Quantum mechanics describes the behaviour of matter and energy at subatomic scales, and it is famously counterintuitive — even Richard Feynman conceded that "nobody really understands" it. This Wired explainer uses the classic double-slit experiment to illustrate why: at everyday scales, objects like tennis balls behave predictably, but electrons fired through two narrow slits produce a pattern that can only be explained if each particle behaves like a wave passing through both slits simultaneously, rather than as a discrete object taking one path or the other.

The experiment, first performed with light by Thomas Young in 1801 and adapted for subatomic particles by Claus Jönsson in 1961, shows that firing electrons one at a time still produces an interference pattern of multiple bands, as though each electron interferes with itself. Physicists describe this as the particle existing in a "superposition" of paths until measured. Crucially, the article notes that placing a detector at the slits to observe which path an electron actually takes destroys the interference pattern altogether, underscoring the strange role that observation itself plays in quantum behaviour.