# If we can make an Alcubierre drive, how fast can it move a ship?

Assuming we ever manage to solve the energy generation problem (and whatever other problems may face it) and build a functioning Alcubierre drive, how "fast" would we be able to travel? That is, from the perspective of an external observer, how much distance could an Alcubierre-driven spacecraft cross in a given amount of time? Or, phrased yet another way, from the perspective of the stellar bodies we're leaving from and arriving at, how quickly can we cross that distance?

Is it simply (not that this is a "simple" matter...) a function of how much energy we can put into it, or does it have its own theoretical limits?

I understand (or at least think I do) that the craft does not exceed c within its local space-time "bubble", but I also understand that the contraction/expansion of the surrounding space-time results in the craft "breaking" that speed limit relative to other bodies around it, and I'm interested in knowing what that speed could be -- hence the multiple phrasings of the question, because I'm not entirely sure what the "correct" way to phrase this question is.

– user
Apr 16, 2014 at 21:44
• That article mentions "arbitrarily large velocities", which I interpret as meaning there's no theoretical limit -- but that statement's from Alcubierre, so White's modifications (and the apparent source of the 10c claim) may have rendered that obsolete. Apr 16, 2014 at 22:32
• @MichaelKjörling I know you're quoting other answers but in doing so you are effectively answering the question. Comments are not for answers, please convert this into a post. Apr 17, 2014 at 12:09
• @called2voyage I'd love to write up a proper answer, but I really don't think I know enough about the subject to not simply plagiarize the existing answer. However, I also don't think this question is a duplicate of the linked one. Also, what appears to be one of the main references of the cited answers doesn't even want to download for me right now.
– user
Apr 17, 2014 at 12:21
• @MichaelKjörling It wouldn't be plagiarism as long as you attributed them correctly, because they are under the cc by-sa 3.0 license: blog.stackoverflow.com/2009/06/attribution-required Apr 17, 2014 at 13:07

My understanding is that you're asking what the theoretical maximum speed would be if you could build and use a Alcubierre drive.

I've done a bit of research and it seems the current thinking is that even though the physics of an Alcubierre drive don't break relativity they do not work in thermodynamics. In a paper by [Stefano Finazzi, Stefano Liberati, and Carlos Barceló] it is suggested that the superluminal speed of the bubble would cause massive heating due to hawking radiation. This heating would be so intense it would not only destroy anything inside the bubble (you and your spacecraft) it would also destabilize the bubble itself.

Ref: Stefano Finazzi, Stefano Liberati, and Carlos Barceló, "Semiclassical instability of dynamical warp drives, Physical Review. Link: http://journals.aps.org/prd/abstract/10.1103/PhysRevD.79.124017

• Well, ain't that a downer! However, while this may be a (or rather, another) strike against the Alcubierre drive, it doesn't really answer the question. I guess what I'm asking rather is how fast can it go assuming we solve all the problems with it, not just the energy generation (which, from my admittedly shallow understanding of it, I had thought to be the primary hurdle to overcome). I've now edited my question to make that clearer, although I also upvoted your answer because it's useful information. Apr 16, 2014 at 15:36
• Well my understanding of this paper is that this method is thermodynamically limited to subliminal speeds and that there would be no way around that as the thermal load is proportional to the speed. That being said there might be a way of syncing the thermal load out of the bubble. So beyond that there is no limit to the speed possible from my research. Although I would expect there to be a limit based on the rate of expansion/contraction of space either side of the bubble - since there is a limit to how much the space could be contracted which I would assume to be based on the plank length... Apr 16, 2014 at 15:41
• ... My next thought would be that the contraction down to the plank length is a one ended limit - you could always start with a larger area of space. But then if contract a large enough proportion of space I would guess there would be problems with potentially creating a miniature black hole. So there's an upper limit for you. Of course the upper limit would be based on how close the space you're travelling through is to a hard vacuum. Apr 16, 2014 at 15:44
• Ah, so your understanding of the heat issue is that it effectively imposes a theoretical limit of something less than c. That's actually really interesting, and could make the drive a good choice for interplanetary distances. I don't understand your comments vis-a-vis plank lengths and hard vacuum, though... Apr 16, 2014 at 15:54
• My comments about the plank length and hard vacuum are just thoughts I had while writing about the thermodynamic aspect. Tempted to do a little more reading around the subject before I reiterate them. If they are new ideas I'll publish and send you a copy of the paper! :D Apr 16, 2014 at 15:57

There is no theoretical limit to the speed allowed by an Alcubierre drive. If sufficient energy is supplied, the drive can push through space time at theoretically infinite speeds. I have a theory that if an Alcubierre drive were to be used in a cosmic string, it would possibly be able to extend the cosmic string and thus allow infinitely faster speeds than the already infinite speeds of an Alcubierre drive, using the 'beyond infinity' idea. I am not sure, though. I don't know that an Alcubierre drive would be able to extend a cosmic string, but if something else were able to extend it, I think that my theory of a multiplicity of infinite speeds would probably work. Basically, my answer is the first sentence. There is no limit provided that sufficient energy is supplied.