Oral History Transcript — Dr. Phillip James Edwin Peebles
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Interview with Dr. P. J. E. Peebles
P. J. E. Peebles; September 27, 1984
ABSTRACT: Early life and schooling in Manitoba, Canada. Undergraduate studies (engineering, later physics) at University of Manitoba; graduate studies at Princeton University, Ph.D. 1962 (Robert Dicke); the gravity research group. Comments on family. Rest of interview is mainly discussion of his published papers on temperature of meteorites and Paul A. M. Diracís cosmology (with R. Dicke, J. Geophys. Res., 1962) done before his thesis work: Blackbody radiation and the formation of galaxies (Astrophysics J. 1965), helium production (Ralph Alpher and Robert Herman); primeval helium abundance (1966); works on pregalactic objects, young galaxies (1967-68), superclusters of galaxies (with Jer Tsang Yu, Astrophysics Journal, 1969) and masses of galaxies (with Jeremiah P. Ostriker, 1974); Big Bang cosmology (with Dicke, 1979). Collaboration with Dicke, his graduate students and other co-workers. Comments on the Space Telescope and on the dark matter puzzle.
Harwit:This is an interview with Professor Peebles at Princeton University. We are talking in Jadwin Hall and itís 9:15 in the morning, and weíre in a little conference room next to Jim Peeblesís office. Itís September 27, 1984. Okay, Jim. I know you were born in Winnipeg in Manitoba. I wonder whether you can tell me anything about early days that you remembered — family background, schools, friends, influences.
Peebles:Well, I was always interested in things mechanical. One of my earliest memories of something that caught my attention was of a steam locomotive. I guess mainly because they have so many moving parts that are out in the open. You can watch the valves moving back and forth ó- the driving rods. Also, of course, it was very big and made a lot of noise, so that was something that I always liked to watch; always begged to be taken down to watch the trains. As a kid I did enjoy building things; learned quickly hi to make gun powder. I built sleighs, forts, houses in the back yards of houses. My father was also quite mechanically inclined, and I guess he was a strong influence in helping me work with my hands with woodworking tools which I enjoyed very much.
Harwit:What did he do?
Peebles:He was a clerk in the grain exchange in Winnipeg. Thatís the main commodity exchange outfit for Canada.
Harwit:Was your mother working also?
Peebles:No, she never left the home - strictly a housewife. My two sisters, I think, were never particularly mechanically inclined. Nor, I suppose, any of my friends. It was something I did on my own.
Harwit:Were you the youngest?
Peebles:I was the youngest of three.
Harwit:When you say you were mechanically inclined. Did you work with things that were available around, or did you...?
Peebles:Strictly informal. I never had formal instruction, aside from the usual school shop on things mechanical. It was what I could find in the basement — the tools of my father; the pieces of wood in the back yard; the scrap yard down the way; taking apart old automobiles, for example; never organized, and never occupied a good deal of my time, although I enjoyed taking apart, say, a speedometer from a car. I never built a car; I never, in fact, repaired a car to make it run.
Harwit:Did you read Popular Mechanics or Popular Science?
Peebles:Popular Mechanics with great pleasure. The ads in the back were endlessly fascinating to me. I never wrote away to a single one of those breed chinchillas for profit and fun, or buy this or that tool, but many a happy hour did I spend with — what were they — Mechanics Illustrated and Popular Mechanics. The ads in the back, I donít know why, but there was just Ůd1ess reading and speculation about why...
Harwit:Do you remember any of them, particularly?
Peebles:Do I remember them? No, they all fade into this vision of curious things being offered for sale.
Harwit:How old were you then?
Peebles:Oh well, this must have started quite early, when I was six or eight, I really canít say and continued until I entered high school when I guess I left such childish things behind.
Harwit:Well, those werenít so childish. A lot of boys start repairing cars around that age.
Peebles:Yes they do, I never did, in fact. I didnít do much in things mechanical in high school, except learn to square dance and other such activities.
Harwit:Now, the school you went to was mainly rural, city?
Peebles:It was suburban, far suburban, an outlying satellite of Winnipeg, which is the major and really the only city in Manitoba and practically half of the population of the provinces is in and around Winnipeg. There are several some towns in the outskirts within which people live and commute into the city. I lived in a small town of that sort, first Norwood and then St. Vital, small tins, small high schools, with not too much going on in them. I had good time in high school, but I donít think I learned a lot.
Harwit:Did you get interested in science then, or in math?
Peebles:No, no neither. The science courses were totally forgettable. The math course, there was a nice course, I recall still, on geometry — the proofs of congruencies of triangles, and so on, that I thought were very neat. That is perhaps the one course in high school that I can remember with pleasure.
Harwit:Was the Canadian educational system at that time influenced by the British system?
Peebles:I donít know. I donít know what the British system is. The Canadian is very much like the U.S.
Harwit:I see. You didnít specifically go through a lot of Euclidís theorems?
Peebles:Well, in this one course we did, whether that was just the taste of our particular teacher, or whether it was standard item in the curriculum, I donít know. One difference from the United States and maybe from Great Britain is that there is considerable variability from province to province in educational requirements, maybe even from town to town. I donít know what standard syllabuses would be in Canada.
Harwit:Do you remember any of the teachersí names...your math or geometry teacher?
Peebles:Now what in the world was his name? I remember his face very well — an ancient Scot, I believe, but I donít remember his name. No, I donít remember a single teacherís name from high school. I was not a very attentive student as you can gather.
Harwit:Did you do sports?
Peebles:I skied enthusiastically. Looking back on it, it seems like a crazy thing to do in Manitoba, the winters are so cold, and itís a semiarid climate, the snow is quite inadequate, and there arenít really any mountains of consequence. But there are hills, and there were skiers, and it was something I very much enjoyed doing. In fact, — this is quite irrelevant — but Iíll tell you that I dropped skiing when I came to Princeton, some 25 years ago. The winter before this past I was on sabbatical leave in British Columbia; took up skiing again. There, itís such a wonderful thing to do because the snow is deep, the weather is always mild and the mountains are high. Itís quite a different scene from skiing in Winnipeg.
Harwit:You probably donít have to stand in line at ski lifts.
Peebles:And on Vancouver Island the crowds were quite low, actually you didnít stand in line for the ski tow in Winnipeg either. There werenít that many of us crazy enough to go skiing in that miserable weather. The other sports, well, what were they? Snowshoeing, hiking, skating -ó we used to skate a lot. There were community clubs in Winnipeg at that time, maybe still are. I was always associated with an outdoor skating rink for the wintertime. There would be dances, parties, as well as skating at these community clubs, and that was the standard place to hang out. And I did hang out a lot at such places.
Harwit:Now, when you got to the university, at what point did you decide you were interested in science, or was that earlier in high school already?
Peebles:No, I wasnít particularly interested in science when I entered university, more in technology. In fact, I entered the engineering school, and lasted there for two years. I would have finished in engineering, but most of my friends — just an accident, I think — were majoring In physics, and I didnít find engineering all that exciting, so I decided to switch over, which I did to the physics department.
Harwit:Why do you say itís an accident?
Peebles:Well, I simply had a random selection of friends. And...
Harwit:Okay, but, you picked those, right?
Peebles:I guess I picked them, and I canít tell you why I picked them, some because they were also from St. Vital, where I grew up. Who was St. Vital? It nest have been some ancient saint. There was St. Nine and St. Mary. It must have been settled by French Canadians. Yes, there are still quite a few around in that area. When I was a child in St. Vital, it was strictly Englishóspeaking Canadian.
Harwit:St. Anne is with an eĒ at the end?
Peebles:Yes, and St. Mary and St. Boniface. There are a surprising number of French Canadians in Manitoba, but isolated. The sociology is very bad on that. Well, I somehow picked friends who were for the most part were majoring in physics, rather than engineering. I somehow had the feeling they were having better parties than we were having in engineering. So it was quite a flip decision, but it turned out to be wonderful. I instantly felt that physics and the math that went with it were great, and I was delighted with the switch, and havenít looked back since.
Harwit:Was the university co-educational?
Peebles:Oh yes. There is only one substantial university in Manitoba — the University of Manitoba. There are small colleges, mostly associated with the university, but as you can imagine for such a small province, one university is enough. Majoring in physics, there were a few women, very few. In my class, there were perhaps ten of us majoring in physics. One woman started out with us — Grace Bloxham — but in fact she decided to switch to...Iíve lost track of her completely...I think she switched to law, is my vague recollection. She was by far the most literate of us in that group. It was a small class of those majoring in physics, and a good class.
Harwit:Have any of the others stayed in physics as far as you know?
Peebles:Oh yes, I donít keep at all close track of them, but I can remember a few of them. Dave Masson was at the University of Toronto, last I saw him, but I havenít seen him in several years. Wilbur Johnsson, Scandinavian, was, I guess, among us, one of the most interested in mathematics, rather than physics. He, last I heard, was at McGill in Canada again. Ian Connel, also an excellent mathematician, Iíve quite lost track of him. Maybe my best friend was John Moore. He, who was more interested in the practical side of physics than research, took a job with an obscure company called Schlumberger, you know the big oil instrumentation company? Itís an enormous company. It must have been rather small then and just starting out, and he was going to go off to Saudi Arabia to design and build instruments for oil, search for oil. Canít imagine whatever became of him, whether he was killed in a riot in Saudi Arabia or has become a high executive of Schlumberger, or what.
Harwit:What does the company do?
Peebles:They build the instruments that are lowered into oil wells, to make measurements that might reveal whether or not one is coming close to hydrocarbons. As you can imagine thatís an enormously profitable thing to have been doing in the last ten years. So Schlumberger has been big in instrumentation and big in Saudi Arabia. The combination means that they have enormous amounts of money. There are now Schlumberger research institutes that lavish ridiculous sums on mathematicians who do percolation theory, for example, as you can imagine.
Harwit:Yes, I see, sure. Any other students that you remember from that time?
Peebles:Well, itís interesting, they were a decidedly mixed bunch. There was one friend whose name Iíve now quite forgotten who, I believe, originated in Hong Kong. His family set up a clothing manufacturing company in the Barbados. His love was physics, so he got into the University of Manitoba. But then he was needed to run the family clothing manufacturing company, so he gave up physics, went back to the Barbados. Iíve only heard from him because he dropped by when his son was looking at colleges, a very prosperous guy, but nonetheless stuck down there, keeping the family operation together.
Harwit:How about professors?
Peebles:Yes, they, of course, were not nearly as active as people in a university like this, but they certainly loved physics, and they transmitted that feeling to us.
Harwit:By which you mean, I guess, that they were not research professors, but ratherÖ
Peebles:They were not research professors, although they did write papers, but their main function was lecturing. Some of them are in fact research professors—Ken Standing—I believe, a nuclear physicists, was quite a young man at the time. He was probably the main motivation for my going to Princeton rather anywhere else, because he was a graduate student at Princeton, and he thought this was a good place to go, and urged me to do the same. And Iím sure wrote a strong letter to people knew here to get me in. He, as I say is a nuclear physicists, still active at that game at the University of Manitoba. Then there were several others who certainly stick in my mind. Sam Neamton was a mathematical physicist. I thought an awfully good lecturer, and broadly knowledgeable in physics. He perhaps inspired me as much as anyone in theoretical physics. Then also very inspiring to me was Harold Coish; I think still active. Sam Neamton died a few years ago. Harold Coish still is active in gravity physics.
Peebles:Yes. Is there a connection with my doing gravity physics here? I doubt it. It was unconnected. Syd Standil. Interestingly enough Syd Standil is an uncle, I believe, of John Bahcall.
Peebles:Curious connections. I see Syd every once in a while at meetings. Havenít kept up with him other than that.
Harwit:John isnít Canadian, though.
Peebles:No, but these curious connections are somehow made. I donít know where the connection developed. Oh, there was a wonderful...now who was our chairman at the time? We used to call him Barney. Bernard...what was his name? At one time we had all made a pact, probably at one of our drunken parties, that the first of us to make a great discovery that required units would use the unit of the Barney.
Harwit:I see. (laughter)
Peebles:Now, what was Bernard...oh my...wonderful guy...beautiful lecturer. As I think back I am impressed with how good the lectures I got at the University of Manitoba were.
Harwit:Better than at Princeton later?
Peebles:Here, of course, the faculty are preoccupied with many things, from deals in Washington, to their own research.
Harwit:I wasnít asking that. (laughter)
Peebles:You know, we at Princeton pride ourselves on being a teaching university, and maybe among the research universities we are one of the better at teaching. But still, we are distracted.
Harwit:I know just what you mean. (laughter)
Peebles:Overly defensive. Bernard Whitmore. I see him still from time to time. Heís gone...heís retired, I think to England and is witty as ever.
Harwit:And these people taught you respectively what types of courses?
Peebles:Well, you know at Manitoba there was much less emphasis on liberal arts than there is at a place like this. I took very few courses in anything but technology. As an engineer, I took perhaps one course in philosophy. Once I started majoring in physics, I took absolutely zero courses outside physics and math. So this was a very unlettered arrangement, but an awful lot— of physics and math. After I transferred, I was in physics for three years. Itís a fiveóyear course there, so I took years three, four, and five in physics. We didnít get too deeply into the twentieth century, if the truth were known, a very good quantum mechanics course, itís true; but it was strictly nonrelativistic and building on things you would have taught if you had taught it in 1940, rather than 1955. The classical physics we learned was thorough, and always proved awfully useful to me, particularly as I do astrophysics, we use a lot of classical physics. I really had it drummed into me. Are you asking which course I had that I might remember?
Harwit:Yes, particularly these that were enjoyable. But also which of these courses were taught by these respective people?
Peebles:Well, In the third year, the most memorable course, maybe, was Whitmoreís properties of matter, which was a course in advanced mechanics, really not too much to do with matter, per se, but rather mechanics. And we learned the most obscure things, like how to treat the vibrations of the pendulum with large amplitude, and such things. We talked about surface tension at great lengths. We talked about rigid body motion. We didnít use Lagrangian formulations, but we did do an awful lot of mathematical analysis, and I enjoyed it a lot, particularly because Whitmore was a humorous, as well as polished lecturer ó- itís his style I enjoyed. Perhaps I unconscientiously try to copy him now. I remember Neamtonís course in quantum mechanics very vividly. It was quite an experience; I loved it. Coish on advanced mechanics through Poisson brackets, and the like, was a lot of fun. They can go on and on... Ken Standingís course on electromagnetism — Maxwellís equations — really very well done. It was a course that would do Princeton proud. He might have even taught it here.
Harwit:So you must have been very well prepared when you came here, perhaps much better a lot of the American students who came.
Peebles:In the classical side I think I was better prepared than the average. On the more modern side of physics I was less well prepared, I suppose, but it was easy to catch up.
Harwit:But In those days there would have been few colleges or universities in this country that would have taught relativistic quantum mechanics. I mean, many students...
Peebles:Thatís very true.
Harwit:...came without any quantum mechanics of any depth.
Peebles:I donít know whether students came to Princeton without quantum mechanics. Some students came knowing a lot more solid state than I, a lot more nuclear physics, a lot more particle phenomenology than I knew. But as I say, if that were the case, it was easy for me to catch up on those details. So, of course, the other aspect of that was I was a big fish in a small pond at the University of Manitoba. It was certainly an interesting and maybe slightly traumatic experience to come to a place like Princeton which was full of people who were similarly experienced.
Harwit:Did you find it tough at first?
Peebles:Yes, I guess tough perhaps more emotionally than on the physics side, the academic side. Being surrounded by so many people who were obviously so very smart, and knew so much more than I, both my fellow students and all those post docs and faculty were really a deeply impressive lot. I remember one of the first courses I took here was quantum mechanics from Mary Goldberger. It was a spectacular course; covered roughly the area of the quantum mechanics course I had taken at the University of Manitoba, but at a rate that must have been an order of magnitude higher and a depth that was comparable. I was very impressed. Maybe a little depressed too, thinking, well, this guy is so smart (laughter).
Harwit:Yes, I know what you mean. Itís always difficult to really appreciate what a head start 20 years of experience gives someone.
Peebles:Itís hard to factor that in. Then I still do believe that Mary is intrinsically smart (laughs). The combination is pretty strong.
Harwit:Let me turn this off for a second.
Harwit:Weíre back on tape; we just stopped for a second here. Go ahead.
Peebles:Where were we? My impressions upon entering Princeton?
Harwit:Yes, and your courses here.
Peebles:And my courses here. The courses are highly variable, were then, still are. Some of the courses I quickly dropped; others I found went surprisingly slowly. Which ones went slowly? John A. Wheeler was always an inspiring lecturer, though I must say he taught a course in thermodynamics, and I guess because he found thermodynamics and statistical mechanics a - well-defined subject, he wasnít about to spend any time talking about the subject at hand, but rather spent the whole term trying to get us to think about the side issues, perhaps the fundamental points, the areas that werenít so well understood. But at the time I didnít appreciate what he was trying to do, and so I wasnít as happy with that course as I should have been.
Harwit:Did he teach general relativity?
Peebles:On an occasion he was then teaching general relativity. By the time I had arrived, his interests had already switched heavily into gravity physics and general relativity. I never took a course in general relativity from him or anyone else. It just happened that when I was interested in taking courses, he was teaching other things. The only course I had from him, as, I say, was a thermodynamics and statistical mechanics course.
Harwit:I guess Princeton was one of the few departments where a course in general relativity was being taught in the physics department. It seems to have been not considered as a serious physics subject at the time.
Peebles:I think you are right. General relativity had been through rather tough times in the years up to the time I arrived at Princeton in 1958. The physics in it was not seen to be too substantial, the mathematical challenges were recognized, and so it was perhaps more considered a mathematical than physical subject. But of course Princeton had a long tradition of general relativity, beginning with Einstein and with H. P. Robertson, with Valentine Bargmann. Certainly, Eugene Wigner had a continuing interest in relativity, and then John Wheelerís interest switched to this direction, I am not sure just when; in the middle 50s, it must have been.
Peebles:Yes, yes, of course...a mathematician. Have you name quite right? (laughter) Yes, and so by the time I had arrived there was certainly a conviction here that there was work to be done in gravity physics and relativity, both John and Bob Dicke having switched their interests to this field, slightly before I arrived. I donít know whether courses in relativity were given at Princeton, say, in the early 50ís, before these two guys had decided that this was an early field, and after Robertson had left.
Harwit:He had gone to Caltech much earlier, hadnít he?
Peebles:He had gone to Caltech, and I am not sure just when. Robertson was here — when, before World War II?
Harwit:I think before World War II. I donít know whether he came back here; he might have.
Peebles:He was back here at least briefly after World War II. Iíve talked to George Reynolds about this and his recollections of Robertson. He was certainly a lively guy.
Peebles:I never met him.
Harwit:I think met him once in 1960, just before he...I think he was killed in a car accident.
Harwit:Which courses interested you most then? I mean, did you go into gravity work right away?
Peebles:No I didnít. No, in fact, the courses that took most of my attention were in quantum mechanics, quantum field theory, and maybe the course that most strongly attracted me — the two courses, I recall — one by Sam Treman on quantum-electrodynamics, beautifully taught, polished, a whole yearís worth of elegant exposition by Sam. Heís got to be one of our best lecturers. And of course by Mary Goldberger on his work on dispersion relations. You recall that was the big project in those days. I think there was one tern that was interestingly done. He was at the time planning to write a book on methods of dispersion relations in quantum field theory. I donít think he ever did...
Harwit:Geoffrey Chew came out with one then.
Peebles:Yes, thatís true.
Harwit:And maybe thatís was...
Peebles:That was later.
Harwit:Geof Chewís book...I think is...
Peebles:I think it was a paperback.
Harwit:Yes, that was around 1961, Ď62, I think.
Peebles:Well, thatís about the time. I would have taken this course in about 1959, 1960, somewhere in there.
Harwit:Who where fellow students with you?
Peebles:Well, who do I remember in particular? Perhaps my closest friend in those days was Ken Turner. He is now at the Arecibo Observatory. You know him, of course.
Harwit:No, I donít know him actually.
Peebles:Oh, well, he is at Arecibo.
Harwit:You know, Arecibo is several thousand miles away.
Peebles:I tend to forget. (laughter) Who else? Well, quite a few people. Jack Benoit. Who else? Quite a few... For example, Bob Pollock, a fellow Winnipeger, now at Bloomington, Indiana University, running their cyclotron, weíre pretty close with.
Harwit:How big a student body was there in the graduate school in physics?
Peebles:I think the average class was 15 to 20, average tenure of four years. There must have been upward of 80 people around at the time.
Peebles:Similarly to what we have now. In fact, it was on the rise. Shortly after I finished up, the number of students reached an extremum of over a 100, and it has been slowly sinking back down since, to around 80 on the premises at the moment.
Harwit:Now, at what point did you get involved with Bob Dicke, because you did your thesis with him?
Peebles:I think I started out...I did start out with an interest more in quantum field theory, particle physics, than relativity. And what attracted me to Bob Dicke was his seminars, an informal evening meeting in which his research students and post-docs described what they were doing. I learned about those meetings and was attracted to then because of another friend Bob Moore, who as it happens is another Winnipeger, with whom I was quite friendly. He was working with Bob Dicke on a research problem as a graduate student. He told me about these meetings. I casually attended, and was instantly taken with the mix of fun physics as well as fundamental questions that Bob Dicke was dealing with. We got to discuss, oh, the motion of the moon, the rotation of the earth, the evolution of stars, the evolution of the sun, the expanding universe. It was fantastic. I mean, we were taking about everything from meteorites and radioactive-decay-ages of them, through to the expansion of the universe. And I was strongly taken with the breadth of neat physics that he did.
Harwit:Is it when you got interested in doing this paper on the temperature of meteorites and Diracís cosmology?
Peebles:Yes, that was one of the things. It was of course Bobís idea to do this. He told me what to do and I did it. At the sane time, I found it very much to my taste, to do this sort of thing, to get to play around with meteorites ó- theyíre a fascinating thing — but then also to use them as a probe for some other fascinating questions. Really, I found very much to my liking.
Harwit:And that you did while you were still a student?
Peebles:Yes, I was a student. But in fact, I think that was part of my Ph.D. Thesis, which was a melange of topics, all having to do with the question of whether the fine-structure constant could vary with position or time. Could you make a model for that that was consistent with constraints such as radioactive-decay ages, the Eotvos experiment.
Harwit:Yes, I guess the Eotvos experiment was going on around that time.
Peebles:Thatís right. People like Bob Moore, Peter Roll, Bob Krotkov were all heavily involved in the Eotvos experiment. Bob Krotkov is now at the University of Massachusetts, Amherst. Peter Roll at, I believe, still at the University of Minnesota, Minneapolis. Bob Moore, I donít know where he is now. Jim Brault does solar astronomy at Sacramento Peak. I worked, I think, for at least for one summer, perhaps two, if I recall, as a research student doing jobs for the Eotvos experiment. I built, for example, an oven to use in degassing the instruments. And I did a fantastic job, except no one had told me that brass screws will sublimate their...what is it tin?
Harwit:Oh, yes, yes.
Peebles:So it was a beautiful oven, but it had to be taken apart and steel screws replaced. So it was odd jobs. I was never close to the Eotvos experiment, watched it from some peripheral distance.
Harwit:But you were doing experimental work then, initially?
Peebles:I donít know if you would call it experimental work; I was doing odd jobs. And I never did get very far into any experiments. There was a time when Bob and I were discussing an experimental project for me to work on, involving the radioactive decay age. I still remember it of Rhenium 187, which decays to Osmium 187, and at the time had the record for the smallest decay energy. Thatís important, of course, if youíre interested in variation of physical constants, because a small change in a physical constant can change the decaying energy by a lot if the decaying energy is small. So we were interested in knowing — at the time it wasnít at all well measured ó- what is the half-life of Rhenium 187, because one had meteorites in which the decay products were measured, and so you would have a handle on possible evolution. I still remember an amusing experience where I drew a little set of plans showing how I would do the experiment. I remember Bob Dicke was over working on the Eotvos experiment. I took them over to him on my bicycle, and he sat and he looked at these plans, holding them a considerable distance from his face and finally saying, ďHave you considered doing theory?Ē(laughter) In any case, I think, theory was always more to my taste than experiment, despite the fact that as a kid I always loved working with my hands and still do. I also very much enjoy pushing a pencil, and I get impatient when I am not instantly gratified by something coming out the other end. So it didnít require much of a push for me to continue doing the theoretical analysis I had been doing and to turn that into a thesis.
Harwit:Has Bob had a number of students whoíve done theoretical work, or has most of it been experimental?
Peebles:The large majority experimental, certainly. Although, as you know, he combines a brilliant skill in experimental physics with wonderful insights in the theoretical side. I was his first theoretical student. There were a few others. Ed MacDonald worked on the theory of oscillations of the sun, particularly in connection with the question of whether the quadruple moment of the Sun, the gravitational quadruple moment, could be large enough to influence the test of the precession of the perihelion of Mercury. He [MacDonald] is now, I believe, at the U.S. Naval Research Laboratory doing plasma physics. He has had, then, a few theoretical students. By far, though, the majority have been experimentalists — always required to learn a hefty amount of theory, though, as part of their experiment.
Harwit:Well, he certainly does very sophisticated experiments that most other people would not even conceive.
Harwit:Now, itís unusual for someone who has been a student to stay on at the same university. How did that come about?
Peebles:It was never planned on my part or so far as I know on the part of Bob or the faculty here. It just happened. Itís often the case, of course, that you will stay on for one or two years as a post-doc...and that was automatically done. There were lots of things that Bob and I wanted to do, and we did it. I, at first, as a non-teaching post-doc. I do remember looking around for other jobs considering possibilities. I even traveled to a few spots for interviews, to give colloquia. There was never any strong offer that I couldnít refuse, so to speak. Always it seemed attractive — letís stay around here another year. Get some more work done; we enjoy the place; weíll leave in a few more years, At no time until I, in fact, got tenured did I ever contemplate staying on here permanently. Just happened.
Harwit:So you sort of slid into an assistant professorship?
Peebles:Yes. Of course, the sliding got strongly pushed by the fact that the microwave background came along, and I became suddenly salable, and I started to get good offers. So very quickly it was decided to accept a professorship here or to go on somewhere else. So I decided...
Harwit:Let me backtrack before we get to the microwave experiment. I noticed you had done on your own this paper on description of particles in quantized field theory.
Peebles:Yes, a hangover from my old love, which I still keep, of particle physics.
Harwit:But this was sort of done on the side and was not specifically an interest of the Dicke group, if there was such a... In fact, there was such a thing, I guess?
Peebles:Oh yes, we called it still do — the gravity research group, and it was nucleated very definitely around Bob Dicke; continued the name, continued the traditions. They had no interest in this; I did it on the side. Talking to other people like Jack Benoit and others, and to the faculty, people like Sam Treman. In fact, that paper was inspired by Mary Goldbergerís lectures on dispersion relations. I remember well his discussion of the asymptotic condition, which he felt was unsatisfactory. I agreed and was inspired to write that paper.
Harwit:Was it that you wanted to try your hand at different things to sort of acquire a tool kit of methods, in addition to the intrinsic interest of the problem, or how did it get started?
Peebles:I think I am not strong on planning. There was certainly no thought on my part that it would make me a better physicist if I attempted to work in more than one field. Iíve tended to work on a problem because it attracted my attention. I thought it was fun to work in quantum field theory; seemed like, and still does seem like, a very neat, fascinating subject. I worked on it because it fascinated me. I guess that paper on the asymptotic condition is the only one in what you might call formal quantum mechanics that I published. I did work on many other topics in that field. At one time it was my great dream to find a quantum theory of gravity, and I spent a lot of time learning about that subject and making equations in it. I even wrote a paper that was, I think, very rightly rejected by Annals of Physics on quantum gravity. I think it was rejected. There isnít a paper in there, is there, onÖyou would have noticed it?
Harwit:I didnít come across anything like that. Unless it was...
Peebles:It would be very early...
Harwit:No, I mean I looked at all the different classes of things that you did, and...
Peebles:I spent a long time on that; but I think that I didnít have the right approach, the approach that people are now taking up.
Harwit:Well, that doesnít necessarily imply that that is the right approach...
Peebles:Thatís true. Thatís true.
Harwit:Öeither. Thereís no guarantee that one will find gravitinos, but.
Peebles:You should go to superunified theories?
Harwit:Yes. Presumably one will find some sort of a scheme eventually that unites gravity with other theories, possibly ten years away, but itís not clear what the format is going to be.
Peebles:Of course. So until we have the answer, we certainly canít anticipate what the methods will be...
Harwit:Letís talk about some of the interests you had in planetary work very early on, because I notice you had a number of papers here. I was wondering what triggered that? Was it the fact that you had worked on meteorites that sort of sucked you into the planetary system?
Peebles:That must have been part of it, but really the main factor was Robert Henry Dicke. He was still my mentor very much. He was interested in Jupiter for reasons of tests of variable G; and certainly if the gravitational constant were varying on a timescale of the Hubble time, it would affect the structure of Jupiter, as he recognized. So he was interested in Jupiter; we got to hear a lot about it because he would talk about it at the gravity group meetings. At that time, W. C. De Marcus [a physicist, now at the University of Kentucky] wrote an excellent paper on the structure of Jupiter, taking the best estimates he could make of equations of state, and then computing equilibrium gas spheres, to try to match to the known properties of Jupiter and Saturn, including the fact that they have known quadrupole and higher order gravitational moments from the motions of the satellites. All I did was to take that paper and redo it, using a computer so one could make the models much more detailed than had DeMarcus. Using, I think, better equations of state, which I mostly made up myself, I guess; and of course computers had just become readily available, so it was so much easier to make models. I could make a whole series of models; and explore parameter space as previously was just not practical. Itís been pointed out to me that that paper is historical because it begins with a statement, ďUsing a high-speed electronic computer I have...Ē (laughter) For a brief interval of time that was stated in the paper, but then of course it was obvious youíd used a high-speed computer, and no longer said it. That was a transition time in physics. It certainly changed the style that I would consider doing physics in and it changed everyone elseís style too.
Harwit:But most of your paper have not been that strongly characterized by computers.
Peebles:No, no, thatís true. The computer was there and I used it. In fact, I suppose in that particular case that if the computer hadnít been there, I wouldnít have written papers on Jupiter.
Harwit:In general, there are people who dive into a computer program at the outset of their papers, and then you canít follow the physics anymore because itís all hidden. Now, that hasnít been true of your papers. You tend to stay analytical as long as possible.
Peebles:Yes. Iím certainly not one whoís ever been deeply fascinated with computers. I quite appreciate their power; Iím very impressed with it; I am impressed with the things they can do, but I have never sat at a terminal and, for the sheer joy of running computers, run them.
Harwit:Do you still sit at terminals yourself, or do your students do most of...
Peebles:Whenever I can avoid it, my students or friends, colleagues do the computation. I guess thatís another aspect of my impatience, and I donít really enjoy sitting staring at something like a terminal. Iíll only do it when Iím required to, when Iím so interested in the problem, that I can see that thatís the only way Iíll get the answers. And then I will sit at a computer for hours. But never for its own sake.
Harwit:Now, when you got started on the early universe problem, that was initiated by Bob Dicke also?
Peebles:Very clearly, yes. No ambiguity. I can remember the day in which...
Harwit:Do you know roughly when it was?
Peebles:Well, I can remember it was a hot day. It was sunnier. Now, it must have been...
Peebles:...the summer, well, I was about to say Ď63. There was a considerable period of...now, remind me of the dates... (Note: See Appendix 1 for a clarification of these dates.)
Harwit:Well, your first papers were sent in...the blackbody paper was sent in, in May of Ď65, and the paper you wrote by yourself on blackbody radiation and the formation of galaxies, that was sent in early March of Ď65.
So it could it have been the summer of Ď64? Only one year. I suppose thatís fair enough. Sure enough, it must have been the summer of Ď64. Of course, prior to that time we had received little lectures from Bob about the beauty and mystery of the expanding universe. But at that time he explained first in simple and elegant terms how it could be, that in an expanding universe with no thermalizing agent, you could adiabatically cool radiation and keep it thermal. His analogy was one we still use, explaining this to people. Take a small section of the universe, enclose that section with reflecting walls, and observe that the reflecting walls donít change anything on the average; they reflect photons that would otherwise pass through, but there is an interchange from one side to the other. Then, let this cavity expand with the expanding universe; it doesnít do anything, but now you see its material inside the cavity as being adiabatically expanded. The number of photons in each normal mode is conserved by adiabaticity. Conserving photon number means that the Planck expression is conserved but the temperature drops, as one over the expansion factor.
He then had this other remark which heís returned to time and again. He dislikes the notion of the universe that starts from nothingness. He said so many times, ďConsider the universe as it is today, you can trace back to the universe as it was yesterday, and then the day before. But in the conventional big bang, you run into the day zero where things stop. So he preferred an oscillating universe. He recognized that in a oscillating universe you must make an arrangement to dispose of the heavy elements. You do that by assuming that the universe then was compressed to a high temperature, which thermally decomposed the elements. He then noticed that the high temperature would leave blackbody radiation as a remnant, and he said, ďWouldnít it be fun if someone looked for this.Ē So he persuaded Dave Wilkinson and Peter Roll to start thinking about...to start first looking in the literature for other measurements that might be relevant, and second to start building an apparatus. And he just said to me, ďWhy donít you go and think about the theory.Ē I donít quite remember his words.
Harwit:Did you start out by thinking about it or by going to the library?
Peebles:No, I started out by thinking about it. I was never strong on the literature. Still am not strong on the literature; I have to have something pointed out to me, often, before Iíll recognize that someone else could have done something. Itís so much more fun to think things through on your own than it is to read someone elseís paper. So we started thinking, and of course one of the first things that would occur to you is that if the universe were very hot, it was like a pressure cooker — in fact, it was like a star. And I vaguely knew that when stars exploded, one predicted that the abundances of the elements would evolve in a way that is computable. Surely the same thing would happen in an expanding universe, so that led to worry about helium and deuterium. And the other thing that would naturally occur to you is, if you have radiation, you have radiation drag; and that will cause an appreciable effect on the evolution of density irregularities. We will certainly be presented with a characteristic epoch.
Harwit:kid that was covered in your paper on the galaxy formation where you pointed out that this would act as a prevention in part.
Peebles:Yes, so it provides a characteristic epoch at which such things .can start to happen, which made galaxy formation a whole lot more interesting to work at. Before, one was entirely dependent on initial conditions. Now, at least part of the problem was taken from initial conditions to physical processes.
Harwit:And this was the essential difference between what you were attempting and what Lifshitz had done — about which you knew.
Peebles:I knew about Lifshitz, yes.
Harwit:Why did you know about Lifshitz?
Peebles:Did I know about Lifshitz? Someone must have told me.
Harwit:You refer to him in the paper.
Peebles:Yes, I believe you. I remember.
Harwit:And that actually is a rather obscure publication, although itís...I mean, itís a wonderful paper...
Peebles:Itís a wonderful paper, yes.
Harwit:...and justly is...I was wondering, you must have then talked to somebo4y about the problem who could have told you about that.
Harwit:...because a very few people knew about this [paper].
Peebles:I bet it was John Wheeler. I certainly talked to him about it. Fortunately, I didnít talk to too many people, because you know at that time it was the received knowledge, the received belief was that the universe is not gravitationally unstable, because as Lifshitz showed, the instability grows only as a power of time, thatís a rather weak effect. And if I had read the literature thoroughly, I might have been discouraged and accepted this belief, that, in fact, gravity isnít what causes galaxies to form. Of course, the fashion has changed since then, and we now believe that gravity is surely playing an important role.
Harwit:Now, Wheeler strangely did not also point out to you the earlier work of Gamow or Alpher and Herman, I mean, you cite one of the review articles that Alpher and Herman had written.
Peebles:When I had published the paper on helium production, I had certainly found out about the work of Alpher and Herman on the same thing. I didnít know about it when I started. Who told me about that? I think Dave Wilkinson, who was being more systematic than I, in plowing through the literature.
Harwit:Well, you did two things, I guess, and...
Peebles:Youíre thinking of the element production and of galaxy formation.
Harwit:Well, you did two things simultaneously, I think, and here I came across a paper that never was published, that I guess you had sent in to the Physical Review, called ďCosmology, Cosmic Blackbody Radiation and the Cosmic Helium Abundance.Ē
Harwit:And in that you did cite a review article by Alpher and Herman in Annual Review of Nuclear Science. Now, that article in an obtuse way refers to their blackbody article but makes no particular play of it. But at least you knew about that.
Peebles:When is that article dated?
Harwit:You sent that off the same day you sent off the one to the Ap. J., as far as I can tell, because they were both received March 8, 1965.
Peebles:And this had to do with helium production, did it?
Harwit:That had to do withÖ
Peebles:Yes, itís helium production.
Harwit:You can look at it if you want.
Harwit:I received this by legitimate means, incidentally. (laughter)
Peebles:No problem, no problem.
Harwit:Itís an interesting
Harwit:It never was published, then.
Peebles:I was excited about it, but in fact, here we are, with helium production. Thatís the main theme of the paper, certainly.
Harwit:Now, itís interesting, and this is one of the things I am puzzled at about the Letter that your whole group, then, Dicke, you, Roll and Wilkinson published, that evidently nobody at The Astrophysical Journal knew about this [earlier] work, whereas, the paper you had sent into the Physical Review, then, was rejected sort of on the grounds that it didnít bring in anything terribly new.
Peebles:I donít know whether thatís true — the Letter. I have vague memories of this paper and about the review.
Harwit:Itís 20 years; itís hard to remember, Iím sure.
Peebles:There were some legitimate complaints about the paper, including the lack of references to earlier work, and maybe there were some errors in that paper. I have vague memories of it.
Harwit:Well, the only thing that the...
Peebles:I do remember, though, being encouraged by the referee to clean it up and resubmit it.
Harwit:Thatís right, yes. And you did that, in fact?
Harwit:But I know that the referee wasÖit wasnít me...(laughter)
Peebles:Well, Iím thinking it was Gamow; am I wrong?
Harwit:No, Iím not going to tell you who it was. It wasnít Gamow, I wonít tell you who it was. But, I mean, the whole thing is fairly straightforward. The referee more or less said you should look at the Alpher-Follin-Herman paper. It didnít say look at Alpher and Herman. Now, itís interesting that when you look at Alpher, Follin and Herman, they donítÖ
Peebles:Yes, thatís the Reviews of Modern Physics paper?
Harwit:No, thatís in Physical Review.
Peebles:Thatís what I meant. Physical Review? No, Iím confused.
Harwit:Okay, let me dig it out. That was published in 1953, and was in the Physical Review. This is perhaps theÖWell, Weinberg has called it the first genuine calculation on the early universe.
Peebles:I think there was a follow-up paper in The Reviews of Modern Physics.
Harwit:They had an almost simultaneous paper in The Reviews of Modern Physics which I also have, and...
Peebles:I have a photograph of the numerical computer they used in this calculation. Have you ever seen that? It was in the days we wrote programs by connecting wires. And it made an impressive mareís nest of wires.
Harwit:Yes, I donít think that computer ever worked all that well. In fact, computers in the early Ď50s didnít. That Review of Modern Physics article is certainly a very beautiful one, and I have it but not right at my fingertips.
Peebles:So they computed an electron-proton abundance ratio versus time. Theyíve not gone on beyond neutron/proton to deuterium capture, if I can make this out right.
Harwit:No, I donít think so. Curiously, they hardly mention the microwave background radiation, and itís interesting that the Hayashi papers which preceded this and which pointed out that you had to take into account not just neutron decay but also the interaction with electrons and positrons, which would make it quite immaterial what sort of a mixture you had started out withÖ
Peebles:Yes, right, that was Hayashi.
Harwit:Hayashi did that. He refers to the original paper that predicts the microwave background, but in a rather curious fashion he simply says...he refers to it only for other literature.
Peebles:Who is he?
Harwit:Hayashi. What Hayashi doesÖlet me see, I may have one more folder here on references that I should...
Peebles:You know, there is the amusing point, not often recognized that the first colloquium I gave on these ideas was at the Applied Physics Laboratory at Johns Hopkins University.
Peebles:By then, I guess all of these people had moved on. There was no memory.
Harwit:Right, thatís right.
Peebles:...no corporate memory.
Harwit:Well, I think there had been a disruption. Incidentally, this is the can see, you had not seen. If you look where the paperclip is, if you had seen that, you would have immediately recognized that that was essentially the curve that you and Dicke...
Peebles:Yes, right. It certainly is.
Harwit:...and the others then published. Itís funny because they almost look identical. You even use almost the sane notation. Hereís the comparison from your article...
Peebles:Which I remember well.
Harwit:Let me...I guess in your article in The Astrophysical Journal, Vol. 142, itís the figure that appears on page 417, and then in the Reviews of Modern Physics article of Alpher and Herman itís page, what?
Harwit:204. Does it give the volume number there?
Peebles:Figure 23 and itís volume 22, April 1950.
Harwit:Itís quite curious. But itís interesting...
Peebles:No surprise, of course. Itís a good physical diagram to write down.
Peebles:And thereís no doubt about it, Alpher and Herman knew that you would expect the temperature of a few degrees Kelvin now, as did Gamow, of course, earlier.
Harwit:But I think that the real thrust here was theirs. I mean, they really had calculated it much more deeply...
Peebles:Oh, sure. Oh, of course. Gamow was always a-back-of-the-envelope man.
Harwit:Right, right. Now, whatís interesting to me, I guess, is that you came across the other review article, which, I believe, was slightly later, but not this one; and if you had, then youíd have seen the whole thing had been done before. Whereas, the other review article for some reason concentrates solely on...it has a preoccupation with the nuclear events — the chemical history.
Peebles:Which is fair enough. Itís an example of how...certainly I have never been...as diligent as I should be about following the literature.
Harwit:Well, in this case, I really tried to figure out how you could have missed it. Okay. (laughter)
Peebles:We didnít try hard.
Harwit:I did some of the...Well, no, itís easy to miss. At least...
Peebles:So you are, in fact, trying to be more logical than we ever were in following our processes.
Harwit:Well, I was trying toÖI was trying to understand what had happened. The article that you wrote by yourself in The Astrophysical Journal had no reference to any of the work of Alpher and Herman or Gamow.
Peebles:This is the galaxy production article?
Harwit:Thatís the galaxy production one. On the other hand, it didnít really have anything in there which dealt with a nucleosynthesis so much.
Peebles:No, thatís right.
Harwit:So, that I guess you refer to Gamow in a 1948 Physical Review article, but nothing he had done with Alpher or that Alpher and Herman had done.
Peebles:Well, I think that in the galaxy formation game, they had not done so much.
Harwit:No, exactly. So thatís fine. The other paper you had usedÖthe one that didnít get published...
Harwit:Öyou had used the review article that makes virtually no reference to Alpher and Herman. It just talks about it in a series of general attributions to work that had been done on the nucleosynthesis. So it wouldnít have been picked out so immediately as something to home in on, if one was looking for that kind of thing. The paper by Hayashi, which I just mentioned before, has a reference to the Alpher and Herman paper, but as I say, he makes use of it for a completely different purpose. Let me see if I can quote that because...
Peebles:I havenít looked at the Hayashi paper in years.
Harwit:Itís a very nice paper. Iím rapidly losing all the various papers IÖthese are just you. Let me turn this off for a second, Okay, we just interrupted here for a few seconds, Yes, Hayashiís paper in Progress of Theoretical Physics, Vol. 5, April 1950.
Peebles:And these are the key equations...
Harwit:Thatís right, yes.
Peebles:Öthat he wrote down.
Harwit:His equations la, b, and c, but he starts out, ďIn the theory of the origin of the elements of Gamow, Alpher and collaborators, reference 1Ē; and then when you go to reference 1, it says see the references in the paper of Alpher and Herman, Physical Review, Vol. 75, 1949, page 1089, which is the paper where they derive the 5į K blackbody radiation.
Harwit:So, even when you came to this, you wouldnít necessarily, I suspect, have been directed towards the remnant radiation. He certainly...Hayashi doesnít make use of a specific value, and the AlpheróFollin-Herman paper, which is based on his, also doesnít. It doesnít have a reference to that paper either, but interestingly does give a ratio of neutrino to photon densities.
Harwit:...at the present time.
Peebles:Not nuclei to photon?
Harwit:No, neutrino. Itís right.
Peebles:Oh, of course thatís...
Harwit:I mean, this is really quite advanced.
Peebles:Oh yes, in fact, they get the right answer, donít they?
Harwit:Yes, they get the right answer, too.
Harwit:I was really struck by that when I first read that paper. Itís just amazing how prescient they were. And it probably also shows that the literatureís pretty difficult to find something in.
Harwit:Still Iím surprised that Chandrasekhar wouldnít have said something too. He was editor of The Astrophysical Journal by the time you people submitted your paper.
Peebles:He had been editor for quite a few years, I think. I donít know when he began...
Harwit:He began around Ď52, I believe.
Peebles:Yes, so this was more than 10 years.
Harwit:The reason why Iím saying that is because Chandrasekhar and Henrich in 1942 had been in this game. They had tried an equilibrium theory.
Peebles:Thatís right. Rather different from.... Gamow was the one who pointed out that the one must be nonequilibrium.
Harwit:Exactly. And so I would have thought that this kind of thing would have stuck in Chandrasekharís mind.
Peebles:Do you say that Chandrasekhar was the referee for this Letter to the Ap. J.
Harwit:I donít know. I was wondering whether you would know.
Peebles:I donít have a feeling for whether he...
Harwit:It got accepted very quickly.
Harwit:I mean, I thought it was printed in the July 1 issue, having been submitted at the beginning of May. I think the date received is May 7. The Penzias-Wilson paper came in the following week, on the 13th. And I think it was in the July I issue, so I thought it could perhaps have just been accepted by Chandrasekhar as something obviously interesting.
Peebles:Yes. I think he must have reviewed many of these papers. I remember a subsequent Letter I submitted to him; it was also published surprisingly quickly, and where the comments I received seemed to indicate that it was his comments.
Harwit:Do you remember any of the comments you got on this?
Peebles:I think there were none. The paper was simply accepted without comment.
Harwit:Now, in this paper interestingly you do refer to Alpher, Follin and Herman, also to the Alpher-Bethe-Gamow paper, but I was wondering where that came from considering that you didnít have that reference in the paper you sent out to the Physical Review two months earlier.
Peebles:Yes. I donít know. All of this is quite lost in the mist of time.
Harwit:SeeÖthe referee for Physical Review called attention to the AlpheróFollin-Herman paper, but if that was the case, if you got it from that, then would only have received it the previous week. And I didnít know whether this could have something you would have just gotten.
Peebles:Itís certainly possible. We wrote that in a kind of a hurry, I remember. And I remember making last-minute adjustments.
Harwit:When did you write this, actually? At what point...I was reading the article that you and Dave Wilkinson wrote up for the Jansky Symposium last year. It doesnít give any of the details of dates of when you first met the Bell Labs group, and how rapidly events then jelled.
Peebles:Yes. When we first met theÖthe Bell Labs, the Holmdel group. Those dates could be discovered. I donít remember them. I do remember that we decided after Bob and Dave went to visit the Holmdel group to satisfy themselves that this was indeed a credible measurement.
Harwit:You donít know when that was?
Peebles:It could be discovered, but I donít even remember whether it was spring or fall.
Harwit:Iíll ask Dave this afternoon. He may remember.
Peebles:I do remember well their coming back and saying, ďThis looks impressive.Ē I didnít go with them. I also remember that many of Bobís old cronnies from his days in his business remembered him. He was with pleasure; he was happy about that too.
Harwit:Bob Wilson or Bob Dicke?
Peebles:Bob Dicke. I also remember that rather shortly after that visit, we decided weíd better do something about this — better write a short letter. And I remember also that it was done in iterations as, I guess, is always the case when four authors are involved. And I do remember last-minute corrections. For example, the graph you drew my attention to. It was made in the first approximation, I think by me, and then I remember at the last minute Bob saying, ďYou didnít get these quite right, the little wiggle in the density of radiation when the electrons and positrons recombine isnít too visible; why donít you notch the thing up a touch,Ē and I remember a last-minute rush to do that.Ē I donít remember any discussion about references.
Harwit:Iím sorry. This was for cosmetic purposes, or...
Peebles:For the graph it wasÖperhaps for cosmetic purposes, he wanted to make sure people saw that little notch.
Peebles:I bet you, and I will try at the end of this talk, that I can find some preliminary drafts of that letterÖ
Harwit:That would be nice.
Peebles:Öand we can seeÖit would be fun to see whatís in them.
Harwit:Yes. This is why I suggested on the phone thatÖ
Peebles:Öweíd be here ratherÖ
Harwit:Öperhaps we could meet once more in the morning, if there is anythingÖ
Peebles:Okay, sure, sure, and Iíll look for that.
Harwit:Yes, I guess in their Letter to The Astrophysics Journal, Penzias and Wilson say that their observations covered July Ď64 to April Ď65, so thatís not too much of a indication of when it was that Dicke and Dave WilkinsonÖor was it Roll who went over?
Peebles:I think the three of them.
Harwit:All three of them, yes, okay. Is there anything else that youíd want to add to sort of insights on the microwave background radiation recollection, other than the ones that you might have written up in this article with Dave?
Peebles:Well, youíve raised some interesting questions, and I will check a few places in my file, but aside from that, I canít think of anything.
Harwit:Well, let me ask you about one impression that you might or might not have. The Alpher and Herman paper still 20 years later is rarely cited. People cite Gamow; they cite the paper by Alpher, Bethe and Gamow, which I guess titillates.
Peebles:But also there were some important ideas in those papers.
Harwit:Thatís true, but no prediction of a remnant temperature.
Peebles:Thatís true also. Thatís true. You could derive it from the Gamow paper, but it wasnít stated in there.
Harwit:Thatís right. And the derivations, unless you did then fairly carefully, could vary quite a bit.
Peebles:Oh, quite true also. Of course, as we have noted Gamow was a backóof-the envelope man.
Harwit:Thatís right, yes.
Peebles:And in any case, he didnít compute with care.
Harwit:Right, right. Was famous for that in fact, I guess. Even when there was a symposium of the Royal Society in London a few years ago to commemorate the discovery of the microwave background radiation, thereís a historical introductory chapter by R. J. Tayler. And he also just refers to the Alpher-Bethe-Gamow [paper], which is sort of the totally wrong paper for that. It doesnít even mention the microwave background radiation at all, as far as I know.
Peebles:Now, remind me, was the Alpher-Bethe-Gamow paper the one in which for the first time Gamow recognized that one should use dynamic rather than static.
Harwit:No, no. The...
Peebles:That was an important idea...
Peebles:...in the Alpher-Bethe-Gamow...
Peebles:...or was it the...
Harwit:They just talked about the cross sections, nuclear cross sections.
Peebles:Oh yes, yes, this is abundances going inversely with cross sections.
Harwit:Exactly. But there is nothing in there...
Peebles:No, thatís right.
Peebles:This is an important paper, but itís more for a supernova theory...
Peebles:...than for cosmology.
Harwit:Gamow did his paper that you talk about in 1946.
Peebles:Thatís the paper I often refer to because of the fundamental idea...
Harwit:Itís not in this file
Peebles:You know I have many copies of these papers next door too.
Harwit:Itís right here. And here he points out that one ought to try to do the nucleosynthesis...
Peebles:Here is a reference, for example, to Chandrasekhar...
Harwit:Thatís right, exactly...
Peebles:Öand Henrich doing equilibrium statistical mechanics, and quite wrongly. So itís appropriate to refer to this paper, but as you point out, this is only the idea, and none of the computation...
Harwit:Well, itís the idea of nucleosynthesis, but not the idea that there ought to be a remnant background radiation.
Peebles:He wasnít yet at a hot universe at this point, was he?
Harwit:I think so, yes. I think everybody recognized you couldnít have the nuclear reactions taking place unless it was very hot.
Peebles:Right. But he had a subsequent paper, didnít he, in which he for the first time estimated the photoproduction of deuterium, which was what constrained the entropy density, relative to baryons that would implicitly, at least, fix the present temperature of the universe.
Peebles:Should I go next door and get that paper, if I can find it? I believe itís in Ď48.
Harwit:This is a Gamow paper in Ď48?
Harwit:I donít have a complete library with me.
Peebles:Itíll just take me a minute.
Harwit:Okay, fine, yes. (Interruption) Okay, weíre back on after a couple of minutes. I found the paper.
Peebles:This is the paper Iíve always felt was significant in the very important order of magnitude, a cross section for deuterium photoproduction, number density, expansion time scale, rms velocity. That fixes a temperature...well, explicitly, I suppose, an entropy per unit baryon, that very quickly thereafter it was, wasnít it, that Gamow and Follin cleaned up this calculation and then pointed out that if oneÖ
Harwit:Alpher, Herman and Follin.
Peebles:Is that the paper?
Harwit:Yes, I think so, I think they...
Peebles:There is an earlier paper than this with a temperature of about 70 Kelvin at the present time. This is the paper, I think, that I have in mind.
Harwit:I see. Can you read references.
Peebles:Let me first make sure itís the right one. No, thatís not right. Yes, here it is. No, no, thatís not it. (looking through papers) There was a quick follow-up paper to Gamowís 1948 paper, and I just canít seem, I think, to put my hands on it. This is not it.
Harwit:It may be a paper in Nature, perhaps?
Peebles:Perhaps, although what I am thinking, Iím not confused, is of a paper of a Letter to the Physical Review commenting on Gamowís 1948 paper.
Harwit:Could it be this one here?
Peebles:It could indeed.
Harwit:This is a paper by Alpher, Herman and Gamow.
Peebles:Yes. Physical Review, Vol. 74, 1948. This is not what Iím remembering. We shouldnít spent anymore time, I guess, nowÖIíll try and refresh my memory of the paper Iíve tried to think of.
Harwit:Good, good. The question, I guess, that initiated much of this was...
Harwit:Öwhat is it with scientists who usually are fairly careful in documenting or citing the correct papers, that for some reason or another, the main paper that describes this...
Harwit:...is very seldom cited correctly.
Peebles:Is that unusual? I thought...
Harwit:I donít know.
Peebles:Can you find other examples where people will choose to refer to some subset of the papers in a rather conventional way and slight other papers that might be just as important, in fact, more so?
Harwit:Well, that happens where sometimes a better-known person is cited at the expense of a lesser known.
Peebles:And once itís done a few times by authoritative people, it will be repeated.
Harwit:Thatís right. Now, that explains why a lot of it is attributed to Gamow, I guess. Oh, you found the paper.
Peebles:I hate to interrupt. You were saying Nature and Nature it is.
Peebles:On checking the results presented by Gamow in his recent article, referring back to Gamowís; in fact, an article by Gamow in Nature but the same substance in the Physical Review paper of 1948.
Harwit:Yes, I remember this paper.
Peebles:They redo the computation and then they extrapolate the entropy to the present date and get a temperature of 50 Kelvin.
Harwit:Thatís right. This is in Nature, Nov. 13, 1948, Vol. 162, and itís by Alpher and Herman. Now, I think this must have been done just about the same time as they were doing their Physical Review paper, except itís earlier. The Physical Review paper was published in 1949.
Peebles:I assume this was the beginning of the work.
Harwit:Yes, thatís right.
Peebles:That, I believe, is the first published reference to the present temperature.
Harwit:I think thatís correct, yes. Thatís right. (Note: added in proofs the Gamow paper, Nature 162, October 30, 1948, pp. 680-682, referred to by Alpher and Hermanís note contains no explicit extrapolation to, or mention of, a present-day cosmic temperature.) Well, why donít we go on nowÖYou went on then and did a paper on the primeval helium abundanceÖ
Harwit:Öwhich was important, of course, to do. One of the things is that as far as I can see you had a different aim now. This was 17 years later, one knew about production of heavier elements in stars.
Harwit:But one was puzzled about the production of helium, and here I have to actually go over to the next tape, so maybe we can continue on tape 2. I just asked you at the end of the last tape, Jim, about your paper on the primeval helium abundance, and how you were led to that. That was written...it was received in February Ď66, approximately a year after you had submitted your first papers on the microwave background.
Peebles:By that time, there was the measurement at Princeton of the microwave background spectrum available, as well as that of the original of Penzias and Wilson, so there was some evidence that the spectrum truly was blackbody. So it clearly was evident that one should follow up the consequences of the measurement of this temperature. By that time astronomers had told me that there was a helium problem. I was not aware of that. I had some vague knowledge of measurements of helium because that was important in the study of Jupiter. But I was not aware that computations of nuclear synthesis in stars encountered problems in making helium. I was not aware that there was some data showing that the oldest stars still had some appreciable helium in them, even though they had not many heavy elements, but these things were told to me by astronomers. And clearly, the thing that I could do that would be interesting would be to try and compute with as much care and accuracy as possible how well helium was produced. And, of course, maybe my main motivation in doing the detailed helium calculations was in using helium as a test of cosmology.
Harwit:Let me interrupt you because I would like to jar your memory here a little bit.
Harwit:You had been aware the previous year of the Hoyle/Tayler paper in NatureÖ
Harwit:Öon helium abundance, So you must have known that that was an important problem as least from that I would suspect.
Peebles:When this appeared, I was already heavily involved in the computations of helium. So it was certainly a reinforcement of my beliefs that this was a good computation to do. By this time, though, I had talked quite a lot to people like Martin Schwarzschild and to George Field and to Nick Woolf, all of whom were at the astronomy department here at the time. Of course, Martin still is. They et al. explained to me the observational situation with respect to helium, and the fact that it was conceivably of great interest to how to make helium in the big bang and deuterium also.
Harwit:So, you were...well, what Iím saying is you were aware of this paper when you wrote the ones on the galaxy formation andÖ
Peebles:I donít think I was aware of this paper at galaxy-formation time.
Peebles:Is it referred to in there?
Harwit:Itís not referred to in there, I believe, let me check, but it is referred to in the paper that you had sent in... No, itís not referred in there. It is referred to in the paper you had sent in to the Physical Review.
Peebles:The long paper onÖ Oh, this one that was rejected.
Peebles:I see, I see.
Harwit:So, you must have started thinking about it around the same time, I wouldÖ
Peebles:Iím surprised. Of course, by the time we had sent in this paper that was rejected, I was already heavily involved in these helium calculations.
Harwit:Yes, okay, I see. All right.
Peebles:Sure, sure. So I guess already by then I was aware of the helium problem. But I must say that the main driver in doing these computations wasnít so much to solve an astrophysical problem with helium, but rather to use the production of helium as a probe for cosmology. Because one recognized if you adjusted the cosmological theory, you would adjust the predictions perhaps very dramatically of what was produced during the big bang. That seemed to me to be more exciting. I didnít recognize, I thinkÖWell, I donít know when I was fully convinced that there was a problem with making helium in stars. It did take quite a while for me to appreciate that point.
Harwit:You somehow thought perhaps that there could have beenÖ
Peebles:It seemed conceivable to me that there were helium-producing stars, I didnít know.
Harwit:A first generation of stars, yes. There was that sort of aspect in the air at the time, certainly, since the Burbidge, Burbidge, Fowler and Hoyle paper was so successful.
Harwit:kid also I guess the steady-state universe people had no other recourse, and so from that point of view it looked as though perhaps one could have formed the helium, at least that was the hope the steady-state people had.
Peebles:It was certainly an advertised hope.
Harwit:Yes. But you also started talking about deuteriumÖ
Harwit:...which is interesting. Can you recall any...
Peebles:Well, of course, deuterium is made along with helium, so why not keep track of how much of it is made as a function of the cosmological model. I had no reason to concentrate on deuterium. I was aware that it was very difficult to know what is the true cosmic deuterium abundance. Also, that it is much more susceptible to evolution through astration than is helium, He4.
Peebles:So I didnít, I think, at the time, push it very much as a test for cosmological models, except that, as I now recall, the deuterium abundance could be a very sensitive function to the cosmological parameters.
Peebles:And so one did have a way to put some bounds, not only on expansion rates but also baryon density, even with the crude limits we had then on what the deuterium abundance might be and how it might have evolved due to stellar evolution, which I think I mitt have referred to in the paper on helium production that was published. And, of course, exciting to me then, and I guess still now, is that if you changed your theory of cosmology to have a very different expansion rate, you could have disastrous production of deuterium.
Harwit:By astration you mean the burning up of the deuterium in stars because itís destroyed at these very low temperatures.
Peebles:Thatís right, or converted to helium.
Harwit:...or converted to helium, yes. Okay, now, you then went on and together with Partridge, you started working young galaxies.
Harwit:Perhaps youíd like to say something about the background of that; how you met Partridge and so on.
Peebles:Bruce Partridge was an undergraduate at Princeton. As it happens, though I might have known him, I believe I didnít. — No, he was an undergraduate.— He then went off to Oxford as a Rhodes scholar, came back to Princeton as a post-doc, started working with Bob Dicke, and with...
Harwit:Do you remember what year that would have been?
Peebles:No, I donít. How did he and I get into this game of making models for young galaxies? Well, I donít know, except the ever-present figure of Robert Henry Dicke has to be mentioned. From the beginning he was interested in the question of galaxy formation and cosmology. Certainly that was a subject that was in the air, so far as this group vent ó- the puzzle of how did galaxies form, what was the sequence of events that led up to galaxy formation? And always with Bob Dicke you took a great chance if you speculated on such things without concentrating on those areas that were susceptible to observations. He always laughed at theorists who made theories that did not have a remote chance of being tested. Time and again I saw him use his sense of humor on that point. So certainly, Bruce and I would have been, careful to keep our attentions directed toward those aspects of galaxy formation that might be testable. And because Bruce is an observer, we would have been further inclined to think of those aspects of the subject that were testable. I donít remember whether Bruce was at the time planning to do any observations on young galaxies, to try and discover the things. But certainly later he turned to that. We simply, in a rather casual way, started talking more and more seriously of about how you would...what signals there might be for the presence for young galaxies, and, of course, now you would test for them.
Harwit:I think thatís when I first became aware of some of the things you two were doing, because we at Cornell were going through similar calculation on background sources of radiation since we were looking at the near-infrared background at the timeÖ
Harwit:...and you came along with this idea that young galaxies might be contributing. Previously, I guess, I remember, although I donít know whether it ever appeared in print, hearing Fred Hoyle muse about where radiation might be which stars would have given up over the aeons in converting hydrogen into helium. And in fact one of the reasons for my going into far-infrared background work — this was before the microwave background radiation had been discovered ó- was because it was quite clear that somewhere there ought to be that energy — it would have been redshifted — one didnít know how much, and so this was an interesting thing to look for.
Peebles:And still is, of course.
Harwit:And still is, yes.
Peebles:I think itís an idea that was in air. I know that Ray Weyman, independently, also, was having thoughts along similar lines, at about the same time at the University of Arizona. Earlier, Whitrow and Yallop had a paper that I think Bruce Partridge and I referred to, on this question of where is the radiation left over from distant galaxies. And was there not also a paper by George McVittie.
Harwit:Youíre right. McVittie? Yes, 1965, with errata, from Ap. J.
Peebles:Certainly the idea was around.
Harwit:What was interesting when you take a combination of what you and Partridge did, and then also what you had done in your work on the background radiation, was that you were careful to point out in one of your papers that there should be no confusion between the background radiation in the microwave regime and the 3į Kelvin background equivalent density from starlight. And Alpher and Herman have sometimes been criticized for having in their paper in the Physical Review, 1949, a statement about the 5į Kelvin radiation to be sure, but some worry about whether the almost equal density of energy in starlight would be easily disentangled from that. Now, people tend to say, ďWell, thatís ridiculous; they should have known better.Ē On the other hand, considering that we now know that something like half of the radiation from the galaxies immediately is converted into far-infrared radiation, something you recognized.
Harwit:I mean, you didnít know, of course, one didnít know at the timeÖ
Peebles:We didnít know wavelengths. What fraction or what wavelengthÖ
Harwit:But one doesnít know what wavelength. You calculated it would be around a hundred microns which turned out to be correct.
Peebles:Thatís about where it is.
Harwit:Thatís about where it is. It could have easily been somewhat further outÖ
Peebles:Öand could have caused problems.
Harwit:Öand could have caused problems.
Harwit:In the 60s Mayo Greenberg was predicting models for interstellar grains that had temperatures that could go as low as 9į Kelvin - this kind of thing. And without knowing at the time too much about the size of the grains, it could have been an interfering factor.
Peebles:Of course the anisotropy would have been the clue. We were convinced then, and I think itís turned out that the dust emission would be concentrated toward the plane of the Milky Way.
Harwit:Sure, thatís true, yes.
Peebles:And so one of the things you emphasized very early was the remarkable isotrophy of this background radiation, as arguing for an extragalactic origin.
Harwit:That is something that probably Alpher and Herman might have known, butÖ
Peebles:Of course they didnít have the data, so they didnít know.
Harwit:They didnít have the data. I am just saying that because I donít think that that sort of hindsight should be considered a criticism of their work.
Peebles:No, thatís fair enough. You know, we did write and ask Gamow whether they had considered looking for this radiation, and I remember that itís in this...
Harwit:I know that they asked people, and the radioastronomers at the time felt that it couldnít be done. This was in the early 50s.
Peebles:I see. You know more then I. You know that, in fact, Dicke had almost done it in the Ď40s.
Harwit:1946. I know about the paper; I read the paper in Physical Review it appears in the same volume, in fact, as some of Gamowís early work. But I think it was partly the upper limit of 20į that Dicke had achieved which made people in the early Ď50s feel that 5į would be too difficult.
Peebles:Mother factor of 4.
Harwit:I once talked to Dave Wilkinson about whether one could have done it without a liquid-heliumócooled radiometer. I have to ask him again this afternoon. At the time he said he didnít think so, but I notice that the historical paper, that the two of you submitted to the Jansky volume, suggested it could have been done. So Iíll have to try and straighten...
Peebles:Youíll have to straighten that out with him.
Harwit:...that discrepancy out.
Peebles:But whether or not helium...helium was available in 1940s.
Harwit:Oh, but the techniques werenít there. Helium was much too difficult to handle at the time, I think.
Peebles:I see, you didnít want to have an open dewar, is that what you are saying?
Harwit:Well, what Iím saying is that we brought a liquid-helium-cooled dewar to Kitt Peak in the late Ď60s. It was one of the first times that the people there had to transfer liquid helium on the mountain.
Harwit:They had an ageóold transfer stick that didnít work —all that got cleared within a couple of years. kid when we started flying liquid-helium-cooled telescopes in the latter half of the Ď60s, that was considered a fantastic sort of thing to even attempt to do, even by the people who did it in the lab.
Peebles:But that was in a rocket, wasnít it?
Harwit:It was in a rocket.
Peebles:That was another order of magnitude and more difficult.
Harwit:But the laboratory people didnít think it was trivial, and so I think in the early Ď50s there just wasnít that much stuff around. There wasnít that much equipment; it wasnít as prevalent as it is today. I think it would have been more difficult, perhaps. At any rate, these two papers you did with Partridge really are still at the forefront of interest now, and one hopes that with the cosmic background explore satellite, one might be able to delve into the interesting parts of the near infrared.
Peebles:But of course it wonít have the angular resolution to see point sources. It will only improve the integrated background measurements.
Harwit:Thatís right. But you pointed out that there ought to be a background flux from those that one should be able to see.
Harwit:So thatíll be interesting because at the moment there are only upper limits available, as far as I know.
Peebles:Yes. Steadily improving upper limits, but still only that.
Peebles:I guess the best now in the far infrared from the IRAS satellite.
Harwit:Sure. The IRAS satellite gives fairly good upper limits, but theyíre still crude compared to the ones one expects from the cosmic background explorer. In any case, I would think you still believe the galaxy-forming epoch have been more of the order of Z = 10, rather than 200, or something like that?
Peebles:Oh, I still think itís wide open. I could imagine forming the bulk of the galaxies at a Z = 200.
Harwit:The young stars too?
Peebles:Sure. Assemble them in pregalactic objects, then make the galaxies from dwarf galaxies.
Harwit:All right. Well, this leads smoothly into the next paper I was going to ask you about, namely the one about the globular clusters, which you did with Dicke, and essentially are the pregalactic stages.
Peebles:Thatís right. I guess thatís why I made my last remark.
Harwit:The thing you just mentioned. Yes, exactly. Thatís what I figured. We might as well go into that.
Peebles:Okay. What is the story behind that paper? Back as early as the 1965 paper on galaxy formation, I had recognized that one had a Jeans length from the known temperature of the microwave background which would fix the temperature of the hydrogen and the roughly known mass density of hydrogen. I had argued as early as Ď65 then that the first objects that form could be the objects that were as small as possible, at the Jeans mass. It was Bob Dicke who said, ďHey, wait a minute. You ignorant physicist; donít you know that the mass of those objects youíre computing, and densities, are remarkably similar to those of globular star clusters.Ē You should ask Bob about globular star clusters. His first paper in Science, I think, or one of the first, was on globular star clusters.
Harwit:Really? I see.
Peebles:So he was well aware of them. Knew that they were enigmatic, in the sense of being so uniform and so common around such diverse galaxies. He recognized the coincidence that the gas clouds I was computing had just the properties of these globular clusters; so that led us to this paper you referred to, wherein we try and explore in as much detail as possible the consequences of this idea. Itís characteristic of my style of operation that I wrote a rash early version of the paper that was roundly rejected because I had missed some good physical points. Donít know who the referee was, but the referee made a big contribution to this final paper in pointing out to us that very important to the history of a gas cloud would be the production of molecular hydrogen. He pointed out how it would be produced; I think he did. Yes, by catalysis by free electrons. Iím going to have to check to make sure just how we slid (?) out of that.
Harwit:Thatís correct. There are two mechanisms, one of them is with free electrons óe that was known at the time — and the other one wouldnít have been relevant, that was the grains.
Peebles:Right, right. So of course once you make molecular hydrogen youíd dramatically change the expected evolution of one of these gas clouds, and that was a very important element in that paper. I also remember that we had a little trouble getting the paper published, in that Chandrasekhar couldnít find a referee who would both declare himself competent in the subject and also declare that it had any chance of being right. Finally, Chandrasekhar reviewed it himself, in the process discovering weíd slipped in a few jokes; and he sternly instructed us to remove those jokes.
Harwit:What kind of jokes?
Peebles:We had remarks about the life of one of these clouds being nasty, brutish and short, and he didnít like that. Itís not much of a joke, not much of a joke. And it was an important point. We had to explain why if the first objects to form were these gas clouds, most of the mass of the universe wasnít tied up in globular star clusters. We had to argue that the chances were very high that the gas clouds would be disrupted by collisions before they could form themselves into star clusters. That was the remark about the life of a cloud being nasty and brutish and short. The paper was received with not much excitement by the astronomical community. They rightly point out that there are star clusters around the Magellanic Clouds that look for all the world like young globular star clusters, How did we account for them? Either through coincidence that star clusters can form in more than one way as we know is the case, and that these things around the Magellanic Clouds are not true globular star clusters. Certainly their mass is down considerably from that of a classical globular star cluster. Also, not inconceivably there are a few of these gas clouds left over, that are just making it to the collapse stage.
Harwit:Sure. I donít see a contradiction there, necessarily.
Peebles:Not necessarily, no — special pleading, perhaps, to save some gas clouds to a very late collapse time, I think the idea still has merit. It has never been considered the leading contender for a theory of globular cluster formation, and I think perhaps thatís right. Itís highly speculative.
Harwit:Well, itís interesting, though. I mean, other people have taken this sort of thing seriously as well. I know Tommy Gold feels -Ö
Harwit:Öhas at times felt, I donít know whether he does today — that globular clusters could have been formed first. Some of that, I think, was related to finding at one time, which then proved to be erroneous, that some of the clusters might have escape velocities from the Galaxy, and I think he was speculating that they might just have gone through the Galaxy.
Peebles:Right. Like a comet.
Harwit:And originated elsewhere. Exactly.
Peebles:I think you have to admit, by and large, that the globular clusters are concentrated around galaxies more strongly than is the dark mass, which is an argument I think for forming globular clusters in the galaxy. But there are ways around that. These days I tend to turn the argument around and to point out that under quite a broad variety of conditions you would expect these gas clouds to form; and if they do form, you have to explain what became of them all if you donít see them as globular star clusters. So it is an important concept either way, either as a constraint on scenarios, or maybe as evidence of the way the scenario did go. Just recently Iíve returned to this primeval globular clusters game in a recent paper in The Astrophysical Journal, with a new twist. If you make a universe out of inos, this new magical panacea for all of our problems in cosmology.
Peebles:Inos. You know what I have in mind, the super-symmetric partners of the ordinary particles or perhaps axions, perhaps massive neutrinos. One of the consequences again would be, under a pretty broad variety of conditions, of formation of these gas clouds, but this time with the difference that theyíd have a halo of dark matter around them. So I was led to write a paper on globular star clusters with dark halos of mass around them. That again annoyed some of my astronomical friends a bit, I think.
Peebles:Well, finding dark halos under every bed seemed too much for them, and to find dark halos around globular clusters seems strange. In fact, thereís very little evidence for globular clusters to have dark halos, but itsí very difficult, also, to rule out the possibility that they have them.
Harwit:It would presumably affect the escape of stars from the...
Harwit:...globular clusters, but it might make them spherical faster and act as a damping mechanism.
Peebles:It also acts as a way to produce the sharp edges seen around globular star clusters. Usually thatís interpreted to imply a temperature gradient ó- a gradient in the velocity of dispersion of the stars in the globular cluster. Theyíre cool near the surface, and that provides a relatively well-defined surface.
Harwit:And also escape, right?
Harwit:Öthrough tidal action.
Peebles:Tidal action, right. So you strip off the outer envelopes through tides, and then you have a relatively well-defined surface, because the velocity of the dispersion is low at the surface. Another way to get a relatively well-defined surface is to have a strong gravitational field of the sort that would be produced by a dark halo around the globular cluster. And you can make pleasing-looking models for globular clusters in which the surface is not cool — in fact, is warm — but the density gradient is provided by the mass of the dark matter. And that can be checked. You can measure the velocity dispersions of stars in the globular cluster as a function of radius. I have some hope that perhaps in the next decade enough will be known about the star orbits to test this idea.
Harwit:Well, weíll come to the missing mass a little later. You did a paper with Ostriker that I want to come to also. Iím sort of going in chronological order through some of the things that struck me as interesting, but I may miss some of the papers and maybe later on Iíll still ask you also to talk about ones that...I might not...I mean, you know youíve got this fantastic publication list. (laughter) We canít talk about each of the papers, but okay... You started in around late Ď69 in the paper with Yu, talking about superclusters, and I think this was...let me back track. Iím sorry. I still wanted to talk about an earlier paper that year, ďAngular Momentum of Galaxies,Ē and you wrestled with that problem; and had suggestions for how you might bring the angular momentum in, suggesting various schemes. What interested me there was that you felt the angular momentum had to be produced at that late a stage.
Harwit:Was that because you wanted to have almost perfect homogeneity and isotropy initially, or?
Peebles:Thatís in the right direction, although I would put it a little more strongly. One could have two opinions about the early state of the universe, either chaotic or very smooth. In an earlier paper I had argued that as I looked at the evidence, I could only see that the universe could start out smooth and grow chaotic, rather than the other way around, because the universe, I argued, is gravitationally unstable. Isnít there a paper in that list called something like ďThe Gravitational Instability of the UniverseĒ?
Harwit:Maybe you can find it faster. Oh, right on top here, maybe?
Peebles:Yes, there it is. Early on I became convinced that it was difficult to imagine a primeval-turbulence type scenario of the sort that had been invoked by von Weizsacker and many others to account for the rotation of galaxies. The simple thought that if the universe is gravitationally unstable, then it means that the universe must be growing more clumpy and must have been more smooth in the past. If thatís right, then the angular momentum was an apparent problem and an embarrassment. Where did it come from? The only thing I could think of was this tidal interaction; so I computed it, I only learned that the idea had been invented earlier by Fred Hoyle when I submitted the paper and the referee said, ďYou should have referred to Hoyle.Ē Iím not sure whether the reference got into the original paper because I remember having a devil of a time finding the article the referee had mentioned. By then I was getting a little sensitive about my poor performance in referring to previous work, and I remember spending at least a whole day in the library ó- I was visiting Caltech at the time — trying to find that darn paper by Hoyle. I guess you donít have theÖ
Harwit:I donít have that one with me. No, Iím sorry.
Peebles:Never mind. I did finally discover the reference and itís there.
Harwit:Okay, let me just read the paper that weíd been talking about into the tape recorder, ďThe Gravitational Instability of the Universe.Ē It was published in The Astrophysical Journal, Vol. 147, page 859, 1967. I suppose that when people were talking about Fred Hoyle, a referee would have perhaps referred to his fractionation during collapse in star formation.
Peebles:Oh, Iím sorry I confused you. I was referring to the angular momentum paper; recall theÖ
Harwit:No, I understand.
Peebles:...and now the paper on angular momentum, due to Fred Hoyle, wasÖ
Harwit:Oh, I see, you were not referring to...okay.
Peebles:Iím not referring to the gravitational instability, but ratherÖ
Peebles:Of course, at the time Fred Hoyle - and I guess still is was arguing for a steady-state cosmology, he needed also a mechanism for transferring angular momentum.
Peebles:He had an early paper in which he outlined tidal torques, and I canít remember where itís published now; Iíll have to look it up.
Harwit:I donít remember that either. The paper weíre talking about of yours is the one that is entitled, ďOrigin of the Angular Momentum of Galaxies,Ē also in The Astrophysical Journal , Vol. 155, page 393, 1969.
Harwit:Now, before we go on from there, this was 1969, were you married by then?
Peebles:I was married in Winnipeg, just as I finished college, and have been ever since— same wife, Alison. She is Canadian, Winnipeger also. By that time I had had children.
Harwit:You have three girls.
Peebles:Three girls, and I am terrible on dates, so I canít even tell you how old they were at that time. The last, must have been just born inÖno, in fact, the last was a year or so old.
Harwit:You could tell me how old theyíre now, thatíll do. (laughter)
Peebles:I can give you an order of magnitude.
Harwit:Refer back from there.
Peebles:We can compute back. So the oldest, Lesley, must be 24; the next, Ellen, 22-23; and the youngest, Marion, 17Ömust be about right. Itís better than an order of magnitude, plus or minus a year.
Harwit:Does your wife work?
Peebles:She does now. In those days she worked part time on a variety of interesting things associated with the university.
Peebles:This university. Took on a full-time, or almost full-time, job at Town Topics, our local throw-away newspaper. If youíd lived at Princeton any extended period of time, youíd surely know it. Itís the local gossip sheet and ads for cribs and all the rest. And she started, I guess middle Ď70s; must be 10 years she has been working at that job. So where were we?
Harwit:Okay, now, let me just ask you. Do you discuss any of your astrophysical things at home?
Peebles:No, no, I donít.
Harwit:Is your wife interested in science?
Peebles:Only in a sociological way, but not research science, no.
Harwit:So you leave your work at work?
Peebles:I leave my work at work, or if I take it home, I go into a quiet place and do it alone.
Harwit:You have a study or something like that?
Peebles:In fact I donít; I use the living room when I want to work at home.
Harwit:Do people have to tip-toe around, or can you concentrate?
Peebles:No, they have to be out of the house. I only work at home when it happens to be empty, or close to empty.
Harwit:Good, Well, letísÖthis was sort of an aside because itís sort of interesting to know howÖ
Harwit:Are any of your children interested in science?
Peebles:No, I think none of them will turn out to be scientists. The oldest two both majored in history in college. Lesley is now working for a venture capital company, developing software. The middle child is working as an editor of the law review, or an assistant to the assistant editor of a law review in Boston.
Harwit:She went to law school?
Peebles:No, no, just to liberal arts. In fact, they both went to Princeton, and did the standard thing, but not an interest in science.
Harwit:Does Princeton have a facultyóchildrenís tuition?
Peebles:It is to weep. No, we paid the full shot.
Harwit:Do you? I see.
Peebles:Itís an incredible flow of money.
Harwit:I see. I know. Well, at Cornell we fortunately do have that. All our children have gone to Cornell for that reason.
Peebles:Thatís an enormous benefit.
Peebles:A large sum of money involved.
Harwit:And before taxes.
Peebles:And before tax money.
Harwit:What about Marion?
Peebles:Well, now the university is somewhat more liberal in its tuition grants. It does give a grantóinóaid to any school, that has been up till very recently $750 per year, per child. Almost in the noise.
Peebles:Now itís roughly a third of the tuition which is appreciable. So above and other than that we were prepared for the shot for Marion. Itís an amazing flow of money; it never really hurt because there has been such a broad variety of low-cost loans available.
Harwit:Yes, thatís true.
Peebles:So itís worked readily. The low-cost loans are drying up, and thatís going to be a problem in the future.
Harwit:And one does have to pay that back. (laughter)
Peebles:And one does have to pay them back. And inflation has turned off so thatís not going away as rapidly, as it wereÖ
Harwit:What is Marion interested in?
Peebles:Cars, boys, parties.
Harwit:All right. Let me go back to the paper with Yu, then, on superóclusters of galaxies which you had in The Astrophysical Journal , volume 158, page 103, 1969. I think that was when you started in on a very long line of work there that stretches to the present time.
Peebles:To the present time, yes. Thatís historically interesting, otherwise not. I can tell you how I got into that game. I was visiting the University of Toronto, where Sidney van den Bergh was at the time. We discussed among other things, the question of whether the universe truly is homogeneous on large scales, and he pointed out to me a map of the distribution of Abellís cluster across the sky. And said, ďLook, even at this depth that how lumpy things are.Ē And I said, ďCan you tell that that distribution is not simply a random distribution of points, and that the fluctuations are not simply accidents?Ē And he said, ďI canít tell\; why donít you check it?Ē And I said, ďI think I will; sounds like a fun project.Ē I also remember that I worked out the statistic that Jer Yu and I used on the plane on the way back from Toronto.
Peebles:Yes. I had the idea and worked it out; we did it shortly thereafter. I also remember that sitting next to me on the plane was a lady who didnít say anything to me the whole flight, and I was occupied, scribbling away working this out; but at the end of the flight she leaned over and whispered to me, ďYoung man, youíre getting all your homework done, arenít you?Ē (laughter) Okay, we went out for a coffee break, and you said that the paper with Yu was more of historical interest than anything else.
Peebles:Well, we got the wrong answer in that paper. We concluded, we could see no evidence of clustering of Abell clusters. Of course, later on others invented other tests. We came back to this method, and saw pretty clear evidence that clusters do cluster. The method also was only of, I think, historical interest. At that time I was convinced that power spectra, Fourier transforms, were better than the configuration distribution. In the statistics It became apparent once I started doing this subject more thoroughly that, in fact, the distribution in configuration space was more useful, just as a practical matter than, the distribution in transform space.
Harwit:Well, one canít guess that sort of thing ahead of time.
Peebles:No, itís trial and error.
Harwit:Yes. Itís...well, you went down, then, and started working in terms of first two point and then higher number of point correlation functions. Perhaps you could say something about that, because thatís an approach which, as far as I know, is original with you. Isnít that correct, at least in this context?
Peebles:Yes. Letís see. Where are we now; weíre starting this long series of papers I wrote on statistics.
Harwit:Thatís right, yes.
Peebles:When I wrote the first paper in this series, I certainly couldnít envision how the last was going to be written. I didnít envision at that time using higher order correlations functions. I only could see that there were a lot of catalogs that seemed suitable for statistical analysis, so why not embark on a program of that. The first paper in the series was theoretical, and it certainly didnít foreshadow even the methods we were going to use. It did emphasize more of the Fourier transform than the autocorrelation function statistic, although one is a transform of the other. Still, it was the distribution in positions, rather than in Fourier coefficients that was most useful. It had nothing, I think, of higher order moments in it. I was thinking just of a way to measure the large-scale mass distribution, because I could see that the large-scale mass distribution looks smoother — looks more simple, therefore, than a small-scale distribution, so it might be easier to untangle to trace back evolution in time, to get a measure of initial conditions. That was the motivation, and that much I could at least see at the beginning. How did I get into the game of higher moments? I can remember only one hint; namely, I happened to read a fascinating paper about the physics of ocean waves, which to a first approximation...Now what was the point of that paper? Ocean waves are nonlinear. They act as solitons, but they also scatter off each other. And a description of that scattering was conveniently done using a statistic that they called the bi-spectrum, whose Fourier transform is the third moment of the distribution, the three-point correlation function. It was a paper by Walter Munk and appeared in a book published by MIT, and otherwise quite lost to me.
Harwit:Thatís not the Munk and MacDonald book?
Peebles:Thatís the Walter Munk of ďMunk and MacDonald,Ē but not the ďMunk and MacDonaldĒ book. That was the hint to me that it would be profitable to go to higher order correlation functions. It was also becoming apparent that we were not getting the whole story by any means in measuring only the two-point correlation function. The galaxy distribution is so far from a random Gaussian process, that the higher moments are essential, and so, I started thinking about higher moments. The method I developed, it turns out, is just what you use in the physics of nonideal gases. The higher moments are reduced by subtracting off appropriate combinations of lower moments. I did that independently of the methods of nonideal gases for very different reasons. I subtracted off — I used reduced moments, as I discovered later they were called — because it turned out to be practically useful in going from an angular distribution to a spatial distribution to subtract off those moments. One could discover that the moments you subtracted off in going from a full n-point correlation function down to the reduced function were just the moments that would be generated by projection effects, rather than by clustering in space. So I was interested to see how, after I had worked these reduced moments out, that that was just a game people played in the theory of gases. One other influence, only, can I remember. Bill Saslaw wrote a paper that influenced me on the BBGKY hierarchy. Now, I donít remember just where I was in this higher moment game when Saslawís paper came out, but I was certainly still fumbling around, and that paper helped crystallize my thoughts.
Harwit:Can you spell out BBGKY. What is it an acronym for? I always forget.
Peebles:Yes. Born, Bogoliubov. Green, who is a famous statistical mechanician; Kirkwood is next; also of statistical mechanics fame; and Yvone. And I donít know who Yvone is. I suspect French. Bill Saslawís the one responsible for drawing my attention to the existence of the BBGKY hierarchy in plasma physics. And once one knew that it would be useful to do higher moments, there was a lot to be done, and so lots of papers in that series were devoted to the higher moments. I think we exhausted it by the time we got to the four-point function.
Harwit:Did you have any trepidation about going into a statistical analysis, in the sense that...well, as an observational experimental worker, I tend to worry that if I canít see something right away with a naked eye in the data, that Iím going to perhaps reach a wrong conclusion in doing a deeper statistical analysis?
Peebles:Thatís important, and itís something I try to bear in mind. Itís so easy to get so sophisticated in the statistics that youíre kidding yourself.
Harwit:Particularly when itís a unique...
Peebles:Particularly when itís unique. But it was important, you know, that I could see fairly early on that there were several catalogs at different depths that gave you separate, almost independent samples, so that was an important part of my considerations from the first — Iíd be able to check by reproducibility, samples of different depths. And indeed, one of the things I stressed in the first paper was how to do scaling to test consistency of results from different depths. Actually, remember, Sidney Van den Bergh was the catalyst in getting into this game originally, and his remark was, ďI look at this and it looks lumpy.Ē And my statement, ďWell, it looks kind of random to rue.Ē We were both making a visual judgment, coming to somewhat different conclusions. So yes, I was interested in testing the eye, so to speak. Not only do I trust the eye, but I also distrust it — an example are filaments, after all, of galaxies. The eye sees them; are we to trust the eye or not? Well, I think until I have a sophisticated statistic that is unambiguously pointing toward filaments, Iíll be cautious about believing my eye. Thatís not to say that I put down the eye, itís also fantastic at picking out patterns, but itís also subject to all sorts of human biases, of course.
Peebles:In any case, as I said before, I donít think Iím a reflective type - I didnít hesitate to jump into this. I didnít know the quality of the catalogs, whether they were full of systematic errors or not. I knew they must be at some level. I just blindly hoped that there would be enough catalogs that one could have enough tests of reproducibility, that one could be convinced, if one had a true affect, a true measurement.
Harwit:Have you been disappointed in the reliability of the catalogs?
Peebles:No, I mean, from the first I was somewhat skeptical about the reliability always: Is the Abell catalog uniformly complete across the sky? I mean, it does have definite gradient running across the sky, in the density of Abell clusters. I can still imagine that thatís due to a systematic error on Abellís part. He ran out of steam, perhaps as he compared one part of the sky to another. But much more impressive to me is how good these catalogs are, considering they were taken by eye and by hand. Of course, when I started out on this game, I didnít know about the full Lick catalog, the catalog in tenóminute cells. And that has been truly impressive. Thatís got to be counted as one of the monuments of great research and great effort. Here is someone who took all these data, without any reassurance that anyone would ever look at them, and took them with great, great care, and with good control on statistics and bias. Iím talking about Donald Shane at Lick Observatory, and in particular about his courageous decision to have lots of plate overlap, which, you know, the Palomar Sky Survey doesnít do, meaning that his project was considerably increased in time and effort; but that if anyone ever wanted to, they could use those plate overlaps.
Harwit:But, Margaret Geller, your former student, was telling me just a couple of weeks ago, that she talked with you about some of her findings in the Shane/Wirtanen catalog which suggests somewhat different sensitivities in different plates, either because of plate materials or perhaps conditions under which the plates were taken. How much to do think that will affect results, considering, as you say, that thereíre a number of different catalogs around?
Peebles:I think, you know, both Margaret and I are making good points when we come to rather different conclusions about the validity of the Lick catalog. It was taken by hand, and itís got to be full of bad errors, and I think her main point is itís time we had a new catalog, and I am really encouraging her every minute of the way to do that. But the catalog still was immensely valuable, and many of the conclusions, I think, from it are reliable; and as you say, weíre convinced theyíre reliable because we have other catalogs — the Zwicky catalog, the Jagellonian sample; the deeper samples, some British groups are taking — that all seem to point in the same direction. We have two pretty good redshift samples these days, the Center for Astrophysics sample, and the Durham, Australia redshift sample, that are large enough that one has pretty good statistical measures; and this time with three-dimensional information, and they again give similar answers. So, I think the main conclusions, derived from Lick and the other catalogs, are standing and firm. Thatís not to say that the Lick is blameless; it has errors in it, we know.
Harwit:But if I understood Margaret correctly, this lovely wall chart that one sees in most labs now, which suggests filamentary structures of galaxies with big voids in betweenÖThose voids seem to be correlated with individual plates, the way she was talking.
Peebles:Yes, she can find some examples where filaments run up against plate edges as if it were a discontinuity, and where perhaps holes overlap with plates, and also where particularly dense spots — one of the particularly most dense spots — does line up with a plate. Some of those are surely artifacts of the data-taking. Is the general visual texture of that map an artifact? I still think not, even after carefully studying her results which I think are valid. For example, the densest spot on the map is in the Serpens Virgo cloud. It just accidently happens to sit square on one of the astrographic plates. But Donald Shane already recognized that and took careful pains with that plate to make sure it wasnít biased, so he was convinced that itís a real dense spot, and I guess I have to believe him. A few of the filaments do line up, as I said, with plate edges, and I am willing to believe those are accidents. Others run across plates and surely are real. So, yes, the appearance to some extent must be affected by the systematic errors in the catalog, but Iím betting the general texture is as it is.
Harwit:Speaking of Margaret, one of the cutest papers — maybe the one I thought was just lovely — is one you did with her called ďTest of the Expanding Universe PostulateĒ in The Astrophysical Journal, Vol. 174, page 1 in 1972, in which you took on the test of the tired-light model, and rather than using the usual excuse — well, the redshift is really the only thing that makes any sense — you looked at predictions of how angles would scale with increasing redshift, and maybe you could say something about that paper.
Peebles:What can I think to say about that paper?
Harwit:Well, has it, for example, been followed up at all, in terms of observations?
Peebles:Yes, it has. The observations are more difficult than Margaret and I had appreciated at the time, but there have been some pretty experiments done — one in particular by Philippe Crane and Alan Hoffman. That is already a few years old; the experiment should be done again, perhaps Space Telescope will give us some more believable data. The thing that struck me about that paper most is that it has generated more than any other paper what I might almost call hate mail — letters from people who are convinced that the universe is not expanding and strongly disliked our conclusion that when we looked at the evidence as best it is, it seemed to point towards expansion. Quite a few letters...
Harwit:Itís probably sufficiently understandable so that people who have axes to grind, and thereís a surprising number of these people who have a deep astronomical interest in this country.
Peebles:Yes, yes, it surprised me. I might also remark that I learned after the fact that the basic physical idea behind this test was already understood by Tolman.
Harwit:Interesting. Where did you find that?
Peebles:Well, I just happened to cross it; perhaps I happened to cross it in Tolmanís book, you know.
Peebles:You know the fat book. I did refer to the paper; Tolman wrote with Hubble...
Harwit:Yes, I remember that Hubble turned to Tolman in the early Ď30s, I think, for advice on statistics, cosmology, and so on.
Peebles:Yes, yes. The interaction didnít seem to me to be all that fruitful. Hubble was rather locked to his observations — fair enough. But out of that collaboration did come this nice test for expansion which then got lost, I referred to it, though, in a review I wrote of the test in Comments on Astronomy and Astrophysics.
Harwit:Itís the kind of thing that belongs in textbooks.
Peebles:Yes, it does; I quite agree, and the next time I write a book, Iíll emphasize it - where is it, (long pause) I thought I remembered writing it in Comments. Here it is, ďTwo Old Cosmological Tests.Ē In fact, it appears in this list before the paper with Margaret and me, but it was written after.
Harwit:Yes, thatís in Vol. 3, No, 6, page 173, 1971.
Peebles:There I explain the connection to the idea of Tolman and Hubble. The second test I have in mind there is galaxy counts, which have always seemed to me to be a wonderful test, not because theyíre so sensitive to anything, but because you can go so darned deep.
Harwit:Yes, except people always worry that youíre then going to uncover a new population of objects.
Peebles:Itís a problem. I think we know that in fact, the deep counts do go to higher redshift, because we have some redshifts measured for these faint objects. And you know...
Harwit:Hewish was playing that game in the early Ď60sÖ
Peebles:Öwith radio counts.
Harwit:Yes, and itís always been a problem.
Peebles:Well, radio counts have the extra problem that one has even less [of an] idea of the luminosity function than for galaxies. Thatís a fair remark.
Harwit:Öand the identity.
Peebles:The identity is so much more difficult.
Harwit:You just mentioned the next time you write a book. You have two that I own, at least...
Peebles:Then you have a complete set.
Harwit:A complete set, okay, which are very nice actually, I think.
Harwit:Both on Physical Cosmology and The LargeóScale Structure of the Universe. Can you say something about how long it took you to write those - any motivation, and so forth?
Peebles:I can still recall vividly the motivation for writing the first book. I had decided I would like to give the course in cosmology. In fact, to get this right, a course in cosmology, one term, graduate level, was created and I was assigned to teach it, John A. Wheeler decided to attend this course, and he started taking notes.
Harwit:That must have been frightening. (laughter)
Peebles:Well, the frightening part was, at the end of each lecture, he would provide me with a pile of paper, in his beautiful handwriting, with everything I had said written down, saying, ďIím going to keep doing this until you agree to write a book. I was so destroyed that I quickly agreed (laughter). I then startedÖI couldnít stand this great man, sitting humbly in the last row, taking down everything I said, so I started writing up the notes of my lectures as I went along and distributing them to the class. At the end of the term I had then a great thick pile of notes, and soon I couldnít resist polishing them up and producing a book.
Harwit:Did you get feedback from the students?
Peebles:Oh yes, that was helpful. And from Wheeler. Mostly encouragement from Wheeler. This constant threat that he would take the notes if I didnít. The students were very good, mostly by asking questions, rather than commenting on the material I had produced. The material I produced was mostly good, simply as a framework around which I could wrap a better version, I wrote the thing very quickly. You can tell; itís very informal in style, and it wasnít the plan, but I guess that was the successful thing. Itís a book thatís easily read.
Harwit:Yes. I was going to say, itís easily read. It had the aspect of having been filtered by students, thatís why I was asking you.
Harwit:I think books that are written without an exchange tend to be a little more opaque.
Peebles:Yes, yes. This is certainly written in the open. I got lots of feedback on successive drafts from my friends around here. I did write it very quickly. I started in January; and it was done by spring.
Harwit:Really? This was after you had finished the course?
Peebles:After I had finished the course and I had let it mellow for one full year, not doing anything with it, but I suppose thinking about it quite a bit — I would put words to pencil to paper if I ever got up the strength to do it. Then I remember my wife and children, after Christmas, went off to visit her family in Montreal, so I had a week alone, and I just sat down at the living room table and started writing...or the dining room table, in fact. By the time she got back, I had already a sufficient pile of stuff that I couldnít turn back.
Harwit:So you didnít have any pressing distractions that would have made you spend a week in the office because you had taken the luxury of spending a week of writing.
Peebles:That was time when not too much was happening at the university. I bet my course that year, and I donít even remember what it was, was poorly given. No, it was a course in electromagnetism for non-majors, and I gave practically zero preparation to each lecture.
Harwit:You ďwingedĒ it.
Peebles:I ďwingedĒ it. (laughter)
Harwit:Itís been known to happen.
Harwit:But actually thatís the only way that, I suspect, one can write...at least I find itís difficult unless you do things in a lump.
Peebles:I can only do things in lumps. Iím very impatient when I have something halfway done to get it finished, and so I will often put off a project until I can see a block of time that I can hope to have. Then Iíll just work long hours until itís done.
Harwit:How about the LargeóScale Structure of the Universe?
Peebles:Well, that started in a similar way I gave a course, By that time, I had spent a lot of time doing statistics of the galaxy distribution and making the first attempts at a dynamical theory of how this distribution could arise. It seemed to me it would be good to write it all down, not that I felt that it was by any means in a final state, but that it had reached a plateau where all the easy things had been done. You know, I could skim off a lot of cream from these catalogs, very quickly. Then one could see how one would try to milk them or else make new catalogs, but that would be long-term projects. And similarly on a theoretical side I could see how many questions remained open, but they were all hard, and I had a chance to skim through the easy questions because no one else had tried this game before. So, I gave a course — same deal — although this time I didnít have John Wheeler taking notes, but I gave the course with the intention of writing a book. So I produced course notes, and I find that when I write, I do much better if I have successive approximations, each larger than the last.
Peebles:The course was an excellent way to get a first approximation. I then, the following year, took a sabbatical leave; went to the Institute for Advanced Study. The first term I somehow just couldnít face that pile of notes. I did other things; sat around and watched the birds. Then around about January again, I started buckling down, and by spring I had the thing done again.
Harwit:Thatís very fast. I find it takes me eight years for each book, but you know there are always observing trips for which you have to suddenly jump in, a month ahead of time, and prepare a rocket payload or something.
Peebles:You lose a lot of time when that happens, because you have to pick up the threads — at least I lose a lot of time.
Harwit:Iíve got to find the threads. (laughter) I forget theyíre still there.
Peebles:Yes, yes, you have to find them, review them. So when I say it took me perhaps five months to write the book, thatís five months almost every day, sitting in a quiet office at the Institute, and that doesnít count all the many hours I must have spent ahead of time thinking through all these things, and preparing a course and so on.
Harwit:Let me ask you, do have grants that require you to write X papers per year (laughter) to...I mean, this is one of the things that does interfere with a large-scale project. How do you handle that?
Peebles:Well, I am very fortunate there. We have what we call an umbrella grant, perhaps thatís a common term. We have from the NSF one research grant that covers the activities of Dave Wilkinson, me, Bob Dicke, Steve Boughn, many people. For many years Bob Dicke wrote the research grants, wrote the progress reports. Then when he became a member of the science board, he had to give that up. Dave Wilkinson took it over, and has carried it on to this time. I am entirely fecklessÖthe money flows in and I spend it. (laughter) But I spend very little time on bureaucracy of that sort or any other. I am not a very good citizen in the department, but I do spend time on committee work. For example, this year I am running the graduate general examinations, but I always try and wangle on a committee assignment that has a rather finite duration to it, and so graduate general examinations will occupy me fully for a couple of weeks and then theyíll be done until spring, and then thereís another week or so of intense activity and then itís done again. Thatís the sort of assignment I like to try and get.
Harwit:Have you ever had or wanted to have administrative positions in the department?
Harwit:How about outside committees in Washington, things like that? Do you do much of that?
Peebles:On occasion I get entangled in that web ofÖ(laughs) how should I characterize it? On occasion I have been involved in such activities, of course. I find that Iím...I believe Iím not at all good at it. My attention wanders. I donít really take an interest in it, and so I try to avoid it as much as possible. So some years I am, yes, heavily involved with committee work, other years Iím not.
Harwit:Can you remember specific ones that youíd like to mention?
Peebles:Wall, there is a Space Science Board that Iíve sat in on as a member for some time in the distant past. The NSF has had...
Harwit:Thatís the National Research Council?
Peebles:Thatís right. Do I have the right name — The Space Science Board?
Peebles:It sounds familiar, yes. What else have I...
Harwit:Who was the chairman at the time, just to approximately date it?
Peebles:Who was the chairman? Harlan Smith toward the end, and before Harlan Smith...who the devil was it? Was it Al Cameron?
Harwit:Al Cameron came later, I thought.
Peebles:No, I donít think it was Al Cameron. Who was it? I donít even remember.
Harwit:That would be in the mid-í70s, though, anyway, or the late Ď70s.
Peebles:Yes, the late Ď70s, maybe mid-í70s. I just donít remember.
Harwit:It doesnít matter. I just wanted to place it.
Peebles:You see, itís an example of my attitude toward these things. I really have trouble getting excited about committee work.
Harwit:Fine. (laughter) I must say I agree with you.
Peebles:Itís important and in a sense I am a parasite on the people who are willing to give up weeks of time to go and fight the battles that must be fought. At the same time I think I recognize that theyíre better at it than I am, so, let us stick to our skills.
Harwit:When you say itís important, are you paying lip service?
Peebles:Vast sums of money flow into this department and I spend it on computers, post-docs, and whatever. Certainly, I am grateful for it; I teach half time, because the university is willing to support me full time because, I guess, I a influential in getting this flow of money into the department. In the sunnier Iím paid an enormous sun by the NSF to walk up and down on the campus and think — trying to decide what paper to write next. Iím grateful for it; I recognize that Iím in the lap of luxury. I canít think of a better position for me than this — minimal teaching and the ability to skip much of the committee work that might be involved in other universities, and lots of opportunity to just hang around and think. and thatís paid for.
Harwit:But do you think the committees make that possible?
Peebles:Well, the NSF makes it possible. This university couldnít afford to treat me the way it does if it werenít for this flow of money from the NSF. That must be fairÖ
Peebles:...even though my academic salary is not paid by the NSF. It must be that indirectly.
Harwit:Well, there is a lot of overhead that comes in, and I imagine it easily covers the half of your time that the university sets you free.
Peebles:Thatís what I am thinking of. Yes. Well, so the NSF is immediately important to me — to my style of life, kid Iím grateful for it. Now, why does the NSF operate in such a rational way? I assume because itís...
Harwit:No, no, thatís not what I was asking. I was asking whether it could act as rationally without all the committees?
Peebles:Thatís the question I was just about to ask, and I donít know the answer. I have the impression that if the NSF were left to its own devices, it would only respond to those pressure groups that are pushing on it which would be in Congress, and it would move in weird ways — weird, I would say, perhaps Congress would differ. I assume itís important that there are these committees of scientists who will go to Washington to testify; I assume thatís important. NASA is a great example of how bad things can get even when scientists are pushing hard. Vast sums of money spent on what is called science, but that seems maybe not, to you and me, quite so productive immediately. The lack of unmanned satellites seems to be a scandal, such an economical way to do great science. Well, there are those scientists like Dave Wilkinson who do spend time leaning on NASA to keep providing funding for rockets and balloons, for example. I would assume that without the strong voice of scientists ballooning would be dead in the United States. So is that fair?
Harwit:Yes, yes. I think so.
Peebles:And I think itís fair to say that there is still a lot of science still coming out of ballooning.
Harwit:No question. Particularly Daveís work.
Peebles:Thatís what I had in mind, but others are doing good science too — with ballooning, U-2 flights, rockets — still a lot to be done. Particularly when you consider how good it is that you have science that can be done by graduate students, getting involved in the space program. Suddenly itís big science and itís awfully difficult to do; itís time-consuming, expensive, and graduate students canít be involved.
Harwit:Do you have any fear of the growth of National Centers and the centralization of science, perhaps also in government laboratories — NASA laboratories - from the point of view dissociating funds, equipment, the best people from the places where graduate students are educated? Is that a worry as far as you can see?
Peebles:Itís something we should be worried about, isnít it? Is it happening? The Institute for Theoretical Physics is an example of such an institute. I think so far itís only been beneficial. The number of permanent members there is quite small; the number of visitors is enormous — seems to be doing just what you would hope. The Space Science Institute in Baltimore has many more permanent members, but theyíre heavily involved in, at the moment, nonscientific things. I donít know that they compete with the universities in research.
Harwit:Well, but thatís a building-up phase there.
Peebles:Thatís a buildingóup phase. How it will turn out, I donít know. Is Kitt Peak a detriment, a competitor to science in the universities? The students weíve sent there have had only good experiences. The one thing Kitt Peak is good at is catering to the visiting astronomer who will perhaps have just a few days every year to take plates and then will go and analyze them.
Harwit:What I had in mind, since Iím an instrumentalist, in part, is that you send a student to Kitt Peak, he uses a Kitt Peak telescope, a Kitt Peak computer to reduce the data with a Kitt Peak program.
Peebles:Thatís not good. To use the Kitt Peak telescope is inevitable; thatís where the telescopes are, conveniently located and yet in relatively good seeing. One would hope that that student would take that data back and analyze it himself. Now if the universities get so starved for computation and scanning devicesÖ
Harwit:Itís not that so much. The question is, who becomes a member of the next generation of instrument builders? Where are those going to be educated? In theory that in the theory area, itís not that big a problem, perhaps, but we just were in Dave Wilkinsonís lab a little while ago. The fact that thatís at Princeton is a great inspiration to the students you have here who can become the next generation of instrument builders, because theyíve worked with the most sophisticated equipment and built it themselves.
Peebles:Right. Theyíve built it themselves. Students were heavily involved in building that thing, and itís a frontier technology on a very exciting problem.
Harwit:The question is how long will that last?
Peebles:It is a function, isnít it, of the style of research and sort of problems you choose, if youíre thinking of high-energy experiments, then weíre in a very nasty situation, I think, where the projects are so big that the students are lost in the backwash, many of the faculty also. Thatís a serious problem. One of the reasons I enjoy doing what I do is that itís still a small science area. And I value that very highly.
Harwit:Sure. Well, you see, the Space Telescope Science Institute might be approaching the high-energy physics model, which you were just deploring.
Peebles:The Space Telescope will be in the high-energy physics model, wonít it, because its big money and thereíll be enormous pressure for time on it. I could imagine that you will have to form a consortium before you can even get near the controls of the thing — a problem.
Harwit:Well, I just mentioned that as a question you might have thought about.
Peebles:Not really. As you will observe from my list of publications, I kind of tend to work alone. I donít know, more than half of those probably are single publications.
Harwit:But youíve had quite a number of students.
Peebles:Yes, yes. Always though small numbers; I never had a large group.
Harwit:One or two at a time?
Peebles:Thatís the idea; thatís the most I can handle.
Harwit:Stu Shapiro was one of your students, wasnít he?
Peebles:Thatís right. Now doing very well at Cornell.
Harwit:But you never published anything jointly.
Peebles:Could that be? I guess youíre right.
Harwit:I didnít find anything on the list.
Peebles:Yes, yes. Fair enough.
Harwit:Why was that?
Peebles:Well, I donít know. It happens. One reason, of course, is that he is a very energetic and well-motivated person, so if you give him a direction, he will start moving in that direction, without any need for further prodding. [That happens] not very often. Students are remarkably reluctant to take the next step. You say, ďWalk in this direction,Ē and they will until you stop talking and then theyíll stop.
Harwit:But there comes a threshold where they suddenly are self-motivated.
Peebles:...Self-propelling ó- a magic moment.
Harwit:And then they start teaching you, rather than...
Peebles:Yes, yes. And that comes earlier or later in a studentís career. If it comes later, then it seems more likely that you will publish joint papers. With Stu it came very early. And always with him it was more a question of saying, ďWhy donít you look at?Ē and him going off and moving ahead very quickly. So none of his research really had all that much of me involved in it.
Harwit:Do you have other students like that who might not appear in your publication list...?
Peebles:I would have to review the data, but certainly .he is the most prominent example of a self-starter that Iíve enjoyed working with.
Harwit:Could you give me sort of a list of students you recollect — it doesnít matter if itís complete or not.
Peebles:It would certainly be incomplete. Jer Tsang Yu, also a self-starter, although we did a lot of work together.
Harwit:Is he Chinese-American?
Peebles:He is ChineseóHongóKong; and in fact, is in Hong-Kong now.
Peebles:Names. Other students. Dan Hawley, an amusing experiment to measure possibility of alignment of long axes of galaxies; Vincent Ruddy, in a starlight in clusters of galaxies, is the dark mass stars? By now weíre pretty convinced it isnít, but it wasnít so obvious in those days. Margaret Geller. You know her well — another self-starter. As a side remark, the second woman to successively complete the Ph.D. program at Princeton. You know, we have an abysmal record in women in the physics department.
Harwit:Oh, in physics?
Harwit:Okay, I mean, there were other women before that, or not?
Harwit:In other departments?
Peebles:In other departments, yes, there certainly were through the years, and by the time Margaret was here, we were nominally co-educational, although we were still proud to get ten percent women on our undergraduate majors.
Harwit:Mike Hauser was not a student.
Peebles:Mike Hauser was not a student. He was not a post-doc. He was an assistant professor. He began in particle physics.
Harwit:We were just talking about Mike Hauser and your collaboration with him. He was at Caltech already when you wrote papers up, but had been here before.
Peebles:He had been here before. He, as I say, began in particle physics as a graduate student óó I think at Princeton, although Iím not sure, then decided to switch to astrophysics, and was an assistant professor when he and I collaborated. He then moved on to Caltech, and we finished up writing the papers while he was at Caltech. The research was all done before heíd left, Ray Soneira, heavily involved with the statistical games weíve been playing, a wiz at the computer — an example of how itís so good to have someone around with other talents. He is a real marvel at computers, and our game of making statistics that would reproduce galaxy distributions and could be compared to the Lick map was only possible because he had the energy and the ability to handle the computers and the picture processing involved. Mike Seldner, also heavily involved with statistics, and with the setting up of the Lick map.
Harwit:Was Ed Groth one of your students?
Peebles:Ed Groth was a student of Dave Wilkinson — did an experimental thesis. Ed Groth is another of these polymaths, both good at theory and experiments, so Iíve had his theoretical ear and Dave the experimental one. Heís on the faculty, a tenured member of the faculty.
Harwit:He also was a student here and stayed?
Peebles:He was a graduate student here and stayed, yes. Perhaps there areÖthatís more common here than at some places.
Harwit:At Cornell we havenít had anybody, butÖ
Peebles:There are other examples.
Harwit:Here, you mean?
Peebles:Here, yes. Arthur Wightman, mathematical physics. Kirk MacDonald, particle, experimental. Stu Smith, particle, experimental. Yes, I guess we tend to be a little more along that line than usual.
Harwit:You also worked with Mark Davis?
Peebles:Oh yes, again he was not a student with me. He was a student with Dave Wilkinson, looking for young galaxies, the primeval galaxy search again. Again, another of these flexible people, brilliant in the lab and also very good in experiment, so after he finished his thesis work, he stayed on as a post-doc for a year or two; came around and asked me if we had anything to do; and we got into this game. He, maybe, regretted soon talking to me.
Peebles:Jim Fry, yes. A good graduate student with me, statistics and dynamics again.
Harwit:You published quite a lot with him.
Harwit:Does that mean he was a late self-starter?
Peebles:No, I donít know how one characterizes these things. Partly, you know, I tend to impose on these people. In Jim Fryís case, as in Ray Soneiraís case, they had talents that I was eager to exploit, in particular the ability to talk to a computer and the willingness to sit for hours doing that. In both cases they were awfully beneficial. In a sense you could say I exploited then, but of course they also had fun things to do.
Harwit:Thereís a paper with Clutton-Brock whom I donít know.
Peebles:Martin Clutton-Brock, yes, he is at the University of Manitoba, though he wasnít there when I was a student at the University of Manitoba. I only happened to meet him...heís one of these rare delta functions, a very good scientist in a rather obscure spot. I met him when I was visiting the university to give a colloquium, invited back as an old boy, I guess; and weíve continued a friendship and collaboration through the years. I guess weíve written only one paper together, but weíve certainly stimulated in each other, other papers and other research.
Harwit:And I donít know if you mentioned Schechter.
Peebles:Paul Schechter. He was never a student here; he was a visitor at the Institute for Advanced Study. A graduate student at Caltech, then a post-doc at the Institute, and exceedingly bright; now at the Carnegie Institution.
Harwit:Let me go to one of the papers which has been cited a lot recently that you did with Bob Dicke, ďThe Big Bang Cosmology — Enigmas and Nostrums,Ē where I think you first laid out the horizon and flatness problems in a way that was sufficiently clear for people who were mainly theorists with not much connection with observational astronomy to of really worry about. At the time you wrote that, were you thinking that you were saying something significant, or were you thinking that this was something that you knew and it was obvious?
Peebles:Yes to both. Also you will have noticed that the tone is a bit flip in that paper; and we were being perhaps even consciously a little flip. The ideas go back to my earliest memories. Itís hard to remember when I wasnít puzzled about these two problems, and when Bob Dicke wasnít. Surely other people have worried about them too through the years. I donít know of any references to earlier discussions. It was surely known to others besides us. He didnít anticipate that suddenly these problems were going to become topical. It was just a coincidence that we happened finally to write down the puzzles at a time when it was inflation, when one would see an answer to them, or a potential answer, at least — just a coincidence. Although I have the feeling there is a curious sociology to the way science is done, in the sense that when the time is right for an idea, it will emerge; and sometimes spontaneously in more than one place, independently. Iíve just learned that there is a good example of that in the discovery of the inflation scenario that one can now find very distinct predecessors of that idea in the literature.
Peebles:I canít tell you the literature. There is an article in Physical Review by someone at the University of Pennsylvania, clearly setting forth many of the ideas, but in fact, making mistakes also, and not nearly as clean and elegant as Alan Guth.
Harwit:This was a long time ago?
Peebles:No, no, just recently. But, in fact, ten years before inflation it is said Linde had many of these ideas. You know, Linde, the Russian theorist. He is very big on zeroópoint fluctuations and the zero-point energy of the vacuum, and attempts to compute it, which, you see, are very irrelevant.
Peebles:Also, tíHooftís advisor, whose name is Veltman, it is said, had some of these ideas even published before Guthís. An example of an effervescence of the subject — bubbling up of ideas — more or less independently at about the same time, perhaps one canít say that the paper Bob Dicke and I wrote is in this class, because we... But in a sense it is; we could feel a resurgence of interest in cosmology. Yes, quite a change. After all, when I began in this subject, cosmology was really almost on the fringes, as we remarked, so was general relativity theory.
Harwit:Yes, they were considered geometry...
Peebles:Not physics so much.
Harwit:Actually the bringing of chemical structure, chemical abundances into cosmology. You know, one could have assumed it was God-given.
Peebles:Yes, or just initial conditions, if you like.
Harwit:Thatís right. And the fact that there is physics in there has really enriched cosmology. The interesting place to look, I guess, is in Bondiís book...
Harwit:Öwhich was written around Ď48 or so, and unfortunately, in the updated version by Lyttleton, nothing much had been added. But that is sort of the last time that people in cosmology were working strictly in geometric terms and the McVitties, and Eddingtons, and the Lemaitres, and all these people were putting out different versions with slightly different concepts, and you could generate new models like this. And I thinkÖeven the Lifshitzís paper we talked about before still was only a slightly moreÖ
Peebles:I remember my reaction to cosmology when I first learned of the subject as a graduate student, preparing for the graduate general exams here — exceedingly dull, I thought, and ad hoc, and unbelievable, I was repelled by it. In any ease, I made this remark because I was reflecting that when we began, cosmology was a very small field, Weíve seen it grow, perhaps exponentially, certainly at a rapid rate; and by the time of the paper, you referred to by Bob Dicke and me, the subject had become very crowded, very lively. I guess there was this feeling on our part, if weíre ever going to discuss these ideas, weíd better do it now.
Harwit:Yes, sometimesÖI donít know whether youíve had the same feeling, but sometimes one writes something which has been discussed over lunch tables. People who are in the field know it, and when you write a review article, itís obvious that that should be put in then and stated. And all of a sudden people applaud that, and you feel a little surprised because you figure you havenít sweated enough to deserve theÖ
Peebles:Yes. (laughter) A little startled. Thatís right. You canít tellÖyou canít anticipate these things fully, and yet you are motivated to put that in for reasons you might not be able to articulate.
Harwit:Of course, it belongs there, yes. Itís funny. Iím continually puzzled by the lack of correlation between sweat and acclaim. You know, one works so hard at something and people donít seem to give the work the respect that you feel it might deserve; and then you turn around and do something which you dash off in a couple of weeks, and all of a sudden people remember that.
Harwit:Or you get hate mail. (laughter) By the way, did the hate mail on the Geller paper come from anyone in universities?
Peebles:There was one from a university in South America, but I have no idea which one, nor which country even.
Harwit:I saw the paper, also, that youíd written in the conference on Einstein here in 1980, which Harry Woolf organizedÖ
Harwit:Öin the volume ďSome Strangeness in the Proportion.Ē What was your impression of that meeting? Do you have recollections of that?
Peebles:Well, my most vivid recollection is that the Institute hired a new chef for the occasion; kept the chef, and itís been the best thing thatís happened there ever. Their lunches now are really great. I remember the food at the conference as being particularly good. The chefís name is Hans. I donít know his last name — a local treasure, I donít remember the conference, except that it was rather crowded, and I have an aversion to crowded conferences — little opportunity for discussion from the floor, for example - rather one had a sequence of set pieces presented by the speakers.
Harwit:Did you find any of it interesting?
Peebles:Well, I remember John Wheelerís paper which was as imaginative as ever, and I remember the very gracious discussion of the paper by Freeman Dyson. I donít know whether Feynmanís talk is reproduced in the volume. He made a very apt point - he recalled his reaction to Feynmanís method of computation in quantum electrodynamics, Feinman saying, ďWell, you just put an arrow here, you put an arrow there. The photons and electrons are running backward and forward in time,Ē And Dysonís reaction was, ďYouíre crazy.Ē But of course Feynman wasnít crazy, and Dyson drew the parallel to John A. Wheeler who was saying these things that sound so mystical. Oneís first reaction is youíre crazy, but of course heís not crazy. Heís paid his dues, and heís pointed to something important if only we could appreciate it.
Harwit:Well, Dyson is a very receptive person for that kind of thing.
Peebles:Just appropriate, the right humor and also the right imagination and receptiveness to new ideas.
Harwit:And knowledge, also, I thinkÖ
Harwit:Öwhat sounds right, He describes his reaction to Feynman in this very nicely done autobiographical book.
Peebles:Oh yes, I had forgotten — Disturbing the Universe.
Harwit:Thatís right, yes.
Peebles:Other remembrances of the conference, no I donít have too many.
Harwit:Wheelerís paper in leafing through the book, I didnít read it cover to cover, was the most interesting one of a wide-class there, andÖ
Peebles:Thatís characteristic, isnít it?
Harwit:A lot of the other things I wasnít that interested in. Some of them were just straight historical things that could have been found elsewhere in perhaps greater depth. What about your paper with Ostriker on masses of galaxies? I guess that was one of the early papers that dealt with galaxies that were more massive than one had originally suspected them to be.
Peebles:Well, of course, the game was you had to put the mass not in the disk but in a halo, so it couldnít be starlight that was dominating our galaxy. The history of that is easily told on my part; Iíd gotten interested in n-body simulations for a simple reason: Around about 1971, I spent an academic year on leave at Caltech. On the way back I was invited to stop off and spend a month as Los Alamos. Los Alamos had big computers. I had recently become interested in the dark mass problem, and that involved, among other things, modeling the motions of galaxies in clusters, so I used their computers to make n-body models. Took the technology back to Princeton and started modeling the expanding universe, but then at that time, Jerry Ostriker was doing analyses of stabilities of rotating objects, first with respect to the maxima of velocity, I believe, of a white dwarf and its breakup.
Harwit:He was probably an assistant professor then?
Peebles:He must have been an assistant professor then. No, he was probably tenured by then, but Iím not sure. Well, he had the theoretical problem that he couldnít find stable solutions for flattened things, for flattened mass distributions that were rotating. I had the technology to do n-body simulations, and now letís see, the proximate cause was even more direct than that. I had written a paper on angular momentum of galaxies. I had obtained what I thought was a pretty reliable estimate of the specific angle of momentum. It required that a good deal of the mass in a spiral galaxy be not in the disk but the halo, because otherwise you didnít have enough angular momentum per unit mass. Jerry remarked to me that coincided with his results, He was having trouble putting the mass into the disk because it was unstable. We then saw that we could readily test this by doing n-body simulation which I knew how to do, so I banged out some n-bodies, and they were dramatically unstable — n-body disks, Itís important to remember that that paper didnít argue that galaxies were more massive than we used to think, because one is only concerned with the mass interior to the outermost orbit of the disk and that is fixed by the motion of the disk; but rather the way that mass is distributed, not in the disk, rather in a dark halo; so one had more of the question, what is the dark matter? — which is still bedeviling us, of course, waiting for a brilliant solution.
Harwit:What about Amos Yahil?
Peebles:Oh, thatís a separate paper. We were talking previously about the stability of a disk, right? And thatís a paper by Jerry and me alone.
Harwit:Yes, I guess I had talked about the one you did with Yahil, but you also did one alone with Ostriker.
Peebles:This is the one I had been talking about.
Harwit:Yes, okay, the one you talked about was Ap. J., 186, page 467, 1973, and then the one with Yahil is in the Letters, Vol. 193, page L1, 1974.
Peebles:This paper with Yahil was a much more loose collaboration. All three of us had been worrying about the mass problem on scales of galaxies and larger, and remarking that the mass problem was growing more or less linearly with scale. You could already find that in my book on cosmology, and this was simply a collection of loose ideas. In fact, written, I believe, exclusively by Jerry Ostriker.
Harwit:And then sort of collaboratively read by the rest of you?
Peebles:No, then I agreed to have my name on it.
Harwit:There are lots of papers we havenít discussed, but are there any that youíd like to talk about?
Peebles:You might think Iíd have a favorite paper, wouldnít you? Is it your reaction, though, that by the time youíve finished with a paper, youíre sick of it; and by the time it comes back to be proofread, you can hardly stand the sight of this pile of junk. (laughter)
Harwit:Yes, to some extent.
Peebles:With a few exceptions, I always am unhappy to see the proofs come in, just because it means that Iím going to have to look at this wretched paper once more. Then it happens with a small subset that I look fondly on them after five or ten years, such as the paper with Jer Yu on linear perturbation theory and the evolution of linear density fluctuations in an expanding universe, taking account of realistic description of the radiation propagation. That was a lot of fun, and Iíve turned back to it many times since. What is it called?
Harwit:Letís see. Is it ďPrimeval Adiabatic Protuberation in an Expanding Universe,Ē Ap. J., 162, page 18, 1970.
Peebles:But probably the most satisfying thing Iíve written is my first book, Physical Comology. That turned out to be written at a time that was entirely propitious, the subject just exploded, but hadnít yet gotten overwhelmingly complicated. I could write an informal interview quickly and cover the field. Couldnít do that now. A similarly thorough book would be twice as long now and much harder to write. So many things have happened all of the particle physics contribution, so much of theÖ
Harwit:Well, also at the time you wrote it things had stabilized for a short while, you didnítÖnow with the inflationary or in models, with all kinds of different versions.
Peebles:Thatís open-ended, so you could never...
Harwit:And axions and so on, it makes itÖ
Peebles:There are many more loose ends now because the subject is so much larger, so much richer. When I had written that book, the cream had been skimmed from the old ideas and the new application of then. But the subject was still fairly small, self-contained.
Harwit:What do you feel about the future of the problem of condensations, how close we areÖ
Peebles:...to a theory of galaxy formation?
Harwit:...or clusters firstÖ
Peebles:...or cluster formation? Sure.
Harwit:...or globular clusters. You know, condensations.
Peebles:I have the impression that weíve getting close to an answer. The way it will come, Iím hoping, is that we will accumulate clues ó- hints ó- and that these will finally be so numerous and connect so well in only one direction that weíll convince ourselves that weíll have the answer. That hasnít happened yet. There is, as I described at the colloquium I gave at Cornell, quite a few, quite a large number, of nice hints to how galaxies and clusters of galaxies and globular clusters might form. They donít yet constitute a network that ties together in what reasonable people would have to consider a convincing way. But I think weíre getting close, and weíre getting to the point where each new clue added is, in fact, N intersections with other clues. You can imagine that rather abruptly this phase of collecting clues will end with a coalescence of networks, thatís so tight, that weíre convinced. And Iím optimistic that that will happen in the next few years.
Harwit:You think within a decade?
Peebles:A decade ago I was betting Joel Silk that it would happen within five years, so Iíll answer, ďYes, within a decade.Ē Of course, we wonít know...we wonít be able to put a date for when we have convinced ourselves we understand cluster formation, galaxy formation, only after the fact will we be able to say, ďWe had it then; and we didnít have it earlier.Ē Maybe we have it now. Maybe the outline is cast in one of the scenarios that have been put forth.
Harwit:It would be difficult to believe after what you were telling us in the two colloquia you gave at Cornell in the spring. I mean, there were a lot of clues.
Peebles:A lot of clues.
Harwit:There was a pretty bright assembly of people thereÖ
Peebles:...and no one jumped up and said, ďHere is the answer.Ē
Harwit:Thatís tight. (laughter)
Peebles:Do you know, I was half hoping that might happen.
Harwit:With Tommy Gold there, it could have, for example, heís very quick.
Peebles:Yes. There could have beenÖ
Harwit:With Ed Salpeter there, it could have come out within two weeks, maybe. You know, I mean, he always thinks perhaps more deeply...
Harwit:cautiously than Tommy. No, I havenít heard anythingÖ
Peebles:You know, but itís interesting, I had that in mind.
Harwit:Well, I was hoping you had. You know, thatís the kind of thing we invite people for, of course, to...as I wrote you afterwards, it was a really provocative set of lectures.
Peebles:All right, so it wonít happen this year — perhaps next. The clues keep coming in. New discoveries are coming so much more rapidly than they did 20 years ago.
Harwit:And it probably wonít deÖwell, it might, I donít know, I was going to say, probably wonít depend so much on the intricacies of the matter...
Peebles:It depends on how lucky we are, isnít it?
Harwit:Yes, it depends on whether there are strings for example.
Peebles:If itís very specific, for example, something so way out as that may take a long time to reveal itself. On the other hand, if it is strings, it may become manifest in linear arrangementsÖ
Harwit:Öin the background radiation.
Peebles:Look for lensing but rows of lensed objects. That would be fantastic.
Harwit:But in some models of strings one also gets discontinuities in the background, I believe.
Peebles:Yes, right, thatís right, because of the motion of the strings, so youíll see a discontinuity in brightness.
Harwit:That might be a hope.
Peebles:That might be a very distinct clue. Well, if weíre lucky, thereíll be something distinctive showing itself, but weíll only know after the fact.
Harwit:Iím more pessimistic than you perhaps because I donít see the tools. Except for the cosmic background explorer, I donít see where the tools will come from in the next few years that would give a [new clue]...unless you think the Space Telescope mightÖbut that has such a restricted field of view.
Peebles:Well, one of the things Space Telescope will teach us is what a galaxy at a redshift of one looks like, Does it have spiral structure? Are ellipticals and spirals segregated at that time? Are galaxies still forming? Will we be able to see perhaps at a redshift of three galaxies in the process of forming? Or perhaps clusters forming? I suspect that the microwave background has been milked, hasnít it? Each new factor of 2 is going to be an enormous effort, and so unless this clue is hiding just on the threshold of present detectability, itís going to be a long time coming.
Harwit:Well, It could be the near-infrared background.
Peebles:But, for example, the near-infrared background, when we have a panoramic detector, we might start seeing something very interesting.
Harwit:Thatís sort of what I was talking about when I mentioned the cosmic background explorer.
Peebles:Well, I didnít understand, but of course the background explorer will give us an absolute flux level, perhaps, or a good upper bound. When we have panoramic detectors at 10 microns that we can fly, who knows what we will see.
Harwit:What do you mean by panoramic?
Peebles:I mean two-dimensional detectors with good angular resolution.
Harwit:Well, the good angular resolution requires very large telescopes.
Peebles:Well, give me a second of arc; give me even 10 seconds of arc.
Harwit:Well, in the infrared thatís...
Harwit:Ten seconds is not that bad.
Peebles:...at 10 microns.
Harwit:Ten microns, 10 seconds, you can do with a 1-meter telescope.
Harwit:...without trouble, so that would come out of the Space Infrared Telescope with certainty now.
Peebles:Alright, whatever. Perhaps someone will get clever and find an unambiguous way to determine the nature of the dark matter, depending on whatē the dark matter is. For example, could the dark matter be Jupiterís? The question Iíve been asking people is, would Jupiterís be detectable as radio sources? Our Jupiter is, a very prominent radio source, and it is because it has a fossil field, magnetic field, and it captures plasma from the solar wind. If there were lots of Jupiters around galaxies, they will occasionally blunder into to 14 regions. Will they become strong radio sources? Well, I donít know how to do the scaling. Thereís lots of plasma around.
Harwit:Iím afraid the galactic radio emission is pretty high already...
Peebles:Well, this would be a very distinctive sort because around, say, an elliptical itíll be a very smooth halo of radio emission, not the blobs. If the dark matter is some exotic particle, perhaps that particle will have a very weak decay and weíll detect that by some method or other. That was a hope for a while with neutrinos. If the dark matter were neutrinos, they could annihilate and give photons -ó the cross section is pathetically small, but you could hope for something there. One has to be an optimist; one has to hope that somewhere thereíll be new measurements to be made and that they will open up new vistas for us theorists to play with.
Harwit:Well, thank you very much.
Harwit:If anything else comes up overnight...
I feel Iíve been on the psychoanalystís couch for three hours (laughs).
APPENDIX 1. Notes made on September 28, 1984, the day following the interview (typed up, slightly paraphrased on December 31, 1984) Peebles had looked in his files to see if he could date some of the events that had led up to the papers dealing with the early universe and micrave background: Notes from the spring of Ď64 still deal with quantum field theory — an attempt to quantize general relativity. This was a sequel to a manuscript submitted October 1963 to Annals of Physics but rejected; Peebles subsequently decided not to pursue that approach any further. February Ď64 has notes on a set of problems on active gravitational mass and a manuscript to the Gravity Research Foundation. The Physical Review Letter 1964 work was submitted March 1964: Paper with Dicke entitled ďEvolution of the Solar System and the Expansion of the Universe.Ē The paper ďThe Big PlanetsĒ published in Science and Technology, 1964, was submitted in the spring of Ď64. Peebles remembers the first session with Dicke on the background radiation as taking place in an informal seminar on a summer evening, ďhot as hellĒ and Dicke explaining the background radiation problem on a blackboard. Peebles is sure that they got onto the problem rather quickly thereafter and that it must have been the summer of Ď64, rather than a year earlier. A springbound notebook marked ď1964Ē in his possession shows many preliminary calculations. Much is undated, but halfway through the notebook is the date October 10, 1964; here plasma recombination is considered. October 20, 1964, deals with the thermalization of strongly nonthermal particles. November 14, 1964, deals with the neutron/proton thermal abundance ratios. The computer sheet at the end is also dated November 14, 1964. The pace appears to be about half the notebook per month, suggesting that the notebook was probably started in early or mid-September 1964. A draft of the paper submitted to the Physical Review and received there on March 8, 1965, is in Peeblesís files dated January 1965. It is very similar to the final version submitted: ďCosmology, Cosmic Black Body Radiation, and the Cosmic Helium Abundance.Ē