Loop Representation of Quantum Gravity

Abstract

A hyperlink is a finite set of non-intersecting simple closed curves in $\mathbb {R}^4 \equiv \mathbb {R} \times \mathbb {R}^3$, and each curve is either a matter or geometric loop. We consider an equivalence class of such hyperlinks, up to time-like isotopy, preserving time-ordering. Using an equivalence class and after coloring each matter component loop with an irreducible representation of ${\mathfrak {s}}{\mathfrak {u}}(2) \times \mathfrak {su}(2)$, we can define its Wilson loop observable using an Einstein–Hilbert action, which is now thought of as a functional acting on the set containing equivalence classes of hyperlink. Construct a vector space using these functionals, which we now term as quantum states. To make it into a Hilbert space, we need to define a counting probability measure on the space containing equivalence classes of hyperlinks. In our previous work, we defined area, volume and curvature operators, corresponding to given geometric objects like surface and a compact solid spatial region. These operators act on the quantum states and, by deliberate construction of the Hilbert space, are self-adjoint and possibly unbounded operators. Using these operators and Einstein’s field equations, we can proceed to construct a quantized stress operator and also a Hamiltonian constraint operator for the quantum system. We will also use the area operator to derive the Bekenstein entropy of a black hole. In the concluding section, we will explain how loop quantum gravity predicts the existence of gravitons, implies causality and locality in quantum gravity and formulates the principle of equivalence mathematically in its framework.