A person is holding on to a helium balloon in an elevator. The elevator cable snaps, and at the exact same moment, the person lets go of the helium balloon. In the perspective of the person in the elevator, what happens to the balloon the moment the cable snaps?
Buoyancy is a direct consequence of gravity. In free fall, the effective gravity within the elevator is zero (assuming perfect free fall of the elevator, with no air drag) and thus the balloon stays where it is relative to the observer.