Space-time noncommutativity, discreteness of time and unitarity

Violation of unitarity for noncommutative field theory on compact space-times is considered. Although such theories are free of ultraviolet divergences they still violate unitarity, while in a usual field theory such a violation occurs when the theory is nonrenormalizable. The compactness of spacelike coordinates implies discreteness of the time variable which leads to the appearance of unphysical modes and violation of unitarity even in the absence of a star-product in the interaction terms. Thus, this conclusion holds also for other quantum field theories with discrete time. Violation of causality., among others, occurs also in the case of the nonvanishing of the commutation relations between observables at space-like distances with a typical scale of noncommutativity. While this feature allows for a possible violation of the spin-statistics theorem, such a violation does not rescue the situation but makes causality violation scale as the inverse of the mass appearing in the considered model, i.e., it becomes even more severe. We also stress the role of smearing over the noncommutative coordinates entering the field operator symbols.