Geometric Gravitational Forces on Particles Moving in a Line
  In two-dimensional space-time, point particles can experience a geometric, dimension-specific gravity force, which modifies the usual geodesic equation of motion and provides a link between the cosmological constant and the vacuum $\theta$-angle. The description of such forces fits naturally into a gauge theory of gravity based on the extended Poincar\'e group, {\it i.e.\/} ``string-inspired'' dilaton gravity. 
Daniel Cangemi and Roman Jackiw