Copenhagen (11 page)

Heisenberg
  The one thing I was wrong about.

Bohr
  You were twenty times over.

Heisenberg
  The one thing.

Bohr
  But, Heisenberg, your mathematics, your mathematics! How could they have been so far out?

Heisenberg
  They weren’t. As soon as I calculated the diffusion I got it just about right.

Bohr
  As soon as you calculated it?

Heisenberg
  I gave everyone a seminar on it a week later. It’s in the record! Look it up!

Bohr
  You mean … you hadn’t calculated it before? You hadn’t done the diffusion equation?

Heisenberg
  There was no need to.

Bohr
  No need to?

Heisenberg
  The calculation had already been done.

Bohr
  Done by whom?

Heisenberg
  By Perrin and Flügge in 1939.

Bohr
  By Perrin and Flügge? But, my dear Heisenberg, that was for natural uranium. Wheeler and I showed that it was only the 235 that fissioned.

Heisenberg
  Your great paper. The basis of everything we did.

Bohr
  So you needed to calculate the figure for pure 235.

Heisenberg
  Obviously.

Bohr
  And you didn’t?

Heisenberg
  I didn’t.

Bohr
  And that’s why you were so confident you couldn’t do it until you had the plutonium. Because you spent the entire war believing that it would take not a few kilograms of 235, but a ton or more. And to make a ton of 235 in any plausible time …

Heisenberg
  Would have needed something like two
hundred million separator units. It was plainly unimaginable.

Bohr
  If you’d realised you had to produce only a few kilograms …

Heisenberg
  Even to make a single kilogram would need something like two hundred thousand units.

Bohr
  But two hundred million is one thing; two hundred thousand is another. You might just possibly have imagined setting up two hundred thousand.

Heisenberg
  Just possibly.

Bohr
  The Americans did imagine it.

Heisenberg
  Because Otto Frisch and Rudolf Peierls actually did the calculation. They solved the diffusion equation.

Bohr
  Frisch was my old assistant.

Heisenberg
  Peierls was my old pupil.

Bohr
  An Austrian and a German.

Heisenberg
  So they should have been making their calculation for us, at the Kaiser Wilhelm Institute in Berlin. But instead they made it at the University of Birmingham, in England.

Margrethe
  Because they were Jews.

Heisenberg
  There’s something almost mathematically elegant about that.

Bohr
  They also started with Perrin and Flügge.

Heisenberg
  They also thought it would take tons. They also thought it was unimaginable.

Bohr
  Until one day …

Heisenberg
  They did the calculation.

Bohr
  They discovered just how fast the chain reaction would go.

Heisenberg
  And therefore how little material you’d need.

Bohr
  They said slightly over half a kilogram.

Heisenberg
  About the size of a tennis ball.

Bohr
  They were wrong, of course.

Heisenberg
  They were a hundred times under.

Bohr
  Which made it seem a hundred times more imaginable than it actually was.

Heisenberg
  Whereas I left it seeming twenty times less imaginable.

Bohr
  So all your agonising in Copenhagen about plutonium was beside the point. You could have done it without ever building the reactor. You could have done it with 235 all the time.

Heisenberg
  Almost certainly not.

Bohr
  Just possibly, though.

Heisenberg
  Just possibly.

Bohr
  And
that
question you’d settled long before you arrived in Copenhagen. Simply by failing to try the diffusion equation.

Heisenberg
  Such a tiny failure.

Bohr
  But the consequences went branching out over the years, doubling and redoubling.

Heisenberg
  Until they were large enough to save a city. Which city? Any of the cities that we never dropped our bomb on.

Bohr
  London, presumably, if you’d had it in time. If the Americans had already entered the war, and the Allies had begun to liberate Europe, then …

Heisenberg
  Who knows? Paris as well. Amsterdam. Perhaps Copenhagen.

Bohr
  So, Heisenberg, tell us this one simple thing: why didn’t you do the calculation?

Heisenberg
  The question is why Frisch and Peierls
did
do it. It was a stupid waste of time. However much 235 it turned out to be, it was obviously going to be more than anyone could imagine producing.

Bohr
  Except that it wasn’t!

Heisenberg
  Except that it wasn’t.

Bohr
  So why …?

Heisenberg
  I don’t know! I don’t know why I didn’t do it! Because I never thought of it! Because it didn’t occur to me! Because I assumed it wasn’t worth doing!

Bohr
  Assumed? Assumed? You never assumed things! That’s how you got uncertainty, because you rejected our assumptions! You calculated, Heisenberg! You calculated everything! The first thing you did with a problem was the mathematics!

Heisenberg
  You should have been there to slow me down.

Bohr
  Yes, you wouldn’t have got away with it if I’d been standing over you.

Heisenberg
  Though in fact you made exactly the same assumption! You thought there was no danger for exactly the same reason as I did! Why didn’t
you
calculate it?

Bohr
  Why didn’t I calculate it?

Heisenberg
  Tell us why
you
didn’t calculate it and we’ll know why
I
didn’t!

Bohr
  It’s obvious why I didn’t!

Heisenberg
  Go on.

Margrethe
  Because he wasn’t trying to build a bomb!

Heisenberg
  Yes. Thank you. Because he wasn’t trying to build a bomb. I imagine it was the same with me. Because
I wasn’t trying to build a bomb. Thank you.

Bohr
  So, you bluffed yourself, the way I did at poker with the straight I never had. But in that case …

Heisenberg
  Why did I come to Copenhagen? Yes, why did I come …?

Bohr
  One more draft, yes? One final draft!

Heisenberg
  And once again I crunch over the familiar gravel to the Bohrs’ front door, and tug at the familiar bell-pull. Why have I come? I know perfectly well. Know so well that I’ve no need to ask myself. Until once again the heavy front door opens.

Bohr
  He stands on the doorstep blinking in the sudden flood of light from the house. Until this instant his thoughts have been everywhere and nowhere, like unobserved particles, through all the slits in the diffraction grating simultaneously. Now they have to be observed and specified.

Heisenberg
  And at once the clear purposes inside my head lose all definite shape. The light falls on them and they scatter.

Bohr
  My dear Heisenberg!

Heisenberg
  My dear Bohr!

Bohr
  Come in, come in …

Heisenberg
  How difficult it is to see even what’s in front of one’s eyes. All we possess is the present, and the present endlessly dissolves into the past. Bohr has gone even as I turn to see Margrethe.

Margrethe
  Niels is right. You look older.

Bohr
  I believe you had some personal trouble.

Heisenberg
  Margrethe slips into history even as I turn back to Bohr. And yet how much more difficult still it is to catch the slightest glimpse of what’s behind one’s eyes. Here I am at the centre of the universe, and yet all I can
see are two smiles that don’t belong to me.

Margrethe
  How is Elisabeth? How are the children?

Heisenberg
  Very well. They send their love, of course … I can feel a third smile in the room, very close to me. Could it be the one I suddenly see for a moment in the mirror there? And is the awkward stranger wearing it in any way connected with this presence that I can feel in the room? This all-enveloping, unobserved presence?

Margrethe
  I watch the two smiles in the room, one awkward and ingratiating, the other rapidly fading from incautious warmth to bare politeness. There’s also a third smile in the room, I know, unchangingly courteous, I hope, and unchangingly guarded.

Heisenberg
  You’ve managed to get some skiing?

Bohr
  I glance at Margrethe, and for a moment I see what she can see and I can’t—myself, and the smile vanishing from my face as poor Heisenberg blunders on.

Heisenberg
  I look at the two of them looking at me, and for a moment I see the third person in the room as clearly as I see them. Their importunate guest, stumbling from one crass and unwelcome thoughtfulness to the next.

Bohr
  I look at him looking at me, anxiously, pleadingly, urging me back to the old days, and I see what he sees. And yes—now it comes, now it comes—there’s someone missing from the room. He sees me. He sees Margrethe. He doesn’t see himself.

Heisenberg
  Two thousand million people in the world, and the one who has to decide their fate is the only one who’s always hidden from me.

Bohr
  You suggested a stroll.

Heisenberg
  You remember Elsinore? The darkness inside the human soul …?

Bohr
  And out we go. Out under the autumn trees.
Through the blacked-out streets.

Heisenberg
  Now there’s no one in the world except Bohr and the invisible other. Who is he, this all-enveloping presence in the darkness?

Margrethe
  The flying particle wanders the darkness, no one knows where. It’s here, it’s there, it’s everywhere and nowhere.

Bohr
  With careful casualness he begins to ask the question he’s prepared.

Heisenberg
  Does one as a physicist have the moral right to work on the practical exploitation of atomic energy?

Margrethe
  The great collision.

Bohr
  I stop. He stops …

Margrethe
  This is how they work.

Heisenberg
  He gazes at me, horrified.

Margrethe
  Now at last he knows where he is and what he’s doing.

Heisenberg
  He turns away.

Margrethe
  And even as the moment of collision begins it’s over.

Bohr
  Already we’re hurrying back towards the house.

Margrethe
  Already they’re both flying away from each other into the darkness again.

Heisenberg
  Our conversation’s over.

Bohr
  Our great partnership.

Heisenberg
  All our friendship.

Margrethe
  And everything about him becomes as uncertain as it was before.

Bohr
  Unless … yes … a thought-experiment .… Let’s suppose for a moment that I don’t go flying off into the
night. Let’s see what happens if instead I remember the paternal role I’m supposed to play. If I stop, and control my anger, and turn to him. And ask him why.

Heisenberg
  Why?

Bohr
  Why are you confident that it’s going to be so reassuringly difficult to build a bomb with 235? Is it because you’ve done the calculation?

Heisenberg
  The calculation?

Bohr
  Of the diffusion in 235. No. It’s because you haven’t calculated it. You haven’t considered calculating it. You hadn’t consciously realised there was a calculation to be made.

Heisenberg
  And of course now I
have
realised. In fact it wouldn’t be all that difficult. Let’s see .… The scattering cross-section’s about 6 × 10
⁻²⁴
, so the mean free path would be … Hold on …

Bohr
  And suddenly a very different and very terrible new world begins to take shape …

Margrethe
  That was the last and greatest demand that Heisenberg made on his friendship with you. To be understood when he couldn’t understand himself. And that was the last and greatest act of friendship for Heisenberg that you performed in return. To leave him misunderstood.

Heisenberg
  Yes. Perhaps I should thank you.

Bohr
  Perhaps you should.

Margrethe
  Anyway, it was the end of the story.

Bohr
  Though perhaps there was also something I should thank
you
for. That summer night in 1943, when I escaped across the Sound in the fishing-boat, and the freighters arrived from Germany …

Margrethe
  What’s that to do with Heisenberg?

Bohr
  When the ships arrived on the Wednesday there were eight thousand Jews in Denmark to be arrested and
crammed into their holds. On the Friday evening, at the start of the Sabbath, when the SS began their round-up, there was scarcely a Jew to be found.