Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.
Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity. NOT to be confused with theory-of-everything.
A requirement of quantum gravity springs from the fact that classical gravity and quantum matter cannot go along together, due to the Einstein Field Equation $G_{\mu\nu}=\frac{8\pi G}{c^4}T_{\mu\nu}$.
However, quantising gravity in the traditional way, leads to a non-renormalisable theory.
