Space/time non-commutativity and causality

Field theories based on non-commutative spacetimes exhibit very distinctive non-local effects which mix the ultraviolet with the infrared in bizarre w ays. In particular if the time coordinate is involved in the non-commutativ ity the theory seems to be seriously acausal and inconsistent with conventi onal hamiltonian evolution. To illustrate these effects we study the scatte ring of wave packets in a field theory with space/time non-commutativity. I n this theory we find effects which seem to precede their causes and rigid rods which grow instead of Lorentz contract as they are boosted. These fiel d theories are evidently inconsistent and violate causality and unitarity. On the other hand open string theory in a background electric field is expe cted to exhibit space/time non-commutativity. This raises the question of w hether they also lead to acausal behavior. We show that this is not the cas e. Stringy effects conspire to cancel the acausal effects that are present for the non-commutative field theory.