Grabby Aliens Simulator

A playable version of the grabby aliens model of Robin Hanson, Daniel Martin, Calvin McCarter and Jonathan Paulson (2021): loud civilizations appear at a rate rising as a power of time (the hard-steps law), then expand as spheres at a fixed fraction of light speed and prevent any later origin inside the volume they have already taken. The simulation is deliberately simplified: a static Euclidean box with periodic boundaries, no cosmological expansion and no curvature, so distances are plain Euclidean and the closed-form nucleation theory it is checked against stays exact.