  Revised translation of Kaluza's historic 1921 paper, "Zum Unit\"atsproblem
der Physik," on 5-dimensional spacetime, used to unify gravity and
electromagnetism. This version is based, in part, on a 1984 translation
provided by T. Muta, but revised and formatted using LaTeX to closely match the
original paper in appearance and pagination. Kaluza's original notation is
restored.
