Bohr Margrethe!
Margrethe No! When he first came in 1924 he was a humble assistant lecturer from a humiliated nation, grateful to have a job. Now here you are, back in triumph—the leading scientist in a nation that’s conquered most of Europe. You’ve come to show us how well you’ve done in life.
Bohr This is so unlike you!
Margrethe I’m sorry, but isn’t that really why he’s here? Because he’s burning to let us know that he’s in charge of some vital piece of secret research. And that even so he’s preserved a lofty moral independence. Preserved it so famously that he’s being watched by the Gestapo. Preserved it so successfully that he’s now also got a wonderfully important moral dilemma to face.
Bohr Yes, well, now you’re simply working yourself up.
Margrethe A chain reaction. You tell one painful truth and it leads to two more. And as you frankly admit, you’re going to go back and continue doing precisely what you were doing before, whatever Niels tells you.
Heisenberg Yes.
Margrethe Because you wouldn’t dream of giving up such a wonderful opportunity for research.
Heisenberg Not if I can possibly help it.
Margrethe Also you want to demonstrate to the Nazis how useful theoretical physics can be. You want to save the honour of German science. You want to be there to reestablish it in all its glory as soon as the war’s over.
Heisenberg All the same, I don’t tell Speer that the reactor …
Margrethe … will produce plutonium, no, because you’re afraid of what will happen if the Nazis commit huge resources, and you fail to deliver the bombs. Please don’t try to tell us that you’re a hero of the resistance.
Heisenberg I’ve never claimed to be a hero.
Margrethe Your talent is for skiing too fast for anyone to see where you are. For always being in more than one position at a time, like one of your particles.
Heisenberg I can only say that it worked. Unlike most of the gestures made by heroes of the resistance. It worked! I know what you think. You think I should have joined the plot against Hitler, and got myself hanged like the others.
Bohr Of course not.
Heisenberg You don’t say it, because there are some things that can’t be said. But you think it.
Bohr No.
Heisenberg What would it have achieved? What would it have achieved if you’d dived in after Christian, and drowned as well? But that’s another thing that can’t be said.
Bohr Only thought.
Heisenberg Yes. I’m sorry.
Bohr And rethought. Every day.
Heisenberg You had to be held back, I know.
Margrethe Whereas you held yourself back.
Heisenberg Better to stay on the boat, though, and fetch it about. Better to remain alive, and throw the lifebuoy. Surely!
Bohr Perhaps. Perhaps not.
Heisenberg Better. Better.
Margrethe Really it is ridiculous. You reasoned your way, both of you, with such astonishing delicacy and precision into the tiny world of the atom. Now it turns out that everything depends upon these really rather large objects on our shoulders. And what’s going on in there is …
Heisenberg Elsinore.
Margrethe Elsinore, yes.
Heisenberg And you may be right. I was afraid of what would happen. I was conscious of being on the winning side … So many explanations for everything I did! So many of them sitting round the lunch-table! Somewhere at the head of the table, I think, is the real reason I came to Copenhagen. Again I turn to look .… And for a moment I almost see its face. Then next time I look the chair at the head of the table is completely empty. There’s no reason at all. I didn’t tell Speer simply because I didn’t think of it. I came to Copenhagen simply because I did think of it. A million things we might do or might not do every day. A million decisions that make themselves. Why didn’t you kill me?
Bohr Why didn’t I …?
Heisenberg Kill me. Murder me. That evening in 1941. Here we are, walking back towards the house, and you’ve just leapt to the conclusion that I’m going to arm Hitler with nuclear weapons. You’ll surely take any reasonable steps to prevent it happening.
Bohr By murdering you?
Heisenberg We’re in the middle of a war. I’m an enemy. There’s nothing odd or immoral about killing enemies.
Bohr I should fetch out my cap-pistol?
Heisenberg You won’t need your cap-pistol. You won’t even need a mine. You can do it without any loud bangs, without any blood, without any spectacle of suffering. As cleanly as a bomb-aimer pressing his release three thousand metres above the earth. You simply wait till I’ve gone. Then you sit quietly down in your favourite armchair here and repeat aloud to Margrethe, in front of our unseen audience, what I’ve just told you. I shall be dead almost as soon as poor Casimir. A lot sooner than Gamow.
Bohr My dear Heisenberg, the suggestion is of course …
Heisenberg Most interesting. So interesting that it never even occurred to you. Complementarity, once again. I’m your enemy; I’m also your friend. I’m a danger to mankind; I’m also your guest. I’m a particle; I’m also a wave. We have one set of obligations to the world in general, and we have other sets, never to be reconciled, to our fellow-countrymen, to our neighbours, to our friends, to our family, to our children. We have to go through not two slits at the same time but twenty-two. All we can do is to look afterwards, and see what happened.
Margrethe I’ll tell you another reason why you did uncertainty: you have a natural affinity for it.
Heisenberg Well, I must cut a gratifyingly chastened figure when I return in 1947. Crawling on my hands and knees again. My nation back in ruins.
Margrethe Not really. You’re demonstrating that once more you personally have come out on top.
Heisenberg Begging for food parcels?
Margrethe Established in Göttingen under British protection, in charge of post-war German science.
Heisenberg That first year in Göttingen I slept on straw.
Margrethe Elisabeth said you had a most charming house thereafter.
Heisenberg I was given it by the British.
Margrethe Your new foster-parents. Who’d confiscated it from someone else.
Bohr Enough, my love, enough.
Margrethe No, I’ve kept my thoughts to myself for all these years. But it’s maddening to have this clever son forever dancing about in front of our eyes, forever demanding our approval, forever struggling to shock us, forever begging to be told what the limits to his freedom are, if only so that he can go out and transgress them! I’m sorry, but really .… On your hands and knees? It’s my dear, good, kind husband who’s on his hands and knees! Literally. Crawling down to the beach in the darkness in 1943, fleeing like a thief in the night from his own homeland to escape being murdered. The protection of the German Embassy that you boasted about didn’t last for long. We were incorporated into the Reich.
Heisenberg I warned you in 1941. You wouldn’t listen. At least Bohr got across to Sweden.
Margrethe And even as the fishing-boat was taking him across the Sound two freighters were arriving in the harbour to ship the entire Jewish population of Denmark eastwards. That great darkness inside the human soul was flooding out to engulf us all.
Heisenberg I did try to warn you.
Margrethe Yes, and where are you? Shut away in a cave like a savage, trying to conjure an evil spirit out of a hole in the ground. That’s what it came down to in the end, all that shining springtime in the 1920s, that’s what it produced—a more efficient machine for killing people.
Bohr It breaks my heart every time I think of it.
Heisenberg It broke all our hearts.
Margrethe And this wonderful machine may yet kill every man, woman, and child in the world. And if we really are the centre of the universe, if we really are all that’s keeping it in being, what will be left?
Bohr Darkness. Total and final darkness.
Margrethe
Even the questions that haunt us will at last be extinguished. Even the ghosts will die.
Heisenberg I can only say that I didn’t do it. I didn’t build the bomb.
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 Ex
cept 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.
Copenhagen Page 7