  Gravitational and electroweak interactions can be unified in analogy with the
unification in the Weinberg-Salam theory. The Yang-Mills framework is
generalized to include space-time translational group T(4), whose generators
$T_{\mu}(=\p/\p x^{\mu})$ do not have constant matrix representations. By
gauging $T(4) \times SU(2) \times U(1)$ in flat space-time, we have a new
tensor field $\phi_{\mu\nu}$ which universally couples to all particles and
anti-particles with the same constant $g$, which has the dimension of length.
In this unified model, the T(4) gauge symmetry dictates that all wave equations
of fermions, massive bosons and the photon in flat space-time reduce to a
Hamilton-Jacobi equation with the same `effective Riemann metric tensor' in the
geometric-optics limit. Consequently, the results are consistent with
experiments. We demonstrated that the T(4) gravitational gauge field can be
quantized in inertial frames.
