Mechanism for Alcubierre engine?

[jerich1000]

The theorized Alcubierre engine is purported to be capable of transporting a ship faster than the speed of light.

I'm aware of the theory's problems associated with how much energy would be required to operate such an engine. This isn't my question.

Apparently gravity waves, waves in the Quantum field (ZPF), and perhaps even waves in dark energy all travel at c.

If all of the above is true, then what is the medium that allows the Alcubrierre engine to operate superluminally even if the energy problem were resolved?

I realize that dark energy is expanded behind the ship and compressed in front of the ship. But, what good does that do if waves of dark energy travel at c?

Thanks

 

[Drakkith]

The drive isn't making a wave which then propagates in spacetime, it is causing spacetime to expand/contract which causes movement. Since this movement isn't caused by a propagation through spacetime then it wouldn't have to move at c. At least that's the way I see it. In any case this is a questionable subject to be talking about on PF and probably violates several of the rules, so I would be careful.

 

[Matterwave]

Alcubierre's engine is a hypothetical warping of local space-time around the ship so that the light cones are tilted in such a way as to allow the ship to move at less than light-speed locally, but traverse extremely large distances in a very short time. It requires some sort of "negative energy density" which isn't physical as far as we know. (In GR, energy density actually is not arbitrary like usual, but has a 0 point).

 

[jerich1000]

Thanks Drakkith and Matterwave.

Aren't gravity waves, which sensors on Earth receive routinely, at different frequencies,

(see for example: http://arxiv.org/ftp/arxiv/papers/1003/1003.3899.pdf)

are they not ripples in gravity created by varying cosmic events? How are they different than the basic idea of an Alcubierre engine?

Is it because ordinary gravity waves are so small that they travel at c, whereas an Alcubierre engine creates much larger waves, and the larger the wave, the faster the velocity? This makes intuitive sense to me. (This is where you're supposed to say, 'yes') :)

 

[bcrowell]

Fundamentally, speeds greater than c are only prohibited locally in relativity, not globally. GR doesn't even have a uniquely defined way of describing the velocity of one object relative to a second, distant object.

Is it because ordinary gravity waves are so small that they travel at c, whereas an Alcubierre engine creates much larger waves, and the larger the wave, the faster the velocity? This makes intuitive sense to me. (This is where you're supposed to say, 'yes') :)

No, actually it's the other way around. Low-amplitude gravitational waves propagate at c. High-amplitude gravitational waves need *not* propagate at c. For example, GR predicts that a gravitational-wave pulse propagating on a background of curved spacetime develops a trailing edge that propagates at less than c.[MTW, p. 957] This effect is weak when the amplitude is small or the wavelength is short compared to the scale of the background curvature.

MTW - Misner, Thorne, and Wheeler, Gravitation

In any case this is a questionable subject to be talking about on PF and probably violates several of the rules, so I would be careful.

I don't think it violates any rules. The original paper was published in a peer-reviewed journal:
Alcubierre, Miguel (1994). "The warp drive: hyper-fast travel within general relativity". Classical and Quantum Gravity 11 (5): L73–L77. arXiv:gr-qc/0009013. Bibcode 1994CQGra..11L..73A. doi:10.1088/0264-9381/11/5/001.

However, I hear that past attempts to discuss it on this forum have resulted in lots of low-quality discussion. I would suggest that anyone posting in this thread try to post only information that they are sure is correct, not speculation based on partial understanding of this topic or of GR in general.

The WP article is very detailed and has a ton of references to the peer-reviewed literature: http://en.wikipedia.org/wiki/Alcubierre_drive

 

[Time Suspect]

The Warp Drive doesn't violate GR, it requires negative energy density but that's not a law of GR.

There are the so called energy conditions, weak, strong and null and I think that only the weak energy condition forbids negative energy density, this conditions are not part of general relativity and they aren't even physical laws, they are simply assumptions on how matter and energy should behave.

 

[Matterwave]

The Warp Drive doesn't violate GR, it requires negative energy density but that's not a law of GR.

There are the so called energy conditions, weak, strong and null and I think that only the weak energy condition forbids negative energy density, this conditions are not part of general relativity and they aren't even physical laws, they are simply assumptions on how matter and energy should behave.

Nonetheless, currently, we don't know of any situation in which negative energy densities arise.

@OP: The Alcubierre drive is not creating some kind of "gravity wave" in space-time. The idea is to create a local curving of space-time so as to tilt the local light cones. I'm not sure why gravity waves traveling at "c" is important to this discussion.

 

[Time Suspect]

Well Kip. Thorne and S.Hawking believe that the Kasimir effect is a case of negative energy density but nevertheless the point was that negative energy densities have nothing incompatible with GR, GR has nothing to say about the sign of energy densities, that's why people invented the energy conditions which are independent of Einstein equations and therefore the Warp Drive is completely consistent with the Einstein Equations, it is not consistent with the weak energy condition but that is another statement.

This point is very important, say also for those that don't know this, the Einstein Equations are completely time reversible, even though there isn't a known case of time travel. Time travel is consistent with the Einstein Equations.

 

[bcrowell]

Nonetheless, currently, we don't know of any situation in which negative energy densities arise.

Actually that's not true. Here's a nice paper on that: Twilight for the energy conditions?, Barcelo and Visser, http://arxiv.org/abs/gr-qc/0205066

Basically there is no energy condition that is both strong enough to be interesting and universally valid. Some of them, however, are strong enough to be interesting and valid in various interesting contexts.

 

[Matterwave]

Well Kip. Thorne and S.Hawking believe that the Kasimir effect is a case of negative energy density but nevertheless the point was that negative energy densities have nothing incompatible with GR, GR has nothing to say about the sign of energy densities, that's why people invented the energy conditions which are independent of Einstein equations and therefore the Warp Drive is completely consistent with the Einstein Equations, it is not consistent with the weak energy condition but that is another statement.

This point is very important, say also for those that don't know this, the Einstein Equations are completely time reversible, even though there isn't a known case of time travel. Time travel is consistent with the Einstein Equations.

I think there's a conjecture that says all closed time-like loops would require also some sort of negative energy density or some such so that "time travel" in the sense of going back to your own past is "not possible" according to this conjecture (just a conjecture).

@bcrowell: I'll take a look at that paper when I have time. Perhaps you can tell me quickly if they actually propose any situation in which negative energies occur and what those situations are?

 

[bcrowell]

@bcrowell: I'll take a look at that paper when I have time. Perhaps you can tell me quickly if they actually propose any situation in which negative energies occur and what those situations are?

Page 2 of the paper (p. 4 of the PDF) gives a pretty good quick rundown.

 

[George Jones]

Lobo has written a review article, Exotic solutions in General Relativity: Traversable wormholes and 'warp drive' spacetimes, on some of this stuff.

http://arxiv.org/abs/0710.4474