Kaluza–Klein unified field theory and apparent four-dimensional space-time

In the 1920s Kaluza and Klein achieved an elegant unified theory of gravitation and electromagnetism by assuming that space-time is really 5-dimensional. Their approach has since been extended to even higher dimensions in an effort to provide a geometrical unification of all the fundamental interactions. Any such scheme must answer the obvious objection emphasized by Einstein: Why, then, does space-time appear to be only 4-dimensional? This paper provides a semihistorical introduction to Kaluza–Klein unification on a level accessible to those with a basic knowledge of general relativity and particle physics, and examines the progress made in answering Einstein’s, and related, objections.