Interview with Edward Mills Purcell
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The interview listed below was either transcribed as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009) or was transcribed in the NRAO Archives by Sierra Smith in 2012-2013. The transcription may have been read and edited for clarity by Sullivan, and may have also been read and edited by the interviewee. Any notes added in the reading/editing process by Sullivan, the interviewee, or others who read the transcript have been included in brackets. If the interview was transcribed for Sullivan, the original typescript of the interview is available in the NRAO Archives. Sullivan's notes about each interview are available on the individual interviewee's Web page. During processing, full names of institutions and people were added in brackets and if especially long the interview was split into parts reflecting the sides of the original audio cassette tapes. 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.
In preparing Sullivan interviews for Web publication, the NRAO/AUI Archives has made a concerted effort to obtain release forms from interviewees or from their heirs or next of kin. In the case of this interview, we have been unable to find anyone to sign a release. In accordance with our open access policy, we are posting the interview. If you suspect alleged copyright infringement on our site, please email archivist@nrao.edu. Upon request, we will remove material from public view while we address a rights issue. Please contact us if you are able to supply any contact information for Purcell's heirs/next of kin.
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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Sullivan
Ok. This is talking with- is it Edward or Edwin?
Purcell
Edward.
Sullivan
Edward Purcell on 16 November ’77 at the University of Washington. I'd like to go back to World War II when you were working in the Radiation Lab, and you were actually leader of the group that [Robert H.] Dicke was working in also, and in particular, do you have any memories as to hearing about radio astronomical sort of things in terms of [James] Hey’s discovery and this sort of thing?
Purcell
Well, not so much that, but we were, let's see, there were two things, in the first place Dicke developed a radiometer there- right in that group. It was used very heavily to study the water vapor absorption, which we had blundered into in K-band. And Dicke did use it for other interesting things. He looked at a partial eclipse of the Sun with it, and he also measured the temperature just looking out in space.
Sullivan
Got an upper limit for it?
Purcell
Got an upper limit as I recall of 20-25K. Something he himself later forgot.
Sullivan
What was the original reason to build this radiometer?
Purcell
Let's see, I'm not quite sure just how it started. Dicke was interested in- the lock-in amplifier which was a great thing then, you see, and he was working sort of off on his own, on things connected with signal to noise, and one thing and another. And I can't remember just, you'd have to ask him about it, I really can't remember. But I can remember vividly when he had it working - we all went down before it was even an antenna thing, we went down there and you could hold a cigarette at the end of the waveguide and the meter would go off scale.
Sullivan
So it wasn't a driving military reason then?
Purcell
No, no, no. In fact, my group was kind of an odd group. It had a sort of free-wheeling license to try things that were far out and things like that. Though at the same time we were engaged in developing the K-band radar techniques.
Sullivan
But the water vapor absorption line wasn't known when K-band was set up.
Purcell
We should have known better, because we could have known about it if we'd only read one of van’s [Sullivan: van Vleck's] papers, and I remember very vividly myself, going down to a meeting at Columbia. You see, Columbia Radiation Lab was making our magnetrons. Magnetrons were coming out at various frequencies. It was time we settled down and said let's have a target frequency so that the plumbing and the magnetron and the LOs are all matched. We had a little meeting down at Columbia to decide which number we were going to pick.
Sullivan
But, what year was this have been?
Purcell
This would have been 1943, I should guess. And we picked 1.25 cm for no very special reason except that some tubes had been coming out fairly close to that, we were using .25 inch by .5 inch OD waveguide which worked all right, didn't have to - and it made .25 wavelength an .125 inch, which is kind of neat. We would have been a lot better off if we had known about the water line, what we really should have done was to pick a longer wavelength. It would have made everything easier and we could have still had a gain- if we'd picked, say, 1.6cm, we would have still had a factor of 2 over X-band resolution and we would have been in the clear.
Sullivan
But in fact, there was very little K-band radar actually getting developed.
Purcell
That's right. And, in fact, as far as radar application was concerned, it wouldn't have made that much difference, because K-band by its very nature was adapted for rather short range application. It just gave us a bad name with the military because, you know, they thought it was terrible to have all that attenuation. But it wasn't all that large and the applications that could have been done on K-band wouldn't have made all that much difference. But at any rate, it led to a feverish activity in measuring this with Dicke’s radiometer. Willis Lamb at Columbia made a copper lined echo room in which he could put steam, and had it festooned with columns of bolometers hanging from the ceiling, and so he'd pulse in with a magnetron into the echo room, and then the heating of the bolometers, of course, would depend on how long the energy rattled around. And [P.] Kusch wound up about 200 feet of waveguide and tried to just "brute-force" measure the attenuation in water vapor in the waveguide; it wasn't a terribly good idea. And Dicke did it very nicely taking tilting curves through the atmosphere and they went to Florida, and they’d fly radiosondes to get it. At the same time, [George] Southworth at Holmdel at Bell Labs was sort of interested in looking at the Sun and things like that. And, of course, Dicke looked at the Sun at K-band. The thing that I did not know about at the time was the sort of long-wave Jansky-type stuff that I just hadn't paid any attention to.
Sullivan
Why was it that no one at the Rad Lab did what Southworth did, essentially, during the war?
Purcell
Well, Southworth didn't do anything that was terribly interesting. He just...
Sullivan
Well, he measured the antenna temperature that you get from the Sun at a couple different wavelengths.
Purcell
Well, so did we.
Sullivan
But all I'm aware of is what Dicke did at the end of the War. Did you do that during the War? In some unpublished report or something life this?
Purcell
You mean... oh, I don't know. Southworth measured the antenna temperature of the Sun at S-band, something like that.
Sullivan
Yes, I think at 3 and 10 centimeters.
Purcell
Yes. Well, it wouldn't have excited us very much; it was a nice thing, you know, there were many things to do that were equally interesting.
Sullivan
But there is no unpublished report or anything this many measurements. Do you know of any other things in Rad Lab reports or something that might bear on radio astronomy that I might not have seen?
Purcell
No. But I think it’s important to recall another aspect of the whole situation. We had an enormous group of people who were working on microwave propagation. And the reason being that it had been discovered that microwaves can be trapped and ducted and bent around the Earth, especially over the ocean. That problem was considered very important for military applications, and enormous amounts of effort were put into it, both experimental and theoretical. It turned out that the trapping was due to a very shallow, very low-level gradient in water vapor that you would get. The first few hundred feet would make a linear gradient in index and then with this index gradient, you could have a waveguide, so the theory of this was just worked on by all sorts of theoretical people and computers. This was absorbing a tremendous effort. In fact, the Mark-1 computer at Harvard, the famous Mark-1, spent most of its time calculating Bessel functions of order one-third which is what you have to work with if you're trying to solve this particular problem.
But now, I mention this because at the end of War, there were some obvious things that one might do going back into scientific life, you see, we were all concerned with that - what shall we do? There were so many things you could do- we had so many new tools. One of the natural sort of obvious things to have done was to continue this work on anomalous propagation, which was well funded by the military and the problem sort of half-solved, you know, and all this stuff. But the smart people didn't do that. In fact, in particular, [E. G. "Taffy"] Bowen went down to Australia to head up the CSIRO Division absolutely had nothing to do with propagation. I told him one time, "Taffy, I admire your intelligence not so much for what you did in Australia as for what you didn't do." That was a problem that one should have turned his back on, absolutely no interest anymore, completely dead and gone. So they went down there and started measuring, using the Lloyd's mirror interferometer on the cliff to measure the solar thing.
Sullivan
I talked with many of them, including Bowen.
Purcell
That's right.
Sullivan
What other fields did you consider?
Purcell
Oh, well, of course, I already had so much physics lined up to do with other things, including nuclear magnetic resonance [NMR], and which we actually did before I officially left Radiation Lab.
Sullivan
Okay, that was one. What others do you remember in the field, in the group?
Purcell
Well, people wanted, of course, to go back to nuclear physics. One field that was ready to roll, of course, was microwave spectroscopy. That went on very fast, because we had the tubes, we had the interest, we had already done microwave spectroscopy, not only the Dicke stuff with water vapor, but we'd looked at the old famous ammonia line in the wave guide, and so on. That was just ready to go.
Sullivan
Do you remember anybody at all thinking of radio astronomy besides Bowen?
Purcell
No. I really don't. Bob Beringer had some nice ideas towards the end, applications of things like the oxygen line, but never thought much about radio astronomy. He didn't go into it. No, we just didn't know much about it, really. Well, in the first place, you might ask were there any astronomers there, which is...
Sullivan
But, of course, there weren't in the labs in England and Australia during the war either.
Purcell
But we had some astronomers!
Sullivan
Oh, you mean in the Rad Lab.
Purcell
Oh sure. In fact, we had, not in my group, but at Radiation Lab, we had, well, John Hall, for one, was at Radiation Lab, but even more important, I can see his face and can't say his name. Now retired, Director of Lick.
Sullivan
Of Lick?
Purcell
Oh...
Sullivan
Oh, I think I know-[Albert E.] Whitford.
Purcell
Al Whitford! Yes, he was in the RF Group. And I think there were a couple more astronomers, but they went back into astronomy.
Sullivan
Yes, but optical astronomers...
Purcell
That was it.
Sullivan
So I might then ask you the question of what are your thoughts on why American radio astronomy just didn't go anywhere till really the late '50s and early '60s?
Purcell
I don't think there's any real theory about it. History is a series of accidents to some extent. It didn't have the man to do it.
Sullivan
Okay. Let's move on then to...
Purcell
Where would I have expected it to happen? I think I would have expected it to happen at Bell Labs because they thought about things in a very fundamental way.
Sullivan
They had the history of [Karl] Jansky.
Purcell
They had the history of Jansky and they were marvelously equipped, but they had other things- everybody had other fish to fry.
Sullivan
Although it is true that they didn't really let Jansky go with his thing. There's a debate as to whether he wanted to, but- well, let's move on to the hydrogen line. As I mentioned to you, I've heard the other view very much and looked into it, and I'm anxious to hear yours. My fundamental question is how did you get into the idea of detecting it in interstellar regions? I could see where you would be interested from an NMR point-of-view.
Purcell
No, well it came about as follows. You can check this with Doc [Harold Irving "Doc" Ewen], but my memory is as follows. Doc Ewen had come to me looking for a thesis problem, and he was interested he thought in doing something about the upper atmosphere with microwaves. He wanted to do it with microwaves and it hadn't anything to do with the nuclear resonance stuff at all, that was a completely separate activity. Then Doc already knew a little bit about astronomy- he had taught navigation during the war to a lot of- in fact, he was a good pal of a number of the Red Sox, Johnny Pesky and Ted Williams because he’d taught them navigation. But he knew that kind of astronomy pretty well, and he had a lot of enthusiasm, so in the middle of this discussion, Doc went off to an astronomy meeting somewhere, I don't know where it was, New York or New Haven or some place. And there were some people there he knew; he's a very jolly, gregarious fellow. He came back and said, "You know, somebody at the meeting was talking about maybe one could detect a hyperfine line from hydrogen in space." Well, the hyperfine line was sort of familiar to me from the Columbia work which, of course, I’d been following.
Sullivan
Do you remember what year this was?
Purcell
It must have been certainly, I should think 1949, perhaps, not earlier than that. And I said, "Well, that's interesting. Let's see if we can calculate whether it could be done." So I did some calculations, and it looked like, you know, maybe- and I looked up some of this stuff about the interstellar gas and I read a little bit- I read one of [Lyman] Spitzer's papers, I think, about where he estimated the temperature of the gas from indirect evidence- turned out to be about right.
Sullivan
Did you hunt up [Hendrik C.] Van de Hulst’s original article?
Purcell
No, because you see, at that point, the name Van de Hulst at that time did not come into it at all. Doc didn't even know where this had come from. He just said, "People were talking." On the other hand, of course, I'm quite certain that it traced back directly it to Van de Hulst.
Sullivan
They were probably talking about the Dutch effort.
Purcell
Sure. There's no question in my mind that if you switch Van de Hulst in and out that our experiment would go along with it. But we didn't at that time. In fact, I don't know if this was as late as when they'd actually begun. But I think it was just that somebody had mentioned it as a kind of amusing possibility. So then we said, "Let's try it." And Doc was thoroughly warned by me, and of course, didn't need to be warned, that it was a really risky thing. I said, "If you don't find it, you're going to have a real job ahead of you because you've got to measure everything and put on strong limits." So that in fact, he did an extremely good job, and although our horn was very simple, it produced as much signal as it could have, and the electronics was really excellent, and I think, actually better than the Dutch receiver at that time. I think Doc would tell you that, and I think he's correct. He had a calibration of the noise figure made with the fluorescent tube that [William W.] Mumford had developed at Holmdel...
Sullivan
I have some pictures here which boggle your mind.
Purcell
Yes, I've got some wonderful ones, I tell you, I have got color stereo pictures and there is Doc just standing in the room in 3-D. It's really very uncanny.
Sullivan
I love this one where it says "Meter Hot" - it shows how it was just all put together and you weren’t trying to be fancy- just get a good set up.
Purcell
That’s right; it was all done in that big waveguide- you don’t have the one that shows the noise source- it was a well calibrated thing, so...
Sullivan
From a technical point of view, was it just a matter of doing everything very well, or were there some differences that hadn't really been done before?
Purcell
Of course now, everything that we did looks so old hat and so common. It was a frequency switching...
Sullivan
That was something different.
Purcell
That was something different, but Doc decided on that quite early, frequency switching, and it was really very carefully measured. We knew what our noise figure was.
Sullivan
But it was off-the-shelf components?
Purcell
Well, it was off-the-shelf; it was mostly off other people's shelves. He was great at scrounging things, and the total expense for the whole damned thing was really ridiculous. He knew how to get things out of the Navy.
Sullivan
Do you think there's possibly any record of some feasibility calculations that you might have done?
Purcell
I have not a scrap of that, and I doubt if Doc has. I think I made most of the first calculations, though he, I'm sure, did them, too. But you can ask him- it may be that he's kept something, it’s possible. I'm sure I haven't got a scrap.
Sullivan
But you said you were basing it on the Spitzer, the latest Spitzer...
Purcell
I can't remember at what point that I started reading about the interstellar temperature. An interesting point later, of course, is that we didn't, at that time, even after we had the line, we did not realize what the main spin temperature "setting mechanism" is, namely the spin exchange that George [Sullivan: Field] and I eventually did a paper on. That came up in a rather amusing place. We had in those days a Thursday night joint seminar with Physics at MIT. It always happened at Harvard, but we had a certain little lecture room - very informal, sort of knock-down-drag-out seminar. And sometime, it must have been in the spring of 1952, I gave the seminar talk that night on the 21 cm observation. So this was a seminar where everybody was very rude, they'd interrupt you or anything like this. I was patiently calculating the radiation time and calculating the equilibrium time to show that the spin temperature was going to be the kinetic temperature. I considered various processes here and I calculated the spontaneous emission time, and all the time I was doing this, Norman Ramsey and Vicky [Victor Frederick] Weisskopf were sitting together in the front row and they were just jabbering back and forth to one another in very coarse whispers. Finally, I got annoyed and stopped and said, "What are you guys talking about?" They said "Wouldn't spin exchange be more important?" And as soon as they said it, you know, it was very obvious.
Sullivan
I got the impression that spin exchange really wasn't really worked out until after the discovery of the line.
Purcell
That's right. No, we already had the line. This...
Sullivan
No, no, no. I mean, but the- I see, so that was the beginning of working out the spin exchange.
Purcell
That was the beginning of working it out. As soon as they said it, you could see that all you had to say is a that during a collision, there is a reasonable probability, not small compared to one for spin exchange and then, of course, you...
Sullivan
That's what led to your 1952 [Sullivan: 1956] ApJ paper?
Purcell
That's right. Then you know that's got to be it. So I always kidded Vicky and Norman that someday I was going to write a paper and put down, "asterisk: publicly communicated by..."
Sullivan
Okay, going back to the discovery of the line, can you characterize for me the role of you versus Ewen in this experiment?
Purcell
Oh, to say that Doc did all the work is an understatement. And really doesn't do him justice. He not only did all the work, but he understood everything and he kept going. I was, at that stage, when the electronics, I was just essentially an interested observer with whom he could discuss his problems and try to think them out. In fact, I wasn't there looking at the record when it came in because it was at two or three a.m. that the Milky Way was coming past in March. And the thing I remember is I came in that next morning and on my desk, there was either a roll of Esterline-Angus or Doc was there saying, "I think I got a thesis." And we went upstairs and took the Esterline-Angus and we rolled it out the length of the hall and got down and sighted down it, which was the way you integrated your signal. And it looked pretty real, but then, of course, we weren't absolutely sure until the...
Sullivan
Did receiver drift or...
Purcell
Well, it was pretty good, and you'll see the original record, it wasn't too bad. But then when it moved in frequency and in time, the way it should have, then, of course, that clinched it. But you see, I had to have Doc - Doc was the guy who knew what galactic coordinates were. I didn't even know about Right Ascension and declination.
Sullivan
That's something I can ask right now. The famous horn stuck out the window at -5° dec, was it simply you're looking because that was a handy place to put the horn, or were you actually picking a declination?
Purcell
No, we picked it. That's a good declination to let the Milky Way go past.
Sullivan
Well, it could be any number of ones, but I was just wondering...
Purcell
We couldn't have it too low or we'd hit another building. It wasn't too high.
Sullivan
And you didn't want to put it too high because then you’re looking away from the galactic center?
Purcell
Yes. It was reasonable. Just out on the parapet over the front door of Lyman.
Sullivan
At what point did you become aware of the Van de Hulst paper and the Dutch effort and so forth?
Purcell
Of course, Van de Hulst then came to Harvard and then we were in complete touch with it.
Sullivan
But you weren't part of his coming there or anything?
Purcell
No, no.
Sullivan
And so he found out about your experiment, and then like you say, you were in touch.
Purcell
My memory is poor on that, but I believe that Doc can fill you in on that better, because I think he was more closely involved and he had talked to Van de Hulst more than I had at that particular point.
Sullivan
The thing that's always struck me is the beautiful exchange took place, not spin exchange, but exchange of astrophysics for techniques. You just said you'd agree with this, namely that the Dutch did not have this frequency switching technique which, of course, is vital to getting these low signals. On the other hand, it's my understanding that in your experiment, you were only going to switch a few kilohertz, you were thinking of only a naturally broadened line, not the Doppler broadening due to a couple hundred K gas, so that you would have missed the line for that reason, and that Van de Hulst was the one that pointed that out.
Purcell
That may well be. That's a point I have simply blanked out on, but you may well be right. It would be a very interesting point. Again, I think that Doc will be able to...
Sullivan
This is mainly from Van de Hulst that...
Purcell
That may be right. I just can't testify to it. I wouldn't have, you know- that point is gone from my memory. Maybe I dropped it for Freudian reasons, but you'll have to ask Doc about that.
Sullivan
Do you remember anything else that Van de Hulst contributed to the experiment?
Purcell
No.
Sullivan
You might not be the person to ask.
Purcell
No, I don't think there was anything else, but that may have been a very important item. If the line had been, of course, ten times weaker, we would have missed it, too. It wouldn't have taken much more than that to...
Sullivan
Well, sure. Along that same line, Frank Kerr happened to be there getting a Master's degree at that time. Do you remember at all the interaction between Kerr and your group?
Purcell
Not especially, I really can't...
Sullivan
The Australians were not working actively like the Dutch were. But nevertheless, once you did discover the line, you notified them as well as the Dutch, I believe.
Purcell
I think, whether we did or whether Frank sent the message back, I...
Sullivan
I think it was the latter.
Purcell
I think it was the latter. Frank came up to our celebration [Sullivan: 25th anniversary celebration in 1976]. I'm so hazy on that. I remember our feeling- we started out, we didn't know whether we'd see the line in emission or absorption.
Sullivan
Which you discuss in this paper...
Purcell
That's right, and that seemed to be...
[Break]
[Resume on 17 Nov]
Sullivan
So what was your real motivation in notifying the other groups after you made the discovery? This is continuing by the way on 17 November.
Purcell
Well, I really can't remember directly. I would have to go back and sort of imagine what it was almost, so it's not very good history. First of all, we were of course very well aware by then of the Dutch work and the really central role of Van de Hulst in generating the idea in the first place. We had clearly some obligation to acknowledge their important position in it. In fact, as I guess you've probably learned from them, if they hadn't had a fire in the antenna or something, they might well have anticipated us, in which case we would have had really nothing to claim much credit for. And then in the case of the Australians, I felt again a personal connection, because Taffy Bowen, among others, was a good friend of mine- I'd worked with him during the war. And Frank Kerr was there, so that it was just that- well, I don't know. Of course, in a way, you might say it was a sign of a lack of confidence if we went out on a limb just on our own and then it turned out that it was just a glitch in the receiver or something, but I don't think that was very much of it, although I can't prove it. I think it was just that our first associations one way or another were with these people and there particularly the important role of the Dutch in the whole thing.
Sullivan
So as you recall, you were quite confident about the reality of the signal?
Purcell
Yes, yes. I think we were very confident after it had moved properly in frequency and in time.
Sullivan
Maybe this is a good stage to ask you a couple of specific questions about the article in Nature. You mention here a detector and a narrow .016 cycles per second filter. Was that essentially an integration constant then of the inverse of that 60 seconds?
Purcell
Yes, that was the post-detection integration time. That was just essentially what we would have called then a lock-in system.
Sullivan
Okay. But this was only a single channel which you were sweeping in frequency?
Purcell
That's right, that's right.
Sullivan
It was rather interesting that this term "spin temperature," which you say here, "For want of a better name," of course, became the standard of the field. Let's see, what else.
Purcell
I'm not sure that we hadn't already used that term in NMR at that point. That's an interesting question, I'm not quite clear.
Sullivan
That would be interesting to know.
Purcell
It was before our negative temperature experiments when we began thinking of spin temperature in a very concrete way.
Sullivan
What were you doing to base the frequency you expected the line on in terms of the galactic Doppler shift- were you looking up the rotation of the Milky Way and expecting a certain shift, or were you just going to search over quite a large band and see where it came up? Do you remember?
Purcell
No, I honestly don't remember. I wonder, let's see, what the direction in galactic coordinates was.
Sullivan
It was about ?= 30 or 40°- modern galactic coordinates- and, in fact, you found a shift of something like 150 kilohertz. But something I should mention, and probably you are aware, if that your switching of 75 kilohertz, although it was much better than what Van de Hulst says you apparently wanted to do, nevertheless, was not sufficient for the broad...
Purcell
That's right.
Sullivan
Galactic lines. So the peak you got was simply the strongest peak and you missed a large baseline.
Purcell
Yes, yes.
Sullivan
But nevertheless, I’ve checked this out and it’s quite consistent with later measurements in this direction.
Purcell
What is the radial velocity in that direction from the simple rotation?
Sullivan
There are several. You're looking through a lot of spiral arms.
Purcell
Through a lot of spiral arms.
Sullivan
And this doesn't correspond to the main one, so that worked out. But I was wondering if you had worked out the galactic rotation.
Purcell
I don't remember, Doc may remember better on that. I don't remember our argument on that.
Sullivan
Now, the main thing that you seemed to be concerned with in this paper, besides, of course, just announcing the discovery, is the business of the opacity of the line and the equilibrium between the line temperature and the background temperature, which, of course, is indeed, quite important. You work out, I believe, what we would call now an "optical depth of one distance" - you call it the absorption length. Is that what you meant?
Purcell
Yes, right.
Sullivan
And I can reproduce those numbers pretty well. You don't mention the observed line width, you mention here you're plugging it in, but you don't give it anywhere in the paper. Do you recall at all the line width that was measured?
Purcell
No.
Sullivan
These figures I have here, I think, are probably from Ewen's thesis. I got them actually out of [Jan Hendrik] Oort’s Archives. And they don't refer to the line width, only the line frequency as a function of time.
Purcell
I don't remember. All that’s water far down the river.
Sullivan
Something else I always find striking is this thing from the Lyman Laboratory, and here's a fundamental paper on the hydrogen atom coming out of there.
Purcell
Yes, yes. Actually, Lyman a was first seen in the basement of the old Jefferson Lab.
Sullivan
I see.
Purcell
And Lyman's spectrograph was a piece of 4 inch pipe.
Sullivan
You also had a sort of pipe stuck out the window! The other thing I find striking...
Purcell
Well, of course, you see, Lyman, at that point, was Director of the Laboratory.
Sullivan
Oh, was he?
Purcell
Yes. And we had to get his permission to stick the thing out the window.
Sullivan
I'm always struck when I look at these two papers in juxtaposition. This is clearly a physicist’s paper and this is clearly an astronomer’s paper. Oort talks about the galactic rotation and so forth and very little about the physics of the line itself, although, of course, later on he went into it quite a bit.
Purcell
Sure.
Sullivan
Okay. Let's move on. You gave the results of this, as I see it, at two different meetings - an APS [American Physical Society] meeting in Schenectady and an AAS [American Astronomical Society] meeting in Washington, D.C. - both in June of 1951. Was that you or Ewen?
Purcell
It was certainly Ewen in the case of this one.
Sullivan
The AAS Meeting?
Purcell
The AAS, yes. And it was possibly, possibly it was me at the other one, but I'm not- let's see...
Sullivan
It's given as Ewen and Purcell.
Purcell
This was in Schenectady- oh, I'm sure this was Ewen. I don't even remember going to Schenectady for that. I think Doc did it.
Sullivan
Can you recall at all in those first few months after the discovery when word was getting out, more or less, the reactions of astronomers and/or physicists?
Purcell
Well, again, it's interesting that I knew mostly physicists and Doc was really our wing among the astronomers. I didn't really know many astronomers. One of the first people I told about it, and I remember telling him, was Rabi, who was, of course, very much interested because of all the Columbia background on the hyperfine splitting.
Sullivan
Who is Rabi? Oh, Rabi, I see.
Purcell
I. I. Rabi. And it was Nafe and Nelson there, who had measured accurately the ground state hyperfine transition in an atomic beam, so that Rabi, was one of the very first people I told. In fact, I remember where I told him. This might help the date some things- I was at the time on the Physical Society Council, and there was a Council meeting at the Bureau of Standards, which would have been in connection with the annual APS meeting in Washington, which was always at the Bureau of Standards in the spring. You know namely Oort and- I remember going to this meeting and Rabi was already there and I came in. I sneaked over and told Rabi, "We’ve got the 21 cm." So that must have been within a week or two after.
Sullivan
Okay, concerning the rest frequency of the line, was this the paper that you quote by...
Purcell
What do we quote?
Sullivan
Kusch and [A. G.] Prodell. Was that vital to the discovery or could you have done it as well with just a calculation of the line?
Purcell
No, no, I think it could have been calculated with sufficient accuracy at that time.
Sullivan
Which is what Van de Hulst did in his original paper.
Purcell
Yes, sure.
Sullivan
And along this same line, did you have any idea that perhaps this was a way to better measure the frequency of the line in any sense?
Purcell
No.
Sullivan
Maybe this could be extended?
Purcell
No, I don't think we ever seriously thought of that, because by then the atomic beam guys could measure anything, and obviously the Doppler shift would be prohibitive...
Sullivan
Of course, now that is happening in molecular spectra where you can actually get some radicals better than you can do in a laboratory.
Purcell
Yes, just because they...
Sullivan
Can't be well produced.
Purcell
Can't be well produced and nourished. No, by then the atomic beam techniques were coming along so rapidly and precisely.
Sullivan
Let's move on to your next touch with astrophysics, believe it is, correct me if I’m wrong, and that is this paper about the 2S2P level in hydrogen and the microwave transitions.
Purcell
Yes, oh yes.
Sullivan
Responding to a publication by [John Paul] Wild in which he considered all sorts of lines, and you were pointing out that Giovanelli had done it incorrectly. Do you think if this had come out all right, you might have wanted to go and look for this line also?
Purcell
No, no, we weren't but the Australians were. They were seriously considering cranking up to look for it, and in fact, I think it was Paul Wild that told me afterward that when they got my paper, they were actually somewhat relieved because it looked like a pretty hard job anyway, and now they could just forget it and move on to something else.
Sullivan
At 10 GHz, too.
Purcell
Yes, yeah.
Sullivan
Along that line Wild in his paper dismisses what we call recombination lines, and...
Purcell
Yes, that's interesting.
Sullivan
And Van de Hulst treated them quite a bit in his original paper, I don't know whether you've ever seen it.
Purcell
No, no.
Sullivan
And he said that they look like they might be feasible. I was wondering if you ever thought about it?
Purcell
No, no. Never did, and you know, when they found them, I could have kicked myself- so obvious an idea. No, never a wisp of a thought.
Sullivan
Now, you mentioned briefly before, but I'd appreciate it if you could tell me...
Purcell
You know, I have so many examples I can find in my own career of a conspicuous lack of foresight.
Sullivan
Well in hindsight that’s always true.
Purcell
Yes, but as I said when I was talking a week or so ago- they had a celebration for Rabi, and I gave a little talk. We were supposed to talk on the future of physics. I said, just to establish my credentials as a prophet, I remember distinctly sitting in my office talking to the head of research at DuPont who had come up to see me to find out if there was going to be anything in nuclear magnetic resonance for the chemist, and I assured him that there was not.
Sullivan
When was this?
Purcell
Must have been about 1947 or 1948. It was only about a year before the fine structure was found. I told him, "Well, there's only one thing. I think you could probably use it to measure the degree of polymerization." Because it was just at the time when our own thoughts were all concentrated on the problem of spin lattice relaxation and we were beginning to understand the role of molecular motion, so I knew you could measure big samples and we'd just measured the proton line, you know and stuff.
Sullivan
Well, following the discovery of the line, what was the next step as you saw it? What happened, I mean, you've got a discovery, are you going to follow it up or what was your attitude?
Purcell
Well, it kept running for a while of course, but then, fairly soon, I can't place it exactly, the center of interest shifted into astronomy even at Harvard, and this happened with [Bart J.] Bok’s return. During this time, you see, Bok was away as I remember. Although- but he remembers it differently. At least when he was back there and talked [Sullivan: 1976 anniversary celebration]- he remembered in a way which Doc and I were both sure was wrong - you know, where he was at different times relative to things. But...
Sullivan
Could you just say which...
Purcell
My impression was that Bok was away, probably in South Africa at the telescope there, in the spring of ‘51. When he came back that fall or summer, whenever it was, and found out what was going on, that then the idea for going into it more seriously at Agassiz Station with making antenna, and so on began to develop. At the same time, there was interest on the part of Merle Tuve at the DTM [Department of Terrestrial Magnetism] in Washington and...
Sullivan
Was Ewen looking for someone like Bok that could have some swing in terms of getting a bigger facility? Or were you? Or was it mainly Bok’s initiative?
Purcell
It was mainly Bok's initiative, and Ewen's I would say. I really played a rather passive role except I encouraged everybody. But I don't remember, you know, doing any of the what-we-would-now-call proposal generating. I don't remember that at all. I had plenty to do at that time, it was a very lively time in the NMR busness. I did not consider diverting or turning my own career in that direction in a big way.
Sullivan
So you were quite content that this is an interesting discovery, but let others carry it on?
Purcell
Yes, yes. I was very pleased in fact when that happened and we used to consult a lot with Bart about things, about the physics and about the plan. There was this deal with DTM when Doc did shut down the stuff at Lyman [Sullivan: Laboratory], some of it was shipped off to DTM.
Sullivan
That original receiver?
Purcell
Yes.
Sullivan
I see, and then later he sent other receivers, I think, did he not?
Purcell
Well, yes.
Sullivan
But I'll talk to him about that.
Purcell
Yes, you talk to him about that.
Sullivan
Is the next time that you are in contact with astrophysics, this 1956 paper with George Fields? Would you say that's fair?
Purcell
Yes, yes. That's right. This problem had been rattling around, and really, I should have written it up before, but I was lazy and didn't- had other things to do. And then George’s arrival sort of triggered...
End of Tape 84A
Sullivan Tape 84B
Sullivan
Ok. This is continuing with Edward Purcell on 17 November ’77. You said George’s arrival...
Purcell
George's arrival sort of triggered it. George kind of got me off dead center, and then when we started to work on it, it turned out that I really hadn't understood the quantum mechanics of the spin exchange right anyway. In my previous calculations, I hadn't formulated that quite properly. And so it was perhaps a good thing that I hadn't...
Sullivan
He was a graduate student at that time?
Purcell
No, he was a junior fellow of the Society of Fellows. And I was a senior fellow at that time of the Society of Fellows, which was how I...
Sullivan
Oh, here I see, "This was pointed out in a private communication that..." [Sullivan: Weisskopf and Bamsey] So it was really more of a public communication?
Purcell
That's it, that's it. It was really more of a public communication. It was pointed out in a loud whisper, let’s say.
Sullivan
And I was interested that once again, Dicke's name crops up here, because not only theory, but also when I looked at that paper, that was measuring the hydrogen line transition yet better.
Purcell
That's right. I had a number of conversations with Dicke about formulating this stuff in here.
Sullivan
But is it fair to characterize your career in astrophysics as sort of dipping in whenever the problem fascinates you?
Purcell
Yes, very opportunistic, and un-fundamental, I’d suppose.
Sullivan
Okay, the final thing before the early 1960s that I am aware of is your jumping into the argument that was raging in the pages of Nature in 1956 and 1957 on the Hanbury Brown-Twiss effect. Everyone seemed to be happy to have it at radio wavelengths, but then when you changed it to optical, it seemed like it was violating quantum mechanics, and you go through here and say it's exactly what you should expect from bosons unlike fermions. It's my impression- would you say this is a fair characterization- that this sort of settled the argument, reading through the papers back and forth it seems like...
Purcell
I think it did, yes. I'm rather proud of that paper because I can remember exactly where I was sitting in my house when I suddenly understood the Brown-Twiss [Sullivan: effect]. I sitting there worrying about it on a certain couch and finally I said, "Now I understand what's going on" - then of course, this stuff came, this followed.
Sullivan
How did you become aware of this controversy?
Purcell
Oh, I read about it in Nature and the thing that really triggered it was this, as I said, was this Braunen and Ferguson thing.
Sullivan
So you had not been involved in interferometry?
Purcell
Oh no. This was, for me, a question of basic physics.
Sullivan
So, I gather then, that the astrophysics you've done has been independent of the Astronomy Department except that you will take someone like George Field who, I guess, was connected with the Astronomy Department more than the Physics Department - or is that true?
Purcell
Yes, he was.
Sullivan
But you're not really part of the Astronomy Department?
Purcell
No.
Sullivan
Well, I think that covers things pretty much unless I've forgotten something that’s related to radio astronomy.
Purcell
Well, let me think. Oh, well, of course, I have always had an interest in antenna design. I've been less confident and made less contribution, I think, in the electronics- I don't consider, never considered, myself an electronic expert. I always had to turn to someone else for that. But on the matter of antenna design and so on, I do consider that I was an expert. The design of that horn was not, it was just a pyramid. You think nowadays, pretty trivial but we knew exactly what to do with horns, I measured the gain of horns in the Radiation Lab, we invented antennas, we had done all kinds of tricks with antenna structures - including, we were extremely familiar with the antenna problems of the big parabaloid.
Sullivan
And you knew that the horn was the right antenna, also.
Purcell
I knew the horn was the right antenna because it didn't have much back lobe and all that sort of stuff. So we knew, we really were very sophisticated I think. In fact, basically, we knew as much about designing that kind of antenna as anyone does now. We knew that the far field was the Fourier transform of the illumination across the aperture and what you did to get rid of side lobes - all of that stuff. And I've always had a continued interest in the design of large antennas. For the last many years, I have been connected with the NEROC outfit, and when we were plugging the very large antenna which never got funded...
Sullivan
This is the 400 foot dish in a radome?
Purcell
Yes, the 400 in a radome. I have stacks of my own calculations on the radome design and the antenna design, on the natural frequencies of the radome, and all that kind of stuff. I even had a design for the azimuth bearing that I thought was kind of cute. I was amused to see why the engineers dismissed, it because they didn't understand it. So I've been very much interested in that and I've kept my association with NEAOC all this time.
Sullivan
Have you contributed to designs of other antennas that have been used in radio astronomy, NRAO antennas or anything like that?
Purcell
No, I can't point to anything and say, "That's my design" - that's the thing that I enjoy from an engineering point of view and I've been pretty close to it. I was interested in understanding why the Buckminster Fuller dome is a good structure and what its mechanical properties are and what its stability is and so on, and I understand that now. It really is a fantastic structure. I don't see why people don't use it more.
Sullivan
You got into this in the design of the NEROC dome?
Purcell
Yes, the NEROC Dome. Which of course, the basic design was done by Herbie Weiss and his engineers at Lincoln, but I felt that as one of the instigators of that, I wanted to be able to go back to sleep later if there was a high wind and it rolled away. But it would have been a very frightening structure to build if you didn't understand the strength of it.
Sullivan
You weren't involved with the Haystack antenna at all?
Purcell
No, not with the construction of it at all. I was involved with NEROC’s operation of it, at least with the oversight of it. No, Haystack’s a marvelous dish. It still is after all these years.
Sullivan
Well, thank you very much. I appreciate your time. That ends the interview with Edward Purcell on 16 and 17 November 1977.