Causal sets as discrete spacetime

Causal set theory is an approach to the problem of quantum gravity in which spacetime is fundamentally discrete and in which causality is a primary concept. The concrete kinematics of the approach makes it relatively straightforward to build phenomenological models that produce observable effects of the underlying discreteness. An example is a model of a massive free particle moving on a causal set background which gives rise to a Lorentz invariant diffusion in the particle's momentum, a novel and potentially fruitful effect.