single metric, though, a result of Weinberg offers a path for deriving the equivalence principle [@Weinberg]. The “soft graviton theorem” shows that a Lorentz-invariant, massless spin two particle that can scatter nontrivially must couple universally to a single conserved stress-energy tensor. One must be careful of assumptions here; see the discussion below of the Weinberg-Witten theorem. In particular, Weinberg’s result are only relevant if Lorentz invariance has already emerged. But the theorem suggests that if only one metric is present, universal coupling may not be arbitrary, but may be associated with the universality of Lorentz invariance.

For type II models, the primary question comes earlier: does a dynamical spacetime emerge at all? If the fundamental degrees of freedom generate such a spacetime at some scale, a metric description offers a natural way to describe the dynamics. As in ordinary general relativity, one might then expect a single spacetime to have a single metric. On the other hand, if matter emerges from the same degrees of freedom, Weinberg’s soft graviton theorem, with its requirement of Lorentz invariance, is the only reason I know to expect universal coupling. Since very few type II models are yet able to describe the coupling of 