Can string/brane theory generate spacetime warps with gravitons?

[selfAdjoint]

In this new paper, http://arxiv.org/abs/gr-qc/0406083 , Lobo & Visser apply linearized gravity to the famous warp drive solutions of Einstein's equations. They are able for the first time to consider an extended (not pointlike) spaceship of finite mass. They find, no surprise, that the warp drives are technologically improbable in the foreseeable future.

 

[DW]

In this new paper, http://arxiv.org/abs/gr-qc/0406083 , Lobo & Visser apply linearized gravity to the famous warp drive solutions of Einstein's equations. They are able for the first time to consider an extended (not pointlike) spaceship of finite mass. They find, no surprise, that the warp drives are technologically improbable in the foreseeable future.

That was not their conclusion, there are errors in certain assertions anyway, and they left out the inclusion of a distance variable lapse function.

 

[selfAdjoint]

That was not their conclusion, there are errors in certain assertions anyway, and they left out the inclusion of a distance variable lapse function.

The following is copied from their paper.

We do not mean to suggest that such a “reaction-less drive” is achievable with current technology — indeed the
analysis below will, even in the weak-field limit, place very stringent conditions on the warp bubble. These conditions
are so stringent that it appears unlikely that the “warp drive” will ever prove technologically useful. The Alcubierre
“warp drive”, and the related Nat´ario “warp drive”, are likely to retain their status as useful “gedanken-experiments”
— they are useful primarily as a theoretician’s probe of the foundations of general relativity, and we wish to sound a
strong cautionary note against over-enthusiastic mis-interpretation of the technological situation

As for the errors you cite, do you plan to address them?

 

[DW]

The following is copied from their paper ... As for the errors you cite, do you plan to address them?

They can make no such conclusions concerning technology whether they said so or not because they did not consider Alcubierre's expression for a more general warp drive. My primary complaint, their error carried down on every relavent line, which conserns this is that they like Natario left out something that makes all the difference. I have said before what that is. They left out a time and distance variable lapse function. What they needed to do and what has already been done at "Modern Relativity" is to write the metric according to the ship frame wherein it is most convenient to insert this variable and then allow it to vary throughout space. As it is they have it everywhere always equal to 1. If they wanted no lapse then what they should have done is allowed it to be 1 at the location of the ship and far from the ship and rise from one toward the warp shell. This single expression I am beginning to suspect is always left out on purpose can be manipulated to arbitrarily lower the magnitude of the negative energy requirement even when considering the Pfenning-Ford inequality. As long as this is left out, all such speculations on whether the warp bubble is unachievable with todays technology is unwaranted and I am saying is in error. If you want to know what I am talking about concerning the lapse function and its impact on the exotic matter requirement see pages 166-173 and especially pay attention to equations 13.2.15, 13.2.16 and 13.2.23 at
http://www.geocities.com/zcphysicsms/chap13.htm#BM166

 

[selfAdjoint]

Thank you. I have defended your lapse function version, and especially its drastic lowering of the negative energy requirement before this, on another forum. Perhaps you should email Lobo and/or Visser and bring this to their attention. I would do it myself, but I don't think I am sufficiently in possession of your theory to carry on a dialog with an expert. You obviously are.

 

[selfAdjoint]

Here's a question for you DW. It seems likely that Einstein's cosmological constant $$\lambda$$ is the generator of the "dark energy" which is responsible for the accelerated expansion of spacetime and is the largest component of the energy of the universe.

Now do the Alcubierre and other warp metrics include $$\lambda$$? Could they? Isn't this a form of energy in being, even if we don't understand it yet, that could achieve the selective warping which is the core of these drives' behaviors?

 

[DW]

Here's a question for you DW. It seems likely that Einstein's cosmological constant $$\lambda$$ is the generator of the "dark energy" which is responsible for the accelerated expansion of spacetime and is the largest component of the energy of the universe.

Now do the Alcubierre and other warp metrics include $$\lambda$$? Could they? Isn't this a form of energy in being, even if we don't understand it yet, that could achieve the selective warping which is the core of these drives' behaviors?

They don't, but they could include it. If it were constant throughout the warp spacetime the metric would have to approach frame transformations of the DeSitter spacetime instead of the Minkowski spacetime in the appropriate limits, but if you could somehow manipulate it as you suggest and make it a function of distance from and angle about the ship then yes it could hypothetically be used to produce warp drive effects. But, no one has any idea how to do that, well except Jack Sarfatti claims to know how.

 

[alpha_wolf]

While we're talking about warp drives...

Two years ago, I was part of an online group that was trying (and still is, I beleive) to show that the warp drive is physically possible, even if not in its original form. The group has these three articles at arXiv:
Reduced Total Energy Requirements for a Modified Alcubierre Warp Drive Spacetime
A causally connected superluminal Warp Drive spacetime
On the Problems of Hazardous Matter and Radiation at Faster than Light Speeds in the Warp Drive Space-Time
I would like to here your opinion on these... Thanks.

EDIT: Hmmm... DW, are you by any chance one of the authors of the first two?

 

[DW]

Two years ago, I was part of an online group that was trying (and still is, I beleive) to show that the warp drive is physically possible, even if not in its original form. The group has these three articles at arXiv:
Reduced Total Energy Requirements for a Modified Alcubierre Warp Drive Spacetime
A causally connected superluminal Warp Drive spacetime
On the Problems of Hazardous Matter and Radiation at Faster than Light Speeds in the Warp Drive Space-Time
I would like to here your opinion on these... Thanks.

EDIT: Hmmm... DW, are you by any chance one of the authors of the first two?

It doesn't matter if I am. I will say that a good portion of the first two is valid. As for the third, obsticles aren't a serious problem for the Alcubierre warp due to frame dragging. There is really no need to resort to a Broeck warp drive.

 

[Chronos]

Any warp drive mechanics would be matter dependent. Assuming matter is positive energy, the warp engine would have to generate a negative energy field with sufficient negtive energy to counteract that mass. A multiple of that mass could produce superluminal velocities of the warp field. Finding the negative energy is, however, problematical. Relativity says it can't be done, quantum physics does, however, suggest the possibility.

 

[Brad_Ad23]

Somehow I have a sneaking suspicion that we won't ever truly be able to proclaim 'warp drive' as possible or impossible until we obtain a quantum theory of gravity.

 

[selfAdjoint]

Somehow I have a sneaking suspicion that we won't ever truly be able to proclaim 'warp drive' as possible or impossible until we obtain a quantum theory of gravity.

Well string/brane theory claims to BE a quantum theory of gravity. I wonder if they can generate any spacetime warps with their gravitons? Anybody know of any papers?

 

[Brad_Ad23]

Well string/brane theory claims to BE a quantum theory of gravity. I wonder if they can generate any spacetime warps with their gravitons? Anybody know of any papers?

That could be an interesting area to lookup, but of course you still have to ask the question as to whether 'M-Theory' as it is, is actually a valid theory of quantum gravity. Last I checked that was still not settled. Certainly though, I don't doubt it can provide us with more insight.