When is quantum gravity necessary?

It is true that at the Planck energy scale, the action for the gravitational field becomes of the order \hbar, and hence quantisation of gravity becomes essential.

However, while this is a sufficient condition for a non-classical treatment of gravitation, it is not necessary. Magnitude of the action of the gravitational field is not the only criterion for deciding whether spacetime should be treated non-classically. The point structure of the spacetime manifold also needs to be attended to.

Even at low energies, if every matter source is quantum and in a superposition of position states, the produced gravitational field will be in a superposition of different classical gravitational fields. At a given spacetime point, the gravitational field does not take a unique classical value. This implies that the point structure of the underlying spacetime manifold is no longer operationally defined, and is hence lost. This is happening even though the action of the gravitational field is much larger than \hbar. As a consequence, it becomes necessary to reformulate quantum theory without making reference to classical spacetime.

Our conventional formulation of quantum theory takes for granted that the universe is dominated by classical objects; so that it has a definite classical metric. This allows a spacetime with a definite point structure to be assumed. But this is an approximate situation.

The standard model of particle physics has so many undetermined free parameters precisely because it is formulated on a spacetime with definite point structure. This is an approximate description. When this approximation is removed, and spacetime replaced by non-commutative spacetime [at low energies] we begin to see evidence that the parameters of the standard model are not free.

There are two aspects to quantum gravity. One is the quantum nature of the metric. The other is the point structure of spacetime. The former becomes significant only at Planck energy scale, and is not relevant to low energy particle physics. However, the latter - the classical point structure of spacetime - is already lost at low energies if all matter sources are quantum in nature. The spacetime is non-classical even though gravity is weak. This is relativistic weak quantum gravity, and its relevance for the standard model of particle physics is non-trivial. This fact has been overlooked thus far.