I Is the Alcubierre Warp Drive possible?

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The Alcuiberre warp drive, and most other similar solutions require matter with a negative energy density, sometimes called exotic matter. No such matter has ever been detected and it is unlikely that such material would be stable if it did exist.

These solutions should be seen as interesting mathematical solutions that likely have no physical realization.
 
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This Quora post seems to say that this type of warp drive could allow faster-than-light travel, but somehow I think this is junk science:
Here is a video by the Author himself. See if you think he takes it seriously!
 

pervect

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Here is a video by the Author himself. See if you think he takes it seriously!
An interesting video. I haven't had the time to watch the whole thing, but his remark that this one just one paper he did back in 1994 and that the majority of his work is in gravitational waves gives a a good indication to me of how seriously he takes the paper. The word "serious" is a bit ambiguous, but the message I get from what I did watch is that this one paper is not the focus of his work by any means, but I think he still stands by the logic and mathematics of this paper, or he wouldn't have agreed to present it.
 
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I agree. His logic and math is correct. It is a valid solution to the EFE. I don’t think that he ever claimed that it was feasible to construct.
 
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The Alcuiberre warp drive does not actually result in faster than light travel in it's local reference frame. What it does is to provide a region of spcetime which in which the distances in front and behind it are shortened and lengthened respectively while inside the bubble, the occupants remain at rest.
 
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What it does is to provide a region of spcetime which in which the distances in front and behind it are shortened and lengthened respectively while inside the bubble
But the warp bubble must already exists in order to do that. Is it possible (in theory) to chose a random target and reach it faster than a light signal outside the bubble?
 
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To answer that question would be to speculate on the possibilities of a device which itself is highly speculative and thought to most likely not be possible. Or, in other words, since the device itself depends on physics that might not be possible in this universe, there is no way to say what might or might not be possible if I assume something that is already probably incorrect.
 
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Whatever the technical issues, does not the existence of FTL not lead to causality violation? That's a good enough reason to think it's not possible.
 
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There is no faster than light travel using this so called warp drive.
 
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Is it possible (in theory) to chose a random target and reach it faster than a light signal outside the bubble?
As I understand how the Alcubierre spacetime works, yes, this is possible, but in a curved spacetime with exotic matter in it, this does not mean anything is traveling "faster than light" locally. A light ray emitted inside the bubble will still outrun any timelike object inside the bubble. All it means is that curved spacetimes with exotic matter in them can have highly counterintuitive properties.
 
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As I understand how the Alcubierre spacetime works, yes, this is possible, but in a curved spacetime with exotic matter in it, this does not mean anything is traveling "faster than light" locally. A light ray emitted inside the bubble will still outrun any timelike object inside the bubble. All it means is that curved spacetimes with exotic matter in them can have highly counterintuitive properties.
But if it is outrunning light just outside of the bubble then it is FTL for all practical purposes
 
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if it is outrunning light just outside of the bubble then it is FTL for all practical purposes
The term "FTL" is misleading. I would say it's moving in a curved spacetime geometry with exotic matter in it that has highly counterintuitive properties. (And most physicists do not think exotic matter is possible, so this kind of spacetime geometry cannot actually exist.)
 
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Whatever the technical issues, does not the existence of FTL not lead to causality violation? That's a good enough reason to think it's not possible.
The technical term is a closed timelike curve. It is not FTL anywhere, but it does have that causality issue.
 
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As I understand how the Alcubierre spacetime works, yes, this is possible, but in a curved spacetime with exotic matter in it, this does not mean anything is traveling "faster than light" locally.
Of course there is nothing travelling faster than light locally. But the bubble propagates faster than light in the flat space around it. This implies all the nasty things of FTL travel, including violation of causality.

But that's not my point. I wonder if the warp drive could move the ship to a random destination faster than a light signal in flat space after the destination has been selected.
 
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the bubble propagates faster than light in the flat space around it.
Again, "faster than light" is misleading. It is a curved spacetime with exotic matter in it that has highly counterintuitive propereties.

This implies all the nasty things of FTL travel, including violation of causality.
No, there is no violation of causality; there is simply a curved spacetime with exotic matter in it.
 
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One such vehicle leaves Earth and goes to alpha centauri, arriving sooner than a photon emitted from earth at the same moment. Also lets say you have another such vehicle leaving alpha centauri toward earth, also arriving sooner than a photon emitted at the launch event. As the two vehicles pass one another, interactions could occur which violate causality.
 
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One such vehicle leaves Earth and goes to alpha centauri, arriving sooner than a photon emitted from earth at the same moment.
A photon emitted from Earth along a different path that doesn't cross the path of the warp bubble.

As the two vehicles pass one another, interactions could occur which violate causality.
Like what? Remember that "causality" in a curved spacetime is not the same as in flat spacetime. Spacetime with warp bubbles in it is not flat.
 
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A photon emitted from Earth along a different path that doesn't cross the path of the warp bubble.



Like what? Remember that "causality" in a curved spacetime is not the same as in flat spacetime. Spacetime with warp bubbles in it is not flat.
The bubble region is a tiny % of a sufficiently large flat-ish region of the universe, and so the standard FTL / causality issues arise.
 
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The bubble region is a tiny % of a sufficiently large flat-ish region of the universe
The spacetime is not flat. Waving your hands and saying "tiny % of a sufficiently large flat-ish region" doesn't change that.

so the standard FTL / causality issues arise
You're going to have to be more explicit: just saying "the standard FTL/causality issues" doesn't cut it. Give a specific scenario that you claim violates causality. Remember that the standard rigorous meaning of "violates causality" is that objects move outside the light cones, and that does not happen in a spacetime containing Alcubierre regions.
 
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the standard FTL / causality issues arise
If you're referring to the possible presence of closed timelike curves, "causality violation" is a misleading term to use for those. They certainly violate many common intuitions about causality, but that's not the same thing. There has been significant work, such as that associated with the Novikov Self-Consistency Principle [1], that shows that it is perfectly possible to have consistent models that include closed timelike curves. Whether those models can actually be realized is a separate question (but that's just the same question as whether an Alcubierre warp drive can actually be realized).

[1] https://en.wikipedia.org/wiki/Novikov_self-consistency_principle
 
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Remember that the standard rigorous meaning of "violates causality" is that objects move outside the light cones, and that does not happen in a spacetime containing Alcubierre regions.
I think that closed timelike curves could also reasonably be described as “violating causality”. Those can occur in Alcubierre spacetimes.
 
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Again, "faster than light" is misleading. It is a curved spacetime with exotic matter in it that has highly counterintuitive propereties.
I'm not sure if I understand you correctly. Are you saying there is no way to keep the reference light signal in flat spacetime and reach the target first?
 

PAllen

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If you're referring to the possible presence of closed timelike curves, "causality violation" is a misleading term to use for those. They certainly violate many common intuitions about causality, but that's not the same thing. There has been significant work, such as that associated with the Novikov Self-Consistency Principle [1], that shows that it is perfectly possible to have consistent models that include closed timelike curves. Whether those models can actually be realized is a separate question (but that's just the same question as whether an Alcubierre warp drive can actually be realized).

[1] https://en.wikipedia.org/wiki/Novikov_self-consistency_principle
Even with Novikov, CTCs allow the possibility of e.g. a play or symphony that has no author. I think many, if not most, physicists would consider this a form of causality violation.
 

George Jones

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I think that closed timelike curves could also reasonably be described as “violating causality”. Those can occur in Alcubierre spacetimes.
The Alcubierre's warp drive spacetime does have closed timelike curves. This spacetime can be modified to produce spacetimes that do have closed timelike curves, but loads of spacetimes, including Minkowski spacetime, can be modified to produce spactimes that have closed timelike curves.
 

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