Natural Complexity, Computational Complexity and Depth
  Depth is a complexity measure for natural systems of the kind studied in statistical physics and is defined in terms of computational complexity. Depth quantifies the length of the shortest parallel computation required to construct a typical system state or history starting from simple initial conditions. The properties of depth are discussed and it is compared to other complexity measures. Depth can only be large for systems with embedded computation. 
Jon Machta
physics.pop-ph cond-mat.stat-mech nlin.AO