A months-old energy leak found in a custom GPU physics engine

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Nora Kinetics is the poster's eight-month passion project, a custom GPU-driven physics engine and renderer, mostly hand-written. Glued structures developed a bug where energy leaked in and small clusters twitched and drifted instead of coming to rest, and months of effort with and without AI had not found it.

When Fable 5 came out they gave it the problem with their notes. After about 15 minutes it diagnosed a GPU reduction accumulating physics quantities in fixed-point integer maths, a holdover from the original Python prototype that survived the port to Metal. For small clusters the rounding noise was larger than the signal. The poster says it was right, though it sounded wrong at first.