The First War of Physics (14 page)

The Norwegians’ response, when it came, was rather puzzling:

IF YOU CAN ASSURE US THAT IT IS OF IMMEDIATE IMPORTANCE TO THE PRESENT WAR WE WILL GET THE INFORMATION YOU REQUEST AT ONCE STOP IF IT IS ONLY FOR THE ICI THEN PLEASE REMEMBER THAT BLOOD IS THICKER THAN HEAVY WATER STOP

Norsk Hydro and Britain’s Imperial Chemical Industries (ICI) were commercial rivals. The Norwegian resistance simply wanted to ensure that they weren’t engaged in industrial spying on behalf of British commercial interests. Tronstad was subsequently identified as the author of this message, although it seems that he did not join Operation Skylark until after the response was sent.

Tronstad did not immediately see the significance of Welsh’s request. He had trained in chemistry in Berlin, Stockholm and Cambridge and had been appointed professor of inorganic chemistry at the Norwegian Institute of Technology in May 1936. Although he had been intimately involved with the development and operation of the Vemork facility, and few scientists knew the physical properties of heavy water better than he,
he was not a nuclear physicist and was probably unaware of the substance’s potential as a moderator in a nuclear reactor.

In any case, he didn’t have much time to ponder. The Gestapo uncovered and shut down Operation Skylark in September 1941, and Tronstad was obliged to escape with his family to Britain via neutral Sweden the following month.

In Britain, Tronstad met with Welsh, who briefed him about the significance of heavy water and reassured him that details of the design of the Vemork plant were required in order to ensure that production was stopped, not because ICI were interested in obtaining Norsk Hydro’s industrial secrets. Tronstad was convinced, and told the SIS all he knew.

Barbarossa

Hitler’s sustained aerial bombardment of Britain began to wind down in May 1941. At the end of this period over 40,000 civilians had been killed and one million homes damaged or destroyed. Britain was not beaten, but it was bowed. Some among Hitler’s military advisers urged that Britain should be finished off completely, but the country was on its knees and hardly likely to pose a threat in the immediate future. Besides, Hitler himself had become impatient. He had written in
Mein Kampf
of his intention to invade Russia and he didn’t want to put this off any longer.

He launched Operation Barbarossa on 22 June 1941, deploying about three million German soldiers against a similar number of Soviet troops. The Nazi–Soviet pact was shown to be what everyone knew it to be the day it had been signed by Molotov and Ribbentrop – a sham.

Stalin had anticipated a German invasion, and had used the Nazi–Soviet pact to buy time. The Red Army was still recovering from the consequences of the Great Purge, and had not fared particularly well in the invasion of Finland in the winter of 1939—40. The contrast with the ruthless efficiency of the German war machine in its conquest of the rest of mainland Europe was stark, and plain for all to see. Stalin needed time to restructure his armed forces and rearm them, but had been wary of committing to a full mobilisation for fear of precipitating the very attack he was hoping to postpone.
It may be that he had at least anticipated a warning, an ultimatum from Berlin that would have allowed time to mobilise his forces.

There was no warning. The Red Army fell back in confusion as German forces swept through the Baltic States and into Russia itself. The German invasion was organised around three army groups supported by the Luftwaffe. Army Group North forged north-eastwards from East Prussia towards Leningrad. Army Group Centre headed for Moscow. Army Group South headed south-eastwards from Poland towards the Crimean Peninsula and the oilfields of the Caucasus.

Stalin was utterly shocked by the speed and ferocity of the attack. He had justified his brutal purges as necessary preparation for the defence of the Soviet Union. Now that defence had been tested, and found wanting. He waited for twelve days before broadcasting an unprecedented appeal to the Soviet people, calling them brothers and sisters and urging them to rally to the call of the party of Lenin and Stalin.

The Soviets declared a Great Patriotic War. The invasion led to the opening of the Eastern Front, set to become the largest theatre of war in human history.

I’m afraid it went badly wrong

Progress had been slow for the Uranverein through the summer and autumn of 1941. The German War Office had placed an order for 1,500 kilos of heavy water but by the end of the year only 360 kilos had been delivered. The German physicists now had plenty of powdered uranium metal, obtained from the Degussa company, but Heisenberg elected to use the first of the heavy water that was now becoming available in conjunction with uranium oxide.

In the late summer of 1941 Heisenberg and Döpel set up experiment L-II using the same reactor configuration they had used almost a year before, this time in an aluminium sphere about 75 centimetres in diameter containing a little over 140 kilos of uranium oxide and about 160 kilos of heavy water. The results were again negative, but after correcting for the absorption of neutrons by the aluminium their calculations suggested the
merest hint of neutron multiplication. The physicists sensed they were on the right track. This was little more than intuition, a ‘gut feeling’, but Heisenberg later stated that: ‘It was from September 1941 that we saw an open road ahead of us, leading to the atomic bomb.’

Heisenberg probably did not doubt that German science was already far ahead of anything that physicists might have done in Britain or America. This left him in a unique and, possibly, quite uncomfortable position. It was Weizsäcker who urged him to consult his former mentor, Niels Bohr. But Heisenberg’s reasons for seeking such a meeting may have been quite complex.

Bohr had preferred to remain at his Institute for Theoretical Physics in Copenhagen following the German occupation of Denmark in April 1940. Of Jewish descent, Bohr was considered a ‘non-Aryan’ by the occupation forces but, like all 8,000 Danish Jews, was protected – at least temporarily – by the agreement reached with the Danish government designed to preserve the fiction that the Nazis were there by invitation.

That Heisenberg and Weizsäcker were concerned for Bohr’s welfare is not in doubt. Growing moral qualms about the work the Uranverein was doing may also have played a part in Heisenberg’s decision to visit Bohr in Copenhagen. In his memoir, written almost 30 years after the event, Heisenberg recalled Weizsäcker’s proposal. ‘It might be a good thing,’ Weizsäcker had said, ‘if you could discuss the whole subject with Niels in Copenhagen. It would mean a great deal to me if Niels were, for instance, to express the view that we are wrong and that we ought to stop working with uranium.’

According to Heisenberg, his primary purpose was to seek guidance from Bohr on the morality of working on scientific problems that could have ‘grave consequences in the technique of war’. As Heisenberg’s Uranverein colleague Peter Jensen later put it: Heisenberg, the high priest of German theoretical physics, sought absolution from his Pope. Or, perhaps, as Peierls later suggested: ‘[Heisenberg] had agreed to sup with the devil, and perhaps he found that there was not a long enough spoon.’

Heisenberg looked forward with great eagerness to the prospect of talking to his former mentor. To him, and indeed to many other physicists
of his generation, Bohr had long been something of a father-figure. As Heisenberg’s wife Elisabeth later wrote:

In Tisvilde, the beautiful vacation home of the Bohrs, he had played with their children and taken them for rides on a pony wagon: he had gone for long sailing trips on the ocean with Bohr, and Niels had visited him in his ski cottage; together they had grappled with the problems of physics, and he thought he could talk about anything with Bohr.

But there may have been other reasons for seeking a meeting. It is possible that, alarmed by reports of American efforts to build an atomic bomb that had appeared in the Swedish press, Heisenberg and Weizsäcker may have also wanted to discover what Bohr knew.

Having decided to pay Bohr a visit, Heisenberg now faced a number of practical hurdles. Although Germany dominated much of continental Western Europe, travel was restricted and the authorities were initially reluctant to let Heisenberg make the trip. Weizsäcker proposed a potential solution. He had already given several lectures in occupied Copenhagen, the most recent being a lecture on the philosophical implications of quantum theory at Bohr’s institute. At Weizsäcker’s prompting, an invitation was issued to both Heisenberg and Weizsäcker to participate in a symposium on astronomy, mathematics and theoretical physics at the newly-formed German Cultural Institute in Copenhagen.

The proposal was initially rejected by the Reich Ministry of Education but, after some arm-twisting by the German Foreign Office (whose officials suggested that State Secretary Ernst von Weizsäcker – Carl Friedrich’s father – might intervene), approval was granted, provided Heisenberg kept a low profile and stayed only a few days.

Despite this proviso, Heisenberg arrived in Copenhagen early on Monday, 15 September 1941, four days before the conference was due to start. As he described it in a letter to his wife that he composed at various times during his trip, it was a journey into the recent past:

Here I am once again in the city which is so familiar to me and where a part of my heart has stayed stuck ever since that time fifteen years ago. When I heard the bells from the tower of city hall for the first time again, close to the window of my hotel room, it gripped me tight inside, and everything has stayed so much the same as if nothing out there in the world had changed. It is so strange when suddenly you encounter a piece of your own youth, just as if you were meeting yourself.

Such was his eagerness to see Bohr that he made his way to Bohr’s Carlsberg residence that first night, walking through the darkened city under a clear and starry sky. He was relieved to discover that Bohr and his family were doing fine. Their conversation quickly turned to the ‘human concerns and unhappy events of these times’. In his letter to his wife he expressed some dismay that ‘even a great man like Bohr can not separate out thinking, feeling, and hating entirely’, but then added: ‘But probably one ought not to separate these ever.’

But Heisenberg remained stubbornly insensitive to the perceptions and feelings of his former colleagues. In occupied Copenhagen in September 1941, with much of Europe conquered by Axis forces, with the German Army Group North just seven miles from Leningrad, with Kiev encircled and the assault on Moscow about to begin, it was not difficult to conceive a German victory. There was a certain startling inevitability about the impending Nazi domination of Europe, and everything that this implied.

Heisenberg, the pragmatist, had made his bargain long ago. And here he now was, a representative of German culture, in Copenhagen at the behest of the German Cultural Institute, promulgating what many Danes may have perceived to be thinly-veiled Nazi propaganda. Surely, Heisenberg may have reasoned, in the face of the inevitability of Nazi victory it was in the best interests of themselves and of physics for his former colleagues in occupied countries to make their bargain too?

Bohr and his colleagues boycotted the formal conference proceedings, but Heisenberg still sought them out. He visited Bohr’s institute and joined some of the physicists for lunch on a couple of occasions. Among those present were Christian Møller and Stefan Rozental. They later had bitter
recollections of the discussion: ‘[Heisenberg] stressed how important it was that Germany should win the war … the occupation of Denmark, Norway, Belgium and Holland was a sad thing but as regards the countries in Eastern Europe it was a good development because these countries were not able to govern themselves.’

It was during Heisenberg’s second meeting with Bohr on the Wednesday evening that he raised the issue of atomic weapons. Their subsequent recollections of this highly-charged meeting were vague and contradictory. Heisenberg remembers that they took an after-dinner stroll, principally to avoid the risk of Gestapo surveillance. Bohr believed the conversation had taken place in his study. It would have made sense that Heisenberg would want to talk somewhere more secure, away from prying ears or listening devices, because when he raised the question of the military application of atomic energy with Bohr he was in principle committing an act of treason.

The conversation got off to a poor start. And then it got rapidly worse. Bohr had heard about Heisenberg’s insensitive remarks, and became angry when Heisenberg not only defended Germany’s invasion of the Soviet Union but further argued that it would be a good thing if Germany were to win the war. When Heisenberg finally raised the question of working on an atomic bomb, Bohr was completely and utterly shocked.

As far as Bohr understood, he had already shown in 1939 that achieving an explosive nuclear chain reaction would ‘take the entire efforts of a country’. Yet here was his friend and former colleague, with whom he had shared some of the most thrilling moments of scientific discovery in his life, explaining with some impatience that a bomb was possible and that he was working on it for the Nazis. In a letter that Bohr composed to Heisenberg long after the war, but which he never sent, he wrote:
⁶

… 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 said that there was no need to talk about details since you were completely familiar with them and had spent the past two years working more or less exclusively on such preparations.

Heisenberg may have even drawn a sketch to explain his work, though this now seems quite doubtful. When Bohr produced this some years later it appeared to be a sketch of a reactor, but whether Heisenberg intended it or Bohr misunderstood, Bohr assumed it was a sketch of an atomic bomb. Even worse was to come. Heisenberg appeared to be probing Bohr for any information about an Allied bomb programme. Was this, after all, an intelligence mission? On whose authority was Heisenberg now acting?