Smolin makes the case that GR is the Equation of State of a given region's geometry considered as a thermodynamic system whose microscopic degrees of freedom are those of Spin Foam QG. In short: GR=EoS of SF The paper is here: http://arxiv.org/abs/1205.5529 General relativity as the equation of state of spin foam He uses a family of accelerating observers to define the boundary of his region. Their worldlines describe a 3D surface S in his Figure 1. Time goes vertically in the Figure. Two dimensions are missing, necessarily, from the 2D picture. You can see how the 4D region R is bounded on one side by S, on the other side by the Rindler horizons H which form behind any accelerated observer. A rough analogy is the Gas Law PV=nkT viewed as the EoS of a bunch of little molecules whizzing and bouncing around in a box. Here instead of molecules we have a bunch of little bits of geometric information (area, volume, angle) intersizzling and exchanging excitement inside this region R which Smolin gives the boundaries of. And now instead of the Gas Law, the coarse overall description is the GR equation.