Copenhagen interpretation

The Copenhagen interpretation is the oldest and, for decades, the textbook-standard way of reading quantum mechanics, associated above all with Niels Bohr and Werner Heisenberg in the 1920s. Its core attitude is pragmatic: the wavefunction is a tool for computing the probabilities of measurement outcomes, and we should not ask for a detailed picture of what the system is doing between measurements. When you measure, the wavefunction collapses to the result you find, and that is simply how the theory connects to the lab.

Two ideas sit at its heart. Complementarity says that some descriptions, such as the wave and particle pictures, are both needed yet cannot be used at once; which one applies depends on the experiment you choose. And there is a working division between a quantum system and a classical measuring apparatus described in everyday terms — the experimental arrangement, including the apparatus, must be specified before a quantum question even has a definite meaning.

Honesty requires admitting that 'the Copenhagen interpretation' is not one tidy doctrine but a loose family of views; Bohr, Heisenberg and later writers disagreed on details. Its great strength is that it lets you do physics without metaphysical commitments. Its great weakness, the one its rivals attack, is the measurement problem: it never says precisely where the quantum world ends and the classical, collapsing world begins.