The closest thing I can think of is [Geant4](https://en.wikipedia.org/wiki/Geant4 https://en.wikipedia.org/wiki/Geant4), which is a monte carlo-based particle s

The closest thing I can think of is [Geant4](https://en.wikipedia.org/wiki/Geant4 https://en.wikipedia.org/wiki/Geant4), which is a monte carlo-based particle simulator. It doesn't have facilities to work with geometric models of spacetime, of course, but it's used extensively in the high energy physics community and medical imaging communities.