measurement?”) Fine-grained histories effectively relax the
irreversible criterion. Mathematically the many-histories approach is isomorphic to Everett’s many-worlds. The worlds split or “decohere” from each other when irreversible events occur. (See “Why do worlds split?” and “When do worlds split?”.) Correspondingly many-histories defines a multiply-connected hierarchy of classical histories where each classical history is a “child” of any parent history which has only a subset of the child defining irreversible events and a parent of any history which has a superset of such events. Climbing up the tree from child to parent moves to progressively coarser grained consistent histories until eventually the top is reached where the history has *no* defining events (and thus consistent with everything!). This is Everett’s universal wavefunction. The bottom of the coarse-grained tree terminates with the maximally refined set of decohering histories. The classical histories each have a probability assigned to them and probabilities are additive in t
