severe restrictions on such models. Most of the pitfalls I describe below arise, directly or indirectly, from these requirements.

Challenges for models of emergent gravity
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The preceding section established a set of conditions that a model of emergent gravity should meet to reproduce general relativity. While such a criterion may be too strong at cosmological or sub-millimeter distances, general relativity is extremely well established at intermediate scales [@Will; @Willb], and any disagreements are strongly constrained by observation. Let us now try to understand the extent to which these conditions create obstacles for emergent gravity.

Lorentz invariance
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We start with Type I models, in which the fundamental degrees of freedom live in a space or spacetime environment. The basic problem is clear: unless that background is itself Lorentz invariant, it must decouple from observable quantities strongly enough to match observations. Since experimental limits on Lorentz violation are very strong [@Mattingly], this is a severe restriction.

Perhaps surprisingly, we have simple examples in which such a decoupling occurs, at least to lowest order. In many analog models—models in which curved spacetimes are mimicked by phenomena such as fluid flows—small perturbations in the flow satisfy Lorentz invariant equations, with an effective 