I know that Bismuth is not stable, Bi209 has a lifetime of 19*10^18y. I also know that before decay was experimentally confirmed, it had been theoretically predicted. My question is - HOW? Is color chromodynamics advanced enough so it can calculate for any number of neutrons and protons the lifetime of the nuclei? Bi-209 contains more then 6 hundreds quarks - is it really possible to calculate a system with that many 'particles' just based on the QM equations? Or may be, as there are so many particles, artificial models (droplet, shells) are used? But this is really strange: energy margin between Bi-209 being stable and having lifetime of 10^19y is so tiny that these ad-hoc models must be fastastically accurate! Please help.