wave-particle duality

Wave-particle duality is the discovery that the tiniest pieces of nature, light and electrons and everything else, behave sometimes like spread-out waves and sometimes like tiny concentrated particles, depending on how you look at them. Everyday image: think of a single actor who can play two very different roles; which role you see depends on the scene, but it is always the same actor. The question it answers is a deep one: is light a wave or a stream of particles? The honest answer turned out to be 'both, and neither, exactly.'

Precisely, the same object shows wave behaviour, spreading out, bending around edges (diffraction) and adding up in bright and dark bands (interference), in some experiments, and particle behaviour, arriving as countable lumps with definite energy and momentum, in others. Light shows its wave side in the double-slit experiment and its particle side in the photoelectric effect. Matter does the mirror image: electrons, normally pictured as particles, form interference patterns when fired through a double slit. The two faces are tied together quantitatively by E = h f (a photon's energy from its wave frequency) and by the de Broglie relation lambda = h / p (a particle's wavelength from its momentum).

Why it matters: duality is the heart of quantum mechanics, and it forces us to give up the comfortable idea that a thing must be either a wave or a particle. Honest caveat: it is not that an electron is half wave and half particle at once, nor that it secretly chooses; rather, our two classical pictures are each incomplete, and the full quantum description simply predicts the probabilities of what a detector will register.