He spent the war serving as an assistant to Enrico Fermi and Leó Szilárd working on the Manhattan Project.
After World War II, he returned to Columbia University to receive his PhD in 1945 with thesis advisor Willis Lamb.
During this time, he was President of the Albert Einstein Peace Prize Foundation, editor of the Bulletin of the Atomic Scientists, and head of the American Pugwash Committee.
[3][citation needed] Feld was a Ford Foundation Fellow and a visiting scientist at the European Center for Nuclear Research (CERN) in Geneva, Switzerland.
[citation needed] "One month after the election of Ronald Reagan, Feld being an editor of 'Bulletin of the American Atomic Scientists' reported that his publication had decided to move the hands on the Doomsday Clock featured on its cover from seven to four minutes to midnight, because, as 'the year drew to a close, the world seemed to be moving unevenly but inexorably closer to nuclear disaster.'