when Muller came, he first of all got the thing working and doing some normal continuum work, and of course, at the same time he started to construct equipment for the 21-cm, which for the present technique, the technique of the time was a pretty short wavelength.
Sullivan
So I see, the continuum work was at much longer wavelengths?
Van de Hulst
Yes. I’m absolutely certain because de Voogt had also indeed a hole in the ground covered with wire mesh, which must have been working at that time, at 1½ or 3 meters. I really don’t know, but I think you can always reconstruct it if necessary.
Sullivan
Yes, I’d like to see those reports. Ok, so when did the push for the line begin? Or was it always in the back of everyone’s mind? [Note added by Van de Hulst, January 1984: I do not have 'those reports' (about the results obtained with various solar receivers at Kootwijk, including the hole in the ground) in my files. De Voogt must have written one or more papers about it in the publications of the 'Nederlands Radiogenootschap', They may also survive in the PTT archives from before the move to NERA (1952) or in the personal collections of Fokker er Stumpers or Neubauer.]
Van de Hulst
It was well, not even in the back of the mind, but we always knew that perhaps sooner or later it would be successful, and Muller was working for it. Well, he could not do everything at the same time. You first need a continuum receiver before you get a line receiver. Then, in fact, the actual trigger came when I spent a semester in early 1951 in Harvard and there we were, the three of us together – Kerr and Ewen and I. And Ewen was a student of Purcell’s. Apparently Purcell had gotten the idea entirely independently that this might be something to observe from the heavens. I don’t think when he got the idea he knew about my paper nor did Shklovsky know about my paper. So it was very much in the air, but it was also clear from our conversation at Harvard at that time that… I mean Muller did not know the newest tricks, which Ewen did know, about the switching back and forth between the different frequencies. Ewen had been actually working on the cyclotron at Harvard and had all the newest electronic techniques in his recent experience. That gave him a certain headstart. Then on the other hand Ewen did not know enough astronomy to realize how wide this line would actually be. He was thinking in terms of a ridiculously short interval between his switching frequencies and we had to tell him you better forget about it, but take it wider because of the Doppler shifts. So there was a kind of certain amount of give and take and letter writing and that to me explains why the actual time interval between the different first observations was not very long.
Sullivan
I see… and you were the facilitator really.
Van de Hulst
Well, Kerr also did a lot of writing.
Sullivan
Well, he was writing to the Australians presumably.
Van de Hulst
Right. Oh, but we did a lot of talking together.
Sullivan
They [The Australians] came in very shortly afterwards, yes.
Van de Hulst
I sometimes quote this as a typical type of cooperation you do get in those fields where the competition element doesn’t count too strongly. You could never think that in industry things would go this way.
Sullivan
Yes, that’s right. But now… so Ewen was in the Physics Department, I guess -
Van de Hulst
He was in the Physics Department.
Sullivan
And he had never done any astronomy before?
Van de Hulst
No, but I mean in a good University you know about astronomy, you know where to find them.
Sullivan
Purcell is still alive, isn’t he?
Van de Hulst
Oh, yes! Very much alive.
Sullivan
I’m afraid I don’t know him. I’ll have to try to talk to him about where he got the ideas.
Van de Hulst
Purcell has had several students and himself working on interstellar grain orientation and so on.
Sullivan
In the last few years?
Van de Hulst
Yes.
Sullivan
Ok, so you were actually away. When you went to Harvard, that had nothing to do with the line though, that was just an accident?
Van de Hulst
I was asked to give a course there. And in fact, at that time, well, not entirely, accidentally, I did choose as a subject, radio astronomy, and I think that was the first course ever given in the world on that subject.
Sullivan
I have those notes, yes. Had you given that course before?
Van de Hulst
Yes, I’d given just the semester before – I’d given something similar at Leiden.
Sullivan
But nevertheless you went to Harvard not knowing about this Ewen/Purcell experiment?
Van de Hulst
Yes, I learned about it a little before that time with, I think, through Gorter and other physicists here. They came with a rumor which seemed all wrong, but it had part of the truth in it, so I inquired. Somebody had visited Philips and had told about somebody else doing something, and when we finally reconstructed it, it was this line.
Sullivan
I see. Ok, were you back in Holland when the line was discovered?
Van de Hulst
No, I was still over there.
Sullivan
Oh, that’s right. I heard how that happened: you sent a telegram and Kerr sent a telegram.
Van de Hulst
Yes, actually – no, I was there at Harvard and one time Ewen came into my office and unrolled one of his narrow strip charts, 4’’ wide of 5’’ wide, and rolled it over the floor and said, “If you look along with your eye you can see the wiggle.” [Note added by Van de Hulst: January 1984: Your (Woody Sullivan's) question in your letter 3 of 19 Dec. 1983, item (6) whether my memory about the contact preceding that discovery date is correct. Certainly, through Gorter I knew that I must contact Purcell as soon as possible after I had settled at Harvard Observatory for a semester’s course, starting mid- or end- January 1951. I did not know Ewen even by name before that, but Purcell introduced me to him and Ewen showed me around both to the separate cyclotron building (where he had worked and picked up his modern electronics knowledge) and to the attic of the laboratory where he had his equipment. I agree with you that the descriptive sheet which you found in my files shows that this visit was made before the actual discovery and the fact that it was referred to in the SRZM minutes of 13 March places the date, say, between early Feb, and 3 March 1953. I can guess two reasons why Ewen did not recall (according to your statement) the preceding discussion about the frequency difference necessary in the switching. The first is that Ewen was not present at that time, but that it was a conversation between Purcell, Kerr and myself. It was in front of a blackboard and it may have been at Harvard Observatory rather than in Purcell’s lab. The second reason is that it did not impress Ewen, who was at that time not closely interested in the astronomy of it.]
Sullivan
And that was the line, yes.
Van de Hulst
That was the line, yes.
Sullivan
And why didn’t they follow up any? That’s always puzzled me.
Van de Hulst
That’s the crazy system of education in the U.S., that’s part of it. Because here we teach the languages when you need them, before you study, but there they wanted to have language requirements at the time you get your degree. So I know that Ewen vanished for months to Poughkeepsie to study German words. (laughter)
Sullivan
I see, but that still doesn’t explain why they didn’t follow-up in the long term.
Van de Hulst
No, no. Well, Ewen actually later founded a private company like so many people do, the Ewen-Knight Corporation and it was, well, I don’t know how successful he was from a business end, but certainly he made quite a few of the early receivers for radio astronomy. And so that was his way of following up.
Sullivan
Well, that’s true, he did contribute the receivers for Agassiz, didn’t he?
Van de Hulst
Yes. And I mean Bok’s start there at Harvard from which later many of the big names in American radio astronomy have come, like Dave Heeschen for instance. That was possible only because of the good cooperation with Ewen, you see, so there is a follow-up. I mean a little delayed.
Sullivan
So you came back to Holland, then you got involved in, I would think anyway, the analysis of the Van de Hulst, Muller and Oort paper.
Van de Hulst
Yes. I hesitate to say whether that was my main work at that time. You see, I don’t remember too well whether what I did mainly -
(short interruption)
[Note added by Van de Hulst, January 1984: What I did ‘mainly’ is not clear from the transcript. My memory is that the discovery of the 21 cm line did not change the emphasis of my work, apart from bringing some pleasant publicity, a trip with the minister of science to Kootwijk, etc. My main research effort during nearly a year was to finish the review on the Corona and Chromosphere (Lit.51) which must have been submitted spring or summer 1952. Another major item was the book on Light scattering by small particles, which appeared 1957 (Lit.82) but about which I had been working off and on since my thesis, as you can see from Lit. 59, 60, 68. In the same year 1952 the preparation for the Cambridge, Mass. Symposium on Cosmical Gas Dynamics (with Burgers) was already starting. The symposium was held in July 1953 (Lit.62). My own paper in there took an enormous effort to describe the state of the interstellar medium without bias by all kinds of models. If asked at that time, I probably would have answered that this was my main field. Couple with the fact that earlier I had been able to make a significant contribution to the theory of magnetohydrodynamic waves (Lit. 40), I would have ended up in what is now called plasma-astrophysics if radio astronomy had not diverted me. References are to my number Literature list. Sorry for the elaboration.]
Van de Hulst
One of the things I was very heavily involved also in ’49, you mentioned the polarization of interstellar dust, at that time I did together with Burgers the preparation and then later the editing of that first "Cosmical Hydrodynamics" symposium, which editing and was an enormous job, and certainly the way Burgers wanted to do that. And it was at that time still relatively new to do a symposium so completely with discussions. And I have the feeling that that took much more of my input than practical radio astronomy. Because eventually these things get together somewhere.
Sullivan
But what was your interest? You hadn’t worked in galactic structure and so forth before?
Van de Hulst
No.
Sullivan
How did you get involved in that?
Van de Hulst
Well, I got of course very much interested. Of course I kept very much up-to-date and I did a fair amount of the practical work, helping out in practical arrangements myself. And in fact in 1953, two years later, we were so far that we could actually make a first map of spiral arms, and that had to be ready for a meeting in April. We spent at that time, during the Christmas vacation, we spent with everybody at the Observatory several days in the Lecture Room reading off these long rolls of paper and plotting them up on a uniform scale. That is that set of profiles which is in these older papers. [Note added by Van de Hulst, January 1984: This must be the Easter vacation. The meetings at Schoonebeek, Drente (Lit. 54), at Washington, January ’54 (about which Hagen wrote a review, I think, in JGR [Journal of Geophysical Research]), and at Oxford (Lit. 55) were held in rapid succession on one trip and I carried the three rolls of paper in a cylindrical cardboard container to all these places.]
Sullivan
Westerhout has told me about herding everyone into the room and -
Van de Hulst
Right. And in fact, I must still have somewhere a couple of the original profiles, because it sat in this room with one of these desk lamps under a plate of glass to copy them late at night still on the same scale in order to be able to use them the next day in those lectures.
Sullivan
But this was mainly for a meeting that you were trying to?
Van de Hulst
It was for three meetings. There was a special meeting of the Academy of Sciences, one of those decentralized meetings. It was actually in Drente, but not in Dwingeloo. I had been asked to give a review and so these results ought to be included. And then there was a meeting of the National Academy in Washington.
Sullivan
That was in 1954?
Van de Hulst
That was in ’53, I think. And there was a meeting, well, actually I gave the Halley Lecture in Oxford also.
Sullivan
That’s right, that’s in Observatory.
Van de Hulst
Yes. That was all in succession in the same months essentially. So those fresh results were being used for that then. [Note added by Van de Hulst, January 1984: see note 13]
Sullivan
That’s right, and the Halley Lecture is published in Observatory?
Van de Hulst
Yes, right.
Sullivan
Yes, I’ve seen that. Well, that paper was published in ’54 and that, of course, I think even up to now is sort of the basic paper for the structure of the Galaxy. I mean that sort of established it. It’s been done later in a lot more detail, but the methods are still basically the same.
Van de Hulst
Yes. Well, of course, it’s not quite an accident if you know that, of course, Oort’s thesis was on the differential rotation. And it was obvious that Oort would be the first of anybody to realize when you had a line in the radio domain you could get out of the linearized portion of that thing and get everything.
Sullivan
Right. Yes, that’s right, it’s no accident. Well, what is the next step in your involvement in radio astronomy?
Van de Hulst
Well, of course already at that time we were – much earlier we were heavily planning for a really big telescope and “really big” meant 25-meter, and so that planning to prepare took a while and the construction was well on the way already in the years we talk about because the thing was open in ’55 or ’56, the Dwingeloo telescope. And that took endless meetings and discussions also. And one of the discussions I remember vividly was whether the investment was justified in view of the fact that perhaps after mapping the sky we would have nothing else to do.
Sullivan
You’d already done it all.
Van de Hulst
Yes. If there was only continuum radiation, and only to a certain sensitivity, then of course you could perhaps map the sky and it’s over. But we very soon convinced ourselves and others that other things were likely to follow and there were many frequencies and there were possibly lines and all these things together gave already in ’49 the conviction that it was fully justified to ask for a big telescope.
Sullivan
But that’s interesting that there was doubt in your mind as to whether -
Van de Hulst
Well, I don’t know if that discussion has been recorded, but if the minutes of our meetings would be complete, then you could probably find that discussion. [Note added by Van de Hulst, January 1984: My memory about that discussion is distinct. It was brief but explicit appraisal of the envisaged work load for such a giant instrument! This may have been anywhere between 1948 and early 1951, for it definitely preceded the discovery of the line. I have not gone over the minutes myself to find out.]
Sullivan
So your next involvement in radio astronomy was with the Dwingeloo data?
Van de Hulst
Yes.
Sullivan
And that was M31?
[Note added by Van de Hulst, January 1984: This is a leading question. What I continue to say about M31 is correct. But it made me jump two other items in radio astronomy which at that time were more in the center of my attention. The first was this textbook, the extension of my 1951 course notes, on which I spent a lot of effort but never finished it. The other is related to it: an effort to define and measure in very precise terms the absolute intensities (flux densities) of continuum discrete sources. The appendix to BAN 472 (Lit. 71) arose from endless discussions with Seeger, Westerhout, and others, but in my memory I did the final writing. I think at the time it was an important contribution to putting things straight. The importance arose from the fact that bends or bumps in the spectrum (to which Hagen had called attention) could be used in the redshifted domain for cosmology, provided you could accurately measure them in the first place. This is still true from the 1984 perspective. In my files is also a memo by Seeger of 4-5 pages, dated 11 Jan. 1953, in which he explains the technical requirements to make such accurate flux measurements at Dwingeloo.]
Van de Hulst
The one thing where I was personally very strongly involved, because I was often very strongly involved in helping and suggesting work for students, and so on, but where I was personally strongly involved was the M31, with Ernst Raimond. And again I remember that I did two things, I did make the computations behorehand of what might be expected, actually quantitative. That turned out to be correct on the dot. If I remember well it was 7 degrees’ maximum brightness temperature. But then also it took me some persuasion at Dwingeloo to actually go after it, because several people had tried on runs and nothing visible came, nothing clearly visible came, so on the basis of that calculation I convinced them let’s do something very unorthodox and really sit for fifteen minutes on one spot and fifteen minutes somewhere else and fifteen minutes back. And when we did that, then it came out.
Sullivan
Really integrate -
Van de Hulst
Really integrate, yes. At that time to do something for fifteen minutes was strange. The normal integration time was something of the order of one minute.
Sullivan
Everything was sort of drift scans before that?
Van de Hulst
It was mostly drift scans, yes. That was quite unconventional and of course that was only possible, and I now say it as if it were my idea, but this was made possible on because Muller had made such a good receiver.
Sullivan
Such a stable receiver?
Van de Hulst
Yes. And that was absolutely clear, but it was also clear to me that having such a stable receiver, one should go to the bottom of it and try to use it in this way, and then as you remember in that paper we then used that method on all these points and got a very nice record out of it.
Sullivan
Besides the Magellanic Clouds, that was the second extended extragalactic object?
Van de Hulst
Yes.
Sullivan
Harvard had picked up a few galaxies they couldn’t resolve.
Van de Hulst
Yes, right.
Sullivan
And would you say that changed any ideas about the nature of galaxies or spirals?
Van de Hulst
No. Parallel to this is the development of what you would call the non-thermal radio astronomy.
Sullivan
The radio galaxies?
Van de Hulst
And that started already immediately at ’46 with the discovery of the first point sources, Cygnus A and Cassiopeia A. And that line you can also – I have been vividly involved with that, but I have done nothing specific about it. But it was clear that in all of the work, with the early work with Dwingeloo of course, we picked up those brighter galaxies and we actually stopped the construction of the Dwingeloo telescope at that occasion of one of the first occultations of the Crab Nebula by the moon.
Sullivan
Gart Westerhout told me about that, yes.
Van de Hulst
And so we were very much aware of that other side, but you could not do everything at the same time. But the excitement certainly was also in those distant nebulae.
Sullivan
Right, but I’m asking now – M31 we think is sort of like our own Galaxy in some respects. What do you think were the additional things that were learned from the Dwingeloo observations – or was it pretty much as you expected?
Van de Hulst
No, it was a surprise in M31 that the hydrogen extended so far out and that it also was so relatively empty near the nucleus. Of course Gart had in his thesis on the HII regions something similar, but it was not quite clear that this was different.
Sullivan
That this was true of HI?
Van de Hulst
Yes. And that we could not with any certainty see from our own Galaxy.
Sullivan
Right.
Van de Hulst
I would characterize that as a nice result, but it was sort of the first neutral hydrogen from outside our own Galaxy but it was not -
Sullivan
Well, Magellanic Clouds -
Van de Hulst
Yes, that may have been a bit earlier, right. But it was not anything which was terribly exciting. But, of course, you always have to follow those two lines, some things which are too novel might not link up with anything, and this linked up so strongly with other things that you could say it’s hardly anything new.
Sullivan
It’s a natural extension.
Van de Hulst
Yes. You have to do that. And I must confess that I personally tend to stay a little on the conservative side. Things that do link up – rather than speculate.
Sullivan
Well, we need both kinds. Well, let’s see now, starting in ’60 or so you got very much involved in space research, didn’t you? Or is there more radio astronomy that you directly worked in after the M31 paper?
Van de Hulst
No, I don’t think so.
Sullivan
Except, like you say, you’ve been a member of the Board of Stichting.
Van de Hulst
And, of course, more importantly that I’ve been a Professor at Leiden Observatory, and so of course have been in thousands of discussions of what to do and what not to do, what to propose and what not to propose. And although my official duty was to teach theoretical astrophysics, we have more or less on purpose neglected the theoretical end of it in order to be able to man the observing staff for this type of work. In fact, the theoretical astrophysics has remained a little underdeveloped in Leiden since that time, because you have put a certain priority.
Sullivan
Right. So you’ve been involved in the whole development of Westerbork and so forth.
Van de Hulst
Yes.
Sullivan
Well, I’ve talked to a few other people about that. Is there anything else you can think of that you might want to comment on?
Van de Hulst
Well, the only thing is that I could give you further documentation for the (?)
Sullivan
I’d like to see that. Ok, thank you very much. That ends the interview with Henk Van de Hulst on 26 November 1973.
End Tape 25B