Interview with George B. Field

Sullivan: 00:00

This is talking with George Field on 10 January 1979 at the AAS Meeting in Mexico City. Now, can you tell me when you first came into contact with radio astronomy?

Field: 00:12

Yes, I was sitting at my desk as a graduate in Princeton and was reading an article about 21 cm emission which had just been discovered by the Dutch of course. And it occurred to me, on the basis of work that was going on observatory at that time, that 21 cm absorption should be observable also. And I contacted the Naval Research Laboratory, at that time, had one of the big dishes in this country, and proposed such an experiment. Well, in fact, I never got a response to that. However, I read sometime later that Lilley, McClain, and Hagen [2024 ed. note:  Hagen, Lilley, McClain, 1955] had detected it. Ed Lilley has the rest of that story because he was there.

Sullivan: 01:04

Yes, right, I know about that story, but I didn't realize that you had, you tried to get them interested in terms of writing them a letter.

Field: 01:10

Yes, I had written a letter. And I think the letter which they published in ApJ gives me credit for having had conversations with them, which is not correct. What I've done is I've written a letter to them explaining that this effect should exist. But you had to look for it.

Sullivan: 01:21

But you never did publish anything on or give a talk or anything like that?

Field: 01:24

Not that particular discovery.

Sullivan: 01:26

Now, this, of course, was a theme throughout the '50s in your work, was the 21 cm line, the whole excitation mechanism, and so forth. Did you work on this anymore at Princeton, or was it only one?

Field: 01:39

Yes, I continued to be interested in the subject. And I remember a discussion at the AAS meeting in Princeton in the spring of 1955. I was just finishing my thesis at that time on plasma oscillations as part of the solar atmosphere. And this meeting was interesting because of a number of different events. First of all, it was the one where the discovery of the radio emission from Jupiter was announced, Franklin and Burke.

Sullivan: 02:07

Burke and Franklin, yeah.

Field: 02:09

And Dave Heeschen was at that meeting. At that time, he was either a student or a postdoc at Harvard. I guess maybe a student [inaudible].

Sullivan: 02:16

About this [inaudible].

Field: 02:19

And the thought had occurred to me that intergalactic matter could be a major component of the Universe. I've read some papers that indicate that might be true, and I talked to him about the possibility of observing the 21 cm line of absorption. And he very kindly suggested that I come up to Harvard and look at their Radio Astronomy Observatory, which at that time, I think, had a 28 ft or something.

Sullivan: 02:46

That's right.

Field: 02:47

And he was very encouraging that maybe I would be able to do something like that. Well, as it turned out, I won a fellowship to Harvard and one of the things that I was immediately attracted by was the vitality of the 21 cm group under Bok, which, as you know, contained many of the people who then went on to do a lot of work in radio astronomy. And that then led to a kind of two-pronged approach on this intergalactic medium problem. First, to understand the physics of the problem, I undertook a theoretical study with Ed Purcell, who's a professor of physics there and then I undertook these observations at Agassiz. And that went smoothly except for the time that the Nautical Almanac which was leaning on the limit switch to allow me to go very far over in my observations of Cygnus A. I forgot that it was--

Sullivan: 03:47

It was purposely there.

Field: 03:48

It was purposely there. It was standard practice at that time. And I drove the antenna into the pedestal at that time. Now, by that time, it was a 60 ft I believe it had been upgraded by the time that I got there.

Sullivan: 04:02

The paper here, yes, was 60 ft. And you published an ApJ in ‘59.

Field: 04:07

Right.

Sullivan: 04:07

Right.

Field: 04:08

But it was a lot of fun. And not too much damage was done. And I did get some results which were published. And they provided a constraint. And since that time, that ground has been gone over many times. I came back to it myself in 1957, I believe it was. I'd been at Harvard as a postdoc for two years. And then I was offered an assistant professorship at Princeton and went back there, continued this interest, and made plans to use a larger telescope, which is, of course, important for this kind of observation.

Sullivan: 04:42

Absorption measurement.

Field: 04:45

Absorption measurement, right. Of course, beyond a certain point, it really doesn't help any because it's a percentage effect that you're looking for. And once you get your source temperature up above your noise temperature in your system, it doesn't help anymore. But in those days, we were still reaching for that. Yeah. And as I remember, it was the spring of '58, if that's possible.

Sullivan: 05:12

I can double-check that.

Field: 05:13

Yeah. It's probably in the logbooks and so on. I remember things were very primitive and--

Sullivan: 05:19

That's precisely, I think, when Frank Drake was did his Ozma. Was it that same season? Do you remember?

Field: 05:24

Well, I know that Frank was observing Venus at the time. Was it Venus or Jupiter? It was Jupiter. And I remember being in the control room when Frank asked me if I would look at the chart recorder. It was giving a temperature for Jupiter which was several times greater than had been predicted on the basis of the--

Sullivan: 05:49

This was the microwave, 10 cm--

Field: 05:51

It would have been 3 cm.

Sullivan: 05:54

I think that's right. Yeah.

Field: 05:55

It was a fairly high frequency. And that's how I got interested in the Jupiter problem.

Sullivan: 06:01

Oh. I see. It wasn't from back in Burke and Franklin's announcement that you mentioned earlier.

Field: 06:05

Well, I suppose that, too. I had that in the back of my mind. But the specific thing, of course, the very exciting discovery of what was then called the decimeter radiation which then makes me think maybe it was—no, as I recall, the situation--

Sullivan: 06:19

I think you're right. I think it is 3 cm.

Field: 06:21

The first one was 3 cm and then other people rapidly followed it up at 10 cm. And then the effect was much bigger, of course, because the thermal radiation would be falling off at that point. And so, I got into that and published in that area. But getting back to the conditions at Green Bank, there were a lot of people there then who then went on to become good radio astronomers. Gart Westerhout was there using the dish. Dave Heeschen, of course, Frank Drake. Kochu Menon, as I recall.

Sullivan: 06:53

But you were a visitor.

Field: 06:55

I was a visitor. That's right.

Sullivan: 06:55

Did you submit a proposal like one does today, or?

Field: 06:58

That's a good question. I don't think so. I think that it was done on a basis of private conversations. Green Bank was just starting up. Conditions were very primitive. I seem to remember slogging around in the mud a good bit. And it had not yet revved up to the point that one needed to have a formal procedure [inaudible].

Sullivan: 07:18

It was still such that a user who was not particularly electronically oriented could go and use the telescope, I gather.

Field: 07:26

That's true. Of course, I had the advantage that Dave and Frank particularly I remember helping me with the electronics. And there was some particularly tricky business about getting rid of the image frequency, it being a superhet system. It was important, the way I was doing the experiment, to just have a single channel and not both channels. The image frequency as well. And so there was a system to do that. And, also, I had to retune the whole system every time I made an observation because I was covering, well, for that time was a fairly wide frequency band, like 5% or something like that. And they were very helpful in getting that.

Sullivan: 08:14

But of course, you had experience at Agassiz also.

Field: 08:16

Yeah, sure.

Sullivan: 08:17

But I'd be interested in your opinion as sort of a semi-outsider at Agassiz, I've talked to several people that were more closely associated with it. They had a number of spurious results. And on the other hand, it was also a place where the receiver was built primarily by Doc Ewen and the users came in and used it. And I get the impression - do you think this is at all right - that they didn't really understand quite how far to trust the baselines and so forth, and therefore they optimistically interpreted their things. Do you think that's a fair assessment?

Field: 08:56

I think there were some very competent people there. Both Dave and Frank had had experience in the service and knew what they were doing in this area. What you say is right. That equipment was largely built by Doc Ewen and it wasn't kind of a physics operation where the equipment was built by individuals. So, he knew all the ins and outs. I also think that Bart Bok was a very enthusiastic leader, and he deserves a tremendous amount of credit for getting that all going. He was not particularly familiar with the workings of the apparatus or indeed the ins and outs of interpreting data of this kind. And so maybe he wasn't as critical as--

Sullivan: 09:39

The final review of the leader of the group more or less.

Field: 09:43

That may have been a problem there. I suspect it was. Actually, there was a situation there at Harvard where, it was odd in a way that Ed Purcell didn't take more of hand in it because Ed, after all, had been involved in the discovery thing. It was his student who then went off and formed a company that built the receivers. It would have been a natural for him to get involved very directly, but he never did.

Sullivan: 10:07

I've talked to him, and he simply says he just wanted to go on in physics. He just didn't want to follow it up. It always seems strange to me, too.

Field: 10:14

That's right.

Sullivan: 10:14

The Dutch really ran with the ball, and at Harvard it wasn't till Bok came along a few years later that really got going there.

Field: 10:20

That's right. Now, of course, Ed spends a lot of his time in astronomy. He has become very fascinated with the problem with interstellar grains and polarization spends a lot of time on that.

Sullivan: 10:31

Well, the intergalactic medium we've talked about. What about the Purcell and Field paper '56, can you tell me, was that pretty straightforward from your point of view, how that had to be worked out? Or were there particular difficulties that you felt that weren't being appreciated by the workers in the field?

Field: 10:56

No, I think it was straightforward. I have to tell you that the basic ideas were Ed’s, but I did the work and I refined the ideas. Then I published a later paper that I think was referred to often by people in the field on question spin temperature was kind of synthesis of all the people had done up to that time. It was one that was in the preceding--

Sullivan: 11:18

In the IRE.

Field: 11:19

IRE '58.

Sullivan: 11:20

That special issue.

Field: 11:22

It was an odd place for it to appear as it turned out that it wasn't that accessible to people. But there I did have some new things on my own. In particular, I got very involved in the whole question of coupling of Lyman alpha to this line and worked out a solution to that that led to a problem of radiative transfer which I was able to solve when I was at Harvard and it was rather unique for the time. It was time-dependent--

Sullivan: 11:50

Right. I have a note on that one. Yeah, what was the inspiration of that? Why time-dependent [?] turning on? What were you thinking of?

Field: 11:58

No, basically the goal was to solve a spatial dependent problem of hydrogen gas scattering a resonant line photon like Lyman alpha, and seeing whether, in fact, you would get a very flat central maximum as had been predicted by several people. And I found that I could not solve that problem. I just didn't have the mathematics to do it. And I still don't have the mathematics to do it exactly. But I found, to my delight, that a very closely related problem, namely of injecting a photon … delta function distribution of photons in a cloud, an infinite cloud, and then watching it relax in time could be solved exactly. And that's referred to sometimes because it's one of the few things in transfer theory that can be solved exactly. And then it's pretty easy to reason how that time-dependent scattering problem then applies to the spatial scattering problem because the time and space become closely correlated. So that did, I think, have quite an impact, that calculation of Lyman alpha coupling to 21 cm and the synthesis in this paper of '58 was referred to a lot as giving me justification for assuming, in most cases, at the spin temperature was equal to the kinetic temperature.

Sullivan: 13:25

Was the kinetic temperature?

Field: 13:26

It was kinetic temperature. Yeah.

Sullivan: 13:29

Let me ask you a general question about this era. You are primarily a theorist, but doing observations also in both radio and optical regimes, so to speak. But this is not usual at that time. If you can put yourself back then, did you just see radio as a perfectly legitimate part of astronomy that should be treated like everything else, or?

Field: 13:50

Sure.

Sullivan: 13:51

So, there was no bias whatsoever in your part?

Field: 13:54

No. The problems that interested me, for some reason often, ended up having the closest relationship to radio astronomy, so, I went out and tried to do what I could to observe things. And I think if it had been the other way, I would have tried to do optical observations as well.

Field: 14:11

I've never regarded radio astronomy as anything, but eventually, a branch of astronomy.

Sullivan: 14:15

Well, other people did, especially at that time in the 50s. Did you see yourself at all as unusual in that respect, or?

Field: 14:22

Well, you say that they did, but I wonder if that's right. For example, the AAS, as far as I know, never had a separate division of radio astronomy.

Sullivan: 14:29

No, that's true.

Field: 14:30

And it's true that the people-- though some people were very sensitive to that because I think they had come through training, particularly engineering departments. You think of people like Fred Haddock, for example.

Sullivan: 14:43

Kraus and all the NRL people.

Field: 14:44

Kraus and McCain. Exactly. And they felt more comfortable with other people who had sort of an engineering background rather than astronomical background. And so, to some extent, yeah, they did feel apart. But since I have been trained in astronomy, I just regarded this as a new technique that is part of astronomy. And I think that's how it was regarded at Harvard as far as I know. That was definitely part of the program, and it was the exciting part of the program.

Sullivan: 15:15

But Harvard is somewhat unique. Leiden is another example where it really was incorporated right into astronomy as opposed to being in an engineering lab or in an engineering department or physics department.

Field: 15:27

That's true.

Sullivan: 15:28

It's usually the way it was in most laboratories around the world.

Field: 15:31

That's true. And yet as we were discussing earlier the fact that maybe these results were overinterpreted may have been, to some extent, the result of that, that there wasn't the engineering input that might have been in some of these other places.

Sullivan: 15:44

Yeah, yeah. So, as you look back on it, anyway, you don't see yourself as one who bridges the gap between the two subdisciplines?

Field: 15:54

I never thought of myself that way, to be honest with you. I always felt that-- well, in a way, I suppose it's right because I certainly had many friends and colleagues in the radio astronomy community. I would go to special sessions on radio astronomy but I was very much involved in other parts of astronomy too. And there weren't that many people were doing that at that time.

Sullivan: 16:15

Let's just talk then about the Jupiter to work, which you alluded to. First of all, you were not concerned with the decimetric bursts and so forth. It was this microwave enhancement. And you had a couple of papers in JGR and ApJ and so forth on the idea that it was cyclotron radiation from electrons trapped in the Jovian magnetic field. But in the abstracts that I went through, I did mention at one point that this required the 1,200 Gauss field for Jupiter. So, I presume then that-- well, I don't know. Did this hold up and what was the status of that time about our understanding of what's going on?

Field: 16:54

Well, Frank Drake, I think, said right away that it has to be synchrotron radiation. And that was, after all, shortly after discovery of the Van Allen Belts. Okay. I don't quite know why I went off in the direction of synchrotron radiation. I think I was particularly intrigued by the fact that if you confine yourself to cyclotron radiation that is [inaudible] electrons, then you can predict the spectrum from the particle pitch angle distribution. The point is that if particles are all circling at 90-degree pitch angle they stay in the equatorial plane, is in a sharp line at a frequency corresponding to the cyclotron frequency at the equatorial magnetic field, but--

Sullivan: 17:43

It's a more tractable problem.

Field: 17:45

It's an exactly calculable problem and there's a one-to-one correspondence between the spectrum which was then being measured and the observed-- sorry, pitch angle distribution, which at that time was very interesting to people in connection with the Van Allen belts whether they travel near the equator or did they penetrate into the planet, all this kind of thing. But then having written that paper in JGR, I realized that there was a problem which basically was that the energy loss compared to the energy stored in these electrons was too great. And I was able to show, in fact in a fairly good way, I think in another paper, that it couldn't work for that reason. So, I think it was kind of a possibility that was then shown to be not interesting. And then other people at about that time came in and showed that the synchrotron thing would work. And I believe that relativistic particles have been demonstrated at that point in the Universe. So as I remember, there was a guy-- well, it's one of the interesting things that Kip Thorne was  primarily known for his work in relativity, I think was the first person to calculate that correctly synchrotron radiation in a dipole field. He did that, I think, in his Ph.D. thesis [inaudible].

Sullivan: 19:05

I didn't realize that. And I think it's generally thought would you agree with this that it was Radhakrishnan's measurements of the polarization that really pinned down the synchrotron.

Field: 19:14

Yeah, sure.

Sullivan: 19:15

But more generally about synchrotron emissions, since you were quite following radio astronomy closely in the last '53 to '60 period, were you aware of the Russian results on synchrotron radiation?

Field: 19:27

Oh, sure. Yeah.

Sullivan: 19:28

And did it seem to you like the way to go from the start or had you become a believer over a long time?

Field: 19:34

No, I remember giving shortly after I arrived at Harvard that would have been in '55, giving a seminar. I recall a series of two or three talks on the interpretation of the spectrum of grand nebula and it was evident to me that this was the right way. And I think other people were being convinced too at that point. I'm not sure when the original publications of [inaudible].

Sullivan: 19:57

[inaudible] Field then said he was very early convinced that the synchrotron radiation was the way to go. And that's the end of the interview.