The usual formulation has “irresistible” rather than “unstoppable” and I always took it that (1) “irresistible force” means something that substantially affects everything it interacts with, (2) “immovable object” means something on which no force has a substantial effect, and (3) “meets” means “interacts with in the way forces in this general class interact with objects in this general class”.
So if they “pass through each other”, that means the object remained immovable but the force wasn’t in this case irresistible.
The usual formulation has “irresistible” rather than “unstoppable” and I always took it that (1) “irresistible force” means something that substantially affects everything it interacts with, (2) “immovable object” means something on which no force has a substantial effect, and (3) “meets” means “interacts with in the way forces in this general class interact with objects in this general class”.
So if they “pass through each other”, that means the object remained immovable but the force wasn’t in this case irresistible.
(It’s an amusing answer, though.)
You forgot the citation!