This paper relates Einstein's mass-shell equation to our proposed combined spacetime 4-manifold M[3], which has the local geometry of {(t + it, x + iy, y + iz, z + ix)}. In solving the mass-shell equation for the energy E linearly, our approach, unlike that of the Dirac equation, is free from the presence of negative energies. Moreover, we find that a left-handed positron is identical to a right-handed electron with the time parameter's sign reversed, which coincidentally is supported by our finding that the Dirac equation does not square to the mass-shell equation unless one identifies anti-matter with matter. Thus, under the premise of the mass-shell equation the Dirac 4-dimensional spinor reduces to a 2dimensional object, but by our geometry the remaining two dimensions of the spin states refer to the same physics by two different physical frames. Consequently the Dirac spinor becomes a 1-dimensional wavefunction that is subject to the description of the (first columns of) three Pauli matrices, which we re-interpret as a 3-dimensional motion in our combined 4-manifold with (0 + iy, 0 + iz, 0 + ix) in the wave universe coinciding with (y, z, x) in the particle universe. In this way we recast the Standard Model in our combined 4-manifold.