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Robert Bacher’s Interview – Part 2

Manhattan Project Locations:

In 1943, J. Robert Oppenheimer recruited American physicist Robert Bacher to join the Manhattan Project as head of the experimental physics division at Los Alamos. Bacher went on to direct the bomb physics division at Los Alamos from 1944 to 1945, helping oversee the design of the implosion bomb, known as “Fat Man,” that was dropped on Nagasaki. In this interview, Bacher recalls how the Los Alamos laboratory was forced to shift gears from the gun-type design for the plutonium bomb to the implosion-type method. He also describes his post-war service as a member of the Atomic Energy Commission.

Date of Interview:
March 29, 1983
Location of the Interview:


Robert Bacher: I presented this [the discovery of the neutron] in the seminar and there were a good many questions. Some people were skeptical. I convinced Ed Condon almost immediately. In fact, within the week we had written a note together on the spin of the neutron because you could work it out. This part of it fit into the things of nuclear spins, hyperfine structure, nuclear moments, and so on. A good many other people came around to it. I think more than half the people were convinced at that time. That prompted other people to go and read the thing. This is an absolutely fantastic piece of work, you see.

Martin Sherwin: These kinds of breakthroughs after the thirties just have not been taking place. It was really a different kind of period in the history of physics, I suppose.

Bacher: I mean the breakthroughs are of different sorts. If anybody had talked about the existence of a black hole –

Sherwin: In a sense, [J. Robert] Oppenheimer did.

Bacher: Yes. That is true. I think he deserves great credit for that. This was almost fantasy. This was I think taken at the time as almost fantasy. The idea that high energy particles and high energy physics could play such an important role in our whole study of the universe and so on – that has come gradually over a period of time. You would not have been able to get very many people to believe that not too many years ago.

Sherwin: The thing that I am impressed about in terms of this period and the interwar period is that there were these stunning new insights that seem to appear suddenly. This is as opposed to the post-war physics, which if you looked at ‘45, ‘55, ‘65, and ’75, you would see tremendous differences. It is an incremental process of measurement.

Bacher: Part of this simply comes from the fact that the problem of understanding excited states of nuclei is just enormously much more complicated. The whole system of particles is just enormously more complicated than anybody thought it would be. Both of these subjects are just so much richer than anybody ever had any idea that they could possibly be.

I can remember during the war that some of our theorists at Los Alamos – I am sure they would not remember this now because it would be too painful – but there were many of them who felt that if they knew the proton-proton scattering up to 10 million volts, that they would have a pretty good understanding of nuclear physics.

It shows you what a view of such a small area existed at that time, and yet we were working on nuclear energy at the point. This is I think in part why it is so difficult to look back at that period. It is hard to look back with an understanding of how little we really knew about the fundamentals at that time. The fact was that we were doing work on fundamental physics at Los Alamos in nuclear cross sections and so on because they just were not known.

Sherwin: You did a lot of that work. Did you not?

Bacher: I did not do it. That is the wrong way to put it. People in the laboratory did it. I was the head of the physics division for a year, and then when the laboratory was reorganized, I set up the bomb physics division.

Sherwin: Let us sort of move up to the war itself. I would like to know in a little bit more detail, for example, about the problems that arose that you explain on page 283 with plutonium-239 and plutonium-240 as a result of the high neutron flux in the reactor. The discovery that a gun-type bomb would not be possible with plutonium. This, I gather, caused something of a panic or of a series of problems.

Bacher: I think there certainly was not a panic, but to say that it was a series of problems would be a complete understatement of the situation.

Sherwin: It is somewhere in between those two. Is that because it was quite clear, for example, that there would not be enough U-235 available for anything except perhaps one bomb? Or is it that all this money had been poured into the production of plutonium?

Bacher: I think both of those questions came into it. One is the production rate of uranium was sufficiently low that to make gun type bombs would be a very slow thing. On the other hand, we did not know very well what the production rate would be. Remember this was now spring of 1944 or thereabouts. We did not know what it was going to be.

I think I am going to have to go back just a little bit. Seth Neddermeyer at that conference in the spring of 1943 suggested that we ought to look into an implosion bomb. He actually, with some helpers, set about trying to do some experiments in this direction. I think that it was one of the conclusions of that conference that that was such a tremendous job to make a bomb of that sort, and it was beyond the scope of what one could hope to do in the Los Alamos project in the time that was available. Most of the effort was focused first at trying to develop a simpler type of bomb. That was put aside. Now, what was found in the spring of 1944 – this was found by Emilio Segre – you are familiar with this.

Sherwin: Yes.

Bacher: Emilio Segre had been studying the possible fission background of various samples. He had studied some of the plutonium that we had available. It was a very small amount, and it had been prepared mostly very laboriously from Chicago. It did not show any serious effect. What happened was that they then built a prototype reactor at Oak Ridge. This was then used as fast as possible to produce a small amount of plutonium, which was then extracted.

One of the earliest samples was sent to Los Alamos for study. That sample showed a spontaneous fission background. At first, quite a lot of work was done at trying to figure out how one could get around this by making a faster assembly, and by using material that was not eradiated so long in the reactor. It just became clear over a period of time that this could not be done.

Sherwin: Yes, there is some of it in here. I know from other reading also. Let me see if my gross understanding is essentially correct. As I understand it, the problem with the plutonium in a gun type of arrangement would be that there was no way to bring the two parts of plutonium – the bullet and the target – together fast enough without it going critical.

Bacher: Precisely.

Sherwin: It would be a fizzle.

Bacher: Yeah, it would be a fizzle.

Sherwin: With the implosion device –

Bacher: It is a faster assembly.

Sherwin: It could be kept loose enough.

Bacher: I guess what you should say about it is that you can assemble it more rapidly.

Sherwin: Assemble it is meaning bringing it into –

Bacher: A critical state. Do I say anything there about the information on this subject getting to the Chicago people? Is that in that discussion? I do not think it is.

Sherwin: I do not think so.

Bacher: It is really amazing. It shows you how things did not get around in the project. [Arthur] Compton first heard about this – though Groves had been informed of this situation and kept almost daily informed. He did not hear of it until I came on a visit there to report in one of their meetings. When I told him first, he had not heard this before. He went just as white as that sheet of paper, because this was the first he had heard of it. Of course, this meant that he was very dubious about why they did not hear about it and what should they do to report about it.

My instructions from Groves were very clear on this. I was to transmit information of this sort to them. He called Groves on the telephone. Groves said, “Let me speak to Bacher.”

I spoke to him, and I said, “Well, this is clearly in the area that they need to know about. It falls in the category of things that are clearly set out, namely nuclear properties that are important to the success of the project.”

He said, “Yes, I can understand that.” He asked me then to report it to a half a dozen of the senior people in the laboratory after the meeting. I guess it took probably all of 15 minutes for it to get around the laboratory after that meeting. It was a very serious thing.

Sherwin: This idea of Neddermeyer’s, which had been set aside, then became central?

Bacher: The point was that we had to say the method that was being undertaken would not work for plutonium. We therefore had to go back and take a method that had been decided as being too difficult to develop. The experimental part of finding out what was going on in an implosion was not an easy thing. That all had to be developed in very short order. A very serious part of it was the part of trying to get suitable explosive charges. That led to difficulties. There were additional nuclear properties that had to be developed. It just suddenly became a much more difficult problem than anybody had imagined.

Sherwin: Let me ask you a question. Here is this extraordinarily difficult problem with implosion and plutonium. Yet, the uranium-235 bomb and plutonium bomb are ready at the same time. Is that because enough U-235 could not be produced until that point in time when the bomb was ready? The physics of it and the engineering problems involved with the U-235 bomb were not particularly complicated.

Bacher: I would put it this way. I was even told by some of the people before Los Alamos was started that the problems were trivial. It was not even necessary to set up a laboratory to do it.

Sherwin: That is for the U-235.

Bacher: For anything.

Sherwin: It was for anything. And you needed 100 people and that would do it.

Bacher: No, they needed less than that. I think it was extremely hard to get an overview of parts of the project that people were not working at.

Sherwin: Therefore, the problems seemed simpler than they actually were.

Bacher: No. It is one thing when you say, “I think we can do this,” but we do not see any way of doing another problem that is an alternative thing.

Sherwin: Right.

Bacher: If you come to this situation, you know that other way is not going to work, period.

Sherwin: Yes.

Bacher: Then you try to do something else just as well as you can.

Sherwin: Into the post war period and the GAC [General Advisory Committee of the Atomic Energy Commission]. Oppenheimer is on the GAC from ’47 to ’52.

Bacher: He is chairman.

Sherwin: He is chairman. You were in what position at that time?

Bacher: Member of the Commission.

Sherwin: In that whole period?

Bacher: No. I was a member of the Commission, of the first Atomic Energy Commission. I left in the middle of May of 1949.

Sherwin: Okay, that is good. It was May of ’49.

Bacher: One of the complications of my going to Washington was that it was made clear to me – I do not know whether it was correct or not, but at least I was told that if I did not go to Washington, they were not going to have a scientist on the Commission. I felt very strongly about that.

Sherwin: Yeah.

Bacher: That is one of the reasons I went. I stayed about a year longer than I agreed to stay when I went on the Commission. First, I had a very great respect for Mr. [President Harry] Truman. I thought he was doing a very good job in a very difficult situation. Before I left, I went to see him to find out how he felt about having a scientist on the Commission. He assured me that it was clear that he hoped I would stay. He was very nice about it. He realized that I had already stayed longer than I promised I would. He assured me that as far as he was concerned, there would be a scientist on the Commission.

Sherwin: Did [Henry DeWolf] Smyth follow you?

Bacher: Smyth followed me.

Sherwin: Okay. As one of the commissioners, you got to know [Lewis] Strauss and I guess you got to know everybody else. You had mentioned that Oppenheimer’s security folder first came to your attention at that time.

Bacher: Yes, the Commission at that time in the early days had to set up one of the things that took a long time to do and was very difficult to do. It was to set up the rules and procedures by which clearance would be given. We had good lawyers, fortunately.

Sherwin: Was [Herbert] Marks one of them?

Bacher: Marks was one of them. [Joseph] Volpe was one of them. Several more were there. [Edwin E.] Huddleson was there.

These were very good people, and I think they did a very good job of setting things up. This was extremely hard to do. The fact was that nobody had set up a hearing procedure before. A provision was done for this, and was made to have hearings for people to decide on clearance.

Sherwin: Can I bore in on the first time that the commissioners were faced with Oppenheimer’s security file? What happened? I do not want to put any words in your mouth. One day this came to your attention in some particular way. How?

Bacher: I mean we had to go over. After all, the FBI was engaged in investigating all of the people who were associated with the Atomic Energy Commission. At first, it was a very small group. I do not know whether they went over the commissioners themselves with the same fine-tooth comb that they went over others or not.

Sherwin: I am sure they did.

Bacher: I do not know about that. Some of the cases that people had then heard about causing trouble were things that had come out one way or another. At any rate, these cases started coming in. The FBI said, “We have derogatory information on this.” One of the things that happened was that they would look into one point, and you would get back an investigation on that. That might raise another point.

This thing was a continual business of sending it back to get clarification. This gets to be a long process, and so on. That had to be done entirely by the Commission, because we did not have any other way of doing it. We spent a lot of time going through some of these things.

Sherwin: There is a lot of stuff [in Lewis Strauss’s papers] that was from his government days, government activities, etc. He also had a lot of time to go through his papers between 1958, the time of his death, and when he wrote Men and Decisions. I think that there was a culling process.

The story is not as full and as many-sided as it should be, which is why I am asking you. I think I would get a fuller story. One of the things that anybody who is concerned with history being accurate has to realize is that history is created by historians, which is created in part by information that is available. If one party who has a very biased view says, “I am going to tell you everything,” and the other party has a greater concern for privacy than the other party, one side gets emphasized. That is the reason that it is important.

Bacher: Okay, I have no comment.

Sherwin: Okay.

Bacher: I do not know what I would comment on that. I had corresponded with Lewis at the time of the Oppenheimer hearings. I had a letter from him in answer to that. When those hearings were held, each person was told – and it is in the record – that these are classified hearings, and they are not to be published and so on. They were published almost immediately. I never quite understood why that was done.

Sherwin: My impression is that Strauss felt that it was advantageous for him, the government, however you want to put it, to put this out the way the hearings were. They were using the excuse of [John] Wheeler, I think, in temporarily losing a copy of the manuscript on the train ride back to Princeton.

Bacher: It was a copy of what manuscript?

Sherwin: It was the hearing.

Bacher: Oh, I see.

Sherwin: What happened was that Wheeler had taken it from Washington.

Bacher: Was this a complete file?

Sherwin: It was pretty complete, yeah.

Bacher: I did not realize that.

Sherwin: He had fallen asleep on the train, apparently had gotten off, gotten to the station, and bolted off the train or something. Then he realized he did not have it. For 24 hours, this was missing. The thought was that it had been lifted or stolen. Finally somebody had enough brains to look in the lost and found department in Grand Central Station, and there it was. But Strauss pushed through very strongly the idea that it had been compromised. The concern was that Oppenheimer’s group might put this out selectively.

Bacher: I remember there was a manuscript, but I did not remember just exactly what it was.

Sherwin: Is there any Project Vista material at Caltech that is not classified any longer?

Bacher: As far as I know, there is not a copy of the Vista report around here. I do not know where there is one. If you run onto one, let me know.

Sherwin: Let you know, okay.

Bacher: I think that there is not. As far as I know, there is not a copy of the Vista report around here. The part of the Vista report with which Oppenheimer was associated was the part on nuclear weapons. I was the one that was sort of responsible for preparing that chapter. [Robert] Christy and I worked on this. 

Robert and [Isidor] Rabi and Charlie Lauritsen – Charlie Lauritsen probably almost more than anybody else worked on this. In fact, the three of them discussed this question not only at the time of the Vista report, but over a long period of time in subsequent years. There are a lot of things about it that they had views on and were interested in. It was related to the problem, of course, of nuclear weapons, what role they played, and so on.

At the end of the war, I think my view was that I thought it would be a little short of a miracle if we could go for ten years and have no international control of nuclear weapons. If we could go for ten years, I thought it would be a miracle without a conflagration. On this I was wrong. I mean I was just plain wrong. It turned out to be a much more stable situation than I estimated at the time. Nevertheless, I was very strongly in favor of that.

I went to work with [Richard] Tolman, who was the Chief Scientific Advisor to [Bernard] Baruch. I spent quite a lot of time and effort with him. This was extremely tough going. I guess my view on it is that this is something when having gone past this period, we came into a period that was not quite as worrisome. My view now is a view I have held for some time. The eighties are very critical in this. I am very concerned about  the international situation and what is going to happen. The situation on a day-to-day basis now is a very, very difficult thing.

Copyright 2005 Martin J. Sherwin. This transcript may not be quoted, reproduced, or redistributed in whole or in part by any means except with the written permission of Martin J. Sherwin. Rights granted to Atomic Heritage Foundation.