# Von Neumann and the Problem of Quantum Measurement

# Von Neumann and the Problem of Quantum Measurement

The ‘Collapse of the Wavefunction’

Heisenberg was an ‘anti-realist’. Although Bohr was infamously obscure in many of his writings, it seems that he adopted a generally anti-realist interpretation, too. As their debate became more bitter, in early June 1927 Pauli was called in to mediate. With Pauli’s help, they forged an uneasy consensus, which became known as the *Copenhagen interpretation*. Einstein didn’t like it at all, setting the stage for a great debate about the quantum representation of reality. Although von Neumann’s formalism broadly conforms to the Copenhagen interpretation, he saw no need to introduce an arbitrary split between the classical and quantum worlds. But eliminating the split poses the problem of quantum measurement: when scaled to classical dimensions, a superposition of different measurement outcomes appears contrary to our experience, exemplified by the famous paradox of Schrödinger’s cat. Von Neumann was obliged to break the infinite regress by postulating the ‘collapse of the wavefunction’.

*Keywords:*
Werner Heisenberg, Niels Bohr, Albert Einstein, John von Neumann, Copenhagen interpretation, quantum measurement, collapse of the wavefunction, projection postulate, Wigner’s friend, Schrödinger’s cat

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