 Oppenheimer was not reappointed to the GAC when his term expired in 1952 , and Rabi succeeded him as chairman , serving until 1956 . Rabi later testified on Oppenheimer 's behalf at the Atomic Energy Commission 's controversial security hearing in 1954 that led to Oppenheimer being stripped of his security clearance . Many witnesses supported Oppenheimer , but none more forcefully than Rabi : 
