[yarvos]

I am honestly shocked at the number of comments strongly asserting the impossibility of FTL based on simultaneity arguments from special relativity. Yes, faster than light travel through space is prohibited in both special and general relativity by simultaneity. But not so for sub light speed travel (or even being stationary) on a space which itself has the property where distances between the passenger and destination are decreasing faster than the speed of light, which is precisely the class of metric solutions that the paper is investigating.

The fact that certain galaxies are receding from us faster than the speed of light due to the expansion of the universe (again just changes in the metric) provides a clear example of this kind of 'FTL' travel that is permitted in general relativity.

[323]

Not to mention that everybody seems certain that the current theory, Einstein GR, is the final theory and that no new radical theory is possible that could upend everything in terms of possibilities.

[analog31]

I've found that that belief in future violations of present-day theory is actually quite commonplace. Judging from conversations with colleagues at the lunch table, not only are violations of theory considered to be possible, but specific violations such as time travel, and warp drive, are considered to be as good as certain.

Every HN thread about FTL travel includes multiple comments warning us not to treat present-day knowledge as final.

[lumost]

Time travel, warp drives, worm holes, and even infinite energy machines are permissible under GR. These aren’t theory violations, but either lack a physically plausible mechanism, are I observable, or require energies outside of experimental capabilities.

[DennisP]

How does GR allow an infinite energy machine?

[ben_w]

I'm a long way from knowing (treat my attempts to teach myself as fresher-level), but I think energy conservation is supposed to be equivalent under Noether's Theorem to time translation symmetry, but GR doesn't have universal time, so energy isn't conserved? (Assuming I'm even correct about any of that, I definitely don't understand what gets conserved instead of energy in GR).

[lumost]

This is pretty much it, GR does not generally conserve energy due to lack of time symmetry. Energy is conserved in specific space time geometries (which happen to be common).

[russdill]

The galaxies receding faster is a bad example as the motion will never "outrun" a ray of light so no causality issues come up.

[phkahler]

>> on a space which itself has the property where distances between the passenger and destination are decreasing faster than the speed of light

Doesn't it take considerable effort/energy to make that happen? And is spacetime elastic, so it would spring back?

[xwolfi]

Their image is receding faster. It's likely they are not.

[ncmncm]

If their image were receding faster, we could not see it. So, no image. We just happen to know the space they are in is going that fast now, based on how far away and thus how far in the past, the image we see is.