Monday, May 1, 2017

Bell did not show nonlocality

A new paper argues:
A die-hard tendency in and out of the area of the foundations of quantum mechanics periodically re-emerges, attempting to claim that after all the Bell theorem did not provide a demonstration that the microphysical world, whatever it is, is nonlocal, but simply an elegant rephrasing of a truism, namely that quantum mechanics is not a classical theory! In this note, I argued against the most recent reformulation of this stance, exemplified by the Boughn 2017 paper, by showing that its flaws concern both the logical structure of the nonlocality theorem and its conceptual content.
That's right, most physicists believe that the microphysical world obeys locality. Bell's theorem is just a clever way of showing that quantum mechanics is not a classical theory.

More specifically, Bell showed that quantum mechanics does not match any classical theory of local hidden variables.

There continues to be a minority of crackpots who continue to claim that Bell proved that quantum mechanics was incompatible with any local theory. No, he only ruled out the classical theories of local hidden variables.

Go ahead and read the above paper, for an example of one of those crackpots. He is some Italian named Federico Laudisa. There seems to be some part of quantum mechanics that he does not accept, but his paper is too incoherent for me to figure it out.

Wikipedia explains:
Bell's theorem states that any physical theory that incorporates local realism cannot reproduce all the predictions of quantum mechanical theory. Because numerous experiments agree with the predictions of quantum mechanical theory, and show differences between correlations that could not be explained by local hidden variables, the experimental results have been taken by many as refuting the concept of local realism as an explanation of the physical phenomena under test. For a hidden variable theory, if Bell's conditions are correct, the results that agree with quantum mechanical theory appear to indicate superluminal effects, in contradiction to the principle of locality.

These three key concepts – locality, realism, freedom – are highly technical and much debated. In particular, the concept of realism is now somewhat different from what it was in discussions in the 1930s. It is more precisely called counterfactual definiteness; it means that we may think of outcomes of measurements that were not actually performed as being just as much part of reality as those that were made. Locality is short for local relativistic causality. (Currently accepted quantum field theories are local in the terminology of the Lagrangian formalism and axiomatic approach.) Freedom refers to the physical possibility of determining settings on measurement devices independently of the internal state of the physical system being measured.
That is a pretty good overview. Bell only disproves locality for classical hidden variable theories. Quantum locality is fine. Bell only disproves "realism" if the term is expanded to include the reality of measurements that were not actually performed. Quantum mechanics is silent about unperformed measurements.

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