What are present problems with alcubierre drive?

[dpa]

firstly it is supposed to travel at the speed of light and not FTL right? That is the limit to gravitational waves as well right?

And what are present problems on it?

 

[DaveC426913]

firstly it is supposed to travel at the speed of light and not FTL right? That is the limit to gravitational waves as well right?

I don't think it's limited by c. Its principle is that it stretches and compresses space, which is not subject to SR.

And what are present problems on it?

Well, what isn't?

It's science fiction.

I'm not saying it's not based on physics; the laws of physics do not forbid the principle. It's that a principle and invoking that principle are utterly different animals. We have absolutely no idea how we might go about stretching and compressing space. And it is looking pretty unlikely we will have any ideas about it in the next few centuries.

 

[PAllen]

It involves no violation of GR. However, GR mandates that you need exotic matter with negative energy to achieve the drive. Classical theories of matter have no such animal. Quantum field theory allows it in very limited circumstances (casimir effect), but there are inequalities strongly limiting such effects (in QFT). As a result, best present understanding suggests it is precluded by current theory. To change this, new physics would be needed to achieve large amounts of exotic matter, or a variation of the drive needing only tiny amounts of exotic matter would have to be found.

 

[Nabeshin]

I don't think it's limited by c. Its principle is that it stretches and compresses space, which is not subject to SR.

Indeed, the velocity of the alcubierre bubble can be made arbitrarily high.

One problem which I am aware of (beyond the exotic matter issue) is that of how exactly do you turn on or off the drive? As a static solution it is OK, but how to transition between being a normal observer in minkowski space to one sitting inside the bubble is nontrivial. Furthermore, it has been suggested that were you to accomplish such a transition, it would be accompanied by a tremendous burst of radiation, so when you arrived at your destination planet you would release a surge of gamma rays which would fry any living thing on the surface!

 

[dpa]

Indeed, the velocity of the alcubierre bubble can be made arbitrarily high.

Is not the drive curvature in spacetime and hence gravitational wave? That would limit would not it?

About radiation, is there any prediction as to how hot it might be?

Is Glenn RC Nasa(/ That is correct, right) considering and doing any resarch in it?
Or is it just left as hypothetical for the necessity of exotic matter?

 

[PAllen]

Is not the drive curvature in spacetime and hence gravitational wave? That would limit would not it?

About radiation, is there any prediction as to how hot it might be?

Is Glenn RC Nasa(/ That is correct, right) considering and doing any resarch in it?
Or is it just left as hypothetical for the necessity of exotic matter?

Your first question is very interesting. Gravitational waves travel less than or equal to c. Alcubierre drive is more like space expansion behind and contraction ahead. As with expansion of the universe, this can lead to effects which, while locally subluminal, are globally superluminal. At no point is any matter moving superluminally relative any possible nearby observers; further, no metric disturbance moves superluminally locally. But the distance between two distant objects can grow or shrink superliminally (e.g. your craft and planet). Someone might have clever analogy, but I can't come up with one right now.

On the radiation, the temperature inside the bubble (where the ship is) is said to reach about 10^30 degrees Kelvin (million trillion trillion degrees; center of the hottest stars or H-bombs incomprehensibly cold compared to this) in the following: http://arxiv.org/abs/1001.4960. This paper discusses the issue of radiation incinerating the destination : http://arxiv.org/abs/1202.5708.

As for research, only theoretical research is possible. There is not yet a first clue of anything to try in a lab.

 

[DrStupid]

firstly it is supposed to travel at the speed of light and not FTL right? That is the limit to gravitational waves as well right?

This is true for the average speed measured from the decision to fly to a specific destination until arrival. But the warp bubble may propagate with any speed. If the velocity of the warp bubble exceeds c the ship have to wait after starting the engine.

For example:
- The engine starts working at to = 0.
- The ship starts moving with v = 2·c (measured outside the warp bubble) at t1 = 1 a.
- The ship arrives after traveling s = 2 LY (measured outside the warp bubble) at t2 = 2 a.
-> Average speed: (t2-t0)/s = c

The reason for the delay from t0 to t1 is the speed limit for the information that tells the space time that it has to bend. That means that space travel as shown in Star Trek is impossible even with alcubierre drives. But warp high ways might be possible.