The simulation hypothesis — the idea that our universe might be an artificial construct running on some advanced alien computer — has long captured the public imagination. Yet most arguments about it rest on intuition rather than clear definitions, and few attempts have been made to formally spell out what “simulation” even means. In a new paper, SFI Professor David Wolpert introduces a mathematically precise framework for what it would mean for one universe to simulate another — and shows that several longstanding claims about simulations break down once the concept is defined rigorously.