Revisiting the expressiveness of metric temporal logic : A tale of "Je t'aime, moi non plus."

The expressiveness of Metric Temporal Logic (MTL) has been extensively studied throughout the last two decades. In particular, it has been shown that the \emph{interval-based} semantics of MTL is strictly more expressive than the \emph{pointwise} one. These results may suggest that enabling the evaluation of formulae at arbitrary time points \emph{instead of} positions of timed events increases the expressive power of MTL. In this paper, we formally argue otherwise. We demonstrate that under standard models of finite or non-Zeno infinite (action-based) timed executions, the interval-based and the pointwise semantics are incomparable, and therefore disprove a twenty-year-old result. We then propose a new \emph{mixed} semantics that embeds both the pointwise and the interval-based ones.