Compton asked theoretical physicist J. Robert Oppenheimer of the University of California, Berkeley, to take over research into fast neutron calculations —the key to calculations of critical mass and weapon detonation—from Gregory Breit , who had quit on 18 May 1942 because of concerns over lax operational security.  John H. Manley , a physicist at the Metallurgical Laboratory, was assigned to assist Oppenheimer by contacting and coordinating experimental physics groups scattered across the country.  Oppenheimer and Robert Serber of the University of Illinois examined the problems of neutron diffusion—how neutrons moved in a nuclear chain reaction—and hydrodynamics —how the explosion produced by a chain reaction might behave. To review this work and the general theory of fission reactions, Oppenheimer and Fermi convened meetings at the University of Chicago in June and at the University of California, Berkeley, in July 1942 with theoretical physicists Hans Bethe , John Van Vleck , Edward Teller, Emil Konopinski , Robert Serber, Stan Frankel , and Eldred C. Nelson, the latter three former students of Oppenheimer, and experimental physicists Emilio Segrè , Felix Bloch , Franco Rasetti , John Henry Manley , and Edwin McMillan . They tentatively confirmed that a fission bomb was theoretically possible.