Authors: Michael Frayn
Margrethe
No, and why didn’t you? I’ll tell you that, too. It’s the simplest reason of all. Because you couldn’t. You didn’t understand the physics.
Heisenberg
That’s what Goudsmit said.
Margrethe
And Goudsmit knew. He was one of your magic circle. He and Uhlenbeck were the ones who did spin.
Heisenberg
All the same, he had no idea of what I did or didn’t understand about a bomb.
Margrethe
He tracked you down across Europe for Allied Intelligence. He interrogated you after you were captured.
Heisenberg
He blamed me, of course. His parents died in Auschwitz. He thought I should have done something to save them. I don’t know what. So many hands stretching up from the darkness for a lifeline, and no lifeline that could ever reach them …
Margrethe
He said you didn’t understand the crucial difference between a reactor and a bomb.
Heisenberg
I understood very clearly. I simply didn’t tell the others.
Margrethe
Ah.
Heisenberg
I understood, though.
Margrethe
But secretly.
Heisenberg
You can check if you don’t believe me.
Margrethe
There’s evidence, for once?
Heisenberg
It was all most carefully recorded.
Margrethe
Witnesses, even?
Heisenberg
Unimpeachable witnesses.
Margrethe
Who wrote it down?
Heisenberg
Who recorded it and transcribed it.
Margrethe
Even though you didn’t tell anyone?
Heisenberg
I told one person. I told Otto Hahn. That terrible night at Farm Hall, after we’d heard the news. Somewhere in the small hours, after everyone had finally gone to bed, and we were alone together. I gave him a reasonably good account of how the bomb had worked.
Margrethe
After the event.
Heisenberg
After the event. Yes. When it didn’t matter any more. All the things Goudsmit said I didn’t understand. Fast neutrons in 235. The plutonium option. A reflective shell to reduce neutron escape. Even the method of triggering it.
Bohr
The critical mass. That was the most important thing. The amount of material you needed to establish the chain-reaction. Did you tell him the critical mass?
Heisenberg
I gave him a figure, yes. You can look it up! Because that was the other secret of the house-party. Diebner asked me when we first arrived if I thought there were hidden microphones. I laughed. I told him the British were far too old-fashioned to know about Gestapo methods. I underestimated them. They had microphones everywhere—they were recording everything. Look it up! Everything we said. Everything we went through that terrible night. Everything I told Hahn alone in the small hours.
Bohr
But the critical mass. You gave him a figure. What was the figure you gave him?
Heisenberg
I forget.
Bohr
Heisenberg …
Heisenberg
It’s all on the record. You can see for yourself.
Bohr
The figure for the Hiroshima bomb …
Heisenberg
Was fifty kilograms.
Bohr
So that was the figure you gave Hahn? Fifty kilograms?
Heisenberg
I said about a ton.
Bohr
About a ton? A thousand kilograms? Heisenberg, I believe I am at last beginning to understand something.
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.