Interview with Martin Ryle on 19 August 1976
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The interview listed below was conducted as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009) and was transcribed for the NRAO Archives by Sierra E. Smith in 2015. The transcript was reviewed and edited/corrected by Ellen N. Bouton in 2016. Any notes of correction or clarification added in the 2016 reviewing/editing process have been included in brackets. During processing, full names of institutions and people were added in brackets when they first appear. Places where we are uncertain about what was said are highlighted and indicated with parentheses and question mark, plus a notation of the time on the audio e.g. (? 00:50) or (possible text? 10:32). If researchers are able to suggest correct text, please contact the Archivist. Sullivan's notes about each interview are available on Sullivan's interviewee Web page. We are grateful for the 2011 Herbert C. Pollock Award from Dudley Observatory which funded digitization of Sullivan's original cassette tapes.
Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons, including subsequent experiences, interactions with others, and one's feelings about an event.
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Begin Tape 59A
Sullivan
Martin Ryle on 19 August ’76 at Cambridge. Before we discuss your first work here at the Cavendish Laboratory, can you just tell me what your background was before then?
Ryle
Well, I took my degree in Oxford in ’39, just a couple of months before the war started. And immediately after the war started, those of us who had any experience with electronics were mysteriously whisked away into the radar game. And there I stayed for six years, until autumn ’45.
Sullivan
Was your degree in physics?
Ryle
Yes.
Sullivan
At this was, I think, at the TRE [Telecommunications Research Establishment] was the name?
Ryle
TRE, yes, which was concerned, well I was concerned with airborne radar and countermeasures...
Sullivan
And I suspect that this was rather useful in terms of the techniques you learned and so forth for radio astronomy.
Ryle
I think this may be somewhat over emphasized, this point. I think it was very important. I mean one stopped doing physics for six years. Physics had almost evaporated by 1945 but one learned a lot of engineering, what things could work. And, of course, when I talk about people, that was important too.
Sullivan
But you think the point is overemphasized that the development of radar and the people that learned it really is what made radio astronomy take off after the war, at least in some places.
Ryle
Well, obviously there were techniques that evolved in radar which were of use when work on receivers had gone quite a way. I was working with antenna quite a bit and that obviously was very important. But a lot of these things weren’t all that clever. They were basic things but the important thing is you got a feel for what was practical, what could be made. No, obviously one had learned a lot about various techniques, but in fact the actual straight transfer of technology to radio astronomy wasn’t as important as is often made out. Obviously one gained a lot from bits of ex-radar equipment, which would be much too expensive to build with the sort of budgets one had in universities just after the war, which was virtually nil.
Sullivan
So you’re saying that it was more of the methodology or more just a feel for what could be down in the area, rather than specific...
Ryle
Yes, I think that was really the most important thing. Of course, it was incredibly good training, this war time work, messy training but... The small group of people that was at TRE, the proportion that’ve gone to the tops of their profession in all sorts of different fields is remarkably non-statistical. This has nothing to do with the type of people brought into the game. It’s because the training was very good.
Sullivan
Right, very intense. I guess six years all together.
Ryle
Yes. It was six years. You had enormous responsibilities at an early age. You had to make decisions, there is no need to ask because you knew more about it than anyone else. This was the sort of thing which one ought to do in one’s Ph.D. program, if you can think of how to do it without having another war.
Sullivan
Well so the war ended and then what was your next move?
Ryle
Well, I got the possibility of one of these temporary fellowship things, which were set up to get people back into universities, which were, of course, in a rather run down state by then. So I came back to Cambridge with Jack [John] Ratcliffe, who had been running an ionospheric group before the war. He also had been at TRE, so we’d kept in touch and we’d started off observing the Sun. The particular point was, at that time, the Sun had been observed at times of great activity at meter wavelengths, and it had been observed at all times at centimeter wavelengths. So the first thing we set out to see was if it was in fact the Sun there all the time at meter wavelengths. And so we build rather more sensitive equipment and we built an antenna system for doing this.
Sullivan
Not knowing what intensity level it would be at.
Ryle
No. You were just making a system as good as you could. And we used an interferometer to distinguish between the contribution from the relatively compact source of the Sun, compact in those days, to distinguish that from the Galaxy. And this was the purpose of using an interferometer, which we rapidly extended to measure the angle and dimensions of the sunspot sources, to show that they were, in fact, a few minutes of arc only.
Sullivan
By "we," you mean yourself and [Derek D.] Vonberg I assume.
Ryle
Yes, that was just the two of us. Vonberg had also been at TRE and he came back, now I can’t recall what state he was in, whether he’d had completed his undergraduate course, because now during the War, the undergrad course was chopped off at two years and people tended to go back and do one year more. Anyway he started out a little after me. I can’t remember the exact date. But we had some sort of equipment going I suppose by early ’46.
Sullivan
Now, let’s take this for an example, the first Michaelson Interferometer in radio astronomy which you just talked about. Was that idea something that was apparent from any work that had been done in radar or just rather... For instance did you ever have to discriminate between a point-like source and background?
Ryle
It didn’t arise that way. It arose because, I think, the... now, how did it arise? I don’t think it arose by analogy with an optical Michaelson because, as I said, I’d forgotten all the physics by then. I think it arose from the idea that if you have a null by introducing two aerials out in antiphase then this could fairly easily tell you something about a compact source. And if you make that null narrow by separating the elements, well, I suppose we were reinventing the Michelson interferometer as it were, but I think it came from a sort of rather simple-minded thinking of aerials, rather than saying, "Ah, I remember in my physics book - optics."
Sullivan
That’s interesting. Let me also ask when you came here how was it that you chose radio astronomy? Was this basically Ratcliffe’s idea or your own idea?
Ryle
Well, during the last months of the war, he had suggested I might come back to Cambridge. Of course, he was very anxious to build up a research team because they were going to have to teach undergraduates as well. And there was, as I say, a very run down state in the Cavendish at that time. And I had already told him that I wasn’t terribly excited about the ionospheric side of things. I thought, well wrongly I thought as it turned out, that there wasn’t too much more to do in that field. But in any case, this seemed a more exciting one, the fact that there had been these observations, wartime observations, of the Sun and more observations of the galactic background, which clearly were interesting.
Sullivan
Ok, now, did you know about [Karl] Jansky’s and [Grote] Reber’s observations at this time late in the War?
Ryle
I don’t think we knew about Jansky, and I should think we probably didn’t know about Reber. One didn’t read literature during the War. There was no time for it. One was living on a very short time scale of course. There were things that needed to be done in weeks, and get into aeroplanes and fly. And that time didn’t allow one the luxury of keeping up to date with the literature. Anyway there wasn’t very much. No, I think probably it had been triggered off by the work of Hey. Well, there were two bits of it. One was the rediscovery of the importance of the existence of the Galactic background as it affected the emitting sensitivity which radar receivers could reach. That was a thing which happened at TRE, I remember.
Sullivan
Well, hold it now then. That must have been someone coming across Jansky’s paper or something. Or it was measured?
Ryle
No, well, you were trying to make receivers better and better. And it didn’t get any better, it measured better on a bench.
Sullivan
Oh, I see.
Ryle
It didn’t get better when you stuck them on an antenna.
Sullivan
It was completely independent of Jansky’s work as far as you know.
Ryle
I think so. Because as I say, people weren’t interested in what had been done in some way off thing. It was in a lower frequency anyway. This is now in the (200?) megacycle region where the (grounded grid triodes? 9:00) were coming in. These were the people who invented these things on the bench said, "This is a marvelous receiver. It’s this much better than the previous one." But you took it out and put it in a radar and it wasn’t. The range wasn’t increasing evidently. This was because at that time you’d been able to get the receiver noise in the 20 megacycle region down to levels where the Galactic background began to matter. So that in a sense was a rediscovery by people who were just concerned with increasing the range of a radar set. Now that, of course, was immediately tied up with people who remembered pre-war things. I imagine I wasn’t one of them because I wasn’t aware of it. That was one thing.
The second thing was the work that [James] Hey did. Well, he did two things. He observed the meter wave radiation from the Sun, which was discovered again accidently on an operational radar system. He also observed variation of the system noise, as you would now call it, with azimuth. It was a non-horizontally looking radar. But by plotting the azimuth distribution at various times of day he was able to draw out a crude map of the Galaxy, which again, I don’t know whether or not he had read Jansky’s work or not. He may well because he was older than me and therefore he would have been in research before the War.
Sullivan
And is this during the War you’re talking about Hey’s? This is not the thing that the published in ’46 or so, the first map, Hey, [James W.] Philips, and [Sydney John] Parson?
Ryle
That’s right. That was the one. It was done just towards the end of the War. It wasn’t published until after the War.
Sullivan
Now do you think there might be some reports at TRE describing this effect of the Galactic noise doing the limiting thing? That would be very interesting to me to see what they had to say at that time.
Ryle
I doubt it.
Sullivan
What you say is that it was sort of common knowledge amongst those who needed to know.
Ryle
Well, I think you have got all together a too academic view of what that period was like. You say this was an effect which happened. It wasn’t that at all. You’d say, "For Pete’s sake, how can I increase the range of this radar a little bit," or whatever it may be. That was the best you can do. "Right, that’s the best you can do. Now get on and do it." And you get on. This aeroplane is depending on it.
Sullivan
So you are saying that there was no time to write a report.
Ryle
Oh, God no. I mean they might have at some time written a report, but I should think it’s not very likely. I mean, it was a different sort of life all together. The amount of paper flowing was rather small.
Sullivan
Rather much more enjoyable in that respect anyway.
Ryle
Now the other thing, of course, which happened during this time was that, I think it was [George] Southworth. I don’t know whether that was an accident or...
Sullivan
That was purposeful. He had a small dish, and he would actually detect the Sun at three different wavelengths.
Ryle
That clearly did owe a lot to Ratcliffe because there would not have been any receivers there if it hadn’t been for centimeter radar. One wouldn’t have known about that except for radar. Of course, that’s really back to the invention of the magnetron too, because if there hadn’t been a magnetron, there wouldn’t have been centimeter radar. So people wouldn’t have made good mixers.
Sullivan
That’s true too. Now you say that Ratcliffe - of course, he had worked in the ionosphere. But you weren’t interested in the ionosphere, so how did this other option come up?
Ryle
Well, we knew of Hey’s work and we’d read of Southworth’s work by then I think, hadn’t we?
Sullivan
Well, you may have had a report from Rad Lab [Radiation Laboratory at MIT] in the States or Bell Labs.
Ryle
Yes, I think we had heard of that by the end of the War. Just these things looked like they might be interesting.
Sullivan
I see. But Ratcliffe himself didn’t want to work in these things?
Ryle
No, he obviously felt, and indeed he was right, that there was a lot more to do in the ionosphere, again, with again techniques that could now be greatly improved.
Sullivan
So the first thing you did, as you’ve already said, was set up this Michelson Interferometer, established that there was small, less than 10 arc minutes bursts in the first paper is what you say. Also that it was circularly polarized, which is a rather interesting sort of thing. Why did it strike you that that would be something to measure, or was it just that you were trying to measure everything that you could on this strange radiation?
Ryle
I can’t recall if at that time anyone else had measured circular polarization.
Sullivan
Well, I don’t think so. You didn’t know anyway. In Nature in two weeks there were three papers: [Edward] Appleton and Hey, yourself and Vonberg, and, who was the other one, Martin and somebody. So apparently all of these people had the idea. I’m just wondering -
Ryle
I think quite likely we did realize that if you put... we did it with an interferometer with opposite polarizations, which would have shown up if it was linearly polarized or circular. And found that it appeared circular.
Sullivan
And were you thinking that the magneto-ionic theory and so forth might be applicable to this?
Ryle
Sure, yes. Obviously with Ratcliffe’s background, this was obviously going to be relevant because of selective propagation of one mode versus the other or the generation. Of course, we were thinking about generation at that time.
Sullivan
And while you were doing the nitty-gritty of building these antennas and so forth, were you also essentially educating yourself on things like magneto-ionic theory and the Sun and such like this?
Ryle
I think probably that first six months, no. I think we were too busy collecting equipment. Of course one was able to collect a lot of ex-radar equipment. But was also building up the lab as a whole. You know, there were any meters or anything. It was really remarkable how little there was at the end of a six year war.
Sullivan
So just getting that established. But then after that six months are you then implying that then indeed you could begin to sit down and read some books?
Ryle
Well, I think again at that time one’s mental attitude at the time was quite different from what one expects. Of course one was quite old. I mean 28 or something I was. And so, one wasn’t in the sort of learning frame of mind. One didn’t automatically go, "I will study this problem and I will read what has been done about it before." Because (a), nothing had been done about this particular problem before, and if it had nobody had written about it, virtually. No, I think probably that the thing first of all was just to see what there was. Later on, of course, one tried to see what could make circular polarization, or what could make radiation at all, of course, initially. But I think that came quite a bit later, I would say.
Sullivan
Can you say sort of when you think? Because that is a rather different change in philosophy I would think. It’s more or less when the science begins to be a little bit more settled rather than...
Ryle
Yes, I should have thought probably that the first two or three years was that you were finding new things all the time so much. Just to collect fact was really rather a full time occupation for a group of two people.
Sullivan
Well, looking at your publications here I see in ’48 Proceedings of the Royal Society you talked about the theory of how this burst radiation had come about.
Ryle
Well, obviously during that time with help from Ratcliffe. As far as I remember, Ratcliffe gave some postgraduate lectures, well you wouldn’t really call them lectures. It was educating the next generation of ionosphere students. For a long time the ionosphere side was a lot bigger than the radio astronomy side and therefore there was at least this education for people starting work in the ionosphere, which we naturally listened to because clearly it was all relevant. And I suppose it was during those first two years that one gradually began to think about what might be the explanation. But one was very much concerned with actually building equipment. One didn’t buy things out of a catalog in those days.
Sullivan
And like you say, virtually your entire source was just this radar equipment. Is this correct?
Ryle
Well, we had a little money. I think our telescope cost 200 bucks. It was a good telescope. It got 50 sources.
Sullivan
You’re talking about the long Michelson.
Ryle
Yes.
Sullivan
That’s skipping a little bit ahead.
Ryle
But up till then, you see, we’d built little frame things, broadside arrays that we’d built ourselves.
Sullivan
What about the idea of what is called the Ryle-Vonberg receiver where you have a balanced noise and so forth?
Ryle
We started right at the beginning. Up till then all we’d heard about it people that put an integration circuit on the backend of a (diode? 18:00) detector and then tried to stabilize the gains of everything. And this nearly worked when you get a signal so strong they can swamp the receiver noise by a factor of many. But as we had set out to see what the radiation from the quiet Sun would be, and for all we knew they wasn’t any, one wanted to get the sensitivity as good as possible. It seemed clear to me that there was no hope of using this technique of making a microphone on the back end of a receiver that you’re asking from gain - I mean the idea of bandwidths and things very often I don’t think we understood all that, (bandwidth? 18:42) ratios and things...
Sullivan
Did you know about [Robert H.] Dicke’s paper?
Ryle
No, no. But it seemed that the only hope of doing this was to have a system which didn’t require gain stability of a receiver of 1 part of 109 or something. You would just hope to try to control the voltages. And, well I remember reading about that time some of the early American reports, which proudly said that the HD was stabilized to 1 microvolt in a filament to .5 microvolts or something. And on occasion something went wrong. I’d inadvertently set a 300 volt passby to run at 400 volts by reading the meter wrong. I went back to look in the record and couldn’t see it. [Laughter] So we thought ours was a better method. So we had in fact gone straight ahead for a system whereby you were balancing the antenna signal with that of a local noise source. That was the first observation we’d ever made with that system. We didn’t attempt to make any other observations. No, actually (? 19:39) making a crude receiver first. I was of the very strong opinion that it wasn’t worth doing. (a) people had done this sort of thing and (b) it was just too much hard work, trying to do things that way.
Sullivan
So you were in favor of really doing it right so to speak, taking longer if necessary.
Ryle
It certainly did take longer. Well we actually used an old radar (? 20:05) and right at the front of it we put a switch, which incidentally came out of a German nightlight.
Sullivan
Oh really. So it was German surplus equipment also? Of course, the Würzburg are certainly German also.
Ryle
Yes, yes.
Sullivan
Well let me ask you about having established that these sunspots were very small and therefore had fantastically high brightness temperatures as we all know, you did write this theoretical paper in Proceedings of the Royal Society in ’48 and then another one in the Proceedings of the Physical Society in ’49 talking about coherent electron oscillations – had been proposed but you had great difficulty in seeing how they were being maintained. Could you just tell me what your view was at that time?
Ryle
That was a long time ago. I think we were familiar with the concept of an apparent temperature. And then, or soon after, I think we realized that if this was going to be what you might call a thermal mechanism, you’d have to have a gas with particles of temperature divided by 104 or whatever it is. There was considerable opposition because that was daft; you couldn’t talk about temperature like that. There couldn’t be such things, you see. So the only other way of doing it was to do it like broadcast aerials, to (loosen? 21:35 more than one electron at a time?). That was where the idea of - well, of course, they existed anyway. In gas discharge plasmas this sort of oscillation was known to exist. There were great problems in starting it up. And there were great problems in getting it to escape because it always happens at frequencies that can’t get through the boundary, but it doesn’t matter with a plasma in a bottle because the gradients are so fast at the edge. (? 22:00). That was the point of these early papers.
Sullivan
And also in the maintenance, of what was keeping the oscillations going, I gather from looking at the abstract. Were you bothered by that?
Ryle
Oh, yes. I mean one might have had situations with dB by dt in a sunspot field producing ways of accelerating electrons there are plenty of volts around in principle.
Sullivan
What did you say?
Ryle
Well you’ve got a sunspot which is known to have fields of 1000s of gauss from the optical Zeeman splitting. These sunspots are known to change in periods of weeks. And therefore there is a straight rate of change of flux calculation which clearly gets you large voltages. So there is no great difficulty in accelerating charged particles, which basically is how you start off with a gaseous charge tube. But, of course, at that time and even till today, there is very little known about gaseous discharge oscillations. They were very mysterious things. Maybe they still are.
Sullivan
Well, just skipping just a tad ahead now. Once there were a few radio sources or radio stars, as they were called then, known, you then seems to me tried to extrapolate this to making these radio stars sort of super suns. Saying that they perhaps had larger magnetic fields and that in the same way they could produce lots of radio emission, and perhaps produce cosmic rays, which is rather interesting in the light of later developments, which likewise connected radio sources up with cosmic rays, but a rather different sort of thing, namely supernovae and so forth. Is that right that this was just an extension?
Ryle
Well, I think having got one mysterious mechanism for making intensity, high brightness temperatures, obviously one would tend to think the explanation leads to even more mysterious things. And there were optical stars known with magnetic fields, general fields, not localized spot fields, of 5000 gauss. And therefore, this was obviously a supposition we made, that the Sun was a rather feeble thing as far as magnetism was concerned. There were other stars that could produce very much more. And, of course, this was the stage when one hadn’t a clue what was making the galactic background either. And it was perfectly conceivable that if there were a fair population of these things, they could expand the background at the same time, a non-interstellar medium mechanism.
Sullivan
But accepted opinion then was that people were grappling with trying to explain it in thermal, as thermal radiation. They’d say, "Well, Jansky’s measurement is somehow wrong," and then they‘d try to fit the rest of it to a hot gas. Apparently you didn’t go along with that.
Ryle
Well, we probably hadn’t thought about it very much until then. We were very much thinking about the Sun. But I think when the sources were discovered and these were also shown to be small angular sized things and therefore very high surface brightness temperature things. The possibility that these could at the same time explain a couple of other thing. The spectrum was such that there wasn’t any hot gas explanation that really worked.
Sullivan
Right. They were always straining to make that work.
Ryle
Yes. Well, it was really even straining. It was hopeless. But the spectrum of the sunspot radiation could be this funny, steep gradient, and would therefore be possible to fit to the galactic background. If there were enough of these stars around, with that boost that radiation could be enough.
Sullivan
I suppose the agreement of the radio spectrum of the solar bursts and the galactic background was also suggested.
Ryle
Yes, immediately. It was this funny steep spectrum, a non-thermal spectrum.
Sullivan
But now what about the nature of these stars? That must have been very puzzling. In this paper you say that they must be very under-luminous and yet they are tremendously radio luminous. That was just something you had to accept as a necessary consequence.
Ryle
Yes. Well to make the discrepancy with the Sun less acute one would obviously like to have them fairly near. In which case the lack of optical, even with a crude to the one degree or whatever it was, accuracy at that time, the low optical luminosity was a worry.
Sullivan
Right. So you might have expected for instance that the half dozen brightest radio sources would line right up with Sirius and Vega and so forth.
Ryle
No, not necessarily because they were clearly quite different things from the Sun. I mean they might have lined up with magnetic stars. There were quite a few stars by then known to have 5000 plus magnetic fields, which changed.
Sullivan
But with one degree accuracies and positions you could not -
Ryle
No, no -
Sullivan
- associate with individual stars.
Ryle
No, there were a number of these known, you see, a number of these strong fields were known. And at that time were as rare as Sirius, I think. As far as I recall, we looked at those. There were probably only a dozen or so known and they didn’t tie up.
Sullivan
None of them tied up, I see.
Ryle
But obviously it wasn’t conclusive because there could be a big range of things.
Sullivan
I see what you are saying. Well, we’ve switched over into radio sources a bit. Let me just finish up and see if there is anything else more in the solar. Well, I guess the main thing to ask is why did you switch from the solar? You had worked for a couple years with several papers.
Ryle
Well, I think probably the point was that we’d got quite a lot of observations going. Quite a lot of other people were in the field. It was now clearly becoming a thing which was going to get more and more theoretical. The interpretation was going to be more and more the major part of it. And I suppose personally I didn’t feel very competent to contribute there. I mean we put these fairly simpleminded ideas forward at the beginning and they didn’t seem to work very well. But this new thing which was triggered off by Hey’s discovery of this fluctuating region. And we said, "Well, let’s build an instrument to look for that particular object." Well that was very exciting actually because we built this instrument and we left it running. And overnight, damn it, there were two of them.
Sullivan
The first night?
Ryle
The first night, yes. We saw the record which showed there was a source in the same place as Hey said it was, but there was another one. And that was how Cas A was discovered.
Sullivan
Right. Well, I guess it would come in the first time you had the thing working.
Ryle
Well sure. There was no declination, no resolution.
Sullivan
Right. I’m wondering in this paper why you did not give a new position for Cygnus, because that was of great interest at that time.
Ryle
Well the point is that the instrument was not really suitable for it. It hadn’t enough resolution and (? 29:11) elements to sort of which was the central fringe. There were just four yagi, weren’t there?
Sullivan
I’m not sure. Aerial spacing of .5 kilometer.
Ryle
Well, anyway, as far as I recall we just rigged up four yagi at each end and connected them together. Oh yes, it must have been, because we were able to do the polarization right away too, weren’t we?
Sullivan
That’s right.
Ryle
They weren’t circularly polarized like the sunspot radiation was.
Sullivan
I think you also tried to split the polarization question up into a steady component and a fluctuating component.
Ryle
Yes, well, the point about Cygnus was that it showed these fluctuations that we now know to be (? 30:03) region scintillation. And that fact that this one did and this one didn’t was primarily a factor of time of day. These things have a marked maximum of 01 local time. And there was I suppose a suggestion from this that there seemed to be little bit more fluctuation in that bit of record than this bit. But there was no clear interference pattern seen, and perhaps there was therefore some evidence that was a local sunspot source contributing that much that was polarized. But the general radiation clearly wasn’t. Of course there was nothing showing here. There was just a bump showing total integration radiation but it wasn’t correlated between the two oppositely polarized antenna.
Sullivan
Now, you say by chance when you first observed, which was May ’48 according to this 1948 Nature article, Ryle and Smith, by chance Cas was coming through at a time when there was not so much scintillation. Which is around noon, is that right? I’m not so much of an ionospheric -
Ryle
Yes, well -
Sullivan
Where Cygnus was coming through around midnight.
Ryle
It was variable anyway. It wouldn’t necessarily - what time of day would it have been? These are GMT anyway, aren’t they? So yes that is 03 or something and that’s 07 or 06 in the morning.
Sullivan
So Cygnus was coming through at 3am.
Ryle
Yes. So that’s when there would have been quite a lot of surround. But it didn’t always do this.
Sullivan
Now, how long did it take? I’m a little confused. One can look at the 1950 paper here with the Cambridge and Jodrell observations next to each other, which most people say established the ionospheric nature of these scintillations. But I’m confused as to when people actually were convinced they were ionospheric, as opposed to this official date of March 18, 1950. Or was there a lot of controversy up until this time?
Ryle
No. Well, you see, this point is that Hey had recognized this because of the variations, not because you could see a worthwhile bump on his record because he had such a low gain instrument. It was a very small excess at this region, and it was all muddled up with the Galactic plane. But the point is in one direction only and in one beam width’s worth he got this fluctuating signal. Now I think probably one was already aware, I’m not sure if he actually said so but I’m sure fairly soon afterward one was aware that the fluctuating source means a source that is physically small.
Sullivan
Yeah, he said so.
Ryle
Yes. If that was the right interpretation, it went right away into the modern game of super-high velocities or what. At that time it was all that and therefore the possibility of this not being that, of being in transit, I think quite likely was around, after one knew about - I mean the ionosphere clearly had an effect at, this was 18 megacycles, wasn’t it? I mean the distance of this spread f stuff was known. But I don’t think you would say, "I believe this is the ionosphere. I’m going to do an experiment." I can’t remember whether we, Tony Hewish and I, had longish paper where we tied it all up, looking at four sources.
Sullivan
That’s right in 1950 in Monthly Notices.
Ryle
Well actually, of course, that was -
Sullivan
That was the result of -
Ryle
Well, it wasn’t much different in time actually because those took a long time to publish in those days. This actually was doing more though, wasn’t it? It was talking about - did it have long baseline experiments?
Sullivan
Yes, that’s right.
Ryle
Well you, the question was, was the something which was intrinsic in the source, because we realized that was very important if true, and so we did the combined experiments with Jodrell and us, which was twenty miles down the road, or ten miles down the road, whatever it was. And by in large there was no correlation except for a few strange events. And of course these are still with us...
Sullivan
Yes. I wrote a letter to [Francis Graham] Smith asking if this had ever been resolved and he said -
Ryle
Well, you see, a lot of work has been done recently by John Shakeshaft with, well, both Weber’s gravitation pulses and cosmic ray shower bursts, which cosmic ray showers--could these things have been due to higher shower producing. And this was part of the Jodrell program. It was why they built the 218 vertical foot dish originally was to look for radiation from a cosmic ray shower. And there are high altitude showers which might have produced correlations over these sort of distances. But as I say, this is work that has been going on in connection with cosmic ray measurements with ever increasing baselines. We had a great program a couple of years back going as far as Malta and all over the place. We tapped into Malta, Jodrell, Cambridge.
Sullivan
I see. Has this shed any light on what these things were?
Ryle
Not really. It’s all marginally significant. Not really, no.
Sullivan
As I say, I wrote Smith. I guess it’s been two years ago now and he said that they were still a mystery to him. Well, something that before we go any further with the science, we should mention that several more people were joining your group as you went along.
Ryle
Yes, well, after Derek Vonberg, Ken [Kenneth E.] Machin. There was Tony [Antony J.] Hewish and Graham Smith, I think... Was it ’47, by ’47? I can’t quite recall. Do you want me to look that up?
Sullivan
Actually David Edge has given me the cards with date on who joined when. But I’m just trying to get at now is that these were all research students that were coming, rather than staff members?
Ryle
They’d all been at TRE as a matter of fact. I was the last batch that had completed their degree before the war, you see. And as soon as the war started I think they converted to a two year, and I think maybe later only one year course, just to give some people some basic education and get them doing something useful. And then when they came back they then completed with the third year’s undergraduate work and took their finals. And that was the case with Tony Hewish and Graham Smith. I’m not sure whether Ken Machin did the same or not, probably. So at the end of the war they had to do this extra year of work.
Sullivan
But then they continued with postgraduate study?
Ryle
Yes, they all got PhDs except for me.
Sullivan
Oh you didn’t?
Ryle
No.
Sullivan
Why not?
Ryle
It was too difficult.
Sullivan
Too much bother after all that -
Ryle
Too much physics.
Sullivan
That’s very interesting. A question has popped into my mind. Going back to the explanation of the radio stars producing cosmic rays and so forth, what was your feeling as to why they were not bursty? Or did you think that the Cygnus A fluctuations could be intrinsic? This was 1948 again.
Ryle
I think probably - there was a Physical Society paper, wasn’t there?
Sullivan
Yes.
Ryle
I think probably the feeling was that if you had a star which had a general magnetic field of 5000 gauss rather than the .5 gauss of the Sun, you might get an analog of the quiet Sun radiation, which was strong. And, ok, you might occasionally get bursty things, but after all the Sun much of the time isn’t doing it. So you wouldn’t know. But, of course, I think it must have been in the back of our minds to explain these things, the fluctuations there, before we knew they were ionospheric.
Sullivan
Right. Another thing about this paper just occurred to me is -
Ryle
Sorry, can I just finish that off? This was one thing of course, showing there was a moderately convincing anticorrelation, I mean no correlation, when you move the things apart. But the final thing was the one, or whatever it was, year observation of the four sources which showed a very nice identical plot for all sources plotted in time of observations. So all our measurements were with transit instruments and therefore the right ascension determined the time of day which observed it. And they showed this beautiful curve which all was identical for all sources. You shifted them to local time and they all fit on the same curve. All sources did this.
Sullivan
It was a local solar effect?
Ryle
It was clearly ionospheric.
Sullivan
I mean it was in solar time.
Ryle
Solar time, yes.
Sullivan
That was Ryle and Hewish 1950, right?
Ryle
Yes.
Sullivan
Did you know about the Australian work going on in the ionospheric stuff at that time?
Ryle
What the spare separate receiver thing, you mean? It was probably going on in parallel, and I’m not sure. No, I think this probably came out before anything they did. I’m not sure.
Sullivan
It came out before they did, yes. But what I’m wondering is as you were working on this, were you aware of similar projects going on in Australia?
Ryle
I don’t think we were. No, I think we realized that between here and Jodrell, we could do this quite nicely. Well, I think we first of all did this short baseline thing and most of the effect disappeared (? at a 30? km? baseline with a tiny portable receiver 39:05) we could get good signal to noise if we went from here to Jodrell.
Sullivan
I’d like to ask you about the paper that has the discovery of Cas A in it. You have a source which is about twelve hours different in RA and I’ve heard it said that this was Cas A in the back lobe.
Ryle
That’s right. It’s a lower combination Cas.
Sullivan
And how did you straighten that one out eventually? Long Michelson?
Ryle
Yes. Well, I think probably finally when we did the accurate positional work on Cas and Cygnus. Then the lobes go the other way.
Sullivan
I see.
Ryle
Well we looked anyway. We had the Würzburg tipped over, that’s right. You looked where that was and there wasn’t anything but obviously the back was measure the position of Cas. And then we realized almost certainly happening, it was coming in.
Sullivan
And that’s why you say Cas is a more variable source than Ursa Major. It was variable because -
Ryle
Because it was very low elevation. It was about 5 degrees below the horizon.
Sullivan
I asked about knowing about the Australian ionospheric work. Let me just ask the more general question of which groups you had contact with at that time?
Ryle
I think probably one didn’t have the sort of contact which has become - Well, there were no conferences continually very few weeks of course, like there are now. And you relied much more on publication. I think the normal thing was to publish it, and I think most of these were in Nature were very short. We didn’t know, for example, of their cliff interferometer as a means for discovering the angular size of the source and they didn’t know of our Michelson interferometer. And those appeared, I forget which order, both in the diameter of sources and in the angular sizes of some of the radio sources. They were just techniques which developed independently. They were radar people anyway and they happen to have a radar on top of a cliff which they could use to scan in azimuth. So you obviously could use that very easily, which is how they started. Obviously when these first observations had been made, then we used to write to each other. Not particularly in the sense of a collaborator program because it wasn’t at all clear what the program ever was in those days. It was moving fairly fast. You did what you could with the resources at your disposal.
Sullivan
Did you exchange preprints as we call them?
Ryle
No, you couldn’t. I mean you didn’t have all these typists and things around.
Sullivan
No Xerox machines.
Ryle
No, no.
Sullivan
I see. Any other groups besides the Australians? I mean were you aware of the little bit of solar work going on for instances at NRL [Naval Research Laboratory] in the States or anything like this? Or the French efforts right after the war -
End of Tape 59A
Begin Tape 59B
Sullivan
Continuing with Ryle on 19th August ’76. So about the other groups, which other groups...?
Ryle
We used to obviously have quite close contact with Jodrell Bank, meeting at RAS [Royal Astronomical Society] meetings and the like. I suppose we probably met up with the French. I forget when the first sort of international meeting was, probably 1950 or so. And from then on there was quite a lot of contact with the French.
Sullivan
Were you reading their stuff in Comptes Rendus?
Ryle
Yes, I think so. Yes. And I’m sure we were aware of the NRL work. ’54 I went there.
Sullivan
Oh, by that time, yes, but I’m thinking of in the ‘40s. There wasn’t very much but they did a few things. But it sounds like it was not terribly influencing what you were doing.
Ryle
No, I don’t think it did. I think we were all going all with what we thought we could do, and just getting on with it. And that’s the way perhaps science often ought to go.
Sullivan
Rather than worrying about what the other fellow is doing.
Ryle
Yes. Jumping on the latest bandwagon -
Sullivan
What about with optical astronomers? Could you tell me what sort of contact you had with them in those days?
Ryle
Well, we had quite a lot of contact with our own, at that time the Harald Von Klüber, I think he was here then. But anyway there was a strong active solar group in the Observatories here which is trying to correlate the bursts with sun spot and (? 01:38) activity with certainty.
Sullivan
And who were the people there?
Ryle
I’m not sure when Von Klüber was there, maybe as early as ’48. But certainly there were people - [Frank J.M.] Stratton I suppose was the Director then.
Sullivan
I think he was. That’s alright. I can check. But did useful information ever come out of these correlations of optical and radio?
Ryle
Well, I don’t know how sure the correlation between radio bursts and optical was at that time. They probably did help in that. Because the Sun doesn’t always shine in Cambridge, but on the occasion when it did then we used to try to compare the observations.
Sullivan
Now once again relative to optical astronomers in April ’49, you gave a talk at an RAS meeting talking about a part of what was to become the 1C survey, namely that you had found 23 radio sources and they were unresolved and so forth, positions of ±1 degree and so forth. How did optical astronomers receive this? You were talking at their Society. But was it just something that was entirely outside anything they could ever work on?
Ryle
Obviously there was interest, but I don’t think it really could have been much more than that simply because if you can only tell them where to look to a degree and it isn’t something as obvious as the Sun or the Crab Nebula, then what are you to do about it. One degree is a rather big part of the sky. And, of course, it went on a rather long time like this. They didn’t, I think, feel that there was anything much they could do about these funny things and it might go away it they waited. That’s a little unfair.
Sullivan
But still it was a fantastic amount of energy being produced in these cases...
Ryle
But one didn’t know anything about energy because you didn’t know the distance. They could be quite local.
Sullivan
But even if they were local stars you had to make them be super suns.
Ryle
Well it had to be much more energy than comes out of the Sun in radio but not out of the Sun in total energy. The actual total energy probably wasn’t very severe. The Sun happened to be a particularly feeble source. That was the interpretation I think.
Sullivan
Right. It really wasn’t until Cygnus A and so forth...
Ryle
That we got the distances, yes. Even the Australian identification of (M87? 4:37) and the Crab, I think everybody, possibly us as well, weren’t all that convinced by the identification. I mean, ok, they fitted within a degree or so, and they were unusual objects, rare objects, but if that had been the case perhaps one ought to have seen some other super novae. Although fairly soon it was evident it was right because everyone got those positions, but even at the first announcement of that I think probably the optical astronomers took that with a pinch of salt.
Sullivan
I see. And you say you did also?
Ryle
I think we did originally, yes.
Sullivan
Even for the Crab? That’s surprising.
Ryle
Well we didn’t know how remarkable the Crab was, not the optical astronomers. Ok, nowadays someone said, probably one of the [Geoffrey R. or E. Margaret] Burbidges, there are two astronomies, one of the Crab and one of everything else. I mean, ok, we know that now because we know a lot about the Crab but at that stage we didn’t know anything about it except that it was a little fuzzy thing on a photograph.
Sullivan
Well along this same line can I ask you what did you consider that you were doing in the nature of the science. Did you consider you were doing ionospheric physics extended to outside the ionosphere? Or something that we would call radio astronomy or radio physics?
Ryle
I think radio astronomy. I’m not sure, but I think we invented the term incidentally, and certainly I consulted my father about the propriety of mixing up Greek and Latin terms. In 1950 when I wrote the Physical Society, Reports on Progress in Physics, or whatever it was called, that I think was the time that the term radio astronomy was used in a definitive way. And my father said it was alright to mix up Greek and Latin derived terms.
Sullivan
There are probably a lot of other examples. But in any case you did definitely think you were becoming astronomers essentially?
Ryle
Yes.
Sullivan
But what about this term radio physicists which I’ve heard used quite a bit.
Ryle
Well radio physics is a term which has been used for quite a time, I think, which includes ionospheric physics and then tropospheric propagation physics, all these things. It’s a much broader term. The Australians were in a radio physics laboratory because I mean they converted their radar research group into a thing which was studying all these things, which was definitely concerned with tropospheric propagation, and proper scatter, and all those things which developed out of it, as well as ionosphere.
Sullivan
Something else that you just brought to mind, I’d be very interested in your opinion on why it was that radio astronomy did not take off in the States like it did here and in Australia. You had the same sort of thing, I mean, tremendous radar push during the war and at the end you had all these trained people and all this surplus equipment.
Ryle
Myself, I believe the difference to be the way that radar development happened. It was, I think, a fantastic occurrence to a young chap to get into radar research in England. Now as I say there were maybe 200 of us at the beginning of the War and the number of fields where radar had application was extremely broad. So you found yourself with responsibility for a rather surprisingly large amount of this for one’s age. And even two or three years later, you’d find young people two or three years after, well not even a proper degree because it would only be a one or two year course, running groups with 25 people in them. It was a great field. Now that was a way of growing up, a feeling of responsibility of what was possible, "All by myself, I will build up this great thing with the help of some engineering boys," but a lot of it was your own work, (sewing up a de Graaff? 08:23) tube. And you’d build something, a prototype, and then you’d try it. And then maybe you’d apply it. And this is a way of growing up that really is a most fantastic thing to happen to anybody.
Now in America the war situation was different. You came in in a much more organized way. Most of the work was done in large, existing laboratories and then you had some commercial ones. You had no contact day by day between the operational people, the RAF [Royal Air Force] that was flying your stuff, the operational needs, what was possible. There was no interaction whereby a young chap with a couple of years physics training and a couple of years of messing around and deploying a (de Graaff? 09:00) tube could tell an Air (? 09:09) Marshall, "That’s a bloody silly thing to do. You want to do it like this." You didn’t have that you see. Someone higher up in the Air Force wrote a specification for equipment, you presumably had this from all these places, and then it was put through like an ordinary production run. There was no way in which, that young people had this direct contact with operational flying. And I think it was this contact with, as it were, life in the raw, being able to talk to the people who were going to be flying, who were going to be killed in these machines, that made one grow up fast. And I think there is this feeling that one had a training to give one confidence, that one could do things, rather than saying that one could write a specification for a radio telescope and get a bid for it and it due course it will come. I mean if you want to be nasty you’d say that this is what happened to the 600 foot at Sugar Grove, you see. That was designed by a lot of people, none of whom were young enough.
Sullivan
Very interesting. Although, of course the Sugar Grove thing was not really for science. That was sort of tacked on to placate Congress but there are other examples one could think of. That indeed is very interesting.
Ryle
There was a huge difference in cost too. The only way radio astronomy was going to get off the ground in this country was if a small university group with a few people in it could scrounge enough bits and pieces to do something. Well that never seemed to happen in the United States. Much too soon you lept at the idea of a national observatory, and that was a great mistake.
Sullivan
Even before then though -
Ryle
Yes, and I don’t know why it didn’t start to get off the ground before then. Hendrik van de Hulst and I were invited over by the NSF [National Science Foundation] to talk about the formation of a national observatory and we both came out stronger. The one conclusion that we reached and put forward most forcefully was they must get cracking now, the small university groups, training people, to decide what was wanted, what was going to be the instruments that mattered. But they didn’t ignore it, they said that was not the way they were going to go about it. So I said, "What are you going to do about the radio astronomers when you’ve got the equipment?" He said, "We’ll always be able to attract enough people from Europe, you see." Now that was an admission not so much of defeat, that was the cheapest way of doing it. And that shocked both of us actually.
Sullivan
Who was the other person?
Ryle
Hendrick van de Hulst.
Sullivan
I see, yes. Very interesting.
Ryle
(? 11:34)
Sullivan
Well anything that I might want to quote you I’d certainly ask your permission. Henry Palmer yesterday made the interesting remark to me that he thought science or virtually anything was best done in groups of sort of ten. Five was a bit too small. Twenty was a bit too big, and I guess you would agree with this.
Ryle
I think probably I would put it a bit bigger than that simply because I think it’s important to have more than one thing going on. I think one thing is done by five people, but I myself always thought 50 is the right size for a research group. So you have a number of lines going on which can interact and help each other. But the group itself, I agree, is about five. And I think we’re too big here actually. We’ve got 80 or something. I think 50 is about right. And independence, I’m a great disbeliever in national instruments.
Sullivan
Well, also on this independence, it seems to me that Ratcliffe must have given you virtually completely independent hand.
Ryle
Yes, obviously he helped us an enormous amount. He didn’t want to interfere with the way we were going. We were very small of course. We were only sort of 20% of the group as a whole.
Sullivan
At the beginning?
Ryle
Yes, yes. He saw that or perhaps saw (? 12:58) -
Sullivan
- leave you on your own. Well, let’s go back to science. I think the next thing indeed is that you obviously got fascinated with these radio sources having picked up a couple with this thing that was lashed together. What was the next step?
Ryle
Well I think as soon as we realized that Cygnus wasn’t unique, there was another one of them, we then built this Long Michaelson instrument, which now had just about enough resolving power so that you could recognize the central maximum, whatever it was 10 to 1 or something. Well it went in two stages. We doubled after a time because we wanted [more?] gain and we weren’t of course able to recognize the central maximum. To go back a step, it was evident on the first morning that what a funny thing. That period is different from that, you see. We hadn’t realized that here was a way of measuring declination until it actually appeared. So you can get both coordinates from a system with no resolution in the North-South direction. And, of course, it had advantages because you didn’t have moving structures. It’s an incredibly cheap thing. You could measure declinations moderately accurately except at low decs. as with all these things. And therefore you get coordinates in the straight away.
Sullivan
Who was it that actually worked on the building of this? The three of you?
Ryle
A chap called Ryle, a chap called Smith, and a chap called [Bruce] Elsmore as far as I recall. No, we had an assistant. What was his name?
Sullivan
Well, I’m asking just in order to get a feel for what the operational style was. You had an idea that you wanted to do something like this and basically the three of you just went out and built the thing?
Ryle
Yes, virtually. There was quite a pile of angle iron needed so we ordered that from down the road and got into (? 15:11) because you didn’t have a power drill. And then we put them into the ground and poured concrete around them. We put the guide poles up. We got ahold of some insulators from an old German radar, posts with dielectric tops to mount the dipoles on.
Sullivan
Now this paper was submitted in August ’50 but the observations were carried out when? Over the previous year or two years?
Ryle
Oh no, it wouldn’t have been that long. I can’t remember when that was built exactly.
Sullivan
Since 1948 May is referring to the first array you had for radio sources.
Ryle
Well yes, but we also did measure the spectrum. We build simple instruments on these three wavelengths but those are much smaller than this. This was a thing with whatever it was, 29 meters.
Sullivan
What I was wondering was about how long did it take to do this survey?
Ryle
It was done in one night you see.
Sullivan
Well in fact is that what you did?
Ryle
Well not in fact. The observations because it covers the whole sky (? 16:39) declination strips. The point really was that the cheapest way of getting gain was to make it long and thin and cover a declination range to avoid the need for rotating it. And as soon as this was evident that we could measure declination of the source from the period there was no need to have resolving power this way. And therefore, (? 17:00) it was the best thing to do to get -
Sullivan
So in fact the observations were done in a very short time is what you are saying.
Ryle
Oh yes. I don’t imagine the observations took very long.
Sullivan
And you came out with 50 radio sources. I might ask you how many might you have expected? Did you have any idea? Of course, [John G.] Bolton you knew he had six sources. So you knew there were six at his sensitivity level.
Ryle
Yes, well one didn’t expect anything. One didn’t know what was going to happen. I mean clearly you were going to get some. But I think one knew enough about things that suggested if you got two or six or whatever up to distance A, if you go four times weaker in flux you should be able to get whatever it is, 9 times as many sources. That sort of sum we could do.
Sullivan
Well, that still is a presumption, though, that things are continuing out in an isotropic manner.
Ryle
Yes. Of course the question whether it was isotropic or not was also... I think fairy soon after this we were also thinking that the galactic background was these things, in which case, of course you wouldn’t expect to see any anisotropy. For a long time, there is an awful lot of collective background with these things.
Sullivan
It takes an awful lot of stars.
Ryle
Yes, and we did some sums on what the space density would have to be to...
Sullivan
And what flux density you’d have to get down to before you could detect an anisotropy.
Ryle
Yes, that would have come out as well.
Sullivan
And you weren’t expecting it at all here.
Ryle
No, no. I think probably the point was the identification that the Australians had made, the lack of the identification of the brightest two which required a lot stronger, of course, than the ones that they had identified, was one mystery. But clearly if some had been identified then you wanted to see what would happen if you looked at more.
Sullivan
Now you are talking about radio stars, and yet in the paper you make quite a bit of discussion over coincidences with galaxies, so you must have been also thinking along that line.
Ryle
Yes.
Sullivan
How did that come about?
Ryle
Well I mean of course the Australian had got M87 as one of them, and as I said I don’t know seriously how we were convinced by it, the accurate positions that were then available.
Sullivan
And Centaurus A also.
Ryle
And Centaurus A, yes. That was one of the earliest -
Sullivan
The three of them came together.
Ryle
They did. Yes, that’s right. And clearly one obviously wants to get as many as one can and see optically what you can find at their positions. At that time it was two extragalactic and one galactic so...
Sullivan
So you really were rather open even though these were being called radio stars at this stage.
Ryle
I don’t think we had the connotation of within the Galaxy for the word star so much as here was a thing which was a bright point source distinct from the background radiation. And I think people have read too much into the use of the word star in that context. I mean looking at the radio sky you’ve got the great Milky Way and you’ve got hot points of light in it. And I think it was just the analogy with optical stars, one called them stars this early. Although there was this other paper, I forget when it was, where we did a sort of limits of row?). Was it ...
Sullivan
I don’t think so. That was... maybe it was... [shuffling of papers]
Ryle
No, because there was graph. Maybe it was in...
Sullivan
The 2C?
Ryle
No. In the -
Sullivan
The 1950 review.
Ryle
That’s the thing I’m thinking of. (? 20:55) should give a straight line if they are uniformly distributed without any falling off or anything. And then I think from this we said how near they’d have to be if you were going to explain the background with them.
Sullivan
Right. That’s rather interesting because I noticed that Edge’s thesis, the 3C survey, in 1959 is also called a survey of radio stars. And what you are saying is that there is no meaning in the interpretation of that except radio source? It really means nothing different?
Ryle
I think it’s meant to imply a compact radio source as opposed to an extended Milky Way.
Sullivan
But it seems confusing to me that this terminology was kept along, if that were the case because there were, of course, these competing ideas about whether they really were stars or not. And why was there not a neutral term? I guess eventually -
Ryle
Well there was, but I don’t think at this time this was really developed, this conflict of views.
Sullivan
Well by Edge’s, by ’59 -
Ryle
By ’59?
Sullivan
Edge’s thesis I’m talking about. It’s also titled radio stars. I was surprised to see this.
Ryle
We didn’t call 3C radio stars, did we?
Sullivan
Not in the paper. But his thesis was titled -
Ryle
Well what he writes means no difference to me. No, I think we did call it sources by the time of the 3C.
Sullivan
I’m not sure but I think that’s probably right.
Ryle
But I don’t believe, if he called his thesis that, I don’t believe he did it for any particular...
Sullivan
I asked him that and as a matter of fact, he said he didn’t. Let’s see, about the 1C survey, is there anything else that I wanted to... Well, a general question which I want to ask is that the 1C begins a long chain, which is still continuing. We’d heard at this conference here about the 6C. It just keeps on going. When you were developing this and saying, "Well, let’s do a survey of the sky," did you realize that you would just be wanting to go to fainter and fainter levels assuming that you got some sources here?
Ryle
It was obviously dependent on what came out of this. It also depended on whether anybody was going to give us any money. Because up till not we hadn’t had any money. I mean literally it was hundreds of dollars, our annual budget. And all of this was done on that sort of money because we’d got bits of ex-radar receiver, we’d got Würzburgs free. We had to mount them but we got the structures free. Most of it, lots of the electronic components were free. Obviously before we went on to the next stage, which was in fact the 2C survey mixing in the paraboloid structures, there was going to be a lot of money involved. At this stage there was no knowing whether that sort of money, which clearly was not coming from university funds, would ever be available. And, in fact, as it turned out it depends on making a good case from what you’d done with your hundreds of dollars. And we did eventually get whatever it was, 11,000 bucks or something, to build the telesopes that made first 2C and 3C surveys.
Sullivan
Was this sort of a breakthrough, you’re saying, in funding in Britain, to get this amount of money? There were not precedents for...
Ryle
Well there wasn’t any money at all. There wasn’t any money, full stop. But I forget what time it was when, what is called the Science (? 24:35) now. It was called the Department of Scientific and Industrial Research. They started getting funds for the development of university research.
Sullivan
It must have been the early ‘50s or mid ‘50s.
Ryle
Yes, thereabouts. Early ‘50s I suppose. But you didn’t have great ambitions early on. You did with what you had.
Sullivan
Ok, well that was the attitude then but was there any time later on when the 2C results were beginning to come in or the 3C that you began to see sort of a 10, 20 year plan? Looking back at your career one can make a nice case that it all develops very logically, et cetera. Is it fair to say that you saw that beforehand?
Ryle
I think perhaps the greatest discontinuity, as it were, was the identification of Cygnus A because that showed that we were in the cosmology game. As Geoff Burbidge says and will continue to say, "Some people’s minds are shut." But that I know to me was the point where one said, "Well, this is now something much more interesting than it might have been. It’s much more interesting than them being galactic objects and much more interesting than them being M87s." Here is something, even with the little instruments of that stage, was quite likely seeing that things like Cygnus existed, as well as Cygnus. Quite like seeing things as far as the 200 inch. It was a crummy little instrument, costing 200 bucks you see. That was a very important thought and from then on, certainly my own involvement in the cosmology game started in 1950, ’51, which was part of [Rudolph] Minkowski’s 200 inch measurements...
Sullivan
Right, ’51, yeah.
Ryle
...based on the two Würzburg interferometer positions that Graham Smith got, which were the first ones to get down on a fraction of a minute of arc.
Sullivan
So you think at that stage it became clear to you that here was something that could really go -
Ryle
It went right out into extragalactic space in a way which nobody could have foreseen until that observation, [Walter] Baade and Minkowski’s.
Sullivan
Then you mentioned that the funding for the 2C was the first time you entered the big time as far as funding went so to speak.
Ryle
Well, it was the first time we got money outside the small university Cavendish grant, yes.
Sullivan
But I don’t think you still answered whether you could sort of see in your mind’s eye over the next 10-15 years that you would be continually pushing to fainter and fainter levels and more and more sources and so forth.
Ryle
I don’t think one thought like that. I think it was clearly important to get more sources than this could see and get them at better precision, because here was something which the further you could go was back in time.
Sullivan
So each decision was made only made on the basis of the previous few years’ experience for the next few years?
Ryle
I think clearly so, because you didn’t know what distance the next instrument after this of this was going to show. And until you’d got that, you clearly couldn’t design another instrument. You needed to know what one wanted to do. And this affects things like sensitivity versus resolving power and all these things. You hadn’t a clue how to design the instrument, the hard fact of what frequency to work on or anything.
Sullivan
Ok. Well I think we better get some lunch.
[Break for lunch]
Sullivan
So continuing a couple hours later. I see at the August ’50 RAS meeting that Bolton, well that's independent, but Bolton did talk about Australian results. But you said some of the fluctuations of the radio sources were ionospheric. It’s still not clear if some of them might be intrinsic?
Ryle
Well that was referring to these things, which we were -
Sullivan
These are night time bursts?
Ryle
Yes, and they didn’t seem to be something that could only be interpreted in terms of intrinsic.
Sullivan
And we’ve already mentioned your paper with Tony Hewish in Monthly Notices in 1950 on the -
Ryle
Yeah, that was the real proof that it was ionospheric.
Sullivan
And you also measured an actual jitter in the positions of the sources, which I think may have been the first time that was done.
Ryle
Yes, I think...
Sullivan
2 or 3 arc minutes.
Ryle
Yes, yes.
Sullivan
Was that done with the two Würzburg dishes?
Ryle
No I think not. I think that would have been working on 210 MHz. It would have been done at 80, I suspect, or even 38, with a small array which we had.
Sullivan
Well one thing I did want to ask you about, in this article according to abstract, I have actually looked at the article, you talk about interstellar matter being accreted by the Sun may be actually what is disturbing the ionosphere as opposed to what we would now call the solar wind, solar particles. I was wondering why you were thinking that way? What was the objection to solar wind?
Ryle
There wasn’t a solar wind at that time. People had talked quite a lot about the accretion of interstellar matter by the Sun as a form of supply of energy to the corona. I think origin of this and probably that was sort of in full flight at the time. This is [Hermann] Bondi I think. And I think the feeling was that you measure 1,000,000 degrees coronal temperature, which, of course, about the hydrostatic value you’d have to have for free escape or free fall in. It’s the same naturally. Therefore the coronal temperature distribution tells you no way which the matter is going or the energy is going. And I believe that’s probably why the reason for that remark.
Sullivan
So in other words the source of the heating for the corona was no understood and this was one possibility that was being talked about at that time?
Ryle
Yes.
Sullivan
Your review paper in Reports in Progress in Physics in 1950 is really the first major review of radio astronomy. I wanted to ask you as you wrote that, if you can try to remember back 25 years, did things seem to be coming together in any sense or did it just become more confusing as sat down?
Ryle
No, I think obviously there were very few problems solved but it was about the right time to write it I think. There were a number of fields becoming apparent that were obviously going to be interesting, which obviously the most interesting at the time was the nature of the radio sources. But I think it was an important time to put things together.
Sullivan
In what sense? Do you think?
Ryle
Enough observations had been made to be able to say something worthwhile about them. Quite a lot of groups had got going by then; quite a lot of techniques had been developed. And the overall picture was becoming interesting. There was clearly a future in this game and it worth giving a title, radio astronomy, which I think I coined.
Sullivan
And it was also worthwhile to sit down and say, "Now where do we go from here?" You were obviously going to stay in the game.
Ryle
Yes, yes.
Sullivan
Something that you did not publish anything with, I don’t think, but nevertheless were involved with, of course, were the two Würzburg dishes that Graham Smith used. I don’t think you published anything relevant to that.
Ryle
No, that was primarily his work. It was built specifically for, we want to push it to a higher frequency both because of the evident satisfactoriness of ionosphere at 80 Mhz where the regional array was, and because its electrical center was much better defined. You see in this measurement of declination by the periodicity of the interference pattern you are required to know the spacing. Now a phased array of dipoles in phase can have imperfections where the electrical center is displaced along it. And that comes in first order as an error in declination. So (A) the higher frequency gets us to the ionosphere and (B) something that had a much more definite electrical center was the right thing. We had these variable dishes available to us and this seemed the right way of going about making, well, the first astrometric instrument.
Sullivan
Right. Now was this phase switched? I can’t remember.
Ryle
Oh, yes.
Sullivan
That was the first instrument to be so.
Ryle
No, I think this long Michaelson thing was -
Sullivan
That was also? Ok, yes.
Ryle
Well, phase switching was written up when, 19...
Sullivan
Yes, that was ’52.
Ryle
That was sort of a Royal Society write-up, so it was in use sometime before then I think. This certainly didn’t start with phase switching because, you see, this record is not a phase switched one. But I’m pretty sure by the time we built the 210 MHz thing -
Sullivan
Right, alternatively in phase and in anti-phase, right. Can you tell me about how that idea came about?
Ryle
Well basically I think the point was that we realized that you are trying to measure something small in the presence of something large of identical characteristics electrically. And you have just got to do something that changes the thing you want and doesn't change anything you don't want or vice versa. The switching gets a noise source with one such thing where whatever you did didn’t affect the receiver noise. Now therefore receiver noise could be eliminated apart from all the ordinary noise it produces. The next part which became evident from the picture in there is that if you are going to get sources that are pretty weak then obviously you are limited by the thing not overloading the system and that limits what you can see on this record. So if you can get rid of that as well, and one clear way is to switch the alternative in and out of phase which will not affect anything which is large compared with the...
Sullivan
Constant off set is what you are referring to, right.
Ryle
Well, it’s the gains of things as well you see. Before we had the first order receiver gain and now we’ve got all the gain of the post-detector thing still present as a deflection, which can vary by small amounts because it isn’t all that much gain but it is still an important variation. But the real thing gets rid of the thing you don’t want right at the beginning. Therefore it was a fairly easily modification to make the switch to put in a halfway phase between the two and cut the noise diode.
Sullivan
Really one could say that the original Michaelson interferometer back in ’46 was the same sort of reasoning that you were trying to detect something small compared to something large.
Ryle
Yes it was. We knew that the Sun radiation was going to be very small compared with the sunspot radiation. We didn’t know how small. In fact, it would have been quite difficult to detect with the telescope we had, you know, only a few dipoles each.
Sullivan
There is a story about the acquisition of the Würzburg dishes which I’ve heard in the hallway so to speak. I’d like to hear from the horse’s mouth, so to speak. Could you tell me that story?
Ryle
Well, I knew these things were brought over at the end of the War and went down the Royal Aircraft establishment to be assessed. One came from Holland, I forget where the other one came from. And we knew of the existence of these, so we got an agreement that we could have then. So we went down to arrange for their transport but found that they’d unfortunately just been sold to a scrap metal merchant, which was sad. But we drove around to see the scrap metal merchant who was a very nice chap, he was a very nice guy. He asked what we wanted it for. We said for scientific research. He said, "You can have them. I like science." So then we were in a bit of a fix because gifts to the University of Cambridge I think have to be recited to the Senate house in Latin. We were quite sure how we would translate all this stuff about German radar sets. However the problem was solved, because we swapped him for a big German trailer which we’d also picked up and some other stuff. We didn’t want the trailer. So we all parted happily. He gave us the two.
Sullivan
I see. Did you finally have to go through the procedures at the University?
Ryle
No, because it was a straight exchange, you see. We’d given him the trailer so that solved this problem.
Sullivan
I see. Ok, now having the 1C survey and seeing that it worked, it was obvious, you already mentioned, that you had to get some outside funding, could you tell me about how that proceeded?
Ryle
Well, here Ratcliffe was an enormous help in making a strong case for us. He, I think, had really realized probably, though he wasn’t in the game, that this was going to be an important field and so he helped make our case to the Department of Scientific and Industrial Research it was then called, now it’s the (SRC?). And we put up the case presenting the evidence that we’d got so far on these things and, well, of course before we put the case up we’d designed an instrument which had considerably greater collective power and element resolving power. And we’d decided to go up to a four element system because it was a large area of possibilities for using interferometric techniques in various ways by cross connecting them in a phase switch to make a phase difference and relative amplitude of all four connected in different phasing. There were various possibilities of getting accurate declinations, which clearly the periodicity method was not satisfactory as it fell off at low decs. So we designed that in some detail with a local engineering firm, a maker of electric circuits very cheaply by stretching wires across parabolic frames, which parabolic frames are virtually two dimensional structures with stretch wire filling up the third dimension. And this has very low windage. Stretch wires are very much better than mesh, but you only needed stretch wires for one polarization. So it all seemed a very practical way of making it. So we got the design fairly well developed and costed and put in this application for, I think, it was 6,400 pounds, $14-15,000 at the time. And that was granted, and we went out and built that.
Sullivan
And how long did it take to build?
Ryle
It took about a year to build.
Sullivan
And having built it you began what became the 2C survey.
Ryle
Well, the notorious 2C survey was what came out of it first, and what we did wrong was to try to analyze it too deep. And as a result the [overlapping?] abilities gave too many weak sources. But I think that I’d like to make it quite clear that we presented the case that the distribution of sources in depth didn’t seem compatible with the uniform distribution, we were sufficiently worried about the analysis of these faint sources that we invented this alternative method, the P(D) analysis, which was at the time, as it still is, entirely independent of this overlapping image problem. It's dependent, it has to be realized, on the noise of the system, which is quite small. But it did give an absolutely unambiguous answer. The source distribution was not, in fact, obeying a 3/2 power law. And the first time we ever made any public statement, publication, about the distribution sources, we always said that we had confirmed this effect by the statistic method of analysis. Well unfortunately nobody understood the method, if they even read the papers, which they probably didn’t. And as a consequence, 2C is mud and the conclusions from 2C are also mud. And that was not, in fact, really fair because this other method was put forward right from the beginning. We never put the source counts from 2C alone as an argument that there was something funny happening.
Sullivan
But I think it would be fair to say, though, that if one looks at those papers now that the main emphasis was logN-logS and P(D) was sort of back it up rather than the other way around.
Ryle
No, the Haley lecture which was the first time this was talk about, that the two are presented as alternative ways of doing it.
Sullivan
I see. Why do you think it was that people were reluctant to look into P(D)? Or did they look into it and have objections to it?
Ryle
It’s fairly complicated. I mean it’s just a fairly difficult statistical problem. You can do it just by a crude Monte Carlo type procedure, which is tedious. Or you can do it analytically, which Peter Scheuer eventually did and proved it all precisely. And then Tony Hewish actually applied it in much more detail than had been done before. Let me just give the paper... That was the source counts themselves and that’s the other thing.
Sullivan
So looking at the Haley Lecture now.
Ryle
Here is the probable distribution of amplitude D over recorded phase. [looking at paper] And these are the experimental curve, the hatched bit, and these are experiment curves with spatial density. "The ordinary (? 43:00) instrument might give us an apparent explanation of sources."
Sullivan
I’ve noted here that you did not explicitly mention the concept of confusion and so forth. You must have been aware of it.
Ryle
We certainly were or we would not have invented a system.
Sullivan
Right, that was the motivation for the P(D)?
Ryle
Well, it doesn’t actually explicitly say that, does it?
Sullivan
Right.
Ryle
Well the other paper was not far behind actually.
Sullivan
The Bakerian Lecture? Or you mean the Scheuer...?
Ryle
No it wasn’t that one.
Sullivan
Was it Ryle and Scheuer that you are thinking of?
Ryle
What was that called?
Sullivan
That was the one that interpreted 2C in Proceedings of the Royal Society. I don’t have the title here. I don’t think I have that one. In ’55, the same time as the... Here it is. I think that is the one you mean.
End of Tape 59B
Begin Tape 60A
Ryle
…number of sources. (? 00:05) for the weaker sources. It’s actually confusion (? 00:08) decrease the number of sources found. It there shown that the difference (? 00:14) that distance greater than that that the results could be observed individually.
Sullivan
So it is mentioned explicitly in Ryle and Scheuer but it doesn’t look like it is in the….
Ryle
Well, it’s the same year you see, the same time.
Sullivan
That’s true. It is the same time.
Ryle
It was a public lecture therefore you can’t go into as much detail as you can in…
Sullivan
Yes, that’s true.
Ryle
But that was May the 6th and that was…
Sullivan
A couple of months before, ok.
Ryle
It was all understood at the time and then here was the curve for the range of predicted curves on the uniform special distribution and the observed one.
Sullivan
So P(D) then was an answer to the confusion problem which you were realizing was a serious one even when you were analyzing your 2000 sources.
Ryle
Yes, that’s what I was saying. We and [Bernard Y.] Mills and Bolton had used the counting of sources but…it was paper two, but anyway it was before that actual… well, they were kind of the same work. They were written up at the same time.
Sullivan
Now as you were designing the instrument however, you apparently were not aware you were going to get some many sources or else you would have designed it differently. Is that a fair statement?
Ryle
Of course we didn’t have any idea how many sources we were going to get. I mean we could extrapolate from the 1C data on the assumption of uniform distribution, which I think is was we did as far as I remember. But the point is that we didn’t only intend to use it on 18 MHz. If we were going to higher frequencies then the situation was change. We would get a rapid increase in resolving power and a decrease in sensitivity. Of course, receivers at that time fell off fairly dramatically with increasing frequency. Well, perhaps not quite at that frequency because galactic background… well, away from the galactic plane that is true that they were quite a lot worse. And the source flux we knew was falling off anyway. So it wasn’t as it were overdesigned because signal to noise issue as we were going to go to higher frequencies as far as we knew of course.
Sullivan
Right. Now why did you chose to do the low frequency first?
Ryle
Just because it was easier, fewer dipoles to make. You got to make a line of dipoles along each…
Sullivan
So it wasn’t for scientific reasons then?
Ryle
No, we’d done the earlier work at 18 and everything had worked there without many problems. It was obviously going to be more difficult to go to higher frequencies, cable length and everything else, amplifiers, front ends and things.
Sullivan
Now you mentioned before that once you had the identification of Cygnus A then it became clear that this was really good stuff, that you could really probe distant parts of the universe.
Ryle
No, we didn’t know that yet. All I said was that if there were other sources like Cygnus, you could see them at distances of cosmological (? 02:56). Now, of course, Cygnus type sources need only represent 1% of total source population. We didn’t know that.
Sullivan
That would be unlikely perhaps.
Ryle
Well, I don’t know. I mean the actual numbers of stars and galaxies in the sky optically is a complete fluke. They happen to come out whatever they come out as. They don't know anything because here you have a new sky. You don’t know how many there might be which we invisible optically. So this is just a possibility which had to be demonstrated before you even have the possibility of being in the cosmology game, but you didn’t know that you were until you had found out what most radio sources were.
Sullivan
So in your grant application for the 2C instrument for instance, did you mention this cosmological possibility?
Ryle
I should imagine so, yes.
Sullivan
But you weren’t clear that it was going to be useful for that.
Ryle
Well when did we build the instrument? Well, this was ’55, wasn’t it?
Sullivan
’51 was the identification of Cygnus.
Ryle
Yes. I’m just wondering when we did in fact build the 2C, 3C aerial. Well anyway, I’m sure it would have been after 1951, I guess, and therefore we certainly would have mentioned the possibility, but only the possibility, until one knows what sources...
Sullivan
And it was only when the hundreds of sources began rolling in you said, "Wow we can really…"
Ryle
No, because even that doesn’t tell you whether they are galactic or extragalactic or whether they are near or far, you see. It awaited the analysis that the Bakerian Lecture did to really know what radio sources are. [Note added in 2015: The Bakerian Lecture is a prize lecture in physical sciences given annually at the Royal Society in London since 1775. Ryle's 1958 lecture was on The Nature of the Cosmic Radio Sources." Proc. Roy. Soc. London Ser. A, 248, #1254 (Nov. 25, 1958), pp. 289-308.]
Sullivan
But already here you were talking about cosmology in 1955, three years earlier.
Ryle
Well, one year earlier, I think. Wasn’t it?
Sullivan
No, ’58 is the Bakerian Lecture.
Ryle
Perhaps it was, yes. Yeah, that’s probably fair but the point is having got a strange result of this source and having known of the existence of Cygnus it is evident that one possible interpretation is that you really are seeing first order cosmology with most of the sources being things like Cygnus. Then of course the whole heavens dropped and everybody said, “You know they must be stars. You said they were stars yourself three years ago. What are you talking about now? You’re obviously wrong whatever you say.” And then all the difficulties began. Yes, I remember now. One did, in fact, say, "Right, we will go back to square one and prove it step by step," which took 15 years. The Bakerian Lecture was the first of these steps, which said step by step they’ve got to be mainly extragalactic and they’ve got to be mainly powerful extragalactic sources. And that’s what Geoff Burbidge didn’t understand this morning.
Sullivan
In 1976.
Ryle
Goldstein didn’t understand it. I spent five minutes just before that thing, “That’s very good. I’ve got a copy. Read it.” Nobody’s read it and the point is that nobody read that one either, because they were both printed in the Royal Society.
Sullivan
Now, did you have any predilection towards cosmology or anything like that, or did you just say, well, here’s something that we can do with the radio data that we have, and if it had been something else you would have went into that. Or had you always been interested in cosmology?
Ryle
No, I think it’s part of astronomy like anything else and obviously I think in a sense the radio astronomer was always an opportunist. He went ahead with was seemed like possible. And if you have an instrument that could do things you used it in all the things that it would do. And you don’t try to do things that it won’t do. Because this is a lot of trouble with some people. They try to use their instruments for the wrong purposes. And incidentally it is perhaps one of the differences we’ve always had here compared to some other places. We have tended to build instruments on purpose for specific programs. We haven’t gone for the all-purpose, all singing, all dancing giant paraboloid which costs the Earth and doesn’t do any of the jobs very as well. That’s slightly unfair, I agree. But it was true in the 1950s of construction of telescopes that the giant paraboloid was going to be "the answer." Now if money was no object that was quite a reasonable bet. But money certainly was an object in this country and it was much more sensible spending 6,000 quid doing this than 600,000 building a 100 meter dish.
Sullivan
Although I think you would have to admit that now with the 5 kilometer, that’s a pretty general purpose thing now.
Ryle
It’s a very special instrument to produce higher resolving power. I mean it is very special in that context but I mean everyone is trying to build higher resolving power telescopes.
Sullivan
But it can’t just do one survey and then you close shop with it such as with the 2C and 3C aerial.
Ryle
But this was a thing which wasn’t designed to look at radio galaxies, it was designed to produce higher resolving power. It wasn’t something that could do high resolving power or detect Sputniks or measuring the brightness of the 3 degree background, that sort of generalization I feel. We’ve always reckoned that you could build far cheaper a range of instrument to tackle different things than one instrument to do them all because it was always a compromise. Engineering is always a compromise. And you end up build a vastly more expensive instrument that doesn’t do anything quite as well as a specialist instrument at a fraction of the price. However that’s an irrelevant leap.
Sullivan
No, that‘s not irrelevant at all. Did you have any idea, I can’t refrain from asking you, the hornets’ nest you go into when you got into the cosmology game.
Ryle
No, that was a considerable shock actually because, of course, the trouble of cosmology up until then was that it had been a playground of mathematicians. Did space curve this way or that way and all these things. It had nothing to do with the real world very much. And observations had never, and apparently would never, make any effect on it. It was a game which mathematicians could play safe from all possible attack. And the development of the steady state model was a breakthrough, an important breakthrough, in that here was something that made specific predictions in a wide range of not necessarily thought of possible measurements. It said that the universe was in a state which would remain the same through time as well as space. And that was a completely general thing which would allow one in any future observation to test it. Now here straight away was an observation which, as soon as you know that you’ve got Cygnuses, could be applied because straight away it predicts something quite definite. As soon as you know can detect sources at redshifts large enough for things to happen on other cosmologies then you can detect a difference. Now, in fact the difference we found was considerably larger than the various geometrically models, none of which differ very much from the steady state with ordinary sort of sources. And it implied, in fact, that one had to have source evolution as well at a very dramatic rate which meant that it was this sort of thing that you were going to be investigating rather than the subtle differences in space curvature. So I think when we made this very tentative proposal here it was remarkable what an absolute storm it provoked. Well, of course, it wasn’t helped by the fact that the press got hold of the story.
Sullivan
I’ve heard about this headline at the RAS. Was is “Cambridge Professor Says World is Flat,” or something? No, that was Sputnik. It was about the same time.
Ryle
It was the same sort of thing. You got it with a radio telescope. It was very significant. But, you know, one felt that having put forward a cosmological model which was unique in allowing itself to be tested, they should have at least had the grace to admit at least that here was an interesting test that they would like to look at carefully rather than the explosion with occurred. Well, anyway when that did happen, of course, then we said, “Right, we won’t say another word until we’ve proved it step by step by step.” And the Bakerian Lecture was the first of these steps.
Sullivan
And the 3C survey I suppose…
Ryle
The 3C survey, that followed straight after. We just changed the frequency in these four element machines. And the 3C survey was by and large not much different from 3CR. There were some extended sources missing which were found with… The 3CR was in fact, as you probably know, based on the first non-(Cygnus observations? 11:42) with the 4C telescope and included (? 11:57) measurements as well as interferometer ones to get over the problem of the possibility of being an important population of extended sources.
Sullivan
It seems to me in trying to analyze the whole 2C, Mills, Ryle, and everything else controversy, Hoyle, that there was a great confusion of issues involved here. That you one instrumental technique versus another. You had one cosmology versus another. You had whether P(D) was valid. You had the influence of extended sources and how much that might be changing things. People were talking at cross purposes often. Do you agree with that, or do you think the (?) were very clear?
Ryle
No, it wasn’t all that clear. I mean obviously if there were a significant number of extended sources at the 3C stage, before 3CR, before the 4C was built, the sensitivity for extended sources was quite a lot less. On the other hand, and this, in fact, turns out to be quite important, the Mills Cross observations undoubtedly included sources which were in fact fluctuations of the galactic background, which shouldn’t have been counted as sources at all. And that can have equally an effect in the opposite direction, which, in fact, was rather more serious I think. So that was one thing, which as you said was a direct outcome of the different technologies which we invented. We at some phase had built this technique to make this clear distinction between what was galactic background and what was source but obviously the boundary here is marginal. There are structures which get finer and finer in the Galaxy and as they get weak enough that by the time they got so fine that there is no confusion with the real sources. So that was certainly one difficulty. I think the side lobes of the Mills Cross were also more of a problem than they realized at the beginning. Over-analysis of the 2C was obviously a disastrous fault. But the P(D) did try to avoid that. But that in a way wasn’t so serious. Obviously here was something that had to be cleared up, the difference in radio observations. It was really the sort of theory of the steady state cosmology and not wanting to believe proof. Ok, it is fair enough that it took some years. They obviously wanted to explore all possibilities. And then this game of on the one hand step by step confirmation of what we thought would be there was in fact there. And on the other hand investigating each crazy level of which the steady state model was distorted to.
Sullivan
Right, I was talking with Paul Scott. He has two papers. I don’t think you are coauthors. One answered [Dennis W.] Sciama’s and one answering [Robert] Hanbury Brown.
Ryle
That’s right. I mean it went on and on and on. By the end the steady state model was so ridiculous that it was more complicated than the evolutionary model…
Sullivan
It had lost its original beauty.
Ryle
Yes. There was no point in it anymore it seems to me.
Sullivan
Now you said a while ago that beginning this step by step approach that just recently I think you implied that you have more or less come to your goal of really establishing the result, and I’m not sure exactly which observation you think has finally done it.
Ryle
No, not recently. I’m talking about the stage when we’d done that…
Sullivan
The Bakerian Lecture…
Ryle
…which meant that any explanation in terms of little local irregularities had to extend to about (30=1? 15:30) and we had to be bang in the center of it to about 1% of volume which would allow itself, which you can’t disprove but it is no longer a sensible cosmology, because you are in a special place then. And that had to be the answer Scheuer wrote down.
Sullivan
It’s not a good post-Copernican cosmology…
Ryle
No, exactly. And I think it was Peter Scheuer wrote a paper that went into all of that and showed how stupid it was. Then, of course, there was tidying up the logN-logS story. Another thing, of course, which went wrong was everybody always said that the discrepancy between (? 16:05) and the predicted 1.5 was so small that I can cover it by doing this and this, adding a few extended sources. But, of course, it was never 1.5. It’s more like 1.3 over the relevant range if what one is talking about has any sense. I’m talking simply about the velocity of the source. You know what distances you are talking about and that there is a considerable drop off any model, steady state included. And the discrepancy was therefore considerably worse. And I think Bernie Mills has never understood that.
Sullivan
That steady state really…
Ryle
No, it doesn’t need a 1.5. Already with powers of the order which you must be having there are considerable spectral fluxed emissions, flux that is going down a lot faster than Euclidian. It is curling off quite rapidly over this flux range and therefore the discrepancy of this line and that line goes between that line and this line. And that was quite a big difference which I think Mills could never appreciate, certainly in those days.
Sullivan
Didn’t the bandwidth, doesn’t that also make the slope less or am I thinking wrong? The fact that the bandwidth as you go to higher red shifts…
Ryle
That’s all a part of it. It’s the energy per unit bandwidth at the receiver which you are measuring. And that’s a part of these curls. Yes, that’s right. But there are several other things which come is as well. The volume of space is limited by red shifts and things.
Sullivan
When would say this sort of became resolved?
Ryle
Well then of course 4C came along, which of course went a lot further, showed the same sort of thing. And that had a pencil beam sort of thing to look for extended sources. And then there was a P(D) on that which Tony Hewish did, which said the same thing. And then finally there was this very small area of sky which Ann [A. C.] Neville and I did at the North Pole, which was simply trying out the synthesis principle. Which we used bits of the 4C aerial as a variable spacing thing and mapped this little area around the North Pole in an absolutely unambiguous, unarguable way because here was a pencil beam method you see. A good old, proper method not an indirect method. A pencil beam method that you could draw on a map and see things on it, without any argument. It then became very hard for people to say, “They are all blends,” and the resolving power is very high indeed. You know, 25 beam widths to source.
Sullivan
So that was ’65 I think or so, that North Polar survey? Is that right?
Ryle
No it was before then was then because 1 mile was built by ’62, ’63, ’64. It must have been ’61.
Sullivan
I don’t have any publications after ’60 here.
Ryle
I see. As I said it was probably ’61. [opens drawer]
Sullivan
So Ryle and Neville was ’62. So really you would say seven years before this got resolved from the time of the first presentation of the 2C survey.
Ryle
Well I think after the P(D) on the 4C survey data…
Sullivan
Which is Hewish in ’61.
Ryle
Yes, that’s right. Which was more or less the same time as the, we thought the definitive paper on all this, Clark and myself which was talking about P(D) as above a gain and all this stuff again about isotropy…
Sullivan
This is Ryle and Clark…
Ryle
In ’61 January. Doing this sort of thing which was done…. The luminosity factor has to derive from the (? 20:00). It’s got to get up at least that much or more if you take the identifying sample of sources and things like that.
Sullivan
It’s much steeper.
Ryle
And the difference between that observed and these theoretical ones which you are talking about, not that on a straight line which you can draw through there or something.
Sullivan
This is figure six of the Ryle and Clark paper. Right. I must admit I’ll have to look at that.
Ryle
And that coupled with this one here, which actually took it a bit further with more sky and more everything else starting putting the limits in the cut off here.
Sullivan
In the Hewish paper now.
Ryle
Yes. This is saying specifically that as well as having high initial slope it’s got to converge rapidly from the P(D) method. Now the North Pole survey was important in confirming that by a method that nobody could not understand. It’s rather a nice method.
Sullivan
Yes, I’ve seen that.
Ryle
That was also a little later. And, then of course, fairly soon after that… well, no, two years after that the 5C survey came along, which sort of cinched the whole thing. But about that time, ’62, ’63 I suppose I gave a final account of this I think to an RAS meeting. I said that as far as I was concerned that this was a course that we’ve been taking for the last ten years or so. And it had been a stormy one but we’ve gotten as far as it could get. And by and large it hasn’t of course changed very much since then.
Sullivan
You’ve brought up aperture synthesis which of course I want to talk about. Can you tell me when you first began thinking about the possibilities of aperture synthesis and its various types?
Ryle
Well that goes back a very long time actually. Of course, the early way you understand to get the distribution of the quiet Sun. I mean having done this work on the Sunspots which it is in a sense much easier because it is so strong, we then set about this program first of all with Harold Stanier, later with Ken [Kenneth E.] Machin and (? Brown? 22:13), a series of papers in which we attempted to discover the brightness distribution across the Sun at different wavelengths as a means of getting the temperature distribution in the corona, which, of course, was interesting back to what I was talking about just now, where the energy is coming from. And I think it was recognized that it was free-free emission from the quiet Sun if not the other one. Therefore it tells you an important thing about the actually physical situation at various heights in the corona.
Sullivan
And there was a controversy about limb brightening and so forth.
Ryle
That’s right. Well, you’ve got to have a lot of resolving power for that and this did in fact show up. But at that time the measurements we made did not have phase stability, because of the difficulty with quite big distances and what at that time quite high frequencies. And cabling everything that wasn’t good enough, nor indeed was the location of the telescopes, we had to move them. And therefore, which is perfectly legitimate for the quiet Sun, we assumed symmetry of the source. Initially it was just east-west distributions. Subsequently we extended this to all around, and we in fact used the same technique which is used in synthesis, we observed at different hour angles. We were limited at that time by the piece of land which you couldn’t get much action north-south anyway. But we used observations at different hour angles to give us resolution in declination. And went one step further and assimed initially that -- sorry.... The first stage we just assumed had circular symmetry. Clearly you might have polar equatorial symmetry, so we then did north-south resolution by observing across the meridian. But they still assumed east-west symmetry (? 12:30) well and north-south. You don’t have to have a phase in either method. And I suspect the relationship between what an interferometer measures and the Fourier transform of the brightness of the sky was realized by various people. I think the Australians...
Sullivan
The [L. L.] McCready…
Ryle
McCready, [Joseph] Pawsey, [Ruby] Payne-Scott paper…
Sullivan
Is the first time it is mentioned in print.
Ryle
Yes, which I don’t know if they actually used that at that time at all. I can’t recall. But anyway Harold Stanier was the first I think to actually apply it...
Sullivan
With variable spacings.
Ryle
With variable spacings to actually get the distributions.
Sullivan
He certainly was.
Ryle
And that was sort of what went on. But also, we built an instrument aimed at measuring an arbitrary sky. Not just of course nicely the Sun in zero brightness background, but to actually look at an arbitrary piece of sky by using a long east-west syn array like that which we moved north-south from it.
Sullivan
Right. What we call a T aerial now.
Ryle
That’s right. That was done by John Blythe and that was the first, I think, synthesis in the sense that it was synthesizing a true telescope aperture without any preconceived knowledge of the sources that might lie in it. Now it was a proper mapping instrument and that had whatever it is, 38 spaces of something. And it was really quite a big instrument. It was used at 38 MHz where the phase problem and the surveying problem was easy. You could have errors at 6 inches in the position of the Earth and it didn’t matter much. And it did produce whatever it was, a 2 or 2.5 degree beam which we felt quite proud of at the time because, with a few little bits of string, you’d made this huge great telescope, which made quite a nice map in fact. So the analysis of this was very tedious. In fact, it is interesting all through the history of synthesis we’ve been just about at the limit of what computers can do. If they’d come along a few years later we would have been limited by them rather than what we thought we could do instrument wise. But that was the beginning of proper synthesis and it was somewhat related to the points in the phase switch paper showing that you could fill apertures with far less hardware than you would have thought necessary. That paper had an array in a one dimensional case had bits like that one end and then some like that which did in fact together fill up along that.
Sullivan
Yes, you need all the spacings there.
Ryle
That, was, of course, part of the same story. It was showing that you could build a telescope you hadn’t actually got. This was taking it… well, it wasn’t quite the same but it was related to it. But, of course, in this case the analysis was done automatically by the sky going around. In this case you had to do a Fourier transform in a computer.
Sullivan
Were you aware at all of the work that [Jacques] Arsac and I think someone else in France were doing at this time on optimum arrays and that sort of thing?
Ryle
I didn’t think he’d done it in 1952.
Sullivan
Not ’52.
Ryle
Well this is in ’52.
Sullivan
That’s true.
Ryle
And that’s ’54 and I think Arsac was after that.
Sullivan
He was ’55 or so. I think you’re right. Now I wanted to ask, I’ve heard from other people that Ratcliffe’s lectures on Fourier theory were rather influential. Was this true with you also?
Ryle
Well, obviously they were. I think that I’m not very good at mathematics and therefore the full import didn’t hit me. But I’m very good at visualizing what things are physically and I saw no difficulty in making a thing like a big instrument with a precise procedure whereby you mathematically treated the data was a Fourier transform. But I could have done it some other way without understanding. But sure, he clearly had a profound influence on Harold Stanier, for example. I think the whole concept of how you do these things was obviously very important. A Fourier transform is a nice technique for coping with the delta. But I’d always thought of this as a way of filling up the aperture, taking that if I had to have all that aperture it would have to have these distributions of spacing present. And I thought much more here and that’s why in the early lectures I use say how we can build the telescope rather than saying this thing measures 110 to the Fourier transform. That was evident that if I did Fourier inversion I would get the sky out. That just was the way that I was brought up, very mathematically challenged.
Sullivan
Can you tell me when the idea about the Earth rotation came to you?
Ryle
That was about the same time as a matter of fact. It is very interesting that it was so early. ’54 I think.
Sullivan
So we are looking at a notebook called “Future Programs, etc.” When does this notebook date from?
Ryle
No, that’s ’58. This may be volume two. I’m not sure. This is volume two. This is designing the 4C aerial. Background radiation… [flipping through notebooks]
Sullivan
So we're looking at… may I have a copy of that? This is the Nobel Prize lecture and the quote is from June 8, ’54. And this says, "possible research student and other projects." And it has the North Polar survey at 81 MHz, effective resolving power area about 1 million square feet. And then June ’54, it says to "try to do a North Polar survey in all directions where 180 degrees rotation is available. It might be possible by directing aerials in successively different directions, i.e. observation not on meridian." So indeed fully eight years before it was carried out or so you were discussing this possibility.
Ryle
Yes and the actually notebook goes into the design of it. We were going to do it with a Würzburg actually on east-west railway track. We’d had realized that a thing would have difficulty in some declinations in filling all the aperture you need. The point is that we were faced at that time with… let’s see, where we were… The question was whether we were going to get the Lord’s Bridge site. This was still on the old, small site a couple of fields away over here. The Grange road [site]. And that was running out of room. We could have got this instrument on it, just, because we would have used it on 200 MHz or thereabout. And it would have been quite a powerful instrument. The fact that we had the availability of the much bigger area at Lord’s Bridge allowed us to consider other instruments and coupled with the difficulty we foresee of doing the much more sophisticated two-dimensional Fourier inversion which this would have involved. Well, in fact, there wasn’t a computer which would have done it even in 196… when did we say we did the North Pole survey?
Sullivan
’62 it was published.
Ryle
Even that to put that together took, I think, 36 hours of the university computer’s time. And this was one of the biggest computers in the world at the time, EDSAC 2. And that was, well, whatever it was, eight years after we were talking about this instrument. And therefore, I’m sure quite rightly looking back at it, to keep to one-dimensional things until the new telescopes were built at Lord’s Bridge which were the 4C aerial and the big 38 megacycle array were both built on this one-dimensional principle.
Sullivan
Right. We haven’t explicitly talked about Lord’s Bridge. That was once again probably as 1C to 2C and then 2C to Lord’s Bridge.
Ryle
No, 3C was done with the 2C aerial you see. The 3C was done with that. The 3CR was simply tidying it up having got this big instrument built but not yet working as a synthesis instrument.
Sullivan
So 4C was the first major thing done at Lord’s Bridge. Is that correct?
Ryle
Well we observed the sky again not as a synthesis instrument, which made 3CR. 4C was the first big survey done with a large synthesis telescope, yes.
Sullivan
And it was the first major thing done at Lord’s Bridge?
Ryle
Yes. Well, there was two of them actually. The 4C survey and a thing just like this but it was, that way round, but it was much bigger. This was one kilometer or so. And it produced the big 38 megacycle survey, Carman Costain and John Baldwin.
Sullivan
Yeah, I’ve talk to him about that. And the Mullard thing was once again a question of financing where in this case apparently the government couldn’t come through.
Ryle
Well the government was not able to buy land. The university has to acquire land. It will provide scientific equipment. So we were stuck until we could get some funds to get a piece of land and build the basic laboratory equipment, bring electricity to it and all these things. And that’s where the Mullard money was very important.
Sullivan
Just a couple more questions. I’ve made a note here. In the 2C survey paper in 1955 you mention that 100 of the 2,000 sources agree with NGC-IC objects and, of course, most of those turned out not to be right. But what I wanted to ask you is, did you find that very discouraging that still with all these sources there were very few even possible optical identifications?
Ryle
Well I don’t think it was discouraging. In a sense it would have been discouraging if these were the things that we knew all about. I think that nowadays that when one sees optically information on radio objects it is terribly important. But the exciting thing at that time was that here was something quite different.
Sullivan
It just made it more tantalizing, you’re saying.
Ryle
Sure, the fact that they were unidentified when, of course, you knew they in fact were very powerful, the explanation for the poor identification rate was clear. But equally it was that much more exciting.
Sullivan
Ok. Now getting into the late ‘50s, you did a project which I’m not quite sure the purpose of it was. It was Elsmore, Ryle, and [Patricia R. R.] Leslie [-Foster], ’59, in which you had a system of calibration sources at different declinations, 64 radio sources at 179 MHz, and I wanted to ask what the purpose of that was.
Ryle
This was the first astrometric survey, wasn’t it?
Sullivan
Well, I’m not sure. I just looked at the abstract and it said that they were for reference purposes. That was the wording.
Ryle
That’s right. They were to present all over the sky sources which you could calibrate other instrument with. It was done with the 4C telescope. The east-west arm is like that, he’s one rails. And first of all that is on the central position and then, of course, you have this higher resolution interferometer which can fix your positions to whatever it was 10 seconds of arc or something for some sources. Rather than doing a complete synthesis of huge areas of sky, we did a quick program in which you covered all the sources every night virtually, at each of a number of phasing along here. Now looking along that axis you therefore have got a parabola which you are going to move like there, and what you are doing is measuring the phase difference between the signals you received from a given source at transit at many different positions. And therefore that allows you to get the declination with the resolution appropriate to a 1000 foot aperture. And the point observationally was that if you now stick the telescope in that position and in one 24 period observe all these 64 sources, I don’t think you can actually do it as fast as that, I think we had to do it in several nights. But the point is that over a short period of time you observe all those sources then move 180 degrees and do it all over again, the instrument effects all disappear by in large. The phase of the system and even the (? 38:00)) things disappear and you have a coherent system of objects all tied to the same positional astrometric system. So it was just simply to get 64 sources whose positions were really rather accurately known.
Sullivan
And so your recent interest in astrometry is really nothing new either?
Ryle
Oh yes. No, obviously astrometry always was important because the identification was clearly entirely dominated by the poor positional accuracy you get. If you got a series of sources around the sky which you measured by independent means very accurately then if you come to do a survey and, of course, the survey is in strips anyway, if you’ve got a source in each of those strips to calibrate it with you’re alright. You can tie the whole strip down in absolute coordinates, absolute measured by these 64 sources.
Sullivan
So that was used for the 4C survey?
Ryle
Oh, yes.
Sullivan
Well, one final question that I’m pretty interested in. It seems to me that the Cambridge group has been remarkably successful in the people that have been trained here and gone all over the world as well as stayed here later one. And I’m wondering how you are able to pick people? You usually just pick people after they’ve finished three years of undergraduate. Or is it something that happens here? What are your views on that?
Ryle
I hope it is a bit of both. Obviously there is some positive feedback in these situations. Directly one group starts to get some nice results, the best people tend to want to come here. And so you probably get a better selection of people that [try?] to come. So you can use a tighter selection procedure. But I don’t know. Perhaps we train them better.
Sullivan
Well that’s certainly true. You have a definite philosophy in training, which you were telling me about at lunch, namely that they should get their hands dirty and actually see what an aerial is like. But in the selection process I was wondering what it is that you look for particularly?
Ryle
I’ve never very happy about selecting people. I’m never know whether we’ve made the right choice for a year, which I suppose is what everybody finds. I think we’ve probably put more emphasize on the guy’s outlook on life compared with his academic record than some places do. We are interested in knowing whether he goes climbing or gliding or something rather than playing chess. No, that’s not fair because playing chess is quite good for theory. But being a radio astronomer, certainly in the early days, not quite so much now, isn’t just a question knowing your physics well. You’ve got to have some enthusiasm, dedication. But it is awfully depressing work sometimes and you’ve got to be able to survive these periods.
Sullivan
So it is much more than, like you say, the formal curriculum vitae that you’re interested in.
Ryle
Yes. Anyway applicants from America, we never understand all these things that they write on those.
Sullivan
Well thank you very much. So that ends the interview with Martin Ryle on 19 August ’76.
End of Tape 60A