Temporal logics for continuous time.
Yoram Hirshfeld 
 
We prove a conjecture by A. Pnueli and strengthen it showing a
sequence of modalities none of which is expressible
in the temporal logic generated by the previous modalities, over the
real line, or over the positive reals. We use
this sequence to prove that over the real line there is no finite
temporal logic that expresses all the natural
properties that arise when dealing with systems that evolve in
continuous time.