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So, whenever I see people or articles discuss the properties of the warp bubble surrounding an Alcubierre drive, there is always mention of extremely high (yet undefined) tidal forces.

What I want to know, is what kind of tidal forces are we talking about exactly (we'll assume it's a 100m sphere sufficient for 1c velocity)? For example, if I fire a .30-06 bullet at the field, will the bullet be vaporized on impact, or will it's path merely be severely altered, or will it continue through the bubble with relatively little change in direction?

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    $\begingroup$ Possible duplicate of Does the Alcubierre drive/negative energy manipulation imply "antigravity" - check Thucydides answer $\endgroup$
    – LinkBerest
    Nov 16, 2018 at 12:57
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    $\begingroup$ @JGreenwell I have mixed feelings about calling this a duplicate. The question doesn't ask about shields, only antigravity. Thucydides' answer veers off on a tangent to get to the concept of cloak and shield, it has no corroborating evidence, and has neither up- nor downvotes, suggesting no one is sure whether it is a worthwhile answer. Additionally, I think each subject deserves its own time in the light. I'm going to refrain from voting on this question as a result. $\endgroup$
    – Frostfyre
    Nov 16, 2018 at 13:51

3 Answers 3

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The warp bubble created by an Alcubierre Drive is causally disconnected from the outside universe. To put it simply, from the perspective of the outside universe, the bubble and everything in it do not exist.

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  • $\begingroup$ So what happens when you fire a projectile where you think it should be? Also, the best answers have a citation and quote supporting the point. $\endgroup$
    – RonJohn
    Nov 16, 2018 at 3:24
  • $\begingroup$ @RonJohn Doesn't the Alcubierre Drive manipulate space, so you can arrive at your destination faster than light because you are directly manipulating the path light travels? Why wouldn't this also apply to the bullet? $\endgroup$
    – Shadowzee
    Nov 16, 2018 at 4:11
  • $\begingroup$ @Shadowzee photons are massless, whereas bullets aren't. Anyway, it's so theoretical that properly applied technobabble should solve any plot problems you have. $\endgroup$
    – RonJohn
    Nov 16, 2018 at 13:15
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    $\begingroup$ @RonJohn I'll see if I can rustle up a citation. However, to answer your query: The projectile continues on its path unhindered because from the projectile's perspective, there is nothing to intersect. $\endgroup$
    – Arkenstein XII
    Nov 18, 2018 at 19:29
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The bubble could be a deflector shield.

There is a scholarly article on the issue of what happens to matter (and light) which is encountered by a moving bubble. The popular press seized on the last paragraph where the author notes that particles overtaken by the bubble are swept along, accumulating energy and then are release when the bubble stops with an energetic radiant burst.

The rest of the article I really struggled with. I tried to see if there was a popularization, or perhaps a talk given by the author that explains the graphs better. No luck so far.

graph

The Alcubierre Warp Drive: On the Matter of Matter

So you get my take. The author breaks it down for bubbles moving faster and slower than the speed of light.

For particles with initial positive velocity lower than the critical velocity, there are two possibilities, either the velocity is above that of the bubble and hence the particle will catch up to the bubble from behind or the velocity is below that of the bubble and thus the particle will in- teract with the front of the bubble first as the bubble catches up to the particle. In the first case, the parti- cle is ejected from the bubble back out the rear with a reduced but still positive velocity below the ship veloc- ity. For particles with initial velocity closer to the critical velocity, the path follows that of the critical velocity par- ticles for longer, thus getting closer to the ship, before diverging and being ejected from the bubble. Similarly in the second case, the particle is ejected out the front of the bubble with an increased velocity larger than the ship velocity.

Summary: particles and radiation definitely enter the bubble and they definitely can enter the normal space near the ship. They can hit the ship. An energetic particle (bullet?) hitting the front of a fast moving bubble can hit the ship - not with some weird superluminal velocity but a velocity related to that which is initially had.

A thing I struggle with: particles catching up with a bubble from behind are ejected back out the way they came in, SLOWER than they came in. A particle entering the front in ejected back out the front, FASTER than how it came in - here is your deflector shield.

This implies to me that the "moving" bubble would be spraying captured particles out the front the entire time it was moving. The huge spray of particles on coming to the destination is one case of this - all the accumulated particles coming in the front and destined to spray back out the front are released at once.

You want your deflector shield to sit in front of you. This would be a bubble with velocity 0. I do not understand the math well enough to figure out the path of a particle (bullet) entering a bubble at rest.

I am insecure in my interpretation of this paper. I really was hoping for a lay version but no luck. But at least here is some science by someone who does understand; maybe other readers will make more sense of it than I did. Links welcome!

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So, I decided to do some (far more likely that not, incorrect) math, and came up with this:

While the energy required to develop a 100m warp bubble capable of traveling at the speed of light has an uncertainty of up to 80 orders of magnitude [1], I chose a number that would be "difficult but probably not impossible for a decent sized sci-fi space ship to deal with in terms of material consumption", which about equates to 6.12*10^17 Joules, or 6.8 kg of matter converted into energy (antimatter ftw).

Since (presumably) this energy is only used for the warp bubble (barring the inefficiencies that would arise from any real warp drive), we can therefore state that the energy per meter squared over the 100m warp bubble is 4.87*10^12 J/m^2

Now, looking at some side-on pictures of a .50 BMG gives me a rough estimation that the area of the tip of the bullet is about 0.000000452m^2. This means that the energy imparted on the tip of the bullet at impact is about 2.2 MJ. This is about 100 times the muzzle energy of said .50 BMG. Given that .50 BMG bullets (as with any lead bullet) tend to violently break apart due to their own energy when impacting a solid object, I would imagine that something inducing two orders of magnitude higher energy on the bullet would therefore disintegrate the bullet effectively.

Again, this math might be (and probably is) total BS in terms of it being the correct math to use to solve my question, but it at least sounds correct.

The fun side effect of this math being that assuming the warp bubble was not given any additional energy to compensate for what was lost in the bullet impact, it would take at least 2.8*10^11 .50 caliber bullets, which appears to (very roughly) be about 5-10 WWII's worth of bullets (let alone .50 caliber BMG specifically).[2]

Source:

  1. https://www.hindawi.com/journals/isrn/2013/482734/
  2. https://history.stackexchange.com/questions/1711/number-of-bullets-used-in-ww2
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