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The Kochen-Specker and Conway-Kochen Theorems

Ahmed Barbar

Published May 5, 2017• 0 citations
OpenAlex

Abstract

This essay provides an analysis of two important theorems that arise in the context of quantum mechanics: the Kochen-Specker theorem, which challenges the existence of hidden variable theories; and the Conway-Kochen (free will) theorem, which can be seen as an improvement over the Kochen-Specker theorem, but instead focuses on challenging determinism.

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This thesis explores the exact differences between the two most well-known no-hidden variable theories: the Bell theorem and the Kochen-Specker theorem. Furthermore, it tries to place a newer result called `the free will theorem', as developed by John Conway and Simon Kochen, in this context. To compare the Bell and Kochen-Specker theorems a reformulation of the two theorems by David Mermin will be discussed, which turns out to lead to an even stronger no-hidden variable theory. From the discussion of the free will theorem and the critical reactions it has elicited, it will become clear that the free will theorem is actually very similar to this stronger no-hidden variable theory.

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This essay provides an analysis of two important theorems that arise in the context of quantum mechanics: the Kochen-Specker theorem, which challenges the existence of hidden variable theories; and the Conway-Kochen (free will) theorem, which can be seen as an improvement over the Kochen-Specker theorem, but instead focuses on challenging determinism.
This thesis explores the exact differences between the two most well-known no-hidden variable theories: the Bell theorem and the Kochen-Specker theorem. Furthermore, it tries to place a newer result called `the free will theorem', as developed by John Conway and Simon Kochen, in this context. To compare the Bell and Kochen-Specker theorems a reformulation of the two theorems by David Mermin will be discussed, which turns out to lead to an even stronger no-hidden variable theory. From the discussion of the free will theorem and the critical reactions it has elicited, it will become clear that the free will theorem is actually very similar to this stronger no-hidden variable theory.
I begin with description of quantum entanglement and its relevance to the EPR-type experiments, which played a decisive role in rejecting determinism and hidden-variable theories. Thereby I prove the closely-related Kochen-Specker paradox, on which also the main object of my paper, the recently published Conway and Kochen’s Free Will Theorem, is founded. It is being shown there, on the basis of three physical axioms, following from the special theory of relativity and quantum mechanics, that if the choice for a particular type of spin 1 experiment is not a function of the information accessible to the experimenters, then its outcome is equally not a function of the information accessible to the particles. Therefor the response of the particles can be seen as free. In conclusion I mention some critical remarks on the theorem, expressed in subsequent responses to the original article.
One of the statements one would hear pretty early in any quantum mechanics course is that for quantum particles there are properties which do not have definite values. For example, if we measure |+〉 in standard basis, the best we can do to describe the outcome is to say, that with probability 1/2 we will get 0 and with probability 1/2 we will get 1. Yet one could wonder, is this really the way things are or is it just because our description of this quantum particle is incomplete, but in fact there are some hidden variables inside every quantum system that contain enough information to tell the outcome of measurement (see Fig. 1). However, many people disdain the idea of hidden variables, so there has been rather long history of theorems that forbid theories involving hidden variables. Usually if one wants to prove some no hidden variables theorem he has to assume something about the hidden variables theories he is going to forbid. One of such assumptions is non-contextuality, which means that the outcome we get after measuring with an observable is the same regardless of what other observables we choose to measure at the same time. The first theorem forbidding hidden variables theories is due to von Neumann in 1932. Then for a third of a century everyone cited von Neumann’s work carelessly, till Bell in 1966 indicated that there was an unreasonable assumption in von Neumann’s proof and urged for further examination of the problem (see [6] and [1] for more historical details). Bell also constructed a hidden variables model for one qubit that could assign the outcome of any observable in a way consistent with quantum mechanics. He also proved two theorems that forbid non-contextual hidden variables theories using counterexamples with larger quantum systems. Yet surprisingly he himself thought that hidden variables do exists and considered these theorems only as characterizations of the hidden variables theory we should look for. In this essay we will discuss several versions of one of the two Bell’s theorems. The theorem we will consider was also independently proven by Kochen and Specker in 1967 (see [5]), hence it is called Bell-Kochen-Specker theorem. For shortness and in order not to mix this theorem with another Bell’s theorem from now on we will refer to it as Kochen-Specker theorem. We will be interested in proving several versions of this theorem and the following lemma will turn out to be of great use:
In this talk I present a simple derivation of an old result of Kochen and Specker, which is apparently unrelated to the famous work of Bell on hidden variables, but is presumably equally important. Kochen and Specker showed in 1967 that quantum mechanics cannot be embedded into a classical stochastic theory, provided the quantum theoretical probability distributions are reproduced and one additional highly desirable property is satisfied. This showed in a striking manner what were the difficulties in implementing the Einstein programme of a `complete' version of quantum mechanics.