Space-Time Noncommutativity, Discreteness of Time and Unitarity

TL;DR

This paper investigates how noncommutative field theories on compact space-times inherently violate unitarity and causality due to the discreteness of time, leading to unphysical modes and fundamental theoretical issues.

Contribution

It demonstrates that space-time noncommutativity causes unitarity violations even without nonrenormalizable interactions, highlighting the impact of discrete time on quantum field theories.

Findings

Unitarity is violated in noncommutative theories on compact space-times.

Discreteness of time leads to unphysical modes and causality violations.

Violations persist even without star-product interactions.

Abstract

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 space-like coordinates implies discreteness of the time variable which leads to 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 as 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 the scale of causality violation as the inverse of the mass appearing in the considered model, i.e., even more severe. We also stress the role of smearing over the noncommutative coordinates entering the field operator symbols.