It is a pleasure to contribute this paper in honor of Charlie Misner for his many contributions to gravitational theory and for his warm friendship.

INTRODUCTION

The discovery of general relativity by Einstein and its early experimental verification excited at that time both the scientific and lay public alike. However, during the 1930s and 1940s the hope that gravity would be a unifying principle of nature faded. The discovery of the self-energy infinities of Lorentz covariant quantum field theory indicated the insufficiency of the quantum theoretical framework. Subsequently, it was realized that these infinities were even more virilant in the non-renormalizable general relativity. Most significant was the experimental discovery during this period of the weak and strong interactions, implying that the original ideas of Einstein and Weyl to unify gravity with electromagnetism were premature. Perhaps the one idea from this era that has remained in present day efforts to unify interactions was the most radical: the suggestion by Kaluza and Klein that there might exist additional compactified dimensions in space-time. Most remarkable was the work of Oscar Klein who, using dimensional reduction, discovered non-abelian gauge theory and applied it to construct a precursor of present day electro-weak theory. This was a spectacular theoretical tour-de-force which unfortunately did not appear to stimulate further work at that time.

The development of the Glashow-Weinberg-Salam model of electro weak interactions combined with the QCD theory of strong interactions to form the Standard Model, has led to the recent approaches to build models of unified interactions.