  Three degrees of quantum involvement in gravity can be distinguished. The first degree, referred to as quantum field theory on curved  spacetimes, simply takes off the shelf a spacetime provided by  general relativity theory and then proceeds to study the behavior of  quantum fields on this background spacetime. The Unruh effect, which  predicts the thermalization of a free scalar quantum field near the  horizon of a black hole, lies within this ambit. The second degree of involvement, referred to as semi-classical quantum gravity, attempts  to calculate the backreaction of the quantum fields on spacetime  metric by computing the expectation value  \(\langle \Psi \mid T_{ab} \mid \Psi \rangle\) of the stress-energy  tensor in some appropriate quantum state \(\lvert\Psi\rangle\) and then  inserting the value into Einstein’s field equations in place of  \(T_{ab}\).[26]  Hawking’s celebrated prediction of black hole radiation belongs to  this   ambit.[27]  The third degree of involvement attempts to produce a genuine quantum theory of gravity in the sense that the gravitational degrees of  freedom are quantized. Currently loop quantum gravity and string  theory are the main research programs aimed at this   goal.[28] 