Salpeter equation and causality
  Acausal behavior of solutions to free Salpeter equation is considered . It is shown that the formal properties of solutions suggest the acausal propagation of quantum phenomena. On the other hand the same properties of solutions describing macroscopic phenomena can be explained without appealing to the notion of acausality. 
Piotr Kosinski
math-ph hep-th math.MP quant-ph