is only recovered at large distances. After all, ordinary lattice quantum field theory is constructed on a lattice that is not even rotationally invariant, but by tuning parameters to a second order phase transition one can send correlation lengths to infinity, wiping out any memory of the underlying lattice. It is not clear that a similar procedure exists for Lorentz invariance, though, where the correlations must respect the light cone structure. One possible ingredient could be an emergent supersymmetry, which can suppress Lorentz violation [@Nibbelink].

Alternatively, Lorentz invariance could appear as a low energy symmetry under the renormalization group flow [@Nielsen]. Models are known in which different “speeds of light” flow to a single value at an infrared fixed point [@Anber], but this flow is typically only logarithmic in energy, requiring enormous initial energy scales or delicate fine tuning to meet observational constraints.

For type II models, Lorentz invariance may be less contrived. By a century-old argument [@Baccetti], the existence of inertial frames, isotropy of space, and the relativity principle are enough to imply Lorentz transformations with some (perhaps infinite) “speed of light.” For type I models, the presence of a background typically violates either isotropy or relativity, but this 