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Oral History Transcript — Dr. Abraham Pais

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Interview with Dr. Abraham Pais
By Lillian Hoddeson

August 27, 1974

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Abraham Pais; August 27, 1974

ABSTRACT: Niels Bohr and Wolfgang Pauli at Princeton University; early 1950s; Bohr and Dean Acheson; the birth of quantum physics (Erwin Schrodinger); Albert Einstein; Niels Bohr Institute, 1946; Comparison of Einstein and Bohr; character and personality of Bohr.

Transcript

Hoddeson:

Tell me why you don't want to be a full time historian?

Pais:

Oh, because I have so much physics to do, and I got into this just by talking to [???] and a few other people, and I said, "Well, I ought to talk to physicists because they know something." Then it turned out there were a lot of things they didn't know, which had to do with the very early period, essentially from 1896 to 1916. [Chatter] What happened was that one summer in Brookhaven, I found out that they had [???] [???] of the French Academy on microfilm. That's where the story starts. And it is an incredible story. I don't think we ever have realized the connection between the discovery of X-rays and the discovery of beta rays, which there is, one, two months after the other. Well, this is an absolutely insane story. See, [???] discovered the X-rays, and he found out that they emanate from a piece of glass of a [Crookes tube?]. What are you doing?

Hoddeson:

[Inaudible].

Pais:

Yes, that has nothing to do with what you want me for today, but that was what I was about to [???] give background of your [???]. Well, Poincare, the great mathematician, he had gotten an early copy of the paper by Roentgen and he had found out that Roentgen had a Crookes tube, and the electrons came into the Crookes tube, they hit the walls, glass walls, there's luminescence there, and then from the luminescent area, the X-rays come out. That was known. So, Poincare said, maybe X-rays are a byproduct of luminescence and one ought to study luminescent source to see if it has mysterious, invisible rays. Then he did an experiment the likes of which you can't believe including experiments with glow worms, fire flies, and anything that they could lay their hands on. Now, Bequerel came from a very distinguished family. His grandfather had been a professor, a member of the Royal Society. His father was an eminent man at Oxford. And his father was one of the first to study phosphorescence such as the phosphorescence of uranium compound. So when Bequerel heard of this idea of Poincare, he remembered that Papa Bequerel had some uranium salts which he knew to be phosphorescence. So he did the experiments on uranium salts and he found some radiation. You know, he took a photographic plate, wrapped it in black paper, with the source, an object like a coin in front of that, the uranium salts on top of that and the whole thing in the sunlight because if it's phosphorescent you have first to irradiate the source. And he found an effect. But then the next week, he wanted to repeat his effect. There was no sunlight. So he put the whole thing in a drawer. And we know exactly what happened because Sir William Crooks was in the library in his laboratory, — he pulled open the drawer — then after some days, there was still no sun, he got impatient. He said, "Well, I'm going to, just for the fun of it, I'm going to leave it out in the study". He found an even stronger effect. So he found that this radiation had nothing to do with phosphorescence because you didn't need sunlight to stimulate the source. That's how the story started. And the whole story between 1896 and the century [???] it has to be really [???] has been an incredible tale of false clues and well, that's the beginning, what I'm doing.

Hoddeson:

Is it the beginning of the second paper?

Pais:

Yes, exactly, that's what I'm exactly [???]

Hoddeson:

Well, the reason [???] discover things, we haven't really formulated very well yet, this is now in the process of evolving on different styles of sort of understanding the physical world. And that's the one I… The other, the third program is an historical narrative story that tells [???] yes, with lots of editors and people that were involved in the story, [???] discoveries, innovations [???]. And the fourth is just kind of like a resampler [?]. It's not put together into any sort of story but we have long excerpts from speeches and interviews on the early [???] and the ideas [???]. To see if they can put together their own programs having been exposed to the other three programs.

Pais:

Right. You want me to talk about Bohr? You told me you want me to read something?

Hoddeson:

No, just that I wonder if you [???]. [???] very excited about Bohr that I read first in the Moscow.

Pais:

It's all in a book. Several books.

Hoddeson:

Another problem.

Pais:

That's the book.

Hoddeson:

But we have very little on tape, kind of, the style. We'd like to go into that. How [???] with Bohr, to have different people give sidelines on how Bohr worked. Among them, those materials so far, we have Bohr [???] people said. And let's see what else [???]. Well why don't you [???] I suppose that perhaps if I just showed you this, part of this file of papers. Some of the kinds of things that other people have said. If you just look at the things that are marked. These are very tentative. It doesn't mean that we're going to use them. But these are just the kinds of things that I've been marking. These are transcripts. That's Dirac. Or perhaps you don't want to see other's, perhaps you just want to tell us [???].

Pais:

Whatever I like. I mean, I won't need much, how shall I say? You know.

Hoddeson:

Your ideas and maybe this is…

Pais:

My ideas, my ideas are not at this moment, shall I say, all that crystallized and succinct so there'll be lots of improvisation.

Hoddeson:

You might look at some of the things…

Pais:

Oh, that's the thing, that's right...

Hoddeson:

Your paper. But generally what we're interested in is Bohr's approach to understanding nature, his approach to his work, how he worked with people, how he worked with words, the interplay between his philosophy and the theories that he developed, how he sought out people who [???]. And then, that Einstein, in a funny sort of way, too...

Pais:

Well, Einstein was of course a wide source. As I said in this thing, it was of course good luck that he had a man like Einstein whom he deeply respected. This forced him to specialize and refine his own thinking in such a way that he could convince Einstein. And I remember discussing it with Bohr, after Einstein died and Einstein's name would come up and Bohr would get into a fictitious argument with Einstein as if he was still alive. He needed the rubbing, you know, the sparring partner. As far as Pauli is concerned, that was quite a different relation. Pauli of course was the great critic of the early part of the 20th century. But the relations between Bohr and Pauli was much more, I would say, of the fatherly kind it seems from Bohr. I remember one year when Pauli and I were in Princeton and Bohr came to the Institute for Advanced Studies. Bohr had been on a ship. He had crossed by boat with Mrs. Bohr and he had nobody to talk to. To talk with physicists was an imperative for Bohr. He needed this kind of discussion in order to sharpen his own thoughts. And he had a curious quality, I think, of what you might call the therapeutic quality: he simply needed somebody to talk to in order to find out what he was thinking. Bohr came to Princeton and Pauli and I were walking along the corridor and Bohr came in. He hadn't seen anybody yet. He saw us walking there. He shepherded us into an office. He told us to sit down. He turned to Pauli and said to Pauli, "Pauli, which means, "Pauli, shut up." Pauli hadn't said a word yet. And Bohr started to talk and he talked for several hours and you couldn't stop it. You know, we had this... I always like to compare Bohr's talking with the way the waters come down in the mountains after the snow melts. You know, they come down inexorably. You can't stop them and Bohr's flow of conversation at certain times had that quality. You just couldn't stop the man.

Hoddeson:

Let's talk a little bit about Gamow. Do you remember him?

Pais:

Yes, that's long before my time.

Hoddeson:

"Pauli [???] " ... is that 1926?

Pais:

That's right, exactly. That's what he said. What I'm talking about is more like oh, it would have been the early fifties.

Hoddeson:

I have a question. Somehow I'm having trouble with the image of Bohr's language flowing out. Picking up some of the transcripts, they talk about how he got stuck for words.

Pais:

Well, that's something else. It's the difference between Bohr in conversation and Bohr giving a lecture or Bohr writing a paper. There's a distinct difference. I remember, I helped a few times, I helped Bohr because he always needed somebody to talk to when he was making, preparing a talk. It was a talk for the bicentennial in Berkeley in 1946. I had just come to the United States and Bohr carne a week later. We met at Princeton and Bohr had to give this talk. It had to do with the foundations of quantum theory and also the foundations of quantum field theory. That's a vague memory I have. And Bohr expressed himself very hard and very well and in great detail. And, then, he gave this talk and the talk was terrible because it had a certain elegant superficially of incoherence to it. In fact, it did have an element of incoherence to it but the trouble with Bohr was that he was thinking very hard while he was talking and he had prepared himself completely and I could follow him very well because I knew his steps. The only trouble was that sometimes he went from Step A to Step C. He was thinking about Step B which carne in between but he simply forgot to talk about it. So that you know, as a lecturer, he was not that outstanding. He was very inspiring because the quality of Bohr giving a talk had to do with seeing a man struggling with the material. You know, you had this sense the man is struggling. He did not lack, you know, the qualities of being lucid in principle but he simply was thinking so hard about the concept that he forgot to say half he had to say. There is an antidote if you wish to hear about what happened between Bohr and Acheson. Well Bohr had an appointment with Dean Acheson who was then Secretary of State to talk about the future of the atom. That must have been in the Truman years so that must have been, I would say sometime between '48 and '52. So Bohr got an appointment with Acheson and Acheson received him courteously and Bohr sat down and began to talk. And, after Bohr had talked for about 15 minutes, Acheson interrupted him and said, "Professor Bohr, there are a few things I ought to tell you. The first thing is that I am very interested in what you are telling me. The second is that I'm a very busy man and that I only have half an hour I can spend with you and then whether I like it or not, I have the next appointment. And, the third thing I want to tell you is you have talked for fifteen minutes and I have not understood a single word you have said." At that point, Bohr got so furious. He got so angry that in the next 15 minutes he lucidly and with great clarity, you know, exposed his ideas. Now, that's typical for Bohr. If you leave Bohr alone and you are talking with him, you know, he struggles with words enormously and tries to say it better and so on. But, when something rouses him, he was perfectly capable of delivering a…

Hoddeson:

Give some other examples. Was there something with Einstein of that nature too?

Pais:

I was not present. I was only once briefly present. It was not really an interchange. The thing with Einstein was always, you know, that Einstein, to put it in simple language, Einstein did not believe that quantum mechanics was the last word. That it was some heuristic intermediate step toward a more refined description. And Bohr had the problem to explain to Einstein that quantum mechanics, in Bohr's own inimitable English, was what he called "Irrevocable." Which means irrevocable. He had a certain way of using certain big words and pronouncing them wrong. He was a very funny man that way. I always had the idea, we talked sometimes together about the FBI, which had a lot to do with atomic energy in those days. He never called it the FBI. He always called it the FIB. And, I sometimes think that there was a certain, that he knew damn well, you know, that there was a certain element of making fun there which he was very well aware of but just didn't let on to.

Hoddeson:

About that Acheson story, who told that story, Bohr or Acheson or someone else?

Pais:

I don't know if I got it from Aage Bohr or from, I think I got it from Robert Oppenheimer.

Hoddeson:

How did he...?

Pais:

Oh, Oppenheimer knew everybody you know. Oppenheimer was ablaze with glory and power in those years and, I'm sure that Acheson must have told it, certainly from Oppenheimer.

Hoddeson:

I still have certain problems understanding it. My general feeling is that Bohr really mumbled that he could not have the ability to speak that precisely.

Pais:

Well, yes. When I came to Copenhagen, I quickly realized that I'd better learn Danish in order to understand Bohr. That helps a great deal. You ought to know that the Danish language is a language which some people call a disease of the throat. And, I can say this without partiality since my own native language, which is Dutch, is also a disease of the throat. Now, you swallow a lot. If you compare the written word in Danish with the spoken word, the written word is about twice as long as the spoken word very often. That's a typical trait of the language. Bohr talked softly and gently so you had to kind of sometimes get physically close to him to hear him. That's all very true. And, then there was of course some, especially at times when he was preparing a manuscript, then he would struggle with words, very very carefully. I remember that, I was in Denmark; I lived with the Bohr family for about six weeks or two months in their country house. Bohr was then writing some articles and he asked me to discuss it. And, there was one moment Bohr was completely stuck and he couldn't find the next word and he thought about it and suddenly his face lit up and he said, "Now I know what I want to say," and with a great triumph in his eyes, he said, "however". That was the word he wanted to have, "however." He could be so happy when the word he wanted came to his mind.

Hoddeson:

So he was terribly concerned with the precise, what a subtle change of words would mean for the meaning?

Pais:

Very much so. Very much.

Hoddeson:

Of course, you know the correspondence between him and Rutherford on the manuscript, you know.

Pais:

Yes I saw part of it in [???] 's book, I think.

Hoddeson:

Probably. And there you know, he ran to England to defend his use of every word.

Pais:

Oh yes.

Hoddeson:

So each word in each sentence, however convoluted to someone else, was very important to him. I see him dictating a paper trying to struggle with somebody about a word and you started to talk about this in your article, where would let that word, repeat, repeat, repeat...

Pais:

Yes, he would walk around like a maniac, like an absolute maniac. He would walk around the table and there was, the last work he had just in his mind, and he would repeat that and repeat it.

Hoddeson:

Like children do when children learn a new word?

Pais:

Well, that brings us to a very important subject. That brings us to a very important subject about which I will write in the future. In another ten years now. I mean, how I do my physics and occasionally I write about history but then I will write about people. There is no question in my mind that the really creative people are children their whole lives long. Not infantile, not juvenile but they have maintained a certain freshness that never gets lost. That's the key.

Hoddeson:

"Beginner's Mind." Roshi Suzuki calls that a "beginner's mind." It's part of Zen philosophy.

Pais:

That's a very beautiful thing because Bohr's approach to physics was "beginner's mind." There was a period he talked very very much for a couple of months. There was something that had to be prepared for the bicentenary celebration of Newton's birth in Cambridge. And damn it, Bohr, every day would start with the discovery of quantum mechanics. He would start every day. He would say how Schroedinger was so surprised when he had found out what he had done and how, you know, Schroedinger didn't understand it and how he explained it to him and Einstein would crop up very soon. Then he would slowly kind of warm himself up and come to the issue at hand but he would always start at the beginning and, in fact, it is a very interesting question to ask — what precisely was the contribution of Bohr to the birth of quantum mechanics? As people have told me, they went through a period, in the span of a few months' time, their life changed. Everybody's life changed and what Bohr did was to essentially give the logic of the times, to see, you know, how, first of all the relations between quantum mechanics and classical mechanics came about. And then, to make clear that quantum mechanics was an inexorable step. You had to make that step and once made, you couldn't go back. You came to a new domain in which the logical concepts had to be defined very, very carefully. The other man who did something about that was of course Heisenberg. I mean Heisenberg's [???] relations, that's the key.

Hoddeson:

Although in a way that follows from Bohr's…

Pais:

Well, that is the question which I am not sufficiently historically informed. I do not know if Heisenberg's [???] experiments came first or if Bohr's elaboration of complementarity came first. That has to be looked up and that I will not be responsible to make a statement there. But that was Bohr's thing, to clarify concepts. That happened in 1925 and it happened again around 1932 or so. Because what happened after quantum mechanics, there was the question of quantification of the electromagnetic field, what we now call quantum electrodynamics, and that was the work of again Heisenberg and Dirac and Pauli. Now then [???] and Landau came along and pointed out certain types of things which they considered logical paradoxes which came bout if you had a quanti description of the electro-dynamic field. There was a certain amount of confusion at that time and it was Bohr who struggled immensely with the question — how to make not only the quantum theory of material systems, like atoms and so on, you, how to make the logical connections here, but also the logical connections for quantum field theory, and that is put down in a long paper which is published in the Danish Academy. It is by Bohr and Rosenfeld. And that, many of Bohr's papers went through a number of galley proofs but that paper I believe went through at least a dozen proofs before it was in shape. But Bohr has, what did you call it, a "beginner's mind"? That is just exactly what Bohr was like.

Hoddeson:

[???] had a theory that he sometimes appeared to be like a driven man. I've heard that.

Pais:

Driven yes, but not obsessed. You can be driven in a variety of ways and sometimes you are driven to the extent that it goes at the price of the personality. You know what I mean? You can be driven so that it is like a cramp, you know, a person who's tight. That is not the way Bohr was driven. Bohr certainly was driven. The times that I stayed with him, he never finished a meal. He would get up, "Margaret please excuse me but something just came to my mind", and he was gone, you know, or he would say to me, "can't we just do something or other?" and then he was just gone. Yes, I would say he was a driven man but he was not an obsessed man.

Hoddeson:

Do you want to go back to the choice of words, the Bohr who repeats and repeats a word. Now, you know, up to a point, I thought that that would be seeking for the right precise word as you used the example of "however". But then there's this wonderful story, which I do want you to put on the tape, of the time when the word he was repeating was "Einstein."

Pais:

That's right.

Hoddeson:

OK now before you talk, because I don't want to interrupt you, it then occurs to me that maybe there were times when he said one word out loud, as when he gave lectures and thought about points but skipped it when his mouth was talking, that maybe he just said a word while his mind was off doing a totally different trip?

Pais:

Yes, what he would do is he would say some word, then he would kind of think while he was talking. He would simply forget the next sentence or two.

Hoddeson:

That's in a lecture.

Pais:

Yes, in a lecture.

Hoddeson:

Now, as he's dictating, you described this feeling at the Princeton Institute where the word he was repeating, as he was trying to write a paper, was Einstein's name. What was in his mind then?

Pais:

My own impression would be that, see, there was practically no name in Bohr's life which had that particular charge as the name of Einstein and it is idle to speculate what precisely goes through a man's mind but I would be willing to make a conjecture that he was saying, standing in front of the window, you know "Einstein... Einstein...," that in his mind of these dialogues he had and arguments he had with Einstein kind of went through his mind. It was a particularly charged word for him as I have said before because one of the principal things in Bohr's life was to be able to convince Einstein that he was damned well right.

Hoddeson:

How would you describe that particular scene? The whole story is really quite amusing.

Pais:

Well, it came about, first of all, it started like this. During the Bicentennial Conference, this man Schilpp came to talk to Bohr. I remember that because I was there and he was explaining to Bohr that he was preparing this book on Einstein which is an excellent book, and he asked Bohr to contribute to that. And Bohr thought about it and it was clear that he wanted very much to do that and he did it and then, the time I talk about now I think is a couple of years later. Bohr has come to the Institute of Princeton. He was a long term member. Whenever he felt like it, he was always welcome to spend a month there. And I was there and Bohr came to my office and said, could I not help him to, you know, formulate some sentences? He had a very absolutely charming way of doing that. He came into my office and he said," [???]" which means, "Pais, you are so wise." Which was, you know, a nice way to say "Can't you give me some advice?" and so on. So I said "Sure", you know because I had a very pleasant personal relation with Bohr and I think I understood him somewhat. So we went down to his office which was actually Einstein's office. See, Einstein had a big office at the Institute and next to that was a small assistants' office and Einstein didn't like big spaces. He liked to work in a small and close space. In fact, I edited the Einstein University issue for the [???] of Physics in '49 and there is a photograph of Einstein at the opening, beginning of that issue, in which you see Einstein sitting in a room and the door is half open and he's sitting in the little assistants' room. It was taken by Eisenstein. Well, anyway, I went down with Bohr and Bohr said, "Now, you sit down because I need always an origin of my coordinate system". So I got to be the origin of his coordinate system. There was a big table in the room and Bohr would walk around the table. He would walk around endlessly you know. He had at that time ever so slight a stoop and he would just walk around. And that was the [???] article in this book. So then Bohr came to a sentence which started with the name Einstein and Bohr ran around the table saying, "Einstein, Einstein, Einstein," and then there was a window looking out over the field and Bohr at some point, walked up to the window, stooped, looking out of the window, and just stood there saying "Einstein... Einstein." And I just had a fine time. I just sat there and smoked by pipe and found the whole thing very enjoyable when the door opened. Bohr stands in front of the window saying "Einstein", and the door opens and Einstein comes in. Now I did not know why Einstein came in. Einstein pointed to me and puts his finger on his lips telling me to be very quiet. So I didn't know what to do. It was a strange situation because Bohr was standing there saying "Einstein" and Einstein was walking in. And I will tell you that there was something slightly uncomfortable in the situation. It was, it had a mythical touch to it you might say. So Einstein walks into the room, walks toward the table and Bohr stands there in front of the window saying "Einstein". Suddenly Bohr turns into the room and says, "Einstein" and there by God Einstein is standing in the room. Now, I have seen Bohr in many occasions but I have never seen him lose his composure, except for that moment. Because suddenly, good old Bohr, he blushed and he stood there, you might say, like a school boy. Einstein stood there and then quickly explained what was happening because in the middle of the table at which I sat was the tobacco box. He was not allowed to smoke by the doctors but he interpreted it in the following way: he was not allowed to buy tobacco but he was perfectly well permitted to steal tobacco and the reason for his coming in the room so quietly was he wanted to steal some tobacco from this tobacco box. So he explained that and everybody started to laugh and the spell was broken. And that was that little incident of which I wrote.

Hoddeson:

Do you remember when you first met Bohr yourself?

Pais:

Very well, very well. I remember if very well. I came to Copenhagen, I was the first Second World War physicist to get an Oersted fellowship, first foreign physicist, you know, youngster to come there. That was in January '46. I came to the Bohr Institute and I went to Bohr's wonderful secretary Mrs. Schultz, just to say that I was here and so on. "Yes" said Mrs. Schultz, "Mr. Bohr knows that you're coming today and why don't you go to the library, the little journal room and just sit there a while and then when Bohr is free, I'll call you." So I went to the library and I looked at some journals, and after a little while somebody knocked at the door and I said, "Come in." And there was Bohr. I have a very vivid memory of him standing there in the door because Bohr had really a remarkable face. He had these very heavy bushy eyebrows and you know, big jowls and it was a remarkable face and my first impression was that his face was somber. And, to this day, I can't bring that first impression back to live because the moment after, as Bohr stood there, he began to talk and that face, which had this great heaviness in repose, turned out to be very light because the moment he began to talk, his eyes began to sparkle and something began to exude from his personality and all my memories of Bohr had completely the opposite of anything related to somberness. There is a great light, a great charm, great warmth. He was a man of great compassion.

Hoddeson:

You know, they call me Joan Bohr sometimes because I adore Bohr, never having met him.

Pais:

I tell you; the thing about Bohr was that in some ways he was able to bring out the best in people, I believe. And it was in part due to the fact that he was, what psychologists would call, the ideal father figure. He was just made for that. And he was also, unlike anything that's happening in present day physics, he was a certain indisputable, undisputed leader. There was no harshness or littleness, no questions of who did what or priorities or anything that ever surrounded Bohr or what he was doing.

Hoddeson:

Would you tell me something else? I sometimes think of Einstein as being rather a 19th century physicist and Bohr being much more modern. Does that make any sense to you?

Pais:

Not quite. They are different types of physicists. I would say that. If I think of Einstein, I think of somebody who is absolutely timeless. There is no doubt in my mind that Einstein stands alone. He is unparalleled in his courage, his genius. Bohr had a different style. Einstein was driven by a few leading ideas which he pursued with tremendous courage and persistence which pinnacled in the general theory of relativity and which afterwards, to some extent, led him astray. Bohr was a much more flexible person. He stayed very much more close to the phenomena. He was much more, in his own words, "prepared for surprise." It was one of his favorite dicta to say, "You ought to be prepared for surprise every new day." I don't think Einstein was prepared for surprise. Einstein was more; he was more of a prophet. He had a vision that physics ought to go according to a certain pattern. It wasn't always toward more unification and so on. Bohr was much more ready to, much more prepared to see new surprises in new phenomena as the experiments revealed themselves. Bohr, in some ways, was somewhat more phenomenologically minded than Einstein.

Hoddeson:

But I think you've convinced me, you see, somehow made me feel again, there are other things that I… Lots of things, what time is it by the way?

Pais:

Shall we go on another 15 minutes or so?

Hoddeson:

Talking about the childlike nature... Yes, I wanted to ask something about the ability to be able, well, I want to ask a question about the moments in between his intense focusing on a particular issue. How did he relax in between?

Pais:

Well, you see, the only way Bohr relaxed....

Hoddeson:

How did he recharge?

Pais:

Yes, I know. The only way Bohr relaxed was to do, with great intensity, something else... Bohr was intense in some ways; he was totally devoted to what he was doing at the moment he was doing it. I remember being in the kitchen with him in Johnny Wheeler's house in Princeton. He had rented the house. Johnny was away and Mrs. Bohr had gone off to New York and Bohr and I had dinner together and afterwards Bohr said, "Now, we're going to wash the dishes". Now, you should see Bohr wash dishes. He washed dishes with the same intensity with which he talked physics. When I lived with them in their country house, you know at certain moments, you just get tired. Even Bohr was capable of getting tired. Then he would go and start weeding in the, you know, he had bushes, berry bushes, and he would start weeding. But he would weed with a complete ferocity. You know, everything Bohr did was charged. He liked to walk and we lived several miles from the sea. In the afternoon, we would take bicycles and we would go through the woods. We would ride to the beach or take the bike. He was also an incredibly interesting driver. One day we drove from this place where we lived back to Copenhagen. It was quite warm and Bohr was at the wheel and we got into some argument. I forget what it was all about and Bohr said you know, it was very warm, and he let go of the wheel to take off his jacket. If Mrs. Bohr wouldn't have intervened at that, "please, Niels watch what you're doing," he would have just forgotten that he was driving the car.

Hoddeson:

They appear to have retained, between the two of them, Mr. and Mrs. Bohr, a kind of [???] quality. Is that true?

Pais:

Yes. It was nothing short of beautiful to see them together.

Hoddeson:

What about the loss of his son Christian? Did he ever....?

Pais:

No, that was before my time and he never ever mentioned it or it never ever came up in discussion. It was a sailing accident. I know that at the time, when, I think he went overboard and that Bohr had to be helped to jump after him. I do not know the detailed circumstances of this. I'd better not talk about it. I don't know.

Hoddeson:

I have a kind of summary question I'd like to ask you. Could you possibly express the impact that Bohr had on your life.

Pais:

Oh Bohr changed my life. There's just no question about it. It was something like this. When I came to the Bohr Institute, I had my doctor's degree and I was a cocky young man like physicists are today and I thought I knew pretty damn well what it was all about.

Hoddeson:

How old were you then?

Pais:

I was 28 and Bohr started to talk to me about quantum mechanics. Now, I knew quantum mechanics. I could solve simple problems. I could do some independent work and so on but when I listened to Bohr, there was something that I found out I had not understood yet. That was how tremendous the struggle was to go from classical theory to quantum mechanics and in some ways Bohr, by telling me over and over and over again what the paradoxes were what the conflicts were between the classical and the quantum mechanical point of view, he in some ways taught me quantum mechanics in a much deeper way all over again. I know that I have very much benefitted from that. I would not say that I learned much from Bohr in a sense of the techniques of theoretical physics, because that was at the time I'm talking about, not Bohr's main strength. His main strength was to be able to get the right order of magnitude including even coefficients, pi and so on, straightened out, by half-intuitive reasoning and at this he was a master. And what I learned from Bohr was, not always to start to compute so very hard, but to do little thinking first, to see if you could not, as it were, guess the answer, more or less before you started to sit down with pencil and paper. There was also a general style of personality which made this very great impact on me and this combination of being a completely devoted scientist and at the same time, a very live human being, much in touch with the events of the world and much in touch with the people around him. These things made a lasting impression on me.

Hoddeson:

I remember reading with pleasure things about the playfulness of Bohr too.

Pais:

Oh yes.

Hoddeson:

One for example, probably from an earlier time but at any rate, the table in the library, the ping pong table.

Pais:

You're right.

Hoddeson:

And the boards were sort of worn down at each end from all the ping pong that had been played on the table and once Bohr wanted to go in and use the library and he walked in, there were two people playing ping pong. He said, "oh, excuse me, will I disturb you?" Like their playing ping pong was terribly important to him, right?

Pais:

Oh yes and he could say these things with great earnestness. I remember many incidents; we were working, Bohr and I, in his villa. He had a little pavilion which was away from the house. It was like a one room cottage and Bohr and I were there when a servant came in with a can of sardines and he said that Mrs. Bohr couldn't open the can and would Bohr please open the can? So Bohr said sure, he could do that and he took the little key and started to open the box and he made a mess of it. So, I said to him, "Bohr, why don't you let me do that?" So, I opened the can. Such a thing he took with complete grace and so on. I opened this can of sardines and Bohr looked at this with great admiration. He said, "Let's go back to the house," and Bohr took up the can and carried it back. He walked over the fields toward the kitchen with this can of sardines in his hand and he walked into the kitchen. There was Mrs. Bohr, and he said to Mrs. Bohr, "Margareta, now look what Pais has done!" Great admiration, you know. He had this quality, there was nothing trying to be funny about that. He meant it. He meant it; he thought that was just fantastic. That was typical Bohr.

Hoddeson:

He enjoyed cowboy movies. There were certain things that he...

Pais:

Yes I know these stories. I can't add to them. That was before my time too. I know very funny stories about this.

Hoddeson:

In other words, what I'm trying to get at is this all parts of his life he could enjoy a corny movie, know that ping pong was important and be able to open a can skillfully was wonderful.

Pais:

Oh yes, I remember the grandchildren came in. They had some new toy and before you knew it, Bohr said, "Let me explain how this works" and before you know it, Bohr was sitting there, on the grass, playing with the toy. Because that was the type of playfulness that was in him. That is unlike Einstein all together. Ya, ya, it's typical. I know that story, it's a little top that goes upside down. You've never seen them? You've never seen a tip-a-top? Oh, they're fantastic. A tip-atop looks like this.

Hoddeson:

That's why it's called [???] in Danish.

Pais:

It's called a tippy top. It looks something like this, you're right, you start to spin it with this thing here and it begins to spin and then all of a sudden, it turns upside down and it spins on this end.

Hoddeson:

Oh yes, I think I might have seen them.

Pais:

Oh, I must have a tippy top at home still somewhere. I don't know where, but....

Hoddeson:

Well, I think we've covered a lot. It's been very, very helpful. Thank you. It was a pleasure. Yes and a pleasure.

Pais:

Well, you know, this man, there is no one like him now. Not even remotely.

Hoddeson:

Not even just as a physicist. There's something very extraordinary about the man as a human being and a thinker.

Pais:

That's the thing, as a physicist, and it is also, the leader. There was something central about Bohr. There was something central about the tone he set at a meeting. He had a compelling sense of honesty and simplicity. For Bohr, life was physics had to be reduced to understanding as simple a viewpoint as possible.

Hoddeson:

What I enjoyed the most today was the contrasting image of... You get one picture of Bohr from the static things that unfortunately are the only things we have. We have photographs, static pictures, these papers, which are in a sense also static, but the quality of Bohr, the flowing quality, that picture you give, that's what I enjoyed most today.

Pais:

Yes, in these kinds of things, it communicates it so well. Physics was something, like a constant crisis, but a very pleasant crisis. You know, one of Bohr's favorite statements was, "Tomorrow things will be much better because today I really don't understand a word of anything." But Bohr's life was one constant crisis.

Hoddeson:

I'm reminded of the things Heisenberg talked about in this interview when he first met Bohr and they went for a walk and Bohr talked to him about the terrific problems in the old quantum theory and so when you talked about how Bohr took Schroedinger and Dirac and Heisenberg and made some logical connections, in a way, no one more than Bohr knew what the problems were with the old theory.

Pais:

Oh, did he ever know. Did he ever know? Because it was also a great act of courage, you know the paper of 1913, crazy but it was right.