Wednesday, January 15, 2014

The reality of the quantum state

Many physicists have had this idea:
Quantum mechanics predicts observations without telling us what is really going on. Perhaps the elusive truth is hidden in variables that are yet to be discovered.

So look at some reasonable class of theories, and find that they either conflict with quantum mechanics or have unphysical properties that make them much stranger than quantum mechanics.
Ever since Von Neumann in 1932, the consensus has been that the hidden variable theories do not work.

Nevertheless, people keep redisoovering some argument against hidden variables, and acting as if it a profound result. The latest is Notes on the reality of the quantum state
Notes on the reality of the quantum state

The physical meaning of the quantum state is an important interpretative problem of quantum mechanics. A long-standing question is whether a pure state relates only to an ensemble of identically prepared systems or directly to the state of a single system. Recently, Pusey, Barrett and Rudolph (PBR) demonstrated that under an independence assumption, the quantum state is a representation of the physical state of a single quantum system [1]. This poses a further interesting question, namely whether psi-ontology can be argued without resorting to nontrivial assumptions such as the in dependence assumption (cf. Ref. [2-4]). In this Letter, we will show that protective measurements [5,6] already provide such an argument.
A protective measurement is one that gets some info from a quantum state without destroying it. He is trying to imply that the quantum wave function must be some objective physical reality, just because quantum mechanics works so well. But this is no argument against a subjective psi-epistemic interpretation, as discussed in QBism below.

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