[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 AAS Meeting in Haverford, Pennsylvania
June 22, 1976
Interview Time: 25 minutes
Transcribed for Sullivan by Bonnie Jacobs

Note: 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.

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.

Sullivan

OK, this is 22 June, 1976 at the AAS Meeting in Haverford, talking with Robert Dicke. Can you tell me just a little bit about your educational background, during the Rad Lab or before?

Dicke

I started as an undergraduate at the University of Rochester, transferred as a junior to Princeton and then went back to Rochester for my graduate work. Again it was in physics. I got my PhD in 1941.

Sullivan

What field was that - microwave?

Dicke

That was physics. It was a nuclear physics thesis, actually. It was the only nuclear physics I ever did, was that thesis.

Sullivan

And then you moved into the Rad Lab?

Dicke

Well, actually before the War I went to the Radiation Laboratory in 1941, I guess, September '41.

Sullivan

What were your main responsibilities there and so forth?

Dicke

I was assigned to, I think it was called, the Fundamental Developments Group, under leadership of Ed [Edward Mills] Purcell at Harvard. We were supposed to be about five jumps ahead of the other groups and when I arrived, we were working on 3 centimeter radar and then got into the development of the 1 centimeter radar after that. I had input into a number of areas. Magic T symmetric junctions, things like that.

Sullivan

I always think of you in terms of low noise receivers. Was that sort of your specialty would you say during the War?

Dicke

Not exactly. As I said, I was in a number of areas. One of my interests was on antenna design, which was to worry about the antenna pattern of the feed. At the time I started, they'd just took the overall antenna and were worried about its pattern. And I broke this down into two parts, (1) the pattern of the feed and then what the reflector would do to that pattern. And so I was interested in measuring a radiation from feeds. I developed a probe sensitive little probe for that purpose so it wouldn't disturb the field. And that used a lock-in amplifier and I guess was one of the first real uses of a lock-in amplifier. I was impressed that the lock-in amplifier was capable of doing, fishing out signal from noise. And then it occurred to me one day by opening up the bandwidth of the IF you should be able to produce a very sensitive detector of microwave radiation. So I built a model of that kind. And then the model worked so well and actually made some water vapor measurements with that crude breadboard model.

Sullivan

Would this wide-banding have been of any use for the military applications?

Dicke

There were regular wideband receivers that were used for microwaves, anyway. But the normal thing that you think of in improving signal-to-noise is the narrow band. Here the trick was the wide band before you detect, and then narrow band afterward. And that seemed to be the new contribution. Since the breadboard worked well, we designed a little better-engineered three versions of the radiometer. They were 1 cm, 1.25 and 1.5. We took those three to Florida to get a measure of the water vapor absorption problem, looming as an already difficult problem.

Sullivan

In the Earth's atmosphere?

Dicke

Yes.

Sullivan

This is still during the War, you are talking about? And what did you find?

Dicke

Yes. Well, we had a measure of the contribution of the water vapor which was quite good. Out of that measurement, there's also an upper limit on the radiation coming in from beyond the atmosphere. I've forgotten what the number was, some 20° or something like that.

Sullivan

And this is what you are referring to as in a Rad Lab book?

Dicke

That is a paper published in Physical Review.

Sullivan

Oh, that's the one with [Robert] Beringer?

Dicke

Yeah, Beringer , [A. B.] Vane, and [Robert L.] Kyhl. And I think the upper bound on the radiation coming from space is 20K something like that. And about the same time, one night I scanned around the sky on a conical scan to see what kind of isotropy measure you could get for radiation coming in from outer space. I set a limit of about 1° on that.

Sullivan

And you were telling me this is published in one of the Rad Lab books, but it's not credited and you can't remember which article.

Dicke

The only thing that's there as I recall now is that in one of the Rad Lab series [Sullivan: Vol. 24, p. 107], there is a picture which is said that we picture produced the radiometer, which scans around the sky at several different elevations so that you first see the horizon, contributions of buildings; a little higher, you get dips as you hit the buildings; and finally up so high that you’re missing the building. But the numbers that are in that figure are not the best measures of isotropy because one night, I just spent the night, the whole night, if you like, hours at least, just scanning around at an angle 45° to get a good measure of that. That's unpublished.

Sullivan

You never published that?

Dicke

That was never published.

Sullivan

But you might have some notes on it, possibly somewhere. When did you learn of [Grote] Reber's and [Karl] Jansky's work? Did you know about this? Was this common knowledge at the Rad Lab?

Dicke

Yes, that was known. It was also known that- what was his name?

Sullivan

Haine?

Dicke

At Bell Labs.

Sullivan

Oh, [George] Southworth.

Dicke

Southworth had been able to detect the radiation from the Sun. This radiometer of ours was much more sensitive than that we'd bang it off scale of the Sun. We got radiation from the Sun and from the Moon. It was quite a decent signal-to-noise on the radiation from the Moon with an antenna that was no more than maybe 14 inches across, a little parabola.

Sullivan

These were the first absolute measurements also?

Dicke

The first absolute measurements that I know of the radiation from the Moon.

Sullivan

Right, or of any kind.

Dicke

Yeah.

Sullivan

You knew about Southworth's work but you did not repeat it during the War, anyway? Did you wait until right at the end of the War?

Dicke

My impression was that Southworth's stuff came maybe six months or so before we did it. I remember hearing about Southworth's stuff before this, but I can’t remember how much before.

Sullivan

Was there a Bell Labs report do you know, on Southworth's work? I've only seen an article which he published, finally, just at the end of the War.

Dicke

I can't remember at this late date whether I saw a lab report or whether I saw an article. I think it was a published article I saw, a reprint of it or something like that.

Sullivan

Now what about your own radio astronomy work? Were there any internal Rad Lab reports relevant to this?

Dicke

Well, there was certainly an internal Rad Lab report that described the radiometer. I can't recall any that described the measurements that we made.

Sullivan

Were there any wartime reports that you can remember to do with accidental detection of the Sun, i.e. how it might cause interference with radars and things like this?

Dicke

No. Usually, the contribution of the Sun is a rather minor one...

Sullivan

At these high frequencies?

Dicke

Well, it's not only this. Even if you pointed right at of the Sun, this would increase the noise figure of the receiver, the poor receivers that we had at that time by at most 3 dB. So it would still work quite well. That was not a problem.

Sullivan

But that was because you were working at such high frequencies. For instance, [James] Hey in his accidental detection that he figured out the English radars were at a couple of meters or something. Bursts were much more important then.

Dicke

Oh, in the case of bursts, yes. At the high frequencies you wouldn't see the bursts. It would only be the thermal radiation. I remember we looked at an eclipse of the Sun and that was published. I guess I described it actually at a Physical Society meeting in Washington [Sullivan: Cambridge, Mass? 4/46- see Physical Review, 694(1946)]. Newspaper reporters were there and I pointed out that the temperature we got was not the 5 or 6,000° but 10,000°, pointing out that the radiation was coming from higher in the atmosphere where the temperature was different. But there was a reporter there that didn't get the point so there were huge headlines in the newspapers in Boston, "Sun Observed to be 4,000° Hotter Than Previously Thought."

Sullivan

Getting hotter...

Dicke

Yeah...

Sullivan

I'm familiar with that short ApJ article in '46, I guess it was...

Dicke

On the eclipse?

Sullivan

Yes. And also just the moon measurement. I think it was that same article.

Dicke

Yes.

Sullivan

Can you describe those observations? What the conditions were? You were up on a roof I think at MIT.

Dicke

Yes. Well, the eclipse was due to start reasonably early in the morning, I think. And I think we didn't get the apparatus going till after it was underway. And had the apparatus set up under roof lab. It was actually on the roof, I think, of one of the Temporary Labs of the Radiation Laboratory. It was a parabolic antenna if I remember right, of some 14 to 16 inches in diameter and we just followed it. That same antenna was used for the Moon measurements.

Sullivan

Yes, I think so. And then you didn't do anything more with it when the War ended?

Dicke

No, we made an attempt to get a cooperative program going with the Astronomy Department at Princeton, but didn't get any encouragement. The astronomers didn't seem to be all that anxious. So I felt I'd better get on with my physics research.

Sullivan

Did they know about Reber or Jansky, the astronomers?

Dicke

I think they must have known about it but that was a time, 1946, when Russell was getting very near retirement and Spitzer and Schwarzschild weren't there yet. It was a period of changeover.

Sullivan

Were there any of your colleagues in the Rad Lab who were interested in radio astronomy or tried to do any experiments, maybe unpublished? Do you remember anything?

Dicke

I don't recall anyone. The first one to use this radiometer technique was not one of the group of us that were involved with it. But was Purcell, who was my group leader, was impressed with this thing and he designed an equipment to look for the 21 cm line using the same technique. And that led independently to the discovery of the 21 cm.

Sullivan

Was he thinking about this during the War, are you saying? I guess he didn't know about the prediction of the line at that time. Yes, so it took a few years for him to start working on that project.

Dicke

It was after the War, maybe two years after we broke up. Of the other two radiometers, I think one went with Beringer to Yale. I don't know what happened to the third one. And I don't believe Beringer did anything more than break it up for components as I did ours. I had the 1.25 cm radiometer. With that equipment and with a little more sense as to what I should have been doing with it, why I could have gotten beautiful measurements on the Moon over a whole lunar cycle for example, back in 1946.

Sullivan

Do you have any comments on why American radio astronomy did not take off like British and Australian radio astronomy did in the late ‘40s?

Dicke

I don’t know, only prejudice on my part, but I think that the people that were involved in the development of the microwaves in our country were ones that didn't have the background or interest in astronomy. And the astronomers didn't have that contact with electronics.

Sullivan

Yeah. Well, that was true in the other countries also, the latter.

Dicke

Yeah, but I don't know why you'd say it’d would be different over there. But who were the leaders? It’s rather interesting that one of the leaders here was Bart Bok, who got his motivation I would guess, in part from the fact he was rather close to Ed Purcell.

Sullivan

Well, this was somewhat later though. I'm talking about the late ‘40s.

Dicke

Yeah, but now Ed Purcell’s work on the 21 cm line was quite shortly after that. It must have been '48 or somewhere in there.

Sullivan

Well no, the discovery was in '51.

Dicke

Was it that late?

Sullivan

Yeah. I don't know when he actually began. I haven't talked to him yet.

Dicke

Well, it might have been that late. My remembrance of the time is way off. It was I think Bart's interest in what Purcell was doing that got him interested.

Sullivan

Yeah, oh yeah that was certainly important, indeed. Who were the leaders of microwave work that if they had had interest in astronomy might have been able to carry the ball?

Dicke

Well, Purcell. I should have been doing something. I could have done it. Beringer could have done it.

Sullivan

Of course it could have even been low frequency. It didn't have to be microwave. I mean the stuff that [Martin] Ryle and [Joseph L.] Pawsey and the Australians was all done at very low frequency, 1 meter sort of stuff- radio sources and all this sort of thing. No one picked this up.

Dicke

That's true. I guess maybe the basic motivation- the people that had the knowledge and ability to do the electronics, as I said, had strong backgrounds in nuclear physics, particle physics, and things of that kind. And they were rather more concerned with the laboratory-type physics.

Sullivan

Right. They were not ionospheric sort of physicists...

Dicke

Yeah...

Sullivan

That makes sense.

Dicke

I think the same thing could be said about the Australians and why it happened there. I had a visit one time from Pawsey. He was thinking about getting into this business. He came to Princeton and talked to me about that radiometer.

Sullivan

When was this now?

Dicke

It was shortly after the War. It might have been '48 or so.

Sullivan

Oh, he was in it right from '46, right after the War.

Dicke

Yeah, it was very shortly after the War. I don't think he was really in the business yet but was just getting set up or something like that.

Sullivan

Yeah, that's probably right. Before we skip all the way up to the background radiation story, is there anywhere else that you were connected with radio astronomy- in that gap?

Dicke

Only when the Associated Universities Incorporated got together to form a big committee to talk about what could be done with astronomy. I became interested in what would be a good way in building antennas for it and considered the possibility of phased arrays of parabolas, and aperture synthesis of systems.

Sullivan

And you said you weren't aware of Ryle's work when you were doing this?

Dicke

No, I wasn't aware of that.

Sullivan

What was your perception of American radio astronomy at that time? Was it really hurting?

Dicke

My perception was that we were behind, pretty far behind, in fact.

Sullivan

Now we come to the background radiation. There is a story in [James] Hey's book The Evolution of Radio Astronomy, which you've probably read a couple of pages, in which he talks about a phone call to Burke and this sort of thing. But I'd like to hear it from you.

Dicke

I don't really know exactly what happened there to tell you the truth. I can tell you exactly the relation between our group and [Arno A.] Penzias and [Robert W.] Wilson. But as to how they found out about our work- I don't really know because the story that I got from Walter Sullivan was that he had been told that some mutual friend that we described what we were doing to, had related information to Penzias and Wilson.

Sullivan

Hey says it is [Bernard F.] Burke.

Dicke

It could have been Burke, for all I know. But then there is another story which has a preprint of a paper having been sent to Penzias and Wilson and they got the idea from that. We don't know anything about that. So I think that story is probably wrong. But anyway, what happened was I had this bee in my bonnet for some time. It went something like this: as the Universe expands the horizon beyond which you can't see is expanding more rapidly than the galaxies out there so you keep seeing more and more galaxies. And that's true of all these cosmological models. The question was how could the Universe be so organized that everything when you first come into view is moving in the right way because it seems to imply a causal relationship when there couldn't have been any causal connection. So I thought well I'm getting a causal connection with an oscillating Universe that expands to maximal size and collapses and then re-expands again. There immediately appeared a problem with that hypothesis which was that the heavy elements formed in stars in the earlier Universe would then be still there. So to avoid that, it was quite clear that if you had some radiation and you compressed it, it would get very hot; the temperatures would rise so high that it would thermally decompose these heavy elements. And you go back to primitive hydrogen. That was the motivation. So with the thought that might be a way of cleaning up the gas, cleaning up the stuff, we began to make some back of the envelope measurements as to what the residual temperature, the background radiation could be now. And it was apparent. At that time, I only made an estimate of some 20° or 30° for an upper bound on the grounds that much more radiation than that the Universe would be expanding too much. It would be too closed. You know, the energy density would be too high in the field.

Sullivan

Too open, you mean?

Dicke

No. There would be so much energy density in the Universe- high energy density leads to a closed Universe. It's crazy- the high pressure of the radiation doesn't push the Universe apart. It causes it to collapse.

Sullivan

Well, maybe we'd better go over this afterwards, I don't know that much...

Dicke

Yeah, well anyway there are two things that decelerate the Universe. One is just the energy density. There's a separate contribution which comes from the pressure and that part goes quite opposite of what you think. It makes it slow down.

Sullivan

Ok, I'll have to think about that. Now how did [George] Gamow's work fit into this? Was that influencing you at all?

Dicke

That wasn't influencing me very much. In fact, that wasn’t influencing me at all at that point. I heard Gamow talk at Princeton. He gave a colloquium talk back in late ‘40s. And I had a very clear recollection of what he had described as a Universe starting out with all neutrinos, cold, and there was nothing but a hot Universe in this. I'm afraid I didn't read the literature well enough because I didn't know that he then, came around later to a hot Universe view. At that time I didn't picture any connection.

Sullivan

Right. So you weren't really building on his work in any sense?

Dicke

No, not at all. It was completely independent. Well then, I got the boys interested in this as something worth doing so we started to design a radiometer. At the same time Jim Peebles started to worry about the implications of a hot fireball like this for nuclear [?] formation and he was unaware of the Gamow stuff too. And he estimated the helium production. On the grounds of getting anywhere reasonable amounts of helium, he could set rough upper limits on a temperature now which went well below what I had said earlier. My estimates were based only on energy density.

Sullivan

What were these numbers, roughly?

Dicke

He was getting, I think in our paper, an upper limit of the order of 10° or something like that.

Sullivan

And you had...

Dicke

I had some 30°. So we had a radiometer all designed and built. As I recall the matter, it was almost completely built.

Sullivan

Under your direction?

Dicke

Well, I was also busy with the solar oblateness about this time and I wasn't doing all that much directly on the instruments. I was contributing to the design ideas, and the construction was completely in the hands of Dave Wilkinson and Peter Roll- those two. All four of us were involved in the design of it though. The ideas were going in.

Sullivan

What was special about this instrument?

Dicke

Well, it was really not all that special. It was a regular switched radiometer, but it had some new features. Some new things had come along since the War years so we could incorporate. So it wasn't really just a copy of that radiometer that I'd built during the War.

Sullivan

This was the first time you had built one since the War?

Dicke

Yeah, well the first one we'd built since then. So it had a number of new features. That radiometer was almost ready to go, as I recall the matter, when I had a call from Penzias, one day, saying he'd heard about our work and saying that he had some funny stuff that he wasn't understanding and suggested we come over and look at it. And if I remember right, Dave Wilkinson and I went over and went over the stuff with him. Well, I went through a chalkboard lecture somewhere along the line there and I remember that those were hard boys to convince. They were considering all kinds of things- pigeon, dirt in the antenna. But anyway, somehow or another I think we fairly well convinced them.

Sullivan

They were not thinking at all in terms of this being an astronomical contribution? They were trying to think of all sorts of instrumental or atmosphere affects?

Dicke

Well, there's a long history about that too. As I remember the matter, they were more or less asked by their management to look into this question because there had been earlier work by Crawford and others in which they tried to trace down the sources of noise in that receiver. They wanted to get the lowest possible noise for their satellite measurements. They did a beautiful job, Penzias and Wilson, of tracking down and looking at the noise from this and the noise from that. And the really nice thing they did was to put a helium load on it to give a reference temperature that was very low that they put in.

Sullivan

Right, that was essential.

Dicke

Then they discovered that they had more radiation even relative to that so after they tracked down all the receiver noise there was still a part left undecided. At that point they thought it was really quite impossible- this couldn't be coming in from outer space because the isotropy they were seeing. And the amount was high compared with anything you would estimate for the Galaxy and that sort of thing. Then because of this isotropy it just looked awfully much like it was the antenna itself. So they were, for a long time, convinced that it was noise from their antenna. They rebuilt the throat of it, swept out the pigeon dirt. They tried all number of experiments. They couldn't get it to go away.

Sullivan

So you pretty much convinced them that this was plausible?

Dicke

I think I pretty much convinced them what was coming in. It was not completely crazy.

Sullivan

And then you followed up with your own measurements? You were at a different wavelength than they were?

Dicke

Yeah. We got going very shortly after that. We didn't rush into print at all. I shouldn't say we, it was Dave Wilkinson and Peter Roll actually. They didn't particularly rush into print because long after they had some numbers, they polished them over and looked carefully at systematic errors. It must have been at least six months after they first had results till they published.

Sullivan

What was the reception of the astronomical community as to the proposed explanation for this?

Dicke

It was surprisingly favorable. There were almost immediately other alternative explanations started appearing- radiations from galaxies and so on.

Sullivan

Here Dicke mentions that measurements at two frequencies and later the more greatly increased through reliability of the acceptance with the radiation. End of Dicke interview.


Modified on Tuesday, 16-Dec-2014 15:52:54 EST by Ellen Bouton, Archivist (Questions or feedback)