Copenhagen

by Michael Frayn

  Bohr But, Heisenberg, with respect now, with the greatest respect, you couldn’t even keep the reactor under your control. That reactor was going to kill you.

  Heisenberg It wasn’t put to the test. It never went critical.

  Bohr Thank God. Hambro and Perrin examined it after the Allied troops took over. They said it had no cadmium control rods. There was nothing to absorb any excess of neutrons, to slow the reaction down when it overheated.

  Heisenberg No rods, no.

  Bohr You believed the reaction would be self-limiting.

  Heisenberg That’s what I originally believed.

  Bohr Heisenberg, the reaction would not have been self-limiting.

  Heisenberg By 1945 I understood that.

  Bohr So if you ever had got it to go critical, it would have melted down, and vanished into the centre of the earth!

  Heisenberg Not at all. We had a lump of cadmium to hand.

  Bohr A lump of cadmium? What were you proposing to do with a lump of cadmium?

  Heisenberg Throw it into the water.

  Bohr What water?

  Heisenberg The heavy water. The moderator that the uranium was immersed in.

  Bohr My dear good Heisenberg, not to criticise, but you’d all gone mad!

  Heisenberg We were almost there! We had this fantastic neutron growth! We had 670 per cent growth!

  Bohr You’d lost all contact with reality down in that hole!

  Heisenberg Another week. Another fortnight. That’s all we needed!

  Bohr It was only the arrival of the Allies that saved you!

  Heisenberg We’d almost reached the critical mass! A tiny bit bigger and the chain would sustain itself indefinitely. All we need is a little more uranium. I set off with Weizsäcker to try and get our hands on Diebner’s. Another hair-raising journey all the way back across Germany. Constant air raids—no trains—we try bicycles—we never make it! We end up stuck in a little inn somewhere in the middle of nowhere, listening to the thump of bombs falling all round us. And on the radio someone playing the Beethoven G minor cello sonata …

  Bohr And everything was still under your control?

  Heisenberg Under my control—yes! That’s the point! Under my control!

  Bohr Nothing was under anyone’s control by that time!

  Heisenberg Yes, because at last we were free of all constraints! The nearer the end came the faster we could work!

  Bohr You were no longer running that programme, Heisenberg. The programme was running you.

  Heisenberg Two more weeks, two more blocks of uranium, and it would have been German physics that achieved the world’s first self-sustaining chain reaction.

  Bohr Except that Fermi had already done it in Chicago, two years earlier.

  Heisenberg We didn’t know that.

  Bohr You didn’t know anything down in that cave. You were as blind as moles in a hole. Perrin said that there wasn’t even anything to protect you all from the radiation.

  Heisenberg We didn’t have time to think about it.

  Bohr So if it had gone critical …

  Margrethe You’d all have died of radiation sickness.

  Bohr My dear Heisenberg! My dear boy!

  Heisenberg Yes, but by then the reactor would have been running.

  Bohr I should have been there to look after you.

  Heisenberg That’s all we could think of at the time. To get the reactor running, to get the reactor running.

  Bohr You always needed me there to slow you down a little. Your own walking lump of cadmium.

  Heisenberg If I had died then, what should I have missed? Thirty years of attempting to explain. Thirty years of reproach and hostility. Even you turned your back on me.

  Margrethe You came to Copenhagen again. You came to Tisvilde.

  Heisenberg It was never the same.

  Bohr No. It was never the same.

  Heisenberg I sometimes think that those final few weeks at Haigerloch were the last happy time in my life. In a strange way it was very peaceful. Suddenly we were out of all the politics of Berlin. Out of the bombing. The war was coming to an end. There was nothing to think about except the reactor. And we didn’t go mad, in fact. We didn’t work all the time. There was a monastery on top of the rock above our cave. I used to retire to the organ-loft in the church, and play Bach fugues.

  Margrethe Look at him. He’s lost. He’s like a lost child. He’s been out in the woods all day, running here, running there. He’s shown off, he’s been brave, he’s been cowardly. He’s done wrong, he’s done right. And now the evening’s come, and all he wants is to go home, and he’s lost.

  Heisenberg Silence.

  Bohr Silence.

  Margrethe Silence.

  Heisenberg And once again the tiller slams over, and Christian is falling.

  Bohr Once again he’s struggling towards the lifebuoy.

  Margrethe Once again I look up from my work, and there’s Niels in the doorway, silently watching me …

  Bohr So, Heisenberg, why did you come to Copenhagen in 1941? It was right that you told us about all the fears you had. But you didn’t really think I’d tell you whether the Americans were working on a bomb.

  Heisenberg No.

  Bohr You didn’t seriously hope that I’d stop them.

  Heisenberg No.

  Bohr You were going back to work on that reactor whatever I said.

  Heisenberg Yes.

  Bohr So, Heisenberg, why did you come?

  Heisenberg Why did I come?

  Bohr Tell us once again. Another draft of the paper. And this time we shall get it right. This time we shall understand.

  Margrethe Maybe you’ll even understand yourself.

  Bohr After all, the workings of the atom were difficult to explain. We made many attempts. Each time we tried they became more obscure. We got there in the end, however. So—another draft, another draft.

  Heisenberg Why did I come? And once again I go through that evening in 1941. I crunch over the familiar gravel, and tug at the familiar bell-pull. What’s in my head? Fear, certainly, and the absurd and horrible importance of someone bearing bad news. But … yes … something else as well. Here it comes again. I can almost see its face. Something good. Something bright and eager and hopeful.

  Bohr I open the door …

  Heisenberg And there he is. I see his eyes light up at the sight of me.

  Bohr He’s smiling his wary schoolboy smile.

  Heisenberg And I feel a moment of such consolation.

  Bohr A flash of such pure gladness.

  Heisenberg As if I’d come home after a long journey.

  Bohr As if a long-lost child had appeared on the doorstep.

  Heisenberg Suddenly I’m free of all the dark tangled currents in the water.

  Bohr Christian is alive, Harald still unborn.

  Heisenberg The world is at peace again.

  Margrethe Look at them. Father and son still. Just for a moment. Even now we’re all dead.

  Bohr For a moment, yes, it’s the twenties again.

  Heisenberg And we shall speak to each other and understand each other in the way we did before.

  Margrethe And from those two heads the future will emerge. Which cities will be destroyed, and which survive. Who will die, and who will live. Which world will go down to obliteration, and which will triumph.

  Bohr My dear Heisenberg!

  Heisenberg My dear Bohr!

  Bohr Come in, come in …

  Act Two

  Heisenberg It was the very beginning of spring. The first time I came to Copenhagen, in 1924. March: raw, blustery northern weather. But every now and then the sun would come out and leave that first marvellous warmth of the year on your skin. That first breath of returning life.

  Bohr You were twenty-two. So I must have been … Thirty-eight.

  Bohr Almost the same age as you were when you came in 1941.

  Heisenberg So what do we do?

  Bohr Put on our boots and ruck
sacks …

  Heisenberg Take the tram to the end of the line …

  Bohr And start walking!

  Heisenberg Northwards to Elsinore.

  Bohr If you walk you talk.

  Heisenberg Then westwards to Tisvilde.

  Bohr And back by way of Hillerød.

  Heisenberg Walking, talking, for a hundred miles.

  Bohr After which we talked more or less non-stop for the next three years.

  Heisenberg We’d split a bottle of wine over dinner in your flat at the Institute.

  Bohr Then I’d come up to your room …

  Heisenberg That terrible little room in the servants’ quarters in the attic.

  Bohr And we’d talk on into the small hours.

  Heisenberg How, though?

  Bohr How?

  Heisenberg How did we talk? In Danish?

  Bohr In German, surely.

  Heisenberg I lectured in Danish. I had to give my first colloquium when I’d only been here for ten weeks.

  Bohr I remember it. Your Danish was already excellent.

  Heisenberg No. You did a terrible thing to me. Half-an-hour before it started you said casually, Oh, I think we’ll speak English today.

  Bohr But when you explained …?

  Heisenberg Explain to the Pope? I didn’t dare. That excellent Danish you heard was my first attempt at English.

  Bohr My dear Heisenberg! On our own together, though? My love, do you recall?

  Margrethe What language you spoke when I wasn’t there? You think I had microphones hidden?

  Bohr No, no—but patience, my love, patience!

  Margrethe Patience?

  Bohr You sounded a little sharp.

  Margrethe Not at all.

  Bohr We have to follow the threads right back to the beginning of the maze.

  Margrethe I’m watching every step.

  Bohr You didn’t mind? I hope.

  Margrethe Mind?

  Bohr Being left at home?

  Margrethe While you went off on your hike? Of course not. Why should I have minded? You had to get out of the house. Two new sons arriving on top of each other would be rather a lot for any man to put up with.

  Bohr Two new sons?

  Margrethe Heisenberg.

  Bohr Yes, yes.

  Margrethe And our own son.

  Bohr Aage?

  Margrethe Ernest!

  Bohr 1924—of course—Ernest.

  Margrethe Number five. Yes?

  Bohr Yes, yes, yes. And if it was March, you’re right—he couldn’t have been much more than …

  Margrethe One week.

  Bohr One week? One week, yes. And you really didn’t mind?

  Margrethe Not at all. I was pleased you had an excuse to get away. And you always went off hiking with your new assistants. You went off with Kramers, when he arrived in 1916.

  Bohr Yes, when I suppose Christian was still only …

  Margrethe One week.

  Bohr Yes .… Yes .… I almost killed Kramers, you know.

  Heisenberg Not with a cap-pistol?

  Bohr With a mine. On our walk.

  Heisenberg Oh, the mine. Yes, you told me, on ours. Never mind Kramers—you almost killed yourself!

  Bohr A mine washed up in the shallows …

  Heisenberg And of course at once they compete to throw stones at it. What were you thinking of?

  Bohr I’ve no idea.

  Heisenberg A touch of Elsinore there, perhaps.

  Bohr Elsinore?

  Heisenberg The darkness inside the human soul.

  Bohr You did something just as idiotic.

  Heisenberg I did?

  Bohr With Dirac in Japan. You climbed a pagoda.

  Heisenberg Oh, the pagoda.

  Bohr Then balanced on the pinnacle. According to Dirac. On one foot. In a high wind. I’m glad I wasn’t there.

  Heisenberg Elsinore, I confess.

  Bohr Elsinore, certainly.

  Heisenberg I was jealous of Kramers, you know.

  Bohr His Eminence. Isn’t that what you called him?

  Heisenberg Because that’s what he was. Your leading cardinal. Your favourite son. Till I arrived on the scene.

  Margrethe He was a wonderful cellist.

  Bohr He was a wonderful everything.

  Heisenberg Far too wonderful.

  Margrethe I liked him.

  Heisenberg I was terrified of him. When I first started at the Institute. I was terrified of all of them. All the boy wonders you had here—they were all so brilliant and accomplished. But Kramers was the heir apparent. All the rest of us had to work in the general study hall. Kramers had the private office next to yours, like the electron on the inmost orbit around the nucleus. And he didn’t think much of my physics. He insisted you could explain everything about the atom by classical mechanics.

  Bohr Well, he was wrong.

  Margrethe And very soon the private office was vacant.

  Bohr And there was another electron on the inmost orbit.

  Heisenberg Yes, and for three years we lived inside the atom.

  Bohr With other electrons on the outer orbits around us all over Europe.

  Heisenberg Max Born and Pascual Jordan in Göttingen.

  Bohr Yes, but Schrödinger in Zurich, Fermi in Rome.

  Heisenberg Chadwick and Dirac in England.

  Bohr Joliot and de Broglie in Paris.

  Heisenberg Gamow and Landau in Russia.

  Bohr Everyone in and out of each other’s departments.

  Heisenberg Papers and drafts of papers on every international mail-train.

  Bohr You remember when Goudsmit and Uhlenbeck did spin?

  Heisenberg There’s this one last variable in the quantum state of the atom that no one can make sense of. The last hurdle …

  Bohr And these two crazy Dutchmen go back to a ridiculous idea that electrons can spin in different ways.

  Heisenberg And of course the first thing that everyone wants to know is, What line is Copenhagen going to take?

  Bohr I’m on my way to Leiden, as it happens.

  Heisenberg And it turns into a papal progress! The train stops on the way at Hamburg …

  Bohr Pauli and Stern are waiting on the platform to ask me what I think about spin.

  Heisenberg You tell them it’s wrong.

  Bohr No, I tell them it’s very …

  Heisenberg Interesting.

  Bohr I think that is precisely the word I choose.

  Heisenberg Then the train pulls into Leiden.

  Bohr And I’m met at the barrier by Einstein and Ehrenfest. And I change my mind because Einstein—Einstein, you see?—I’m the Pope—he’s God—because Einstein has made a relativistic analysis, and it resolves all my doubts.

  Heisenberg Meanwhile I’m standing in for Max Born at Göttingen, so you make a detour there on your way home.

  Bohr And you and Jordan meet me at the station.

  Heisenberg Same question: what do you think of spin?

  Bohr And when the train stops at Berlin there’s Pauli on the platform.

  Heisenberg Wolfgang Pauli, who never gets out of bed if he can possibly avoid it …

  Bohr And who’s already met me once at Hamburg on the journey out …

  Heisenberg He’s travelled all the way from Hamburg to Berlin purely in order to see you for the second time round …

  Bohr And find out how my ideas on spin have developed en route.

  Heisenberg Oh, those years! Those amazing years! Those three short years!

  Bohr From 1924 to 1927.

  Heisenberg From when I arrived in Copenhagen to work with you …

  Bohr To when you departed, to take up your chair at Leipzig.

  Heisenberg Three years of raw, bracing northern springtime.

  Bohr At the end of which we had quantum mechanics, we had uncertainty …

  Heisenberg We had complementarity …

  Bohr We had the whole Copenhagen Interpretation.

  Heisenbe
rg Europe in all its glory again. A new Enlightenment, with Germany back in her rightful place at the heart of it. And who led the way for everyone else?

  Margrethe You and Niels.

  Heisenberg Well, we did.

  Bohr We did.

  Margrethe And that’s what you were trying to get back to in 1941?

  Heisenberg To something we did in those three years .… Something we said, something we thought .… I keep almost seeing it out of the corner of my eye as we talk! Something about the way we worked. Something about the way we did all those things …

  Bohr Together.

  Heisenberg Together. Yes, together.

  Margrethe No.

  Bohr No? What do you mean, no?

  Margrethe Not together. You didn’t do any of those things together.

  Bohr Yes, we did. Of course we did.

  Margrethe No, you didn’t. Every single one of them you did when you were apart. You first worked out quantum mechanics on Heligoland.

  Heisenberg Well, it was summer by then. I had my hay fever.

  Margrethe And on Heligoland, on your own, on a rocky bare island in the middle of the North Sea, you said there was nothing to distract you …

  Heisenberg My head began to clear, and I had this very sharp picture of what atomic physics ought to be like. I suddenly realised that we had to limit it to the measurements we could actually make, to what we could actually observe. We can’t see the electrons inside the atom …

  Margrethe Any more than Niels can see the thoughts in your head, or you the thoughts in Niels’s.

  Heisenberg All we can see are the effects that the electrons produce, on the light that they reflect …

  Bohr But the difficulties you were trying to resolve were the ones we’d explored together, over dinner in the flat, on the beach at Tisvilde.

  Heisenberg Of course. But I remember the evening when the mathematics first began to chime with the principle.

  Margrethe On Heligoland.

  Heisenberg On Heligoland.

  Margrethe On your own.

  Heisenberg It was terribly laborious—I didn’t understand matrix calculus then … I get so excited I keep making mistakes. But by three in the morning I’ve got it. I seem to be looking through the surface of atomic phenomena into a strangely beautiful interior world. A world of pure mathematical structures. I’m too excited to sleep. I go down to the southern end of the island. There’s a rock jutting out into the sea that I’ve been longing to climb. I get up it in the half-light before the dawn, and lie on top, gazing out to sea.