Quantum geometry of 2 d gravity coupled to unitary matter

We show that there exists a divergent correlation length in 2d quantum gravity for the matter fields close to the critical point provided one uses the invariant geodesic distance as the measure of distance. The corresponding reparameterization invariant two-point functions satisfy all scaling relations known from the ordinary theory of critical phenomena and the KPZ exponents are determined by the power-like fall off of these two-point functions. The only difference compared to flat space is the appearance of a dynamically generated fractal dimension d h in the scaling relations. We analyze numerically the fractal properties of space–time for Ising and three– states Potts model coupled to 2d dimensional quantum gravity using finite size scaling as well as small distance scaling of invariant correlation functions. Our data are consistent with d h = 4, but we cannot rule out completely the conjecture d H = −2α 1 /α −1 , where α −n is the gravitational dressing exponent of a spin-less primary field of conformal weight (n + 1, n + 1). We compute the moments L n and the loop–length distribution function and show that the fractal properties associated with these observables are identical, with good accuracy, to the pure gravity case.