Bei Lok Hu at the University of Maryland has a review article on the "fundamental decoherence" research topic, which devotes detailed attention to the treatment by Gambini and Pullin. I think it's an interesting topic for several reasons, so I'll give some links. Here's the paper by B.L. Hu et al.: http://inspirehep.net/record/781938 (Intrinsic and Fundamental Decoherence: Issues and Problems) http://inspirehep.net/author/profile/B.L.Hu.1 (profile of Bei Lok Hu) Here are G&P's papers on this topic: http://inspirehep.net/record/645205 47 cites (A Relational solution to the problem of time in quantum mechanics and quantum gravity induces a fundamental mechanism for quantum decoherence) http://inspirehep.net/record/653376 38 cites (Realistic clocks, universal decoherence and the black hole information paradox) http://inspirehep.net/record/674573 12 cites (Fundamental decoherence in quantum gravity) http://inspirehep.net/record/712912 38 cites (Fundamental decoherence from quantum gravity: A Pedagogical review) http://inspirehep.net/record/735013 25 cites (Relational physics with real rods and clocks and the measurement problem of quantum mechanics) I think the gist of it is that in GR natural processes occur at different rates all over the place. There is no official/ideal time, so the best one can do is correlate the other observables to some choice of *real clock*. The definition of unitary evolution is only as good as the clock. It seems that Eugene Wigner came up with a theoretical limit on the precision-lifespan of a real clock (how accurate for how long a time a clock could be made without it foiling you by turning into a black hole). And Gambini Pullin adapted Wigner's limit on real clocks to find a theoretical limit on the lifespan of unitarity. You will have to refer to their "Realistic Clocks" paper, http://arxiv.org/abs/hep-th/0406260, because I can't reproduce their argument in detail, but the upshot seems to be that if one focuses on black hole evaporation the unitarity of evaporation dies out on a timescale comparable to the lifespan of the black hole itself.