Wave-particle duality is a fundamental concept in quantum mechanics, which is the branch of physics that describes the behavior of matter and energy at the smallest scales. It suggests that particles, such as electrons or photons, can exhibit both wave-like and particle-like properties, depending on how they are observed or measured.
Traditionally, particles were thought of as discrete, localized entities with definite positions and velocities, while waves were described as extended disturbances propagating through space. However, experimental observations in the early 20th century, such as the famous double-slit experiment, challenged this classical view and led to the development of the wave-particle duality concept.
According to wave-particle duality, particles can exhibit wave-like properties, such as interference and diffraction, where they can interfere with themselves or exhibit a wave-like spreading out. Conversely, waves can exhibit particle-like properties, such as being quantized and carrying discrete packets of energy called "quanta" or "photons."
The concept of wave-particle duality is deeply intertwined with the mathematical formalism of quantum mechanics. In quantum mechanics, particles are described by wavefunctions, which are mathematical functions that can be thought of as waves. The wavefunction encodes the probability distribution of finding a particle in different states or locations. When a measurement is made, the wavefunction "collapses" to a specific value corresponding to the observed outcome, and the particle is detected at a particular position, revealing its particle-like behavior.
The reason we can't say that everything is either a wave or a particle, but not both, is because the behavior of particles on the quantum scale is fundamentally different from the classical macroscopic world we are familiar with. At the quantum level, particles do not possess well-defined properties until they are measured. Instead, they exist in a superposition of states, represented by the wavefunction, which describes the probabilities of different outcomes. It is only upon measurement that a specific outcome is observed, and the wavefunction collapses to a particular value.
In essence, wave-particle duality reflects the limitations of classical concepts in describing the behavior of particles at the quantum level. The true nature of particles is not limited to being solely waves or particles but rather a combination of both, depending on the context and experimental setup. This duality is a fundamental aspect of quantum mechanics, and it has profound implications for our understanding of the nature of reality.