would require so much energy that it is (for now(?)) impossible to achieve.

It’s not just excessive quantities of regular energy. These schemes all require negative mass-energy or other exotic forms of energy. We have no theory of physics that predicts the existence of these forms of energy, and we don’t know how they would work.

Physicists who write papers about these things use neutral, professional language, like “exotic matter,” to describe this stuff, but it’s really pretty firmly in the realm of magical make believe.