Question

Cellular automata provide interesting models of physics: Google Scholar gives more than 25,000 results when searching for "cellular automata" physics.

Google Scholar still gives more than 2.000 results when searching for "quantum cellular automata".

But it gives only 1 (one!) result when searching for "relativistic cellular automata", i.e. cellular automata with a (discrete) Minkoswki space-time instead of an Euclidean one.

How can this be understood?

Why does the concept of QCA seem more promising than that of RCA?

Are there conceptual or technical barriers for a thorough treatment of RCA?

Answer 1

Check out Mark Smith's PhD thesis titled Cellular automata methods in mathematical physics, specifically Chapter 4: Lorentz Invariance in Cellular Automata.

The conclusion part of the chapter:

Symmetry is an important aspect of physical laws, and it is therefore desirable to identify analogous symmetry in CA rules. Furthermore, the most important symmetry groups in physics are the Lorentz group and its relatives. While there is a substantial difference between the manifest existence of a preferred frame in CA and the lack of a preferred frame demanded by special relativity, there are still some interesting connections. In particular, CA have a well-defined speed of light which imposes a causal structure on their evolution, much as a Minkowski metric imposes a causal structure on spacetime. To the extent that these structures can be made to coincide between the CA and continuum cases, it makes sense to look for Lorentz invariant CA.

The diffusion of massless particles in one spatial dimension provides a good example of a Lorentz invariant process that can be expressed in alternative mathematical forms. A corresponding set of linear partial differential equations can be derived with a simple transport argument and then shown to be Lorentz invariant. A CA formulation of the process is also Lorentz invariant in the limit of low particle density and small lattice spacing. The equations can be solved with standard techniques, and the analytic solution provides a check on the results of the simulation. Generalization to higher dimensions seems to be difficult because of anisotropy of CA lattices, though it is still plausible that symmetry may emerge in complex, high-density systems. The model and analyses presented here can be used as a benchmark for further studies of symmetry in physical laws using CA.