Many physical laws involve relationships between time derivatives to space derivatives of one or more quantities. For example, thermal conduction relates the thermal energy time rate of change [dQ/dt] to temperature space rate of change [dT/dx]. In fluid flow, the Navier-Stokes Theorem relates the time rate of change of velocity [du/dt] to the first and second space derivatives of velocity [u' =du/dx and du'/dx]. In quantum mechanics, Schroedinger equation relates the time rate of change of the wave function [d(psi)/dt] to the second space derivative the of the wave function [d^2(psi)/dx^2]. In general, what does it say about a phenomenon that time and space derivatives are proportional?