Interview with Thomas Gold
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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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Sullivan
This is talking with Tom Gold at the AAS [American Astronomical Society] Meeting in Haverford on 24 June ’76. Can you tell me when you first became aware of this new field of radio astronomy and so forth?
Gold
[Fred] Hoyle and [Hermann] Bondi and I had discussed possibilities in various areas of radio astronomy during the war when we were working together on radar. Of course, all three of us in the Theory Division at the Admiralty [Sullivan: Signal Establishment, Witley] were interested in astronomy and very much concerned with what would come of radio methods.
Sullivan
I didn't know that you were all in the same group in the radar lab.
Gold
Yes, that's right.
Sullivan
What sort of theory did you work on there?
Gold
Hoyle- radio propagation, ion reflection properties of various things, like airplanes and the rough sea, things like that.
Sullivan
Amazing.
Gold
That's where we got to know each other.
Sullivan
And you knew about [Karl] Jansky's and [Grote] Reber's work?
Gold
A little but it was only sort of - in those days it had not yet come to be regarded as a ground-breaking new field. It was just some strange effects that had been observed. I remember very clearly observing on a radar set that I was running, observing the interference caused by the Sun and thinking and discussing with my colleagues then how interesting it would be if I was only allowed to use that same instrument just for the solar studies.
Sullivan
This is after [James] Hey's discovery?
Gold
Yes. It was very obvious. This was a 10 cm radar set, I had no trouble at all observing, at disturbed times, the Sun.
[Interruption]
Sullivan
Now what was the nature of the discussions that you had?
Gold
We discussed what kind of instruments one would build and what one would do. And also we were very much concerned with what one could do in the solar system with radar. I remember then discussing that one would want to build a very large vertical, essentially vertical-looking dish.
Sullivan
A hole in the ground?
Gold
The hole in the ground idea was already then very much discussed. I discussed it for various purposes including radar to the Moon and the planets and we worked out approximately what it would take and it was clear you needed very large instruments. Receivers were not as good then as they are now so the calculations came somewhat bigger than they now turned out to be. But that was all discussed then.
Sullivan
Were there any British attempts right after the War to bounce off the Moon like the American...?
Gold
Unfortunately not. In fact, I pressed very much for that and I had written a paper, secret, of course, in those days, but now after that many years I'm sure I can safely say, I'd written a paper advising the Admiralty that they could make a secret communication channel to their distant ships at sea by bouncing off via the Moon.
Sullivan
The Moon surface, yes.
Gold
And so that if at high frequency they merely looked from a ship at the Moon it would not give away their position even if they transmitted and it was received back home. It was much harder to position-find on a beam directed upwards.
Sullivan
This would have required an incredibly higher gain though than...
Gold
Well, the dish they could have built back home at the Admiralty could have been a pretty big one. And then the satellite object does not have to be all that big.
Sullivan
Yes, that's true.
Gold
I had, however, not solved- when I wrote that paper I said that it would of course, it will mangle the speech by the different delay times because the big size of the Moon. And I had not appreciated, or couldn't, of course then, appreciate just how smooth the Moon's surface at longer wavelengths would be. And as it turns out now at 10 cm you can, in fact, communicate speech. And it doesn't mangle it too badly, it mangles it a bit but not too badly. So in fact it was better than I had claimed it would be.
Sullivan
Very interesting. You said you discussed, besides the radar, you discussed what one would do and what sort of instruments one would build for radio astronomy but can you tell me in a little more detail of what directions you were thinking at this time?
Gold
Essentially just thinking about the types of antennas that we built for radar purposes and how we would use larger versions of them for radio astronomy.
Sullivan
And what about from the astronomy side? What did it seem like would be most promising to investigate?
Gold
That we really didn't know. We knew the Sun would be interesting and we knew the [Sullivan: lunar] radar would be interesting. And then we had heard that there was some radiation from the Galaxy but I suppose that we then thought, most people then did, that there would be just some diffuse gas component in the Galaxy that would radiate a little and that would be the end of it. Because then we didn't really realize what a wealth of new information that was to come I don't remember that we really appreciated that.
Sullivan
Then what happened when the War ended?
Gold
Well, then the three of us all returned to Cambridge and we started to interest ourselves in cosmology. Although we were all three cognizant of radio methods we somehow did not manage to get anything organized. I was very keen to become a radio, whatever it was then, radio astronomer. But I had no real opportunity. I was too junior still to make my own arrangements and so I didn't succeed.
Sullivan
Why not get involved with [Martin] Ryle's fledgling group?
Gold
Because that was a little bit too much of an in-group already then. [John] Ratcliffe was running it in those days, and I had, to say the least, a rather poor relationship with Ratcliffe. Ratcliffe is a very stern man and a big organizer he was and from the beginning I didn't get on with him. Chiefly because I had firstly had a plan to leave the Admiralty and get to Cambridge on the basis that I was an expert in ultrasound devices having developed them for Doppler radar where I'd built mercury delay lines. And so I'd become quite knowledgeable in that. And I thought that would like to build an instrument which is now, now after 30 years or whatever it is, has now become, only just now, a most interesting thing, namely, a medical instrument to give a pictorial display of an ultrasonic reflection pattern. And I argued that we immediately look into somebody's inside and use it like X-rays, you see. And I also had argued that one could use it for looking through metal components for cracks and indeed it is very good for that, much better than X-rays, of course, because to find cracks with X-rays is difficult because they don't introduce any extra opacity, you see. Well, unless you look along the crack you don't find it with X-rays. With ultrasound, of course, they are completely opaque, the crack is just absolutely- so I wanted to build such a thing and had already raised the money and had agreement of the Cavendish that they would give me space and let me work there with my money that I had raised for the job from a government agency. And just before I was to take up this job there, Ratcliffe informed me that he had a recent look over the space problems at the Cavendish and he found there was no possible accommodation for me and so he had to turn me down, me and the money and this job. Had he not turned me down, I would claim that I would have produced exactly this instrument that we now have after this long time because that was exactly what was the program then.
Sullivan
Well, anyway, this sort of thing drove you to theory, I take it?
Gold
It didn't put me in the Ratcliffe camp, anyway.
Sullivan
That was the only game in town, really.
Gold
Yes, that's right. Well then we worked on various theoretical matters and then we had these sorts of colloquia meetings where the radio astronomers presented their story and frequently, as I think [David O.] Edge reports, there were minor sort of tussles between Hoyle and myself on the one side, and Ryle and some of his group on the other side.
Sullivan
Starting when?
Gold
Starting in '49.
Sullivan
When the first few radio sources were beginning to be found and identified?
Gold
Yes, that's right.
Sullivan
And how would you sort of summarize the nature of these tussles?
Gold
Well, the one outstanding one that I remember clearly is that Hoyle and I made at one evening meeting, I think in perfectly good taste, a suggestion of how one could sort out some particular puzzle that was left from a discussion that Ryle had given us. Ryle had told us he'd observed this, that and the other and we said what I thought was perfectly normal etiquette, we said, "Well, couldn't you observe so and so, then you would settle that question." And instead of saying, "Yes, that's a good suggestion," or, "Yes, we thought of that before and we will do it, or yes, we can't do it for one reason or another." Instead of saying that, he was absolutely annoyed and he said, "Well, you can go out and drag the cables all over the ground to do this work." It wasn't our business to drag the cables over the ground, but he was just annoyed that we should at all interfere to the extent even of making some suggestions which evidently were not erroneous suggestions, otherwise he would have tackled that, you see. He would have been able to say, "Well, no, that's no damn good." "No," he says, "You can go out and drag the cables over the ground to set it up." So there was a sort of air of annoyance anytime we opened our mouths. And it was always clear as every other observer of the scene was, of course described. It was always clear that the Ratcliffe organization was very closely organized system; they didn't like to talk to anybody outside about anything. A few of them did but did so very sort of sheepishly. Some of them would be quite friendly with me and talk to me but, I came fairly soon to be recognized as somebody who had a role to play, you see. And so whether somebody knew me or didn't was of some concern.
Sullivan
And so I guess you would say that - or would you - that the present reputation which the Cambridge group has in radio astronomy of secrecy and so forth grew directly out of the ionosphere group, Ratcliffe?
Gold
Yes. I mean, I suppose that Ryle went along with that in the first place and then augmented it later.
Sullivan
What was the scientific nature of these tussles primarily? Did you have basic agreements there, or was it only a matter of philosophy in any sort of suggestion you made?
Gold
I think the first major matters of disagreement were the two items that you know. One was the discussion in ‘51 or '52- when was the IAU [International Astronomical Union] in Rome?
Sullivan
'52.
Gold
Well, it was previous to that.
[Interruption]
Sullivan
So the ’52 Rome Meeting.
Gold
Well before that, you see, was the meeting in London, [?] organized meeting at University College at which radio astronomy, what was then known, was discussed pretty thoroughly. And it was there that Ryle presented what he thought was the evidence of the sources observed, 50 about by then...
Sullivan
1C
Gold
That they were on a scale of stars and at distances like stars from us.
[Interruption]
Gold
That they were on stellar scale and that his evidence showed that very clearly. And then I got up and I said, "Well, I can't believe that that's what the evidence is because all that you can really say is their angular distribution around us and if that's reasonably isotropic, as it was then, then I can either believe that they're very close like you say, or that they're very far so they’re outside the Galaxy."
Sullivan
But Ryle must have considered that they might be very far, too and not liked it because of too much power required...?
Gold
That's right. And he said, "Oh, but the power requirements are absolutely outrageous," and my reply to that was, "Well, we know of some galaxies that are very different from our own that had greatly broadened emission lines," that was stated then.
Sullivan
The Seyferts?
Gold
Well, what were they were...
Sullivan
Seyfert’s paper was during the War in '42.
Gold
Well, that could be but I...
Sullivan
They weren't called that, anyway.
Gold
No, no. But in any case, I said, there existed galaxies with greatly broadened emission lines suggesting that enormously violent activity went on in them and I was not one to say that they could not be producing that much more power than ordinary galaxies. And in any case, I said, "You'd have to have more power for the star story, you would have to have a certain factor in power up on a star, for the Galaxy story you have to have a certain power up in the Galaxy and I can’t tell which one of those two is more likely than the other." And then Ryle replied to that, he said, "The theoreticians have misunderstood the nature of the evidence," and so on. He was absolutely adamant that he had proved that they were not extragalactic. I merely insisted that both were possibilities and that really there is some evidence for thinking that very different galaxies occur. You'd better look at that because that I think was the sort of best discussion.
Sullivan
Is there a proceedings of this meeting somewhere? I've never seen that.
Gold
Yes.
Sullivan
Do you know where?
Gold
As I say Edge reports substantial parts of it.
Sullivan
So it's a separate volume?
Gold
If you have Edge's book you'll find a lot of it reported there.
Sullivan
Ok.
Gold
But it was not printed, it was duplicated. And I have one of the duplicated copies of it.
Sullivan
How many pages is it?
Gold
Oh, it's quite thick.
Sullivan
Well, I might like to get a Xerox copy myself. It sounds like it could be pretty valuable because it summarizes the state of the field at that time.
Gold
If you want just the radio astronomy parts Xeroxed, that's not so much. There was also a lot of geophysics and so on in the same meeting.
Sullivan
Oh, I see.
Gold
If you didn't want that, then the radio astronomy would maybe be a matter of say 60 pages or so.
Sullivan
That's quite reasonable, yes.
Gold
Well, then I'll tell you the story which is a sort of pretty personal matter. Because we had this so very definite fight at that meeting, that the sources could be extragalactic, it seems that because of that Ryle was particularly upset. When in Rome, [Walter] Baade waved me over as soon as he saw me in the antechamber to the meeting when I'd just arrived. He waved me over and said, "Look, Tommy, you'll be delighted, we have here the picture of the Cygnus colliding galaxies," as they were called then. Then he showed me the picture. "There’s no question," he said, "That we've identified that that is what the radio source is, and there's no question that they're extragalactic, you will be delighted to know." Because he'd known that I'd favored that point of view. So I looked at the pictures and I was delighted. A moment later Ryle appears in the distance and Baade shouts over, "Hey, Martin, come over here, have a look at this." So Ryle comes over and is shown the picture, is given the same story, of course, from Baade, and I stand right by the side still and I didn't make any very biting comment but I said something like, "Well, it seems that they could be extragalactic," you know, just some slight reminder of what was said a few months before. So I said, "Well, it seems they could be extragalactic," or something like that. Would you believe it, Ryle collapsed on the bench, on the settee in the room, buried his head in his hands and sobbed.
Sullivan
Incredible. Was this the first time he'd found out about this? Wasn't there correspondence between Baade and [Francis Graham] Smith?
Gold
This was the first time that he was forced to believe the evidence. Up till then he'd believed that it was a misidentification.
Sullivan
I see.
Gold
This was the first time he really had to believe the evidence.
Sullivan
So he apparently had gotten so wound up with this interpretation you're saying?
Gold
That it was such a matter - I mean, he's a terribly emotional person. It was such a deep concern to him that they really were extragalactic and that he was wrong and that I was right. Because he had remembered the conversation and the fact that he'd been so biting before, saying that, "You've misunderstood the nature of the evidence."
Sullivan
But then it's rather interesting in things like the Bakerian Lecture in '58 that Ryle becomes the chief proponent to show that they must be extragalactic.
Gold
Not only extragalactic but much further than...
Sullivan
Cosmological essentially.
Gold
Absolutely, yes. He was always on the side of passionately wanting his objects to be most dramatic. And in the first place he thought if they were star-like things he would have discovered that there was a whole new world as dense as stars of new things. And nothing interested him more than that the occasional galaxy should be of a funny kind. But then having to have it that it was galaxies, then he preferred to think they were extremely far and therefore were the most valuable cosmological tool. And then, of course, his only case that they were far was that the gradient was not three-halfs. With a gradient of three-halfs, well then you wouldn't know what distance they were. Incidentally, at that same meeting I also said that as an argument in that I thought argued against it being star-like objects, I said that if they were star-like objects then I would imagine it most probable that they would be collapsed stars, just because the high density allows strong magnetic fields to exist and allows the fields to extend into gasses of lower density. And that I said was a particularly favorable circumstance for radio emission. I said all that, it's all in the book.
Sullivan
This was before the synchrotron mechanism.
Gold
Yes, yes. And I said that was a particularly favorable mechanism for radio emission, I would imagine that it's those stars that you will be seeing if they were stars. But in that case, I said, I will be very surprised to find them steady and not fluctuating with time constants appropriate to such small objects. So this really is absolutely a prescription for the pulsars except that I did not see the regularity of them.
Sullivan
Incredible. But now what were you thinking of for the emission mechanism? Something like solar flares? No, they were steady though.
Gold
No, no, flares were bursts. But what I had in mind was that you needed big gradients of electron density and you needed big accelerations of electrons to make radiation. And without understanding in detail how you jiggle the electrons, I knew that a vacuum tube will produce radiation in radio bands if you have a lot of electrons that suffer a high acceleration and if there is a large gradient of electron density because if there's a tenuous electron gas extending out from there, then the radiation doesn't get out. So that much I understood and that was my picture, well, the nearest tube to the vacuum tube that makes radio waves that I could think of was from a very strong field that has electrons in it and then I would have imagined the...
Sullivan
It's a shame you didn't really publish in this game at this time, I mean, in terms of interpretation, because these sort of thoughts were not being thought very much at this time, in print, anyway.
Gold
Yes, yes. That's right. Well, as I say it is at least in those documents, so it's all there. And then I also commented at the same meeting on the galactic radiation that Ryle wanted to attribute to the same stellar sources unresolved, the summation of them in the Galaxy unresolved. And I argued against that point by saying I could perfectly well imagine that as a small by-product, and energetically I said it would be only a small by-product of the cosmic rays that there would be, if there was any mechanism for converting a tiny fraction of the cosmic energy into radio waves, that that would account for it.
Sullivan
Yes, I remember that comment, that's in one of the Observatory things, I think.
Gold
And that, of course, is also correct.
Sullivan
You didn't explicitly say if there was an electron component?
Gold
No. Because I did not really know. I was not really clear about the mechanism. Before synchrotron one didn't really understand how one was going to make it anyway. But I just thought that if the cosmic rays in going through a plasma will make it a little noisier in some way.
Sullivan
There's a lot of energy there, yes.
Gold
A tiny fraction of the energy converted in that plasma in some way into radio waves so they could be sufficient I said, "So therefore with that discrepancy in the energy I didn't really think it was necessary to invent another."
Sullivan
Going back even before logN- logS, there had always been the galaxy count business and controversies with how well you could do that. But when the radio sources came along they turned out to be isotropic and at least you were thinking they might well be extragalactic. Did you think that this might be a good test?
Gold
Oh, yes. Of course, we'd thought a great deal about the galaxy counts and it was quite clear that there would be some. We perhaps were not as clear about the fact that even if one had no idea of the individual nature of the objects that those statistics- many could still be used. I think that I have to say that I did not think of that first myself. It was immediately clear to me when it was pointed out, but I don't think that I thought of that. And that that would be the way around, we would go for the radio sources, I don't think - I don't know, but in any case, it was certainly immediately clear but it was also immediately clear as I think I mentioned to you earlier that if a moment I thought about it then, that if any errors were made that they would tend to steepen the curve.
Sullivan
Could you just say that again?
Gold
Yes. That it was clear that if a uniform background in uniform in depth gives you the three-halfs law that if you make no error for the strong sources but make progressively more and more error for the weak sources then even if each error that you make is an equal probability of going to the weaker, going to the stronger when you measure the intensity of a source it will mean that you will always transport more sources from a weak category to a strong one because there are more in a weak category than vice versa. And so it means automatically that you will shift the upper part of the curve where you make errors, you will shift that over to the right, the lower part you have not shifted and so therefore when you draw a line through you're going to have a steeper line. That we were clear about and in fact, I remember visiting on one of the sort of slightly charged visits that I paid to Ryle's group in the Cavendish. I remember sitting in the Cavendish with Ryle and some of his people around to discuss this and I said something like, "Well, how accurately can you measure the radio sources? Can you give me some estimate of the amount of error that you are likely to make depending on the intensity," with this very point in mind, which I'd understood but I'd not discussed with them yet, you see. And so Ryle's first reply to that was an absolutely ridiculous one of saying, "Oh, well, the errors don’t matter. Don't you understand that the statistics don't depend on the errors?" So I said, "You mean to say the errors don't matter? Why bother to have a radio telescope at all? You can just suck them out of your thumb those figures of yours. What are you doing if not measuring the intensities?" So then he had to think again, and only then did it become clear that what in the statistics did not matter to him was the identification of the distance to any particular source.
Sullivan
Right. The intrinsic luminosity distribution didn't...
Gold
The intrinsic luminosity distribution didn't affect the matter. But the measurements of it, of course, did. If you make an error in the measurement then it does affect it otherwise obviously you wouldn't need to measure.
Sullivan
And you say your recollection is that this was published.
Gold
My recollection is that this was published.
Sullivan
But I don't remember it and you can't remember where. Who were the authors again?
Gold
[Dennis W.] Sciami, Bondi and myself, I think.
Sullivan
Ok, I'll have to check that.
Gold
I'm absolutely certain that we worked on this and I have a recollection of a brief note being published but I’ll check on it once more. This is an item that previously I would keep but only in recent years I’ve completed a bibliography and every now and again I find some item of earlier years that’s been forgotten and this was one of them that I sort of penciled in and said, "Well, we must identify that one."
Sullivan
Now while the 2C survey was going on, were both Ryle and you and Hoyle thinking all the time this was going to be a nice test or was it sort of an afterthought that you can do some powerful cosmology with this? In other words was it a driving thing for the 2C or was it only what was more or less beginning to be completed?
Gold
Well, I think that what we said then was we didn't believe the error situation of the Ryle story. We'd considered that it would be a valuable test specifically and in some writings that he's in, specifically because of the steady-state Universe giving you a precise prescription. While in other cosmological theories you would not have a prescription what you ought to be seeing. You always wiggle out on grounds of evolution of how a galaxy evolves to make radio noise, we haven't a clue. And so we said specifically the steady state is an important cosmological model for tests of that nature. And I still think to this day that that's the right attitude to take.
Sullivan
Very well defined as to its prediction in this case.
Gold
Yes.
Sullivan
And so was it a matter then that you were eagerly awaiting to see what the result of this was going to be?
Gold
Except after the very first announcement from Ryle’s group [Sullivan: 1955], where the gradient of over three, which was immediately coupled to this being a final disproof of the steady-state theory, we were very suspicious of the veracity of this and evidently correctly so. As I say, having after that a meeting with him and finding out he didn't even know...
Sullivan
This meeting was after this big brouhaha?
Gold
Yes. Oh, yes, that was absolutely in the usual fashion of the Cambridge group that that press announcement was the first thing we heard.
Sullivan
You knew there was a large survey going on? You didn't even know that...
Gold
Not much, no. And that was the first thing. So after that, as I say, when I then met with him and I found that he didn't even understand that you really had to measure to a good accuracy- that put me on the side of then saying well, I don't trust the data at all. If somebody produces this big noise without even having realized what the accuracy has to be before the thing works, I wasn't too impressed. And so I told Hoyle that, and I said, "You needn't believe any of that stuff, you see." But Hoyle is always much more cautious than I in that way and always said, "Oh, no, you..." He didn't like Ryle but he always felt, "Oh, but you must listen to the observers. If you don't listen to the observers you're lost." And I said, "Well, I listen to them but they have to be right." But Hoyle was much more concerned that these data were correct and that he had to invent new cosmologies to fit those data. And I just brushed them off and said, "Well, I don't believe they're right."
Sullivan
So you didn't worry about finding other things, problems?
Gold
No. And whatever the state of affairs is now it is quite clear that the gradient of three was absolute bunk, wasn't it? Because the gradient, as it now turns out, is 1.8; well, it's significant, maybe but that's another story.
Sullivan
Yeah, it's not three.
Gold
It's not three and the difference between 3 and 1.5 is nearly the same as difference between three and 1.8. So there's no use saying now, "Oh well, you see, it was indeed a steeper gradient." If it is, I'm still not sure but there's no use saying now....
Sullivan
But are you saying then there were no other major objections that you brought to the 2C survey results other than this one of the errors you mentioned? Before [Bernard Y.] Mills' results, of course, that changed the scene. But before then?
Gold
Before then we just discussed that the errors would be making it steeper and that you didn't know about that correctly. And then that [Peter A. G.] Scheuer discussion which we didn't really understand, that you didn't need to- I don't know, didn't need anything- and that must have been wrong too, because obviously that seemed to agree with the count and then the count is now known to have been wrong. So I don't know what was wrong.
Sullivan
This is very confusing. I haven't been through it all in detail. I think the p of d is basically correct but its application to...
Gold
Well, I tell you there must have been something wrong because at the time we were told, "Oh, well, if you do it the Scheuer way you get the same answer." And now we know that that answer, if it was a gradient of three, was wrong. So there must have been something wrong with that too. That's all I know.
Sullivan
So, was the next major development when indeed the results from Australia came in and what happened then? How did you get wind of those results, first of all?
Gold
Yes. That goes a little deeper in my memory than I can remember. I don't really know.
Sullivan
I'm wondering was there something before a preprint? Some meeting or something?
Gold
No, I don't think I had any particularly close contact in those days with the Australians. I suppose I remember the '58 Paris meeting where there was a straight head-on clash between Mills and Ryle.
Sullivan
Right, but that was a year or two after the [?].
Gold
Yes, but I don't really remember the earlier phases.
Sullivan
But what happened when the word did get to England about this Mills survey, the first part of it anyway?
Gold
Well, the obvious thing is that we just thought, well that's going in the right direction and it shows we can't trust the Ryle survey.
Sullivan
And were you willing to trust the Mills’ simply because it gave the result you wanted to see?
Gold
No, but I still insisted that the errors would always tend to steepen the curve and so therefore anybody getting a flatter curve was more likely to be free of errors, that's all.
Sullivan
Well, maybe you could tell me about this Paris Symposium. There's a couple of papers and some discussion that are recorded.
Gold
My main recollection of my own position at the Paris Symposium was not in relation to the Mills/Ryle story, because I was perfectly willing to let these characters fight that without my intervening in any way. Although I listened carefully to it, but that's all. But I remember then standing up to the viewpoint that the 21 cm observations did not reflect the gaseous mass from the Galaxy because it was more likely to be in molecular form.
Sullivan
I see.
Gold
And the only other person who was supporting this point of view at this Symposium was Bart Bok. He remembers the story very well, he says it to me every time. And now it has taken these many years until last year, from '58, almost 20 years, before that has now been shown to be by true, because by now I think we're pretty clear about the fact that the bulk of the hydrogen is in molecular form. And that the 21 cm maps therefore refer to just those regions presumably where the densities are not high enough to make molecules.
Sullivan
And the people, it was [Jan Hendrik] Oort, I suppose, the Dutch were saying that this was the gaseous component?
Gold
Yes. In those years, it was every time that the 21 cm people discussed the gas in the Galaxy I would always say, the gas that happens to be left in atomic form and I'd sit down again. Every time I would do that.
Sullivan
Were you thinking that this other gas might or might not show these features which were called spiral arms?
Gold
Well, I thought it would be in some patterns of this kind too, but I wasn't sure that they would be the same patterns.
Sullivan
Right, it was just really a caveat that you were getting out everything.
Gold
Yes. That's right. I didn't want to have it said that we've demonstrated where the spiral arms are in the Galaxy from the 21 cm data. It was not true.
Sullivan
When did you actually leave England and go to Harvard, I guess?
Gold
In '56 or '57. '57.
Sullivan
And what was your job?
Gold
And then I was at Harvard at the Harvard College Observatory as a Professor. And one of my duties was to run the radio astronomy there, which in those days just had that 60 foot dish.
Sullivan
That was taking over from Bok? When he was going to Australia?
Gold
Yes. And [T. Kochu] Menon was the sort of young man there. And there I organized that it was equipped with a maser and I got Cooper from Australia, Brian Cooper, and John Jelley from England. I got an NSF grant and within a year we had a brilliantly working maser at the focus on the telescope.
Sullivan
You were mainly running this show, not really directly involved yourself, in terms of dirty work, anyway?
Gold
Not in terms of dirty work but, you know, we had our weekly meetings and program and so on organized.
Sullivan
Were you enthusiastic about this hydrogen line as a way to...?
Gold
Oh, yes.
Sullivan
What did you think were its main...?
Gold
In fact, when I left Harvard in '59 to go to Cornell the then Director of the Observatory, Donald Menzel, wrote me a very complimentary letter about the direction of the radio astronomy project and the construction of the maser and what a marvelous thing it was and how they were all pleased and proud of it and so on.
Sullivan
What did you see as the main things that could be done with the hydrogen line? Apparently at Harvard there was not great surveys done like in Australia and Holland. There were more areas of star formation and so forth. Was this under your influence somewhat?
Gold
Well, it certainly was a little under my influence in that I didn't feel that we were well set up for contributing with any organized survey work. We had, you know, just the odd graduate student who could spend a few hours out there and very little staff, no great organization. Not like CSIRO or somebody. And so I didn't want to steer into anything other than an individual student’s program that he could do in a year and write a paper on it, that's all.
Sullivan
I should ask you why you came to the United States? You're an Englishman?
Gold
Yes. I came to the United States from a job as Chief Assistant at the Royal Observatory. I'd left Cambridge in '53 to become Chief Assistant at the Royal Observatory when Spencer Jones was the Astronomer Royal and so I was the next one in charge, the next person down. And that suited me quite well and I liked it. And then in '56 Spencer Jones had to retire and they appointed [Richard] Woolley, and as soon as he appeared on the scene he was so opposed to every area of work that was other than classical optical astronomy that you wouldn't believe it. I had just built what was called a neutron monitor, a solar cosmic ray measuring gadget and there was the great flare in February of '56 for which we had one of the best records. And I regarded it very appropriate for the Royal Observatory to have long term cosmic ray records. Because like other things there ought to be long term records in existence and this was the place to keep long term records. This was exactly what the Royal Observatory was doing.
Sullivan
If you didn't do it, then no one would.
Gold
Yes. So exactly the thing. So we set up very excellent equipment, the best in the world, built a little building for it and everything. And Woolley appeared on the scene, "Cosmic rays," he said, "That's not astronomy! Get rid of it," in those words. In those words, not in any softer way. They had built this stuff for two years and in that case, yes, indeed, with my own hands, built it and absolutely marvelous equipment it was...
Sullivan
This is side two, continuing with Tom Gold on 24 June ’76. So Woolley just said, "get rid of this..."
Gold
"Get rid of this," and then there was another item. I had wanted the solar department to equip itself with a high definition solar magnetograph. I'd argued that the that most important thing that remain to be done was not just to look at every God damn flare and chromospheric eruption or something, but get to know more about what's going on. And the obvious thing was the magnetic field and so I'd said, "Well, one has to build a much higher-definition magnetograph." It need not be able to measure weak fields. Previous to that, the only magnetograph that people had made using the Zeeman splitting was able to measure down to quite low field strengths and measure the field quite accurately. And I said, "Well that's what's interesting me. I don't care whether the field is 2000 gauss or 3000 gauss but I want it on the scale of the flares."
Sullivan
Right, high angular resolution.
Gold
Yes, high angular resolution, and very coarse resolution in field if you need to accept that. And I had then lined up one Jim Ring who was then prepared to come and join the Observatory and we'd arranged that an appointment would be offered him and so on to build such a gadget. He's a superb optics manipulator and I'm sure he would have done a very good job. And at again Woolley just threw that out, that in fact at the IAU meeting in Dublin just before- well, he was already appointed but before he'd come to the Observatory, he was so rude to Ring that Ring immediately informed me that he was not going to come. So with that attitude, I mean, it was just absolutely clear that I couldn't stay there. And so then when I couldn't stay there, then I dickered a little with other astronomical appointments in Britain and then out of the blue a telegram from Harvard that was offering a full professorship and that sort of clinched the matter.
Sullivan
Was there anything else at Harvard that you should describe relevant to radio astronomy? You were Director there for a year?
Gold
2 years, 2.5 years.
Sullivan
And this was, of course, when many students came through which are now leaders in the field, like [David S.] Heeschen and...
Gold
Heeschen, Frank Drake...
Sullivan
There's about a half a dozen of them. Do you have any idea how you were able to pick what can now be seen as being successfully?
Gold
No, I don't think I can really take any credit for that. That was done with a whole committee working, going over records and so on. It’s also just that to get into Harvard as a graduate student, it tough and you have that selection.
Sullivan
So what attracted you to Cornell?
Gold
Well, the Arecibo situation. I'd been at Cornell for one semester and also on a personal basis liked it better there but only that. And I didn't like the big city life of Cambridge, Mass. I really had nothing against Harvard. I thought them very pleasant, too. But I was not yet so deeply settled there and when Cornell came and said - they'd offered to set up an interdisciplinary center for radio astronomy and various other topics, that would combine engineering and physics and they would want to offer me the directorship of this. Well, I found that was very attractive and that the planning that Bill [William E.] Gordon had meanwhile been doing for building a large radio telescope that that would be part of this package, part of this center.
Sullivan
So this is '59 now?
Gold
This is now '59, yes.
Sullivan
And what stage was Arecibo at then?
Gold
In negotiation with the Defense Department.
Sullivan
That had all been designed and you had a price and you were just trying to get...?
Gold
No, no, by no means. It was just that the money for it was sort of earmarked from the Defense Department. The background of it goes back a bit further and I have to be very careful about that because there was some strife between me and Bill Gordon in later years. I must be very careful that I don't overstep anything. What my complete recollection is that in '56, I believe, or '57, '57 I suppose, I was in Boulder at some meeting with- well, Bill Gordon was also present. I was then at Harvard and he was at Cornell but I knew him. And he was on an NSF [National Science Foundation] committee to concern itself with the question of building large radio telescopes. And he had at that time so far as I know no notion of doing any such job himself. He was merely an engineer concerned with...
Sullivan
Consultant to this committee...?
Gold
Consultant to this committee. And I discussed with him then the situation. I said, "Well, we have a large steerable dish built at Jodrell Bank." I had always wondered whether a fixed dish, very much larger or a steerable one not so large, was the best to do now that there was a large steerable one. I felt that a very large non-steerable one was the next step to take and I gave him the views that I’ve mentioned to you earlier, about radar to the planets requiring a big and not very steerable instrument and so on. And so I persuaded him to carry to this committee the notion of a large non-steerable dish. Now, and to my knowledge, that is indeed what we did. But then within a year of that time or so he developed- he or Ken [Kenneth L.] Bowles and history does not know exactly who or whether both independently, developed a notion of incoherent scatter from the ionosphere. And he had undoubtedly the wrong picture of the physics of it and Ken Bowles what turned out to be the right version namely, the critical matter whether the bandwidth of the scatter signal is that of the thermal motion of the electrons or of the ions. And I remember the debate and Ken Bowles had no idea why he said the right thing either. And I remember debating it with them on a visit that I paid to Cornell and I even had a bet on the subject. I wrote it up on the wall so it couldn't be lost, in which I sided with Ken Bowles and gave in detail the reason that if you are dealing with a wavelength that was shorter than the Debye length, then you would be following the ions and not the electrons. The electrons would be following the ions and the density fluctuations you would be seeing would be the density fluctuations, of the ions, and therefore the bandwidth would be the ions. And, of course, a very critical because the whole thing would not have been doable. If only now with the most sensitive receivers and the whole of our receiver, only now after that many years been able to get an electron signal from the place where the regime is reversed. In other words, if it had been that it really was the bandwidth of the electron, you'd never have seen it, I think. It wouldn't have worked. And Bill Gordon thought that's what it was. It would not in fact have worked. Whatever calculations he must have made at the time must in fact have been in error because he claimed [?]. But Ken Bowles had it right. But it was this incoherent scatter that Bill Gordon then took as the main reason for building the kind of instrument, which however fitted in this approximate description that we had previously discussed, as being the next desirable thing to build.
Sullivan
So you weren't around pushing Arecibo at that point?
Gold
No. I wasn't and I claim no credit for that, but I do claim that I was the person who first stuffed Bill Gordon's ears full of the notion that the fixed dish in the ground was a good thing. And he had what turned out to be really a wrong reason for building it, but luckily it was turned right by the...
Sullivan
When did this get straightened out relative to the construction of Arecibo?
Gold
I think Arecibo had already begun by then, before Bill Gordon was persuaded that it was going to be the ion bandwidth.
Sullivan
So you're claiming that he really didn't appreciate that the whole thing wouldn't have worked if he had been right himself?
Gold
That's what I would say, yes.
Sullivan
When you got to Cornell, Arecibo was in the talk stage?
Gold
It had not started. It was in the talk stage with the Defense Department and shortly after I assumed the directorship of the Center the first construction meetings and so on were held.
Sullivan
Was this under the Center?
Gold
Yes. It was immediately from the beginning put under the Center and therein then lay the problem that when the thing became a very large project, then Bill Gordon felt that he should not have been constrained to be under anybody's center and wanted to be directly under the Vice President for Research as a separate thing. My center had another 20 different contracts but none of them were as big. So at any rate, this produced some troubles. Bill Gordon just wanted to be totally independent. Although I think that I never interfered with his management of his contract in any way. Unfortunately not, because I had misgivings about the feed right from the beginning, which turned out to be correct.
Sullivan
Can you be more specific?
Gold
A contract was given to one Allen "K" in Cambridge, Mass. for the feed which was the only technically difficulty scientifically.
Sullivan
Not this support structure but the feed itself?
Gold
The line feed was the only thing where there was an electrical scientific problem involved. And K Industrial Company claimed that they had solved that, no problem.
Sullivan
This had never been done before, such a long line feed?
Gold
Never been done, not even an object of that general nature. I had some experience with antenna work during the War. I understand antennas, I think, quite well and I was very skeptical of this object right from the start. When it was finally put in there the whole dish was 6 dB below par, that's an awful lot, it's a quarter of the size only that it should be for the collecting area. And I suspected the feed was involved. There was a period where Bill Gordon kept saying, "Oh, no, it will only just be- maybe this is not quite correctly adjusted focally and we'll have to move it up a bit and move it down a bit." But two years went by and it still wasn't any better. Then eventually a little bit of a more careful survey was done of the electrical properties of the whole system and it showed that it was indeed in substantial error. And then I invited Ron [Ronald N.] Bracewell to Cornell and discussed with him whether he would spend some time on the matter of trying to find out what was wrong with it.
Sullivan
What year is this with Bracewell?
Gold
I suppose it must have been '64 or '65.
Sullivan
So the two years before that was when there was this adjustment and troubles?
Gold
I forget. When was Arecibo finished? The main construction?
Sullivan
'61 or '62, something like that? I'm not positive.
Gold
'63? And maybe Bracewell was there in '65, something like that. But anyway, Bracewell then investigated the matter and I also had one Vic Rumsey given a small contract for investigating feeds of this nature and before long Bracewell had already come up with the first key idea of why it was wrong. The azimuth modes of the radiation pattern of the feed and demonstrated. That was an error in that and I think [Victor H.] Rumsey came up with a polarization error calculation that showed that to be also substantially wrong and it was quite clear that the feed was basically no damn good. Then followed a very distressing, to me terribly distressing period. It was a very major item to us. 6 dB was no joke. And of course Bill Gordon was always a little bit constrained to belittle it and say, "Well, we still have the discrimination of the other advantages," but I couldn't really see myself supporting, speaking in favor of a successful instrument that was 6 dB below par. So I organized that ARPA, who was giving us money then, allow me to place a contract for a better feed. We couldn't possibly do that ourselves. We decided we had the input from people like Rumsey who had worked by then for over a year and Wielebinski worked for over a year on this deal. And we had a reasonably good idea of how one might go about it.
We then went to Allen Love at Automatics in California. He's a great antenna designer. We placed a contract with him to go into that. And all was fine, he made some measurements and he then suddenly told me he was momentarily stuck because there was a problem that he could not solve. At the moment I don't place correctly which the electrical problem was. But in any case there was a serious technical problem and he at that moment neither he nor I nor Rumsey nor anybody else saw a clear way through that. So I said, "Well, in that case, we'd better sit back and think for a little while." But you're running a big contract there and if we sit back and think all your staff and everything else gets put on the payroll of this contract and we run out of money before we've done the thinking. So I said, "Should I not interrupt the running of the contract temporarily and so Automatics will not charge any overhead?" And so Love says, "Yes by all means, there's no point in wasting our money that way." So I go to ARPA and I say, "We wish to make a temporary interruption of this contract and we will resume it as soon as we've been able to puzzle out this. At the moment all we want to do is tell Mr. Love think about it." So we do that and a short time later a statement comes from ARPA [Advanced Research Projects Agency], "It is obvious that you have failed with the whole contract work and we've decided instead of temporarily interrupting it we've cancelled it." By the time that this letter comes, Love has already found what is a brilliant solution to this whole problem. It turned out to be a brilliant solution. So I write back furiously and say, "No by now we already know how to do it." Too late. Out. Finished. We then go on for three years before under somebody else's management, [Gordon H.] Pettengill, the Defense Department allows another contract to be placed.
Sullivan
To actually build the feed?
Gold
That other contract is then done to exclude me carefully because I had goofed, is their story. It picks up precisely where we left off, it has big major meetings with all kinds of important antenna consultants. It ends up advising that the Love solution be accepted, which is the one that he had had at that time. It goes back to Automatics, they design the feed, the feed is built, it works brilliantly. And three years were lost and in addition to an extreme measure of annoyance for me.
Sullivan
Was that '68 when the feed finally worked?
Gold
Something like that, yes.
Sullivan
Was this when [?] came into the scene under Pettengill?
Gold
No.
Sullivan
That was still later?
Gold
That feed is nothing to do with [?]. That feed is the circularly polarized feed which is the difficult thing. The linear polarized feeds are regarded as trivially simple right from the start.
Sullivan
And these are the ones with [?].
Gold
That was my prescription from the beginning that [?] was making. It's just say a tailor-maker the width of a guide so it makes the right phase velocity in measuring and you can't go wrong. And that's all [?] been doing. I mean I won't belittle him. He's done it very well. But that never was a problem. The great electrical problem had only to do with how to- when you did not have this freedom because it emits circular polarization and for going through the ionosphere you must do circular. And so we were involved with that difficulty. And then Love had found this brilliant solution with little hacks every few inches, which is what the present feed is. I mean, the final thing after all the heavy meetings and all the nonsense that went on three years clean wasted, it was finally built and the cost must have been enormously more than what it would have been if that initial contract had been allowed to continue. That was the biggest mess I ever got into because an absolute idiot in the Defense Department by the name of Colonel [Dowel?], I know this is going down on record but I wouldn't even mind if he heard it, who must have been the worst manager of large sums of money that I've ever seen.
Sullivan
When did Arecibo do its first astronomical observations?
Gold
I don't remember.
Sullivan
Was it only one feed?
Gold
Oh, no. It was on time with the old feed.
Sullivan
With the 6 dB loss.
Gold
Yes. Bill Gordon, I think, was largely inclined to smuggle the whole problem away, to just say, "Well, it's a very fine instrument and all is well, nobody needs to calculate just exactly how many dB it ought to be or the flux units you ought to be seeing from a source." Well, I wasn't quite willing to do that. Bill Gordon, I suppose, thought that I would pick on the shortcomings of the instrument to irk him. I think he took it as a bit of a personal sort of thing.
Sullivan
What did you see as the major contribution to astronomy that this instrument could make besides the radar astronomy?
Gold
Well I was myself very keen on the radar, of course. I was very keen also that we should have an in-house radar team which finally when NSF took over they did not want. That was also a bone of contention between me and the NSF. I thought it was absolutely ridiculous to have an instrument like Arecibo which was so ideally suited for radar and not have a permanent team on the site to use it for radar. It's very difficult to do radar as a guest observer because you have to do it year after year, every step you take helps you for the next round of observation. You have to get the ephemerides more accurate than they are known before, before you can even observe the next thing, you see. So it was through my intervention there that I got Pettengill onboard in the first place, made him Associate Director. He then got the radar business working and that was my management. And I had hoped to build up from Pettengill a permanent group there.
Sullivan
I suppose you were also interested, you were involved in this controversy over whether space ships would sink in the dust in the moon and so forth and the radar would help to discriminate this thing?
Gold
Well, I was certainly keen on Moon radar. Mind you, I am terribly misquoted on this business of space ships sinking in the Moon.
Sullivan
It's not directly radio astronomy but I'm still curious as to what you'd say about it.
Gold
Well, in fact I used the earliest radar data to say that it was clear that the Moon had a dusty surface which was remarkably smooth. And the earliest data that were relevant to this were the 10 cm data taken at TRE [Telecommunications Research Establishment] in Britain. And they showed a very substantial central brightening, central bright spot on the moon.
[Interruption]
Sullivan
You said you had used the 10 cm TRE data.
Gold
To say that the Moon had a much too smooth a surface at 10 cm to be understood as the impact scarred hard rock surface and I'd taken that to imply a dust covered Moon. I had always insisted that it was dust, that it was a deep that deposit of it, but by no means that it would be soft. I had said in those earliest years such things as it's a deep deposit of small particles that makes up the plains on which Denver stands. Yet when you land on your plane in Denver you don't expect to sink kilometers down. So I definitely said that the deposit is very deep but how firm or loose it is I cannot tell. I would say that the main danger in a landing would be blowing of dust in the exhaust and that indeed it was. That was a danger and the first mission was almost aborted because of that. And that I insisted on would be the case because then I couldn't see that it could be firm enough not to blow up in it. But so far as the bearing strength I said I don't know. My feeling is that in the vacuum it’s much more cohesive. I've written that in many papers- much more cohesive than such a powder would be in air but even in air such a powder might well have enough bearing strength. On the other hand I said one thing, I said, "Without having been treated by wind and water the Moon's surface is not assured of having everywhere been tested with a certain load." Well, on the Earth almost everywhere you know it has been so tested. I said there are a few places that have not been so tested and then one has to walk with the greatest of care, namely glaciers. The snow on a glacier has only been there for a short time and there you don't know when it's safe. And I said well, I would regard it in the same sort of light of saying, if I were to walk around on the Moon that had not been tested with wind and water I'd prefer to be on a rope. That's what I said. But for the bearing strength no, I thought that probably the vehicle would be all right but I would bear it in mind that there is a possible problem.
Sullivan
You were not particularly interested that Arecibo shed light on this...
Gold
Well, you see, let me say one more thing. The newspaper people always came out with this absolute rubbish that when I said the deposit is some kilometers deep, they wrote, "Gold says you will sink kilometers deep." And I can stand on my head, I said, "Look you go to a sand dune and it's so deep and you don't expect to sink to the depth of that." And I found out in the end one reason for that is that they'd all read Mr. Arthur Clarke's rubbish of moon dust who acknowledges he had the idea of dust from me. And then he wrote a novel out of that in which people apparently, I’ve never read it, do sink out of sight. So that's how this whole miserable story comes about. But it pursues me around the world. People saying, "Oh, but you weren't right about the depth of the dust." I said, "Well I'm damn right about the depth of the dust and I never said you would sink that deep into it." But it's always the same story. People always identify the depth of the dust with the sinkage.
Sullivan
What you're talking about is how far you have to go down to get to bedrock?
Gold
Yes.
Sullivan
And what is the current story on that? I guess it's not known really, it's only been probed indirectly anyway, a couple of meters.
Gold
My lunar colleagues always hoped that the bedrock is just below the deepest that they have probed to. The radar evidence from the Moon is absolutely over-powering that there is no bedrock to a depth of several hundred meters. Because at the long radar wavelengths we've seen the Moon at 7.5 meters resolved in fine detail and at 7.5 meters the transparency of the soil that we've measured is such that you would go of the general order of 50 wavelengths-100 wavelengths into the ground. And if there were a rough bedrock underneath you would see the echo from that, it would give you a much brighter Moon than we have and it would give you virtually no limb-darkening. And the moon is limb-darkened by 32 dB.
Sullivan
Was there anything else in your direction of the whole Arecibo building and so forth?
Gold
Well, I then organized- I was always the one to organize more the radio astronomy work there while Bill Gordon was interested almost entirely in the ionosphere.
Sullivan
What was the main thrust of the radio astronomy programs?
Gold
Mixed, all kinds of things. And, of course the moment the pulsars came along we were very hot on the pulsars.
Sullivan
But you weren't particularly directing a program that you had interest in. You were just taking proposals and just deciding their merit?
Gold
That's right. I was taking proposals, I was deciding their merit. I was discussing here and there individual programs with people but that's all.
Sullivan
And you never, I don't think, actually got involved in any radio astronomy observing yourself?
Gold
No.
Sullivan
Rather curiously perhaps, haven't you ever had that urge with your radar background?
Gold
Well, when you’re directing a big instrument like that it’s not all that easy. And also I never spent any long periods of time down there. I sort of got that established, I would go down there once a month, supervise the running of it, straighten out all the personal strife that went on and all the nonsense that went on and then I came back. And I would have had to stay a considerable period of time down there. But I always had a local director and if I stayed down there I was afraid that I would get in his hair. It's always a little touchy business. As far as the overall [?] spent time down there, while Bill Gordon was the local director I wouldn’t in any case because there was a little friction and then later it sort of got established that way.
Sullivan
Sure, I can see that. So basically you just didn't have the time?
Gold
I didn't have the time, yes.
Sullivan
The last thing I guess, if you could just talk about the discovery of the pulsars. You heard about them along with everyone else and then what your interpretation was, how they struck you in those first few months.
Gold
I suppose it was the end of the February, when the Nature article that was published, that was the first that I had heard. Frank Drake, who was then the local director at Arecibo, immediately proposed to do observations and I, of course, immediately concurred with that and that was done. I think three days after the Nature article had reached Arecibo, Arecibo already had better data on the then one known position pulsar 1919, had better data on it than Cambridge had ever been able to obtain, of course , because of [?]. My view was immediately that, went back to what I told you earlier the notion, small condensed objects would be able to radiate well because of magnetic fields with gradients and so on. So I immediately worked out sort of a number of things and I worked out firstly what would be the spinning speed of a neutron star that had collapsed from ordinary stellar angular velocities and what would be the magnetic field. And it was a complete surprise to me how enormous these two figures turned out. And apparently a complete surprise to everybody else too. I just invented that figure of 1012 gauss, which is sort of a canonical figure now.
But I just wrote, "Well, I suppose that ordinary stars may have fields of the order of 100 gauss and by the time you collapse it that's what it comes to." And the spin is the same, you see, and the kinetic energy of the spin. And so just from doing these simply calculations I immediately convinced myself that the only sensible source of energy that a neutron star would possess would be spin. The cooling is trivially easy to come to the conclusion that it would be very fast and so therefore any heat from the convection is immediately lost. So if these objects collapsed stars, if they were neutron stars well then the energy must come from the spin, there's no other energy they have. And I calculated various other things immediately: the brightness temperature [?] on the assumption that the pulse length defined the approximate size of the radiating region as it must do. It could be smaller but not bigger. So when I did that I immediately concluded the fantastic temperatures. I think the first sort of figure was 1021. So I said, "Well, obviously you cannot have 1017 ev electrons. You can't have 1017 ev electrons in the vicinity of a small object. You can neither accelerate them nor would they radiate in the radio waves. They'd radiate like crazy in the gamma rays. Or in any kind of curvature even though the field of, I don't know, 10-10 gauss you still radiate gamma rays. So that's obviously nonsense." So therefore I immediately said it must be a coherent radiation that you see. It's the only way in which you get that intensity. So then I said, "A coherent radiation I would understand best as being due to a cloud of electrons whirling around," and I still think to this day that's the best story. Because then they can readily organize themselves into a spatial pattern of coherence.
Sullivan
Like a travelling wave tube, that sort of...?
Gold
That's right. They’re all going in almost the same speed in any case, you see. But if you have any other accelerating mechanism to make energetic electrons well then I don't understand that they could be so well organized. The rotation is just ideal for that. And then I immediately noted another effect, namely that the clock precision, the mean precision of the clock, was enormously greater than the pulse-to-pulse precision. And that means that you must have an underlying clock on which the individual pulses can make an error without however resetting the clock by that error. And I immediately said, "Well, that argues completely against it being any kind of radial pulsations." Because if anything was radially pulsating then I would understand it that the next pulse would come into a particular phase of the previous one. The next pulse is sitting on top of the previous. There is no underlying clock.
Sullivan
So that's what put you off white dwarfs from the start?
Gold
Yes, yes. There is no underlying clock, you see. I can't have a radial pulsation that has no underlying clock. One pulse is determined by the phase of the previous. And that is so clearly in conflict with the most primitive plot that you make of pulsars. That individual phase errors are enormous but then you take the mean over a day and it's absolutely accurate to 8 decimal places, you see. So it's clearly out. And I was very disappointed that people didn't understand that. Even now in the X-ray stuff I had to get up in an X-ray conference at MIT recently and explain all this story again, that they had to investigate the X-ray by the same analysis to see what the right kind of story was. They didn't understand it even now. But in any case that was certainly the thing that persuaded me that there was an underlying clock, an underlying clock that was surely rotational. So I said, "Well, with its rotation then it must be neutron stars because it's much too fast for the white dwarfs than the standard magnetic field strengths of that order the energy is high for the white dwarf." You never had much energy either, you see, because the collapse the big energy comes from the collapse from the white dwarf density to the neutron stars. That's where the big energy comes from and a lot of that goes into the spin. Well, the white dwarf never has much energy of any kind. So I said it was a neutron star, you understand, an underlying clock, you understand the strong magnetic fields, you understand the large amount of energy available in the spin, and so surely that must be the right story. And then I said immediately then you would expect that there would be young ones that are much faster but they would all be slowing down, but that the youngest would be slowing down the most.
Sullivan
Was this before the slowdown had been found?
Gold
Oh, yes, yes.
Sullivan
How long did that take? How long a time scale?
Gold
Well, I wrote the article in Nature in, it was published in May, and Arecibo discovered the slowdown of the Crab in September or October.
Sullivan
Well, the Crab wasn't found until a year later or so, wasn't it?
Gold
That was that same summer wasn’t it?
Sullivan
Was it that early, only a few months after the first pulsar? I was thinking that it was a year later sort of thing. Was the Crab the first one to have a slowdown picked up also?
Gold
Yes.
Sullivan
It makes sense because it's slowing down the fastest.
Gold
Yes, it was immediately seen from one day to the next, the slowdown was seen.
Sullivan
Well, I'll check that. I have the idea that [D. H.] Staelin and [E. C.] Reifenstein was summer of '69 or something like that.
Gold
Well, it could be and then it could be that I'm all out by one year. Yes, it was probably right.
Sullivan
But anyway, your slowdown paper you think may have been a year later also?
Gold
No, no, that was absolutely the first.
Sullivan
The prediction was still in the spring of '68.
Gold
The prediction was absolutely the first thing, yes. That immediately said that we'd expect to find pulsars- it said everything, it said we will expect to find them in the locations of supernova shells because neutron stars can only be made as a result of a dramatic explosion.
Sullivan
You were not bothered by the fact that there were no apparent supernova remnants near the others.
Gold
No, I took that in my stride and said, "We'll find younger ones that will have supernova shells." With the neutron stars they should be associated with supernova shells. If the supernova shells are still visible, then they will be younger and they will be spinning faster. We will see them, we will see the spindown and that was just all to be substantiated. Yes, but it may be that it was only substantiated a year later, yes. And then I wrote another paper a year later in which I put in the rate of energy loss from the Crab.
Sullivan
That fit so nicely, right.
Gold
That fit so nicely, yes.
Sullivan
Well, I think that's about it unless you can think of any other comments along the long trail that you've followed that you might like to make.
Gold
I must say, do you transcribe all this?
Sullivan
Oh yes, you'll be getting a transcription to correct and embellish if you wish.
Gold
Well, that's wonderful, but also I'd be quite happy to have a transcription because in fact I've been asked by the National Academy to record a biographical few pages. So, I don't have to go through the misery again, at least some part of it, just the radio part at least.
Sullivan
Yes, you can definitely have a copy of the final transcription. Thank you. That ends the interview with Thomas Gold on 24 June 1976.