I have a scene where a fleet of ships activates thier Alcubierre Type Warp Drives to flee from an enemy fleet, but unlike many sci-fi settings where a warp drive instantly pushes you up to superluminal speeds for a quick get-away, in this setting, you have to accelerate to superluminal speeds over time; so, there is a window where you can have an active warp drive, but until you break the light speed barrier, you can still be shot with conventional weapons. This means that as they flee, ships will be getting taken out by enemy lasers while technically moving at warp. So, what I want to know is how the presence of a warp bubble will effect what it looks like when these fleeing ships explode.

An Alcubierre Drive in this setting is defined as a reactionless propulsion system that manipulates space-time such that the area in front of your ship resembles an extreme positive mass density, and the area behind it an extreme negative mass density. This causes the ship to "fall" perpetually in the direction of the positive mass density. For purposes of this question, we will assume the toroidal shaped warp field represented below.

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You can also assume that the Alcubierre Drive's mass density bubble is maintained by (insert clarke tech here) that if spontaneously shut off or destroyed would cause spacetime to snap back to its normally flat self at approximately the speed of light. This would presumably cause some intense gravitational waves to ripple out from the event. This does not necessarily mean that the warp drive of the ship will be the first component to fail; so debris from both before and after the warp drive fails should be considered, but the part of this event I am most interested in is the gravitational wave pattern formed by the collapse of the warp field, and what effect they would have on the debris field pattern of an exploding ship.

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    $\begingroup$ That's not quite how alcubierre warp works... you're describing a bias drive. $\endgroup$ Commented Feb 24, 2022 at 16:39
  • $\begingroup$ @StarfishPrime a Bias drive modifies the constant of G, an Alcubierre drive bends space time in a way that resembles positive and negative gravity. What did I say that makes you think this is a Bias drive as opposed to an Alcubierre drive? $\endgroup$
    – Nosajimiki
    Commented Feb 24, 2022 at 17:01
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    $\begingroup$ Space inside an alcubierre warp is flat... the contents of the bubble don't fall in the direction of travel. I'm probably thinking of the diametric drive rather than the bias drive... all the old (and apparently unpopular) ideas from the breakthrough propulsion peeps all kinda blur together in my mind. $\endgroup$ Commented Feb 24, 2022 at 17:19
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    $\begingroup$ that is not a definition of an alcubierre drive by any means. The alcubierre metric is defined as a solution to the Einstein Field Equations satisfying certain requirements, none of which you mention, whilst the drive is a hypothetical device that might be able to induce such a metric $\endgroup$
    – Tristan
    Commented Feb 24, 2022 at 17:28
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    $\begingroup$ There is some discussion of exactly what the Alcubierre warp drive implies for travel. You can see this discussion on arxiv.org, but I recommend taking a lot of really big maths with you. I did a PhD in particle physics with quite a bit of gravity mixed in, and I can just about follow the conclusions, not the arguments getting there. Anyway, there is disagreement on whether it can be used to travel, even supposing it can be generated. I only mention it because of the "science-based" tag on the question. $\endgroup$
    – Dan
    Commented Feb 25, 2022 at 2:00

2 Answers 2


The Alcubierre metric consists of two causally isolated regions of space: the inside of the bubble, and the outside. Within each region, space behaves as you would expect for normal relativity, only moving at subluminal speeds, with well-understood relativistic mechanics. It is the bubble itself that moves superluminally and paradoxes are avoided because the two regions are causally isolated from one another meaning there is no path by which anything can enter or exit the bubble (which is why, contrary to pop-sci articles, the Alcubierre metric could never be used as a means of propulsion).

If a ship inside the bubble explodes, everyone inside the bubble sees exactly what they'd see if there was no bubble - an explosion in space, whilst people outside the bubble would see nothing, as they cannot see in. If something outside the bubble explodes, the situation is reversed, with everyone outside the bubble seeing a normal explosion, and people inside having no way to see out.

Given your final paragraph, if the ship's explosion destroys the bubble, you'll end up shrinking the bubble to 0-size, destroying all inside, and having to dissipate all the energy stored in the bubble, and the mass of its contents. Absent any net charge, this would through gravitational waves, albeit of a very unusual kind. As nothing outside the bubble can see into the bubble, I would expect this wavefront to be spherical, but you could make it whatever shape you choose by tweaking the way in which the bubble collapses when the ship explodes.

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    $\begingroup$ Alcubierre Drives are not necessarily superluminal. see worldbuilding.stackexchange.com/questions/176134/…. I am specifically asking about a subluminal drive failure in which case you should be able to see in and out of the bubble just fine. $\endgroup$
    – Nosajimiki
    Commented Feb 24, 2022 at 21:29
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    $\begingroup$ I think the causal separation between the two regions would exist even if the bubble were not moving at relativistic speed. Thus, there could not be any interaction between the interior and exterior. Which makes me think that this bubble would necessarily act as a perfect "shield", since no weapon fired at the bubble could interact with anything inside the bubble (again, regardless of bubble's movement). I'm curious whether @Tristan would agree with that. $\endgroup$
    – Tom
    Commented Feb 25, 2022 at 1:36
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    $\begingroup$ @Tom In the zero-speed limit, the metric is Minkowski space, which isn't a perfect shield. The geometry is a continuous function of the speed so it doesn't become a perfect shield when the speed is nonzero. In fact it is not a perfect shield (or causally isolated) even when v>c; this entire answer is wrong. $\endgroup$
    – benrg
    Commented Feb 25, 2022 at 1:47
  • $\begingroup$ The interior and exterior of the bubble aren't causally disconnected. Light leaving the bubble in the forward direction gets stuck in the boundary, but that only happens in the forward direction. The bubble could be opaque if the phlebotinum in the boundary is opaque, but that doesn't make it impervious to explosions. There's no physics of Alcubierre drives; people just write down whatever metric they want, so there's no way to predict what would happen in a situation like an explosion. $\endgroup$
    – benrg
    Commented Feb 25, 2022 at 1:54

No effect at all

You have defined an engine that is "a reactionless propulsion system" and then asked what the reaction of the propelled explosion particles will be. All the forces of general relativity will act on the particles, and no force at all will be imparted onto the particles from the deformation of spacetime. It's actually in your own definition.

Everything will follow the exact vector it was given by the combination of its relativistic momentum plus the explosive force.

I should add that properly, your ship’s relativistic momentum while using a reactionless drive (propulsion is a misnomer here) should be zero, by definition. This is why every sci-fi that used a consultant were told to “instantly go to light speed” in the trope you want to avoid. It begs the question, why use Newtonian forces at all?


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