Probing the quantum microstructure of spacetime

Abstract

The question of how tightly one can constrain the microscopic theory of quantum gravity from the known features of low energy gravity is addressed. To begin with, from the very fact that our universe made a transition from a quantum regime to classical one, it is possible to conclude that infinite number of degrees of freedom had to be integrated out from the fundamental theory to obtain the low energy Einstein Lagrangian. Further constraints can be imposed from the fact that the quantum state describing a black hole has to possess certain universal form of density of states, in any microscopic description of space-time, which can be ascertained from general considerations. Since a black hole can be formed from the collapse of any physical system with a low energy (E « E_p) Hamiltonian H, it is possible to obtain the form the effective high energy (E » E_p) Hamiltonian from general consideration. These results provide the physical reasons for some of the mathematical features underlying string theories and other models for quantum gravity.