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complementarity principleprinciple

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The principle was formulated by Niels Bohr in 1927 as a resolution of the paradoxes of quantum mechanics. The essence: a complete description of a micro-object requires two mutually exclusive experimental setups — one reveals wave properties (interference), the other — particle properties (trajectory). In the double-slit experiment, if we try to peek through which slit the electron passed, the interference pattern disappears. Complementarity is not just a limitation of knowledge, but a way of existence of quantum reality.

How it works

In everyday life: in high-resolution microscopes, there is a fundamental limit to clarity because photons are simultaneously waves and particles. In lasers, the wave nature of electrons in atoms is used to obtain coherent light — but then information about the exact position of electrons is lost.

💡 Niels Bohr was so fascinated by the idea of complementarity that he later tried to apply it in biology (life and physico-chemical processes) and psychology, considering them complementary aspects of reality.
V^2 + D^2 \le 1
V — visibility of the interference pattern (fringe contrast, from 0 to 1); D — path distinguishability (reliability of determining the particle trajectory, from 0 to 1).
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Discovered by
John Archibald WheelerNiels BohrPhilippe Grangier
Related concepts
wave-particle dualityquantum measurement
Related laws
Heisenberg uncertainty principlede Broglie formula

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