Alcubierre drives

[Aranfan]

I found this: http://doc.cern.ch//archive/electronic/ ... 04-110.pdf which seems to imply that we can't rule out whether or not they could work. Unfortunately I am not familiar enough with the math and physics to tell whether or not the authors were pulling it out of their asses or not.

So, could the Alcubierre drive work?



A related question is: If it can, what would be the negative energy requirements for say, ten tons of matter to be sent on a FTL trip, and what speeds could it achieve?

[Admiral Valdemar]

We've had this discussion before, and if I recall, the amount of exotic energy needed would be universal, literally, for any real mass. Additionally, the drive would not be true FTL anyway, since causality needs to be conserved.

[Aranfan]

I thought that the negative energy requirements had been brought down to a few miligrams? And that causality was something that we would like to be conserved but that doesn't necessarily have to be?

[Admiral Valdemar]

I thought that the negative energy requirements had been brought down to a few miligrams? And that causality was something that we would like to be conserved but that doesn't necessarily have to be?

If the energy requirements have been reduced, I'll have to look into it. It's been a while since I entertained Alcubierre's drive idea.

As for causality, at quantum scales it can be meaningless, but at macroscales it still holds true. If it didn't, you've found a way to reverse entropy and cause all sorts of mischief. The way the idea is seen now is that the contraction and expansion process within the warp field must happen at either c or below. This doesn't mean the drive is useless were it possible to be created, since you'd have a handy massless drive system, assuming you could get the energy to maintain the field and power the ship too.

*Sits awaiting Kuroneko*

[Gullible Jones]

Even with the negative energy requirements reduced, and with a "micro-warp" type drive that shrinks down the volume of the bubble, you're talking about something like a solar mass of positive energy to move the volume of a hydrogen atom.

Also, the bubble can't be generated without moving stuff FTL locally, unless you want to generated a naked singularity. The first is of course considered impossible, and AFAIK so is the second.

[montypython]

I've always considered the Alcuberrie drive to be something similar to the LDS drive from I-War, it would allow for warp-like movement near light-speed, but not a FTL system like in Trek.

[Aranfan]

Why is a naked singularity considered impossible?

[Aranfan]

Even with the negative energy requirements reduced, and with a "micro-warp" type drive that shrinks down the volume of the bubble, you're talking about something like a solar mass of positive energy to move the volume of a hydrogen atom.

Really? Wait, is the amount of energy dependent on mass of stuff moved or volume of stuff moved?

[Kuroneko]

To appreciate the significance of the Alcubierre warp drive, one must first recognize just how loose the Einstein Field Equation is when taken by itself. If you know how the stress-energy is configured, the EFE tells you how spacetime curves (although it fixes only half the degrees of freedom of the curvature). This is invaluable; however, the Alcubierre warp takes the reverse appraoch: create a spacetime manifold with the properties sought, and let the EFE define the stress-energy configuration needed.

It's not all that impressive precisely because one can do the same with any of a very general class of four-dimensional Lorentzian manifolds: pick a nigh-arbitrary manifold, use the EFE to define a corresponding stress-energy tensor, and voila--a "solution" of the field equation. In GTR, there are various energy conditions to make sure what you're doing is physically sensible, and the Alcubierre warp drive violates them all.

So, could the Alcubierre drive work?

For an FTL Alcubierre drive? Classically, absolutely not. With quantum mechanics on the scene, that only changes to "extremely unlikely" rather than flat-out "impossible", because quantum fields can violate some of the standard GTR energy conditions to some degree (although far from arbitrarily), and it's not clear there are some other warp drive geometries that are less unreasonable. Still, even van Broeck's "micro-warp" drive involve shells of mass densities nearing a googol-gram per cubic meter, which is plenty unreasonable enough already.

I thought that the negative energy requirements had been brought down to a few miligrams?

That would be more than a bit unbelievable. Alcubierre's original drive required about ten orders of magnitude more than the total mass of the visible universe in negative energy; van Broeck shrunk down the external profile of the drive to microscopic levels, and it still required "just" a solar mass of negative energy (by coincidence or intentional design of the geometry, almost exactly one solar mass, in fact: -2.0e30kg), and even more in positive energy.

Also, the bubble can't be generated without moving stuff FTL locally, unless you want to generated a naked singularity. The first is of course considered impossible, and AFAIK so is the second.

This can't be stressed enough: for an Alcubierre warp drive to FTL, the matter in the warp bubble must be capable of transmitting a superluminal signal--a speed of sound greater than speed of light in vacuum. It's not clear to what degree other warp metrics might fix this, but it doesn't look very promising.

Why is a naked singularity considered impossible?

Because it's not possible to determine what's going on at such points (worse, true curvature singularities are like "holes" in spacetime).

Really? Wait, is the amount of energy dependent on mass of stuff moved or volume of stuff moved?

Volume. You have a situation where the energy requirements of making the bubble are already so high that almost every payload would be just a negligible perturbation to the overall propagation of the warp drive. Basically, until the payload is massive enough to have a non-negligible gravitational field of its own, the bubble is not going to notice it. As long as it fits in the first place, of course.

[Aranfan]

So is the paper in my OP legit or full of crap?


Also, assuming we could get the drive to work FTL, what kind of speeds are possible? And would the speeds have an effect on energy requirements?

[Kuroneko]

So is the paper in my OP legit or full of crap?

Well, let me put it this way. An FTL warp drive requires some rather exotic matter in very incredible configurations, but at least in the sense that QFT doesn't seem to absolutely forbid such matter (although it doesn't predict it either), it "cannot be ruled out". But just because we can't rule it out doesn't mean we should take it as a serious possibility. Squeezing solar masses of matter into microscopic shells less than a femtometer thick is not exactly plausible, no matter how one slices it.

The bulk of the paper seems to concentrate on practical matters of incident radiation, controlling the bubble, etc., and I haven't looked at it. However, if I'm understanding the first part correctly, it looks very fishy: adding spherical shells of positive matter can certainly make the total mass positive without affecting internal structure (GTR also has its own version of Newton's shell theorem), but WEC violation are local, and putting shells over that should simply destroy the warp effect altogether. Locally, the weak-field limit, although referenced correctly, is very dubiously applicable to solar-masses of matter and fm-scale distances. Perhaps I'm misunderstanding something, but that part of the argument seems to me to be completely broken.

Also, assuming we could get the drive to work FTL, what kind of speeds are possible?

Any speed.

And would the speeds have an effect on energy requirements?

Yes. For constant bubble thickness, energy scales as v².

[Aranfan]

I see and how does it scale with size?

[K. A. Pital]

I would be glad if honourable Kuroneko would also comment on the concept of Krasnikov tubes, how plausible are they compared to the Alcubierre drive?

[Aranfan]

Also, what are the options for weaponizing the Alcubierre drive?

[Formless]

Also, what are the options for weaponizing the Alcubierre drive?

Shit man! You already have enough energy in one place and at one time to light up a solar system at least! If you can devote even a fraction of it into a weapons system without damaging the integrity of the drive, I highly doubt anything is going to mess with you or stand in your way.

[Ariphaos]

Shit man! You already have enough energy in one place and at one time to light up a solar system at least! If you can devote even a fraction of it into a weapons system without damaging the integrity of the drive, I highly doubt anything is going to mess with you or stand in your way.

"At least" is a drastic understatement. We're talking about mass-energies measured in kilograms for sub-atomic-nucleus scales.