If you look up the Alcubierre drive on the wiki page there is a singularity on the bubble. Contracting space in front and expanding behind is all well and good, but there seems to be a plane centered on the bubble and perpendicular to the direction of travel at which there is no change in the dilation of space/time. if this were to pass an outside observer then it would be traveling FTL and would include a infinitesimal slice of the traveling spacecraft with a non zero mass. This would seem to contradict general relativity unless this plane had zero width and discontinous functions don't really seem that possible in this context. I am not a theoretical physicists so I hope someone can tell me that I am incorrect (I probably wouldn't understand the explanation unless simple).