Is the Alcubierre Warp Drive possible?

In summary: but if you could find a way to create a closed timelike curve, then theoretically something could travel faster than light inside of it.
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  • #2
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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  • #3
swampwiz said:
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!
 
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  • #4
m4r35n357 said:
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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  • #5
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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  • #6
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.
 
  • #7
bobob said:
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?
 
  • #8
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.
 
  • #9
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.
 
  • #10
There is no faster than light travel using this so called warp drive.
 
  • #11
DrStupid said:
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.
 
  • #12
PeterDonis said:
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
 
  • #13
1977ub said:
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.)
 
  • #14
1977ub said:
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.
 
  • #15
PeterDonis said:
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 traveling 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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  • #16
DrStupid said:
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.

DrStupid said:
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.
 
  • #17
One such vehicle leaves Earth and goes to alpha centauri, arriving sooner than a photon emitted from Earth at the same moment. Also let's 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.
 
  • #18
1977ub said:
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.

1977ub said:
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.
 
  • #19
PeterDonis said:
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.
https://en.wikipedia.org/wiki/Alcubierre_drive#Causality_violation_and_semiclassical_instability
 
  • #20
1977ub said:
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.

1977ub said:
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.
 
  • #21
1977ub said:
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
 
  • #22
PeterDonis said:
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.
 
  • #23
PeterDonis said:
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?
 
  • #24
PeterDonis said:
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.
 
  • #25
Dale said:
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.
 
  • #26
The question is - is this a technology which can be operated at will. That would be different from the issue of whether nature might generate something like this, outside of human intentions. If I have a friend 4 light years away, and one day I've got a black box I invented which can send him a letter in what he and I both agree is 3 years, then we are in causality violation territory. Say we've been sending chess moves back and forth at the speed of light then one day I have a black box (containing Alcubierre drive with warp bubble) and I put my chess move in it and send it to him in 3 years. Then he sends the object back to me in 3 years with his move.
 
  • #27
1977ub said:
The question is - is this a technology which can be operated at will.

Since it isn't a technology, and the mathematics that it is make no overt references to "will" or not . . .
 
  • #28
If you are allowed to choose the metric freely, you can make a patch of space to travel faster than light. That is clear.

Then you can use the Einstein field equation to calculate the stress-energy tensor T which would produce such a metric.

The Alcubierre drive, of course, allows closed timelike loops and causality violations. It breaks the principles of special relativity since we would have a spaceship which travels faster than light. The energy of such an object in special relativity is imaginary. These contradictions strongly suggest that the stress-energy tensor T is impossible to realize.

The stress-energy tensor T for the drive is very weird: there should be something which makes space to contract in front of the rocket and stretch behind it. I think matter with a negative energy density does not accomplish this. Ordinary matter has stretched the radial dimension r around it, and negative energy matter has contracted r. But it is static - there is no dynamic stretching or contracting around such matter.

One may imagine that the rocket would move inside a solid rod where a lever mechanism keeps negative pressure in front of the rocket and positive pressure behind it. One problem is how a rocket can fly inside a solid rod? Maybe we should instead of a rocket use a neutrino which can fly trough solid rock.

The negative pressure has to be huge to win the positive gravity of the material in the rod. Such material does not exist in nature. Can it exist theoretically? I think such material would allow faster-than-light travel for the neutrino. Since faster-than-light travel involves the causality violations, that strongly suggests that the required material for the rod cannot exist, even in theory.
 
  • #29
PAllen said:
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.

But that is a different usage of the term "causality" than the standard one, which is that nothing moves outside the light cones. IMO it would be better to just say "closed timelike curves".
 
  • #30
Heikki Tuuri said:
It breaks the principles of special relativity since we would have a spaceship which travels faster than light.

First, SR is inapplicable since we are talking about a curved spacetime.

Second, as I've repeatedly said in this thread, nothing moves "faster than light" in this spacetime. Nothing moves outside the light cones.

Heikki Tuuri said:
The energy of such an object in special relativity is imaginary.

SR is inapplicable; see above. What makes the stuff that produces the Alcubierre warp bubble "exotic matter" is that its pressure is greater than its energy density in the rest frame of the bubble. There is no imaginary energy anywhere. (There are frames where the energy density is negative, but none in which it is imaginary.)

Heikki Tuuri said:
I think

Don't "think". Do the math and see. The properties of the Alcubierre spacetime are well understood.

Heikki Tuuri said:
One may imagine

Don't "imagine". Do the math.

Heikki Tuuri said:
The negative pressure has to be huge to win the positive gravity of the material in the rod. Such material does not exist in nature. Can it exist theoretically?

As multiple posters have already said in this thread, most physicists think the answer to this is no.

Heikki Tuuri said:
I think such material would allow faster-than-light travel for the neutrino.

Please review the PF rules on personal speculation.
 
  • #31
George Jones said:
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.
All you need to do is link up two bubbles as in the tachyonic antitelephone to create a CTC. Long ago, @bcrowell posted a paper on this.
 
  • #32
1977ub said:
The question is - is this a technology which can be operated at will. That would be different from the issue of whether nature might generate something like this, outside of human intentions. If I have a friend 4 light years away, and one day I've got a black box I invented which can send him a letter in what he and I both agree is 3 years, then we are in causality violation territory. Say we've been sending chess moves back and forth at the speed of light then one day I have a black box (containing Alcubierre drive with warp bubble) and I put my chess move in it and send it to him in 3 years. Then he sends the object back to me in 3 years with his move.
No, that does not involve any causality violation. For a causality violation, you would need two separate inertial frames, in relative motion with respect to each other and sending FTL messages back and forth. This is the result of the simultaneity of relativity.
So let's say that you have two spaceships traveling at 0.5c relative to the Earth, and a string of space buoys at rest with respect to the Earth strung out along its path. Space ship 1 passed Earth when its both clocks read 0. 1 year later by his clock, when he is 1/2 light year from the Earth, as he measures the distance, he sends a instantaneous message (any FTL signal will produce similar results, it's just simpler to assume instant messages) back to to spaceship 2 just as it is passing the Earth. Both spaceship 2 and Earth will agree that the Earth clock reads 0.866 yrs as spaceship 2 passes. The message is transferred to the Earth and it then sends a reply instantaneously to a space buoy that is 0.433 ly away in the same direction as spaceship 1. In the Earth frame, this buoy is right next to spaceship 1 at that moment and the reply is transferred to spaceship 1. Assuming that the clocks on the buoy and Earth are synchronized in the Earth rest frame, the buoy clock reads 0.866 years at that moment. However, according both buoy and spaceship 1, spaceship 1's clock only reads 0.75 yrs, 1/4 of a year before he sent the original message. He gets his reply before sending the message.

I'm not sure how the Alcubierre drive would work in this scenario, as you would need to use it to deliver the messages back and forth.
 
  • #33
Heikki Tuuri said:
The Alcubierre drive, of course, allows closed timelike loops and causality violations.

Give an explicit, mathematical demonstration the Alcubierre warp drive spacetime contains closed timelike curves.
 
  • #34
PAllen said:
All you need to do is link up two bubbles as in the tachyonic antitelephone to create a CTC. Long ago, @bcrowell posted a paper on this.

This is not the Alcubierre warp drive spacetime.
 
  • #35
George Jones said:
loads of spacetimes, including Minkowski spacetime, can be modified to produce spactimes that have closed timelike curves.
Well, Minkowski can only be modified to have a CTC with unusual topology, like a torus. The Alcuiberre spacetime can get a CTC with standard topology.
 
<h2>1. Is the Alcubierre Warp Drive theoretically possible?</h2><p>According to current scientific understanding, the Alcubierre Warp Drive is theoretically possible. It is based on the principles of general relativity and the concept of warping space-time to achieve faster-than-light travel.</p><h2>2. Has the Alcubierre Warp Drive been tested or proven to work?</h2><p>No, the Alcubierre Warp Drive has not been tested or proven to work in real-world conditions. It is still a theoretical concept and there are many challenges and limitations that need to be overcome before it can be tested.</p><h2>3. What are the main challenges to making the Alcubierre Warp Drive a reality?</h2><p>Some of the main challenges to making the Alcubierre Warp Drive a reality include the enormous amount of energy required, the need for exotic matter with negative energy density, and potential causality violations.</p><h2>4. Are there any proposed solutions to these challenges?</h2><p>Scientists have proposed various solutions to the challenges of the Alcubierre Warp Drive, such as using a different form of exotic matter or finding ways to reduce the energy requirements. However, these solutions are still theoretical and require further research and development.</p><h2>5. When could we potentially see the Alcubierre Warp Drive in action?</h2><p>It is difficult to predict when or if the Alcubierre Warp Drive will become a reality. It will likely require significant advancements in technology and a deeper understanding of physics. Some estimates suggest it could take decades or even centuries before we see a functioning warp drive.</p>

1. Is the Alcubierre Warp Drive theoretically possible?

According to current scientific understanding, the Alcubierre Warp Drive is theoretically possible. It is based on the principles of general relativity and the concept of warping space-time to achieve faster-than-light travel.

2. Has the Alcubierre Warp Drive been tested or proven to work?

No, the Alcubierre Warp Drive has not been tested or proven to work in real-world conditions. It is still a theoretical concept and there are many challenges and limitations that need to be overcome before it can be tested.

3. What are the main challenges to making the Alcubierre Warp Drive a reality?

Some of the main challenges to making the Alcubierre Warp Drive a reality include the enormous amount of energy required, the need for exotic matter with negative energy density, and potential causality violations.

4. Are there any proposed solutions to these challenges?

Scientists have proposed various solutions to the challenges of the Alcubierre Warp Drive, such as using a different form of exotic matter or finding ways to reduce the energy requirements. However, these solutions are still theoretical and require further research and development.

5. When could we potentially see the Alcubierre Warp Drive in action?

It is difficult to predict when or if the Alcubierre Warp Drive will become a reality. It will likely require significant advancements in technology and a deeper understanding of physics. Some estimates suggest it could take decades or even centuries before we see a functioning warp drive.

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