Two Theorems on Flat Space-Time Gravitational Theories

The first theorem states that all flat space-time gravitational theories must have a Lagrangian with a first term that is an homogeneous (degree-I) function of the 4-velocity $u^i$, plus a functional of ${\eta }_{ij}{u}^i{u}^j$. The second theorem states that all gravitational theories that satisfy the strong equivalence principle have a Lagrangian with a first term $g_{ij}(x)u^i{u}^j$ plus an irrelevant term. In both cases the theories must issue from a unique variational principle. Therefore, under this condition it is impossible to find a flat space-time theory that satisfies the strong equivalence principle.