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Interview of Maarten Schmidt by Alan Lightman on 1989 March 28, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/33967
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Family background; influence of uncle in early interest in astronomy and construction of first telescope in 1942; discussions between Schmidt's father and Adriaan Blaauw about the possibility of a career in astronomy; copying tables of astronomical data as a boy; early work on galaxies and mass models for galaxies; university education and influence of Blaauw, Oort and Minnaert; work as a Carnegie Fellow on optical structure of open clusters; work at Caltech on helium abundance in HII regions; history of discovery of quasars; learning about cosmology from Alan Sandage; work with the 200-inch telescope; early desire to measure large redshifts; interest in problems with only a limited number of facts and degrees of freedom; introduction to and attitude toward the horizon problem; attitude toward the inflationary universe model; Schmidt's conception of himself as an observer and not a theorist; attitude toward invoking initial conditions to solve cosmological problems; more discussion of the inflationary universe model and reasons why the community has embraced it; introduction to and attitude toward the flatness problem; reaction to de Lapparent, Geller, and Huchra's work and Haynes and Giovanelli's work on large-scale inhomogeneities; possible over interpretation of data; value of being puzzled in astronomy and Schmidt's wonder why people are unhappy when they are puzzled and confused; visualization in cosmology; interplay of theory and observation in cosmology; astronomy as a passive science (as opposed to physics, for example); outstanding problems in cosmology: time scale and distance scale of the universe, horizon problem, large-scale motions and peculiar velocities; ideal design of the universe; question of whether the universe has a point.
Let me start by asking you some questions about your childhood and how you first got interested in science. Do you remember any early influences — either people, your parents, or things that you read as a child — that got you interested in science?
I know that after I got through the stage where I wanted to have the same profession as my father, who was an accountant, that probably at the age of nine or ten or so, perhaps a little later, I became interested in chemistry. I had a chemistry lab at home and did the usual things.
Spots on the rug.
Yes. But it was really my uncle, a brother of my father's, who was an amateur astronomer himself, who got me onto astronomy. As far as I remember, it was in the summer of 1942, no pun intended, that I visited him and my aunt in central Holland. He showed me his telescope and things he was doing with it. Then I went to my grandfather's, my father's father, who lived in the country — he was a house painter, but retired by that time — and he was sort of a tinkerer himself. So I rummaged through his things and I found a big lens, probably about 3-1/2 inches across and thick in the center. I asked him whether I could have it and he said all right. My uncle had told me that if I took a small eye-piece like you use for biology or so, that if you match up the focal points, you could make a telescope. I did that at home back in Groningen, where I lived, with a toilet roll in between — which isolated most of the edge of the big lens, of course, but its edge wasn't any good I'm sure.
But that gave you the right distance between...
It gave me about the right distance.
That was good luck.
Well, it had to be focused of course, so I had another tube in it. It was very, very primitively put together. That's how I first got an image. So I then wrote to my uncle and I said, "I got the thing together. What do I do with it?" And that was really the start, because he wrote back and said, "See whether you can split this and this double star." I had not the faintest idea where to find the thing, so I had to go to the library to find a book on astronomy, and that's what started it. In looking up the double star, I became interested in what was in the book, and the rest is history. So it was really my uncle. He's alive and still, until a few years ago, active as an amateur astronomer and a star occultation observer. He would travel all over the country to find place where the thing is grazing. Really a very active man, with a wide range of interests. So he was the main influence.
Did your parents encourage your interest in astronomy when you were nine, ten, and eleven?
Yes, they were certainly ready to encourage anything I wanted to do. They were always supportive. I'm sure that they wouldn't have minded either in case I'd switched again, but they were always very supportive. It is true that by the end of high school, probably in the year 1946, when it was clear that I wanted to have astronomy as a profession; my father certainly became somewhat concerned as to whether there was a future in that. He talked with — I think together with me but I'm not absolutely certain of that — but he talked with Adriaan Blaauw, the Dutch astronomer, who at that time himself was in the beginning of his career at Groningen as an assistant to Professor [P.] van Rhijn at the lab. The way Blaauw talked to my father was obviously sufficiently positive that there were no objections when I then made clear that I wanted to go on in astronomy. The situation at that time was, by the way, not very favorable. I seem to remember from a book that [Marcel] Minnaert, well-known from Utrecht, wrote about astronomy that there were at that time fourteen permanent positions in astronomy in Holland. When you looked around and you saw vital people like Minnaert and [Jan Hendrik] Oort, younger people like [Jan] van de Hulst and Blaauw, who was just starting, and a number of others, you just wondered how you would ever get in there. So although my father's objections perhaps were alleviated to quite a degree, the situation was not all that positive.
But you were thinking about a career in high school. At that age you were thinking ahead that far?
No, I don't think so.
But your father was thinking ahead.
Yes, I think that that book probably appeared later, and I'm probably now thinking back [about] how it must have been at that time. But [at that time] I couldn't [have] cared less about 14 positions. You're just optimistic, I think.
You were just interested in looking with your telescope.
When you looked at stars with your telescope at this age — ten or eleven or twelve — did you ever think about the universe as a whole? How far space went, or questions like that?
Well, slowly, not so much while you perhaps worked with that telescope, but reading about the galaxy and galaxies and so on, your interest would go that way. But initially I think that I certainly didn't know at all what I wanted to do in astronomy. It was just something of major interest and... By the way, I do remember that since it was the war when I started, 1942-43, that there was a blackout in Holland, which made observing even from the city of Groningen really possible and quite good. Youngsters these days have no chance of really doing it from the cities where most of them live. At that time, it was all right.
When you were reading your astronomy books, did you read anything about cosmology?
Not specifically that I remember. It must have been in the books that I looked at. I remember there were books that I almost copied at that time. [I would copy] whole paragraphs or tables out of [them] onto my own notebook. At that time, in the war, books often would not be available anymore. You would get a copy out of the library. There were no xerox machines. Things were different at that time.
Yes, so if you wanted something, you had to copy it.
I copied whole tables about the solar and the planetary system, masses and sizes of planets. In hindsight, [it was] a strange way of getting into a field. But that's just what happened, that's what happened. I don't remember cosmology particularly well. I think the bigger thing that I had in mind was the galaxy [our galaxy, the Milky Way]. But you start small. You start nearby.
Do you remember when you first got interested in cosmology? Was that in college?
It was certainly not very early. That's a hard question to answer. I think that happened in a very gradual fashion. My interests really blossomed out slowly, first towards the galaxy. Even before I had my Ph.D, I did work on the 21 centimeter spiral structure in the northern hemisphere. That was done mostly in 1953, I think, or 1954. Perhaps it ended in 1955, while I was doing my thesis work, which was a mass model of the galaxy, finished in 1956. So I did those two jobs almost in parallel during my graduate years, and that clearly set my interest in the galaxy. I think that there never came a time, by the way, when I had a major interest in galaxies as a whole. I remember that when I became a Carnegie Fellow in 1956 through 1958 in Pasadena, that I was still interested in mass models. When material came out on M31, I think I published a mass model for M31 then. But it sort of was an extension of my work and my interest in the galaxy itself. When finally radio galaxies and quasars became my area of interest in the early 1960's, it sort of happened that way. There was never a stage in which 1 was really interested in extragalactic astronomy, apart from these [individual objects] and quasars.
Let me back up just a second. When you went to university, did you know from the very beginning that you wanted to go into astronomy?
Yes, that's what I wanted to do. I entered university in 1946, and that was my intention. There were certain options that you had in the university itself, and I concentrated on the one that was aimed at a professional career in astronomy.
Were there any particular people in your university or graduate career that had a strong influence on you?
Yes. Adriaan Blaauw, whom I mentioned [before], although by the time that he got his Ph.D at Groningen, I think in 1946, he probably then left. But he was sort of my role model nonetheless. Then somebody who was also an assistant at Groningen, Lucas Plaut, was also an example for me. He essentially did all the teaching of astronomy at Groningen, because professor van Rijn had tuberculosis during the war and was laid up. During the time that I was at Groningen, until 1949, he essentially worked from home and didn't teach. I think [I] took one of the oral exams with him, but that was in his house. So Lucas Plaut was really, as it were, at that time my teacher, and he of course had an influence on me. Those were the two at that time, except distant people like Minnaert and Oort whom you would see during an annual summer conference for all of Dutch astronomy including the students. I think that was all students interested in astronomy, not only graduate students. Those two, Oort and Minnaert, were, of course, shining examples. No wonder. They were among the very great people. Oort still is, of course.
Let's go to the early 1960's, when you said that you began thinking of galaxies as a group. Was it at that time that you first began thinking about cosmology as a subject — different possibilities for the universe and so on? Maybe 1 should ask first, did you study cosmology in graduate school?
No, never. I never was formally taught in the field of cosmology. 1 must say that when I started on galaxies — and that was in the form of radio galaxies — there was no such lofty purpose as to try and decide about how the universe is built, etc. I wish it were true, but it just came together in a fairly practical way. I'm hesitating at this moment slighty because, in case you're interested, I wonder whether I should say a few things on what I was interested in as a Carnegie Fellow, [before] I started on radio galaxies in 1960. I can be very brief about it.
Okay, yes, please do.
While I was a Carnegie Fellow, I became interested in trying to see — it seemed natural in following up the radio work which I'd done in Holland which established parts of spiral arms — whether one could get the equivalent optical structure, say from open clusters. So I started to work on a few open clusters that seemed of interest because I had hoped that they were very distant. I was, after all, at Mt. Wilson with opportunities for observing that were quite remarkable. So I tried in the northern hemisphere to go to distances that were unusually large for galactic clusters, perhaps 5 kiloparsecs or so. I tried to find clusters that were very small angular-wise and yet that one could hope for a good distance determination. That work never crystalized into a publishable paper. It's, as it were, a tribute to the freedom that one has as a fellow that even when something doesn't pan out as well as it should, you still can undertake it and don't feel too guilty about not finishing it. While I was at Mt. Wilson, I became interested in the consequences of the change of gas density in our galaxy — perhaps in galaxies in general — as a consequence of star formation. It had just been established around that time that star formation was really active, that it took a certain amount of gas out of the interstellar medium. At the same time, it was clear that after the giant stage, stars have to shed most of their mass, [an amount] we thought at that time [to be] in excess of a white dwarf mass. You saw an interesting circulation of gas going through stars and then being partly recycled — I mean heavy element enrichment. So I started on that, and that was what kept me very busy in the year 1957. I wrote a couple of papers that around that time. And when, after having gone back to Holland, I came back to Caltech in 1959 to take up a permanent position, I was interested in the helium abundance, and as soon as I could I did some observational work with the 200 inch. I did some very difficult observations of the helium abundance in HII regions in the center of M31, to try and see whether the helium abundance, as people thought at that time, did go just as we thought metal abundance should go in galaxies. And the discovery, although it was never properly published, was that it didn't at all. Just at that time it became clear that, in fact, it shouldn't. But it all was part of the whole evolving picture of the evolution of stars and of [the process of] chemical enrichment [by stars].
Did you think at that time that helium might have been produced primordially?
Well, in fact, I was told that. Before, in 1957, when this idea had just started, people thought that [abundance distribution of] helium would just go as [that of] the metals. Then when I did these observations in 1960 and found that it didn't and I talked with [William] Fowler and [Fred] Hoyle when Fred was in Pasadena, they hardly gave any attention to what I had to say, and they asserted that indeed the helium was all primordial, so there should be no change.
So they had reached that from independent means?
Yes, indeed. And the place that I published it was at a La Plata conference in 1960, — La Plata in Argentina — [a conference] on stellar evolution etc., a fairly obscure place to come up" with such an independent observation. So that as the area of my interest when I came permanently to Pasadena, but then the following thing happened. In the summer of 1960, [R.] Minkowski retired, just after having discovered [the extragalactic radio source] 3C295's redshift of 46%. He found it in May and he retired on the 30th of June. Then it became a sort of unofficially shared responsibility between a few people like Guido Munch, Jesse Greenstein, myself, and perhaps somebody else, to take care of the radio galaxy identification that Tom Matthews was doing from radio data taken mostly at Owens Valley. Until that time, Minkowski had taken spectra of the identifications of Tom's, and so we had an understanding that people do a bit of it each on their observing runs. But what happened in practice was that nobody [else] did, and I did it all. I became gradually interested, and Tom was feeding me identifications of small angle radio sources because he was interested in going to high redshifts. So that's essentially how I got into the radio galaxy business, because of the availability of interesting identifications and Tom's capability to come up with the good identifications and so on. But that started in radio galaxies.
And that was around 1961 or something like that?
That started in 1961, yes. I think that in 1963 I published a paper with 30 or 35 redshifts already or so, which was the production that I had achieved at that time.
And by that time the quasar...
By that time, the quasars had already started. Let's see, what do you want to do next?
Well, by the early 1960s were you aware of different cosmological models?
Yes, I think by that time I was. I think that came mostly just by proximity, because Allan Sandage, of course, was very much working on his qo test [measurement of the deceleration parameter], the redshift versus the magnitude of the brightest cluster galaxies. I couldn't help but hear about it and read about it. So it really happened not in a formal course, but essentially just by Allan's great activity in that field.
By being around people.
That really did it. So one couldn't help but be interested at that time in the question, particularly, of what qo [the deceleration parameter] was. Somehow at that time the question of what H0 was was slightly less...
Yes, less important. The darn thing changed all the time, of course, as soon as Allan predetermined it. But while these days one would like to know what it really is, at that time it was just changing all the time. But qo was Allan's hope of course and that...
Do you remember having any preference for any particular model — like open versus closed or homogeneous versus inhomogeneous?
No, no. Inhomogeneous was essentially never mentioned at that time, and open versus closed was something that I had no views about. I am totally different now, because my prejudices are definitely in the direction that qo is a half — probably the effect of physicists on us.
I'm certainly going to get to that in a few minutes.
Yes, of course.
But you think at that time that you had no particular prejudice or preference?
Nope, absolutely not. No.
Did you think that you might measure qo yourself some day? I mean were you that interested?
No. In fact, if I think about what I hoped for at that time, it was incredibly modest. I remember that during one of my runs I [collected data from] a particularly difficult radio galaxy. I used the fastest camera at the front focus of the 200 inch, and I'd taken a spectrum which had had a quite long exposure time. I don't remember whether it was four hours or nine hours, and the result was quite disappointing. I remember that Dr. Bowen, the director, came up the next day after lunch. He came to the 200 inch and asked me whether he could see what I'd done that night. And I perhaps had only two plates or so, two spectra, which was not unusual, but the longer one really was very poor. And I well remember how positive he was about it because he looked with great interest at that spectrum. I explained what I'd been trying to do and why I did it and he said, yes, it was very difficult but he appreciated that the 200 inch was being used for things that were exceedingly difficult. I think that he didn't necessarily approve of people who used the 200 inch to do lots of things very fast and use it in order to accelerate the work, [but] for these borderline problems, he really felt that was the way to go. I think he was right in a sense. That was very reassuring. I remember at that time, my greatest expectation was that, in my life, I would get to a redshift of two thirds, I had decided. With radio galaxies.
Yes, with radio galaxies. That was it. Tremendously modest, as you see. [Laughs] That doesn't sound like a lofty purpose, does it? I mean I was not going to solve world problems.
Why did you want to get to a large redshift?
Ah well, that must have been just instinctive. Clearly things became very much more difficult in that work when you got to a redshift of the order of 0.3, and it was in a sense the challenge, I think.
The technical challenge.
Yes. But it was a fairly narrow challenge at that time. I was not thinking of solving the cosmological problem. I was not thinking of solving the problem of where the radio galaxies came from with all their energy. I just wanted to contribute in a particular direction. It sounds very narrow minded, but that's the way it was at the time. I may be underestimating myself. It may be that I had loftier things in mind.
I think your career shows that one can get good results from that approach.
Yes, but, for instance, in the chemical evolution of galaxies, which was the field that I mentioned somewhat earlier, I think my purposes were much deeper and I had a program in mind, namely to truly understand how star formation affected everything, the formation of the elements, the light of the galaxy — where the change in light I hoped could contribute to Allan's q0 problem. There I had a real program. But I got disenchanted by it when it soon turned out that the number of parameters of freedom that you had seemed to' increase so much. For instance, I worked with a closed system, but it soon became clear that infall [of gas] from the outside [the galaxy] could affect things, and I became disenchanted with the fact that there were so many possibilities. I think that, perhaps, in terms of the style or the type of things that I'm interested in, as soon as either there are too many facts, but especially if there are too many degrees of freedom, I think I start to lose interest somehow. I seem to want to work in an environment where there is not too much freedom. Perhaps it's working at the edge where you don't know yet what it's going to be, but where clearly a good set of observations and a conservative interpretation of the results are of value. Perhaps that's why I'm attracted to that. I think you'll see it on Wednesday again when I talk, because I have very few interpretations, but I tried to get a rather solid, interesting set of results together. So, in terms of a good program, it was [such] in chemical evolution, but not in what I then started, which was radio galaxies. Light man: Let me move further ahead in cosmological ideas and ask you whether you remember when you first heard about the horizon problem — the fact that we see regions of the universe very far apart that seem to be at the same temperature and [yet] haven't had time to exchange heat since the big bang?
I cannot remember when I first heard about it. I'll have to guess.
Okay, take a guess.
This is really a guess. I think it must have been 10 or 15 years ago.
So 15 years ago would have put it at 1973.
Yes. Is that possible at all?
I don't know who posed it first.
Jim Gunn was talking about it at Caltech in the early 1970's, but Dicke actually wrote something about it in 1969.
It was talked about at least as early as 1969. So you're memory sounds consistent with what I know.
I think so, yes.
What I really want to know is: when you heard about it in the early 1970s or whenever it was, did you regard it as a serious problem in cosmology?
Yes, very. I can hardly extend on that, but it seemed indeed like a very fundamental problem to me, as soon as I heard about it. It was probably explained to me by people who also... It could have been explained to me by [Charles] Misner of Maryland.
When he was visiting Caltech at some time?
Possibly, or I might have met him somewhere. But anyhow yes, I saw it immediately as a crucial problem that had to be resolved before... so that you need not wonder anymore about what we were doing.
Did you have any ideas about how it might be resolved?
Nope. Absolutely not. Nope.
I know that some people were saying at that time that it might be resolved by appropriate choice of initial conditions, which of course some people are still saying.
Did you hear anybody talking about that as a possible way of solving the problem or did you think that that was not a good solution to the problem?
I can't remember that. I heard people like [Roger] Penrose talk about it and others. But in these areas, I hear what people have to say, but... I might even have some instinctive or intuitive judgment about how it sounds, but...
But you did think that it was a serious problem?
That struck at the foundations of the whole big bang model?
Yes, absolutely, yes.
Has your view of the horizon problem changed as a result of the inflationary universe model? Do you feel like there might be a solution of it now, or do you still think that it's a serious problem for cosmology?
That I don't dare say much about. The fact that [the horizon problem] is addressed so squarely is very good. [The inflationary universe model has] gotten a lot of attention. As to whether one is now on the right road, I have not the faintest idea. I'm impressed by what inflation does all in one fell swoop, but then I must also say that today I'm very impressed, and sometimes alarmed, by the cleverness of theoretical physicists and astronomers. And when I say alarmed, I think what I have in mind — and I'm not meaning to be facetious — is that one wonders at times as to whether in case one invented some observations — which none of us ever thinks of, by the way, — but presented them in the right way, [one wonders] as to whether or not quite soon there will be a number of reasonably authoritative explanations about them.
Whatever they are. [Laughs]
Whatever they are, yes, yes. I'm now thinking of something that was totally fake, and this will not be tried, of course. But somehow one has a feeling that the fact that something can be possibly understood in certain terms, sometimes very complicated terms, need not give one the confidence that it's the right road at all. Sometimes you almost wish as if the things that can just barely be understood with great effort happen to be [understood by] just the right one [theory]. But there is no such matching, I think, of nature and imagination. But, as you know, I'm not one who looks at the theories and can make up my mind independently. I'm an observer of the scene. Hopefully a perceptive one, but no more than an observer.
Let me just ask you a couple of things more about the horizon problem before we move on. Do you feel comfortable or uncomfortable with invoking initial conditions as solutions to cosmological problems?
No, I'm not. I'm not unhappy with that.
You're not unhappy?
No, I'm not unhappy with that. That's pure intuition. I can't explain that. It's just the view you have about the universe as to whether, in any possible case, it had to come about, or [whether] the one in which we live happened to be lightly special — dependent on those initial conditions. And since we have only one [universe], that seems just about the most extraordinarily difficult question to answer, and I see no reason to be unhappy if [initial conditions] have to be evoked.
I can't explain it.
We just brought up the inflationary universe model, and you partly answered the [following] question. How do you regard this model? Do you feel comfortable with it? Do you feel it's speculative or on the right track?
Well, totally without any authority obviously, it looks to me as if it may be all right. But to me it becomes increasingly complicated, especially when I listen [to the lectures] today, where things don't necessarily converge but have to diverge. There were so many different inflationary scenarios that one heard about today. It seems a fascinating possibility, and it might well be right — one of [the variations of the model]. It just sounds as if basically it's a possibly good solution, but...
But you wouldn't bet money on it.
Oh no! Absolutely not. No.
Let me ask you a somewhat sociological question, and of course I'm just asking your opinion on this. Why do you think the inflationary universe model has been so widely accepted, or gotten so much attention? Maybe people accept it the way that you accept it, with a big grain of salt.
Well, I mentioned that not only did it solve the communication problem [horizon problem], but also the prejudice that many physicists have that omega should be 1. I understand, isn't that right, that it resolves [that problem] at the same time. Or am I wrong?
The prejudice that omega should be 1?
That inflation can do that.
Yes, this is the flatness problem that you're referring to.
Yes, right. And there is yet a third problem that is quoted that I can't remember. [The inflationary universe model] seemed to be something that addresses more than one of the problems [in cosmology].
Yes, that's right.
Perhaps the reason it's gotten so much attention partly has to do with its name. I'm now slightly facetious, because I think inflation is a marvelous name for it. [Laughs] But the fact that it did a few things all in one go was important.
Do you feel that most astronomers and physicists accept it — the people that you know? Maybe accept is a word that doesn't have enough content to it. It's not an adequate word.
I would consider [the inflationary universe model] a good working hypothesis, in the absence of a solution, of course.
In the absence of something better.
Yes, I would imagine.
We just mentioned another cosmological problem that I wanted to ask you about, the fact that omega is so remarkably close to 1, which many people call the flatness problem. Why is the universe so close to being flat, 1060 Planck times after the beginning, or whatever the number is. Do you remember when you first learned about the flatness problem? Was it after the horizon problem?
I think so. Again, it sounds to me now like I must have heard about it...
In the 1970's also?
Perhaps slightly later than the other one, but it's also of the order of at least ten years ago, I think.
Do you remember how you reacted to that problem when you heard it? Did you think it was a serious problem, at the same level as the horizon problem?
Yes and no, because there's immediately a solution. I mean you make [omega] 1.
At the beginning. But that requires certain initial conditions, right?
But you could have also given that solution to the horizon problem. You could have said the universe began completely uniform.
Well, here you may well point out that I'm not the deep thinker that you wish I were. On the communication problem I had no such thoughts. In the flatness problem, I felt immediately that omega [equal to] 1 was obviously the preferred solution to the problem, because otherwise omega should have been by now either very large or very small, but anyhow not near 1.
You saw [omega equal to 1] as somehow a special value that might have been singled out by something?
Well, the way I understand it is that if omega at the moment is slightly smaller than I like, for instance, the value of 0.2 or 0.1 that you get from the abundances in the early universe, then if you look backwards in time, omega has been an ever increasing function where asymptotically it was 1 at very early times. As you say, [after] the order of 1060 Planck times, it's extraordinary that it then only went down to 0.1 or 0.2. One would expect it to be 10-80 or 10-30 or whatever. I see that as a very powerful argument. It appeals to me very much.
That omega is equal to 1.
And you don't worry about why it was exactly equal to l at the beginning. That doesn't particularly bother you?
That's correct. And it may even be consistent [with what] I said earlier — that I'm willing to accept initial conditions... I don't know whether I'm consistent or not. No, that doesn't bother me. No. This I find a particularly powerful argument.
So let me just see if I understand what you've said. It sounds to me like you were more troubled by the horizon problem than by the flatness problem.
Indeed, yes. For the horizon problem, somebody had to come up with a solution that I had not the faintest feeling for. I'm just an observer and I've reacted to it the way I did. [With] the flatness problem, I immediately felt an affinity for the value of 1, once [the problem] was explained to me.
And you were willing to consider the possibility that [omega] was just 1 as an initial condition? And that was that.
Yes, indeed. I was willing to accept that. I think that perhaps the basis for this is that I am very impressed by the Copernican principle in our explanations. Of course, that pertains to astronomy and you have to watch it, because the universe is only one [system]. But, in general, our explanations within astronomy should not put us either at a special time or, as Copernicus explained it, a special place in the universe. You can extend this to time. I find that in many cases, this is a particularly powerful argument in astronomy to which physicists in general, well, there's no need for them to consider it. That's why, including the teaching in astronomy, I think it's [the Copernican principle] a very important concept to bring up in certain occasions.
Would this help explain the flatness problem? Or help understand it?
Yes, I think so, because the value of 0.1 for omega, if it has gone down to only [that value] in 1060 Planck times, that really means that somehow in the overall evolution of the universe that we'd be absolutely very close to the beginning, ridiculously close to the beginning, whereas eventually [omega] is going to go to 10 to the minus [a very large number].
That just seems to be a very special time to me. Well, I don't know. You could also argue that the scale length of 1060 is so enormous that it's just unreasonable that what happens in [that] scale length is of such a small value. Anyhow, intuitively, the argument appeals to me.
That omega is equal to 1.
Yes. Because if, in explaining this story to somebody who didn't know what omega was, [you] had told him that he could have 1060 scale lengths and [asked] where is it now, he might have come up with a couple of values. But the probability that he would have mentioned 0.2 or 0.1 would have been very small indeed. But the fact that we can slightly argue about this shows that it is not an iron argument, otherwise it would be a law in physics, and it obviously isn't, yet. But I've clearly indicated at least my preference. It appeals.
I was thinking of something else that you've said — that we have only one universe. In order to make the flatness problem compelling, you have to imagine that there could have been lots of universes and that it's very unlikely that we would have a universe just like this one. Whereas, if you say we have only one universe so we can't talk about lots of universe, then it seems it's easier to accept the fact that omega should be 1.
Yes. That's right.
So that sounds to me like a consistent line of thinking on your part.
Yes. I think so.
Okay, I just wanted to make sure I understand how your ideas fit together.
Now let me ask you about another interesting idea that's come up, this time an observational idea — the results of [Valerie] de Lapparent, [Margaret] Geller, and [John] Huchra and of [H.P.] Haynes and [R.] Giovanelli in the last few years on the large scale structure. Do you remember how you reacted to those observations when you first heard them? Did it change your thinking a lot, or is it something that you already had an opinion about?
It perhaps started, I think, with the void, this big void that...
[The void found by] [Robert] Kirshner, [Gus] Oemler.
Yes, that may have slightly preceded the Geller et.al work. I think [that] there, in the beginning, I was worried. In the beginning, I wasn't just willing to accept it. I could initially imagine that a slight variation of the luminosity function with distance could...
Could have mocked up the [results].
Yes, could have done things like that. I think that I'm still not sure that I'm convinced that the effects have to be as large as we imagine or interpret them to be because we really think of these bubbles and these voids as being entirely empty. But, of course, in the meantime a few objects have been found in them of a certain nature so that obviously they are not entirely empty. But nonetheless, I do think that they have a powerful effect on one's thinking about the structure of the universe as a whole. I think it does change one's idea about the universe thoroughly, about the overall structure.
How does it change your ideas, or your thinking?
I was brought up with strong statements by authorities like Oort and many others who in the 1950's really would insist that the universe has to be homogeneous on the larger scale. So as soon as inhomogeneities were found at that time, they were said to be absent on the next larger scale. I think, in fact, those inhomogeneities were the evidence of [Gerard] de Vaucouleurs for the local super-cluster. You now only have to look at a Shapley-Ames [catalogue] projection on the sky, where you immediately see these big bands of galaxies. Well, Oort himself shows it these days. But at that time it was not something that was preferentially shown.
You think even though the data was there...
The data was there. People didn't give any attention to it. I think the tendency in the 1950's really was very much to believe that things had to be homogeneous on the, larger scale, and so one took it [homogeneity] at a scale which was just above the one at which you observed that [the distribution of matter] wasn't [homogeneous]. And the fact that this darn business can just go on to these very large scales, I think, is a major change in our concept of the universe. Initially I was just somewhat critical of these things. I [wondered if] the people who were interpreting the data realized that at most of these larger distances you look only at the very bright end of the luminosity function, so if there were minor changes from position to position, perhaps you could get [the observed very inhomogeneous structure]. Nonetheless, I think it's a major change, a major improvement in. our concept of the universe. And at the moment, one is not willing to trust, it seems to me, any scale that things are homogeneous over, except for the microwave background, which comes in with this fantastic homogeneity. [For the microwave background] there is, as far as I know, only one observation [showing a departure from isotropy] that the experts believe, but you know it's small. If we didn't have the microwave background, I wouldn't be surprised if some people, perhaps myself too, would be willing to believe — wasn't it de Vaucouleurs who talked about a hierarchical picture, inhomogeneous on all scales? I think I would have believed that by now. I would be a happy believer of that by now if only that microwave background didn't put a lid on things. So, the microwave background has had an enormous effect on our thinking, I think. Anyhow, yes, I'm impressed by it. It is very remarkable. But I think it's also very good. I see it as positive, in an optimistic view, because clearly scenarios of galaxy formation will have to explain that. I imagine there is much more information in all these structures than if it were all, at more than 50 mega parsecs [about 150 million light years], beautifully homogeneous, without structures.
So you see [these structures] as something that are going to have more explanatory power than without them?
Yes, I imagine so.
Even though it shakes up the paradigm.
Oh, but that doesn't matter. The funny thing is that it seems to me in astronomy — I don't know whether it's like that too in physics, perhaps less so — in astronomy so often it happens that when you meet people who have found a problem, in fact they are very down. They say, "Ah, I can't understand this. This is going so badly." And you say, "What's wrong?" [And they say] "Ah, this doesn't fit, and I can't understand that." And your tendency is to sort of commiserate with them. You say, "Yah, that's too bad. No, that's not good," etc. But of course, when people are in that mode, that's very good. That means there is new information to be gained. Clearly there is more to be gained because in the existing picture this and this clearly don't fit. So out of all this moodiness, there is a reason for great happiness, I think. There is more information to be gained. Of course, people only will feel happy for the first time — it's a strange attitude of scientists I think - they'll only feel happy there for the first time once they've come up with a reasonable explanation. They say, "Ah, I had a great day! I explained this and that." But just before it, when they are really in the data-gathering and the difficulty-gathering phase, they are really down. It's really interesting.
And that may be when they're doing their most creative work.
They should be up! Because — well, you hope that a solution will soon come. But somehow it depresses them. So there are these moods, you know, and then, possibly, suddenly the exuberation when there comes a resolution of it.
Let me shift gears just a little bit and ask you about how you visualize things, about mental pictures that you form in doing your work. I'm interested in the extent to which mental imagery plays a role in the work of different scientists. When you work, is it important to you to visualize things you're working on, or is that not important?
If you have in mind whether I have to have an idea in my mind how things came about, like with quasars or in cosmology — even though it might not be the right picture that I must have a ready explanation for everything even if things aren't worked out yet, I can do very well without that.
I don't mean an explanation, but I mean an actual visual picture. You know, if you're working on harmonic oscillators, whether you would need to picture a pendulum swinging back and forth. That's the kind of thing that I'm talking about.
I am theoretically so inept that I need that picture when I think about a harmonic oscillator, yes. I like to have that, yes.
When you think about the early universe as you study quasars of greater and greater redshift, do you have some picture of space, or of these objects, or of the early universe?
No. And that's where I come back to that earlier thing. No, I don't think I do. Although if you probe it further, perhaps one should find that actually I had, but I'm not aware of it. But I don't do that. I don't have that clearly, no. That's a very difficult question to answer, by the way.
Yes, it is.
And I'm not sure I gave the right one, but my first initial reaction is no.
Well, it is a difficult one to answer. I want to ask you about the interplay between observations and theory in cosmology, let's say over the last ten years. Do you think that theory and observations have worked well together, or do you think they're going in opposite directions, or not paying enough attention to each other?
I think that they are well-attuned to each other. I think there is a reasonably good interchange and equilibrium. As a practical fact, I feel that often theory contributes strongly to the exploration of observations once they have been made. I think it is rather rare that theory helps the observers a lot in telling them what to do. Now there are good counter-examples to that, too. But I must say that when I'm thinking of the observational programs done over the last 20 or 30 years, it was rarely so that I found myself very strongly influenced by what theorists are telling me I should do. There is no great pressure on me or other observers to direct the way the observations should go. Only at times, incidentally, like when we thought in a perhaps somewhat unguarded moment ourselves that we had a gravitational lens with a separation of two and a half minutes of arc. Rapidly from the theoretical side came a number of very good tests which we reacted to within a month and found ourselves firmly deeply believing that we didn't have [a case of a gravitational lens] at all. But radio observations were done immediately that would have allowed one to see the wake of a [cosmic] string, you know, in an edge. We immediately started looking for other quasars in the field to see whether there were other doubles or whether one could then draw the string, like somebody tried today with six galaxy pairs. So we reacted very rapidly to that. Perhaps partly because in our case, Jim Gunn and Don Schneider and I were working together, so Jim was very aware of these [problems] anyhow. But that was sort of the exception rather than the rule. In my main programs, I think that I'm not terribly affected by what theorists tell me, but more by the opportunities as they present themselves to me — technologically in terms of observing.
Let me talk specifically about cosmology. Do you think theorists have been sufficiently responsive to the data and to observations and people doing observations?
I believe so. Yes, I do think so. Of course, you have all sorts of theorists. There must be theorists who practically never give any attention to what's going on, and they are among those that I mentioned earlier. I have great admiration for their imagination and what they can achieve, but they are not the best of barometers as to whether something is feasible as a practical explanation of the universe. I think, to put it crudely, you could tell them anything and they could digest it and be very clever about it.
And find an explanation.
Yes. So when I give an answer, I naturally give an answer about those theoreticians that have some contact with the observations. I think many of them are in good equilibrium with the observations. When I mentioned the asymmetry, that [means] that I think they spent a lot of time in understanding observations. It's important, by the way, that the observation be presented properly and be digested to a certain stage so that it is clear, so that it is something which is easily absorbed by theoreticians. But I think they pay proper attention to that and are receptive and responsive. I mentioned just as a practical method that it is rare that the observers heed the theoreticians too much, except in rare cases. That probably has something to do, as I mentioned earlier, with the way you do observing.
But you think that's a good thing?
No, I think it is something that is typical of astronomy. I'm sure that in physics, it's quite different. Most of the experiments that are done come out of theoretical developments on the basis of previous experiments, and now it turns out that you should go for the XY1 particle obviously, so there you go. That follows out of theory that comes out of the previous experiment. But I think the difference has to do with the fact that astronomy is a passive science and physics is active, so often in physics...
You can turn up the knob.
That's right, or you can kick it differently so that it will do what would yield the XY1 particle. But it is rarely so in astronomy that you can in extragalactic astronomy you can never be active. It is rarely that [when] a theoretician has a good idea about what you could try, practical circumstances in the universe — telescopes, sensitivities, and energy ranges and emissions available — allow you to just do that at the right level.
Yes, that's a good point.
So that's a big difference, I believe.
What do you think are the major problems in cosmology today, from your perspective? You can comment either on observational or theoretical problems. What are the things that we should be doing?
A major problem that I see at the moment, which is highly practical, is the time scale and the distance scale of the universe. It is very interesting. I'll be very brief about it, because it's not terribly deep. But it's clear to me, at least, that at the moment the better distance scales that we have in the universe lead to a Hubble constant of the order of 100 or slightly less. The best time scales that I'm aware of lead, if you again express it in terms of a Hubble constant, leads to a Hubble constant of the order of 50. I find it very interesting that this is the third time in history that those two have been out of whack. The first time happened in the 1950s, and we got the steady state theory, and it was a major theoretical perturbation — the big thing about the radio counts and the steady state theory. And then 15 or 20 years later there was [a mismatch] again, simply because the one went [was modified by updated observations] faster than the other. The [time scales] have always become longer or the Hubble constant [has become] smaller, isn't that right. They were again out of whack for a while and the Brans-Dicke theory appeared. To my mind, this time they are out of whack in by far the most serious fashion that we've seen yet. It's very interesting that now there's not even a theory that addresses it. People say [that the problem might be resolved by] the cosmological constant, but again that cosmological constant is also subject to one of these arguments that is somewhat similar to the flatness problem, namely that the value that we play around with in astronomy is just totally out of the expectation value. I mean so much that I also feel that that argument is probably something I'd like to accept. So we're in a very strange situation. One had certainly hoped that gravitational lenses [would give us] Ho, but they won't; they're too complicated apparently. Perhaps very new methods or the space telescope [could help solve this problem]. But I see this as a major problem.
The two time scales.
The second time scale, the one that is not related to distances, that just comes from the cosmic clocks and ages of globular clusters and thing like that?
I suppose that possibly if we were in a universe that was near maximum expansion, that there would be a large adjustment in the age of the universe [as given by] the Hubble constant.
So I guess that would conceivably be one way of bringing the two time scales closer together. I don't know whether that works or not. There are other constraints too.
Yes, it is still not easy.
It's still not easy.
But I find it interesting that whereas in earlier epochs, this was considered such an important problem that whole new theories were developed that took care of it, that were based on it, this time when it may be more serious, there is not much attention spent on it. Everybody says happily, "Of course, we know things only to within a factor of two." (But suppose that persists. We'd have one of these long periods in which we are very down, you know, you remember? Very down, until there comes a solution of this. [Laughs] But who knows? It may be fundamental. We give this at the moment fairly little attention, but I've been impressed by the fact that I like both arguments. I like both the distance scale and the time scale arguments. They appeal to me, and they are different. They lead to different results.
Do you have any opinion as to why this discrepancy is not drawing a lot of attention right now? Do you think it's because people are interested in other questions?
I think partly because the debate between mostly de Vaucouleurs — but then later [Marc] Aaronson, [Jeremy] Mold, and the others — on the one side, and Sandage and [Gustav] Tammann on the other side, got too boring. People are just fed up with it, I think. They both come up with solid statements of what's so bad about the other method etc. As a good Pasadenan, until recently, I had tended to believe that Sandage was probably right, even though it was somewhat funny that where he changed arguments he always got to H0 = 50 ±5. But in the meantime, one or two of the fundamental pillars — which de Vaucouleurs didn't like as you remember, he wanted to have many small arguments — one or two pillars of Tammann and Sandage's argument have not done well lately. And that was the one that gave 50 and would be in agreement with the long time scales. So I think that's one of the major problems. What are other major problems? Well, strangely enough — and that is clear from my earlier statements — I wouldn't say that the flatness problem is a problem. I would just say, well, omega is 1. The communication [horizon] problem I would still consider a problem. I apparently don't feel that it is really resolved, although it's a fantastic attempt that is being done. Then, finally, it is my feeling — although I'm not sure that others share this — that all these big motions and these big mass concentrations that one needs are just an incredible mess. I'm not necessarily inclined to believe that whole story. I don't know where it's going.
The Seven Samurai work?
That's right. And then [Alan] Dressler with his great attractor etc. They are many close and dear colleagues of mine. I'm not absolutely sure that I believe everything that's going on there. But it's fascinating.
You think it's something that needs to be looked into more.
Yes. But it's fascinating and it's certainly true that the Rubin-Ford thing for ScI galaxies gave this funny effect around the sky that you remember. They really started the thing, and apparently what they found is still around. So something is going on, but I consider what's happening in that field — although the people who are in it are probably deliriously happy — as a potential problem, where I wonder whether most of the statements that are made really will hold up very long. But it's just a feeling.
Let me ask you to take a big step back and ask you a speculative question. You need to put some of your natural scientific caution aside, perhaps. If you could have designed the universe any way that you wanted to, how would you do it?
[Laughs] Well, that is surely the ultimate question that doesn't allow one to be cautious. What a question, Alan. That is really bad.
This question was first asked to Dennis Sciama.
You going to leave it at that?
I'm not going to take responsibility for thinking up the question, but I've asked it to many other people and people have given various answers.
God! What a wild question. This is really the worst question I've ever heard, I should say. The first thing I would say is that I would never have constructed a universe in which it was even allowed to ask such questions [Laughter]. That's a question to which I'm not sure I can give an answer. It's like asking me to take off my clothes and start walking around here. I mean one has naturally a certain shyness about things, and I think that it is absolutely in conflict with my philosophy about doing science. I will finish that in one sentence, because that's not what you want. I volunteered to you earlier, even when you didn't ask the question, that I don't have fairly strong ideas about how things in the universe work if I have had no input on which I can base that. So in the very early days of quasars, for instance, even before black hole scenarios with accretion disks, people would often say to me, "Well, you have been so active in quasars. What are they and what do they do?" I say, "I've not the faintest idea. This is possible, that's possible, that's possible. This is what's been brought up, mostly not by me." And they would say, "Well, what do you think?" I say, "I don't know. I have no feeling for it." And here you can see how unreasonable your question is to ask me to construct a universe and to indicate a preference. It's something that is totally out of my mode of thinking and my mode of working.
If I can be allowed to say something else, unless you want to ask another question, since I didn't give you a good answer to this one. Light man: I'm going to ask you one more question, but if this [last question] stimulates another thought, please say it.
Well, briefly, as one of the perpetrators of looking at the distant universe, I find it extraordinary that it is possible with human means, with pieces of glass that are no larger than this room, to see things that are interestingly far out in the universe. Sometimes it strikes me that the universe is much smaller than... All right, here we go. I would have constructed a bigger universe. I think the universe is small. There we go. If I'd had my rathers, I would do that. I find the universe too confined. I find it amazing it's so small.
Can you explain that, what you mean by small?
All right. We have a certain concept of what is a reasonable distance that we have some feeling for. In astronomy, you probably think that many astronomers would have a feeling for the distance to Andromeda [a galaxy about two million light years from the Milky Way]. You talk about it so often and it's fairly big in the sky, and so on. So there you are. It's at one half megaparsecs. Now we know that the universe is of the order of — what is it — 6000 or a few thousand megaparsecs, so apparently...?
You mean the observable universe.
Yes, right, right, to a redshift of 1 or 2 or whatever. But anyhow, it is extraordinary that we can get out to a distance where the light travel times is a substantial fraction of the age of the universe, in only a few thousand times the distance to Andromeda. It could have been any arbitrary number of course. It could have been 44 million times the distance to Andromeda. So, I find the universe small. I'm surprised. I would have made it much bigger, I think. I find it remarkable how small the universe is.
It would be much bigger if the universe were a lot older. If Andromeda was still the same distance but the universe was a lot older.
Absolutely, you can translate it into time and say 1010 years is amazingly young.
Can I say it that way?
Yes, yes. 1012 or 1015 would have been more reasonable perhaps. But I got to that because I was trying to explain to you why I find that with a fairly small piece of glass — considering the size of whatever you consider — you can catch enough light to study fairly distant parts of the universe, and have some chance, even, to observe these things. But that's really the same question. It's the size of the universe. That's how I got to it.
So you would rather the universe be much bigger than we had any...
Of course, I wouldn't rather, but I think that if I, in my innocence, had constructed the universe, I would probably accidentally have made it much bigger. I wouldn't have thought of making such a small universe. Anecdotally — and I'm not sure whether it was right — I heard that when it was clear that the turnover of the counts of radio sources really had been understood well by people in radio astronomy, [Martin] Ryle had expressed that [result] was almost disappointing. Because here was the end of the universe. And I share that. I mean you'd almost feel claustrophobic in a universe that is [so] small [that when] you just look at one of the first results in radio astronomy, you already see the effect of the end of the universe. That's amazing. So perhaps that strikes me.
That's a very nice answer to my question.
I don't know.
Let me ask one final question. There's some place in Steve Weinberg's book The First Three Minutes where he makes the statement that the more the universe seems comprehensible, the more it also seems pointless. Do you ever wonder about the point of the universe or whether the universe has a point?
No, I think, in fact, I would probably say that in terms of my personal experience, the tint of my thinking would be the other way around. To me, the universe becomes more and more incomprehensible, and this probably more reflects a personal view. Weinberg understands more and more of the universe. I understand less and less. When you think you understand things more, I think it's really just an extension of your awareness of the problems.
 M. Schmidt, "A Model of the Distribution of Mass in the Galactic System," Bulletin of the Aatronomical Inatitutea of the Netherlanda, vol. 13, pg. 15 (1956)
 M. Schmidt, "The Distribution of Mass in M31," Bulletin of the Astronomical lnstitutes of the Netherlands, vol. 14, pg. 17 (1957)
 M. Schmidt, "The Rate of Star Formation," Astrophysical Journal, vol. 129, pg. 243 (1959); Astrophysical Journal, vol. 137, pg. 758 (1963)
 M. Schmidt in Symposium on Stellar Evolution, November 7-11, 1960 (La Plata, Argentina: Astronomical. Observatory, National University of La Plata, 1962)
 R. Minkowski, Astrophysical Journal, vol. 132, pg. 908 (1960)
 M. Schmidt, Astrophysical Journal, vol. 141, pg. 1 (1965)
 M. Schmidt, "Spectrum of a Stellar Object Identified with the Radio Source 3C 286," Astrophysical Journal, vol. 136, pg. 684 (1962); "3C 273: A Star- Like Object with a Large Redshift,” Nature, vol. 197, pg. 1040 (1962)
 Schmidt is referring to his upcoming talk on quasars at the Yamada Conference on "Big Bang, Active Galactic Nuclei, and Supernovae," Tokyo, March 25-Apri11 , 1988
 R.H. Dicke, Gravitation and the Universe, The Jayne Lectures for 1969 (American Philosophical Society, 1969), pg. 62.
 C.W. Misner described the horizon problem, and tried to solve it, in "The Isotropy of the Universe," Astrophysical Journal, vol. 151, pg. 431 (1968)
 A. Guth, "Inflationary Universe: A possible solution to the horizon and flatness problems," Physical Review D, vol. 23, pg. 347 (1981)
 Yamada Conference on "Big Bang, Active Galactic Nuclei, and Supernovae," Tokyo, March 25 - April 11, 1988
 V. de Lapparent, M.J. Geller, and J.P. Huchra, "A Slice of the Universe," Astrophysical Journal Letters, vol. 302, pg. L1 (1986)
 H.P. Haynes and R. Giovanelli, "A 21 Centimeter Survey of the Perseus-Pisces Supercluster. I. The Declination Zone +27.5 to 33.5 degrees," Astronomical Journal, vol. 90, pg. 2445 (1985)
 R.P. Kirshner, A. Oemler, Jr., P.L. Schechter, and S.A. Shectman, Astrophysical Journal Letters, vol. 248, pg. L57 (1981)
 G. de Vaucouleurs, "The Case for a Hierarchical Cosmology," Science, vol. 167, pg. 1203 (1970)
 A. Dressler, S.M. Faber, D. Burstein, R.L. Davies, D. Lynden-Bell, R.J. Terlevich, and G. Wegner (referred to as the "Seven Samurai"). Among their series of papers on large-scale motions is "Spectroscopy and Photometry of Elliptical Galaxies: a Large-Scale Streaming Motion in the Local Universe," Astrophysical Journal Letters, vol. 313, pg. L37 (1987)
 A. Dressler, "The Large-Scale Streaming of Galaxies," Scientific American, vol. 257,pg. 38 (1987)
 V.C. Rubin, W. Ford Jr., and J .S. Rubin, "A Curious Distribution of Radial Velocities of Sci Galaxies with 14.0 > m > 15.0", Astrophysical Journal Letters, vol. 183, pg. L111 (1973)
 S. Weinberg, The First Three Minutes (Basic Books: New York, 1977), pg. 154