The Physics of the Undo: Building Universes from Code In the physical universe, time moves in one direction. Entropy increases. A star that collapses cannot be un-collapsed; a glass that shatters on a stone floor does not spontaneously reassemble. The arrow of time is the ultimate, non-negotiable constraint of our existence. Yet, in the digital worlds we build, we have created a miracle that defies the very nature of reality: The Undo. We have cultivated a modern expectation