Stanislaw Ulam (1909 - 1984)
Ulam was born in Poland, where he received his Ph.D. in mathematics. During the Manhattan Project, he worked in the Theoretical Division at Los Alamos.
In 1951, Ulam's calculations proved that Edward Teller's scheme for the "Super" bomb was a "fizzle" and that it would never work. Ulam would later develop a new scheme that would lead to the development of the hydrogen bomb.