In spite of this, observers such as physicists Luis Alvarez and Jeremy Bernstein have suggested that if Oppenheimer had lived long enough to see his predictions substantiated by experiment, he might have won a Nobel Prize for his work on gravitational collapse, concerning neutron stars and black holes. In retrospect, some physicists and historians consider this his most important contribution, though it was not taken up by other scientists in his lifetime. The physicist and historian Abraham Pais once asked Oppenheimer what he considered his most important scientific contributions—Oppenheimer cited his work on electrons and positrons, not his work on gravitational contraction. Oppenheimer was nominated for the Nobel Prize for physics three times, in 1946, 1951 and 1967, but never won.