# Confusion by Murray Gell Mann video and Bell's Theorem

I'm trying to understand 3:02 in this video:

He seems to be saying that the measurement of a property one photon has no effect on the other. So the other photon doesn't change at all from before the measurement to after the measurement of the first photon? So it already had whatever property the experimenter measured before the measurement took place?

Isn't this precisely what Bell's Theorem shows isn't true?

The version of Bell's Theorem I'm using is stated here:

https://en.wikipedia.org/wiki/Bell%27s_theorem

as "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics."

I understand what Murray Gell Mann is saying in the video is that the other photon has certain pre-existing properties. We just learn about those properties when measuring on the first photon. Aren't those local hidden variables?

• can you give a link to what form of Bell's theorem you are referring to? – anna v Jul 16 '17 at 7:27
• The photon as it exists in the entire set of decoherent histories which includes what happened in the counterfactual history where you found a different result for your photon, isn't affected. – Count Iblis Jul 16 '17 at 7:32
• @CountIblis, so these counterfactual histories are "real" in some sense? – Ameet Sharma Jul 16 '17 at 7:37
• local means the same (x,y,z,t) , the two photons have different (x,y,z,t). quantum mechanics is non local, i.e. the wave function describes the whole system at different (x,y,z,t) . see this view of Bells theorem scholarpedia.org/article/Bell%27s_theorem . – anna v Jul 16 '17 at 7:37
• In the Many Worlds Interpretation one assumes that the counterfactual histories are real. If you assume that they are not real, then you still can't assume that the actual history out of the set of all the histories is predetermined as that will lead to a conflict with Bell's theorem. But this view leads to a hidden nonlocality, it's not going to bring QM in conflict with special relativity. I summarized the problem in this question. – Count Iblis Jul 16 '17 at 21:17