xkcd.WTF!?

Explanation ➲

An "unstoppable force meeting an immovable object" is a common expression and thought experiment when two things with mutually exclusive properties are forced to interact. In the comic, this is depicted with three drawings, with an arrow representing an unstoppable force moving toward an object that is immovable. The next two images depict how they interact - or rather, not interact, with the force passing right through the object as if it weren't there. In a caption below the comic, Randall states that he cannot understand why people find this scenario to be tricky

The comic plays on the word "force", which has different definitions depending on context. To a physicist like Randall it describes a fundamental influence between particles of matter. Not all forces interact with all types of matter, nor can they be stopped (only depleted, by interaction and dispersion over their effective distance); as a result, although "unstoppable forces" seem like an impossible thing, they are in fact quite common (again, provided one is using the physics definition), something Randall exploits in line with prior informative comics of this ilk. In casual language however, a "force" is an entity with a large amount of (usually kinetic) energy, hence the question might better be phrased as "what happens if one attempts to apply kinetic energy to an object whose velocity cannot be changed?".

A lot of times, the "unstoppable force / immovable object" expression is just that, an expression, to indicate that two parties who have contradictory goals and are unwilling or unable to compromise. Other times it's an exaggeration for large and powerful forces that are not literally unstoppable but still cause massive damage when they run into each other.

In the title text, Randall clarifies what he means in that the force-carrying particles cannot interact with the matter by definition. If such objects did exist, being intangible to each other is the only possible answer that would be compatible with how we understand physics to work, as force-carrying particles can only interact with some particles. (An further explanation of this situation is described in a video by Minute Physics.) In quantum physics, all forces (electromagnetic, strong-nuclear, gravity, weak-nuclear) are carried by force-carrying particles (photons, gluons, W & Z bosons, Higgs Boson), but this is not usually something that is relevant to consider, when macroscopic objects interact.