 However, it should be noted, finally, that to date neither of the main research programs has been shown to properly reproduce the world we see at low energies. Indeed, it is a major challenge of loop quantum gravity to show that it indeed has general relativity as a low-energy limit, and a major challenge of string theory to show that it has the standard model of particle physics plus general relativity as a low-energy limit. There are promising indications that both theories might be able to overcome this challenge (see Thiemann for the loop quantum gravity case; for the string theoretic case, see Graña, 2006). A similar problem faces causal set theory in the form of the ‘inverse problem’, which roughly amounts to the difficulty of getting continuous manifolds (with their corresponding symmetries) from a fundamentally discrete theory (see Wallden, 2010, for a good recent review of causal sets, including a discussion of this problem, on which progress has also been made).