4 The Main Ideas for Unification between about 1918 and 1923

After 1915, Einstein first was busy with extracting mathematical and physical consequences from general relativity (Hamiltonian, exact solutions, the energy conservation law, cosmology, gravitational waves). Although he kept thinking about how to find elementary particles in a field theory [70] and looked closer into Weyl’s theory [72], at first he only reacted to the new ideas concerning unified field theory as advanced by others. The first such idea after Förster’s, of course, was Hermann Weyl’s gauge approach to gravitation and electromagnetism, unacceptable to Einstein and to Pauli for physical reasons [246, 292].

Next came Kaluza’s five-dimensional unification of gravitation and electromagnetism, and Eddington’s affine geometry.