The surprising origins of wave-particle duality (Ethan Siegel, 2/20/24, Big Think)
One of the most powerful, yet counterintuitive, ideas in all of physics is wave-particle duality. It states that whenever a quantum propagates through space freely, without being observed-and-measured, it exhibits wave-like behavior, doing things like diffracting and interfering not only with other quanta, but with itself. However, whenever that very same quantum is observed-and-measured, or compelled to interact with another quantum in a fashion that reveals its quantum state, it loses its wave-like characteristics and instead behaves like a particle.