dead in your tracks.
Bohr I was horrified.
Heisenberg Horrified. Good, you remember that. You stood there gazing at me, horrified.
Bohr Because the implication was obvious. That you
were
working on it.
Heisenberg And you jumped to the conclusion that I was trying to provide Hitler with nuclear weapons.
Bohr And you were!
Heisenberg No! A reactor! That’s what we were trying to build! A machine to produce power! To generate electricity, to drive ships!
Bohr You didn’t say anything about a reactor.
Heisenberg I didn’t say anything about anything! Not in so many words. I couldn’t! I’d no idea how much could be overheard. How much you’d repeat to others.
Bohr But then I asked you if you actually thought that uranium fission could be used for the construction of weapons.
Heisenberg Ah! It’s coming back!
Bohr And I clearly remember what you replied.
Heisenberg I said I now knew that it could be.
Bohr This is what really horrified me.
Heisenberg Because you’d always been confident that weapons would need 235, and that we could never separate enough of it.
Bohr A reactor—yes, maybe, because there it’s not going to blow itself apart. You can keep the chain reaction going with slow neutrons in natural uranium.
Heisenberg What we’d realised, though, was that if we could once get the reactor going …
Bohr The 238 in the natural uranium would absorb the fast neutrons …
Heisenberg Exactly as you predicted in 1939—everything we were doing was based on that fundamental insight of yours. The 238 would absorb the fast neutrons. And would be transformed by them into a new element altogether.
Bohr Neptunium. Which would decay in its turn into another new element …
Heisenberg At least as fissile as the 235 that we couldn’t separate …
Margrethe Plutonium.
Heisenberg Plutonium.
Bohr I should have worked it out for myself.
Heisenberg If we could build a reactor we could build bombs. That’s what had brought me to Copenhagen. But none of this could I say. And at this point you stopped listening. The bomb had already gone off inside your head. I realised we were heading back towards the house. Ourwalk was over. Our one chance to talk had gone forever.
Bohr Because I’d grasped the central point already. That one way or another you saw the possibility of supplying Hitler with nuclear weapons.
Heisenberg You grasped at least four different central points, all of them wrong. You told Rozental that I’d tried to pick your brains about fission. You told Weisskopf that I’d asked you what you knew about the Allied nuclear programme. Chadwick thought I was hoping to persuade you that there was no German programme. But then you seem to have told some people that I’d tried to recruit you to work on it!
Bohr Very well. Let’s start all over again from the beginning. No Gestapo in the shadows this time. No British intelligence officer. No one watching us at all.
Margrethe Only me.
Bohr Only Margrethe. We’re going to make the whole thing clear to Margrethe. You know how strongly I believe that we don’t do science for ourselves, that we do it so we can explain to others …
Heisenberg In plain language.
Bohr In plain language. Not your view, I know—you’d be happy to describe what you were up to purely in differential equations if you could—but for Margrethe’s sake …
Heisenberg Plain language.
Bohr Plain language. All right, so here we are, walking along the street once more. And this time I’m absolutely calm, I’m listening intently. What is it you want to say?
Heisenberg It’s not just what I want to say! The whole German nuclear team in Berlin! Not Diebner, of course, not the Nazis—but Weizsäcker, Hahn, Wirtz, Jensen, Houtermanns—they all wanted me to come and discuss it with you. We all see you as a kind of spiritual
Bohr I was horrified.
Heisenberg Horrified. Good, you remember that. You stood there gazing at me, horrified.
Bohr Because the implication was obvious. That you
were
working on it.
Heisenberg And you jumped to the conclusion that I was trying to provide Hitler with nuclear weapons.
Bohr And you were!
Heisenberg No! A reactor! That’s what we were trying to build! A machine to produce power! To generate electricity, to drive ships!
Bohr You didn’t say anything about a reactor.
Heisenberg I didn’t say anything about anything! Not in so many words. I couldn’t! I’d no idea how much could be overheard. How much you’d repeat to others.
Bohr But then I asked you if you actually thought that uranium fission could be used for the construction of weapons.
Heisenberg Ah! It’s coming back!
Bohr And I clearly remember what you replied.
Heisenberg I said I now knew that it could be.
Bohr This is what really horrified me.
Heisenberg Because you’d always been confident that weapons would need 235, and that we could never separate enough of it.
Bohr A reactor—yes, maybe, because there it’s not going to blow itself apart. You can keep the chain reaction going with slow neutrons in natural uranium.
Heisenberg What we’d realised, though, was that if we could once get the reactor going …
Bohr The 238 in the natural uranium would absorb the fast neutrons …
Heisenberg Exactly as you predicted in 1939—everything we were doing was based on that fundamental insight of yours. The 238 would absorb the fast neutrons. And would be transformed by them into a new element altogether.
Bohr Neptunium. Which would decay in its turn into another new element …
Heisenberg At least as fissile as the 235 that we couldn’t separate …
Margrethe Plutonium.
Heisenberg Plutonium.
Bohr I should have worked it out for myself.
Heisenberg If we could build a reactor we could build bombs. That’s what had brought me to Copenhagen. But none of this could I say. And at this point you stopped listening. The bomb had already gone off inside your head. I realised we were heading back towards the house. Ourwalk was over. Our one chance to talk had gone forever.
Bohr Because I’d grasped the central point already. That one way or another you saw the possibility of supplying Hitler with nuclear weapons.
Heisenberg You grasped at least four different central points, all of them wrong. You told Rozental that I’d tried to pick your brains about fission. You told Weisskopf that I’d asked you what you knew about the Allied nuclear programme. Chadwick thought I was hoping to persuade you that there was no German programme. But then you seem to have told some people that I’d tried to recruit you to work on it!
Bohr Very well. Let’s start all over again from the beginning. No Gestapo in the shadows this time. No British intelligence officer. No one watching us at all.
Margrethe Only me.
Bohr Only Margrethe. We’re going to make the whole thing clear to Margrethe. You know how strongly I believe that we don’t do science for ourselves, that we do it so we can explain to others …
Heisenberg In plain language.
Bohr In plain language. Not your view, I know—you’d be happy to describe what you were up to purely in differential equations if you could—but for Margrethe’s sake …
Heisenberg Plain language.
Bohr Plain language. All right, so here we are, walking along the street once more. And this time I’m absolutely calm, I’m listening intently. What is it you want to say?
Heisenberg It’s not just what I want to say! The whole German nuclear team in Berlin! Not Diebner, of course, not the Nazis—but Weizsäcker, Hahn, Wirtz, Jensen, Houtermanns—they all wanted me to come and discuss it with you. We all see you as a kind of spiritual