The inner life
Wave-Particle Duality
Direct answer
Wave-Particle Duality: Wave-particle duality is the finding that light and matter show wave behaviour, such as interference, in some experiments and particle behaviour, such as arriving in single lumps, in others.
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At a glance
- Also called
- wave particle duality, is light a wave or a particle, particle wave duality, de broglie wavelength
- Found in
- Consciousness & Spiritual Psychology
- Related
- Double-Slit Experiment, Complementarity, Wave Function, Uncertainty Principle
Wave-particle duality is the finding that light and matter show wave behaviour, such as interference, in some experiments and particle behaviour, such as arriving in single lumps, in others.
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What is wave-particle duality?
Light spreads, bends round edges and interferes, as waves do. It also delivers its energy in indivisible packets, photons, each striking a detector at one spot, as particles do. Electrons, long thought of as particles, form interference patterns just as light does. Neither everyday picture covers all the facts, and which behaviour appears depends on how the experiment is arranged.
Quantum theory does not say a thing switches between two natures. It gives one description, the wave function, whose wave-like evolution fixes the chances of particle-like detections. The duality is therefore a feature of our two inherited pictures and their limits. It is sometimes quoted to mean that everything is vibration or that matter is an illusion, neither of which the physics says.
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What sets Wave-Particle Duality apart
The quantitative heart of the idea is de Broglie's relation: wavelength equals Planck's constant divided by momentum. It turns a slogan into a prediction, telling an experimenter how finely spaced a grating must be to make electrons or molecules interfere, and why heavier things are harder. The second precise point is that the two behaviours never appear together in full. Arrange the apparatus to reveal the path and the interference fades in step, which is the trade that Bohr named complementarity.
- Light as quanta: Albert Einstein, 1905, on the photoelectric effect
- Matter waves: Louis de Broglie, doctoral thesis, Paris, 1924
- De Broglie's relation: wavelength equals Planck's constant divided by momentum
- Electron diffraction: Davisson and Germer, and G. P. Thomson, 1927
- Largest so far: interference of molecules of about two thousand atoms, Vienna, 2019
Often mistaken for something else: Complementarity is Bohr's principle about why the two pictures exclude each other in any one experiment; wave-particle duality is the experimental fact that both pictures are needed.
The story behind Wave-Particle Duality
Thomas Young showed the interference of light in the early 1800s, and the wave theory seemed settled. In 1905 Albert Einstein explained the photoelectric effect by treating light as quanta. In 1924 Louis de Broglie proposed in his Paris thesis that matter has a wavelength too, and in 1927 Clinton Davisson and Lester Germer, and separately George Thomson, saw electrons diffract from crystals. Interference has since been shown with molecules of up to two thousand atoms.
Why Wave-Particle Duality matters
Wave-Particle Duality is the observation the whole of quantum theory was built to explain, and the source of most popular sayings about it.
How to read Wave-Particle Duality
The idea is put to work in electron microscopes, which exploit the short wavelength of fast electrons, and in teaching as the way into quantum theory.
Wave-Particle Duality in practice: an example
A reader who meets the phrase everything is waves can ask what is waving, and learn that the quantum wave is one of probability.
Where readings of Wave-Particle Duality go wrong
Wave-Particle Duality describes how small things behave in experiments; it does not show that solid objects are unreal or that thought is a wave.
Sources and editorial method
This entry summarizes the tradition in the Almanac's own words; where no source is listed, it is an editorial summary rather than a primary text, and symbolic readings are named as the tradition's, not as fact.
- Source
- The Great Almanac Dictionary
- Category
- Consciousness & Spiritual Psychology
- Aliases
- wave particle duality, wave-particle duality meaning, is light a wave or a particle, particle wave duality, de broglie wavelength
- Last updated
- 2026-09-26
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Common questions
What is wave-particle duality?
It is the fact that light and matter cannot be fully described as waves or as particles alone. They interfere like waves and are detected in single, whole units like particles.
Is light a wave or a particle?
Neither old picture fits completely. Light travels and interferes as a wave and is absorbed in discrete photons. Quantum theory treats it with a single mathematics that gives both behaviours.
Do large objects have a wavelength?
By de Broglie's formula everything does, but the wavelength shrinks as mass and speed grow. For a thrown ball it is unimaginably smaller than an atomic nucleus, so no wave behaviour can be seen.
