At the instant of the big bang, for example, the original spacetime structure presumably expanded for a split second and did so much faster than any ray of light could travel. Even today, the expansion continues to drive extremely distant galaxies away at speeds faster than light, which means their light can no longer reach us.
Based on his discovery, Alcubierre surmised that it would only be a small step to a warp drive. If spacetime were contracted in front of a spaceship and expanded behind it to compensate, it would be possible to travel to one’s destination at a speed faster than light. The ship would remain encapsulated in a bubble, and the crew would not sense the magnitude of the interstellar journey. In a 2017 lecture, Alcubierre compared it to being on a passenger conveyor belt at the airport: “You can imagine that the floor behind you is being created out of nothing and in front of you it is being destroyed, so you move along.”
But formulating this idea in the language of general relativity immediately gives rise to major practical problems. First, to deform spacetime so radically, you would need to cram a huge mass into a bubble bounded by a wall thinner than an atomic nucleus. Then you would need two forms of matter to maintain the bubble. The gravity of ordinary mass would cause the space at the front of the bubble to contract, moving the whole structure forward. But at the same time, the space at the back of the bubble would need to expand like rising bread dough. To make that expansion happen, according to Alcubierre, you would need some form of negative energy radiating a kind of antigravity.