  Field theories based on non-commutative spacetimes exhibit very distinctive
nonlocal effects which mix the ultraviolet with the infrared in bizarre ways.
In particular if the time coordinate is involved in the non-commutativity the
theory seems to be seriously acausal and inconsistent with conventional
Hamiltonian evolution. To illustrate these effects we study the scattering of
wave packets in a field theory with space/time non-commutativity. In 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 field theories are
evidently inconsistent and violate causality and unitarity. On the other hand
open string theory in a background electric field is expected to exhibit
space/time non-commutativity. This raises the question of whether they also
lead to acausal behavior. We show that this is not the case. Stringy effects
conspire to cancel the acausal effects that are present for the non-commutative
field theory.

High Energy Physics - Theory