Eventually the energy would run out. The bubble would rupture, with catastrophic effects. Inside the bubble the temperature would rise to about 10^32 degrees Kelvin, destroying almost anything on the bubble.
Anyone watching the ship nearby wouldn't be much better off.
"We know that the warp drive will be destabilized," said Finazzi. "But we do not know if it will in the end explode or collapse to a black hole."
Other physicists agree with the Italians' calculations, up to a point.
"It's a good paper; their results are sound," said Gerald Cleaver, a professor of physics at Baylor University who reviewed the work. The results make sense, at least, when creating warp drive using exotic matter in a universe where 1 plus 1 equals 2.
In a universe where 1 plus 1 equals 3, a possibility with string theory instead of the semi classical physics used by the Italians, a stable warp drive is viable.
Last year Cleaver and co-author Richard Obousy detailed a string theory-based warp drive that creates a bubble of space time by expanding one of the tiny, rolled-up dimensions (instead of a bubble of dark energy) predicted by string theory.