The Irresistible force paradox, also the unstoppable force paradox, is a classic paradox formulated as "What happens when an irresistible force meets an immovable object?" This paradox is a form of the omnipotence paradox, but that paradox is most often discussed in the context of God's omnipotence ("Can God create a stone so heavy it cannot be lifted, not even by God Himself?"). Note that the immovable object is implicitly assumed to be indestructible, or else the question would have a trivial resolution ("it destroys it"). Furthermore, it is assumed that they are two separate entities, since an irresistible force is implicitly an immovable object, and vice versa.
The apparent paradox arises because it rests on two premises—that there exist such things as irresistible forces and immovable objects—which cannot both be true at once. If there exists an irresistible force, it follows logically that there cannot be any such thing as an immovable object, and vice versa.
The paradox should be understood as an exercise in logic, not as the postulation of a possible reality.
According to modern scientific understanding{fact}[1], no force is completely irresistible, and there are no immovable objects and cannot be any, as even a minuscule force will cause a slight acceleration on an object of any mass. An immovable object would have to have an inertia that was infinite, and therefore infinite mass. Such an object would collapse under its own gravity and create a singularity. An unstoppable force would require infinite energy, which does not exist in a finite universe.
An example of this paradox in non-western thought can be found in the origin of the Chinese word for paradox (Chinese: 矛盾; pinyin: máodùn; literally "Spear-Shield"). This term originates from a story (see the Kanbun example) in the 3rd century BC philosophical book Han Feizi.[2] In the story, a man was trying to sell a spear and a shield. When asked how good his spear was, he said that his spear could pierce any shield. Then, when asked how good his shield was, he said that it could defend from all spear attacks. Then one person asked him what would happen if he were to take his spear to strike his shield; the seller could not answer. This led to the idiom of "zìxīang máodùn" (自相矛盾), or "self-contradictory".
The paradox is also commonly referenced in sport. The paradox is generally used to refer to a game between two teams or individuals one of which has a strong defense and the other a strong offense.
