Remarks on time-space non-commutative field theories

We propose a physical interpretation of the perturbative breakdown of unitarity in time-like non-commutative field theories in terms of production of tachyonic particles. These particles may be viewed as a remnant of a continuous spectrum of undecoupled closed-string modes. In this way, we give a unified view of the string-theoretical and the field-theoretical no-go arguments against time-like non-commutative theories. We also perform a quantitative study of various locality and causality properties of non-commutative field theories at the quantum level.