Density proportional to curve in space time. I.e. D=k x l Mass/volume = l x K Energy/volumex c^2= l x k E= V x c^2 x l x k...... Eq1 Gravity proportional to curve in space time I.e. G=p x l l=G/p...eq2 From eq1 n eq2 we get, E=(G/p)x k x V x c^2 From what we can see here if the energy were to be quantized we can also quantize gravity. Hence quantum gravity.