# I Is the Alcubierre Warp Drive possible?

#### swampwiz

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.
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

Mentor
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

Mentor
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

Mentor
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.)

"Is the Alcubierre Warp Drive possible?"

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