Here, German racial policies effectively stymied Nazi pursuit of a nuclear device. A German, Gustav Hertz, was on the cutting edge of research on gaseous diffusion. But Hertz’s uncle, the famous Heinrich Hertz, was Jewish. As a result, Gustav was forced out of his position as head of the physics department at Berlin Technical College. Whatever Nazi functionary replaced him evidently was unable or uninterested in continuing Hertz’s work, and the Germans never developed a uranium isotope separation method.
Both sides faced the question of how much U-235 would be necessary to make a workable atomic bomb. The issue was particularly crucial given the difficulty of uranium separation. Heisenberg and the Germans made a fundamental and critical theoretical mistake regarding the practicality of a U-235 fission device. They thought a bomb required far too much U-235 to make it a practical weapon in any case.
This error remains the source of enduring controversy. It appears Heisenberg only attempted to roughly calculate the critical mass of U-235 that would produce a runaway nuclear chain reaction. He and his colleagues erroneously concluded that it would require about a ton of U-235 to reach critical mass.
In fact, as American scientists determined, critical mass was less than 100 pounds of U-235. The Germans knew that producing a ton of enriched U-235 was a practical impossibility. The more modest actual requirement for an atomic bomb was, however, within their reach, assuming an enormous industrial and governmental effort (as in fact occurred with the Manhattan Project).
No Support From Speer
Whether Germany would launch its own Manhattan Project was not up to Heisenberg or the other German scientists, but the Reich’s armaments minister, Albert Speer. Speer needed to know whether the scientists could deliver a workable weapon before the war ended. Heisenberg met with Speer in 1942 during the course of a conference on the potentialities of nuclear power put on for various officials from the Army and the SS.
What exactly was said during the conference and during Speer’s more private discussions with Heisenberg is not known, but evidently Heisenberg did not encourage Speer as to the short-term possibilities of producing uranium-based atomic weapons or a practical reactor engine for ships or submarines. Heisenberg did not request a dramatic increase in funding for the German project.
Speer concluded that the German atom scientists could make little contribution to the war effort and concentrated the Reich’s experimental energies on the more promising rocket and jet projects that would materialize before the end of the war. Clearly, the German scientists did not believe they could extract sufficient U-235 to make a bomb and so did not urge Speer to commit German industry to an isotope separation project, as was then being done in the United States at the Manhattan Project’s huge Oak Ridge, Tenn., facility.
Working Toward a Plutonium Bomb
Heisenberg, however, was not without hope. German scientists were on their way to producing a working nuclear reactor. For a time, Heisenberg and his scientists considered the possibility of using the reactor itself as a kind of “dirty bomb.” The reactor ran on U-238 and so could not sustain the kind of rapid chain reaction required for a true atomic explosion, but the Germans considered the possibility of running the reactor to a critical stage and then supplementing the intense radiation developed with conventional explosives. The result would be massive radioactive contamination. Like a nuclear bomb with a ton of uranium, the idea itself was impractical: How do you deliver a reactor bomb?
Germany’s last hope was producing sufficient plutonium in a reactor and using it, rather than uranium, as the fissionable bomb material. Such a plutonium bomb was dropped on Nagasaki. The Germans realized that highly radioactive plutonium could be produced as a byproduct of U-238 “burned” in the reactor. Plutonium in sufficient quantities would work as well or better than U-235 in an atomic device. Heisenberg and other scientists had hinted at such a possibility during the conferences with Speer and other Nazi officials.
As Heisenberg’s discussions with Speer implied, German reactor research remained a surprisingly academic endeavor. While the Germans managed to construct a couple of experimental reactors, the various teams did not always coordinate their efforts, and it took the Germans much longer than the Allies to simply determine what shape the uranium pile should take within the reactor. Heisenberg and other scientists pursued atomic research while carrying on other academic duties, visiting occupied countries on propaganda missions, and writing papers.
The German hierarchy never deliberately organized German industry to aid the bomb project. There was nothing in Germany like the scientifically star-studded Los Alamos pressure cooker or the vast Tennessee separation plant. The Germans also had to contend with the insecure heavy water supply and American and British air raids on important industrial, transportation, and research facilities.
By 1945, German research teams managed to build a research reactor in an abandoned cave in the Swabian region of southwest Germany, far from the advancing Soviet Red Army and coincidentally in the general area where Einstein spent some of his boyhood. Yet, despite Heisenberg’s best efforts, sometimes at physical risk to himself and his assistants, the Germans did not succeed in producing a self-sustaining chain reaction in the reactor by the time American troops arrived.
Not only had the German scientists failed, but they were surprised when they realized that the Allied scientists advancing with the American troops were unimpressed by their accomplishments. The German reactor was little more than a curiosity. The Americans were mainly concerned that leftover uranium, heavy water, and Germans physicists not fall into Russian hands.
Why Did the Nazi Atomic Weapons Project Fail?
After the war, Heisenberg and his apologists suggested that the project failed because Heisenberg deliberately sabotaged it. Heisenberg intimated that he knew the proper critical mass for U-235 in 1942, but deliberately misled Speer in order to frustrate the German project. Heisenberg’s famous meeting with his mentor, the great Danish (and half-Jewish) physicist Nils Bohr, in Copenhagen in 1941, has also been the subject of much speculation, and the subject of a prize-winning play that suggests Heisenberg was morally troubled at the prospect of a Nazi bomb.
Beyond Heisenberg’s own circumlocutions on the topic, and quasi-fictional speculations, there are abundant reasons for the German failure that have little to do with postwar claims of moral scruples by German scientists. Most available evidence suggests that the German physicists would have happily produced a bomb if they had been capable. That they did not was due to a number of discrete and explainable reasons.
Most obviously, Germany suffered from a critical brain drain produced by Nazi anti-Semitism. Brilliant scientists in Germany and occupied Europe were exiled, ignored, or executed. Many found their way to England or America where they dedicated themselves to destroying the Nazi regime. Although Heisenberg and some of his remaining colleagues were topnotch physicists, they could not match the basic brainpower deliberately assembled for the Manhattan Project.
Allied organizational ability also outmatched Germany’s. There were no effective German counterparts to General Leslie Groves or Robert Oppenheimer, who together drove the Manhattan Project. The German project bounced around under varying ministries until the end of the war, while the scientists themselves often worked in an uncoordinated, wasteful fashion.
The Germans remained blissfully unaware of American advances throughout the war. This made the Germans complacent. Arrogantly, they refused to believe German science could fall behind American science. Effective Allied security prevented German intelligence from proving otherwise.
Mistakes Made by the Nazi Scientists
Meanwhile, a steady stream of fleeing European scientists, Bohr among them (in 1943), provided a relatively clear picture of German advances, or lack thereof, to Allied intelligence services. After the war, captured German scientists were surreptitiously tape recorded after being informed of the atomic bombing of Japan. The tapes reveal a mixture of surprise, bewilderment, and excuses.
Heisenberg and his colleagues were very smart, but they made fundamental conceptual errors throughout the war that led them to believe producing an atomic weapon was more difficult and impracticable than was the case. These misconceptions ranged from miscalculation of the critical mass necessary to fission U-235 in a bomb to the best shape to form U-238 in a reactor to the relative difficulty of separating the two isotopes. All of these problems were correctly solved relatively early on by Allied scientists.
Finally, the overall cultural prejudices of the regime and of a great number of scientists, functionaries, and soldiers prevented the German project from being anything more than a sidelight, while “real” German scientists worked on jets and rockets. While the jet and rocket projects were impressive and far in advance of Allied efforts, they also proved to be indecisive militarily and even wasteful.
On the other hand, had Germany produced just a couple of atomic weapons before the end of the war, instead of the thousands of V-1 and V-2 rockets lobbed on Britain, the course of history would undoubtedly have been different. For reasons having little to do with morality or luck, we remain fortunate this alternate history did not come to pass.