  With appropriate modifications, the multi-spin Klein-Gordon (KG) equation of
quantum field theory can be adapted to curved spacetime for spins 0,1,1/2. The
associated particles in the microworld then move as a wave at all spacetime
coordinates. From the existence in a Lorentzian spacetime of a line element
field $(X^{\beta},-X^{\beta}) $, the spin-1 KG equation
$\nabla_{\mu}\nabla^{\mu}X^{\beta}=k^{2}X^{\beta} $ is derived from an action
functional involving $X^{\beta} $ and its covariant derivative. The spin-0 KG
equation and the KG equation of the outer product of a spin-1/2 Dirac spinor
and its Hermitian conjugate are then constructed. Thus, $ X^{\beta} $ acts as a
fundamental quantum vector field. The symmetric part of the spin-1 KG equation,
$ \tilde{\varPsi}_{\alpha\beta}$, is the Lie derivative of the metric. That
links the multi-spin Klein-Gordon equation to Modified General Relativity (MGR)
through its energy-momentum tensor of the gravitational field. From the
invariance of the action functionals under the diffeomorphism group Diff(M),
which is not restricted to the Lorentz group, $ \tilde{\varPsi}_{\alpha\beta}$
can instantaneously transmit information along $ X^{\beta} $. That establishes
the concept of entanglement within a Lorentzian formalism. The respective
local/nonlocal characteristics of MGR and quantum theory no longer present an
insurmountable problem to unify the theories.
