For example, Alpha Centauri, the nearest neighboring star system, is 4.4 light-years away. Put a starship in a warp bubble. Shrink the distance in front of the starship to a couple of inches, expand the space behind it to 4.4 light-years and then pop the starship out of the bubble.

Voilà! The starship arrives at Alpha Centauri, in less than the 4.4 years it would take a beam of light to travel that distance.

Dr. Alcubierre likened the concept to a moving walkway in an airport. “You just stand there and you move,” he said. “You don’t have to walk. It just pushes you along.”

The key is that relativity does not impose a speed limit on the expansion of space. “Space ca n expand at any speed it wants,” Dr. Alcubierre said.

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The biggest missing piece is that distorting space-time as described by Dr. Alcubierre requires something not known to exist : matter with an energy density less than zero. All known matter — stars, rocks, hot dogs, people and everything else — has a positive energy density, a consequence of E=mc ² .

Normal gravity contracts space. Negative energy has the effect of antigravity, pushing space apart, which you would need for the aspect of the warp drive that expands space.

Physics does not rule out the possibility that negative energy density can exist. Some have proposed that the Casimir effect — a strange but demonstrated phenomenon of quantum mechanics in which an empty vacuum can generate electromagnetic forces to pull objects toward one another — could be employed to generate a negative energy density.