Could geometry itself break the equivalence principle?

Einstein's equivalence principle — the bedrock idea that all objects fall identically — may break down in metric-affine gravity, where spacetime geometry carries extra structure beyond curvature. The authors show this geometric violation produces a shift in the ratio of gravitational to inertial mass that mathematically mirrors thermal corrections from quantum field theory at finite temperature. A new geometric tool they introduce reveals that no standard reference frame can even be consistently carried along an observer's path in such spacetimes — making the violation detectable in principle.