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
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
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