A question regarding the Copenhagen interpretation.

[DevilsAvocado]

Just to make clear, I do agree that one cannot have a local realistic interpretation of quantum mechanics unless one violates the assumptions of the Bell theorem. For example, one can violate the assumptions of the Bell theorem by assuming that a measurement does not yield a unique outcome, but rather all outcomes occur - even then - it is not clear if the interpretation is local - but I'll certainly grant that its nonlocality is not assured by violation of the Bell inequalities.

Okay, that must be MWI, right?

I also agree that it is not enough to say that the measurement results don't exist before measurement in order to violate the assumptions.

Agreed 100%, it especially goes baloney if the outcome assumption is always [1, 0] or [0, 1] ...

One has to do something drastic like saying that there are not two observers, but only one.

Wow, that's a new one, isn't it?

However, it does seem that Griffiths claims consistent histories to be "realistic" and local. You can find the claim of locality in http://quantum.phys.cmu.edu/CQT/chaps/cqt24.pdf and the claim of "realism" in http://quantum.phys.cmu.edu/CQT/chaps/cqt27.pdf. In fact, both claims are made as points 2 and 3 on p318 of the second link. I don't know enough about consistent histories to comment on the claim, but my impression is that his definition of reality isn't "common sense realism", so if his claim is correct, my bet would be that his definition of reality is actually a form of nonrealism.

Yeah I know... it's quite strange... he writes:

"By contrast, a consistent application of quantum principles provides a positive demonstration of the absence of nonlocal influences, as in the example discussed in Sec. 23.4."​

But chapter 23.4 is about Stern-Gerlach and Measurements of One Spin, not about Bell. And in chapter 24.4 about Bell inequalities, he instead admits that:

"Such a theory must either exhibit peculiar nonlocalities which violate relativity theory, or else incorporate influences which travel backwards in time, in contrast to everyday experience."​

And finish chapter 24.4 with:

"This seems a rather high price to pay just to have a theory which is more “classical” than ordinary quantum mechanics."​

And there it all ends... no explanation, just a 'statement'.

A quite inconsistent story, if you ask me.

 

[DevilsAvocado]

Well its the other thing I often say about Consistent Histories - its defining your way out of problems.

Okay, then there's some serious defining ahead (after post #93).

 

[atyy]

Wow, that's a new one, isn't it?

Old one :) http://arxiv.org/abs/quant-ph/0509061, see the discussion on p9-10, which includes "I suggest that Einstein was quite deliberately not assuming the existence of more than one observer"

 

[Dale]

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