Trope-a-Day: Pocket Dimension
Pocket Dimension: Alas, outside virtual reality, this seems to require basement universes (requires very high-energy physics, being worked on) or dimensional transcendence (requires emergent ontotechnology, also being worked on).
Being worked on very enthusiastically by starship manufacturers, I note, because holy mother of crap what would being able to keep your remass in a pocket dimension do for your mass ratio!
(Let’s hope that the mass of the contents inside doesn’t seamlessly translate to the mass of the dimension “mouth” outside…)