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Oxford University Press eBooks

Von Neumann and the Problem of Quantum Measurement

Jim Baggott

Published Jan 10, 2020• 0 citations
Publisher

Abstract

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’.

DOI
10.1093/oso/9780198827856.003.0012

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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’.
The field of interpretation of quantum mechanics emerged in an attempt to solve the measurement problem. This turned on the perception that Niels Bohr avoided addressing the measurement problem by taking an instrumentalist view of quantum mechanics. I argue that this view is mistaken and Bohr's interpretation of quantum mechanics is realist. Moreover, Bohr's interpretation, which is different from textbook quantum mechanics (which is due more to Von Neumann and Paul Dirac), succeeds in solving the measurement problem. While the claim that Bohr dissolves the measurement problem within the limits of the epistemological framework he assumes has been made by a few authors, rarely has the case been made that Bohr's project unambiguously and completely overcomes the measurement problem. I make the strong case that Bohr eliminated the measurement problem altogether. For this, I put forward two new postulates through which to make sense of Bohr's interpretation. The article thus seeks to single out Bohr's interpretation from the various views that go together under the umbrella of orthodox quantum mechanics, and which have been traditionally considered susceptible to the measurement problem. It shows that Bohr's interpretation should be classified along with those like hidden variable theories, collapse models, modal interpretations etc., which offer a solution to the measurement problem and are committed to a realist ontology.
Bohr’s approach to the measurement process implies a shifty split between the classical and quantum descriptions of the world. The current work presents arguments purporting to show that this split has not been satisfactorily removed from subsequent interpretations of the quantum theory. However, in their attempts to do so, rather obscure notions needed to be invoked, which were anticipated, explicitly or implicitly, and carefully circumvented by Bohr. We discuss von Neumann’s description of the measurement process, the decoherence approach, the de Broglie‐Bohm interpretation, and dynamical collapse models.
In his famous thought experiment, Schrôdinger (1935) imagined a cat that measures the value of a quantum mechanical observable with its life.Since Schrödinger's time, no any interpretations or modifications of quantum mechanics have been proposed which give clear unambiguous answers to the questions posed by Schrödinger's cat of how long superpositions last and when (or whether) they collapse?In this paper appropriate modification of quantum mechanics is proposed.We claim that canonical interpretation of the wave function 1 2 cat live cat death cat c c = +.
In his famous thought experiment, Schrôdinger (1935) imagined a cat that measures the value of a quantum mechanical observable with its life.Since Schrödinger's time, no any interpretations or modifications of quantum mechanics have been proposed which give clear unambiguous answers to the questions posed by Schrödinger's cat of how long superpositions last and when (or whether) they collapse?In this paper appropriate modification of quantum mechanics is proposed.We claim that canonical interpretation of the wave function 1 2 cat live cat death cat c c = +.
Abstract As Heisenberg began to promulgate his version of the new quantum philosophy as the ‘Copenhagen spirit’, John von Neumann undertook to purge quantum mechanics of its defects and make it comprehensible to Göttingen’s prince of mathematics, David Hilbert. The result is familiar to any student of physical science. Von Neumann separated the continuous, deterministic, and reversible evolution of the wavefunction from its discontinuous, indeterministic, and irreversible ‘collapse’ in an act of measurement. He saw no reason why the first process should not continue all the way up the chain to the observer’s conscious mind, where the second process occurs. Bohr had little to say about von Neumann’s theory, other than it was ‘wrongly put’. Von Neumann also closed the door that Born had left ajar. Causality could not be restored to the quantum domain by supposing the existence of ‘hidden variables’, as these, according to his reasoning, were mathematically impossible.
This paper reviews and suggests a resolution of the problem of definite outcomes of measurement. This problem, also known as ‘Schrodinger’s cat’, has long posed an apparent paradox because the state resulting from a measurement appears to be a quantum superposition in which the detector is in two macroscopically distinct states (alive and dead in the case of the cat) simultaneously. Many alternative interpretations of the quantum mathematical formalism, and several alternative modifications of the theory, have been proposed to resolve this problem, but no consensus has formed supporting any one of them. Applying standard quantum theory to the measurement state, together with the analysis and results of decades of nonlocality experiments with pairs of entangled systems, this paper shows the entangled measurement state is not a paradoxical macroscopic superposition of states. It is instead a phase-dependent superposition of correlations between states of the subsystems. Thus Schrodinger’s cat is a non-paradoxical ‘macroscopic correlation’ in which one of the two correlated systems happens to be a detector. This insight resolves the problem of definite outcomes but it does not entirely resolve the measurement problem because the entangled state is still reversible.