The gauge principle and Levi-Civita's parallel transport: The fundamental forces in nature (gravitational, eletromagnetic, strong, and weak interaction) are based on the symmetry of the action functional under local gauge transformations. The corresponding parallel transport of physical information generates the intrinsic Gauss–Riemann–Cartan– Ehresmann curvature which, roughly speaking, corresponds to the acting force (also called interaction). Briefly: force = curvature.