So, my understanding of how Alcubierre drives would theoretically work is not particularly complete. I know it involves compressing and expanding space, but I don't really have an intuitive grasp on what that means. So, if you have the technology to create an Alcubierre drive, can you theoretically bend space such that you can create a barrier that light (or anything else) cannot cross? The idea is to use the technology to create a 'perfect' containment system for a fusion reaction so that waste heat cannot get places we don't want it. Could Alcubierre technology be used for this purpose, or not?
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asked Apr 13, 2020 at 17:58
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$\begingroup$ Is this a worldbuilding or physics question? $\endgroup$– rekApr 13, 2020 at 20:04
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$\begingroup$ The Alcubierre metric is a mathematical solution to Einstein's field equations. Physically impossible solutions exist to most physics equations because the equation is only valid under specific conditions. For instance you will not experience infinite gravitational potential energy at the center of a planet, that is because the underlying assumption of the equation breaks down and a "phase-transition" occurs. That said, if you could manipulate the energy-density field at will, a perfect containment system is completely possible, only problem is that no known physics allows for this. $\endgroup$– user110866Apr 13, 2020 at 20:49
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$\begingroup$ @rek Worldbuilding, because I'm trying to figure out how to get hard-ish science fiction spaceships to do what I want them to do with a minimum of handwaving. If I already have Alcubierre drives, then re-purposing them to handle the heat generation problems associated with fusion reactors/engines gets two birds with one stone. $\endgroup$– FlyingLemmingSoupApr 13, 2020 at 22:14
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$\begingroup$ The Alcubierre drive involves a corridor that is separated from the rest of spacetime. As such, what happens in the corridor stays in the corridor. Whether you accept the physical reality of the Alcubierre drive solution to Einstein's field equations I would feel that it can be considered hard science fiction and would plausibly perform as you suggest. To go any further, you would have to actually do the physics to determine whether the tweaking you want actually does exist as a solution. $\endgroup$– Ponder StibbonsApr 15, 2020 at 6:50
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$\begingroup$ Addendum, in spacetime terms, the principle is a box that you put something into and close it. After that nothing gets in or out of the box for a while, then the box opens (essentially from the inside) and whatever is in there gets out. (I keep thinking of the shout-in-a-box cartoon joke). $\endgroup$– Ponder StibbonsApr 15, 2020 at 6:54
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The Alcubierre drive not only allows but actually necessitates the production of event horizons at superluminal speeds - but not necessarily the sort you're looking for.
It's been shown (Finazzi et al. 2009) that once an Alcubierre drive exceeds the speed of light$^{\dagger}$, two event horizons form, one in the front and one in the back. The horizons are, in a sense opposites, with surface gravities of opposite signs and opposite causal connections with the rest of the universe. The horizon in front is, for lack of a better description, like a black hole: matter from inside the bubble cannot travel forward through it out of the bubble. The horizon in back is, similarly, akin to a white hole: matter from outside the bubble cannot travel through it into the bubble.
These are both true relativistic horizons. While the entire bubble is not wrapped in a single one-way horizon like a black hole is, there are still two boundaries dictating the causal structure of space-time and restricting the motion of light and matter. Whether you could modify the Alcubierre metric to form a single horizon seems unlikely; it's a toy model existing in a universe with no outside matter, and modifications would be quite complicated.
On the other hand, a civilization that can build an Alcubierre drive must be able to handle the exotic matter required for its construction and operation might be able to form artificial event horizons in some other manner. If you look at other spacetime metrics, you might be able to find a starting point.
$^{\dagger}$ At subluminal speeds, these horizons disappear. This has the advantage of potentially making it possibly to actually control and steer the drive.
answered May 19, 2020 at 15:49