  In his search for a unified field theory that could undercut quantum
mechanics, Einstein considered five dimensional classical Kaluza-Klein theory.
He studied this theory most intensively during the years 1938-1943. One of his
primary objectives was finding a non-singular particle solution. In the full
theory this search got frustrated and in the x^5-independent theory Einstein,
together with Pauli, argued it would be impossible to find these structures.
