Authors: Michael Frayn
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.