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If electromagnetism can be incorporated into the standard model, where I think of electromagnetism as derivable from an action principle ala Landau-Lifshitz, at what point does this process stop working when you use gravitation? I was thinking the difficulty lies in the fact that the theory of gravitation in Landau & Lifshitz derives from the equivalence principle implying that one can incorporate curved space into your lagrangian by varying your metric (i.e. this difference in exposition of theory causes problems) yet this link develops the basic theory of gravitation completely analogously to the path followed by Landau & Lifshitz in their electromagnetism section, so I guess there is a more substantial reason problems arise & I'm just wondering what these are. Another way to ask this question would be: why don't GR & QM merge when you look at things from the perspective of Lagrangians? Thanks!

(If I give the impression of knowing what I'm talking about I barely do, still just learning!)