In modern physics, duality most often refers to the paradigm underlying quantum mechanics, according to which matter or energy can exhibit properties associated with wave physics as well as classical particle mechanics.

These two sets of phenomena are mutually exclusive in classical physics, but nevertheless are both needed in order to describe the possible behaviors of matter and light. Therefore one speaks of the wave-particle duality.

This duality also implies that light and electromagnetic radiation, which in classical physics are considered to be wave phenomena, can exhibit particle-like properties. Light has to be described as a wave in order to explain interference effects, whereas the photoelectric effect is usually explained with a particle model.

Dual descriptions of a physical system may have overlapping ranges of validity. In such regions of overlap, two dual theories will not give contradictory predictions. The wave-particle duality is only one example where this requirement applies. Other fields where duality is encountered in this sense are statistical mechanics and string theory.

Duality can lead to counterintuitive conclusions. For example, in quantum mechanics the wave property is assigned to a probability amplitude. The conceptual problem with this particle/probability construct is that when the particle is absorbed, the wave must instantaneously collapse throughout all space.

See also

• Correspondence principle
• Berry phase
• Dualism
• Duality, duality (mathematics)
• Erlangen program
• Ising model
• Photoelectric effect
• Philosophy of science
• S-duality
• Wave-particle duality
• Quantum chaos