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The physics underlying non-relativistic quantum mechanics can be summed up in two postulates. Postulate 1 is very precise, and mentions that the wave function of a quantum system evolves according to the Schrödinger equation, which is a linear and deterministic equation. Postulate 2 has an entirely different flavour, and can be roughly stated as follows: ‘when the quantum system interacts with a classical measuring apparatus, its wave function collapses – from being in a superposition of the eigenstates of the measured observable to being in just one of the eigenstates’. The outcome of the measurement is random and cannot be predicted; the quantum system collapses to one or the other eigenstates, with a probability that is proportional to the squared modulus of the wave function for that eigenstate. This is the Born probability rule.
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