Abstract
Relational Quantum Mechanics is an interpretation of quantum mechanics proposed by Carlo Rovelli. Rovelli argues that, in the same spirit as Einstein’s theory of relativity, physical quantities can only have definite values relative to an observer. Relational Quantum Mechanics is hereby able to offer a principled explanation of the problem of nested measurement, also known as Wigner’s friend. Since quantum states are taken to be relative states that depend on both the system and the observer, there is no inconsistency in the descriptions of the observers. Federico Laudisa has recently argued, however, that Rovelli’s description of Wigner’s friend is ambiguous, because it does not take into account the correlation between the observer and the quantum system. He argues that if this correlation is taken into account, the problem with Wigner’s friend disappears and, therefore, a relativization of quantum states is not necessary. I will show that Laudisa’s criticism is not justified. To the extent that the correlation can be accurately reflected, the problem of Wigner’s friend remains. An interpretation of quantum mechanics that provides a solution to it, like Relational Quantum Mechanics, is therefore a welcome one.
Highlights
The Wigner’s friend thought experiment formulated by Eugene Wigner [15] makes it very clear how hard it is to determine when collapse happens
I will explain why this is the case in Sect. 4, but for it is important to see why Wigner’s friend poses a problem if we use the analysis presented above
Relational Quantum Mechanics (RQM) is developed mostly by Rovelli [11, 12, 14] and, just like QBists, Rovelli argues for a relativization of quantum states
Summary
The Wigner’s friend thought experiment formulated by Eugene Wigner [15] makes it very clear how hard it is to determine when collapse happens. The story of the experiment is as follows: Wigner is standing outside a lab in which his friend is performing a measurement on a quantum system with two possible outcomes. Since Wigner himself is outside the lab, he doesn’t know which outcome his friend measures. He does know that his friend and the system will interact and that
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