[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 1 of the 1976 interview.

Part 2 | Part 3

Begin Tape 59A

Sullivan

Martin Ryle on 19 August í76 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 í39, 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 í45.

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 werenít 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 wasnít 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 youíre 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 thatíve 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. Itís 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 oneís 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 weíd kept in touch and weíd 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 canít recall what state he was in, whether heíd 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 canít remember the exact date. But we had some sort of equipment going I suppose by early í46.

Sullivan

Now, letís 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 didnít arise that way. It arose because, I think, the... now, how did it arise? I donít think it arose by analogy with an optical Michaelson because, as I said, Iíd 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

Thatís interesting. Let me also ask when you came here how was it that you chose radio astronomy? Was this basically Ratcliffeís 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 wasnít terribly excited about the ionospheric side of things. I thought, well wrongly I thought as it turned out, that there wasnít 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] Janskyís and [Grote] Reberís observations at this time late in the War?

Ryle

I donít think we knew about Jansky, and I should think we probably didnít know about Reber. One didnít 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 didnít allow one the luxury of keeping up to date with the literature. Anyway there wasnít 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 Janskyís paper or something. Or it was measured?

Ryle

No, well, you were trying to make receivers better and better. And it didnít get any better, it measured better on a bench.

Sullivan

Oh, I see.

Ryle

It didnít get better when you stuck them on an antenna.

Sullivan

It was completely independent of Janskyís work as far as you know.

Ryle

I think so. Because as I say, people werenít 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. Itís this much better than the previous one." But you took it out and put it in a radar and it wasnít. The range wasnít increasing evidently. This was because at that time youíd 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 wasnít one of them because I wasnít 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 donít know whether or not he had read Janskyís 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 youíre talking about Heyís? This is not the thing that the published in í46 or so, the first map, Hey, [James W.] Philips, and [Sydney John] Parson?

Ryle

Thatís right. That was the one. It was done just towards the end of the War. It wasnít 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 wasnít that at all. Youíd say, "For Peteís 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, thatís 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 itís 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 donít 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 hadnít been for centimeter radar. One wouldnít have known about that except for radar. Of course, thatís really back to the invention of the magnetron too, because if there hadnít been a magnetron, there wouldnít have been centimeter radar. So people wouldnít have made good mixers.

Sullivan

Thatís true too. Now you say that Ratcliffe - of course, he had worked in the ionosphere. But you werenít interested in the ionosphere, so how did this other option come up?

Ryle

Well, we knew of Heyís work and weíd read of Southworthís work by then I think, hadnít 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 didnít 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 youíve 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 canít recall if at that time anyone else had measured circular polarization.

Sullivan

Well, I donít think so. You didnít 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. Iím 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 Ratcliffeís 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 oneís 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 wasnít in the sort of learning frame of mind. One didnít 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. Itís 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 í48 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 wouldnít 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 didnít 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

Youíre talking about the long Michelson.

Ryle

Yes.

Sullivan

Thatís skipping a little bit ahead.

Ryle

But up till then, you see, weíd built little frame things, broadside arrays that weíd 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 weíd 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 wasnít 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 youíre asking from gain - I mean the idea of bandwidths and things very often I donít think we understood all that, (bandwidth? 18:42) ratios and things...

Sullivan

Did you know about [Robert H.] Dickeís paper?

Ryle

No, no. But it seemed that the only hope of doing this was to have a system which didnít 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. Iíd 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 couldnít 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 weíd ever made with that system. We didnít attempt to make any other observations. No, actually (? 19:39) making a crude receiver first. I was of the very strong opinion that it wasnít 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 Würzberg 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 í48 and then another one in the Proceedings of the Physical Society in í49 talking about coherent electron oscillations Ė 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, youíd 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 couldnít talk about temperature like that. There couldnít 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 canít get through the boundary, but it doesnít 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 youíve 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 hadnít 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. Theyíd say, "Well, Janskyís measurement is somehow wrong," and then theyĎd try to fit the rest of it to a hot gas. Apparently you didnít go along with that.

Ryle

Well, we probably hadnít 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 wasnít 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 didnít tie up.

Sullivan

None of them tied up, I see.

Ryle

But obviously it wasnít conclusive because there could be a big range of things.

Sullivan

I see what you are saying. Well, weíve 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 weíd 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 didnít feel very competent to contribute there. I mean we put these fairly simpleminded ideas forward at the beginning and they didnít seem to work very well. But this new thing which was triggered off by Heyís discovery of this fluctuating region. And we said, "Well, letís 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. Iím 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 hadnít enough resolution and (? 29:11) elements to sort of which was the central fringe. There were just four yagi, werenít there?

Sullivan

Iím 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, werenít we?

Sullivan

Thatís right.

Ryle

They werenít 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 didnít 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 wasnít. Of course there was nothing showing here. There was just a bump showing total integration radiation but it wasnít correlated between the two oppositely polarized antenna.

Sullivan

Now, you say by chance when you first observed, which was May í48 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? Iím not so much of an ionospheric -

Ryle

Yes, well -

Sullivan

Where Cygnus was coming through around midnight.

Ryle

It was variable anyway. It wouldnít necessarily - what time of day would it have been? These are GMT anyway, arenít they? So yes that is 03 or something and thatís 07 or 06 in the morning.

Sullivan

So Cygnus was coming through at 3am.

Ryle

Yes. So thatís when there would have been quite a lot of surround. But it didnít always do this.

Sullivan

Now, how long did it take? Iím 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 Iím 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 widthís worth he got this fluctuating signal. Now I think probably one was already aware, Iím not sure if he actually said so but Iím 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, wasnít it? I mean the distance of this spread f stuff was known. But I donít think you would say, "I believe this is the ionosphere. Iím going to do an experiment." I canít remember whether we, Tony Hewish and I, had longish paper where we tied it all up, looking at four sources.

Sullivan

Thatís right in 1950 in Monthly Notices.

Ryle

Well actually, of course, that was -

Sullivan

That was the result of -

Ryle

Well, it wasnít much different in time actually because those took a long time to publish in those days. This actually was doing more though, wasnít it? It was talking about - did it have long baseline experiments?

Sullivan

Yes, thatís 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 Weberís 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. Itís all marginally significant. Not really, no.

Sullivan

As I say, I wrote Smith. I guess itís 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 í47, by í47? I canít 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 Iím just trying to get at now is that these were all research students that were coming, rather than staff members?

Ryle

Theyíd 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 yearís undergraduate work and took their finals. And that was the case with Tony Hewish and Graham Smith. Iím 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 didnít?

Ryle

No.

Sullivan

Why not?

Ryle

It was too difficult.

Sullivan

Too much bother after all that -

Ryle

Too much physics.

Sullivan

Thatís 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, wasnít 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 isnít doing it. So you wouldnít 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 Iím not sure. No, I think this probably came out before anything they did. Iím not sure.

Sullivan

It came out before they did, yes. But what Iím wondering is as you were working on this, were you aware of similar projects going on in Australia?

Ryle

I donít 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

Iíd 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 Iíve heard it said that this was Cas A in the back lobe.

Ryle

Thatís right. Itís 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 Würzberg tipped over, thatís right. You looked where that was and there wasnít anything but obviously the back was measure the position of Cas. And then we realized almost certainly happening, it was coming in.

Sullivan

And thatís 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 didnít 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 didnít know, for example, of their cliff interferometer as a means for discovering the angular size of the source and they didnít 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 wasnít 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 couldnít. I mean you didnít 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

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Modified on Tuesday, 26-Apr-2016 09:03:19 EDT by Ellen Bouton, Archivist (Questions or feedback)