Between the summers of 1943 and 1944, the Theoretical Division, under Hans Bethe, and the Physics Division, under Robert Bacher, collaborated in studying the nuclear physics of the atomic bomb. T-Division's responsibilities included calculating critical mass and efficiency. The lack of hard nuclear-constant data was particularly troubling. While P-Division worked to improve the experimental data using available detectors, accelerators, and other devices, T-Division developed flexible models based on the changing set of available data. To cross-check their results, researchers often used different methods to solve the same problem. For example, the Water Boiler, a nuclear pile using enriched uranium in a water solution, provided a means of checking critical mass calculations. As a backup, Richard Feynman made calculations on uranium hydride, then being considered as a potential active material. Teller's investigation of the hydrogen bomb (the Super) was an alternative approach to a nuclear weapon. The opportunity to conduct physics research on a larger scale than had ever before been attempted gave the Los Alamos physicists the experience of working in well-funded multi-disciplinary groups, which included both experimentalists and theorists, as well as electronics experts, chemists, and metallurgists.

Nuclear Theory: Critical Mass and Efficiency

In September 1943, T-Division was refining its critical mass and efficiency predictions and calculating the damage the bomb could cause. Up to this point, the division had remained somewhat informal in its organization to accommodate changing priorities, but by October it had begun to subdivide into groups: Bethe took on the problem of implosion, Victor Weisskopf led the calculations of efficiency, Robert Serber spearheaded diffusion theory, Edward Teller assumed responsibility for both the Super and implosion, Feynman led the uranium hydride calculations, and Donald Flanders headed the computational effort.