Wave–Particle Duality
The quantum-mechanical fact that entities such as light and electrons exhibit both wave-like and particle-like behavior depending on the experiment.
Wave–particle duality is the fact that quantum objects exhibit both wave-like and particle-like properties depending on how they are observed. The history runs through both sides of the question: Newton treated light as corpuscles, while Young's double-slit experiment of 1801 demonstrated interference and established the wave picture. Einstein's 1905 explanation of the photoelectric effect showed that light is absorbed and emitted in quanta — photons with energy E = hf — which earned him the Nobel Prize in 1921. In 1924 de Broglie proposed the converse, that matter has wave character with wavelength λ = h/p, confirmed shortly afterward by electron diffraction experiments.
The double-slit experiment with single particles makes the situation concrete. Electrons fired one at a time through two slits build up an interference pattern as if each particle interfered with itself; if a detector reveals which slit each electron passed through, the interference disappears. The state of the particle is not a mixture but a superposition of possibilities, and the pattern is an interference effect on probabilities.
The standard interpretation, associated with Bohr and the Copenhagen school, treats the wavefunction as encoding probabilities and measurement as collapsing it to a definite outcome. Alternatives — Bohmian mechanics, many-worlds, objective collapse — remain interpretations rather than experimentally distinguished theories: all make the same statistical predictions.
The quantitative predictions of quantum mechanics, computed from the Schrödinger equation, are among the most precisely verified results in all of physics. The duality is not a paradox to be resolved but the working principle behind technologies from electron microscopy and neutron scattering to quantum computing, and Bohr's complementarity principle expresses the practical rule: which aspect of the phenomenon appears depends on the experimental arrangement.
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electrons light physics quantum mechanics
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