In 1941, with World War II raging in Europe but the United States not yet at war, Feynman spent the summer working on ballistics problems at the Frankford Arsenal in Pennsylvania. After the attack on Pearl Harbor brought the United States into the war, Feynman was recruited by Robert R. Wilson, who was working on means to produce enriched uranium for use in an atomic bomb, as part of what would become the Manhattan Project. At the time, Feynman had not earned a graduate degree.Quote:"I did not even have my degree when I started to work on stuff associated with the Manhattan Project."Later in this same talk, at 5m34s, he explains that he took a six week vacation to finish his thesis so received his PhD prior to his arrival at Los Alamos.</ref> Wilson's team at Princeton was working on a device called an isotron, intended to electromagnetically separate uranium-235 from uranium-238. This was done in a quite different manner from that used by the calutron that was under development by a team under Wilson's former mentor, Ernest O. Lawrence, at the Radiation Laboratory of the University of California. On paper, the isotron was many times more efficient than the calutron, but Feynman and Paul Olum struggled to determine whether or not it was practical. Ultimately, on Lawrence's recommendation, the isotron project was abandoned.