The theory was built in a rush between 1925 and 1927 by a small group of mostly young physicists, including Werner Heisenberg, Max Born, Wolfgang Pauli and Paul Dirac, with Niels Bohr's institute in Copenhagen as their meeting point. The way of thinking that grew out of their discussions became known as the Copenhagen interpretation, and for most of the twentieth century it was simply how quantum mechanics was taught.
Not one doctrine but a family
It helps to know that the label "Copenhagen interpretation" only became common in the 1950s, decades after the theory was finished, and that Bohr and Heisenberg did not agree on everything. Historians now treat Copenhagen as a family of related attitudes rather than a single official position. Four themes run through it.
Complementarity
In a lecture at Como, Italy, in September 1927, Bohr introduced the idea he called complementarity. Light and electrons sometimes behave like waves and sometimes like particles. Bohr argued that wave and particle descriptions are complementary: both are needed for a full account, but they can never be applied in the same experimental arrangement. An experiment built to show interference reveals the wave aspect; one built to track which path a particle took reveals the particle aspect.
The language of classical physics
Bohr also insisted that the results of experiments must be described in ordinary classical terms, because that is how we tell one another what we did and what we saw. For Bohr, the quantum formalism was a tool for linking such reports, not a picture of a hidden world behind them.
The cut and the wavefunction
Heisenberg spoke of a "cut" between the quantum system and the classical measuring device. He stressed that the cut's exact position could be moved without changing any prediction: you could treat the detector as part of the quantum system, or as part of the classical apparatus, and the numbers would come out the same. Along with this went a modest view of the wavefunction. In most Copenhagen thinking, the wavefunction is a tool for predicting the results of measurements more than a picture of what exists. Collapse, on this reading, is not a strange physical event but the moment our information is updated.
The best case for Copenhagen
Defenders make a strong case. Copenhagen works, and it avoids claiming more than experiments can show. It lets physicists calculate without committing to unseen worlds, hidden particles or new laws. Defenders also argue that the demand for a picture of what the electron "really" does between measurements may be a habit carried over from everyday life, one that nature has no obligation to satisfy. In 1989 the physicist David Mermin summed up the working attitude of many colleagues as a blunt instruction to stop asking what it all means and get on with the calculation, though he later turned to the question himself.
Its influence remains large. A survey of more than a thousand physicists published by the journal Nature in 2025 found Copenhagen style views the most popular choice, though held by only about a third of those who answered, with no interpretation near a majority.
Fair criticism
Critics raise several objections. The most common is that Copenhagen never says clearly where the quantum world ends and the classical world begins. Measuring devices are made of atoms; if quantum mechanics is universal, why should they be treated differently? The cut works in practice, but a fundamental theory, critics say, should not depend on a line nobody can locate.
Einstein pressed a related point. According to the physicist Abraham Pais, Einstein once asked him on a walk in Princeton whether he really believed that the moon exists only when someone looks at it. Einstein wanted a theory describing reality as it is, independent of observers. His 1935 paper with Boris Podolsky and Nathan Rosen, which argued that quantum mechanics must be incomplete, is the starting point of QUAN 201.
A third problem appears in cosmology. When physicists apply quantum theory to the whole universe, there is no outside observer or apparatus to perform the measurement. John Bell, one of Copenhagen's sharpest critics, argued that the word "measurement" should have no place in the basic laws of physics at all, since a measurement is just one physical process among others. Defenders reply that these objections assume what they set out to prove: that physics must describe a reality beyond what we can observe and report.
Worked example: watching the slits
Fire electrons one at a time at a screen with two slits. With no detectors at the slits, the dots on the far wall build up an interference pattern of light and dark bands, as if each electron passed through both slits like a wave. Now place a small detector at each slit that records which one the electron passes through. The bands vanish; the dots pile up in two clumps, as particles would.
The Copenhagen reading is calm. The two setups are different experimental arrangements, and each reveals one complementary aspect. Asking what the electron "really" did between the source and the wall, when no detector was there, is asking for something no experiment can supply. A critic replies that the electron must have done something, and a theory that refuses to say what is incomplete. Notice that neither side disputes the data. They disagree about what a theory owes us beyond correct predictions, and that is the question running through the rest of this course.