Copenhagen (5 page)

Heisenberg
  But there is a catch.

Bohr
  There is a catch, thank God. Natural uranium consists of two different isotopes, U-238 and U-235. Less than one per cent of it is U-235, and this tiny fraction is the only part of it that’s fissionable by fast neutrons.

Heisenberg
  This was Bohr’s great insight. Another of his amazing intuitions. It came to him when he was at Princeton in 1939, walking across the campus with Wheeler. A characteristic Bohr moment—I wish I’d been
there to enjoy it. Five minutes deep silence as they walked, then: ‘Now hear this—I have understood everything.’

Bohr
  In fact it’s a double catch. 238 is not only impossible to fission by fast neutrons—it also absorbs them. So, very soon after the chain reaction starts, there aren’t enough fast neutrons left to fission the 235.

Heisenberg
  And the chain stops.

Bohr
  Now, you can fission the 235 with slow neutrons as well. But then the chain reaction occurs more slowly than the uranium blows itself apart.

Heisenberg
  So again the chain stops.

Bohr
  What all this means is that an explosive chain reaction will never occur in natural uranium. To make an explosion you will have to separate out pure 235. And to make the chain long enough for a large explosion …

Heisenberg
  Eighty generations, let’s say …

Bohr
   … you would need many tons of it. And it’s extremely difficult to separate.

Heisenberg
  Tantalisingly difficult.

Bohr
  Mercifully difficult. The best estimates, when I was in America in 1939, were that to produce even one gram of U-235 would take 26,000 years. By which time, surely, this war will be over. So he’s wrong, you see, he’s wrong! Or could I be wrong? Could I have miscalculated? Let me see .… What are the absorption rates for fast neutrons in 238? What’s the mean free path of slow neutrons in 235 …?

Margrethe
  But what exactly had Heisenberg said? That’s what everyone wanted to know, then and forever after.

Bohr
  It’s what the British wanted to know, as soon as Chadwick managed to get in touch with me. What exactly did Heisenberg say?

Heisenberg
  And what exactly did Bohr reply? That was
of course the first thing my colleagues asked me when I got back to Germany.

Margrethe
  What did Heisenberg tell Niels—what did Niels reply? The person who wanted to know most of all was Heisenberg himself.

Bohr
  You mean when he came back to Copenhagen after the war, in 1947?

Margrethe
  Escorted this time not by unseen agents of the Gestapo, but by a very conspicuous minder from British intelligence.

Bohr
  I think he wanted various things.

Margrethe
  Two things. Food-parcels …

Bohr
  For his family in Germany. They were on the verge of starvation.

Margrethe
  And for you to agree what you’d said to each other in 1941.

Bohr
  The conversation went wrong almost as fast as it did before.

Margrethe
  You couldn’t even agree where you’d walked that night.

Heisenberg
  Where we walked? Faelled Park, of course. Where we went so often in the old days.

Margrethe
  But Faelled Park is behind the Institute, four kilometres away from where we live!

Heisenberg
  I can see the drift of autumn leaves under the street-lamps next to the bandstand.

Bohr
  Yes, because you remember it as October!

Margrethe
  And it was September.

Bohr
  No fallen leaves!

Margrethe
  And it was 1941. No street-lamps!

Bohr
  I thought we hadn’t got any further than my study.
What I can see is the drift of papers under the reading-lamp on my desk.

Heisenberg
  We must have been outside! What I was going to say was treasonable. If I’d been overheard I’d have been executed.

Margrethe
  So what was this mysterious thing you said?

Heisenberg
  There’s no mystery about it. There never was any mystery. I remember it absolutely clearly, because my life was at stake, and I chose my words very carefully. I simply asked you if as a physicist one had the moral right to work on the practical exploitation of atomic energy. Yes?

Bohr
  I don’t recall.

Heisenberg
  You don’t recall, no, because you immediately became alarmed. You stopped 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. Our
walk 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 father.

Margrethe
  The Pope. That’s what you used to call Niels behind his back. And now you want him to give you absolution.

Heisenberg
  Absolution? No!

Margrethe
  According to your colleague Jensen.

Heisenberg
  Absolution is the last thing I want!

Margrethe
  You told one historian that Jensen had expressed it perfectly.

Heisenberg
  Did I? Absolution .… Is that what I’ve come for? It’s like trying to remember who was at that lunch you gave me at the Institute. Around the table sit all the different explanations for everything I did. I turn to look … Petersen, Rozental, and … yes … now the word absolution is taking its place among them all …

Margrethe
  Though I thought absolution was granted for sins past and repented, not for sins intended and yet to be committed.

Heisenberg
  Exactly! That’s why I was so shocked!

Bohr
  
You
were shocked?

Heisenberg
  Because you
did
give me absolution! That’s exactly what you did! As we were hurrying back to the house. You muttered something about everyone in wartime being obliged to do his best for his own country. Yes?

Bohr
  Heaven knows what I said. But now here I am, profoundly calm and conscious, weighing my words. You don’t want absolution. I understand. You want me to tell you
not
to do it? All right. I put my hand on your arm. I look you in the eye in my most papal way. Go back to Germany, Heisenberg. Gather your colleagues together in the laboratory. Get up on a table and tell them: ‘Niels Bohr says that in his considered judgment supplying a homicidal maniac with an improved instrument of mass murder is …’ What shall I say? ‘ … an interesting idea.’ No, not even an interesting idea. ‘ … a really rather
seriously uninteresting idea.’ What happens? You all fling down your Geiger counters?

Heisenberg
  Obviously not.

Bohr
  Because they’ll arrest you.

Heisenberg
  Whether they arrest us or not it won’t make any difference. In fact it will make things worse. I’m running my programme for the Kaiser Wilhelm Institute. But there’s a rival one at Army Ordnance, run by Kurt Diebner, and he’s a party member. If I go they’ll simply get Diebner to take over my programme as well. He should be running it anyway. Wirtz and the rest of them only smuggled me in to keep Diebner and the Nazis out of it. My one hope is to remain in control.

Bohr
  So you don’t want me to say yes and you don’t want me to say no.

Heisenberg
  What I want is for you to listen carefully to what I’m going on to say next, instead of running off down the street like a madman.

Bohr
  Very well. Here I am, walking very slowly and popishly. And I listen most carefully as you tell me …

Heisenberg
  That nuclear weapons will require an enormous technical effort.

Bohr
  True.

Heisenberg
  That they will suck up huge resources.

Bohr
  Huge resources. Certainly.

Heisenberg
  That sooner or later governments will have to turn to scientists and ask whether it’s worth committing those resources—whether there’s any hope of producing the weapons in time for them to be used.

Bohr
  Of course, but …

Heisenberg
  Wait. So they will have to come to you and me. We are the ones who will have to advise them whether to go ahead or not. In the end the decision will be in our
hands, whether we like it or not.

Bohr
  And that’s what you want to tell me?

Heisenberg
  That’s what I want to tell you.

Bohr
  That’s why you have come all this way, with so much difficulty? That’s why you have thrown away nearly twenty years of friendship? Simply to tell me that?

Heisenberg
  Simply to tell you that.

Bohr
  But, Heisenberg, this is more mysterious than ever! What are you telling it me
for?
What am I supposed to do about it? The government of occupied Denmark isn’t going to come to me and ask me whether we should produce nuclear weapons!

Heisenberg
  No, but sooner or later, if I manage to remain in control of our programme, the German government is going to come to
me!
They will ask
me
whether to continue or not! I will have to decide what to tell them!

Bohr
  Then you have an easy way out of your difficulties. You tell them the simple truth that you’ve just told me. You tell them how difficult it will be. And perhaps they’ll be discouraged. Perhaps they’ll lose interest.