  Three-dimensional turbulence simulations are used to show that the turbulent
root mean square velocity is an upper bound of the speed of turbulent
diffusion. There is a close analogy to magnetic diffusion where the maximum
diffusion speed is the speed of light. Mathematically, this is caused by the
inclusion of the Faraday displacement current which ensures that causality is
obeyed. In turbulent diffusion, a term similar to the displacement current
emerges quite naturally when the minimal tau approximation is used. Simulations
confirm the presence of such a term and give a quantitative measure of its
relative importance.
