  Between the microscopic domain ruled by quantum gravity, and the macroscopic
scales described by general relativity, there might be an intermediate,
"mesoscopic" regime, where spacetime can still be approximately treated as a
differentiable pseudo-Riemannian manifold, with small corrections of quantum
gravitational origin. We argue that, unless one accepts to give up the
relativity principle, either such a regime does not exist at all (hence, the
quantum-to-classical transition is sharp), or the only mesoscopic, tiny
corrections conceivable are on the behaviour of physical fields, rather than on
the geometric structures.
