

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 [242
, 291].
Next came Kaluza’s five-dimensional
unification of gravitation and electromagnetism, and Eddington’s affine geometry.

