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IMPLAUSIBLY high energy demands and insurmountable engineering difficulties haven鈥檛 stopped people fantasising about 鈥渨arp drives鈥� that could propel spaceships across the Universe faster than the speed of light. But now a Portuguese mathematician says he has proved that the idea is impossible.

Although Albert Einstein鈥檚 special theory of relativity says nothing can travel faster than light, Mexican mathematician Miguel Alcubierre found a way round the problem in 1994. He showed that a bubble with walls made of negative energy would distort the space around it. In theory, if the bubble contracted the space immediately in front of it and expanded the space behind, it could travel faster even than light. No one knows if negative energy exists, but Einstein鈥檚 theory doesn鈥檛 rule it out.

Unfortunately, Jose Natario of the Higher Institute of Technology in Lisbon has spelled out some fatal problems with the idea of warp drive. He began by working out what an astronaut standing on the bridge of a starship would see as the bubble overtook the speed of light. He says light coming from in front would be shifted to bright blue, while nothing would be visible behind because light wouldn鈥檛 be able to keep up. The view to the side would look normal. 鈥淯nlike the films, you don鈥檛 see stripes or flashes,鈥� he says.

The problem comes when you consider how observers in the bubble might send signals to the world outside. The astronauts would overtake any signals they sent forward, generating a barrier or 鈥渉orizon鈥� beyond which they could never communicate. Natario calculated that the horizon would actually cut the wall of the bubble in two (see Graphic). That means the travellers couldn鈥檛 send signals to control the part of the bubble wall in front of them, making it impossible to steer or stop. 鈥淭hey can鈥檛 even generate it [in the first place],鈥� says Natario.

Michael Pfenning, who works on warp drive problems at the University of York, suggests people outside the bubble might lend a hand by controlling the bubble walls. 鈥淭his means someone has to travel to the other end at normal speeds first,鈥� he says.

But even if that works, there鈥檚 another, even bigger problem. Any light or other radiation emitted by the travellers would build up at the horizon, unable to get past. Natario worked out that because the boundary is infinitely thin, the energy from that radiation would get squashed to an infinite density. Under such extreme conditions, physics as we know it would break down.

Pfenning says there鈥檚 a slim chance that light rays don鈥檛 really behave like the geometrical curves Natario considers. So he鈥檚 going to keep working on the problem. 鈥淚t was always going to be an engineering nightmare,鈥� he says. But even if warp drive can never break the light barrier, he says it might still be possible to use it to travel just below light speed 鈥� too slow to compete with Captain Kirk and his crew, but fast enough to reach neighbouring star systems in decades.