Why do some spacetime simulations hit a hard stop?

Imposing rigid boundary conditions on a finite surface in Einstein's gravity — a setup used in quantum gravity and holography — generically causes singularities to collide with that boundary, terminating evolution entirely. The clearest examples appear in cosmological settings, but the authors also show that in 2+1 dimensions, a Dirichlet wall outside a BTZ black hole falls into the horizon and hits a singularity. This suggests a fundamental tension between finite-boundary formulations and well-defined long-term dynamics.