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The Irresistible force paradox is a classic paradox formulated as follows:

What happens when an irresistible force meets an immovable object?

Common responses to this paradox resort to logic and semantics.

• Logic: if such a thing as an irresistible force exists, then no object is immovable, and vice versa. It is logically impossible to have these two entities (a force that cannot be resisted and an object that cannot be moved by any force) in the same universe.

• Semantics: if there is such a thing as an irresistible force, then the phrase immovable object is meaningless in that context, and vice versa, and the issue amounts to the same thing as asking, e.g., for a triangle that has four sides.

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 that He cannot lift it?).

The paradox should be understood as an exercise in logic, not as the postulation of a possible reality. According to modern scientific understanding, there are not and indeed cannot be either irresistible forces or immovable objects. An immovable object would have to have infinite inertia and therefore infinite mass. Such an object would collapse under its own gravity and create a singularity. An irresistible force would imply an infinite energy, which by Albert Einstein's equation E = mc is equivalent to an infinite mass. Note that, in the modern view, a cannonball which cannot be deflected and a wall which cannot be knocked down are both types of the same (impossible) object: an object with infinite inertia.

Another example of this paradox comes from the origin of the Chinese word for paradox (矛盾), literally "spear shield." This word originates from a story where a seller was trying to sell a spear and 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 all spear attacks. Then one person asked him what would happen if he were to take his spear to strike his shield. He could not answer, and this led to the idiom of 自相矛盾, or "self-contradictory".

Pop culture

The irresistible force paradox has infiltrated popular culture. A reference to the irresistible force paradox has been made in a Knight Rider episode (Trust doesn't Rust) where the paradox is wrongly attributed to Zeno of Elea and its meaning is intentionally distorted.

This paradox was also popularized in the 1980s with reference to World Wrestling Federation nemeses Hulk Hogan (the irresistible force) and Andre the Giant (the immovable object).

It also appears in the novel Walking on Glass by Iain Banks, where the solution is: "the immovable object moves; the unstoppable object stops."

The paradox has also been applied in the Marvel Universe of comics, whenever a battle between the Incredible Hulk (immovable object) and the Juggernaut (irresistable force) occurs.

From the 1950s: What happens when an irresistable force meets an immovable object? The bus window stays closed. Those of who remember the buses of that era will appreciate the humor.

• Paradoxes