In quantum mechanics, wave-particle duality is a fundamental concept describing how quantum entities such as photons and electrons exhibit both particle-like and wave-like properties. Traditionally, particles like electrons are described by a wave function, a mathematical construct that encodes the probability of finding a particle in a given location or state.
A famous experiment demonstrating this duality is the double-slit experiment. When electrons pass through two adjacent slits, they create an interference pattern on a screen, indicative of wave behavior. However, when these electrons are observed, they behave like discrete particles impacting specific locations on the detector, collapsing the wave function into a particle state.
The transition from wave to particle is not just a passive transformation, but a dynamic process wherein the act of measurement forces the wave-like nature to "choose" a particular outcome, a phenomenon known as wave function collapse. The exact mechanics of this collapse and what constitutes a 'measurement' are areas of ongoing research and debate in quantum mechanics.
The principle of complementarity, articulated by Niels Bohr, suggests that the wave and particle aspects are not contradictory but complementary, depending on the experimental context. Quantum mechanics therefore defies classical intuition, challenging the notion of particles as defined entities existing independently of observation.
In certain interpretations, such as the Copenhagen interpretation, this duality and the role of the observer bring forward questions about the fundamental nature of reality itself, suggesting that physical systems do not have definite properties independent of observation. This brings to light the peculiar notion in quantum mechanics that observation and measurement play a critical role in defining the behavior and state of quantum systems.