In the Wikipedia article "Quantum Gravity", it claims that there is some experimental motivation for finding a quantum theory of gravity. In one of the experiments it cites, neutrons are found to jump between discrete quantum states in the earth's gravitational potential, similar to the discrete states of an electron in the nucleus's electrostatic potential. I don't know much about quantum gravity, but it seems that from the equivalence principle, we could view the above experiment as simply what would happen to a neutron in an accelerated reference frame of 9.8 m/s^2. In fact, couldn't one replicate this experiment in a spaceship accelerating at 9.8 m/s^2 and get the same result, without any force of gravity? So there seems to be no evidence for the force of gravity in this experiment, let alone quantum gravity.