[Dicke, 1970]
Dicke, 1970 (Photo courtesy of NRAO/AUI/NSF)



NATIONAL RADIO ASTRONOMY OBSERVATORY ARCHIVES

Papers of Woodruff T. Sullivan III: Tapes Series

Interview with Robert H. Dicke
At the University of Washington
January 18, 1979
Interview Time: 20 minutes
Transcribed for Sullivan by Pamela M. Jernegan

Note: The interview listed below was either transcribed as part of Sullivan's research for his book, Cosmic Noise: A History or 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.

Sullivan

Ok, this is continuing our follow up interview with Robert Dicke at the University of Washington on 18 January 1978. The previous interview was in June í76 at Haverford. I wanted to ask you about the atmospheric measurements that you made at one point at 1.0, 1.25 and 1.50 cm, actually, while the war was still on. It was published after the war.

Dicke

Yes, these measurements were made in April of 1945, if I remember correctly.

Sullivan

It's undoubtedly here in the paper exactly when but what was the motivation to make these?

Dicke

The K-Band radar had been planned with the assumption that the atmosphere would be transparent. It was noted empirically that it didnít work very well when you had a lot of humidity, so water vapor was suspected as an absorbing source of difficulty. This radiometer that I had put together without this as a motivation, but- no I'm sorry, that isn't really true. I thought of it and discussed it with Purcell, Professor Edward Purcell of Harvard, about this way of getting a radiometer of great sensitivity, and he thought it was an intriguing idea. And then he came up with a thought that one could use this to get at the absorption problem. What he had in mind, then, was to simply look at the brightness of the Sun as the Sun went across the sky using the radiometer to follow and I saw that one could do a better job by looking simply at the emission of the atmosphere itself. So we built a crude breadboard model of this and the early measurements of the absorption were made at MIT, on the roof of the laboratory there, correlating the effective antenna temperature as a function of tilt angle with the measure of the water vapor above that was obtained from a weather balloon- sounding of the atmosphere.

Sullivan

Right. You said that your radiometer was very sensitive; what were the unique aspects - before you mentioned the wide bandwidths were something that was new, but also the switching technique.

Dicke

Yes, what was done was to interchange the antenna with a dummy radiating source, the absorbing level which was simply dipped down in the guide and this dipping down of the absorbing wheel occurred thirty times a second. So you were effectively interchanging antenna and dummy noise source.

Sullivan

This was simpler to do than a microwave switch that we would think of today?

Dicke

Yes. We didn't have such things as circulators and so on at that time- those were inventions that came later, and mechanical switches had their own problems of course. So this really worked quite well, the chief difficulty was that the absorber was at room temperature so you had quite an offset problem.

Sullivan

Yes. Now, this I believe, is connected with your development of the lock-in amplifier somewhat earlier. How did the idea for the lock-in amplifier come about?

Dicke

This was not something I invented. I vaguely remembered having seen a paper by Michaels on this idea. I don't know whether that was Michaels of Bryn Mawr; he died some five years or so ago. But he published this paper and pointed out the advantages of using this particular technique to amplify and detect a very narrow band frequency because youíre locked right on to the frequency.

Sullivan

Right.

Sullivan

Dicke

And so I designed an amplifier of this kind to use in connection with some antenna measurements I was making.

Sullivan

What were those measurements?

Dicke

Well, previous to these measurements, the antennas were designed at the [?] laboratory in sort of a brute force way where you designed both the feed of the antenna and the dish itself all in one piece. And what I wanted to do is simply examine the feed all by itself and take the problem of fitting it to a parabola as a second problem. So you wanted to be able to look at phase fronts and intensities around the feed. For intensity measurements, what I did was to modulate the klystron on and off, some thousand cycles or so, with a little tiny dipole probe with a coaxial carried signal picked up back to a velometer detector where it simply modulated in a bridge, and then you obtained a thousand cycle voltage which was demodulated in this lock-in amplifier.

Sullivan

I see. So then it was a very natural thing for you when you came up with the broad band idea for a radiometer, to incorporate this superimposing your modulation on...

Dicke

Yes. In fact after the war, I feel a very large percentage of my experimental research used a lock-in amplifier. If I could find a way of modulating the effect, I did it.

Sullivan

Well, in fact, did this not lead to Princeton Applied Research [Sullivan: Inc.] and so forth?

Dicke

That's right. Thatís the company. You know as a result of papers being published, describing the techniques I had all kinds of requests for circuit diagrams from my lock-in amplifier, which I sent out. And finally, this little company was organized without the thought, really, of building the lock-in amplifier- it was a little electronics company- and then finally we suggested manufacturing this thing. Up to that time nobody manufactured a lock-in amplifier, although hundreds of people apparently had built them. In fact, there were far more built than I realized, because at the time this question came up I said to the president of the company, "I really can't do more than promise you something like a hundred potential customers." We sold far more than a hundred the first year.

Sullivan

Right. This was started right after the war?

Dicke

That little company was organized, I can only guess roughly, in 1963 or something.

Sullivan

I see, so this is long afterwards?

Dicke

Yes.

Sullivan

Okay. So the choice of K-band then was really made then without proper investigation of what the atmosphere might be doing?

Dicke

Well, it had been investigated, but a mistake had been made in the analysis of the effect of water vapor. This particular one, the .25 cm line there, it was known that it was there, but the matrix element had been incorrectly calculated.

Sullivan

I see. Now, relevant to the 1946 eclipse that you observed, I wanted to ask you about how did you track the Sun and what kind of mounting did you have for your small dish?

Dicke

Oh, it was an alt-azimuth mounting, very crude. As a matter of fact, it was a nicely designed little tripod that the Engineering Department of the British Laboratory had designed specifically for this job. A couple of little hand cranks on it. I can't remember how we pointed it, but I do know that we followed the Sun in the sky, and it was simply empirical. One had some kind of crude sight on the antenna.

Sullivan

Now, in the previous interview, you mentioned that when you went to Princeton right after the war that you made some effort to get radio astronomy going, I'd like to follow that up. First of all, would you have been quite content to more or less continue specializing in what we now know as radio astronomy if you'd found some encouragement do you think?

Dicke

I think I would have been interested in doing this, but there were two things that made me shy away- first of all, I went to Princeton without a tenured position. I was just an assistant professor and I did have a family to worry about.

Sullivan

This was a speculative field, you mean?

Dicke

Well no at that time 1946, the Physics Department was interested in cosmic rays, nuclear physics, and what would they have thought of an assistant professor off flying in the wild of radio astronomy? That isn't even physics.

Sullivan

Right.

Dicke

I think I'd still have done it if I'd had any support from the Astronomy Department. Years later, I realized how completely mistaken I was about the department because they would have been ever so pleased to have had it. I didn't realize that they really were a remarkably tolerant bunch but I didnít know.

Sullivan

Do you remember who you talked to?

Dicke

Yes, it wasn't that- I had taken a course from John Stewart, and I talked to him about it. He promised to take it up with the department and he reported later that they weren't interested in it. I didn't really go directly to the chairman of the department or anything like that.

Sullivan

Who was the chairman at that time?

Dicke

That was [Henry Norris] Russell, but he was very close to retirement at the time, and I guess with retirement so close, he wasn't about to get involved in something which was new and radical.

Sullivan

So what did you begin working on then?

Dicke

I got interested in a variety of atomic high precision stuff at that time. I got interested in the question of the G-factor electron tube and set up an experiment to try to do that. It turns out very closely related to the experiment that [?] got going.

Sullivan

[?]

Dicke

I finally got going and had an electron trapped in the uniform magnetic field with the quadratic electrostatic potential.

Sullivan

Oh yes, I see.

Dicke

I had nothing but trouble. I could not do it. Then I got interested in the lamb shift. I started to try to measure a lamb shift until I heard Lamb was doing it, so I got out of that. There were a number of things, sort of unconnected with each other.

Sullivan

So we might have had a Dicke shift as well as a Dicke switch?

Dicke

I had to learn too because, you know, you only learn by making mistakes.

Sullivan

Sure. So in essence you did the same sort of things that Purcell worked on in the nuclear physics end?

Dicke

Yes. Well, there wasn't...

Sullivan

Atomic physics, I guess.

Dicke

He was into nuclear magnetic resonance. I was looking at electron transitions, treating the [?] transitions, and looking, for example, at the magnetic transitions of ammonia, trying to devise techniques for getting around Doppler broadening, and these things have coherence effects. It was shortly after that that I wrote a paper on spontaneous release and spontaneous coherent radiation systems.

Sullivan

Well, what I meant was that broadly you were taking these microwave techniques during the war and applying them to physics in the lab. Now, we discussed before the tape went on about this memo that you wrote in 1954 to AUI [Associated Universities Incorporated] and it appears that you explicitly did not mention aperture synthesis, that the idea is very implicit in there. And I was wondering, what sort of reception this got on the committee? Do you remember at all?

Dicke

I didn't get any response at all. I wrote this up after the meeting was over and I don't know if I sent it to all the members of the committee or not.

Sullivan

It seems like you sent it to the whole committee but you didn't say explicitly, no.

Dicke

I doubt if I did, but I don't remember.

Sullivan

But it was never discussed that you remember.

Dicke

It was never discussed; I never heard any more about it.

Sullivan

You had railroad tracks there and everything, just like it turned out twenty years later.

Dicke

It was probably regarded as a wild scheme at the time.

Sullivan

Well, finally, moving up to the microwave background, before you told me about the experiment that you were doing with Rowland and Wilkinson and how you had to do some convincing of Penzias and Wilson that they really had something of interest. Would you have written a paper at that time if it had not been for [Arno A.] Penzias' and [Robert W.] Wilson's discovery, do you think?

Dicke

I rather doubt it. I think our own experiment was ready to come on line soon and in view of that, I think we would have been more inclined to not write anything up at the time.

Sullivan

But you did send these purposefully in together as sort of one?

Dicke

We called them on the phone and told them we'd decided we were going to publish something on this just so they could have a chance to publish also if they wanted to. And they decided they would.

Sullivan

I see. It was just that one was more or less the theory and the other was the observation. Do you think that's a fair characterization?

Dicke

Yes, I think thatís a fair characterization.

Sullivan

Had you published anything previously on the idea of looking for a relic background?

Dicke

No, there was nothing published before that.

Sullivan

Okay, now I don't think there's any other aspect of radio astronomy that you've been involved in- or is there?

Dicke

Let me just cover one thing.

Sullivan

Yes.

Dicke

I think the last time we talked, this paper you have which is showing the dish in that report, as you mentioned.

Sullivan

Right, the AUI report.

Dicke

I was uncertain about the time. If you want to know a real date on that, I have a real date now. I was going through my files a month ago and I came across the information.

Sullivan

Okay. I'll ask you that in a letter that I'll drop you. But are there any other aspects of radio astronomy that you've been involved with that I may not know about?

Dicke

Well, I had a visit very shortly after the war probably 1946, from one of the Australian radio astronomers, the one that died.

Sullivan

[Joseph L.] Pawsey?

Dicke

Yes, right. And I guess I gave a talk or two just about right after the war. I remember giving a Physical Society talk on the radiometer and the measurements, which incidentally got some strange publicity in the Boston area. It came out with big headlines something like, "Astronomer Discovers That the Sun Is 4,000į Hotter Than Previously Thought."

Sullivan

I see. Would there be an abstract of that article in Physical Review?

Dicke

This was...

Sullivan

I mean of your talk?

Dicke

It might have been an invited talk without an abstract, I don't remember.

Sullivan

I'll have to check.

Dicke

It was a Boston Meeting of the Physical Society.

Sullivan

Okay, it was shortly after the eclipse?

Dicke

It was shortly after the war probably, 1946.

Sullivan

The eclipse was summer of 1946, so it would be shortly after that, I guess.

Dicke

I think the eclipse was in the summer of 1945 because I was a radio in Princeton in 1946.

Sullivan

Youíre right, July Ď45. Did you have any measurements of the Sun before the eclipse?

Dicke

Yes. We'd done a number of measurements. And one thing I can say which shows how extremely ignorant I was about possibilities of this, in fact this was connected with scientific interest. I had a perfectly good instrument capable of detecting radiation from the Moon. I could have followed the temperature of the Moonís partial phase and never did it because it never because it never occurred to me that this might be significant scientifically.

Sullivan

Well, [John "Jack" Hobart] Piddington and [Harry C.] Minnett did that a year or two later.

Dicke

Did I tell you about my isotropy measurements?

Sullivan

Yes, you mentioned those and you sent me the Rad Lab thing, although I would love to get hold of your notebook in which you actually discussed precisely what you did.

Dicke

I didn't keep very good notebooks; you might find it pretty thin.

Sullivan

But what you said was that at about an elevation of 45į, you spent several hours- did you have a switching technique involved in that or were you just looking at strip chart output?

Dicke

Oh well, just go around and go back again, switching at about 15 seconds for about several minutes sweep slowly around like the strip chart and then analyzed the strip chart.

Sullivan

And that gave a similar limit to what you have in the...

Dicke

I don't remember; I can't give you a good solid number on that one. That must be in the notebook somewhere.

Sullivan

But of course, you were not thinking as we would today, in terms of a cosmological sort of thing. You were just thinking of anything that might be extraterrestrial.

Dicke

Well, I was thinking cosmological in the sense of radiation from distant galaxies, not from a fire ball.

Sullivan

I see.

Dicke

I was thinking of all those distant galaxies.

Sullivan

I see. Did it ever occur to you to look at hot gaseous regions, what we call HII regions, with this microwave radiometer? That would be the other nice thing you could have done.

Dicke

I didn't even know enough astronomy to think of that. The only astronomy I had was an introductory course from Stewart. My first paper was in astronomy, believe it or not, was a paper I wrote for Stewart in this course, and he was impressed with it and felt it should be published. So my first publication was in astronomy ["The radial distribution of globular clusters," Astronomical Journal 48, 108-110 (1939)].

Sullivan

I see.

Dicke

Globular clusters, stability and the distribution of globular clusters.

Sullivan

I see. And when were you taking this course?

Dicke

1940 or 1941. [Sullivan: A.B. awarded in 1939]

Sullivan

This is as a graduate student?

Dicke

As an undergraduate.

Sullivan

Where?

Dicke

Princeton.

Sullivan

At Princeton, but then you went to...

Dicke

Then I went to Rochester and then back to Princeton. But my first publication precedes the first publication of physics by about five years, six years.

Sullivan

There were several astronomers at the Rad Lab, of course, working on various things.

Dicke

Yes, [Albert E.] Whitford.

Sullivan

Right. Did you have any contact with them on these sort of things?

Dicke

No.

Sullivan

It may not have been very helpful anyway. It was so different from traditional radio astronomy.

Dicke

Yes, that's right. It would have been all optical.

Sullivan

Okay, well thank you very much.

Dicke

You're welcome.

Sullivan

That ends the follow up interview with Robert Dicke on 18 January 1979.


Modified on Monday, 29-Apr-2013 14:34:52 EDT by Ellen Bouton, Archivist (Questions or feedback)