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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 beenthere 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 wasof 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