An Alcubierre drive as I understand it is based on the idea of dangling a carrot in front of a horse with one hand and lighting its tail on fire with the other. Its desire for the carrot and its aversion to the fire drives the horse forward. The salient difference is that an Alcubierre drive would manipulate gravity to achieve the same effect. Expand the space behind the ship and contract the space in front of it. So why can we not propel a spaceship forward by making it, say, positively charged in front and negatively charged in back, and then by dangling a negative charge in front of it and another negative charge behind it? The attraction in front and the repulsion in back would drive the whole thing foward. It would be an EM Alcubierre drive, would it not? I don't doubt for a second that what I just suggested is impossible. I suppose I'm just asking "why is it impossible?"