The 47 million thing is based on a specific machine, which was identified as the current 5-state champion in 1990 and is known to halt. https://bbchallenge.org

The 47 million thing is based on a specific machine, which was identified as the current 5-state champion in 1990 and is known to halt. https://bbchallenge.org/story#goal https://bbchallenge.org/story#goal Since nobody has found any busier 5-state beaver than that since 1990, Scott Aaronson conjectured that this might be the actual maximum for halting 5-state machines.