wave-particle duality

Wave-particle duality is the discovery that light and matter each refuse to fit neatly into either of the two boxes that classical physics offered. A beam of light can spread out and interfere like ripples on a pond, yet it also arrives in tiny indivisible lumps of energy called photons. An electron, long pictured as a tiny ball, can diffract and form interference patterns just as a wave does. Neither the pure 'wave' picture nor the pure 'particle' picture is the whole truth.

The honest way to read this is not that an electron is sometimes a wave and sometimes a particle, as if it switched costumes. Rather, it is a quantum object whose behaviour the old words only partly capture. Which description seems to apply depends on what you choose to measure: ask a wave-like question, such as how it interferes, and you get a wave-like answer; ask a particle-like question, such as where it landed, and you get a single localised hit.

Quantum mechanics resolves the tension by describing the object with a wavefunction that evolves smoothly and wave-like, while the act of detection delivers a discrete, particle-like outcome with a probability fixed by that wavefunction. So duality is less a paradox than a signal that nature at small scales is genuinely a third kind of thing, for which 'wave' and 'particle' are two limited human metaphors.