I thought I was settled at the Radiation Laboratory for the duration of the war, and this would have been quite satisfactory, but Robert Oppenheimer called me and said he needed me for another project. And he made it clear that this was related to uranium fission and I don't precisely remember how he made that clear. Anyway I agreed to join for the summer of '42 and proceeded to travel west to San Francisco and Berkeley, and on the way I visited Chicago. In Chicago in the physics department one part of the building had been closed off and made secret. That was where Enrico Fermi pursued his quest for establishing a chain reaction of fission in natural uranium. And after many tries it was now clear, it was made clear to me that the chances were very high that Fermi would succeed and that then the chain reaction would produce plutonium-239 which was fissionable just like uranium-235 and could be used as the active material in a nuclear weapon. Because of that the project at Chicago which was under the direction of Arthur Compton decided that it was now time to be serious about actually building a nuclear weapon and that to build, to develop the theory of that they had appointed Oppenheimer to lead that project. So Oppenheimer assembled a group of... of theoretical physicists: Teller and myself, and Robert Serber who was already at Berkeley as Oppenheimer's right-hand man, two younger people from Berkeley and my old collaborator, Konopinski. In addition to that, van Vleck from Harvard. So we had a summer study lasting two months or so.

[Q] I believe Edward Teller was there too.

I mentioned him, I think. Anyway Edward Teller was very much there, and it turned out that Serber and his graduate students had the theory of explosion of uranium or plutonium... bomb already well in hand and we just listened and it was clear that they done a... a very good job of that.