# Quantum setback for warp drives

Include quantum mechanics in the calculations and faster-than-light drives become unstable

Bad news I’m afraid – it looks as if faster-than-light travel isn’t possible after all. That’s the conclusion of a new study into how warp drives would behave when quantum mechanics is taken into account. “Warp drives would become rapidly unstable once superluminal speeds are reached,” say Stefano Finazzi at the International School for Advanced Studies in Trieste, Italy, and a couple of friends.

Warp drives have been the focus of science fiction writers for decades. But scientists kept them at arms length until 1994 when the idea was put on a firm (ish) theoretical footing by the Mexican physicist, Michael Alcubierre. His thinking is that while relativity prevents faster-than-light travel relative to the fabric of space time, it places no restriction on the speed at which regions of spacetime may move relative to each other.

Alcubierre imagined a small volume of flat spacetime in which a spacecraft might sit, surrounded by a highly distorted bubble of spacetime which shrinks in the direction of travel, bringing your destination nearer, and stretches behind you. He showed that this shrinking and stretching could enable the bubble–and the spaceship it contained–to move at superluminal speeds.

The conclusion is the result of classical thinking using the ideas of general relativity but physicists have long wondered what would happen if you threw quantum mechanics into the mix? Now Finazzi and pals have worked it. For a start, they say that the inside of the bubble would be filled with Hawking radiation, making life rather uncomfortable for any spacecraft within it.

They have also studied a property of a quantum field called the renormalised stress-energy tensor which should be well-behaved under normal circumstances. But in the front wall of Alcubierre’s bubble travelling at superluminal speeds, the renormalised stress-energy tensor grows exponentially.

That strongly implies that such a bubble would be unstable. So it looks increasingly likely that, after a brief few years of excitement, Alcubierre’s warp drive is impossible.

Shame really.

Ref: arxiv.org/abs/0904.0141: Semiclassical Instability of Dynamical Warp Drives