A Theory of Quantum Gravity may not be possible because Quantum   Mechanics violates the Equivalence Principle
  Quantum mechanics clearly violates the weak equivalence principle (WEP). This implies that quantum mechanics also violates the strong equivalence principle (SEP), as shown in this paper. Therefore a theory of quantum gravity may not be possible unless it is not based upon the equivalence principle, or if quantum mechanics can change its mass dependence. Neither of these possibilities seem likely at the present time. Examination of QM in n-space, as well as relativistic QM equations does not change this conclusion. 
Mario Rabinowitz