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Originally published in “Tuttoscienze” of “La Stampa”, on 28 August, 2002

The successful building of the first American atom bomb was one of the enterprises of the Century that certainly arouses a sense of guilt, but it never provoked the furious polemics which still surround the unsuccessful building of the German atom bomb. These controversies have particularly surrounded two Nobel Prizes winners who are considered not only two great minds in the physics of the twentieth century, but also two great persons: the German Werner Heisenberg, who worked on the Nazi bomb, and the Danish Niels Bohr, mentor and friend of Heisenberg, who in 1943 escaped from German-occupied Denmark and went to America to work on the bomb for the Allies. The debate on this theme has continued without interruption from the Fifties, after the publication of the book “Brighter than a thousand suns”, in which the journalist Robert Jungk proposed the idea that the German scientists had attempted to prevent the building of a Nazi bomb for moral reasons. The debate has remained evergreen over the years thanks to documents, some more illuminating than others, such as the Farm Hall transcript, published only in 1992. From July to December 1945, Heisenberg and other important German scientists were interned as prisoners of the Allies in a British country house (Farm Hall) and all their conversations were recorded, without their knowledge, by the British Intelligence Service. In 1998, the theatre piece “Copenhagen” by the British playwright Michael Frayn was one of a deluge of treatments focusing on the same theme . And “Copenhagen” also arouses strong polemics and criticisms, to some of which Frayn replied: “Bohr will continue to inspire respect and love in spite of his involvement in the building of the Hiroshima and Nagasaki bombs, and [Heisenberg] will continue to be regarded with distrust in spite of his failure to kill anybody”. As a consequence of the controversy that Copenhagen aroused, in February 2002, the Bohr family decided to publish the letters that Bohr wrote to Heisenberg after the publication of Jungk’s book, letters that Bohr never sent to the addressee. Bohr, Heisenberg and almost all the protagonists of the unsuccessful Nazi bomb are dead. One only authoritative witness of those events remains: Carl Friedrich von Weizsaecker, first-rate nuclear physicist, pupil and friend of Heisenberg, and son of Ernst von Weizsaecker, the highest official in the Foreign Ministry under Minister von Ribbentrop from 1938 to 1943. Despite his venerable age of 90 years, von Weizsaecker agreed to tell us his version of the facts in this telephone interview.

Professor Weizsaecker when did you begin to work on the bomb?

In September 1939, when the war started, I was immediately drafted into the army, but after two weeks they recalled me to work on a project regarding the weapons.

What relationships did you have with the Deutsche Physik, the pro-Nazi current of the German physics which condemned Relativity and Quantum Mechanics as “Jewish Physics”?

The Deutsche Physik was an absurdity and I did not have any relationship with it.

How did you know about the possibility of using the chain reaction to make an atom bomb?

Well, I was a nuclear physicist and all the physicists all around the world, who understood the work of Otto Hahn about the nuclear fission, had realised this possibility immediately.

Why did you, Heisenberg and Hahn accept to work on the bomb for the Nazis?

We realised very soon that this weapon was possible in principle and, since we expected that the English and the Americans would try to build it, we thought that Germany should also have that ability. But this was not due to Hitler, for us Hitler had always been an evil person, we thought that the bomb might be necessary for our nation.

As a deterrent?

We could not know.

But your nation was under Hitler’s domination. Did you not fear giving him a lethal weapon?

I can just say that, when I began to work on the bomb, I had the idea that, if perhaps I could make it, I could perhaps speak with Hitler and to convince him to adopt a better policy, but this idea was wrong, and, when we realised that we could not make the bomb, I was very happy not to end up in the difficult situation of having to speak with Hitler.

Why did you not make the bomb?

We did not have the necessary resources. I think that, for the bomb, the Americans spent thousand times the money that we spent in Germany and we could not spend so much. And we were very surprised when we learned that the Americans had made the bomb in such a short time.

However, you also had scientific problems, problems with the separation of the uranium isotopes, and the Allies sabotaged your plans for the production of heavy water.


When did the programme for the building of the bomb finish?

We came to the conclusion after one year and a half, more or less, that we would not be able to make it. And only after having realised this, Heisenberg decided to speak with Bohr.

Let’s speak about that famous meeting between Bohr and Heisenberg in Copenhagen in September 1941.

I accompanied Heisenberg to Copenhagen but not to that meeting. Heisenberg did not have the certainty that the English and the Americans were not able to make the bomb and he could not speak with his American and English friends, because they probably would not believe him. However, he believed that if he had told Bohr that we Germans could not make the bomb, Bohr would have spoken with the Allies and told them not to make the bomb, since we were not able to make it.

When the talk took place, were they free to speak?

In a way, yes, Heisenberg could speak knowing that the German military authorities did not know the scope of the talk and the conversation took place during a walk. However, Heisenberg used very indirect language, knowing that he risked his neck talking about a secret project to a foreigner.

In one of the letters released in February and regarding the meeting in question, Bohr writes to Heisenberg: “ you and Weizsaecker expressed your definite conviction that Germany would win [the war] and that it was quite foolish for us……to be reticent as regards all German offers of cooperation. I also remember quite clearly our conversation in my room at the Institute, where in vague terms you spoke in a manner that could only give me the firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons and that….you had spent the past two years working more or less exclusively on such preparations” (with thanks to the Niels Bohr Archive of Copenhagen for the permission to quote the contents of the letters, which can be consulted on the web site www.nba.nbi.dk). Concluding, from the letters of Bohr, a complete different version of that meeting arises. How is it possible?

When Heisenberg began to speak about nuclear reactions, Bohr said immediately: “I do not want to speak with you about that”, and after the war I knew from one of Bohr’s sons that Bohr thought that Heisenberg wanted to induce him to work on the atom bomb with the Germans, and, since he would not work on it, he stopped Heisenberg immediately.

But according to the letters, Bohr listened to Heisenberg in silence and he did not stop him immediately.

Well, Bohr did not stop Heisenberg immediately. Heisenberg began to speak about nuclear reactions and after a while he felt that Bohr did not want to continue.

You and Heisenberg were two Germans working on the atom bomb and, when you went to Copenhagen, Denmark was under Nazi occupation. Your position could obviously leave room for ambiguity. Why did Heisenberg not go direct to his message?

I was not present at the talk but certainly it was too dangerous, as I said before.

Did Heisenberg say to Bohr that Germany would win the war?

In September ’41, Germany had just defeated France, and therefore Heisenberg could also have said such a thing to Bohr, but at the beginning of the war, he was convinced that Hitler was running into a catastrophe, and this turned out to be true, in the end.

Another mystery of that meeting is the drawing of a reactor that Heisenberg is supposed to have given to Bohr. Do you know anything about this?


According to Thomas Power’s book “Heisenberg’s war”, Heisenberg “boycotted” the German bomb by deliberately hiding his expertise from the Nazis. But did Heisenberg really know how to make the bomb?

All the good physicists knew how to make it , in principle.

Well, in principle, yes, but in practise?

In principle, Heisenberg was able to make it, but he was conscious that we could not make it , given the conditions in which we were working. For my part, I was very happy when I realised that we could not make it, because I would not come to the moral problem.

You say this, but the Manhattan Project for the development of the American bomb was conceived and largely built by refugees from fascist Europe, terrified by the idea of a Nazi bomb. If you never perhaps began to work on it….

The Manhattan Project was absolutely indipendent from our work! It was the result of the same conclusions that we drew in Germany. Since the atomic bomb was possible, we expected that the English and the Americans wanted to make it, and the Allies were expecting a German bomb, and so they started their Project immediately, determined to have a project certainly stronger than the Germans.

The day after the Hiroshima bomb was dropped, according to the Farm Hall transcripts, you said: “History will record that the Americans and the English made a bomb, and that at the same time the Germans, under the Hitler regime, produced a workable engine. In other words, the peaceful development of the uranium engine was made in Germany under the Hitler regime, whereas the Americans and the English developed this ghastly weapon of war”. Do you still think that this is a fair judgement, since you worked on the bomb?

We worked on the problem, but we came to the conclusion that we could not make it and we were very happy.