How to make a warp bubble moving?

Question

In answer to this question it is stated that:

In summary, warp drives move because they already move, or else they need to propel, and their movement is not related to spacetime expansion or contraction.

This means that firstly a warp bubble had to be accelerated at some speed. The speed is higher than speed of light (this is a reason why we try to construct a warp bubble). However, how is it possible to break "light barrier"? Is it thanks to a "special" spacetime geometry of the warp bubble that allows a conventional propulsion system to break the barrier?

Answer 1

Currently, there is no known mechanism to getting the warp bubble moving at all, let alone to superluminal velocities. In Alcubierre's original metric, he assumed that the warp bubble would already be moving (see original paper), and didn't consider how that warp bubble would start moving in the first place.

And firing conventional rocket engines or any other propulsion system in a warp bubble doesn't do anything to this effect either. The "inside" of a warp bubble is flat spacetime, and so firing rockets inside of it would be essentially like firing rockets in interstellar space; it doesn't affect the warp bubble itself.

In fact, this is probably the big issue affecting all warp drives that still remains unresolved. Here is another physics SE post that elaborates more on this issue.

Answer 2

I don’t think @JS4137’s answer is quite complete. The warp bubble moves because its (fictional exotic) mass moves. The energy-momentum tensor of the alcubierre drive has forward-pointing momentum on the inside of the torus and backward-pointing momentum on the outside of the torus. I’ve seen someone elsewhere on a forum liken this to a smoke ring. It kind of is producing a frame dragging effect on the bubble, pushing it along. I’m currently working on an extension of Einsteinpy that demonstrates this nicely and will share my results in the documentation there.