They explain part of that in the article > Today, however, it’s recognized that what’s needed isn’t necessarily negative mass or negative energy; that was simp

They explain part of that in the article > Today, however, it’s recognized that what’s needed isn’t necessarily negative mass or negative energy; that was simply the way that Alcubierre recognized one could induce the needed “opposite type” of curvature to space from what normal mass or energy causes. There's other options, and also we aren't 100% certain negative mass/energy can't exist (but probably not...) Edit: changed my tone because I was being a snarky jerk.