[Martin Ryle, 18 August 1976]
Martin Ryle, 18 August 1976 (Photo from NRAO Archives, Kraus Papers)



NATIONAL RADIO ASTRONOMY OBSERVATORY ARCHIVES

Papers of Woodruff T. Sullivan III: Tapes Series

Interview with Martin Ryle
At Cavendish Laboratory, Cambridge, England
19 August 1976
Interview time: 2 hours 10 minutes
Transcribed by Sierra E. Smith

Note: 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.

Click start to listen to the audio for part 3 of the 1976 interview.

Part 1 | Part 2

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 Würzberg 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, Carmen 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

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Modified on Tuesday, 26-Apr-2016 09:08:03 EDT by Ellen Bouton, Archivist (Questions or feedback)