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man who’d queried his mathematics, and swept him off for a tramp in the country. Walk—talk—make his acquaintance. And when Heisenberg arrived here to work for him, off they go again, on their great tour of Zealand. A lot of this century’s physics they did in the open air. Strolling around the forest paths at Tisvilde. Going down to the beach with the children. Heisenberg holding Christian’s hand. Yes, and every evening in Copenhagen, after dinner, they’d walk round Faelled Park behind the Institute, or out along Langelinie into the harbour. Walk, and talk. Long, long before walls had ears … But this time, in 1941, their walk takes a different course. Ten minutes after they set out … they’re back! I’ve scarcely had the table cleared when there’s Niels in the doorway. I see at once how upset he is—he can’t look me in the eye.
    Bohr   Heisenberg wants to say goodbye. He’s leaving.
    Margrethe    He won’t look at me, either.
    Heisenberg   Thank you. A delightful evening. Almost like old times. So kind of you.
    Margrethe   You’ll have some coffee? A glass of something?
    Heisenberg   I have to get back and prepare for my lecture.
    Margrethe   But you’ll come and see us again before you leave?
    Bohr   He has a great deal to do.
    Margrethe   It’s like the worst moments of 1927 all over again, when Niels came back from Norway and first read Heisenberg’s uncertainty paper. Something they both seemed to have forgotten about earlier in the evening, though I hadn’t. Perhaps they’ve both suddenly remembered that time. Only from the look on their faces something even worse has happened.
    Heisenberg   Forgive me if I’ve done or said anything that …
    Bohr   Yes, yes.
    Heisenberg   It meant a great deal to me, being here with you both again. More perhaps than you realise.
    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, 2 80 specks of dust have been moved. 2 80 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