Michael Frayn - Copenhagen

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.