The inner life
Complementarity
Direct answer
Complementarity is Niels Bohr's principle that a quantum object shows wave or particle behaviour depending on the experiment, and that the two pictures exclude each other yet are both needed.
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At a glance
- Also called
- wave particle duality
- Found in
- Consciousness & Spiritual Psychology
- Related
- Wave Function, Double-Slit Experiment, Observer Effect, Synchronicity
Complementarity is Niels Bohr's principle that a quantum object shows wave or particle behaviour depending on the experiment, and that the two pictures exclude each other yet are both needed.
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What is complementarity in physics?
Light sent through two slits spreads and overlaps like a wave. Light striking a detector arrives in single packets like particles. Bohr held that these are not rival theories to be decided between but complementary descriptions: each belongs to a particular experimental arrangement, no arrangement shows both at once, and together they exhaust what can be said.
He extended the thought beyond physics, with care, to pairs such as justice and mercy or thought and feeling, where attending fully to one blurs the other. That wider use is philosophy, offered by Bohr as analogy. It is the part of his thinking that most invites comparison with Taoist and other teachings of paired opposites.
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What sets Complementarity apart
Bohr's principle is about the conditions of observation, not about hidden properties. A given apparatus is built to show interference or to show paths, and the two designs are mutually exclusive, so the question of what the object is apart from any apparatus has, for Bohr, no answer. That is a stronger claim than saying light is sometimes a wave and sometimes a particle. It also explains his motto: the opposites do not merge, each completes the description the other leaves unfinished.
- First stated: Como, Italy, September 1927
- Classic pair: wave and particle; also position and momentum
- Motto: contraria sunt complementa, 'opposites are complementary'
- Coat of arms: the yin and yang symbol, chosen by Bohr in 1947
- Chief critic: Albert Einstein, in debates from 1927 into the 1930s
Often mistaken for something else: Wave-particle duality names the puzzle that quantum objects show both behaviours; complementarity is Bohr's answer to it, that each behaviour belongs to its own kind of experiment.
The story behind Complementarity
Bohr first presented complementarity in a lecture at Como, Italy, in September 1927, months after Heisenberg's uncertainty principle. It became the core of what is called the Copenhagen interpretation. Einstein never accepted it as final, and the two argued for decades. In 1947, awarded Denmark's Order of the Elephant, Bohr designed a coat of arms bearing the yin and yang symbol and the Latin motto contraria sunt complementa.
Why Complementarity matters
Complementarity is the one place where a founder of quantum physics himself pointed to an Eastern symbol, so it shows what a fair parallel looks like.
How to read Complementarity
Readers borrow it for pairs of views that cannot be held at once and are both true to experience, naming the borrowing as analogy.
Complementarity in practice: an example
A reader may notice that analysing a feeling and simply having it cannot both be done fully in the same moment, a homely case of the idea.
Where readings of Complementarity go wrong
Bohr offered the wider use as a likeness; it does not make Taoism physics, or physics a proof of any teaching.
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
- complementarity principle, bohr complementarity, wave particle duality, principle of complementarity
- Last updated
- 2026-09-26
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Common questions
What is complementarity in physics?
It is Bohr's principle that wave and particle descriptions of a quantum object are both required, but apply to different experimental set-ups and can never be observed together.
Why did Bohr use the yin and yang symbol?
When he was honoured in 1947 he needed a coat of arms, and chose the yin and yang with the motto that opposites are complementary. He saw in it an image of his principle, not a source for it.
Is complementarity the same as the uncertainty principle?
They are related. Heisenberg's principle sets a numerical limit on knowing paired quantities such as position and momentum. Bohr's is the broader idea that such pairs call for mutually exclusive experiments.
