Bianchi Haggard Rovelli just posted a landmark paper showing that QSM rises automatically from the GR requirement of general covariance. Yesterday in another thread Atyy identified their paper as especially interesting. I agree. http://arxiv.org/abs/1306.5206 The boundary is mixed Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli (Submitted on 21 Jun 2013) We show that Oeckl's boundary formalism incorporates quantum statistical mechanics naturally, and we formulate general-covariant quantum statistical mechanics in this language. We illustrate the formalism by showing how it accounts for the Unruh effect. We observe that the distinction between pure and mixed states weakens in the general covariant context, and surmise that local gravitational processes are indivisibly statistical with no possible quantal versus probabilistic distinction. 8 pages, 2 figures I believe the point is that with general covariance you don't have an independent time variable and you don't have "initial" and "final" as realistic predicates. You can't have transition amplitudes between initial and final states. So you have to recast quantum theory in boundary formalism, using a bounded region of space-time. Then the Hilbertspace is associated with the entire boundary (not merely with initial and final slabs) and the amplitudes depend on boundary geometry. Hence entanglement with the outside must blur any distinction between pure and mixed states and statistical mechanics is so to say intrinsic--inherent to the situation--comes with the territory Earlier we had a thread or two about "Tomita flow" time. A very beautiful kind of time that is born from star-algebra. Something I like very much about the BHS paper is that after they do away with time by adopting the boundary formalism, at the very end time reappears as the Tomita flow---"saving the day". This is so nice. Please read the paper and say if you find it interesting too (as do Atyy and I)!