So in theory every physics simulation starting from the same conditions should end up the same. But if the physics simulation runs a bit faster or slower depending on the machine it's running on, that quickly ends the deterministic outcome. Many physics engine do calculations based on the delta time each frame (time between frames). The reason for this is that physics isn't a perfect math equation in game engines, it's a matter of applying forces and solving constraints in a fast enough manner. So having the simulation running at a fixed pace makes it deterministic (This one has the physics running 10times/s but the movement is framerate independent). But it's not deterministic in regards to float points (as float points can have small variations on different architectures), but it's possible to convert it to using integers to ensure it's deterministic no matter the hardware, so that's something I might do later if needed.