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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?

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    $\begingroup$ I wouldn't call it 'exist', as no one has even remotely tried to build one! $\endgroup$
    – Gert
    May 14 at 16:19
  • $\begingroup$ @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. $\endgroup$ May 14 at 17:50
  • $\begingroup$ 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. $\endgroup$
    – safesphere
    Jul 17 at 5:19
  • $\begingroup$ @safesphere Many thanks! $\endgroup$ Jul 17 at 8:44
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Please note that my answer is simplification.

As you pointed out, the spacetime is expanded behind the ship and compresed in front of it. You can imagine this like an expanding air. As a result you are pulled in the direction of stretched spacetime. As a result the warp bubble is moving through spacetime fabric. Since the ship is attached to the bubble, you said correctly, that effectively you are surffing the spacetime.

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  • $\begingroup$ So it's not your ship alone that is attracted by some "virtual mass" in front of it, but the spacetime distortion is a bit more complicated and effectively you are in a bubble that is attracted and moving together with you, correct? However, then there is some real motion invlved and I don't understand why you shouldn't be limited to the speed of light? $\endgroup$ May 15 at 9:52
  • $\begingroup$ @CharlesTucker3: I wouldn't say that there is a virtual mass, the spacetime is warpped so that a "wave" is created, and this wave propagates throught spacetime. As the ship is attached to the wave, it moves together with it. Concerning supeluminal speed, the spacetime can expand (contract) with any velocity, there is no max. limit (this is similar to expansion of the Universe and receding galaxies - very distant ones can move away from us with speed higher than c). Inside the bubble (i.e. locally), everything is governed by special relativity. $\endgroup$ May 15 at 16:14
  • $\begingroup$ Understood! And I know that no technique exist to compress or expand spacetime. However, compression appears more logic since masses do that. But how to expand behind your ship... Is there any theoretic effect that could do that? $\endgroup$ May 16 at 15:28
  • $\begingroup$ @CharlesTucker3: Not only mass but any energy as well as mass and energy are equivalent. I see an experimental setting using electromagnetic field to curve the spacetime, however, it seems that currently we do not have enough sensitive measurement instruments to carry the experiment out. See this thread: physics.stackexchange.com/questions/541362/… $\endgroup$ May 16 at 21:12
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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.

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  • $\begingroup$ But then, are you again limited to the speed of light? $\endgroup$ May 16 at 19:26
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    $\begingroup$ 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. $\endgroup$ May 17 at 16:03

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