 Phillippe Eberhard and Ronald R. Roos (1989) have established a theorem which says that if quantum mechanics is correct, it is impossible to use quantum effects to generate a break in the chain of normal causation. Quantum field theory does not allow any superluminal communication between different observers. Indeed, this is not so strange, since quantum field theory is relativistically invariant whereas superluminal frames of reference are not. But Eberhard and Roos’ theorem does not rule out all forms of backward causation. Two possible scenarios are still open: (1) entangled pairs exchange some form of superluminal information (and energy) below the limits of Heisenberg’s uncertainty relations; or (2) causation may be symmetrical so that the direction of causation in a physical system is determined by its boundary conditions.