# What exactly is the motion in Warp drive?

I know that there exists something like the Alcubierre warp bubble, and there are many threads here about it, but unfortunately the answers didn't help me to understand what the basic principle is...

How I understand warp drive is that it is based on compressing spacetime in front of the starship (like putting a large mass there) and expanding behind the ship (like putting a large negative mass there). But why do you move then at all? My initial feeling is that when switching off the drive, spacetime is flat again and everything is at the original place... so how do you move?

Or do you start “surfing down spacetime” when you compress it in front of your starship, like being attracted by a large mass you put there (a bit like Jim Button's perpetuum mobile fixing magnets to his steam locomotive...). However, this would involve real movement and then you couldn’t move faster then light, could you?

• I wouldn't call it 'exist', as no one has even remotely tried to build one!
– Gert
Commented May 14, 2021 at 16:19
• @Gert Yes, that is absolutely true! What I meant is that the theoretical concept exists and that there are a lot of threads here about, but unfortunately they don't help me with my understanding problem. Commented May 14, 2021 at 17:50
• If space wise the horizon is the center... Is r in that equation above then the distance to the event horizon? Thought it's really the radius of the event horizon as an observer far away would see it” - There $r$ is not radius, but reduced circumference (the length of the equator circle divided by $2\pi$). In a flat spacetime, radius and reduced circumference are the same, but in a curved spacetime, they are different (a sphere with a zero spatial radius can have a finite circumference). Note that the radius isn’t zero, only it’s spatial part is zero. It also has a non-zero temporal part. Commented Jul 17, 2021 at 5:19
• @safesphere Many thanks! Commented Jul 17, 2021 at 8:44

As shown by Natario+2006 (https://arxiv.org/abs/gr-qc/0110086), stretching and compressing spacetime around the Alcuibierre bubble 'is but a marginal consequence of the choice made by Alcubierre' and has nothing to do with how it moves. Bobrick+2021 (https://arxiv.org/abs/2102.06824) argue that warp drives move inertially, just like any other massive objects do.

• But then, are you again limited to the speed of light? Commented May 16, 2021 at 19:26
• Yes, in the same way as ordinary massive particles are. In other words, having a warp drive moving faster than the speed of light requires moving like that from the start. Or else, accelerating past the speed of light requires infinite energy or superluminal matter or negative mass. Commented May 17, 2021 at 16:03