The events in spacetime are not symmetric. The causal structure could never quite emerge from such a starting point. More obviously, the symmetry between the events – points of the graph – creates the impression that the spacetime in quantum gravity may be discrete but it remains fundamental.

The last decade in theoretical physics has simply settled this question – whether someone likes it or not – and the answer is “No”. The spacetime geometry cannot be fundamental at the Planck scale. It is subject to transitions, dualities, and holography, among other phenomena that prove that it must be flexible and it cannot arise from a graph because a graph is too local. In neither of the existing descriptions we can find a discrete spacetime and it seems rather unlikely that there exists a description where it is discrete.

— Lubos Motl

2012.10.22 Monday ACHK