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NAZI leaders were kept in the dark about how far Germany’s nuclear physicists had got in their work on an atomic bomb. According to a new account of German nuclear research during the Second World War, physicist Werner Heisenberg hid information from Nazi leaders about how to build an atomic bomb. The account relies heavily on secretly taped conversations among German scientists interned in England after the war.

Ten German scientists involved in nuclear research, including three Nobel laureates, were interned for six months in 1945 at Farm Hall, a country house near Cambridge. All the rooms contained hidden microphones. Summaries and partial transcripts from the tapes were released earlier this year.

The tapes have provided raw material for both sides in an emotional debate over whether German scientists tried to build an atomic bomb for Hitler. Samuel Goudsmit, a physicist born in the Netherlands but working in the US, had full access to them while writing an account of the German effort in the 1940s. Goudsmit argued that the German physicists showed no moral objection to building a bomb; they simply had no idea how to go about it. When the Farm Hall documents were released in February, press accounts tended to support this view. The transcripts show vividly that Heisenberg and his colleagues were stunned when they heard that the US had dropped an atomic bomb on Hiroshima.

But Thomas Powers, author of a forthcoming book on the German programme and co-author of a new interpretation of the tapes in the Bulletin of the Atomic Å®ÉúСÊÓƵs, says that Goudsmit was blinded by his wartime obsession with the danger of a German bomb. In reality, he says, the tapes seem to confirm Heisenberg’s later claims; that he tried to keep his research programme free of military control, and avoided working on an atomic bomb.

The crucial period in the Farm Hall documents covers the days just after the bombing of Hiroshima. Walther Gerlach, a physicist from Munich who was officially in charge of German research on uranium, acted like ‘a defeated general’, according to another interned scientist, Max von Laue. Later that night, Heisenberg and Otto Hahn of the Kaiser Wilhelm Institute in Berlin discussed Gerlach’s reaction. Heisenberg told Hahn that ‘Gerlach was the only one of them who had really wanted a German victory’.

The conversation then took a remarkable turn as Heisenberg told Hahn how one might build an atomic bomb. He described how a sphere of uranium-235 with a diameter of 54 centimetres, weighing about a tonne, could sustain a chain reaction of 80 collisions, using ‘very fast neutrons’, generating 1024 neutrons. But only a quarter of a tonne would be necessary, he said, if the uranium were covered with a ‘reflector’. The bombs could be made to explode at the right moment by bringing together two halves which were too small to generate a chain reaction when separated.

Both the reflector and the idea of bringing together two subcritical masses of uranium at the right moment are essential parts of the bomb. According to Powers and historian Stanley Goldberg, this conversation reveals how much Heisenberg knew about the bomb, and how much he hid from German political authorities. Hans Bethe, one of the main theoreticians for the American bomb programme, the Manhattan Project, said after reviewing the Farm Hall documents: ‘Heisenberg knew a lot more than I have always thought’.

A week later, Heisenberg gave a seminar on this topic to the assembled scientists. Most of them, especially Gerlach, seemed to be hearing about the possibility of atomic weapons for the first time. According to Goldberg, the transcripts show that ‘for a lot of Heisenberg’s colleagues, this was new. Therefore, he must not have told them.’

After the war, Heisenberg wrote in Nature that there were simple practical reasons why Germany never embarked on a full-scale bomb programme. Under wartime conditions, it would have been impossible to build the huge industrial infrastructure of the US’s Manhattan Project. But he also wrote that the physicists themselves ‘had consciously striven to keep control of the project’ and avoided work on a bomb, preferring to work on reactors and cyclotrons.

At the time, Heisenberg’s claims were met with ‘derision’ outside Germany, says Goldberg. Philip Morrison, who worked at Los Alamos, wrote that Heisenberg and his colleagues ‘worked for the military as best their circumstances allowed. But the difference, which it will never be possible to forgive, is that they worked for the cause of Himmler and Auschwitz’.

Goldberg and Powers, however, say that there is increasing evidence that Heisenberg was indeed telling the truth. ‘I think that Heisenberg guided that programme into a closet until the war was over,’ says Powers.