  In the last 100 years, the most important equations in physics are Maxwell's
equations for electrodynamics, Einstein's equation for gravity, Dirac's
equation for the electron and Yang-Mills equation for elementary particles. Do
these equations follow a common principle and come from a single theory?
Despite intensive efforts to unify gravity and the particle interactions in the
last 30 years, the goal is still to be achieved. Recent theories have not
answered any question in physics. We examine the issues involved in this long
quest to understand the ultimate nature of spacetime and matter.
