Primary purpose explanation

Great news for Star Trek fans: warp drives that can propel starships around the Galaxy faster than the speed of light may be possible after all--with a little help from Dr Who. In 1994 Miguel Alcubierre, then at the University of Wales in Cardiff, startled physicists by showing that the starship Enterprise's famed faster-than-light propulsion system might not be so ludicrous after all. Alcubierre proposed that a starship could "warp" space so that it shrinks ahead of the vessel and expands behind it. By pushing the departure point many light years backwards while simultaneously bringing destinations closer, the warp drive effectively transports the starship from place to place at faster-than-light speeds.

But in 1997 Michael Pfenning and Larry Ford at Tufts University in Medford, Massachusetts, apparently killed this ingenious idea by showing that it needed far more than the entire energy content of the Universe to work. Now Chris Van Den Broeck of the Institute for Theoretical Physics at the Catholic University of Leuven, Belgium, has resurrected Alcubierre's proposal. The trick lies in using a strange form of warped space, involving a "bubble" with a large internal volume but a tiny surface area.

To picture how it works, first imagine space having just two dimensions. Then suppose there is a bubble which is connected to this flat space by a very thin neck. Inhabitants of this two-dimensional world would find that this neck has a very small circumference, but the inside of the bubble has a large surface area. In the real world, says Van Den Broeck, these 2D circumferences become surface areas and surface areas become volumes: "You can have regions of space that appear small from the outside, but have huge internal volumes."

Science fiction fans will instantly recognise this as one of the key properties of Dr Who's Tardis, which looked like a police box but had a spacious interior. But the real appeal of such bubbles is that their small surface area can be created with very modest amounts of energy. In his paper, published on the Los Alamos general relativity pre-print service, Van Den Broeck uses Pfenning and Ford's results to show that a bubble big enough to contain a starship could be formed using just a gram of suitable space-warping material. And with the space warped conveniently around the ship, once again destination and departure are brought together, allowing the ship to move faster than the speed of light.

Van Den Broeck cautions that this space-warping material must possess negative energy--and no one knows how to make such material at present. Even so, the discovery that warp drives may not need vast amounts of energy is already causing a flurry of excitement among theorists. "This is a beautiful result", says Alcubierre, who is now at the Max Planck Institute for Gravitational Physics in Potsdam, Germany. "Of course, there are still some basic questions--like how does one go about constructing this Tardis space-time--but it puts the concept of space warps back on the agenda."