Portrait of Wolfgang Pauli (1900-58), Austrian- American physicist. From his teens, Pauli was a formidable physicist, writing studies of relativity admired by Einstein. In 1925, he announced his exclusion principle. This stated that only two electrons could inhabit a given atomic energy level at the same time, one with positive 'spin', the other negative. This led to his belated Nobel Prize in 1945. In the meantime, he proposed the existence of neutrinos from his study of beta-particle emissions. This was only confirmed by experiment some 25 years later. Pauli moved to the USA in 1930, becoming a US citizen in 1946.