Unification of Gravity and Electromagnetism Revisited

Unification of gravity and electromagnetism based on a theory with an affine non-symmetric connection $\Gamma^\lambda_{\mu\nu} \neq \Gamma^\lambda_{\nu\mu}$ and $\Gamma_\mu = \Gamma^\lambda_{[\mu\lambda]}\neq 0$, proposed by S. N. Bose in 1953, is revisited in the context of modern developments in high energy physics. It is shown that electrogravity unification can be achieved at a {\em premetric} stage, and that this unification based on {\em projective invariance} is broken by matter fields, opening up the possibility of a unified theory of all forces in which gravity emerges as a classical field. The theory predicts $\alpha_{sym} = \frac{1}{9}$ in the projective invariant limit, where $\alpha$ is the fine structure constant