Copenhagen - Michael Frayn [10]
Margrethe It was a pleasure for us. Our love to Elisabeth.
Bohr Of course.
Margrethe And the children.
Heisenberg Perhaps, when this war is over .… If we’re all spared .… Goodbye.
Margrethe Politics?
Bohr Physics. He’s not right, though. How can he be right? John Wheeler and I …
Margrethe A breath of air as we talk, why not?
Bohr A breath of air?
Margrethe A turn around the garden. Healthier than staying indoors, perhaps.
Bohr Oh. Yes.
Margrethe For everyone concerned.
Bohr Yes. Thank you .… How can he possibly be right? Wheeler and I went through the whole thing in 1939.
Margrethe What did he say?
Bohr Nothing. I don’t know. I was too angry to take it in.
Margrethe Something about fission?
Bohr What happens in fission? You fire a neutron at a uranium nucleus, it splits, and it releases energy.
Margrethe A huge amount of energy. Yes?
Bohr About enough to move a speck of dust. But it also releases two or three more neutrons. Each of which has the chance of splitting another nucleus.
Margrethe So then those two or three split nuclei each release energy in their turn?
Bohr And two or three more neutrons.
Heisenberg You start a trickle of snow sliding as you ski. The trickle becomes a snowball …
Bohr An ever-widening chain of split nuclei forks through the uranium, doubling and quadrupling in millionths of a second from one generation to the next. First two splits, let’s say for simplicity. Then two squared, two cubed, two to the fourth, two to the fifth, two to the sixth …
Heisenberg The thunder of the gathering avalanche echoes from all the surrounding mountains …
Bohr Until eventually, after, let’s say, eighty generations, 280 specks of dust have been moved. 280 is a number with 24 noughts. Enough specks of dust to constitute a city, and all who live in it.
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?