In the most basic of definitions, gravity is the attractive force between two objects. Every object, no matter how small, exerts some attractive force on surrounding matter. Einstein’s insight was that this force is a curvature in space-time. Negative energy, though, is gravitationally repulsive. Instead of drawing space-time together, negative energy would push it apart. Roughly speaking, for his model to work, Alcubierre needed negative energy to expand the space-time behind a craft.
Though no one has ever measured negative energy, quantum mechanics predicts that it exists, and scientists should be able to create it in a lab. One way to generate it would be through the Casimir effect: two parallel conducting plates, placed very closely together, should create small amounts of negative energy. Where Alcubierre’s model broke down is that it required a vast amount of negative energy, orders of magnitude more than most scientists estimate could be produced.
White says he’s found a way around that limitation. In a computer simulation, White varied the strength and geometry of a warp field. He determined that, in theory, he could produce a warp bubble using millions of times less negative energy than Alcubierre predicted and perhaps little enough that a space craft could carry the means of producing it. The findings, he says, change it from impractical to plausible.