von Neumann chain

The von Neumann chain is a thought experiment that exposes the slippery heart of the measurement problem. John von Neumann showed that if you treat a measuring device quantum-mechanically, it does not collapse the system; it simply becomes entangled with it, ending up in a superposition of pointing at each possible result. So you bring in a second device to read the first — but that one entangles too, and so does a third, and a fourth, on and on.

The chain can be extended without limit: the apparatus, the photons reaching your eye, your retina, your nerves, your brain. At no point does the pure quantum evolution, all by itself, produce a single definite outcome; it only spreads the superposition further along the chain. Somewhere a 'cut' must be placed, beyond which we declare that a real, classical, definite result has occurred. Where to put that cut — the Heisenberg cut — is exactly what the formalism refuses to tell us.

Different interpretations are, in effect, different decisions about the chain. Copenhagen places a pragmatic cut at the classical apparatus. Many-worlds refuses to cut at all and lets every link branch. Objective-collapse theories insert a real physical break partway along. Von Neumann himself, pressing the logic to its end, suggested the cut might lie at consciousness — a famous but minority view that few physicists endorse today, since it grants the mind a peculiar physical power with no independent evidence.