Michael Frayn - Copenhagen

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.