SPACETIME INFORMATION - COMMENT

A recent paper by Hartle [Phys. Rev. D 51, 1800 (1995)] proposes a def inition of ''spacetime information'' - the information available about a quantum system's boundary conditions in the various sets of decoher ing histories it may display - and investigates its properties. We not e here that Hartle's analysis contains errors which invalidate several of the conclusions. In particular, the proof that the proposed defini tion agrees with the standard definition for ordinary quantum mechanic s is invalid, the evaluations of the spacetime information for time-ne utral generalized quantum theories and for generalized quantum theorie s with nonunitary evolution are incorrect, and the argument that space time information is conserved on spacelike surfaces in these last theo ries is erroneous. We show, however, that the proposed definition does , in fact, agree with the standard definition for ordinary quantum mec hanics. Hartle's definition relies on choosing, case by case, a class of fine-grained consistent sets of histories. We supply a possible gen eral definition of a class of sets that includes all the sets consider ed in Hartle's paper and that generalizes to other cases.