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[ **Challenges for Emergent Gravity**]{}\

[**Abstract**]{}

[The idea of gravity as an “emergent” phenomenon has gained popularity in recent years. I discuss some of the obstacles that any such model must overcome in order to agree with the observational underpinnings of general relativity. ]{}

Over the past few years, the idea of gravity as an “emergent” phenomenon has become increasingly popular. Emergence is seldom sharply defined, and the notion has been described as “vague and contentious” [@Butterfield], but the basic picture is that gravity, and perhaps space or spacetime themselves, are collective manifestations of very different underlying degrees of freedom. Such proposals are not new—Wheeler wrote about pregeometry as early as 1963 [@Wheelera], and Finkelstein introduced a version of causal set theory in 1969 [@Finkelstein]—but historically they were marginal aspects of research in quantum gravity, whose most noted proponents were often outsiders to the field (e.g., [@Pines; @Laughlin; @Volovik]). More recently, though, such mainstream approaches as string theory matrix models [@Seiberg], the AdS/CFT correspondence [@Polchinski; @Witten], loop quantum cosmology [@Bojowald], and quantum Regge calculus [@Loll] have all been described as ß“emergent.”

Given the enormous difficulty of quantizing general relativity, it is natural to consider the possibility that we have 