The article is badly edited, speaking of both "negative density energy" and "negative energy density", when the latter is what was meant.<p>Even though, as I understand it, General Relativity doesn't forbid FTL explicitly, it doesn't get you out of the causality problems of FTL. At least, I've never seen an explanation of a "warp" style FTL method that doesn't allow for causality violation. I'm all ears, if someone wants to convince me that warp drives can't violate causality.