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Feynman 30 Examples of interfering alternatives. I
From Feynman and Hibbs, Quantum mechanics and path integrals:
p.14 "It is not hard, with a little experience, to tell which kind of
alternative is involved. For example, suppose that information about the
alternatives is available (or could be made available without altering the
result), but this information is not used.  Nevertheless, in this case a
sum of probabilities (in the ordinary sense) must be carried out over
exclusive alternatives.  These exclusive alternatives are those which could
have been separately identified by the information."

Feynman's use of "information" can be identified with 1-1 correlations to
orthogonal states of an "environmental" measuring system which are traced
over in the local detection of the phenomenon.

"When alternatives cannot possibly be resolved by any experiment, they
always interfere."

This is necessary to get a quantum connection signal - the two alternative
paths of the transmitter photon for a fixed path of the twin receiver
photon must interfere.  In addition, the exclusive alternatives for the
detection of the transmitter photon which are traced over at the receiver
must not wash out the phase information from the transmitter.  Henry Stapp
says that the latter is impossible - but that is adhoc not in the standard
quantum formalism.  If retarded causality is right then Stapp will be
correct. Therefore, this quantum connection signal experiment is a direct
test of retarded causality at the individual quantum event level.  It is my
position that retarded causality is a statistical macro- emergent
phenomenon essentially identical to the arrow of time and the second law of
thermodynamics.