In 1919, Fowler returned to Trinity and was appointed college lecturer in mathematics in 1920. Here he worked on thermodynamics and statistical mechanics, bringing a new approach to physical chemistry. With Arthur Milne, a comrade during the war, he wrote a seminal work on stellar spectra, temperatures, and pressures. In 1925 he was made a Fellow of the Royal Society. He became research supervisor to Paul Dirac and, in 1926, worked with him on the statistical mechanics of white dwarf stars. He was in 1927, one of the participants of the fifth Solvay Conference on Physics, that took place at the International Solvay Institute for Physics in Belgium.In 1928 he published (with Lothar Nordheim) a seminal paper that explained the physical phenomenon now known as field electron emission, and helped to establish the validity of modern electron band theory. In 1931, he was the first to formulate and label the zeroth law of thermodynamics. In 1932 he was elected to the Chair of Theoretical Physics at the Cavendish Laboratory.