No, that’s a *terrible* “caveat” that people often bring up when talking about warp drives/wormholes/god knows what else. It’s not “negative mass.” It’s magic fairy dust. It’s the equations literally telling you that can’t happen with stuff you know about, and so magically inventing something that *allows* it to happen is entirely circular logic.

The problem with general relativity is that we only know how to include *really* basic stress-energy terms, and even those only at energy densities such that the “gravity” part doesn’t matter. For instance, general relativity *says* you should be able to create a black hole with just an electromagnetic field (a “kugelblitz”). But that’s crap, because by the time you *get* to those energy densities it doesn’t act like Maxwell’s equations anymore – you’re way, way into quantum electrodynamics.

In fact, there are “energy conditions” that get applied to general relativity so that the stress-energy tensor involved is in some way “sane.” Because we have *no idea* what the stress-energy tensor for, say, a neutron star is, so you come up with some ‘sane’ limits to understand it.

Note that those “sane” limits aren’t even enough! See the above kugelblitz example, for instance. It’s perfectly fine from an energy condition standpoint, it’s just that the electromagnetic field that actually exists doesn’t work like that.

The “negative mass” stuff *throws even those sanity conditions out*, which means you’re totally into circular logic territory. You write down a metric, the equations tell you “no, that can’t work” and you say “but… what if it did?”