Home › Forums › 2015 International Workshop on Quantum Foundations › Retrocausal theories › Causality and quantum mechanics (Online 7/15 @ 10 p.m. to Midnight UTC-7) › Reply To: Causality and quantum mechanics (Online 7/15 @ 10 p.m. to Midnight UTC-7)
Fortunately the case we consider avoids the situation you refer to. For example, your example does not begin with a maximally mixed state and it is implicit in our case that each member of the ensemble is available for A, B and C to be performed on them.
When considering transitions, as your example does, the role of final states is a thorny question. There is a brief discussion of this in Feynman’s thesis p.4 where, comparing his (and Wheeler’s) theory of action at a distance (= advanced and retarded potentials) with conventional theory, he says “It is here that the theory of action at a distance gives us a different viewpoint. It says that an atom alone in empty space would, in fact, not radiate.” (emphasis in the original) There have been interesting discussions about this and related matters over the years but it seems minds still differ. Certainly in condensed matter systems an electron can’t tunnel out of a quantum well unless there are empty states nearby but normal quantum mechanics seems to handle that OK without requiring anything retro.