Spin foam

Summary

A spin foam is a two-dimensional cell complex whose faces carry spins and whose edges carry intertwiners. It is a "history" of spin networks. Spin foam models define the transition amplitudes of loop quantum gravity as a sum over such histories, a discrete version of the gravitational path integral. The name was introduced by John Baez in 1998; the Ponzano–Regge model of three-dimensional gravity (1968) is the first example.