Whats the problem between the theory of Relativity and Quantum Physics?

I understand that the Theory of Relativity explains the space/time fabric of the universe, gravity effect on it, energy and mass/the speed of light, and ect. and that Quantum physics is about atomic sized partials (i know less about it than Relativity) but i have herd that the two theory's don't quite agree with each other i heard some explanations but i don't quite understand them.|||General relativity is a classical theory--it explains the behavior of objects at low energy scales.





Quantum mechanics describes the behavior of objects at very high energy and small distance scales.





Because the quantum of gravity carries so little energy, the quantum nature of gravity has never manifest itself in any experiment or natural phenomenon. General relativity works fine. But given enough energy (maybe more than we will ever generate experimentally), GR should break down.





There are theories of quantum gravity that have GR as a low energy limit, just as classical gravity is a low mass, long distance limit of GR. The problem with them, though, is that they are non-renormalizable. There are infinite energies that can't be ignored or hand-waved away as they are in other quantum field theories (because gravity couples to energy). String theory does successfully reconcile classical GR with quantum mechanics, but we are a long way from being able to test it in any other way.