  The dynamics of a massive, relativistic spinning particle could be described
either by the Dirac equation or by the Kerr solution of Einstein equations.
However, one does not know a priori as to which of the two systems of equations
should be used in a given situation, and the choice is dictated by experiments.
It is expected that the Dirac equation holds for microscopic masses, and the
Kerr solution for macroscopic masses. This suggests that Einstein gravity and
the Dirac theory are limiting cases of a common underlying theoretical
framework. Here we propose that such a framework is provided by a geometric
theory of gravity on a Riemann-Cartan spacetime, which includes torsion. The
Dirac equation emerges as the torsion dominated, gravity-free limit of this
framework.
