|Fig. 1: A picture of Werner Heisenberg in 1927. (Source: Wikimedia Commons)|
In the years leading up to World War II, Germany was at the forefront of theoretical and experimental physics pertaining to atomic energy. Actually, in November 1945, the Royal Swedish Academy of Sciences awarded the 1944 Nobel prize in Chemistry to Otto Hahn for the discovery of nuclear fission.  This discovery followed from a set of successive experiments where the newly discovered neutron had been studied in many different cases. In the years after the discovery of the neutron in 1932, a Berlin-based team of scientists recorded a large number of "transuranics" and distinguished U-239, a beta-emitting uranium isotope, formed from resonance capture of U-238.  These experiments led to the necessary proof the nucleus had split, barium was found in the uranium products. Hahn published his results with Fritz Strassmann on January 6th, 1939. Lise Meitner, an Austrian-born physicist of Jewish heritage who had fled Berlin and took a position in Stockholm in 1938, gave the first theoretical interpretation of the fission process in a paper submitted to Nature on January 16th, 1939.  This research put Germany ahead of the United States and the rest of the world in the quest for nuclear weapons.
When looking for a scientist to help lead their nuclear program at the beginning of the war, Germany decided Werner Heisenberg would be a value asset to their ultimate objective of creating the atomic bomb. Heisenberg had won the 1932 Nobel prize for what the Nobel committee had called, "the creation of quantum mechanics."  While he might not have been the sole inventor of quantum mechanics, his uncertainty principle is a fundamental component in the study and understanding of quantum mechanics. Heisenberg is considered one of the greatest theoretical physicists of the twentieth century, on the same level as Einstein and Bohr.  A picture of Werner Heisenberg can be seen on the right.
This was a man of significant scientific intellect while at the same time he and physics in general had to deal with the situation inside Nazi Germany. Balancing German national loyalty in Nazi Germany and scientific obligation was something Heisenberg along with other German scientists struggled with throughout the entire era. Heisenberg was one of the few University professors not to sign the manifesto in support of Hitler, which he did at significant personal risk.  Before the war, an SS publication accused Heisenberg of teaching "Jewish physics" or theoretical physics.  He was saved personally by Heinrich Himmler, through a relationship between Himmler's grandmother and Heisenberg's grandmother, of all disloyalty to the regime. Throughout the war, Heisenberg mainly worked in developing nuclear reactors. He convinced himself that a reactor with natural uranium and heavy water would be self-stabilizing and thus put no cadmium rods into his reactors.  He came to this result due to never considering delayed neutrons, which play a large role in reactor safety. A lump of cadmium was kept on hand if things got out of control, but luckily was never needed.
While Heisenberg was leading reactor development, he spent a significant amount of his time trying to save his students and collaborators from death on the war fronts.  This was done at great personal risk, but he realized the importance of the scientific community once the war had ended. Not only did he save individuals from combat, but he did his best to protect nuclear research labs, especially Bohr's Institute in Copenhagen, from being robbed by German occupying forces.  Due to his intellect and skill, Heisenberg became the highest official credentialed physicists in Germany during the war.
In September 1939, Heisenberg along with other German scientists joined together under military order to create Uranverein or "Uranium Club" to investigate nuclear energy for the war effort.  This team's goal was to determine whether nuclear weapons would be relevant in the near future to warrant the considerable expenditure required to develop the technology. In 1940, C. F. von Weizsacker proposed using neptunium, element 93, as a nuclear explosive, but once it was realized that the element was unstable, plutonium, element 94 and neptium's decay product, was proposed as an alternative.  At the same time, Heisenberg calculated, incorrectly, that the critical mass required for a U-235 nuclear weapon was on the order of a few tons instead of the actual value of 15-60 kilograms.  Even with this error in calculations, there was belief in 1941 that if the war lasted a few more years, a nuclear weapon could be developed. 
In 1941, however, Germany had just invaded the Soviet Union after conquering France, Norway, and Poland in the previous few years. The German nuclear program, at its height, consisted of twenty-two institutes over twelve cities throughout Germany and Austria.  This was a significant manpower and intellectual drain on resources. Thus in December 1941, the German army decided to abandon its nuclear fission project deciding to focus on the development of other new technologies, mainly rockets and jet aircraft, that could make a more immediate impact.  It is possible that Heisenberg's error in calculating the critical mass of uranium needed for a reaction played a part in the decision to withdraw funding. A summary report from February 1942 named "Energiegewinnung aus Uran" covers all aspects of the nuclear work since 1939 including an approximately correct estimate of the critical mass needed for a bomb.  Either way the German nuclear problem had significant obstacles that would have had to been overcome if a nuclear weapon would have been developed.
When considering reasons Germany did not develop the atomic bomb given their leads in both 1939 and 1941, three main issues stand out. These include lack of nuclear physicists, industrial requirement to succeed, and the desire for immediate results. In the 1920s and early 1930s, Germany was a leading nation of theoretical physics, but with the rise of Nazism, a significant number of scientists, Jewish ones in particular, left the German team at a significant disadvantage to the Allied team.  For the scientists that stayed in Germany, the lack of interest in pure science by the regime resulted in almost an entire generation of physicists being lost. On a per man basis, the Allied team was more capable with certain individuals, such as von Neumann, the German team could just not match.  After the war, Heisenberg told Hans Bethe that nuclear energy was a way to save German physicists for when the war ended. 
Beyond lacking a significant number of physicists, Germany was a nation of limited industrial output within a war zone. Two moderators, needed to slow the neutrons from fission in order to create a chain reaction, were believed to be possible for nuclear fission, carbon and heavy water.  Walter Bothe, a German experimental physicist, and Enrico Fermi, working in America, conducted experiments to see if carbon could be a moderator. Bothe concluded that carbon would not work, but while Fermi though that carbon was marginal at best, Leo Szilard, working with Fermi, remember that boron carbide was commonly used to manufacture graphite.  Boron atoms absorb about 100,000 times the number of carbon atoms and this impurity resulted in leading Bothe to believe carbon was not a satisfactory moderator. Thus, Germany believed heavy water was the only capable moderator.  Since the only heavy water production facility was located in Norway and was easily targeted and destroyed by the Allies, the German team did not have the industrial support required to be successful.
With the war turning for the worse after the invasion of the Soviet Union, technologies believed to be implemented in the near future were advocated over long term technologies. Bagge and Diebner stated the critical error was the army requirement in December 1941 that a military product should be generated within 9 months.  The only way funding would have continued would have been by scientists making claims they knew they could not meet. In this situation, the experts decided not to push for increased industrial effort of the nation in support of nuclear weapons.  Considering the Allies nuclear program had better and larger number of physicists, support of the military, and the industrial capacity of the United States, it is not surprising that they developed the atomic bomb before Germany. It has to be mentioned, however, that even with all these advantages, the United States only first successfully denoted a nuclear weapon in July 1945, two months after the war in Europe.
In July of 1945, ten members of the "Uranium Club" were held at a country estate called Farm Hill in Britain.  This estate was bugged so the Allies could gain an understanding in just how close Germany was to creating a nuclear weapon. Prior to the dropping of "Little Boy" on Hiroshima, the scientists fully believed they were the furthest along in the development of nuclear weapons, but nuclear weapons could not be build in the immediate future.  With the knowledge of the nuclear detonation, Heisenberg gave a lecture on nuclear weapons with an accurate back of the envelope measure of the amount of uranium required.  When they learned of the dropping of the bomb's mass that was dropped on Nagasaki , the scientists could not determine how a nuclear weapon could be created with such a small amount of nuclear fuel.  The scientists never even considered the possibility that plutonium could have been used in the detonation. While the German scientists understood the basics of the problem, there were still very significant roadblocks that kept them from every generating a fully operational nuclear weapon.
© Andrew D. Wendorff. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 E. Crawford, R. L. Sime and M. Walker "A Nobel Tale of Postwar Injustice," Physics Today 50, No. 9, 26 (September, 1997).
 T. M. Sanders, "Heisenberg and the German Bomb", Contemp. Phys. 43, 401 (2002).
 N. P. Landsmand, "Getting even with Heisenberg", Studies in History and Philosophy of Modern Physics 33, 297 (2002).
 D. C. Cassidy, "A Historical Perspective on Copenhagen," Physics Today 53, 28 (July, 2000).
 J. Bernstein, "Heisenberg and the Critical Mass," Am. J. Phys. 70, 911 (2002).
 H. A. Bethe, "The German Uranium Project," Physics Today 53, No. 7, 34 (July, 2000).