What is Bell's Inequality?

Continuing the discussion from The Pilot Wave Interpretation of Quantum Mechanics:

As a supplement to a sister thread, some people are probably wondering what we mean by “Bell’s Inequality”. The wikipedia page is dense:

This article from Mark Alford at WUSTL might be a better start. He presented a talk on this to the seminary faculty at Concordia (@CPArand), and they were able to follow him.

Ghostly action at a distance: a non-technical explanation of the Bell inequality

We give a simple non-mathematical explanation of Bell’s inequality. Using the inequality, we show how the results of Einstein-Podolsky-Rosen (EPR) experiments violate the principle of strong locality, also known as local causality. This indicates, given some reasonable-sounding assumptions, that some sort of faster-than-light influence is present in nature. We discuss the implications, emphasizing the relationship between EPR and the Principle of Relativity, the distinction between causal influences and signals, and the tension between EPR and determinism.
[1506.02179] Ghostly action at a distance: a non-technical explanation of the Bell inequality

We have physicists on the forum, so ask your questions if you’d like to understand.

@pevaquark, @dga471, @PdotdQ


As in voluminous, not as in obtuse. :slight_smile:

This may have already been mentioned, but I didn’t see it.

Why not mix in some philosophy? The finding of the BIg Bell Test was basically that if free will exists then some physical events have no causes. Published in Nature in May, but the link to the 36 pages in Arxiv is at the bottom of this MIT Tech review.


I’ve always wondered whether it’s possible to create an account of causation that doesn’t rely on determinism. For example, if I do the Schroedinger’s cat experiment and the cat dies, then in some way I am the cause of the cat’s death, even if there is some inherent indeterminacy in the chain of events causing the cat’s death due to quantum mechanics.

If I may suggest, the Scholarpedia article on Bell’s Theorem is better-informed than the Wikipedia article, having been written by physicists who have studied Bell’s own perspective in detail.

The gist of it is that Bell’s theorem proves that some kind of non-locality is operative in quantum mechanics - there are experimentally well-confirmed predictions of QM that cannot be explained by causal influences propagating no faster than the speed of light.

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