Upper limit to human intelligence

Every other species has a limit to intelligence and humans are no different. So there won't ever be a quantum theory of gravity or unification of all forces. Simply because we aren't smart enough.

The intelligence of a single human is limited. But humans the unique(as far as we know) ability to transmit knowledge form one generation to the next. So out collective intelligence and our knowledge grows incrementally with each generation. therefore i think we will eventually find out the answers to those questions.