Turns out the Lua API exposes functions to take undo snapshots and return to them. I fragrantly abused them to create tachyons, particles that travel back in time.

https://gist.github.com/rdococ/433a3c6461b187403a2f4f1f3f5ec0cd

Tachyons reverse the flow of time until they decay. Time moves forward again, creating a "negatachyon" representing the tachyon in reverse. If the negatachyon doesn't end up within line of sight to where the tachyon was created (meaning the tachyon has prevented itself from existing or taking a certain path), the simulation rewinds to erase the negatachyon and its effects from history.

This save has a machine that emits a tachyon only if it doesn't detect one from its past self, causing a grandfather paradox that manifests as an infinite loop.

https://powdertoy.co.uk/Browse/View.html?ID=2988647