A "Halo Drive" is a concept for sub light speed travel that uses either a spinning black hole, or twin black holes (stationary) to accelerate a ship by slingshotting a laser around the singularity and harvesting the spin from the black hole to go real real fast.
You can accelerate an arbitrarily large mass.
Does that mean that one could just IDK speed up a whole moon and use it like ...ballast for a longer journey? Is there a way to convert extra mass to more speed? (not like fuel)
