I am not a physicist.
For as long as I can remember, I've been told that the main problem in physics is to unify quantum mechanics and general relativity into a single theory.
I once heard someone say, that the reason we haven't combined general relativity with QM, is simply because in GR, quantum operators wouldn't be linear (due to the curvature of spacetime), so that the superposition principle fails.
In other words, that would mean we have no problem understanding how they would be unified, conceptually, we just don't know how to deal with the nonlinear mathematics that would result from this.
Is that understanding correct?