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