== Conflict between GR and QM == 

 The section originally had this line: "Several decades after the discovery of general relativity it was realized that general relativity cannot be the complete theory of gravity because it is incompatible with quantum mechanics".   
 This statement implicitly promotes a point of view: the correct statement is that GR and quantum mechanics are incompatible, and hence either GR is an incomplete theory, or quantum mechanics is. (or both, of course).  It's quite plausible that it may be possible to reformulate QM in a way to incorporate highly-curves spacetime, and that doing so would solve the problem with no changes in GR.   
 I rewrote this section slightly to remove the implied statement that, of the two theories, the one that's in errror is GR, not QM.  
 I'm also not at all sure that the formulation of gravity as a force involving virtual exchange of spin-2 gravitons postdates the (rather slow) realization that QM and GR are incompatible at the fundamental level; in fact, without formulating GR in terms of quantum field theory, you can't really show that GR and QM are incompatible, so I removed a handful of words suggesting this timeline.