Gordon Watson wrote:
1. If that is now all that you are saying (limiting your statement to a single run of the EPRB experiment rather than a single run of an experiment known as a Bell-test), then this statement of yours remains false: "It [QM] makes no prediction whatsoever for the Bell-CHSH quantity E(a, b) + E(a, b’) + E(a’, b) – E(a’, b’), because no experiment can ever be performed (even by "God") which can measure that quantity."
2. I look forward to an explanation of this statement of yours: "The point I am making is rather trivial to understand: a, b, a', and b' are mutually exclusive, or physically incompatible observation directions."
I find this discussion quite tedious, not the least because we (all of us in this forum) have beaten these issues to death, many times over. But let us indulge in it again:
It is quite obvious that (a, b), (a, b’), (a’, b), and (a’, b’) are mutually exclusive pairs of measurement directions. Each pair can be used by Alice and Bob for a given experiment, for all runs, but no two of the four pairs can be used by Alice and Bob
simultaneously. This is simply because Alice and Bob do not have a miraculous ability to be in London and Paris at the same time (
i.e., on the same space-like hypersurface). If you don't agree with this paragraph, then this discussion ends here.
Now consider the following averages E(a, b), E(a, b’), E(a’, b), and E(a’, b’). Each of these averages is well defined and observable. I believe we agree about this.
Let me now unpack E(a, b) to explain what it means: E(a, b) = < A(a) B(b) > , where < > stands for ordinary average of the product A(a)B(b) of numbers A(a) and B(b).
Now, for a single run of a given experiment, consider the quantity A(a) B(b) + A(a) B(b' ) + A(a' ) B(b) - A(a' ) B(b' ). This quantity is actually meaningless because it is not observable for a single run of a given experiment, because (a, b), (a, b’), (a’, b), and (a’, b’) are mutually exclusive pairs of measurement directions, as agreed.
Now consider several different runs of the experiment and the corresponding un-observable quantities similar to A(a) B(b) + A(a) B(b' ) + A(a' ) B(b) - A(a' ) B(b' ).
For clarity, let us label the runs by numbers 1, 2, 3, ... n, so that we have the following set of n un-observable quantities altogether:
A_1(a) B_1(b) + A_1(a) B_1(b' ) + A_1(a' ) B_1(b) - A_1(a' ) B_1(b' )
A_2(a) B_2(b) + A_2(a) B_2(b' ) + A_2(a' ) B_2(b) - A_2(a' ) B_2(b' )
A_3(a) B_3(b) + A_3(a) B_3(b' ) + A_3(a' ) B_3(b) - A_3(a' ) B_3(b' )
*
*
*
A_n(a) B_n(b) + A_n(a) B_n(b' ) + A_n(a' ) B_n(b) - A_n(a' ) B_n(b' ).
Since each of these n quantities is an un-observable quantity, their average,
< A_k(a) B_k(b) + A_k(a) B_k(b' ) + A_k(a' ) B_k(b) - A_k(a' ) B_k(b' ) >
(where the summation is over the index k going from 1 to n),
is also an un-observable quantity.
But the above average is often what is meant by the expression E(a, b) + E(a, b’) + E(a’, b) – E(a’, b’) in the CHSH discussions, which surreptitiously assumes
E(a, b) + E(a, b’) + E(a’, b) – E(a’, b’) = < A_k(a) B_k(b) + A_k(a) B_k(b' ) + A_k(a' ) B_k(b) - A_k(a' ) B_k(b' ) > ......................... (1).
Hence my statement you keep quoting: "It [QM] makes no prediction whatsoever for the Bell-CHSH quantity E(a, b) + E(a, b’) + E(a’, b) – E(a’, b’), because no experiment can ever be performed (even by "God") which can measure that quantity."
Just in case there is still some confusion, let me stress that, individually, each of the averages ,
E(a, b) = < A_k(a) B_k(b) > ,
E(a, b’) = < A_k(a) B_k(b' ) > ,
E(a’, b) = < A_k(a' ) B_k(b) > ,
E(a’, b’) = < A_k(a' ) B_k(b' ) > ,
is a perfectly good, observable quantity. Each E(a, b) can be computed for a given choice (a, b) of a pair of observable directions, on a given space-like hyper-surface.
But if one sticks to individual averages only, then the upper bound of 2 on CHSH cannot be derived. The upper bound of 2 can be derived
only by cheating, as in Eq. (1)
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