  As is well known, there exists warp drives in GR, such as the Alcubierre
bubbles, which achieve an apparent faster than light travel \cite{alcubierre}.
A result due to Gao and Wald \cite{gaowald} suggests that such a travel is
unlikely for GR with matter satisfying both the Null Energy and the Null
Generic Conditions. There exists a generalization of this statement due to
Galloway, that ensures that the Gao-Wald result is true regardless the
underlying gravity model, unless there exists at least one inextendible null
geodesic with achronal image in the space time (a null line). The proof of this
proposition is based on techniques of causal theories, and has never been
released. In the present work an independent proof of this result is presented
by use of the Raychaudhuri equation, and avoiding several technical
complications described along the text. Some consequences of these affirmations
are discussed at last section, in particular their potential use in problems of
causality.
