One issue is that photons exiting the front of a warp bubble, where space is compressed, are blueshifted; they have shorter wavelengths. That means the photons gain energy, but in order to preserve conservation of energy, they’d siphon it from space that already has negative energy, destabilizing the whole bubble.
“In my paper,” says Natarió, “I prove that this problem occurs for all warp drives.”
“Unless one finds a way to tame those instabilities,” says Barceló, “the warp drive would get destroyed in a very short time period.”
Natarió highlights another issue. Even if you did successfully create a warp bubble with negative energy, there’s no known way to move it forward in space, in much the same way you can’t hear a supersonic airplane before it arrives. “You cannot tell space to deform so that the warp bubble can move forward,” he says.