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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.

An example of this paradox in non-western thought can be found in 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".

Another approach to this paradox is to simply state that the object will continue to exist, since by definition an irresistible force is an immovable object.

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 André the Giant (the immovable object).

The paradox was refenced in the 2002 World Cup final between Brazil and Germany. Brazil was the irresistable force as they had scored more goals than any other team. germany was the unmovable object as they had only conceded one goal in the whole tournament. Brazil won.

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

Another answer is given in a MENSA puzzle book by Victor Serebriakoff (the former head of MENSA): by allowing the two to come together, a realistic answer that matches the semantics of the question is "an inconceivable event".

There is a reference to the paradox in World of Warcraft. When a player gets exalted reputation with the Alterac Valley battleground, among the rewards are The Unstoppable Force (a two-handed mace) and The Immovable Object (a shield). While they are specifically made for player versus player combat, they have no special properties contradicting each other, so the paradox itself is not addressed in-game.

The former X-men villain known as Juggernaut used to be known as the unstoppable force. Another X-men villain called Blob, is said to be immovable, though he has been moved before by beings like the Hulk. Many fans have had discussions over what would happen if the Juggernaut ran into the Blob.

Jarvis Cocker references the paradox (in a sexually suggestive way, unsurprisingly) in the Pulp song "Seductive Barry" (This is Hardcore), concluding that "when the immovable (or unmovable) object meets the unstoppable force, there's nothing you can do about it." Many lyrics websites misquote him as saying "unbelievable object".

Solutions

There are many possible solutions including one that has the immovable object never moving and the unstoppable force never stopping. This is where the unstoppable force becomes exponentially slower forever in order to stop from violating the trait of the immovable object. The two objects never actually meet. This solution only works if one is to interpret the paradox as having the immovable object in the path of the unstoppable force rather than the two actually meeting. This solution is not entirely true, as the question is "What happens when an irresistible force MEETS an immovable object?" The solution only works when the two objects never meet. Another Idea is that the unstoppable force will remove a piece from the immovable object. Physicist Ross Ozarka states in his book Factum Forlorn that "the human eye will never be able to witness such an event, since the immovable object a black hole, and the irresistible object any object unable to break the escape velocity of said black hole. Light rays emitting from the black hole will never reach an observer's eye, unless he himself is the irresistible object."

• Paradoxes