String theory
By the equivalence principle, Einstein inferred that being under either gravitation or constant acceleration are indistinguishable experiences that might share a physical mechanism. The suggested mechanism was progressive length contraction and time dilation—a consequence of the local energy density within 3D space—establishing a progressive tension within a rigid object, relieving its tension by moving toward the location of greatest energy density. Special relativity would be a limited case of a gravitational field. Special relativity would apply when the energy density across 3D space is uniform, and so the gravitational field is scaled uniformly from location to location, why an object experiences no acceleration and thus no gravitation.