The Atomic Meeting of 1941 in Copenhagen. In the fall of 1941, the scientist Carl Fredrich von Weizsäcker wrote a secret report to Hitler’s army about his and Werner Heisenberg’s visit to Niels Bohr in Nazi occupied Copenhagen: “The technical extraction of energy from uranium fission is not being worked on in Copenhagen….
Obviously, Professor Bohr does not know we are working on these questions; of course, I encouraged him in this belief.” What Weizsäcker didn’t know (or was lying about) was that Bohr *did* know about their uranium work.
How did he know? Well, there are many conflicting stories about this meeting but in all of them Heisenberg told Bohr outright that he was working on military uses of Uranium fission for Hitler. This is the mystery that people have been trying to solve ever since.
Why, why, why, would someone tell a foreign scientist who was pretty vocally anti-Nazi and half Jewish to boot that they were working on a nuclear bomb *for* Hitler?! As the Heisenberg character said in the play “Copenhagen”: “No one understands my trip to Copenhagen.
Time and time again I’ve explained it…The more I’ve explained, the deeper the uncertainty has become.” (Get it?) However, due to the popularity of this play, new documents have been released that clarify the meeting tremendously, and, if you include Weizsäcker’s point of view and are willing to accept an unflattering view of Heisenberg, this visit and Heisenberg’s wildly inappropriate communication with Bohr can make a lot more sense.
Table of Contents
Heisenberg’s Restoration of Honor
I would like to start 2 years before that meeting in July, 1938. Hitler had been in power for 5 years and had already made it illegal for Jewish scientists to keep their jobs, and had labeled theoretical physics “Jewish science”.
In fact, almost exactly a year before, Heisenberg was attacked in a full-page “article” on “White Jews in Science” that called Heisenberg, “bacterial carriers of the Jewish spirit who must be eliminated just as the Jews themselves.” Even worse, after that article, the SS investigated Heisenberg! What most people didn’t know, including Heisenberg’s young wife, was that Heisenberg insisted on the investigation to “restore his honor” after the “white Jews” article.
It took a full year (and some family connections) but Heisenberg, and quantum mechanics, were given the green light in July, 1938 by Heinrich Himmler, the head of the SS. This, in my mind, was the turning point for Heisenberg. Although Heisenberg wasn’t particularly anti-Semitic, he was decidedly pro-German and also, deeply prejudiced against Russians who he considered barbaric and these feelings were exacerbated by his time in questioning.
Ironically, the SS investigation of Heisenberg gave Heisenberg anti-Nazi cred *while* it caused him to act like a Nazi (although Heisenberg never joined the party as he was always anti-Hitler).
It was also when he made his Faustian bargain: Heisenberg decided that he needed to support the Nazis so that theoretical Physics could survive in Germany, and he would excuse anything (ANYTHING) as long as they didn’t insult his “honor”.
Nine days after being cleared by Himmler, Heisenberg was in uniform as an Army officer and soon he was part of the group getting ready to attack Czechoslovakia. Luckily for Heisenberg, he did not have to go through with the attack as, in September, 1938, Hitler signed the “Munich Treaty” with England, France and Italy to get part of Czechoslovakia for a promise to stop there (bad idea).
Discovery of Nuclear Fission and Possibility of a Nuclear Reactor
In Denmark, Heisenberg’s longtime mentor and friend, Niels Bohr, was deeply concerned about the rise of Hitler. Bohr was a man of action and formed an organization back in 1933 to rescue persecuted scientists that eventually helped over 300 scientists including a young man named Otto Frisch and his aunt Lise Meitner.
I mention those two in particular, because on January 3rd, 1939, Frisch (who was working as Bohr’s assistant) returned from Christmas break with his aunt in Sweden with an amazing story.
Seems that Meitner’s longtime partner, Otto Hahn, had written her that their experiments with bombarding Uranium with slow neutrons with the hope of creating an even heavier element had actually made Barium, a much smaller element. Meitner and Frisch decided that Bohr’s model that the nucleus worked like a drop of water could be used to explain how the nucleus could split.
Meitner even determined the energy produced on a scrap of paper! When Frisch told Niels Bohr, Bohr exclaimed, “Oh what idiots we all have been! Oh, but this is wonderful! This is just as it must be!” Frisch then named the process “nuclear fission” after asking a biologist the name for when cells divide in two.
Minutes after talking to Frisch, Bohr went on a boat to America to teach for a semester in Princeton, and there, in February, 1939, Bohr theorized that fission only happened in this experiment with a rare isotope of Uranium and 99.3% of natural Uranium does nothing (which is why you need enriched Uranium).
On September 1, 1939, Bohr was back in Denmark and he and a scientist named John Wheeler published the seminal work on the detailed physics of fission. On that same day Hitler decided his promise to stop at Czechoslovakia didn’t count because he didn’t want it to count, and he attacked Poland.
Within days England, France and others declared war on Germany, although Denmark and its tiny army remained neutral.
Two weeks later Heisenberg sent a letter to Bohr  Heisenberg then waited his call to the front, but instead he was called to Berlin where he was informed by his old student and confidant, Carl Frederick von Weizsäcker, to join the “Uranium club” to study if a uranium bomb was possible, to which he immediately agreed.
By December 6, 1939, Heisenberg produced the first secret publication for the Nazi Army Weapons Bureau that used the Bohr-Wheeler theories to predict that a nuclear reactor was possible, and that the resulting bomb, “surpasses the explosive power of the strongest explosive materials by several orders of magnitude.
” At the same time, in Copenhagen, Bohr was coming to the opposite conclusion, and gave a lecture that, “With present technical means it is, however, impossible to purify the rare uranium isotope in sufficient quantity to realize the chain reaction.”
By April 1940, the German’s invaded Denmark as a stepping stone to occupy Norway and Denmark fell within hours
. Mostly because the Nazis felt that Denmark was filled with Aryans and they wanted them to be “a model protectorate” they left the King and the Parliament, and didn’t enforce any race laws.
Meanwhile, Hitler’s war was going so well, that, on June 22, 1941, Germany double-crossed Stalin and attacked the Russians, with a plan to enslave, starve and kill the population of Eastern Europe, starting with the Jewish and Romani people.
Heisenberg decided that the Nazis were unstoppable, and he came up with this theory that all “revolutions” are messy, but once they are over, things would be all better, and lead to a glorious German future! A man named Paul Rosbaud, who was a spy for the Allies, said in 1945 that he had a long talk with Heisenberg in 1940 or 41 where Heisenberg talked about how great it would be when Germany wins the war and don’t worry, “When Germany has won the war this ‘purification’ will take place.”
Disgusted, Rosbaud stopped all conversations with Heisenberg aside from superficial pleasantries, and, “as I knew he was completely in accord on every point with his friend von Weizsäcker, I avoided any further contact with him too.”
The Visit to Copenhagen
In Denmark, despite all the Nazis “kindnesses”, the Danes seemed to be constantly defiant of their Nazi rulers.
Therefore, in March of 1941, Carl Fredrick von Weizsäcker (whose father, Ernst Weizsäcker was a high-level minister in the regime) went to Copenhagen to give pro-German speeches at the newly opened Cultural Institute, a Nazi propaganda site.
The Reich Education thought the visit was a resounding success and asked Weizsäcker to return, and to bring Heisenberg with him! Heisenberg agreed and Weizsäcker’s father, Ernst von Weizsäcker, argued that the trip could be used as a test case of “Heisenberg’s suitability for future propaganda lectures.” This is the famous visit to Copenhagen.
By September 1941, Heisenberg and Carl Fredrick von Weizsäcker went to Copenhagen, and Heisenberg wrote his wife, “It is amazing, given that the Danes are living totally unrestricted, and living exceptionally well, how much hatred or fear has been galvanized here.”
He added how sad it was that, “nobody wants to go to the German Institute on principle, [just] because during and after its founding a number of brisk militarist speeches on the New Order in Europe were given.” Heisenberg wrote his wife that felt like it was, “naturally and automatically became my assigned part to defend our system.”
Heisenberg didn’t seem to notice that his “natural” defense of the Nazi regime was repulsive to his Dannish friends. For example, Bohr’s assistant Stefan Rozental (who was a Jewish Polish refugee and also Heisenberg’s former assistant) recalled how disgusted he was when he heard Heisenberg speak, “with great confidence of the progress of the German offensive in Russia [and stress] how important it was that Germany should win the war.”
Rozental even remembered that Heisenberg said that the occupation of countries in Eastern Europe was a “good development” as “these countries were not able to govern themselves,” to which a Danish colleague named Christian Møller retorted, “so far, we have only learned that it is Germany which cannot govern itself.”
This is backed up by a contemporary account as Christian Møller visited Lise Meitner in Sweden a few months later and told her that Heisenberg, “was entirely imbued with the wish-dream of German victory and had developed a theory of higher-level people and serf-people to be ruled by Germany.”
Meitner was shocked! Carl Fredrick was a former student of hers and she had known Heisenberg for years. In April, 1942, Lise Meitner wrote a warning letter to the scientist Max von Laue living in Germany: “Dr. Møller told me a lot about the visit of Werner [Heisenberg] and Carl Fredrick [von Weizsäcker] I was quite sad to hear all this, as at one time I thought highly of both of them.
It was a mistake,” to which Max von Laue tactfully replied: “I have often wondered about the inner attitude of Werner and Carl Friedrich.” Heisenberg wasn’t just putting on a racist front to keep himself safe in Nazi Germany as these views were repeated even after the war!
For example, in 1947, he told a German-Jewish refugee in England who had lost dozens of relatives “starting with an old aunt of 92 and ending with a young boy of 2” to Hitler’s gas chambers that, “the Nazis should have been left in power for another fifty years, then they would have become quite decent.”
After that same trip Heisenberg’s former mentor Max Born wrote a note to Einstein: “By the way, Heisenberg visited us last December, as pleasant and intelligent as ever, but noticeably ‘Nazified’.” (Born later said that he had misunderstood Heisenberg).
Paul Rosbaud, the spy who was disgusted with Heisenberg’s views in 1940 or 41, tried to talk to Heisenberg about politics again in 1950 and wrote a friend, “I am shocked and depressed about the childish and, I would almost say immoral, views of such a great scientist.
He has not changed a bit and has not learned anything.” Heisenberg was one of the greatest scientists who has ever lived, and he was known as a very charming person but his politics, according to many people who heard him directly, was repulsive… and the reason that people were upset by his visit in 1941.
Anyway, in Heisenberg’s trip of 1941, we know that Heisenberg also told Bohr something about his work on nuclear fission. Unlike the pro-Nazi stuff, we have no contemporary reports about it for years, possibly because Bohr wasn’t that concerned about nuclear bombs while he was in Copenhagen (and, this is when the story gets good – buckle up).
15 months after Heisenberg visited Bohr, in January of 1943, James Chadwick, the man who discovered the neutron, sent Bohr a secret message by putting it in Morse code in microscopic pinholes in a set of keys inviting him to come to England to work on “a particular problem”! Bohr replied on a tiny paper that was inserted into the false tooth of a courier that he wanted to stay as long as he could and besides, “to the best of my judgment…any immediate use of the latest marvelous discoveries of atomic physics are impracticable.” Meanwhile, despite all the propaganda attempts, the Danish resistance continued to increase their harassment of the Nazi occupiers.
At the end of August, 1943, the Nazis got fed up, declared Martial law and disbanded the parliament. Some Nazis wanted to put Niels Bohr in a concentration camp immediately but others decided it would be better to do so with the rest of the Jewish population of Denmark so Bohr’s arrest was delayed until October 1st as, “The Germans were hoping that in the general confusion an order for Bohr’s arrest would then attract less attention.” Luckily, on September 11, a Nazi shipping expert named Georg Duckwitz learned about this and tried to get passage for all Jewish Danes to Sweden but was unsuccessful and so he leaked the information to the Danish resistance, who informed Bohr directly and the head Rabbi in Copenhagen.
By September 29th, Bohr and his family were on their way (illegally) to Sweden and the majority of Denmark went to work helping nearly the entire Jewish Danish population remaining go into hiding. When Bohr arrived in Sweden, the British wanted to whisk him away immediately, but he insisted on staying a few days so he could do something to help other Danish Jews escape.
He had several meetings with high level officials, and after an impassioned call from his friend, the fabulous actress Greta Garbo, managed to even have an audience with the King of Sweden! The next day, the King told Bohr that Sweden would accept all Danish refugees, but Bohr was still not satisfied and demanded that it be publicized and King Gustav agreed, and soon, according to a Danish resistor, “Swedish radio announced, around the clock, that Danish Jews were welcome to come to Sweden.” Over the next two months a parade of boats mass evacuated most of the Jewish population to Sweden and, in the end, 99% of all Danish Jews survived the war!
Finally satisfied that he had done his best, on October 4th, 1938, Bohr went to London by a military bomber where he was shocked to be told that a nuclear chain reaction had been successfully created by Enrico Fermi’s team 11 months before and that a huge international group of scientists were working on a nuclear bomb in Los Alamos! By November, Bohr and his son Aage were on a boat to America and by Christmas they were in Los Alamos, and everywhere he went, Bohr told nuclear scientists about his 1941 conversation with Heisenberg, which most found very confusing.
Bohr also told the scientists about the terrors of the Nazi occupation and his belief that a nuclear threat would remove Hitler and keep Europe free from enslavement ever again. Robert Oppenheimer, the head of the Manhattan project recalled, “Bohr at Los Alamos was marvelous.” Even before the bomb was detonated, in 1944, Bohr met with Churchill and tried to convince him to investigating how, “such mighty forces of nature… can best be turned to lasting advantage for the cause of freedom and world security.” By June, 1945, Bohr was back in freed Denmark to a hero’s welcome and he continued his push for international scientific cooperation, which he continued for the rest of his life.
The Bomb, Bohr, Heisenberg, and Weizsäcker
Fast forward to 1955, when an Austrian journalist named Robert Jungk started to write a book about the history of Nuclear bombs called, “Brighter than a Thousand Suns.” Jungk learned about the 1941 conversation from several Allied scientists and asked Heisenberg for “a little more detail about [the] Copenhagen conversations during WW2”.
Heisenberg replied that he told Bohr, “in view of moral concerns, it might be possible to get all physicists to agree not to attempt work on atomic bombs” but, according to Heisenberg, Bohr, “thought it would be hopeless to exert influence on the actions of individual countries, and that it was…the natural course in the world that the physicists were working in their countries on the production of weapons.” Weizsäcker backed up Heisenberg’s story as you can see from this extraordinary video. Weizsäcker also added that Heisenberg had told him about the conversation *that night* and that Heisenberg had told him, “You know, I’m afraid [my conversation with Bohr] went very wrong.”
OK, this story is… this story is completely illogical and contrary to how all three men (Bohr, Heisenberg and Weizsäcker) behaved in every other instance. Bohr, as I have already described, believed that nuclear research should force international cooperation and promote peace and democracy which you can argue was naïve or ignorant, but you certainly can’t paint Bohr as a person frightened of scientists standing up to politicians! Heisenberg was the one who believed in the “natural course of the world” and wrote in his biography that “we ought even to insist that everyone brave what storms there are in his own country”. Finally, Weizsäcker would have had to sign on to this hairbrained idea that would have meant death for all involved and their families if they were caught (and remember Weizsäcker’s father personally vouched for Heisenberg).
Then, once Weizsäcker learned it didn’t work and that Bohr might tell other people decided to *not* clarify things with Bohr but instead to write an official document that outright said that, “Obviously, Professor Bohr does not know we are working on these questions”! Not only was Heisenberg’s and Weizsäcker’s story illogical, but the people who lived through it immediately stood up and said it was a total lie. Neil Bohr’s son Aage said his father told him at the time about the conversation and, “[Heisenberg’s] account has no basis in actual events.” Bohr’s colleague Stefan Rozental, wrote the author that he remembered when Heisenberg told Bohr that he was working on nuclear projects and how, Bohr, “quoted Heisenberg for having said something like, ‘You must understand that if I am taking part in the project then it is in the firm belief that it can be done.’” Niels Bohr, at the center of the maelstrom, wrote a letter to Heisenberg that said: “I have seen a book ‘Brighter than a Thousand Suns” by Robert Jungk… and I am greatly amazed to see how much your memory has deceived you…I remember every word of our conversations, which took place on a background of extreme sorrow and tension for us here in Denmark.
In particular, it made a strong impression both on Margrethe and me, and on everyone at the Institute that the two of you spoke to that you and Weizsäcker expressed your definite conviction that Germany would win and that it was therefore 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 [gave] 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.”
However, Bohr decided against sending this letter, as he worried it would hurt Heisenberg. Even without Bohr’s letter, the author, Robert Jungk, was convinced that he had been bamboozled, and apologized for, “spreading of the myth of passive resistance by the most important German physicists is due above all to my esteem for these impressive personalities, which I have since realized to be out of place.”
See, throughout the war, Heisenberg thought a bomb was possible, but over-projected how much time and money it would take to succeed. He then assumed that no one would divert that much to such a project, and, besides, no one could be as far along as he was, as he was GERMAN, so there were no worries about competition.
That is why, throughout the war, Heisenberg was quite flippant about nuclear secrecy. For example, in November of 1944, Weizsäcker was working in France and escaped just before the Allies took over. When the allies found Weizsäcker’s papers they, “let out a yell at the same moment,” the papers were not in code and even included the address and phone number of Heisenberg’s “secret” location! By May 3, 1945, the Allies used this information to pick up Heisenberg (to keep all German nuclear scientists out of the hands of the Russians) and he was placed, with his friend Weizsäcker, Lise Meitner’s friends Otto Hahn and Max von Laue and six other scientists in an elegant farm house in the English countryside, with a piano, tennis courts, and with every room wired for sound. When the radio announced that the Americans had bombed Hiroshima with a nuclear bomb on August 6, 1945, Heisenberg declared, “I don’t believe a word of the whole thing”.
Then, when the second radio broadcast made it clear that it was real, as well as the size of the American program and that it started in earnest in 1942, Heisenberg said, “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.” This is the morality that Heisenberg was concerned with – he did not care about making a weapon of destruction for Hitler, or for any country – he just felt that he didn’t have the courage to divert that much money and manpower in the middle of a war.
The very next day, after endless discussions of the bomb Heisenberg declared: “How have they actually done it? I find it is a disgrace if we, the professors who have worked on it, cannot at least work out how they did it.” So much for the claims that Heisenberg knew how to make a bomb but withheld it for moral reasons.
In addition, as soon as they heard about the bomb, Weizsäcker came up with his cover that German scientists, “didn’t want to do it, on principle. If we had all wanted Germany to win the war we would have succeeded,” to which Otto Hahn replied, “I don’t believe that!” Neither Heisenberg or Weizsäcker mentioned the 1941 meeting with Bohr while they were under surveillance by the British, but, near the end of their internment, they did have a little discussion that I find illuminating as it demonstrates their relationship. On September 9th, they were worried that Max von Laue would return to Germany before Heisenberg as Weizsäcker thought that Laue was “incompetent”. Heisenberg then told Weizsäcker that it would, “be difficult officially to take away Laue’s authority.” So Weizsäcker offered to “undertake a little intrigue to see that Laue doesn’t get back …until you get back.”
So, given all that, why did Heisenberg mention his work on uranium research to Bohr? My best guess is that Weizsäcker’s father gave Heisenberg a secret mission to see if Bohr was working on nuclear projects or knew anything about the Allies nuclear work. The Weizsäckers couldn’t have been particularly concerned about this: it would be impossible for anyone to make a bomb for the allies *while* they were under Nazi control and it would be idiotic for Allies to tell someone living under the Nazis any secrets, as, under torture, almost anyone, even Bohr, would reveal all.
However, Weizsäcker and his father might have thought it was a good “project” for Heisenberg to demonstrate his dedication to the regime.
Remember, at the time, Heisenberg was trying to prove his worth to the Nazis, and did not see that promoting Hitler was repellent, or that working on creating a weapon of mass destruction for a psychotic racist might be … oh I don’t know… bad. Also remember, Weizsäcker’s father had said that the trip was to test, “Heisenberg’s suitability for future propaganda lectures,” and how better to test his suitability then by giving him a secret mission to spy on his friend?
After 1941, Heisenberg and Weizsäcker did go on more lecture circuits in occupied areas and Heisenberg was often allowed to travel to Switzerland, which was a rare privilege in Nazi Germany. In addition, in December of 1942, Heisenberg joined an elite group of intellectuals called that “Wednesday Club”, where everyone was fiercely pro-German, anti-Russian and disdainful of Hitler (who they called the chimpanzee).
When the Allies started bombing Germany, many of the members decided that they could not wait until the end of the war to remove Hitler, as Germany might lose, and in early 1944, according to Heisenberg’s wife, Heisenberg was asked to join in on a plot to kill Hitler on July 20th but he refused out of fear.
Years later, Heisenberg wrote that he, “always felt very ashamed before the people of July 20th, (some of whom were my friends), who at the time put their effort into serious resistance, sacrificing their lives.” After the plot failed, the Nazis arrested and tortured over 7,000 conspirators but Heisenberg was so well connected politically that no one bothered to even interview him!
If my theory is correct, then in Copenhagen, in 1941, Heisenberg tried to ask Bohr about working on nuclear projects in a roundabout way and Bohr responded that a nuclear bomb was impossible. Heisenberg then got offended and started spouting off how it definitely would work and might change the course of the war.
Then, Heisenberg realized that he had said too much, and added that he didn’t need to talk about details, and ended the conversation without even telling Bohr to keep it secret! After the conversation, Heisenberg only told Weizsäcker that Bohr wasn’t working on nuclear power, which is why Weizsäcker wrote an official document that Bohr didn’t know about their work.
This is also why Bohr and his friends and family remembered the conversation in his laboratory but Heisenberg insisted after the war that it was during a walk in the woods.
If Heisenberg was attempting to help the Nazis in his conversation with Bohr, it was fine to say it in a room that was probably bugged. If his actions were anti-Nazi, however, his words needed to be done in secret – ergo, he changed the location from the laboratory to the woods.
After the war, there was a mad rush among almost all Germans that stayed in Germany to gain “proof” that they were secretly anti-Nazi the whole time. This was particularly true for Weizsäcker’s father, Ernst, who was put on trial in Nuremberg in July of 1947 and sentenced to seven years in jail for sending French Jews to Auschwitz.
The Weizsäcker family and the Heisenberg family then used all their considerable influence to convince people that this was a miscarriage of justice and that, as Heisenberg’s son Martin wrote in a letter to the editor: “[Ernst von] Weizsäcker was closely associated with the German resistance.” This story was pushed more when Ernst’s youngest son, Carl Fredrick’s younger brother, Richard, became President of Germany in 1984!
Combined with the fact that Heisenberg was an amazing Physicist and that the Americans and English were trying to push the qualifications of “their” German scientists (as compared to Russia’s German scientists), Heisenberg’s repellent views were suppressed or the people who mentioned it were discounted as biased.
In June of 1945, Lise Meitner wrote a letter to her friend Otto Hahn who was still in captivity with Weizsäcker and Heisenberg in England: “What we have heard recently about the unfathomable atrocities of the concentration camps exceeds everything we had feared. When I head [about it] I began to wail aloud and couldn’t sleep all night. They should force a man like Heisenberg, and millions of others with him, to see these camps and the tortured people.
His performance in Denmark in 1941 cannot be forgotten.” Don’t let Heisenberg’s and Weizsäcker’s version muddy the truth: in 1941 Heisenberg promoted Nazi propaganda and he spied on his good friend and mentor to help his position within the regime, that should not be forgotten… or forgiven.
One more comment about Bohr: in 1942, a Danish Nazi newspaper published an article titled, “The days of the Jews should be past” which complained about “the Jew Niels Bohr” having a position and “Should it not be possible to find a Dane for this post?” to which a more popular newspaper rejoined, “If Bohr is not Danish then who is?” I have to say that the more I studied Bohr the more impressed I have been with him as a scientist and as a person, and Danes have every right to be proud of their native son. Wish I could say the same about Heisenberg!
Whew, so that was my version of what I think happened (and why) in 1941 – I also have a little 9 minute video about Heisenberg and Bohr’s relationship before Hitler which is on my Patreon page but if you are too broke to join, just shoot me an email (link is in the about me) and I will send you a link [Also, the material will probably be folded into later videos].
I also have 3 choices for the next video: chronologically: How Bohr became famous, the Bohr-Einstein quantum debates, and why Heisenberg didn’t succeed in a nuclear bomb.
 300 scientists according to Pais, A Niels Bohr’s Times (1991) p. 383
 Frisch, O What Little I remember (2019) p. 94
 Heisenberg found in Bernstein, J Hitler’s Uranium Club () p. xxiii
 Bohr Lecture December, 1939 found in Pais, A Niels Bohr’s Times (1991) p. 462
 Rosbaud to Gouldsmit (Oct 25, 1950) found in Rose, P Heisenberg and the Nazi Atomic Bomb () p. 311
 Rosbaud, deposition of August 12, 1945 found in Rose, P Heisenberg and the Nazi Atomic Bomb () p. 310
 According to a record by the German Reich in Denmark to Berlin (March 27, 1941) as found in Cassidy, D Beyond Uncertainty (2010) p. 485
 REM documents from August and Sept 1941 found in Pais, A Niels Bohr’s Times (1991) p.486
 Werner Heisenberg to Li Heisenberg (Sept, 1941) Heisenberg, W My Dear Li: Correspondence: 1937-1946 (2016) p. 168
 Werner Heisenberg to Li Heisenberg (Sept, 1941) Heisenberg, W My Dear Li: Correspondence: 1937-1946 (2016) p. 168
 Christain Moller to Lise Meitner (1941) found in Sime, R Lise Meitner: A Life in Physics (1996) p. 301
 Christain Moller to Lise Meitner (1941) found in Sime, R Lise Meitner: A Life in Physics (1996) p. 301
 Lise Meitner to Max Von Laue (April 20, 1942) found in Sime, R Lise Meitner: A Life in Physics (1996) p. 301
 Max Von Laue to Lise Meitner (April 26, 1942) found in Sime, R Lise Meitner: A Life in Physics (1996) p. 301
 Fritz Simon recalled in McRae, K Nuclear Dawn: F. E. Simon (2014)
 Rudolf Peierl relaying what Francis Simon told him found in Rose, P Heisenberg and the Nazi Atomic Bomb () p. 313
 The description of the secret keys are in Pais, A Niels Bohr’s Times (1991) p. 485-6, the contents of the letter are from the same source and dated January 25, 1943.
 Bohr to Chadwick (1943) found in Pais, A Niels Bohr’s Times (1991) p. 487
 According to Pais, A Niels Bohr’s Times (1991) p. 400 “The Germans were hoping that in the general confusion an order for Bohr’s arrest would then attract less attention”
 According to Bret, D Greta Garbo (2012)
 According to Flender, H Rescue in Denmark (2019) p.
 Translated and found in Bernstein, J Hitler’s Uranium Club (2001) p. 332
Goudsmit, S Alsos (1996) p. 72
 Heisenberg to Hahn, found in Bernstein, J Hitler’s Uranium Club (2001) p. 110
 Heisenberg to Hahn, found in Bernstein, J Hitler’s Uranium Club (2001) p. 122
 Heisenberg and Weizsäcker conversation, Sept 9, 1945 found in Bernstein, J Hitler’s Uranium Club (2001) p. 222
 REM documents from August and Sept 1941 found in Pais, A Niels Bohr’s Times (1991) p.486
 Heisenberg’s interactions with the Wednesday Club was found in Cassidy, D Beyond Uncertainty (2010) pp. 513-516
 Heisenberg to Robert Jungk (November 17, 1956) found in https://fairy-tales-alive.blogspot.com/search?q=heisenberg&updated-max=2012-11-07T02:14:00%2B05:30&max-results=20&start=1&by-date=false
 According to his biographer, “The high-ranking regime officials – Speer, Göring, Himmler – who had supported him previously may have protected him from suspicion” Cassidy, D Beyond Uncertainty (2010) p. 515
 Martin Heisenberg (January 16, 1992) “The Heisenberg Case: An Exchange” The New York Review of Books
 Lise Meitner to Otto Hahn (June, 1945) found in Sime, R Lise Meitner: A Life in Physics (1996) p. 310