Interview with Rudolph Minkowski
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The interview listed below was either transcribed as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009) or was transcribed in the NRAO Archives by Sierra Smith in 2012-2013. The transcription may have been read and edited for clarity by Sullivan, and may have also been read and edited by the interviewee. Any notes added in the reading/editing process by Sullivan, the interviewee, or others who read the transcript have been included in brackets. If the interview was transcribed for Sullivan, the original typescript of the interview is available in the NRAO Archives. Sullivan's notes about each interview are available on the individual interviewee's Web page. During processing, full names of institutions and people were added in brackets and if especially long the interview was split into parts reflecting the sides of the original audio cassette tapes. We are grateful for the 2011 Herbert C. Pollock Award from Dudley Observatory which funded digitization of the original cassette tapes, and for a 2012 grant from American Institute of Physics, Center for the History of Physics, which funded the work of posting these interviews to the Web.
Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event.
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Transcription
Sullivan
Ok, this is talking with Dr. Rudolph Minkowski at his home in Berkeley on 7 June 1974. Now, why did you get interested in radio astronomy and when did you get interested, I should ask?
Minkowski
I mean, it was obvious when Reber's research became available, that this was a thing which had astronomical importance.
Sullivan
Well, it was perhaps obvious to you, but many of your colleagues didn't think that way, did they?
Minkowski
You’re right. It's hard to determine, but the thing is the idea that radio observations of astronomical objects might be of interest is really quite old. I mean I don’t know, this went back to the early years of the century, maybe before, I am not quite sure.
Sullivan
Yes, but there were some people around 1900 that looked for radio radiation from the Sun.
Minkowski
Yes. Actually, I don't know whether you know of Edison's attempt?
Sullivan
All I know is the letter that [Alex] Shain published in PASP [Publications of the Astronomical Society of the Pacific]. In fact, I want to talk to him. I'm going over to Santa Cruz and I want to see if he knows anything more about that. It's not certain if that experiment was actually carried out.
Minkowski
It was carried out, as far as I know.
Sullivan
Do you know more about it than just that letter?
Minkowski
I don't remember that letter.
Sullivan
This was a letter than Edison wrote to the Director of Lick [Observatory], [Edward Singleton] Holden, telling about this proposal that he and [Arthur E.] Kennelly wanted to do and saying, "Can you tell us when the Sun is active, because we think that might be the best time to look?" And that's all I know about it.
Minkowski
Well, what I know about it, but I don't know to whom that story goes back, is that Edison actually made an attempt. A rather interesting one, he was supposed to have used an iron ore hill.
Sullivan
Right, a field...
Minkowski
A field and string a wire around which is, of course, the type of aerial which you have in little portable radios.
Sullivan
Exactly. That's what this proposal was. I'll have to try to do research to see if it actually was done.
Minkowski
As far as I know but that may be wrong, I don't know. But as far as I remember, he tried it and didn't get anything which is not surprising because the intensities are not there.
Sullivan
Well, and also the frequency was lower than the ionosphere will allow through.
Minkowski
Sure, he could never gotten anything through the ionosphere at that frequency.
Sullivan
It's still amazing. This was 1890 after all. This is only 10 [Sullivan: 3] years after Hertz had discovered the waves.
Minkowski
Yes and so the idea was there, and now this was, of course, a problem for radio engineers. The decisive thing were Reber's observations.
Sullivan
What about Jansky's observations? Why did they not have more of an impact on astronomy?
Minkowski
He showed that there was something, but he did not place it into any astronomical objects.
Sullivan
Well, he had it peaking at the galactic center and he showed that it was...
Minkowski
Was that Jansky? I thought it was Reber.
Sullivan
No, Jansky had that, 1935. And he showed also that it came from the Milky Way in general.
Minkowski
Yes, yes.
Sullivan
Reber had higher resolution and better signal-to-noise.
Minkowski
He had higher resolution, better signal-to-noise.
Sullivan
He also made a contour map on the sky...
Minkowski
Yes.
Sullivan
Which Jansky never did.
Minkowski
Yes.
Sullivan
Perhaps that was...
Minkowski
That was the thing which made it more convincing. Well why nobody took it up- the thing is there was one difficulty of course, this was a thing which had to be done by radio people.
Sullivan
Right. In other words, the technical difficulty...
Minkowski
The thing is, there was no easily available instrumentation, say, which you could have put up and tried to repeat that experiment. And actually the interesting thing was Baade and I tried to stimulate some interest with the radio people then at Caltech.
Sullivan
And when was this?
Minkowski
That must have been about the time when Reber's paper came out.
Sullivan
1940?
Minkowski
1940.
Sullivan
I see. And what happened when you tried to do this?
Minkowski
The main man at Caltech at that time was Potapenko.
Sullivan
How do you spell that name?
Minkowski
P-O-T-A-P-E-N-K-O.
Sullivan
Potapenko.
Minkowski
And he listened to the story and in thinking it over said, "Well, I one would need a rather large aerial to do this," which is correct, taking the technique of that time, and he was not interested. He didn't think it was a very fruitful experiment.
Sullivan
I see. There's no written record of that anywhere?
Minkowski
No.
Sullivan
This was already, I see, 1940 or so.
Minkowski
That must have been 1940... I'm not quite sure. When was Reber's paper?
Sullivan
1940.
Minkowski
Was it in 1940?
Sullivan
The first one.
Minkowski
It must have been at about that time.
Sullivan
Well, then, what was the next connection with radio astronomy that you had? Obviously you kept interested in Reber's observations and then many other people.
Minkowski
Well, the thing was, of course, somewhat complicated. I don't know whether you know this story of, I think, Reber's paper.
Sullivan
Well, tell me what you know about it. I've heard different versions.
Minkowski
He sent it to the Astrophysical Journal and Struve who was then Editor, really didn't know what to do about it. He didn't know anything of Reber nor did anybody else. And the idea which he got was he wrote a letter to the Institute of Radio Engineers.
Sullivan
Yeah, the IRE.
Minkowski
And he got an answer back that he had always paid his dues on time.
Sullivan
That's all they could say, of course.
Minkowski
Well, Struve somehow had the spirit to take this paper and publish it.
Sullivan
Right, and I've heard that he sent a delegation actually out to see Reber's telescope.
Minkowski
That is possible.
Sullivan
With [Gerard] Kuiper and [Bart J.] Bok, I think, also. I hadn't heard about this letter before, that's interesting.
Minkowski
And I think then in Reber’s stuff there was, of course, included the possibility that there was something which was not directly connected with the Galaxy as a whole. And that was, of course, he had Cygnus A in his record.
Sullivan
Well, you could see its influence on the map, but he did not recognize it as a discrete source.
Minkowski
Yes, but you could not fit it with the main feature, which was clearly the Galaxy.
Sullivan
Right, and then it had these peaks.
Minkowski
And this peak was obviously nothing connected with the optical structure of the Galaxy. That made it a little bit more interesting.
Sullivan
I see. Well, there is the Cygnus rift pretty close to that direction. The great Cygnus rift, is that not true?
Minkowski
Yes, but there was nothing which really looked convincing having anything to do with the Galaxy. So this was somewhat of a remarkable feature with which one couldn't do anything.
Sullivan
The same with Cassiopeia, I suppose?
Minkowski
Well, Cassiopeia was another part, but these were obviously things which were not tied in with the structure of the Galaxy. And you couldn't do much with that. And then I think the decisive thing, that was a very important one, was Hey's finding that these sources scintillated.
Sullivan
Right, Cygnus A, specifically.
Minkowski
Cygnus A, specifically. This, whatever it was, it fitted of course in the picture that it was not a feature of galactic structure.
Sullivan
Right.
Minkowski
And so there was something else. The thing stuck there more or less. We got some information from Sydney because E. G. ["Taffy"] Bowen, who was the head of CSIRO [Commonwealth Scientific and Research Organization], came occasionally to Pasadena and told us about what they were doing. They had, of course, the cliff interferometer. Bowen, he gave us a position for Cygnus A, of which nobody was ever able to find out where he got it. It was incorrect.
Sullivan
Yes, I know the position you mean. In fact, they published a paper and they actually had an optical photograph and they had where their position was and it was quite a bit off.
Minkowski
You see, this photograph was...
Sullivan
Oh, that's right, you took the photograph!
Minkowski
I took the photograph and that was taken in this position which E. G. Bowen gave us.
Sullivan
But actually you can see the galaxies we now know to be Cygnus A on that photo, but it's well away from their position.
Minkowski
Yes, but it was too far off in position and there was no indication that a distant galaxy could be a strong radio source.
Sullivan
Right, the second strongest.
Minkowski
And I think that is in a footnote in this paper by Baade and myself on Cygnus A.
Sullivan
The '54 paper you mention this?
Minkowski
Yes. Mills noticed the thing and I think I told him [???]. Then Baade got the positions which really mattered in the IAU [International Astronomical Society] in Rome in 1950. [Sullivan: Rome meeting was actually 1953.]
Sullivan
From Ryle and Smith?
Minkowski
Right. Graham Smith and that then set it off.
Sullivan
Right, let me go back a little more first. So the first time you looked for a radio source was this Cygnus A from Bolton's position?
Minkowski
Yes.
Sullivan
And that was '48 or '49, I guess.
Minkowski
That must have been about '48, '49, yes.
Sullivan
I can check that and then you didn't do any more until the...
Minkowski
No, well, the thing it was... It was a little more then. By that time, about '49 I think, clear indications came about some radio sources because Bolton had identified the Crab Nebula. That was '48, I think, or '49, I'm not sure.
Sullivan
Yes, this was '48 actually, the Crab Nebula, Centaurus A.
Minkowski
And Centaurus A...
Sullivan
There was a third one.
Minkowski
And Virgo A. Now you see, this was very instructive because you now knew that you had to deal with a variety of objects. The Crab Nebula was a supernova remnant, so you knew that. Centaurus A and Virgo A were obviously peculiar galaxies. So you knew that peculiar galaxies could be strong radio source emitters. The moment you could tie in a source with a galaxy, you knew that this was a distant thing so that the source had to be very strong.
Sullivan
Intrinsically, yeah.
Minkowski
But to go on from there they are- well, it was also clear... I don't think that was published or mentioned or not. You knew that in general peculiar galaxies are not radio sources.
Sullivan
I see, just from looking at the 1C survey.
Minkowski
You had, Centaurus A is obviously a rather peculiar galaxy. As a matter of fact, it was not clear whether it was a galaxy or a galactic source.
Sullivan
Or two galaxies.
Minkowski
No, galactic source.
Sullivan
Oh, I see, in our Galaxy.
Minkowski
In our Galaxy. That was really settled only in about '54.
Sullivan
When you got a spectrum?
Minkowski
When I got the spectrum and the radial velocity. But there they were other peculiar galaxies, but they in general were not strong radio sources.
Sullivan
Was this only from looking at the lists that radio astronomers had?
Minkowski
The thing is- it was obvious that you needed precise positions.
Sullivan
Right, that was the key.
Minkowski
You knew by that time there was a variety of objects which had the capability of being strong radio sources. And you knew that some of them were easily recognizable as something unusual but you also knew that this was not enough.
Sullivan
Right. You needed a small error box, yeah.
Minkowski
So it was clear that positions were of great importance. I remember - this was kind of amusing - this must have been the first symposium on radio Astronomy, it was in Washington in the Carnegie Institution in '54. And I know after that symposium we sat around in [Merle] Tuve's place and just gabbed about things, and somebody raised the question how precise I thought radio positions should be so that one really could hope to identify a larger number of sources. And I said, "Well, about a second of arc," and everybody shook their heads and looked at me.
Sullivan
Said, "Impossible."
Minkowski
For being completely crazy and out of line. But that is exactly what happened. When the positional accuracy got to that level of a second of arc, then sources became identified in quantity.
Sullivan
Exactly, but it took ten years after '54.
Minkowski
Yes.
Sullivan
But let me ask you, back when you were realizing when there were many peculiar galaxies that were not radio sources, did you ask the radio astronomers specifically to check them at or did you only look at their published catalogues?
Minkowski
No. Some of it was just off-hand. See you knew some peculiar galaxies, so you asked radio astronomers, "Do you have a source there?" Generally, the answer was no. I think the most extensive experiment of that kind was made in '56... Let's see '55 was the IAU.
Sullivan
In Dublin.
Minkowski
In Dublin... yes. In '56, I went to Sydney and took along a large number of prints of the Sky Survey, which then was not finished. It was underway, but I had access and could take prints along. And I took along prints covering a fairly substantial area of Mills' survey. This was, of course, a doubtful thing at the time whether Mills or Ryle...
Sullivan
That whole debate, yes.
Minkowski
But Mills’ data were more accessible. So I checked over a fairly substantial area of the sky, checking over each source of Mills with the sky, and what came out was just the same picture that in general the peculiar galaxies were not radio sources.
Sullivan
Was that published?
Minkowski
Actually, that was published, but where? I know... in Washington and Green Bank.
Sullivan
Oh, the National Academy of Sciences?
Minkowski
Yes.
Sullivan
That was 1960 or so. Yes, and you had an article in there, yes, I remember. I know that article.
Minkowski
It may be in that article.
Sullivan
But you'd done that a couple of years before, actually?
Minkowski
Yes. It is also possible that there was an earlier one. I am not quite sure.
Sullivan
Ok, you'd have a reference to it, if there were, in that article?
Minkowski
Yes.
Sullivan
Ok, let's go back to around 1950. What was the relationship? You said that Bolton sent you some positions. Did Cambridge send you positions also? Were they in close contact with you?
Minkowski
They were not as productive as the Sydney people at that time.
Sullivan
And did you find out about their positions that they finally did get in '51, I guess?
Minkowski
Well, you see the thing is the state of affairs was at that time that you had these very few precise positions, essentially Cygnus A and Cassiopeia A, which Graham Smith produced. They were precise enough to locate objects. And the others were at best usable for statistical purposes. The positions were too uncertain.
Sullivan
±10 arc minutes or something like that.
Minkowski
Something like that- to really make identifications. But somehow you could use these things for statistical problems. The thing is that actually it didn't make much difference you used whether you used positions you got from Cambridge or what you got from Sydney. They were all so uncertain it didn’t matter. It was the 2C catalogue at Cambridge and the errors were just too large to do anything.
Sullivan
I want to go back even further though. Back to the 1C catalogue.
Minkowski
The 1C catalogue is kind of amusing because they were, of course, entirely wrong.
Sullivan
Right.
Minkowski
One of the bright galaxies was identified with a source, which eventually actually turned out to be 3C48. If you call that the 1C- it had some information, I think, it came out that Messier 31 was a source. This was correct, I think.
Sullivan
Yeah but I think that was by luck really.
Minkowski
That one, yes.
Sullivan
It was really [Robert] Hanbury Brown that finally got it.
Minkowski
Yes, but that was really of very little use.
Sullivan
In any case, you couldn't go and look anywhere on the basis of the 1C catalogue?
Minkowski
Well, you could look but you couldn't find it.
Sullivan
Right, well it wasn't worth looking.
Minkowski
This was this thing that if sources in general had been things like Virgo A, Centaurus A, and so on, then you could have found them. You see...
Sullivan
Even with errors of + 10 or 20 arc minutes, you could have found those?
Minkowski
Yes. You would have found very peculiar objects. This could have been significant and we would have noticed.
Sullivan
Right, and chances of having two objects like that in the error bars were small.
Minkowski
Yes.
Sullivan
Your work on Cas and Cygnus were based on Graham Smith's positions?
Minkowski
Yes.
Sullivan
Were you working closely with them or did you just read about them in Nature when the article came out?
Minkowski
No, he told Baade about it in Rome. [Sullivan: 8/51 letter in fact, 9/52 IAU] Then Baade took the plates and located things.
Sullivan
Right immediately more or less after that meeting?
Minkowski
Yes.
Sullivan
This would be the fall of '52, I guess.
Minkowski
Yes.
Sullivan
Then the work that went into the 1954 paper was from '52 to '54, I guess?
Minkowski
Yes. [Sullivan: Wrong dates]
Sullivan
Something I want to ask. Maybe it just a difference in style between the radio astronomers and the optical astronomers, but it was two years before you published any of all at this very interesting information. Why wasn't there a couple short papers?
Minkowski
That's kind of a question of habit.
Sullivan
Just your style?
Minkowski
In general before you publish something you want to be fairly sure and have some real information.
Sullivan
So it's a matter of your style that you don't believe in popping off something to Nature?
Minkowski
No. Well, you wonder if this was the general attitude. You could find, there was a funny-looking nebula seen in the position of Cassiopeia A. Actually, this is not at all so terribly faint and it's funny, but somehow the statement that there is a funny little nebula in that position isn't quite enough.
Sullivan
Well, for some people it is.
Minkowski
What?
Sullivan
Some people publish things like that. But you're saying your style is to want to really understand?
Minkowski
No, we wanted to have something- some information, what kind of object is it. And well, in Cassiopeia A, even with waiting in effect for about two years or so before publishing this, we burned our fingers very nicely.
Sullivan
Why?
Minkowski
Why? We concluded that this thing was something very funny. It was not a supernova remnant.
Sullivan
Right. But that’s an easy mistake to understand.
Minkowski
Oh, it's easy to understand.
Sullivan
Because it was a very strange sort of supernova remnant.
Minkowski
Well, it isn't even so strange. This was the type of thing we wanted to avoid.
Sullivan
Even though you waited two years, yeah.
Minkowski
We didn't want to put out information which would be wrong the next week. The thing is we should have waited longer. What happened is that in '55 the information was still quite inadequate and it became adequate the next time around in '58.
Sullivan
What was the thing that changed the picture? Was it the proper motions?
Minkowski
Observations. It was mainly that Baade's proper motions gave a clearer picture, and I took radial velocities, and getting more radial velocities in this thing, it ended up by showing that it was an expanding shell.
Sullivan
Right, you could get the pattern of it better, yes.
Minkowski
And it was just more observations and this couldn’t have been done faster. Of course it takes time with faint stuff - you need a large number of spectra.
Sullivan
And you could only do it on the 200 inch.
Minkowski
We could do it only on the 200 inch. I think in '58 at the Paris meeting- offhand, there must have been something of the order of 50 or 60 radial velocities. Now in many cases, you got two features simultaneously on the slit, but still this meant somewhere around 20 or 30 nights of observation.
Sullivan
What was a typical exposure? The whole night?
Minkowski
These were all long exposures. This went 20 or 30 nights. And 20 or 30 nights means two years. You are limited. It a can be done only at a certain time of year. So it was just a case we thought we knew enough, and we published it first and we burned our fingers.
Sullivan
What about the controversy over the distance with the 21 cm absorption and so forth?
Minkowski
That was involved in it, believing that the thing was not a simple expanding shell, meaning supernova remnant. You had to fish for some method of assessing the distance. Now we did this essentially assuming that the scatter of the proper motions was somehow a random scatter. In that case you could use the random scatter to establish the distance.
Sullivan
Knowing the turbulent velocity width?
Minkowski
Yes. But the thing is this turned out to be wrong. The thing was an expanding object. The errors in Baade's proper motions, small as they were, are too large to get the distance that way and so the distance was wrong. This is just almost an automatic consequence of being mistaken about the nature of the object.
Sullivan
What did you think when the hydrogen line people said that it was 3 kiloparsecs away?
Minkowski
Well, the thing is these were the first distances from 21 cm, so I don't know whether we said anything, but the obvious thing was that you had to consider some things wrong.
Sullivan
The method wasn't proven, you're saying?
Minkowski
The method was not proven and the other methods seemed to be reasonable so ... But it just shows you how dangerous it can be to put out information too fast...
[Interruption]
Sullivan
What about the other sources in the '54 paper?
Minkowski
Let's see...
Sullivan
Cygnus?
Minkowski
Cygnus, this was a very much simpler case, I mean observationally, because you knew what you see there is a galaxy and you got velocities and you could get the distance of the galaxy. This was much more straightforward.
Sullivan
Let me ask you about supernova remnants. At this time you had studied the Crab Nebula very well, but were there other supernova remnants that were known besides the Crab Nebula? Is that the only one?
Minkowski
No, but the Crab Nebula is actually a rather unusual object.
Sullivan
Yeah, it's not typical.
Minkowski
There is no conspicuous supernova remnant of the same kind, so to speak, as the Crab.
Sullivan
Were there others that were known in the early 50s?
Minkowski
Oh, there were supernova remnants known, but some of them were not observed, were not easy. I don't know what is in the '54 paper.
Sullivan
When you were trying to understand Cas A you were probably thinking of the Crab as your model for what a supernova remnant should be like?
Minkowski
In a general way, you knew that the supernova remnant had to be an expanding nebulosity. In the beginning Cassiopeia A didn't seem to fit that picture.
Sullivan
Yes, so you just ruled it out right from there?
Minkowski
So, it did not seem to be a systematically expanding thing. So it was not clear what it was.
Sullivan
Let’s see, what else? Puppis A was in that paper also?
Minkowski
Puppis A, but this was done as a matter of course, having what you had at that time, little as it was, meant that if you got a position of some source- some I think were reliable enough- you would at least take a look at the Sky Atlas picture of it. In general this didn’t give you anything, but occasionally it did. Puppis A was one of the things which was not very visible, but there was something.
Sullivan
Even on the Sky Atlas?
Minkowski
On the Sky Atlas.
Sullivan
Let's see, what else? I think those are the main sources. Then there was a following couple of papers also, I think you and Greenstein had a paper on the nature of the radio sources which immediately followed after the main paper.
Minkowski
Yes, well that was just an assessment of the total energy.
Sullivan
Right, looking at the energy requirements.
Minkowski
Looking at the energy.
Sullivan
Let me ask, at this time amongst the people in Pasadena, were the only ones interested in radio objects at all you and Baade and Greenstein?
Minkowski
Yes, more or less.
Sullivan
And of course Greenstein goes back all the way to...
Minkowski
Well, Greenstein actually was one of the first...
Sullivan
Right, I want to talk to him sometime in the near future also.
Minkowski
And he actually was one of the first ones.
Sullivan
He and Whipple wrote the first [Sullivan: theoretical paper]- in ’37 they tried to explain Jansky’s results using dust.
Minkowski
Yes.
Sullivan
Well, what was the next stage then, after your '54 paper?
Minkowski
You had a situation that you got positions which got gradually a little bit better and all you could do, if you got a position, you could look at the sky and see whether there was anything. In general there wasn't and sometimes there was. And you got some very misleading information. One of the things, which goes back, of course, to the beginning, was the question of double galaxies. And the situation there was that - this became eventually clear- the thing is that it was much easier to recognize a double galaxy than a single galaxy.
Sullivan
By "double" you mean the colliding galaxy idea?
Minkowski
No, I mean anything which is two galaxies...
Sullivan
Close together, yeah.
Minkowski
What came out essentially was- in the beginning you found double galaxies, but this is very clear and is a definite thing to see, and if it's near a source you get the idea this might be the identification. However, all that’s necessary is that one of the two galaxies is a sufficiently strong radio source to be observed. And that is what happened. It is just a case of wrong statistics, if you will, that when you saw two galaxies it seemed likely that one galaxy ... the positions weren't good enough.
Sullivan
Yes, so you took it as coming from the two...
Minkowski
So this thing played a role and for a time it looked as if double galaxies were a kind of object which was likely to be a radio source. But this was not it - the thing is that if there was a double galaxy, you were likely to assume that it was the radio source.
Sullivan
And I suppose you liked the idea also because this could explain where the energy came from?
End of Tape 32B
Sullivan Tape 33A
Sullivan
With Dr. Minkowski on 7 June ’74. So that's how the idea of colliding galaxies or double galaxies happened?
Minkowski
This is one of the things which was misleading. Some of these things are very difficult. It's a similar question of which role clusters of galaxies play.
Sullivan
Today, you mean?
Minkowski
Well, I mean that also started very early. You had this thing and it seemed as if there were sources connected with objects in clusters of galaxies. Again, you get into the statistics of the problem there because you can recognize a cluster of galaxies?his doesn't necessarily mean that it has anything to do with it.
Sullivan
Are you saying that the size of the source was not well enough known to tell whether it was coming from a single galaxy...
Minkowski
We did not know about size.
Sullivan
Or from many galaxies?
Minkowski
The whole thing can be very misleading. If you look for a source and you find a cluster, you always have to remember that there are lots of galaxies. And if you have a source position in a cluster of galaxies, you’d like to identify one of these galaxies with the source, and then you have a source in the cluster of galaxies. But it may not have anything to do with it. So there are statistical difficulties which are very difficult to overcome.
Sullivan
And that particular question is still not resolved today.
Minkowski
No.
Sullivan
The nature of radio sources in clusters.
Minkowski
Yes.
Sullivan
Well, let's see, I guess we are up to about 1960 or so, or are we? Is there anything else you can think of before that point?
Minkowski
No, I would not - I don't think so.
Sullivan
Were you doing any work in the early ‘60s connected with radio sources?
Minkowski
Well, let's see, in the ‘60s... I do not think I did anything.
Sullivan
Of course, the quasars came along in '63 and ’64, but you weren't involved in them?
Minkowski
No. Then came the quasar story and this was done in Pasadena with Schmidt and others.
Sullivan
Right, I need to talk to them about that.
Minkowski
I did some things on sources, but that somehow got lost. This is a problem- I don't know whether I can recall. There is, of course, always the question of what is the Crab. It's an unusual object.
Sullivan
The different sorts of filaments?
Minkowski
Well, what kind of a remnant is it. And if you look through the various papers, it's mainly, I think, in my Annual Reviews article in '64, a paper on supernova remnants.
Sullivan
By whom?
Minkowski
Myself. And in '68 in Stars and Stellar Systems.
Sullivan
Right.
Minkowski
And in '70 in the Crab Nebula [Sullivan: IAU] Symposium. And there was also a Crab Nebula Symposium which was local, in Tucson. It’s in the ASP [Astronomical Society of the Pacific].
Sullivan
When was that?
Minkowski
That must have been May '70 or so. If you look at these papers, you will find that the Crab Nebula was in the beginning called a supernova of Type I. The next time around, it was uncertain whether it was. And I think the last one is "definitely not." Actually, this Crab Nebula situation is by no means simple.
Sullivan
The pulsar has helped a lot I think, hasn't it?
Minkowski
Well, the pulsar, no - this is just an added thing.
Sullivan
But it shows where the energy comes from.
Minkowski
Oh yeah, well it answers the question where the energy comes from, but it does not answer the question of what kind of a supernova it was. But what happens is that the Chinese information is just not quite good enough. And I think it is probably so that the Crab Nebula really is not a supernova of Type II, but something unusual.
Sullivan
Yes, it's been misleading us since it's so close and so much studied and yet it’s not typical at all.
Minkowski
But then you get the question- well, all right, it may be a rather unusual object, but is it really so unusual that there is no other supernova remnant of that kind at all in the Galaxy? Now this answered itself: 3C58 is that kind of a source.
Sullivan
Right, the same sort of spectrum.
Minkowski
Same type of spectrum. And there is one very interesting thing. You have 21 cm distance for 3C58 which is uncertain as to...
Sullivan
I think it's a lower limit of 8 kiloparsecs or something like that.
Minkowski
Yes.
Sullivan
So it could be extragalactic.
Minkowski
It is not likely to be extragalactic.
Sullivan
But you can't rule it out on the basis of...
Minkowski
You can't rule it out...
Sullivan
On 21 cm.
Minkowski
Except that if you make it extragalactic, it gets too strong.
Sullivan
Yeah.
Minkowski
But take that 21 cm distance - you saw that most recently re-determined by Dave Williams in Astronomy and Astrophysics. But take that distance, then you find a very remarkable situation- that is that the intensity of 3C58 is very closely similar to the intensity of the Crab Nebula.
Sullivan
The total flux?
Minkowski
The total flux.
Sullivan
That's interesting.
Minkowski
And it's very interesting to look into that in some detail - there are now some uncertainties that it is likely to tell you something on the future evolution of the Crab Nebula, because 3C58 must be very much older.
Sullivan
Larger?
Minkowski
It's larger. And if you assume, which is arbitrary, that 3C58 is really exactly another Crab Nebula, then you can make definite statements from that. And I was working on that when I got sick and now I've forgotten most of it. Whether I can recover it, I don't know.
Sullivan
Let me ask about your 1942 paper.
Minkowski
But you see in years after '60, I was working more on the supernova than on the radio sources.
Sullivan
Your early 1940s paper with Baade. You mention in that paper, I think, this south-preceding star and how it had an unusual spectrum. I think there are no spectral lines that...
Minkowski
No. That's right.
Sullivan
And that was the pulsar, right?
Minkowski
Yes.
Sullivan
I just find that rather amazing that you had studied it at that time.
Minkowski
Yeah. Well, the thing is it would have been not quite impossible to notice that this was a pulsar.
Sullivan
How?
Minkowski
Well, you see the rate of 30 per second is too fast to see the scintillations, but you can see it if you would get some kind of a stroboscopic effect, if you would see that star on a moving mirror.
Sullivan
Yes. Right, if you had something that was moving.
Minkowski
Now, you have such things in a spectrograph and it would not have been entirely impossible to notice that.
Sullivan
That's an interesting point.
Minkowski
Well, we were very lucky that it didn't happen.
Sullivan
That would have really...
Minkowski
Because nobody would have believed it. The natural explanation would have been that we had been drunk.
Sullivan
But you could have demonstrated it for them at any time, though.
Minkowski
No! It would not have been at all easy to demonstrate it.
Sullivan
Why not?
Minkowski
Because you didn't have the electronic techniques.
Sullivan
But couldn't you just put on a spinning disk.
Minkowski
It’s not so easy to do, you see, because even after some minutes, it’s even then a rather faint object.
Sullivan
Yes, that's true.
Minkowski
So you would have to have a stroboscopic disk which keeps the frequency constant over considerable lengths of time.
Sullivan
Yes, that's true.
Minkowski
So it would not have been impossible, but it would have been a darn hard experiment to show that this is really so.
Sullivan
Let me ask a little bit about Walter Baade. Do you know anything about why his strong interests developed in radio astronomy in the early days?
Minkowski
Well, just because it seemed to be an interesting phenomenon. It was obvious.
Sullivan
I'm trying to understand what separates the very few optical people that were interested and those that weren't. What was the insight that they had?
Minkowski
It's awfully hard to say, but I think there was this - that it was clear that if there was observable radio emission from any of these objects, it had to be a very strong non-thermal process.
Sullivan
Right, therefore unusual.
Minkowski
And therefore something very unusual, and something you had to investigate.
Sullivan
Ok.
Minkowski
I mean it's awfully hard to say why as other people didn't see this. I mean the fact was that it was obvious with a little thinking and that there was something very unusual.
Sullivan
And you wanted to understand it?
Minkowski
Yes. It was obvious that there was a process of which you didn't know anything.
Sullivan
Well, I think that covers it pretty well. Do you have anything else that you might like to say?
Minkowski
I would not say at the moment.
Sullivan
Ok, well thank you very much. That finished the interview with Rudolph Minkowski on 7 June ’74 at his home in Berkeley.