Complementarity principle

The complementarity principle is a philosophical thesis introduced by Niels Bohr to interpret the formal results of quantum mechanics. It states that quantum objects possess complementary properties that cannot be observed or defined simultaneously — not because of technical limitations, but because the experimental arrangement required to measure one property precludes the arrangement required to measure the other. The principle is not a mathematical theorem like the Heisenberg uncertainty principle, but a conceptual framework asserting that the quantum world cannot be captured by a single classical picture. Wave and particle, position and momentum, time and energy: each pair requires mutually exclusive experimental contexts. The principle forces a rethinking of what it means for a physical theory to be complete — a debate crystallized in the Bohr-Einstein debates over the nature of physical reality.