As a nitpick, negative energy densities are indeed uncommon but they aren’t unphysical. In fact, every quantum field theory admits at least one state with negat

As a nitpick, negative energy densities are indeed uncommon but they aren’t unphysical. In fact, every quantum field theory admits at least one state with negative energy density—its a theorem. But even in quantum field theory there isn’t enough negative energy density to actually form acausal (faster than light) shortcuts through the spacetime.