Is the Alcubierre Warp Drive possible?

[PeterDonis]

I really don't see any basis to claim that you can't join two sections of spacetimes each globally hyperbolic in such a way that the result is not.

I'm not making this claim as a general claim; as a general claim it would be way too strong. I'm only making it in the specific case of Everett's construction.

Can you try to clarify why you think there should be a problem with Everett's construction?

I'm not saying there's a problem with the construction in itself. I'm questioning whether the construction Everett describes will actually contain CTCs. I need to go through the paper again and see if I can fill in the details he leaves out.

 

[PAllen]

I'm not making this claim as a general claim; as a general claim it would be way too strong. I'm only making it in the specific case of Everett's construction.
I'm not saying there's a problem with the construction in itself. I'm questioning whether the construction Everett describes will actually contain CTCs. I need to go through the paper again and see if I can fill in the details he leaves out.

One thing I found is that several papers published in major journals decades after Everett's paper cite it as an established, non-controversial result.

 

[DrStupid]

Your interpretation of this is that "causality" is violated in Alcubierre spacetime.

I know it is limited to Minkowski spacetime. That's why I added above that it is only works within the spacetime that is not affected by the Alcubierre warp drive. It just means that an observer who is not aware of the bubble would conclude that causality is violated and that the spaceship travels faster than light. For some observers it would even look like the ship travels back in time. In that case it should always be possible for the ship to go all the way back and connect event X at position D with another event X' at position A with X' preceding X.

 

[PeterDonis]

In that case it should always be possible for the ship to go all the way back and connect event X at position D with another event X' at position A with X' preceding X.

This is a spacetime with two warp bubbles, not one. One warp bubble can only go in one direction.

 

[DrStupid]

This is a spacetime with two warp bubbles, not one.

Yes, I assumed that to be obvious.

 

[swampwiz]

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.

What if the bubble denies any ability for a photon to traverse it in the lateral direction? Then it wouldn't violate causality.

 

[1977ub]

What if the bubble denies any ability for a photon to traverse it in the lateral direction? Then it wouldn't violate causality.

no photon is traversing it. if it is a small unit carrying a communication at "FTL" then causality will be violated.

 

[rrogers]

Due to an egregious error in an earlier posting, I have been studying Kerr metrics. It appears (although I have doubts) that the mathematics of spinning black holes has: escapes clauses, something similar to negative energy (repulsive regions), and closed time-like orbits/areas/curves. To me, that indicates that Alcubierre type of arrangements is not excluded by GR.
So if you have a spare couple of solar systems laying around to convert to a spinning black hole...
or you can wait for the great meeting at the "great attractor"; you could see if the physics is correctly described by GR.
Really..really (??) closed timelike curves occurring naturally without new physics? I am still studying.

 

[PeterDonis]

the mathematics of spinning black holes has: escapes clauses, something similar to negative energy (repulsive regions), and closed time-like orbits/areas/curves

Yes, but only inside the inner horizon, which is not considered a physically reasonable portion of the spacetime.

To me, that indicates that Alcubierre type of arrangements is not excluded by GR.

Of course it's not "excluded by GR"; the Alcubierre spacetime is a perfectly valid solution to the Einstein Field Equation. Nobody is disputing that. But this perfectly valid mathematical solution is not considered physically reasonable, just as the region of Kerr spacetime inside the inner horizon is not.

 

[rrogers]

Yes, but only inside the inner horizon, which is not considered a physically reasonable portion of the spacetime.
Of course it's not "excluded by GR"; the Alcubierre spacetime is a perfectly valid solution to the Einstein Field Equation. Nobody is disputing that. But this perfectly valid mathematical solution is not considered physically reasonable, just as the region of Kerr spacetime inside the inner horizon is not.

Yes, but if the model, GR, is broken inside and somebody came up with an alternative, I would expect that the alternative would be testable in gravity wave signatures. I realize our detection is primitive so far, but one can hope for future experiments. Gravity probe B; that took a long..long time to implement.

 

[rrogers]

Yes, but if the model, GR, is broken inside and somebody came up with an alternative, I would expect that the alternative would be testable in gravity wave signatures. I realize our detection is primitive so far, but one can hope for future experiments. Gravity probe B; that took a long..long time to implement.

It's not true that you can't communicate or have to wait till the end of time; the detection of gravity waves indicate that one could probe a black hole with disturbances and detect the resulting gravity waves. A little presumptuous in terms of technology but when has that stopped dreaming?

 

[PeterDonis]

if the model, GR, is broken inside and somebody came up with an alternative

This is way out of scope for this thread, and indeed this forum.

 

[PeterDonis]

the detection of gravity waves indicate that one could probe a black hole with disturbances and detect the resulting gravity waves

Yes, in principle this could be done, but it's going to be a while before we have the technology to throw neutron stars into black holes and watch the gravitational waves that get produced.

(Btw, it's "gravitational waves", not "gravity waves"; "gravity waves" refers to something completely different.)