The First War of Physics (51 page)

On 6 July, a few days after their arrival, Diebner wondered if there were any hidden microphones installed there. Heisenberg laughed.

Guests at Farm Hall

The principal purpose of Operation Epsilon was to discover the full extent of the German atomic programme. It was anticipated that in informal conversation between themselves the Uranverein physicists might reveal details of the programme that they had perhaps chosen to withhold during interrogation and which were not revealed in the many documents seized by the Alsos mission.

Rittner prepared detailed, top secret reports to accompany the transcripts of the physicists’ conversations. The reports commented on the morale of the physicists, their personalities, attitudes to the Allies, allegiance to the Nazi Party, their hopes and fears for the future, speculation on the reasons for their detention, their views on current events and, of course, their discussions on atomic physics.

The physicists were still ‘guests’. They could not be considered as prisoners of war, as none of the ten scientists had been in the German
military. They were neither suspected nor accused of any crime. The British authorities had instead cited a particularly flexible wartime law which allowed detention of specified individuals for up to six months at ‘His Majesty’s pleasure’.

The physicists had committed in writing to remain at Farm Hall and understood that if any one of them attempted to escape the result would be a considerable restriction of their liberty. But life at Farm Hall was comfortable. Each internee had a prisoner-of-war batman. The food was good. There was a small library and a tennis court in the grounds. There was a piano in the common room that Heisenberg would play. The physicists once again settled into a routine.

It was still not clear to them why they had been detained, and speculation inevitably inflated their sense of their own importance. They knew nothing of the Manhattan Project and had no idea of the frantic energy being expended in New Mexico as their erstwhile scientific colleagues prepared for the Trinity test. They imagined their work on the ‘uranium problem’ to be so far advanced and therefore so important that their fates were surely to be discussed and decided at the forthcoming meeting of the Allied heads of state at Potsdam.

‘It’s quite possible that they just don’t want to say anything’, Diebner said on 6 July.

‘Then of course they will have to wait until everything has been settled by the “Big Three”,’ said Korsching, a reference to the impending meeting of Truman, Churchill and Stalin.

‘I think the right thing in that case,’ continued Diebner, ‘would be for the English to give us a hint in some way. They may not be able to say it openly because of Comrade Stalin.’ Diebner’s biggest fear was that they would be sent back to Germany, where they might be forced to work on atomic physics for the Soviet Union.
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Heisenberg echoed Diebner’s fears. ‘It is possible that the “Big Three” will decide it at Potsdam,’ he said, ‘and that Churchill will come back and say: “Off you go, the whole group is to return to Berlin” and then we’ll be in the soup.’ Berlin was in the Soviet occupation zone.

That they led the world in atomic physics was an impression further emphasised by the young physicists Bagge and Korsching on 21 July, five days after the successful Trinity test.

‘I am convinced [the Anglo-Americans] have used these last three months mainly to imitate our experiments’, said Bagge.

‘Not even that’, said Korsching. ‘They used them to discuss with their experts their possibilities and to study the secret documents. They probably examined a few specimens of our uranium blocks. From these specimens they can see for instance whether the [reactor] has been running already. It could have been run; the blocks must have undergone some internal chemical change.’

Korsching’s logic was that they were being detained because their knowledge presented the Allies with a threat. ‘But there are many military men in England,’ he continued, after some further exchanges with Bagge, ‘who say “Once we let those swine go back then they’ll construct the uranium [reactor] and in the end they’ll blow it up.” They might also say “These people are so clever that our guard troops will be blown up with it, but not they themselves.”’

Their illusions were about to be completely shattered.

The announcement

Just before dinner on 6 August, Rittner took Hahn to one side and advised him that the Allies had used an atomic bomb against Japan earlier that day. Hahn was distraught. As one of the principal discoverers of nuclear fission he felt personally responsible for the deaths of hundreds of thousands of people. He contemplated suicide, before calming down with a few drinks. ‘If the Americans have a uranium bomb then you’re all second raters’, he told the rest of the group over dinner. ‘Poor old Heisenberg.’

Their reactions went through the classic cycle of shock, denial, dawning realisation, incredulity and, eventually, comprehension. At 9:00pm, they listened to the radio:

Here is the news … The greatest destructive power devised by man went into action this morning – the atomic bomb. British, American and Canadian scientists have succeeded, where Germans failed, in harnessing the basic power of the universe … The bomb, dropped on the Japanese war base of Hiroshima, was designed for a detonation equal to twenty thousand tons of high explosive
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… The Allies have spent five hundred million pounds
³
on what President Truman calls the greatest scientific gamble in history – and they’ve won … Up to a hundred and twenty-five thousand people helped to build the factories … Mr Stimson, American Secretary for War, announces that
uranium
is used in making the bomb …

The speculation now raged as to how the Allies had done it. Was it really true? Was it actually a uranium bomb? Or had they separated enough plutonium to make a plutonium bomb? If so, then surely they must have succeeded in getting a uranium reactor to work some time ago?

‘I think it is dreadful of the Americans to have done it’, said Weizsäcker. ‘I think it is madness on their part.’

‘One can’t say that’, Heisenberg replied. ‘One could equally say “That’s the quickest way of ending the war.”’

‘That’s what consoles me’, said Hahn.

Their discussion turned to practicalities. Heisenberg remained doubtful. ‘I still don’t believe a word about the bomb but I may be wrong. I consider it perfectly possible that they have about ten tons of enriched uranium, but not that they can have ten tons of pure U-235.’

Heisenberg’s comment may have belied a degree of forgetfulness on his part. In their February 1942 report to German Army Ordnance, the Uranverein physicists had correctly estimated that a fission weapon could be constructed from between ten and 100 kilos of fissile material. Heisenberg himself was reported to have told the audience of high-ranking military figures at Harnack House in June 1942 that the mass of active material for an atomic bomb would need to be about the size of a pineapple. And yet now he seemed to have drifted back to a much earlier, even pre-war conclusion, that a bomb would require tons of U-235. He doubted that the Americans had managed to separate anything like this quantity.

Hahn, for one, was puzzled. ‘I thought that one needed only very little “235”’, he said.

‘If they enrich it slightly,’ Heisenberg replied, ‘they can build [a reactor] which will go but with that they can’t make an explosive which will—’

‘But if they have, let us say, 30 kilograms of pure “235”’, Hahn interjected, ‘couldn’t they make a bomb with it?’

Hahn was right, of course. Frisch and Peierls had discovered this much in March 1940. And, just as clearly, someone in the Uranverein had discovered much the same by early 1942. But Heisenberg was still uncertain.

‘But it wouldn’t go off,’ he declared, ‘as the mean free path is still too big.’ This was a reference to an altogether different – and erroneous – approach to calculating the critical mass, based on so-called random-walk diffusion theory. Heisenberg elaborated this approach in the subsequent discussion. If followed to its logical conclusion, the method suggests a totally impractical critical mass of about thirteen tons of U-235. Heisenberg was clearly confused, and it seems that he had at some stage worked on a different line of reasoning, as Hahn now reminded him.

‘But tell me why you used to tell me that one needed 50 kilograms of “235” in order to do anything’, Hahn asked.
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‘Now you say one needs two tons.’

Heisenberg needed more time to think. ‘I wouldn’t like to commit myself for the moment,’ he said, ‘but it is certainly a fact that the mean free paths are pretty big.’ His confusion and his doubts probably reflected the fact that he had paid relatively little attention to the nuclear programme in the latter stages of the war. He may have given no further thought to atomic weapons after mid-1942.

With the possible exception of Diebner (and Gerlach), the German physicists had after 1942 concluded that an atomic bomb was out of reach in any timeframe likely to have an impact on the war. By the modest objectives they had set for themselves, the German nuclear research programme could be considered relatively successful. But when compared to the achievements of the Manhattan Project, it was difficult to see the German programme as anything more than a failure. The German physicists now started to grasp for reasons for this failure.

‘We wouldn’t have had the moral courage to recommend to the government in the spring of 1942 that they should employ 120,000 men just for building the thing up’, Heisenberg remarked, a reference to the BBC news broadcast which had mentioned that up to 125,000 people had helped to build the atomic industry in America.

Weizsäcker followed with a comment that was eventually to shape the
Lesart
, literally the ‘version’ or ‘party line’, on the reason for their lack of success, a comment that was to provoke highly-charged debate for the next 60 years.

‘I believe the reason we didn’t do it,’ he said, ‘was because all the physicists didn’t want to do it, on principle. If we had all wanted Germany to win the war we would have succeeded.’

Hahn didn’t accept Weizsäcker’s argument. ‘I don’t believe that but I’m thankful we didn’t succeed’, he said.

As they cast about in the search for someone other than themselves to blame, Heisenberg pointed the finger at their political and military masters. ‘The point is that the whole structure of the relationship between the scientist and the state in Germany,’ he said, ‘was such that although we were not one hundred per cent anxious to do it, on the other hand we were so little trusted by the state that even if we had wanted to do it, it would not have been easy to get it through.’

Diebner agreed. ‘Because the official people were only interested in immediate results. They didn’t want to work on a long-term policy as America did.’

‘Even if we had gotten everything we wanted,’ Weizsäcker said, ‘it is by no means certain whether we would have gotten as far as the Americans
and English have now. It is not a question that we were very nearly as far as they were but it is a fact that we were all convinced that the thing could not be completed during the war.’

Heisenberg disagreed. ‘Well that’s not quite right’, he said. ‘I would say that I was absolutely convinced of the possibility of our making a uranium [reactor] but I never thought that we could make a bomb and at the bottom of my heart I was really glad that it was to be [a reactor] and not a bomb. I must admit that.’

‘If you had wanted to make a bomb we would probably have concentrated more on the separation of isotopes and less on heavy water’, said Weizsäcker.

At this point Hahn left the common room. Weizsäcker continued: ‘If we had started this business soon enough we could have got somewhere. It they were able to complete it in the summer of 1945, we might have had the luck to complete it in the winter of 1944–45.’

‘The result would have been that we would have obliterated London but still would not have conquered the world,’ remarked Wirtz, ‘and then they would have dropped them on us.’

Weizsäcker responded, picking up his earlier theme. ‘I don’t think we ought to make excuses now because we did not succeed, but we must admit that we didn’t want to succeed. If we had put the same energy into it as the Americans and had wanted it as they did, it is quite certain that we would have not succeeded as they would have smashed up the factories.’ This was probably a reference to the Allied sabotage operations against the Vemork plant, and subsequent bombing of German factories towards the end of the war.

Later in the same conversation, Wirtz further elaborated Weizsäcker’s
Lesart.
‘I think it is characteristic that the Germans made the discovery and didn’t use it,’ Wirtz said, ‘whereas the Americans have used it. I must say I didn’t think the Americans would dare to use it.’

Upset by Korsching’s criticism of the project’s leadership, Gerlach had left the common room and headed for his bedroom, where he was later heard sobbing. Hahn, Laue and Harteck joined him to offer their consolations. In his report, Rittner observed that Gerlach, who had once hoped
that their work on uranium would help to ‘win the peace’, now acted like a defeated general, whose only remaining option was to shoot himself.

As Harteck entered Gerlach’s room, Geriach asked: ‘Tell me, Harteck, isn’t it a pity that the others have done it?’

‘I am delighted’, he replied.

‘Yes,’ said Gerlach, ‘but what were we working for?’

‘To build [a reactor],’ said Hahn, ‘to produce elements, to calculate the weight of atoms, to have a mass spectrograph and radioactive elements to take the place of radium.’

‘We could not have produced the bomb but we could have produced [a reactor],’ said Harteck, ‘and I am sorry about that. If you had come a year earlier, Gerlach, we might have done it, if not with heavy water, then with low temperatures. But when you came it was already too late. The enemy’s air superiority was too great and we could do nothing.’

Lesart

Few of them appeared to get any sleep on the night of the announcement. Laue in particular was concerned about Hahn’s state of mind. Others were concerned for Gerlach. They stayed awake until the early hours of the next morning, so that they could be sure that Hahn would not take his own life. In his diary, Bagge wrote of a conversation with Laue at one o’clock in the morning: ‘When I was young,’ Laue had told him, ‘I wanted to do physics and experience world history. I have done physics, and I have witnessed world history. I can really now say that, in my old age.’