Final answer:
Albert Einstein's theories indirectly led to research into nuclear fission, as his letter to President Roosevelt urged research that resulted in the Manhattan Project.
Step-by-step explanation:
The physicist whose theories led to research into nuclear fission was Albert Einstein. Although Einstein did not directly discover fission, his establishment of the mass-energy equivalence principle (E=mc^2) underpins the vast energy potential in nuclear reactions. In 1938, German physicists Otto Hahn and Fritz Strassman discovered nuclear fission, and the potential for a chain reaction was soon understood. Knowing the gravity of this discovery, especially in the prelude to World War II, and concerned about Nazi Germany's potential development of nuclear weapons, Einstein signed a letter drafted by physicist Leo Szilard, which was sent to President Roosevelt, prompting the US to increase its investment in nuclear research. This ultimately led to the initiation of the Manhattan Project, a massive mobilization of resources and scientific talent, including J. Robert Oppenheimer, known as the "father of the bomb" for his leadership role in the project.